material science and engineering - khanna publishers

1.1. ROLE OF MANUFACTURING ENGINEERIt is the duty of manufacturing engineer to bring the ideas and designs into reality by proper selection of material, machine, and manufacturing process.

To meet the challenges imposed on manufacturing engineers, it is desirable for him to have thorough knowledge of material sciences, design and manufacturing processes. To keep the production competitive he must select optimum tools and machines. He must see the whole picture, both technically and economically. He must be smart enough to visualise the problems likely to be encountered in production right from selection of materials and tooling, manufacturing, planning process layouts, plant layouts, equipment speci cation, tool design, methods development, work standards, value analysis, cost control, nal assembly, inspection, packing and dispatch/shipment. Since computers and automatic controls are nding more and more usage in production processes, the manufacturing engineer must also have good knowledge of computer and microprocessor systems. Computers are also used in manufacturing planning to handle vast amount of data needed to utilise raw information, such as sales forecasts and customer orders, keeping inventory of raw materials, tools and other equipment, information about pricing, details of competitors etc. Computer-aided-design and computer-aided-machining are obvious choice for large industrial houses because of the advantages offered by them and to remain competitive in the market.

Present day manufacturing demands high degree of accuracy for the part to work satisfactorily when assembled. Tolerances and surface nish desired demand great care in selection of manufacturing process, complete quality control and quality assurance programmes, strict inspection at all stages. Some surfaces call for wear resistant properties for which coating of tungsten-carbide may have to be sprayed. Emphasis should be laid on the safety and should be conscious of health and environment.

Needless to say, manufacturing engineer has to be fully aware of the use of optimum cutting speeds, feeds, and depth of cut, selection of proper cutting tool materials to have substantial savings in machining time, and also be conscious of conservation of energy.

1.1.1. Manufacturing can be defined as the transformation of materials and information into goods for the satisfaction of human needs. Manufacturing

can be considered as a system in which product design is the initial stage, and the delivery of nished products to the market is the nal output. The eld of manufacturing integrates many disciplines in engineering and management.

1.1.2. Manufacturing Processes, Equipment and Systems. Manufacturing processes alter the form, shape and/or physical properties of a given material.

Manufacturing equipment is used to perform manufacturing processes. Manufacturing systems are the combination of manufacturing equipment and humans bound by a common material and information ow.

1.1.3. Classi cation of Manufacturing Processes.Manufacturing processes can be classi ed as—

(a) Forming processes—original shape is created from a molten stage or from solid particles by creating cohesion among them.

(b) Deforming processes—convert original shape of a solid to another shape without changing its mass or material composition, maintaining cohesion among particles.

(c) Material removing processes—material removal occurs.

(d) Joining processes—unite individual work pieces to make sub-assemblies or nal products.

(e) Material properties modi cation processes—These purposely change the material properties of a workpiece in order to achieve desirable characteristics without changing its shape.

1.1.4. Manufacturing Attributes to be Considered for Making Manufacturing Decisions. The four classes of manufacturing attributes for decision making for manufacturing systems are cost, time, quality and

exibility.Costs related to manufacturing are : equipment

and facility costs, material costs, labour costs, energy, maintenance and training, overheads and the cost of capital and nances.

Time refers to how quickly a manufacturing system can respond to changes in design, volume demand etc. ( exibility) and how quickly a product can be produced by the system (production rate).

Flexibility. In present times, mass production is being replaced by the era of market nitches. The key to creating products that can meet the demands of a

Material Science and Engineering1

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diversi ed customer base is a short development cycle yielding low cost, high quality goods in suf cient quality to meet demand. This makes exibility an increasingly important attribute of manufacturing. Flexibility refers to the ability of a system to cope with both external changes (jobs to be processed) and internal changes (disturbances). The exibility of a manufacturing system is determined by its sensitivity to change.

Quality. It broadly relates to customer satisfaction. In manufacturing, quality refers to how well the production process meets design speci cations related to different features and properties of a product.

1.1.5. Safety, Health and Environment. Safety needs to be built into design and manufacturing. Training for safe practices is important. Occupation health of employees is a matter of concern. To keep environment clean by all means has attained serious proportions now-a-days.

1.2. ROLE OF MATERIALSA vast range of materials are available today at the disposal of engineer. A proper selection has to be made to suit the requirements. A large variety of steels to suit any application, plastics to resist attack by acids and capability of fabrication into a variety of shapes, ceramics to withstand high temperatures, metals to stand up to the environment in a nuclear reactor, semi-conductors for use in computer circuits are available. Effective design in engineering calls for our ability to put them to the best use by selecting the right material for a given job. We have to understand why different materials behave differently in service and the principles involved. By careful selection and treatment it is possible to impart different properties. It is important for us to understand all these principles in order to be able to make best use of materials available to us. To achieve the optimum blend of properties, an engineer may have to make use of a variety of metals, organic materials and ceramics.

The selection of a material for a particular application involves consideration of factors like service requirements (strength, the manner in which load is applied—steady/

uctuating/sudden, wear, corrosion resistance, electrical properties, aesthetic considerations, etc.), manufacturing requirements (ease of machining, finish desired, fabrication technique to be adopted, casting, moulding, welding etc., method of forming—hot or cold, method of joining various sub-assemblies, need of heat treatment to achieve or restore properties), cost of raw material. More than often one may have two or three possible solutions in selection of an appropriate material. The nal decision then should be based on preference and experience of designer and user and considerations like ease of repair on occurrence of faults, availability of repair facilities, skill of personnel, useful life, etc.

1.3. MATERIAL SCIENCEMaterial Science and Material Engineering provide information and understanding pertaining to inner structure of material upon which manifestation of various behaviours are based. There provide understanding about

various properties, behaviours, processing, protection and disposal of commonly used materials in engineering practice. Preparation of manufacture of the materials, controlling and modifying their properties during shaping, sizing or use is also integral part of the subject of Material Science. New materials with exotic properties are produced and methods of modifying the properties of new and existing materials are been deviated the design engineers and manufacturing technologists readily make use of newly developed materials, and methods of modifying the properties for purposes of economy, load carrying capacity, temperature tolerance, ease of manufacturing of substitution.

Materials for use as memory components, easy to crystallize according to preconceived programme and those having desired electronic properties, magnetic behaviour and super conductivity are still being searched and developed.

Materials considered for making engineering products are generally solids and these are classi ed broadly as metallic, plastic or polymeric and ceramic materials. Many metals are mixed with non-metallic elements like carbon, nitrogen, oxygen, phosphorous and sulfur. When a metallic element is having additives the resulting material is called an alloy. Normally an alloy has a base element and small proportions of additive and is known as alloy of base element.

All metallic materials are crystalline in nature in the sense that atoms are arranged in a particular order which is repeated all through its structure. The metallic materials are strong, conductors of heat and electricity can be shaped by application of force of cut by tools. Their mechanical strength; thermal and electrical conductivity; electrical, electronic and magnetic properties depend upon crystalline structure.

Polymeric materials or plastics have gained considerable popularity as engineering materials. Though inferior to most metallic materials in strength and temperature resistance, they offer the advantages of convenient manufacturing into several shapes and sizes right from molten state. They also offer the advantage of smooth surface which is not attacked chemically by atmosphere. Since plastics are not bio degradable then present the problem of nal disposal. These materials are bad conductors of heat and electricity. They nd use as electrical insulators in several applications.

Polymeric materials are long chain molecular organic compounds. The chains many times form network. They are basically non crystalline but some plastics may contain regions of crystalline and non-crystalline structures.

Ceramic materials are basically inorganic compounds of metallic and non metallic elements. They may be crystalline and non crystalline in structure. Ceramicmaterials are well known for their thermal resistance, insulative properties, high hardness, light weight and brittleness. For these reasons they become good materials for furnace lining and liquid metal container. They are also being used to fabricate combustion chamber and bladings in gas turbine plants.

MATERIAL SCIENCE AND ENGINEERING 3

The basic nature of composite material has existed for several years. In reinforced concrete steel rods are ller or reinforcing materials in the matrix of concrete. The basic principal is that reinforcing material is stronger than the matrix material. The principle has been extended to strong graphite bres in the matrix of relatively weaker plastics such as epoxy polyphenylene sulphide. The strength of composites is directional and better than the matrix material which is greater bulk. The metal bres of stronger materials in softer matrix like aluminium are also used to obtain composites stronger than matrix materials. It is important to note that bres and matrices should not dissolve or react with each other.

New metallic materials are being developed in conjunction with new processing techniques like isostatic pressing and isothermal forging capable of imparting better fatigue properties to-aircraft components. Powder metallurgy technique while producing nished surfaces is also capable of imparting improved mechanical properties under several loading conditions. Rapid cooling technology achieving high cooling rates is used to produce and hot isostatic pressing to yield temperature resistant parts. Several molybdenum and aluminium alloys as well as alloys of titanium and nickel are being produced to meet anti-corrosion properties at elevated temperatures. A new trend in plastic technology is the production of synergistic plastic alloys which have better properties than two individual members producing the alloy. Ceramics have mainly been used as high temperature low load carrying materials. Their major drawback is the brittleness and dif culty in cutting and shaping. Ceramic specially when mixed with metal powder like molybdenum, producing cermets are expected to be useful cutting materials. Alumina, a well known ceramic is likely to be successfully reinforced with bres of molybdenum.

1.3.1. Chemical Bonding. The chemical properties of the elements depend mainly on the reactivity of the electrons in outermost shells or energy levels.

Electronegativity is understood as capability of an atom to attract electrons to itself. By losing electron an atom becomes positive ion or cartion. By gaining electron the atom becomes negative ion or anion. The electron af nity is the energy given out when an electron is added to an atom and an anion is formed. The energy required to remove the most loosely bound electron from an atom is called ionization potential.

Electronegative elements have high values of ionization potential which decreases from top to bottom. Fluorine, having highest ionization potential, is most active chemically. The electronegative elements are described in terms of electron af nity rather than ionization potential.

Oxidation Number is another measure of above two characteristics. The number of electrons given up by an electropositive atom is represented by a positive oxidation number whereas a negative oxidation number indicates number of electrons accepted by an atom. Carbon, silicon, germanium, arseniac and antimony are some elements belonging to groups IVA and VA which act both as electronegative and elctropositive. Thus they show the characteristics of both metal and non-metal.

Electronegativity is defined as a degree to which an atom attracts electrons to itself. Each element is assigned an electronegativity number, measuring on scale from zero to 4.1.

1.3.2. Chemical Bonds are as a Result of Binding Forces between Atoms and Molecules. The atoms and molecules are held together by strong mutual forces of attraction which are electrostatic in nature and depend upon the electronic structure of the element.

Chemical bonds may be either primary or secondary bonds. Primary bonds are interatomic bonds (these bonds are stable and strong). Secondary or molecular bonds are formed due to intermolecular attraction forces or van der Waals forces between molecules. These bonds are weak and unstable.Primary bonds have further three classi cations.

(i) Ionic bond is formed due to the attractive force between a positive ion and a negative ion when they are brought into close proximity. Ionic bonds are unidirectional and they form compounds which general characteristics like crystalline in nature, high strength and hardness, high melting and boiling temperature due to strong electrostatic forces binding atoms non-conduct of electricity and highly soluble in water.

(ii) Covalent bond is formed by sharing of electrons between the atoms of non-metallic elements as the atoms of these elements, usually have incompletely lled outer electron orbits. Covalent bonds are directional in nature and covalent compound can be solids, liquids and gaseous. These tend to produce materials with high strength but brittle high melting and boiling temperature, soluble in nonpolar solvents instead of water.

(iii) Metallic bond is formed by the partial sharing of valence electrons by the neighbouring atoms.Secondary bonds are also of 3 types

(i) In dispersion bonds, the distribution of electrons is not symmetrical about their nuclei, which causes the displacement between the centres of positive and negative charges, creating electronic imbalance of the charge (polarization which uctuates with motion of electrons and is known as dispersion effect). Thus in dispersion bond, there is a weak force of attraction between two molecules of the same element.

(ii) In dipole bonds, unequal sharing of electrons creates opposite charges on the molecules, producing permanent dipoles. These are weaker than primary bonds but they are stronger than dispersion bonds.

(iii) Hydrogen bond is a special type of dipole bond and it is produced between covalently bonded hydrogen atoms and oxygen atoms.1.4. PHYSICAL METALLURGYMost of the material properties are dependent on its atomic structure. The changes in atomic structure do not in uence the surface appearance and as such no judgement about material properties can be made by seeing the surface appearance. For de nitive comparison, microscopic and X-ray examination are used to show the relationships between structure and properties of materials. All materials are made up of atoms. An atom of an element is the smallest particle that retains the physical characteristics of that element. (Iron atom has


a diameter of 1.24 × 10–10 m). Matter is a collection or agglomeration of atoms whose position and behaviour determines its properties and characteristics.

In metals the atoms occupy xed positions and strong bonds exist between them. A degree of order (lattice structure) exists in the atomic arrangement in metals. The atoms are located at sites which are decided by the structure of the atoms and the nature of the bonds. The combination of the properties of the atoms and the way in which they are assembled into collections determines the essential characteristics of a material. The mechanical properties of a particular metal are related to the patterns found in its lattice structure. In iron, the atoms are arranged in regular lines, in layers throughout the thickness (long-range order) (Refer Fig. 1.1), called crystalline structure. Some non-metals like common salt, graphite also have a crystalline structure, but most non-metals are not crystalline. In non-metals, there is no recognisable repeated grouping, but molecules are distributed in random manner as in the case of glass. Materials which do not show long range order are often termed non-crystalline or amorphous.

Million Atoms

in each Direction

Fig. 1.1. Layers of atoms in iron.

The ability of an atom to bond with other atom depends on the number of electrons in the outermost shell (valence electrons) which varies from one to eight. If there are less than eight electrons in outermost shell, it is incomplete and contains vacant sites. In most non-metallic compounds (as with plastics) atoms are joined together by covalent bonding (sharing of valence electrons). Two or more atoms bonded together form a molecule. More than one atoms can combine to form a molecule having completely different characteristics than the constituent elements.

Some atoms can enter into a different type of bond which relies on their ability to gain or lose electrons. An atom after losing/gaining electron acquires positive/negative charge and is called ion. Ionic bond is established by attraction between electropositive and electronegative ions.

1.4.1. Crystallography. Crystallography is science dealing with internal structure of crystals, and properties of crystalline metal external and internal symmetries possessed by crystalline solids are also studied. The atoms in any crystalline material are arranged in a regular three dimensional space in repeating pattern. A crystal is a 3-D translation ally periodic arrangement of atoms. The three dimensional pattern of atoms present in a crystalline material is called crystal or space lattice.

All metals and alloys have their atoms arranged in a regular order, at the point of a space lattice which is

periodically repeated in three directions. The smallest unit of this lattice is a unit cell. A crystal is composed of these unit cells, piled upon one another like, bricks, and reproducing at their corners the points of the lattice.

c

a

b� � �

Fig. 1.2. Crystal lattice.

The edges of the unit cell (parallelepiped) are denoted by a, b and c and the angles between the edges by a, band g (Fig. 1.2).

Crystal Structure. When a metal is melted, metallic atoms detach themselves from other metallic atoms and vibrate rapidly in random directions. For solidi cation of metal from liquid stage, all that is required it that a suf cient number of atoms may exist in the proper arrangement so that a crystal may grow. Various crystals or grains grow and join in a dendritic columnar structure. In fact growth is in a three-dimensional pattern so that atoms exist in straight lines and in planes. Fig. 1.3 shows schematically the way unit cells formed at places grow in all the three directions to produce a crystal, or grain. A boundary, known as a “grain boundary” is formed when development of growth of crystals is stopped by interference with adjacent structures, (Refer Fig. 1.4).

Fig. 1.3. Growth to produce grain.

Grain

Dendritic

Structure

Grain

Boundary

Fig. 1.4. Formation of grain boundary.


Metallic Bond. Metallic bond exists between a large number of atoms in close proximity. In a piece of metal, the valency electrons of all the atoms are shared mutually in a complex system of orbitals. Metallic structure can be visualised as comprising metallic ions occupying xed positions in a cloud of electrons which are moving along a number of limited paths. The interaction between a large number of atoms leads to the formation of a three-dimensional lattice structure which is a characteristic of all metallic materials. Metal atoms in a lattice may occupy the corners of a cubic space or hexagonal arrangement. In hexagonal arrangement there is less space between the atoms and it is thus also called close packed. These two unit cells (cubic and hexagonal) feature in most of the lattice structures found in metals. The shape and dimensions of the lattice structure in a metal play an important role in the control of mechanical properties. X-ray diffraction studies show that the lattice parameter of the cubic unit cell (i.e. the length of one of its sides) in case of iron metal is 2.86 × 10–10 m. X-Ray diffraction techniques also reveal that there are following three basic lattice arrangements for majority of common metals.

Space LatticesThe four forms or crystal structures are:1. Simple cubic crystal structure: In it there is one

lattice point of each of 8 corners of the unit cell.2. Body centered cubic (b.c.c.): This form has an

atom at each corner of the unit cell and one atom at the geometric centre of the volume of the cube. This type of lattice is found in chromium, molybdenum, vanadium and alpha and beta iron.

3. Face centered cubic (f.c.c.): fcc lattice has an atom at each corner of the cube and one at the centre of each face; for example, nickel, manganese and gamma iron.

4. Close packed hexagonal (c.p.h.):An atom is located at each corner and in the centre of the hexagonal face with three more atoms in the middle plane of the hexagonal prism; for example, zinc, magnesium and cadmium.

b c c. . . lattice f c c. . . latticec. . . latticep h

Fig. 1.5 Fig. 1.6 Fig. 1.7Space lattices.

Imperfection in Lattice Structure. Theoretically above lattices should be found uniformly throughout the length, breadth and depth of metal.

Any crystal with perfectly regular crystal structure is called ideal crystal while real crystals always have certain defects or imperfections. Natural crystals contain different types of defects due to conditions under which these are formed.

Various types of imperfections present in solids are:Point imperfection: These could be Zero-dimensional

defect (a) vacancy, (b) substitutional impurity, (c) Interstitial impurity (d) Frenkel’s defect and (e) Schottky’s defect.

Line imperfection: These are of three types One-dimensional defect (a) Edge dislocation, (b) Screw dislocation and (c) Mixed dislocation.

Surface imperfections: These are of following types Two-dimensional defects (a) Grain boundary, (b) Twinning, (c) Low angle boundary, (d) High angle boundary, (e) Twist boundary, (f) Stacking fault and (g) Interphase.

Volume imperfections: These could be Three-dimensional defect (a) Pores, (b) Foreign particle inclusion, (c) Non-compatibility regions and (d) Dissimilar natured regions.

Due to some interruption in the growth of the crystal from molten metal or due to inclusion of atom of another metal, imperfections do occur. Imperfection may be in the form of dislocation, or a vacancy in the crystal lattice. Dislocation refers to a break in the continuity of the lattice. In edge dislocation, one plane of atoms gets squeezed out. A plane of atoms comes to a stop and at this point the two neighbouring planes move closer until they are at the correct interatomic spacing [Refer Fig. 1.8 (a)]. Dislocations are an essential element in the forming of metals because they allow plastic deformation to take place at achievable stress level. Vacancy is another imperfection which occurs due to an atom not taking up the prescribed site [Refer Fig. 1.8 (b)]. It plays an important role in the diffusion or movement of atoms, through the lattice.

DislocationVacancy

(a) Dislocation in a Crystal. (b) Vacancies in a CrystalLattice Lattice

Interstitial

Atom

(c) Interstitial atom.

Fig. 1.8

Another crystal-lattice imperfection is interstitials, i.e., extra atoms that t into the interstices between the normal atom structure. Such a structure exists when carbon atoms (much smaller) are introduced in iron atoms and they squeeze into the structure.


Elastic and Plastic Deformation. Below elastic limit, the crystal structure yields by a small amount temporarily and recovers when load is removed. But beyond this limit, the atomic structure slips along certain crystal planes called slip planes. Metals having few slip planes and few directions of slip (like HCP) are dif cult to form. It may be noted that slip systems are 3 in HCP, these are 12 and 48 respectively in FCC and BCC metals. However, HCP metals experience deformation by twinning by plastic deformation. All the atoms in the twinned region move a given amount and change orientation.

Strain Hardening. External force causes slip to occur in the atomic structure at the points of imperfection or dislocation and twinning. With increase in external force, more crystals deform, and more dislocations occur along the slip planes. As more and more dislocations are forced to intersect on various slip planes, strong interactions start occurring offering resistance to further deformation (strain hardening) and metal becomes harder and stronger.

Fracture. Every solid material resists stress upto a point but nally fractures suddenly. Brittle fracture is characterised by the small amount of work absorbed and by a crystalline appearance on the surfaces of fracture. The dislocation defects make the metals vulnerable to crack formation. On application of high load, if no slip is taking place, then atomic bonds are subjected to great stress. Beyond their design limit, a few grains rupture, forming a tiny crack. Once a crack is started, it propagates at a fast rate leading to rupture.

1.4.2. Electronic Properties of Materials. With respect to conduction of electricity the materials are divided into three categories, viz. conductors, semiconductors and insulators. Insulators ceramic and polymeric materials do not permit the ow of current through them.

The measure of conductivity depends upon number of charge carriers n and the charge on each carrier q. Thus

1/ρ = s = nq m.

m in the above equation is the mobility of the charge carrier. Mobility may be considered as a net or drift velocity (v) of the carrier. The drift velocity arises from applied electric eld E in V/m. Thus

m = v/E

Mobility is often expressed as m2/V. s.The charge is carried by the ions or electrons.

Electrons carry the negative charge as they are negative charge carriers. An ion can carry positive or negative charge. A cation carries a positive charge whereas an anion carries a negative charge. Yet another positive charge carrier is an electron hole, i.e., an absence of an electron from its regular orbit.

Superconductivity: Superconductivity is sudden drop in resistance of a conductor as temperature approaches zero K. The phenomenon occurs at a temperature called superconducting transition temperature denoted by Tc. As long as the temperature of conductor will be maintained below Tc it will be superconductor. Superconductivity is

destroyed by magnetic eld either applied from outside or produced by current owing through the conductor.

Dielectrics: Dielectric materials used as insulators in electrical equipment. Polyethylene, polyvinylchloride, neoprene and many other polymeric materials as well as some paints and varnishes are used as insulating materials in electrical machines and transformers. Many insulating material serve the double purpose of insulation and protection from hostile chemical atmosphere. They may be required for use from very low temperatures to as high as 1000°C. Dielectrics are also used for storing electric charges in capacitors. In this capacity they have to be able to cope with ranges of temperature and frequencies.

Polarization: If a material is placed in electric eld the charged particles interact with the eld. If the material is conductor the free electrons simply move to the nearest positive electrode. No field is, thus, left within the material. The displacement of charged particles occurs almost instantaneously bringing about the equilibrium. If the material is non-conducting or insulator or dielectric the electrons are only locally displaced, because they are bound to individual atoms. The local displaced polarizes the material. The negative electron cloud is displaced in each atom with respective positive nucleus. This creates a small dipole whose negative pole is towards the positive side of the eld. All dielectric materials are subject to such electronic polarization. Ionic polarization is induced in those materials which are ionically bonded. The negative ions are attracted towards the positive side of the eld while the positive ions are attracted towards the negative side. The relative displacement of the negative and positive charges induces the polarization in the material.

Break-Down: After relative permittivity and loss factor the third important property of the dielectric or an insulator is the dielectric strength. The dielectric strength of an insulator is the maximum voltage gradient which the dielectric can withstand without failure. The geometry of the specimen, the type of the electrodes and procedure of the testing may affect the dielectric strength resulting into a large scatter in test result.

1.4.3. Insulating Materials. Electrical insulating materials are required to have low dielectric constant low power factor, high dielectric strength and high insulation resistance. Dielectrics used in capacitors and having high dielectric constant is desirable for a greater amount of stored energy in relatively thin insulation. While an insulating material must have adequate mechanical strength to face service condition, the resistance to heat and temperature, low moisture absorption and good chemical stability are necessary requirements.

Liquid and solid insulating materials are commonly used. Liquid materials are mainly used as impregnants for high voltage cables and capacitors. The are also used as lling media in transformers, circuit breakers and terminals. Petroleum oils, being non-polar compounds, are best suited liquid insulators. However, better oxidation resistance at higher temperature is provided


by such electrical insulators as silicon oils, uorinated hydrocarbons and other uorochemicals.

Organic high polymers and ceramics together make the solid electrical insulators. A large spectrum of plastics, rubbers, waxes, papers bres and fabrics is used as solid insulating materials for electrical applications. These insulators assume different forms depending upon the need of machine or equipment. Films, sheets, tapes, slab, sleevings, tubings of exible or rigid type, rods and mouldings are common forms of insulating high polymers. Plastics of the types of polythylene, PTFE, PMMA polystyrene and silicones are used as insulating materials in various applications and they have low disipation factor and high intrinsic dielectric strength. Their dielectric constant is low between 2 and 4. They provide the facility of moulding and machining.

Ceramics are most widely used electrical insulators and include such materials as glass, porcelain, alumina, quartz, mica and asbestos. They have dielectric constants higher than plastics. Very high values of dielectric constants are found in mineral rutile (TiO2) and in ferroelectric and piezoelectric materials. Several of these are used as llers in moulded slabs and other forms of insulators.

Mice splittings, sandwiching paper of fabric and bonded with organic varnish, resin or silicone varnish is a very common insulator in winding applications. They are composite insulators.

The ceramics are very popular insulators in electrical and electronic industries. They have both good mechanical and electrical properties. Ceramic materials have ionic or covalent bonding which make them good insulators. The chemical composition and microstructure of ceramics need closer control for being useful as electrical strength.

Common ceramic materials used for insulations are porcelain, stealite, fosterite and alumina.

Ferro-Electricity: There are a few materials which have a special crystal structure which permits spon-taneous polarization. Such materials are known as ferroelectric. Their spontaneous polarization is like spontaneous magnetization of ferromagnetic materi-als which will be described in next chapter. However, there is no relationship between ferroelectric and fer-rous materials.

Ferro-electrics show spontaneous plarization due to slight irregularity which is induced by thermal agitation in the crystal structure. Barium titanate (BaTiO3) is the classic example of ferroelectric. The permanent polarization is because the central titanium ion is displaced towards one of the oxygen ion. The oxygen ions are also slightly displaced with respect to barium ions. Parallel displacement occurs throughout the crystal lattice. In a polycrystalline structure the individual displacements are aligned by the application of a strong external eld and thus the sample develops an overall electric dipole moment. Suf cient thermal energy may, however, destroy such an arrangement and the permanent polarization may be lost. The temperature at which this occurs is termed ferroelectric Curie temperature. At temperatures: below the Curie point the individual moment are easily aligned,

thereby increasing the permittivity. The ferroelectric Curie temperature of BaTiO3 is 120°C the polarization begins at sacttered points throughout the material. If no

eld is present the orientation at these points is random. As polarization spreads from these nuclei, domains are formed in which the/directions of polarization are same as those of the nuclei. Since there is a possibility of more than one direction, the domains form in such a way that the net polarization is zero.

Piezo-Electricity: In many dielectric materials polarization is accompanied by changes in atomic or molecular positions, resulting in changes in volume. Contrarily an applied stress causing changes in volume will generate an electric field in the material. Such materials are called piezoelectrics and the phenomenon of “pressure-electricity” is termed piezo-electricity. Consequently the measured dielectric constant of piezoelectric materials will assume different values under load and without load.

The piezoelectric effect occurs only in crystals which do not have symmetrical structure. In such crystals the pressure can cause relative displacement of the negative and positive charges in the lattice. Although the strains in the piezoelectric crystals causing changes in the electrical eld make the charges appear in the surface, these charges are soon neutralized by external charges which are attracted to the surface.

Pyroelectricity: Some crystals, notably tourmaline show a net polarization when heated uniformly. This effect is known as pyroelectricity. Extremely sensitive thermometers having sensitivity of 10–7°C can be made using pyroelectric crystal such as tourmaline.

Electrostriction: All materials changes shape to some degree when under the in uence of an electric

eld. The mechanism is known as electrostriction. The polarization of the atomic and molecular components in the structure is the cause of such deformation. If in a very simplistic model, the polarization is considered to change the spherical atom into ellipsoidals the expansion would occur in the direction of the major axis of the ellipsoids accompanied by the contraction along the minor axis. Reversing the electric eld naturally changes the shape of the atoms in exactly the same nammer, so that the electrostictive defect is independent of the eld. The deformation is proportional to the square of the electric

eld applied. The reverse of this is not true that is the applied stress will not produce the polarizations of the type involved in an electrostriction. The electrostriction deformations are much smaller than those involved in piezoelectric effect.

Extrinsic semi-conductor: If an impurity is present in a semi-conductor such that it occupies substitutional position and has a valence greater than that of the original material than an extra electron will be present which does not enter the bonding state. This extra electron is attracted to the positively charged region of the impurity atom. The binding energy between this electron and the positively charged region is, to conduction band by thermal energy or externally applied voltage and impurity of this kind is called donor since it donates conduction


electrons without producing holes in the valence band. The donar level represents the ground state of the extra electron of the impurity atom. This type of semi-conductor is known as an n-type extrinsic semi-conductor.

If impurity atom in an extrinsic semi-conductor has valency 1 less than that of the original atom then one of the bonds surroundsing the impurity atom will be missing. This will result into existence of a hole in the valence band. If the hole moves away from the impurity atom a net negative charge will remain at it. The liberation of the hole from the impurity atoms is equivalent to excitation of an electron from the valence band. During this process the impurity atom accepts an electron and is, therefore, called an acceptor atom or in acceptor state. The excitation of electron dur to this acceptance becomes the cause of

ow of electric current. This type of semi-conductor is designated as a p-type extrinsic semi-conductor.

Ge and Si are intrinsic semi-conductor they become n-type extrinsic semiconductors when substitutional impurities like P, as or Sb are added to them. Si and Ge are tetravalent while P, As and Sb are pentavelent. Similarly the trivalent elemets such as B, Al, Ga are substitution-ally added to Si and Ge, they convert them into p-type extrinsic semiconductor. The process of adding small amounts of substitutional impurity to produce extrinsic semiconductor is called doping. The impurity atoms are called dopants.

In many intrinsic semi-conductor materials the impurity concentration is less than one part per million. If this concentration increases from 100-1000 part per million they become extrinsic material. The donor impurities like P, As and Sb have their levels close to conduction band. On the other hand the acceptor impurities like B, Al and Ga establish their levels close to valence band.

1.5. INTRODUCTION OF ENGINEERING MATERIALSBroadly, engineering materials can be classi ed as (a) metallic materials and (b) non-metallic materials. Metallic materials could further be classi ed as (a) ferrous metals (consisting of iron and alloys of iron) and (b) non-ferrous materials and alloys. Non-metallic materials include plastics, asbestos, rubber, wood, concrete, ceramics etc.

The selection of a material for a particular application is governed by the working conditions to which it will be subjected, ease of manufacturing and the cost considerations. Pure metals find few applications in engineering, rstly because they are dif cult to produce in pure condition and secondly they generally have poor strength in pure form. The various desired and special properties can be achieved by addition of different materials to form alloys. Alloy comprises of a base metal (usually more than 50% content) and one or more alloying elements. The typical properties associated with working conditions are tenacity, elasticity, toughness and hardness and typical properties associated with manufacturing processes are ductility, malleability and plasticity. The various properties can be determined by

testing techniques e.g. tensile strength is determined by tensile test, ductility by bend test, resistance to abrasion by hardness test, toughness by impact test and other special properties like fatigue and creep by fatigue tests and creep tests.

The materials can be manipulated in several ways; the choice being governed by the material, properties desired, shape to be produced, accuracy desired, quantity to be produced and cost aspects.

The mechanical properties of a material determine its usefulness for a particular job. Understanding of mechanical and physical properties of metals and how these are measured help in the selection of materials. Understanding of characteristics like creep, scaling, brittle transition, corrosion, fatigue, etc. is of help to be more aware of problems experienced in severe environments. The great utility of metals is due to their elastic behaviour to a certain level of stress followed by a plastic behaviour at higher levels of stress.

It has been established that the life (tool wear) of cutting tools, surface finish, and machinability of metals have no direct relation in any simple way with such physical properties of the work piece as hardness, yield point, or ultimate tensile strength ; but these are directly related to the microscopic structure of the metal as revealed by metallographic examination. However, physical properties of metals are also of much interest in many other ways. Therefore, we will now study the most commonly used metals in respect of their composition, physical properties, metallographic structure and engineering uses.

The structure and characteristics of pure iron are the simplest. How the structure of pure iron will look at a magni cation of 1000 × is shown in Fig. 1.9 which shows the grain boundaries. All metals are made up of many individual crystals or grains, the strength and hardness of a given material depending upon its mean grain size. Pure metals are relatively weak and soft compared with ordinary structural materials, but their hardness and strength increase with decrease in grain size. It may be noted from Fig. 1.9 for pure iron, that the atoms in any one grain are uniformly aligned whereas the alignment varies from grain to grain. Further the grain boundaries consist of atoms with alignment that gradually change from that of one neighbour to the other.

Fig. 1.9. Pure iron at 1000 × magni cation.


Two important characteristics of metals are their tendency to strain harden and to recrystallise. Whenever any metal is plastically deformed its grains change in shape from a roughly spherical form to an elongated one. The metal becomes harder and stronger as it is deformed. This effect is known as strain hardening and is most pronounced in the grained metals.

When a strained metal is heated, there is a temperature at which new spherical crystals rst begin to form from the old deformed ones. This temperature is known as the Recrystallisation temperature and process is called Strain recrystallisation or Process annealing. For iron it occurs at about 650°C.

1.6. ALLOTROPIC FORMS OF IRONIron has got the following three allotropic forms of crystal at different temperatures. When iron is heated from normal temperature to high temperature (molten state), it undergoes all the allotropic forms.

1. Alpha Iron. It occurs from normal temperature of 910°C and has got body-centered cubic lattice crystals.

(a) Ferromagnetic alpha iron which occurs from normal temperature to 770°C.

(b) Paramagnetic alpha iron which occurs from 770°C to 910°C.

2. Gamma Iron. This occurs from 910°C to 1400°C. It has got crystal structure of face-centered cubic lattice.

3. Delta Iron. This form occurs from 1400°C to 1539°C (molten state). The crystal structure is body-centred lattice.

All these forms have been shown in graphical form in Fig. 1.11.

Fig. 1.10. (a) Body-centered Fig. 1.10 (b) Face-centered cubic α-iron. cubic g-iron.

1539°C

1400°C

Molten Iron

Delta Iron

A4

Gamma Iron

A3

Paramagnetic

Alpha IronA

2

Ferromagnetic

Alpha Iron

Time

770°C

910°C

Tem

p.°C

Fig. 1.11. Allotropic Forms of Iron.

Points of Arrests. In the graph there appear points of discontinuity which are called Points of Arrests or simply A points. In order to distinguish between the various A points, these are numbered as A1, A2, A3 and A4.

The change from ferromagnetic alpha iron to paramagnetic alpha iron is called A2 and it occurs at 770°C. The change from paramagnetic alpha iron to gamma iron which occurs at 910°C is called A3. When gamma iron changes to delta iron at 1440°C the change is called A4.

The point A1 does not represent any allotropic change and does not occur in pure iron. It occurs in iron containing carbon, that is, cast iron and steel.

Physical Properties of Iron. Pure iron is soft and has got silvery white colour. It is strongly magnetic in the presence of a magnetic eld or electric current. The induced magnetism in pure iron, due to the presence of a magnetic eld or an electric eld, is not retained by the pure iron, when the inducing eld is removed. This power of retentivity of magnetism of pure iron is improved by the addition of other elements such as carbon, cobalt, or nickel. Iron loses its magnetic properties when heated to 770°C. It again attains its magnetic properties when cooled below 770°C.

1.7. CLASSIFICATION OF IRON AND STEELIron as available commercially is not a high-purity metal, but contains other chemical elements which have profound effects on its physical and mechanical properties. The amount and distribution of these elements are dependent upon the method of manufacture. The most important commercial forms of iron are :

(i) Pig iron. It is the product of the blast furnace and is made by the reduction of iron ore.

(ii) Cast iron. It is an alloy of iron containing so much carbon that, as cast, it is not appreciably malleable at any temperature.

(iii) White cast iron. It contains carbon in the combined form (cementite of Fe3C) which makes the metal hard and brittle, and the absence of graphite gives the fracture a white colour.

(iv) Malleable cast iron. It is made by changing all the combined carbon in a special white cast iron to free or temper carbon by suitable heat-treatment.

(v) Grey cast iron. This one as cast, has combined or cementitic carbon not in excess of a eutectoid percentage—the balance of the carbon occurring as graphite akes.

(vi) Ingot iron. It is an open-hearth iron very low in carbon, manganese and other impurities.

(vii) Wrought iron. It is a ferrous material aggregated from a solidifying mass of pasty particles of highly re ned metallic iron with which is incorporated, without subsequent, fusion, a minutely and uniformly distributed quantity of slag.


(viii) Puddled iron (steel). It is wrought iron made by the pudding process.

(ix) Steel. It is a malleable alloy of iron and carbon, usually containing substantial quantities of manganese.

(x) Carbon steel. It owes its distinctive properties chie y to the carbon that it contains.

(xi) Alloy steel. It owes its distinctive properties chie y to some element or elements other than carbon, or jointly to such other elements and carbon.

(xii) Bessemer steel, open-hearth steel, cruicible steel, and electric-furnace steel. These are the names given according to the process from which steel is made, irrespective of carbon content.

(xiii) Electrolytic iron. It is produced in the form of thin-wall large-diameter tubes by employing large revolving mandrels as cathodes and ferrous-chloride as electrolyte. It is extremely brittle and can therefore be readily pulverised to a ne powder.

1.8. PIG IRONAll iron produced in blast furnace whether in molten state or cast into pigs is called pig iron. Iron ore, coke and limestone are charged into a blast furnace to produce pig iron. The composition of pig iron varies with the quality of ore and operation of the blast furnace. Due to the reducing conditions in the furnace, the impurities in the ore get reduced and incorporated with iron. These impurities are mainly phosphorus, silicon, manganese, sulphur and carbon. The composition of slag, percentage of fuel and regulation of the blast furnace determine the quantities of phosphorus, sulphur, silicon, manganese and carbon. The presence of these impurities in the pig iron effects the properties of iron and these must be removed to improve the quality of iron. Pig iron can be regarded as an impure form of cast iron and it is the raw material for practically all iron and steel products.Impurities in Pig Iron and Their Effects:

1. Phosphorus. The quantity of phosphorus present in pig iron varies from 0.1 to 2.0%. It combines with iron to form Fe3P which embrittles cast iron. Phosphorus is introduced into iron from phosphate in the ore. Phosphates get reduced in the reducing atmosphere in the blast furnace and phosphorus is formed. This phosphorus combines with iron to form Ee3P. Some of the phosphorus may get oxidised too, into P2O5. This oxidation of phosphorus to P2O5 is an exothermic reaction and takes place at low temperature. P2O5, if formed, is removed by the slag which is basic in character. In this way the quantity of phosphorus in iron is reduced. Its amount is kept as low as possible.

Phosphorus when present in pig iron increases the uidity of molten iron and thus makes the lling of the

moulds in a better way. However, wrought iron or steel made from phosphatic iron is brittle when cold.

2. Sulphur is introduced from coke used in production of pig iron. The quantity of sulphur present in ore varies from 0.4 to 1.0% Presence of manganese in iron lowers the percentage of sulphur by combining with sulphur in iron and forming MnS. MnS formed thus combines with CaO and is removed. In this way the percentage of sulphur in iron is reduced. The reaction of sulphur with manganese is as follows :

FeS + Mn → Fe + MnSMnS + CaO → MnO + CaS

(Slag)MnO + C → Mn + CO

It may be noted that FeS has a low melting point and it forms between the grains that make up the alloy. Iron sulphide being very brittle, whole alloy becomes brittle.

Presence of sulphur tends to make iron hard and produces unsound castings. Wrought iron and steel produced from iron containing sulphur makes wrought iron and steel to be brittle when heated. It makes steel cold-short as well as hot-short and such steel can neither be cold worked nor hot worked.

3. Silicon. Percentage of silicon present in pig iron varies from 1.0 to 4.0%. Source of silicon in iron is from its presence in any of the raw materials.

At low temperatures some silicon is oxidised into silica and basic slag removes it. In this way there may occur some elimination of silicon from iron.

Silicon affects the hardness and strength of iron. Both of these properties worsen by the presence of silicon in iron. Presence of silicon in steel increases its electrical resistance. Silicon when present in steel promotes the decomposition of cementite to form graphite.

4. Manganese. The quantity of manganese varies from 0.2 to 1.5%. The source of this impurity in iron is from the ore. Manganese dioxide in ore is reduced at higher temperature as the reduction reaction is endothermic in nature. Manganese thus formed gets mixed up with iron. Some of the manganese is removed by acid slag.

Presence of manganese in iron reduces the sulphur content of iron by forming MnS thereby improving the quality of alloy since MnS is not as harmful as FeS. Manganese increases the tensile strength of iron. Since Mn promotes combined carbon, it increases hardness of cast iron.

5. Carbon. The quantity of carbon varies from 4 to 4.5 per cent. Source of carbon in steel is coal. Carbon is present in pig iron either in free state as graphite or in combined state as iron carbide.

Carbon in pig iron increases its hardness.Due to presence of impurities in pig iron, pig iron is

too brittle and possesses very little strength and ductility. So most of the pig iron is converted into steel.

Distinction of Pig Iron from Cast Iron. Pig iron is the direct metallic product from the blast furnace without any change in it while cast iron is prepared from pig iron with or without re ning or alloying treatments, or it may be prepared from mixtures of pig iron with steel or scrap or alloying agents.


1.9. FACTORS DETERMINING THE CHOICE OF MATERIAL

The various factors which determine the choice of material are discussed below :

(i) Properties: The material selected must possess the necessary properties for the proposed application. The various requirements to be satis ed can be—weight, surface nish, rigidity ability to withstand environmental attack, attack from chemicals, service life, reliability etc.

The following four types of principal properties of materials decisively affect their selection : (a) physical, (b) mechanical, (c) from manufacturing point of view, (d) chemical.

The various physical properties concerned are: melting point, thermal conductivity, specific heat, coef cient of thermal expansion, speci c gravity electrical conductivity, magnetic properties, etc.

The various mechanical properties concerned are: strength in tensile, compressive, shear, bending, torsional, and buckling loading; fatigue resistance, impact resistance, elastic limit, endurance limit, modulus of elasticity, hardness, wear resistance, and sliding properties.

The various properties concerned from the manu-facturing point of view are : castability, weldability, brazability, forgeability, machinability.

The various chemical properties concerned are—resistance to acids, oxidation, water, oil, bases, greases, petrol, etc.

It may be stressed here that the consideration of properties alone is not suf cient, but there are always other factors which affect the choice of material. However, the following instances cite as to how the selection of material is affected by consideration of properties?

When large forces are involved, the usual choice is steel, but which grade to use will be decided by other factors. In design of a frame for a small appliance, the materials which can be considered are steel, cast iron, bronze, brass, aluminium or magnesium or zinc alloy, moulded plastic, etc. ; but any nal decision will be governed by considering other factors.

In applications where light weight is the consideration, then structural materials like aluminium alloys or magnesium alloys, or plastics lighter than cast iron or steel must be considered. In applications where movement and acceleration are experienced, as in fan blades, pistons and connecting rods in I.C. engines, the use of light metals yields economies which derive from the reduced weight.

However, in the use of aluminium, the designer should bear in mind that its modulus of elasticity is low and thus deformations will be larger leading to elastic instability calling for careful watching of Euler values. Also its thermal expansion is twice that for steel, which calls for special attention when composite methods of construction are used. Although light weight construction materials can be used and designed to offer same degree of safety and stiffness, and at the same time lead to reduced power demand, simpli ed bearing design due to less weight, yet the initial cost and bene ts to be obtained

in the long run should be considered. Aluminium alloys are widely used for aircraft parts ; magnesium alloys having low speci c gravity of 1.8 are used for automotive applications like oil-pump bodies, gear box top covers, crankcases and brake shoes.

For applications calling for cooling or heating by heat transfer through tubes, the material of tubes must have high thermal conductivity. Obviously brass, copper, aluminium and stainless steel are the choice; but other factors like corrosion, erosion, space considerations and economic aspect will decide the selection of one of these.

The use of cast iron, which is best to take up compressive loads and can be easily cast, should be avoided above 300°C because of “growth” set in the cast iron above 300°C. Sometimes some gen-eral rules and past practices and experiences about selection of materials should be followed, e.g. cast iron is exclusively used for machine beds, but never for rods and levers in the larger size.

For longer service life, the parts should be dimensioned liberally to give reduced loading and due consideration given to its resistance to thermal, environmental and chemical effects, and to wear.

A unit required to stand in the open where it is exposed to weather will need to be resistant to atmospheric corrosion.

If there is a possibility of mechanical damage through impact or shock, then the material chosen must satisfy certain conditions in regard to impact strength, notch toughness and elongation.

(ii) Size: Usually size alone does not decisively affect the choice of material used, as both cast iron as well as steel are used for fabrication of large items like machine frames, e.g. the machine frames are now-a-days being usually fabricated in steel by welding, can be cast also.

(ii) Manufacturing Ease: Sometimes the demand for the lowest possible manufacturing cost, or surface qualities obtainable by the application of suitable coating substances may demand the use of special materials.

(iv) Quantity Required: This generally affects the manufacturing process and ultimately the material. For example, it would never be desirable to go for casting of a less number of components which can be fabricated much more economically by welding or hand forging the steel.

(v) Availability of Material: Some materials are scarce- and in short supply. It then becomes obligatory for the designer to use some other material which though may not be a perfect substitute for the material desired. The delivery of materials and the delivery date of product should also be kept in mind.

(vi) Space Consideration: Sometimes high strength materials have to be selected because the forces involved are high and the space limitations are there.

(vii) Cost: As in any other problem, in selection of material also, cost of material plays an important part and should not be ignored.

Sometimes factors like scrap utilisation, appearance, non-maintenance of designed part also dictate in the selection of proper material.


1.10. TESTING OF MATERIALSA material must have suitable mechanical and physical properties in order to be of value to the engineer. It must also be capable of being shaped by any of the production methods like casting, welding, machining, etc. without loss of strength and at the same time also give long and ef cient service at low cost.

Tensile Testing: Elasticity is the property of material to return it to its original shape after deformation.

Plasticity is opposite of elasticity and it enables the material to retain the shape imposed by a force even after it is removed.

Tenacity is the ability of a material to resist breaking under a tensile force.

Ductility is the property of a material to withstand volumetric deformation without fracturing. A ductile material can be drawn into wire form and it can be worked into shape without loss of strength. Such a material yields and becomes deformed when subjected to a shock load. Ductility of material is judged by noting the elongation of material under the action of a gradually applied force.

These properties can be determined by the tensile test on universal testing machine using specially prepared test pieces.

Testing Malleable Materials: Malleability represents the ability of a material to withstand beating ancl get formed without cracking. It may be noted that a malleable material can be formed into sheet or strip, but can’t be drawn into wire; whereas a ductile material can be drawn as well as formed into sheet or strip. This explains the difference between malleability and ductility.

Malleability of a material can be determined by a cupping test in which a dome or cup is formed in a portion of sheet or strip metal by making a plunger to force the metal into a die. After test the specimen must not crack or rupture and must not show orange peel effect if it is to be further surface nished.

The malleability and ductility, particularly of sheet metal, wire and rivet material can be determined by bend test.

The reverse bend test consists of bending a sample of metal through 180° over the radius edges (radius of block being 3 times the gauge thickness of metal) of clamping blocks a speci ed number of times.

Impact Testing. Brittleness is the tendency of a material to fracture when subjected to shock loading. There is no warning before the failure as no permanent deformation takes place before fracture.

Toughness is opposite of brittleness and a tough material resists fracture under load, after the elastic limit has been passed.

Brittleness can be judged by the Izod impact test using standard test pieces.

Hardness Testing: Hardness is the behaviour of a material under the action of cutting tools, and it depends upon the composition, tensile and shear strength of the materials.

The hardness of metals can be determined on Brinell, Rockwell and Vickers Diamond pyramid machines. Sometimes Shore hardness test is also used for metals and rebber.

1.11. CAST IRONPig iron on account of its impurities is very weak and cannot be shaped into different articles by processes such as forging or hammering. It is melted in cupola furnace with scrap steel or scrap iron to control the percentage of carbon and impurities and then cast into moulds of the desired shape. It is then called cast iron. The properties of cast iron are regulated by the control of the amount, type, size, and distribution of the various carbon formations. The important factors are casting design, chemical composition, type of melting scrap, melting process, rate of cooling in the mould, and subsequent heat treatment. It may be noted that cast iron is generally not speci ed by the chemical composition but on the basis of the properties. Cast irons have relatively high carbon content (1.5 to 5%) whereas steels contain upto 2% carbon. The carbon in cast iron can be present in two forms : (i) Combined carbon as iron carbide and (ii) graphite, as a mechanical admixture. Graphite is in the form of dispersed akes occupying from 6 to 10% of the volume of the typical grey iron. These akes impair the continuity of the matrix to such an extent that they exert a very pronounced effect upon the mechanical properties of the metal. However, increase in ake size or unfavourable distribution of the graphite may adversely affect the strength of the metal.

In the absence of silicon (or its presence in very little quantity) most of the carbon in cast iron is in the chemically combined form and the iron is called white iron. The presence of silicon causes softening effect and reduces the ability of the iron to retain carbon in chemical combination. Silicon acts as softener in cast iron as it increases the free carbon and decreases the combined carbon. More silicon is required for light sections as these cool more rapidly, resulting in formation of more combined carbon with consequent increase in hardness. This is counteracted by presence of silicon. Metals like manganese, chromium, molybdenum, titanium and vanadium promote the retention of carbon in the combined form (carbide stabilizers) and counteract silicon, thereby rendering them harder. Nickel and copper improve the matrix and increase the strength of the iron, but they do not lessen the amount of graphite and keep the iron readily machinable. In foundry, the various elements therefore should be so adjusted as to obtain machinable and strong casting.

Melting of pig iron to form Cast Iron. Melting of pig iron is carried out in cupolas which are miniature blast furnaces. The cupola has a column of about 8 metres and is quite uniform in diameter and is lined with refractory bricks from inside. The hearth portion is provided with tuyeres to blow in air. Molten metal is tapped from the bottom.

Pig iron along with coke and lime as ux are charged from the top. The molten metal is collected from the bottom. As the atmosphere inside cupola is oxidising, some of the impurities are removed by oxidation.


Different Types of Castings1. Chilled Casting. The mould used for chilled

casting is either of a metal having high melting point or the mould may be given a lining of such a metal. This results in rapid conduction of heat from the surface of the casting with the result that the casting will have harder outer surface and softer inner core. Chilled castings are very useful in making railway carriage wheels.

The degree of graphitisation is in uenced by the presence of silicon which tends to cause the cementite to break down during cooling to form ferrite and graphite. Since graphitisation requires time, the effect of silicon can be offset by rapid cooling or chilling of metal. Thick sections are faced with chills to conduct away heat. Chills are also used to produce local surface hardness.

2. Centrifugal Casting. The casting produced by pouring the metal in speedily rotating mould is called Centrifugal casting. The crystallisation of the molten metal takes place from the farthest end unlike the normal casting where the crystallisation takes place from sides which are exposed to cooling. Therefore, centrifugal castings are uniform and strong. Even thin castings symmetrical about the axis of a cylinder can be obtained from this method of casting.

3. Malleable Cast Iron. It can be obtained by annealing the castings.

The cast product is packed in an oxidising material such as iron ore or in an inert material such as ground

re-clay. The pack is put into an oven and is heated to a temperature of about 870°C. It is kept at that temperature for about two days and is then allowed to cool at the rate of 5 to 10 degrees per hour.

Iron ore acting as an oxidising agent reacts with carbon and carbon dioxide escapes. The annealed cast product is free from carbon.

If the cast product is packed in an inert material, slow cooling will separate out the temper carbon.

Malleable cast irons are used for complicated structures.

4. Inoculated Cast Iron. The molten pig iron before casting is inoculated with soluble silicon compounds like calcium silicide. The silicon added in this way has got a better effect. Pearlitic cast iron having microstructure of small akes of graphite set in pearlite is formed in this way.1.11.1. Production of Different Grades of Cast Iron and their Physical Properties. Carbon can be present in cast iron as part of the ferrite, part of the cementite or free carbon (graphite).

1. Grey Cast Iron. It is produced by melting together low quality foundry pig, scrapped casting and coke in a cupola which is quite similar to a small blast furnace. The salvaged cast scrap is used to control the alloying elements in the nished cast iron.

When this type of cast iron is fractured, it gives a grey appearance. Therefore, it is called Grey iron. Grey cast iron has got most of its carbon in graphite form. It is quite possible to produce such a type of cast iron with all

its carbon in the form of free graphite akes but it is not always desirable. Generally in castings, 0.8% of carbon is in the form of iron carbide Fe3C, and the rest 2 to 4% is in the form of graphite. As a matter of fact, a complete series of cast iron is possible ranging from cast iron with all the carbon in graphite form to the cast iron with a good share of the carbon in combined form. As the carbides give hardness and strength to the iron, it is possible to have wide range of properties.

Cast iron having all the carbon in graphite form is soft, easily machinable metal having high damping capacity and high compressive strength and has self-lubricating characteristics. But tensile strength, ductility and impact strength are much lower than steel owing to the weakening effect of the graphite akes. In such cast irons there is no well-de ned yield limit and modulus of elasticity. It is used for basic structures of machine tools and structural members loaded in compression.

Cast irons which have got high content of carbon in the form of carbide are hard, brittle and unmachinable and have got resistance to wear. Close-grained iron containing graphite and pearlite is the strongest, toughest and best

nishing type of cast iron. Medium grey irons contain some ferrite with graphite and pearlite and thus have poor strength and poor nish.

A high phosphorus grey cast iron pours very easily and is very cheap also, but is a low quality cast iron. It is used for covers of switch boxes and rain water goods requiring no machining and good physical appearance. High phosphorus cast iron is suited for producing ornamental castings.

The relative amount of free and combined carbon is determined by the variations in composition, melting practice and casting practice. Another factor which causes variation in composition is the rate of cooling of iron in the mould. Slow cooling helps the formation of graphite and when the rate of cooling is rapid, cementite is formed.

Cast iron has lower melting point (1135 to 1250°C) compared to steel (1500°C). The mechanical properties of grey cast iron depend upon its composition, but tensile strength varies between 150 and 400 MPa, hardness between 155 and 320 HB and compressive strength is 3—4 times the tensile strength.

According to IS : 210, grey iron castings are designated by letters FG followed by ultimate strength in kg/mm2, e.g., FG 15, FG 20, FG 25, FG 30, FG 40. It also speci es the important properties like minimum ultimate tensile strength, BHN, results of transverse test such as breaking load, rupture stress and de ection. Where chemical composition is more important, as the percentage of silicon, then in the designation this percentage is also speci ed, e.g. FG 30 Si 12 which means grey iron casting having UTS of 300 MPa and containing 12% silicon.

The basic composition of grey cast iron is described in terms of carbon equivalent which is equal to total carbon % + 1/3 (Silicon % + Phosphorus %). This factor gives the relationship of % age of carbon and silicon in the iron to its capacity to produce graphite.


2. White or Chilled Cast Iron. It has no graphite and is, therefore, white in colour. The whole of carbon content in this type of cast iron is in the form of either free cementite or cementite in lamellar pearlite.

White or chilled cast iron is prepared by two methods :(i) The grey iron is cast in such a way that it is

cooled rapidly.(ii) By adjustment of the composition in such a way

that carbon and silicon content are low.For manufacturing such a kind of cast iron, low

phosphorus pig iron and steel scrap are melted together in an air furnace which is heated from above, or in a cupola furnace. However, for getting best results generally duplexing or triplexing processes, which are combinations of cupola, air furnace, Bessemer converter and electric furnace, are adopted.

White cast iron is very hard, brittle and wear resistant iron. Hardness of 400 Brinell can be obtained by keeping silicon below one per cent and carbon to about 2% in cast iron.

When chromium is present above 3% in cast iron, it prevents formation of graphite. White cast iron produced in such a way has got better high temperature strength, grain growth resistance and corrosion resistance besides having ordinary properties of white cast irons.

The toughness and strength of white cast iron is doubled by small additions of nickel and chromium (for example, 4.5% Ni and 1.5% Cr). hardness thus obtained is of the order of 700 Brinell.

This being almost unmachinable, is used in parts requiring high abrasion resistance.

3. Malleable Cast Iron. Malleable cast iron is produced by annealing the white cast iron. The annealing process consists of heating it slowly to 870°C and keeping at this temperature for 25 to 60 hours, depending upon size and then cooling slowly. Process of annealing is carried out by two methods.

(i) Annealing in which castings are packed in an oxidising material. In this way some carbon is removed and the malleabilized castings are known as white heart.

(ii) Annealing in which castings are packed in inert material such as ferrous silicate scale or slag; such malleabilized castings are called black heart and consist almost entirely of graphite and ferrite.

It is tougher than grey cast iron and more resistant to bending and twisting. It is used for various automobile, tractor and plough parts, gear housing etc. Malleable cast iron as per BIS is classi ed as black heart, pearlitic, and white heart and accordingly designated by letters BM, and WM respectively followed by their respective ultimate tensile strength in kg/mm2.

4. Ductile Cast Iron. Ductile cast iron is produced by small additions of magnesium (or cerium) in the ladle. By doing so graphite content is rendered nodular or spheroidal in form and is well dispersed throughout the material.

Composition of ductile cast iron is in the following range:

3.2 to 4.5% C1.0 to 4.0% Si0.1 to 0.8% Mn0.1% P0 to 3.5% Ni0.05 to 0.10% Mg

Magnesium controls the graphite form as stated above but there is no effect of magnesium on matrix structure. Since graphite in a spheroidal shape occupies minimum surface for a given volume, there are less discontinuities in the surrounding metal giving it far more strength and ductility. Nickel and manganese increase the strength but lower the ductility. Manganese content is kept low as the sulphur is generally very low in ductile irons. Silicon is used as an alloying element.

This kind of cast iron has got very high uidity, castability, strength, toughness, wear resistance, pressure tightness, weldability and machinability. It can be heat-treated in a manner similar to steel. Because of its excellent casting quality, it is suited for both intricate castings as well as big size castings.

Spheroidal graphite iron according to BIS is designated by letters SG followed by UTS in kg/mm2 and the percentage elongation. For example, SG 42/12 means spheroidal graphite iron having ultimate tensile strength of 420 MPa and percentage elongation of 12%.

5. Special processed Iron. Numerous types of cast irons (known as Mechanite) each having a different combination of mechanical and engineering properties suitable for speci c purposes, are produced by licensed or patented controlled processes. Four general classi cation types are : (i) general engineering, (ii) heat resisting, (iii) wear resisting, (iv) corrosion resisting. These have high tensile strength of the order of 1,650 to 3,650 kg/cm2 ; and when oil quenched and tempered, strength of 5,000 kg/cm2

can be obtained.Nodular iron, or ductile iron, is cast iron with the

graphite substantially in spherical or nodular shape and substantially free of ake graphite. There are two grades of nodular iron : (i) cast grade, and (ii) graphitizing annealed grade. It is produced by adding alloys of magnesium or cerium to molten grey iron. The addition of these alloys causes the graphite to take form of small nodules or spheroids instead of the normal angular akes. Tensile strength varies between 440 and 700 MPa depending on composition. Nodular iron has the advantages of cast iron ( uidity, low melting point, good machinability) in addition to high tensile strength.

Various types of special processed cast irons are speci ed as follows by BIS :AFG Ni 16 Cu 7 Cr 2 → Austenitic flake graphite

iron casting having 16% Ni,7% Cu and 2% Cr.

ASG Ni 20 Cr 2 → Austenitic spheroidal graphite iron casting having 20%Ni and 2% Cr.


ABR 33 Ni 4 Cr 2 → Abrasion resistance iron casting having minimum ulti-mate tensile strength of 33 kg/mm2 and containing 4%nickel and 2% chromium.

6. Alloyed Cast Iron. Alloyed cast irons are produced either in cupola by melting together the alloying compounds or by adding the alloying metal to the pouring ladle after drawing the molten iron from the furnace. Still better methods in which uniform compositions are obtained are by heating the allowing compounds in an air furnace or electric furnace.

The advantages of alloying cast irons are the improvement in strength, hardness, corrosion resistance and response to heat treatment.

Nickel is added upto 5% for improving machinability. Addition of nickel may also increase hardness and strength simultaneously. Presence of nickel also improves the corrosion resistance. It promotes graphitization, and thus offsets the effect of thin sections by producing uniformity over thick and thin sections. It also lowers the hardening temperature, and so enables cast iron to be quench-hardened without cracking.

Chromium is added upto 3%. It checks the formation of graphite and promotes the formation of carbides. Thus higher percentages of chromium harden the iron by increasing the percentage of combined carbon. It also increases corrosion resistance. It also increases hardness without the extreme brittleness.

Nickel and Chromium when added in the ratio 3 : 1 (total 4%), make their graphite and carbide-forming tendencies neutralize each other. The resultant cast iron obtained has improved grain re nement, hardness and strength without any loss in machinability.

Molybdenum upto 1.5% in cast iron improves strength and wear resistance but decreases machinability. By the addition of molybdenum, graphitization is slowed down, critical transformation is retarded and thus there is improvement in the uniformity of structure. It causes cast iron to become tough.

Vanadium upto the extent of 0.5% improves carbide formation and thus increases the strength and hardness of cast iron very much. Copper in small amounts produces an improvement in the resistance to atmospheric corrosion.Heat Treatment of Cast Irons

1. Stress Relief. It is carried out by heating the cast iron article to 430—450°C, keeping it at this temperature for 30 minutes to 5 hours and then cooling the article slowly in a furnace. Though the internal stresses are decreased yet there is a slight decrease in hardness or strength at room temperature.

2. Annealing. Cast iron is sometimes softened to facilitate machining. This is carried out by annealing process which consists of heating to 760°C—825°C (up to 980°C in case of alloyed iron), maintaining at this temperature for some time and then cooling slowly.

Annealing increases the free carbon but decreases the strength though in case of alloyed steels, the strength reduction is less.

3. Hardening. Hardening is generally carried out in case of alloyed steels and accomplished by heating above transformation temperature of 815°C to 870°C, quenching and then tempering to improve hardness and resistance to water. By hardening 0.5 to 0.8% combined carbon is converted to pearlitic or sorbitic structure. Quenching is usually done in oil but sometimes water and air quenching are also used.

Alloyed irons of special compositions are also nitrided to get high surface hardness and wear resistance. Nitriding is carried out at 510°C to 600°C in contact with anhydrous ammonia gas. Time taken is generally 20 to 90 hrs, depending upon the depth and size of hardening contemplated.

4. Malleabilising. This is a lengthy heat treatment process to improve the strength of cast iron by changing the shape and size of the graphite. This treatment suits best on white cast iron in which carbon is more evenly distributed throughout the structure. Three types of cast irons, viz. white heart, black heart and pearlitic are produced by this treatment.

In the white heart process, the castings are packed into boxes with haematite ore, slowly heated to about 900°C, held at that temperature for several days, and

nally cooled to room temperature. In this way these sections are completely decarburised. White heart cast iron is used for motorcycle sockets, agricultural machines, etc.

In the black heart process, air is excluded during heating by surrounding the castings in the containers with an inert substance, to prevent decarburisation. The cementite is broken down to form rosettes of graphite in a matrix of ferrite. Casting for this treatment should not contain more than 2.5% carbon. As melting point of cast iron increases with decrease in carbon content, this iron is dif cult to cast. Black heart malleable iron is used for axle-boxes, rear axle housings, wheel hubs, etc.

The treatment for pearlitic cast iron is similar to black heart iron, but pearlitic structure is produced by increasing the manganese content is about 1%, or by heating a quenched and tempered black-heart malleable iron. This process produces cast iron structure similar to that of steel and such cast iron is used for axle and differential housings, gears and camshafts.

The tensile strength and elongation of three types respectively are 350, 300 and 450 MPa and 5%, 10%, and 5%.

1.11.2. Important Properties of Cast Iron(i) Mechanical Properties. The tensile strength of

cast iron varies from 135 to 53m MPa. The elastic limit is close to its ultimate breaking strength. Grey iron can sustain inde nitely a static load just short of the tensile strength without distortion or breakage. Grey iron has low ductility and breaks with perceptible distortion. Since grey iron does not distort prior to breaking, it is essential that service stresses be known or else a conservative safety factor be employed.


With static loading the ultimate strength of cast iron in tension is less than that in compression ; the impact strength of most cast irons is low. The damping capacity, or the ability to absorb vibrations, is high. The ease of machining grey iron is usually inversely proportional to the strength of the casting. Chilling, heat treatment and alloy additions reduce the machinability. The white iron or chilled-iron casting is widely used for machinery parts to resist wear. High-alloy cast iron of the chromium, nickel and silicon type is specially resistant to sulphur and acid corrosion.

Cast iron is most widely used in engineering and allied industries because of the ease with which it can be cast and its wide range of useful properties. Cast iron is a very general term. Actually it is available in the forms of soft, weak, hard, brittle, and strong irons.

(ii) Machinability. Cast iron has wide range of machinability, that is, from very good to the most unmachinability. Annealed permanent mould iron has got the highest machinability as the carbon in such a cast iron is in the state of nely divided and dispersed graphite akes and not in the combined state as carbide, and moreover it is free from burned-in sand at the surface. Ductile cast iron has also got very high machinability. The following is the decreasing order of the machinability of the various types of cast irons :

Pearlitic ferritic irons.Pearlitic irons.Motteled iron with pearlitic and massive cementite

white iron.White iron is particularly very dif cult to machine

because its structure is largely massive carbide.(iii) Weldability. The weldability of all the cast irons

is quite low. Forge and submerged-melt welding cannot be used for cast irons. Gas and arc welding can be employed with special rods especially when sections have more than 6 mm thickness provided casting is heated red hot before welding and then cooled slowly to room temperature. Bronze welding is used for grey irons and for white irons before malleabilizing and without preheat, provided temperatures obtained are 810°C to 860°C.

(iv) Corrosion Resistance. Though cast irons are not resistant to rusting yet the formation of rust is very slow and slower in comparison to alloy steels. Cast irons with high silicon and high chromium content are quite resistant to acids. However, both these and also the unalloyed grades have very little resistance to alkalies. High nickel austenitic irons are resistant to acids, (excepting nitric acid) to stress corrosion in hot and to alkalies if stresses are low. High silicon (11 to 17% Si) cast irons are remarkably good for withstanding all acids except hot hydrochloric acid.

(v) High Temperature Usefulness. For pressure vessels, grey cast irons are useful upto 340°C and for other applications they can be usefully employed upto 425°C. When grey cast iron is heated repeatedly above 425°C, grain growth, distortion and brittleness are caused. Too much scaling takes place if it is heated above 580°C. Lower

carbon, lower silicon and more chromium contents in cast irons increase their permissible temperatures.

Q. 1. Explain the effect of silicon on properties of cast iron.

Ans. Silicon occurs in cast iron as FeSi dissolved in the ferrite. It acts indirectly as softener, promotes graphitization, fluidity and roundness but reduces strength and hardness. It promotes the formation of graphite. The effect on silicon and carbon on graphitization is shown for typical cooling rate in Fig. 1.12.

Carbon

%

4.3

Ferrite

Pearlitic

Ferrite

Pea

rlite

Mottle

d

White

4.0Silicon %

Fig. 1.12Flake graphite in cast iron may be present as

uniformly distributed and random oriented, or as rosetta grouping (mottled iron) or as super imposed ake sizes, or as inter-dendritic segregation and random or preferred orientation.

Silicon increases the scaling resistance and corrosion resistance of cast iron.

Q. 2. S.G. iron (or nodular iron) has better mechanical properties than gray iron and excellent heat transfer properties. It can replace steel castings for several applications. How it is produced?

Ans. S.G. iron is obtained from cast iron by addition of spheroidising agents like magnesium or cerium (which make carbon to occur in nodular form) to the molten steel before pouring the metal into the mould. In S.G. iron, the graphite nuclei are completely surrounded by austenite grains and thus their growth as ake graphite is prevented. They grow by diffusion of carbon through austenite grains and because of the restriction to the growth, they obtain spheroidal form.

Q. 3. Why cast iron is very popular material?Ans. Cast iron has high demand because of its

versatility and the following properties:1. Wide range of strength.2. Resistance to wear, abrasion and corrosion.3. Resistance to warping and cracking in the

presence of heat.4. Ease of machining.5. Ability to take good casting impressions.6. Damping capacity to damp out vibrations in

machine structures.7. Most economical of other cast materials.8. It can be cast into complicated shapes easily.9. Cast iron melts at round 1,200°C, which can be

attained with coke.


Q. 4. Explain in brief various types of cast irons?Ans. (i) White cast iron. White cast iron contains

most of carbon combined with iron to form cementite, which is extremely hard and thus renders the iron unmachinable. Rapid cooling and a low silicon content favour the production of white iron. When it is factured, the iron presents a silvery-white crystalline appearance. It serves as a raw material in the manufacture of the malleable cast irons.

(ii) Grey cast iron. In grey cast iron dissociation of the cementite has taken place (Fe3C = 3Fe + C), with precipitation of carbon in the form of graphite. Slow cooling and a high silicon content favour the formation of graphite, the amount increasing with the increase in silicon content. When it fractures, the iron shows a grey colour due to the presence of the graphite. Grey iron is readily machinable.

(iii) Chilled cast iron. If iron is cast into a metal mould, the portion in contact with the cold surface gets rapidly chilled, and appears white. The interior of the casting, however, is unaffected and remains grey.

(iv) Malleable iron. This material is produced by heat-treating white cast iron with the object of either eliminating the free carbon or changing its shape from

ake form to small rosettes. Such castings exhibit a fair amount of malleability and toughness and are readily machinable.

(v) Spheroidal graphite (S.G.) iron. This iron, also known as nodular iron, is grey iron which has been treated in the molten condition with a small amount of magnesium or cerium which causes the carbon to assume a nodular form rather than akes. Nodular iron possesses mechanical properties almost equal to those of mild steel.

(vi) High duty cast iron. Improvements in properties of ordinary cast iron can be attained by reducing carbon content (graphite); incorporating steel scrap in the melting stock ; casting in hot moulds in order to avoid chilling. One type of high strength iron known as Meehanite, is made by inoculating the molten iron, as it runs into the ladle with calcium silicide. The inoculant has a bene cial effect on the shape and distribution of the graphite akes. Such irons possess high tensile strength and are known as high-duty irons.

1.12. WROUGHT IRONIt is prepared from pig iron by burning out C, Si, Mn, P and sulphur in a puddling furnace. So wrought iron is a purer form of pig iron. Pig iron contains 6% or more of these impurities but their percentage is reduced to about one per cent in wrought iron. Carbon content is reduced to about 0.02% . In the process of puri cation of pig iron into wrought iron, a minute quantity of slag is incorporated into wrought iron and is uniformly distributed in it. The presence of slag gives brous structure to wrought iron.

Properties. It can be readily worked and welded at temperature close to its melting point. It is ductile when cold and has good forming qualities. In resistance to corrosion, it is superior to mild steel. It has many practical applications owing to its ability to take on and

hold protective metallic and paint coatings and its good machining and threading qualities. The mechanical properties of wrought iron are dependent upon the form of nished product.

Wrought iron does not contain impurities and is thus intensely soft. Due to very low carbon content its melting point is high and it cannot be used as casting alloy. Due to poor strength it has very little use. It cannot be heat treated for changing its physical properties. Due to the presence of slag in wrought iron, it resists corrosion. It can be obtained in the form of plates, sheets, forged billets, structural shapes, bars, piping and tubing. The bres tend to halt the crack slightly, rather than fracture outright if overloaded, and so give a warning of danger.

Uses of Wrought Iron. 1. It is used for pipe making due to its superior corrosion and fatigue resistance and better welding and threading qualities.

2. It is used for making bars for stay bolts, engine bolts and rivets etc. because properties demanded in these applications are corrosion and fatigue resistance.

3. For making plates.4. For making special chains and crane hooks due to

its good weldability and high impact strength.5. It is also used extensively for general forging

applications.

1.13. STEEL

Steel is an alloy of iron and iron carbide. It is initially cast into a malleable form, and then it can be changed in shape by forging, rolling or other mechanical processes.

The essential difference between cast iron and steel is that steel never contains graphite or free carbon. Carbon exists in very small quantity in ferrite and majority in cementite.

Besides carbon, steel contains many chemical elements which are added into iron to form steels of different kinds having different physical properties. The wide range of hardness that is possible in steel is a consequence of the carbon present ; the manner in which it is associated with the iron ; and the aggregation of the resulting phase. A phase is related to any homogeneous mechanically separable constituent of an alloy ; and in steel there are three important phases ; Ferrite, Cementite and Austenite. The other elements which are added are given below.

1. Carbon Steels. The following are the constituent elements which form steel :

Iron ... Over 90% in most steels.Carbon ... The element which mainly deter-

mines the physical properties of steel.

Manganese ... Very essential for steels of all types.Phosphorus, sulphur and silicon are present in

varying quantities. The proper adjustments and various combinations of the above elements give a wide range of properties.


2. For alloy steels and stainless steels. Many other elements are added in the above basic elements in order to widen the range of physical, chemical and magnetic properties. Various combinations of the following alloying elements are used :

Aluminium Molybdenum BoronTitanium Chromium Tungsten

Columbium VanadiumCopper Zirconium NickelThe elements which are added for special purposes

are:Bismuth Silver CobaltTantalum LeadRare earth elements of atomic numbers 57 to 61 are

used for making stainless steels.Selenium or tellurium are also used in order to replace

or augment sulphur.Alloys having 50% or more iron are categorised as

steel, and below 50% these are categorised as nonferrous alloys.

Manufacturing Steel. Pig iron contains lot of impurities due to which it cannot be used for machine tools and industrial construction. So pig iron is re ned and mixed with desired elements in a de nite proportion to make steel. The impurities are removed by oxidation using a uxing stage. When the process of re ning involves only oxidation of carbon, manganese and silicon, the process is known as acid. When oxidation is supplemented by the use of lime or other strong base which removes phosphorus, sulphur and silicon, the process is called base. It is produced from pig iron by the removal of impurities in either an open-hearth furnace, a Bessemer converter or an electric furnace, or by the spray technique.

Steel Ingots: After manufacture of steel, it is tapped into a ladle and poured into iron ingot moulds. A great care is required at this stage because many defects in rolled steel products are likely to be introduced by incorrect ingot practice. In order to facilitate withdrawal of ingot, the mould half has to be made tapered.

When mould is placed with big end up, then shrinkage cavity is formed at top and only small portion has to be discarded. When mould with big end down is used then primary and secondary piping effects are observed at top and the portion to be discarded is excessive. Therefore, former arrangement is commonly used.

When molten metal is poured from top, splashing and turbulence are worst. This effect can be minimised by making the molten metal to enter from the bottom.

Killed steel. High quality machine and tool steels are deoxidised in the ladle with silicon and aluminium to such an extent that there is no gas evolution in the steel upon solidi cation and they lie quietly in the mould as it cools. Such steels are called killed steels. Usually a cavity is found in the upper portion of the ingot because of the cooling to steel in the mould and shrinkage of steel on solidifying. The minimise this condition, a large-end-up mould is used together with a refractory “hot-top” which

supplies molten metal to the main body of the ingot while solidi cation proceeds. Killed steel is characterised by relatively uniform chemical composition and properties. Sheets and strips made of killed steel have excellent forming and drawing qualities.

Semi-killed steel. In order to reduce the cost of hot tops and large percentage of metal discard when making mild steel for structural purposes, the steel is not fully deoxidised. This results in blow holes in the steel on solidi cation. The presence of these blow-holes minimises piping by distributing small voids throughout the ingot instead of having one large pipe in the upper centre of the ingot. If not exposed at the surface of the ingot, these blow-holes weld together during rolling. Steel deoxidised in this manner is called semi-killed steel. It is suitable for drawing operation (except severe drawing).

Rimmed steel. If the steel is deoxidised to still less extent in the ladle, a reaction takes place during solidi cation in which the oxygen and carbon in the steel form carbon monoxide which is freely evolved from the ingot at the outer rim of the ingot. The intensity of this reaction affects the ingot structure greatly. If the reaction is allowed to go to completion, the product is called rimmed steel. It is also known as drawing quality steel.

Capped steel. If the above reaction is stopped after a short while by preventing, in a mechanical manner, further evolution of gas from the top of the ingot, the steel is called capped steel. The gas evolution results in an outer skin on the ingot which is clean and very low in carbon. In capped steel, the skin is thinner and there is less segregation or concentration of impurities than in rimmed steel. The presence of this nearly pure iron skin enables the production of an excellent surface nish on the rolled product, and therefore sheet and strip are made nearly exclusively from rimmed or capped steel.

The defects which are likely to occur in steel ingots may be classi ed into two categories, (a) those which can’t be remedied to any large extent (pipe, segregation, and inclusion), (b) those which can be removed by chopping the surface with air hammers (seams, laps and scabs). Segregation is the concentration of impurities which occurs in all steels upon solidi cation. It can be minimised by proper mould design and low pouring temperatures.

Non-metallic inclusions consist of sulphides, silicates etc., and are found to some extent in steels and are introduced in the re ning and deoxidation of the steel.

If the surface of the mould is rough or if it contains cracks or cavities, then these interfere with the normal contraction of the ingot, thus resulting in transverse cracks in the ingot. Cracks thus produced have their surfaces oxidised, and when the ingot is rolled out, these defects are elongated in the direction of rolling and are called seams. Improper pouring conditions such as splashing of steel in the moulds forms scabs.

When rolling with grooved rolls which are not properly designed or set up, ns are liable to result from the ow of metal between the at bodies of the rolls. If the

n is thin and wide, if will be folded over when the steel passes through the next set of rolls and will form a lap.


1.13.1. Role of Carbon in SteelsCarbon is the most important alloying element in steel. It has profound effect not only on the properties of the steel but also on the way in which these can be altered by heat treatment.

Iron and carbon combine together to form a compound, iron carbide (Fe3C) in which one carbon atom is bonded to three iron atoms. In the structure of iron carbon alloys, the carbide is usually called cementite.

The amount of cementite present in an iron-carbon alloy depends on the carbon content. If carbon is not present (or present in small traces), the micro-structure consists of uniform grains of ferrite (pure B.C.C. iron with a very small carbon in solution) which is soft and ductile due to absence of cementite. Cementite consists of 6.67% carbon and it is very hard structure with virtually no ductility and no commercial use. Table 1.1 shows how the ductility of steel varies with increase in carbon content (i.e. proportion of cementite in the microstructure.

Table 1.1. Ductility of Plain Carbon Steels

Carbon content Elongation in tensile testNil 42%0.2 37%0.4 31%0.6 22%0.8 17%1.2 3%

If manganese is also present, then elongation value will be still lower than the values listed in above table for plain carbon steel.

How the properties of carbon steel change with increase in carbon content would be appreciated from following examples:

Steel with 0.08% carbon : It has good ductility with reasonably low yield stress. It can be pressed accurately into shape, especially around sharp corners. Suitable for car body panels with 0.3% manganese.

Steel with 0.18% carbon : It has good impact strength at sub-zero temperatures (to avoid catastrophic brittle fractures) and is also weldable. Added with 0.8% Mn and 0.1% Si, it is used for ship’s hulls and can withstand the stresses experienced in service in heavy seas.

Steel with 0.4% carbon : Has good strength in bending and torsion. Surface layer can be hardened to improve resistance to wear. With 0.8% Mn and 0.1% Si, it is used for axle shafts.

Steel with 1.0% carbon : It is capable of being rolled into rods. It has good ductility and coils can be formed. With 0.6% Mn and 0.3% Si, it is used for helical springs.

Steels normally contain 0.1% to 2.2% carbon and cast iron contains 2.4% to 4.2% carbon. Steels with 0.1% to 0.8% carbon are used for general engineering and with 0.9% to 1.2% carbon are used for wear resistance purposes.

Strength vs. Carbon Content in Steels. With very low carbon contents, the metal consists of ferrite grains having tensile strength of 250 MPa. At 0.4% carbon, just over half of the area of microstructure consists of pearlite and the strength is around 540 MPa. With 0.8% carbon, the microstructure is completely pearlitic and the maximum strength of 850 MPa is attained.

Beyond 0.8% and upto 1.2% carbon, the steel continues to have a pearlitic structure but cementite forms a network at the grain boundaries. Since it is not an integral component of the pearlite, it is referred to as free cementite. This network of a hard brittle compound at the grain boundaries is associated with an increase in the hardness, but it causes a marked deterioration of the ductility and this is undesirable for most commercial purposes. The strength in this region more or less remains constant, but may fall little bit.

1000

850

750

500

250

0 0.2 0.4 0.6 0.8 1.0

Carbon %

Ten

sil

estren

gth

,M

Pa

Fig. 1.13100

50

0

%of

pearli

te

in

mi c

rocon

sti t

uen

t

Fe C+

pearlite

3Ferrite +

Pearlite

0.2 0.4 0.6 0.8 1.0

Carbon content %

1.2

Fig. 1.14


Liquid + delta solid

solution crystals

N

J

B

A

1,600

1,539°

1,500

H

1,400

Delta solid

solution

Delta solid

solution + gamma

solid solution

1,300

1,200

1,100

1,000

900

G

800

M

700

600

Q

Alpha solid

solution

(ferrite)

Ferrite + cementite

(tertiary)

Liquid

E 1,130° CF

D

Liquid + cementite

(primary)

Liquid + gamma

solid solution

crystals

(austenite)Gamma solid solution

(austenite)

Au

sten

ite

+

cem

en

ti t

e

( procu

tectoi d

)

Austenite +

+ Ferrite

O

Tem

pera

tu

re,

°C

0 10 20 30 40 50 60 70 80 90 100% Fe C3

1 2 3 4 4.3 5 6 6.67% 7% C

P SPearlite + cementite

(proeu-

tectoid) + ledeburite (pear-

lite + cementite)

Cementite (primary)

+ ledeburite (pearlite

+ cementite)

Eu

tectic

(led

ebu

ri t

e)

K

723°

Ferrite+

Pearlite

780°

Cementite (primary) +

+ ledeburite (pearlite

+ cemen

Austenite + cementite

(proeutectoid) + ledeburite

(austenite + cementite)

L

Fig. 1.15. The iron carbon equilibrium diagram concerns transformations that occur in alloys having compositions from pure iron to cemen-tite having 6.67% carbon. Point A (1539°C) on the diagram is the melting point of pure iron and point D is the melting point of iron carbide (Fe3C). Points N (1400°C) and G (910°C) correspond to allotrophic transformation of iron to γ iron. Point E shows the solubility limit of carbon in γ iron at 1130°C and 2% Carbon.

From above it would be obvious that there is some relationship between the presence of pearlite and the increase in the strength of the steel. (Refer Fig. 1.14). Pearlite has a laminated structure. It consists of alternate layers of ferrite and cementite. As carbon content increases, the amount of pearlite produced also increases, and it increases the strength.

This pearlite is an important constituent in the structure of a steel and it is better to understand how this constituent is formed and controlled.

It is observed that with increase in carbon content, the hardness, tensile strength and yield strength increase ; but impact strength, reduction of area, and elongation are reduced.

1.13.2. The Constitution Phase Diagram for Iron Carbon Alloys. A phase diagram is a chart which shows the number and nature of phases that are present in a given alloy at any temperature and composition under equilibrium condition.

Fig. 1.15 shows the different constituents of steel.

There are four main phases of steels, viz., ferrite, cementite austenite and pearlite. It has been noticed that various other elements, besides carbon and iron, which are present in steels as impurities or as alloying elements, have no appreciable effect on the iron carbon constitution diagram shown in Fig. 1.15. However, by their presence the position of the boundary lines are changed. These three phases are discussed below.

1. Ferrite. It is a solid solution of upto 0.025% carbon in the solvent α-iron. This phase is indicated in

the diagram by GSP. γ phase is converted to ferrite due to slow cooling of the solid alloys. Ferrite generally contains no carbon but many other elements such as Mn, Si, Cr in the solid solution.

Ferrite is soft, weak and ductile.The hardness of the ferrite is as low as 50 to 100

Brinell. Ferrite is most prominent by its high ductility.2. Cementite. It is an intermittent compound

consisting of a de nite lattice arrangement of iron and carbon atoms, the relative number of each of the atoms present in a given sample being in accordance with the formula Fe3C. This phase is formed due to slow cooling of solid alloys within the area ESK. In chemical composition it is a compound of carbon and iron carbide, Fe3C. Other elements which may be present in steel are also in the form of their carbides. In such a steel, cementite is very hard and the hardness is of the order of 1400 Brinell. In the annealed steel, (that is, in the steel which has been cooled in a controlled manner) cementite is found as spheroids (rounded particles) or parallel plates (lamellar layers) or as a covering over the pearlite grains. Cementite is quite brittle, but hard and strong.

Ferrite and cementite tend to form a laminated structure called pearlite. The amount of pearlite in plain carbon steel (carbon 0.1 to 1.5%) increases with increase in carbon content. When carbon content is 0.83%, all the grains will be pearlitic. Carbon is excess of 0.83% forms free cementite at the grain boundaries, causing a reduction of the tensile strength, due to the localised brittleness.

3. Austenite. It is a solid solution of upto 1.7% carbon in gamma iron. Austenite is obtained by heating


carbon-iron-steel above the range GSF. The formation of austenite takes place due to interface reaction of ferrite and cementite. At rst, nuclei of austenite are formed and then they go on growing by the further reaction of ferrite and cementite. The speed of formation of austenite is increased by the increase of temperature.

The iron-carbon diagram tells which of the three steel phases are preset, at a given temperature and carbon concentration, when the alloy is cooled or heated slowly enough so that it remains in a state of equilibrium.

4. Pearlite. As already indicated earlier, iron at room temperature has a body-centred-cubic lattice (α-iron), and on heating above 910°C, it adopts face-centred-cubic arrangement (γ-iron also called austenite). These two allotropic forms of iron have different physical properties like coef cients of expansion). The change from one allotropic form to other is called transformation or reaction. Energy is expended in the change from γ to α iron, and the cooling curve shows a thermal arrest at the temperature at which the event occurs. For pure iron the transformation occurs at 910°C but with addition of carbon it occurs over a range of temperatures, the limits being known as critical points—(A3 the higher temperature, i.e. the one at which transformation starts during cooling, and A1—the lower point which is 723° for all types of iron/steel). At 0.8% carbon A3 merges with A1and rises at lower as well as upper carbon levels.

Carbon has different solubility in austenite and ferrite. At 1130°C, austenite can hold upto 1.7% carbon when all the carbon is in solution. The carbon is dissolved interstitially i.e. the carbon atom does not replace an iron atom to form part of the cube but nestles between iron atoms. In the case of body-centred-cubic pattern (for ferrite), the space between the iron atoms is inadequate and the carbon atom is ejected from its site. This is what reaction occurs at A3 point.

Carbon atom in

space between

two iron atoms

Fig. 1.16

At A3 point, the b.c.c. ferrite grain starts nucleating at the edge of an austenite grain, setting up a boundary between the two constituents. As the temperature is lowered between A3 and A1 limits, the boundary advances further and further into the austenite grain. When the carbon is rejected from the ferrite lattice, it diffuses into the body of the austenite grain and is retained in solution. The untransformed austenite thus gets progressively richer in carbon upto a limit of 0.8%. On further cooling at A1 limit, the remaining austenite is completely changed to ferrite and virtually all the carbon is rejected from the solution to form layers of cementite which are sandwiched between ferrite.

If a piece of plain carbon steel (carbon upto 1.5% and no alloying elements) is heated at a uniform rate in a furnace, its temperature will rise at a uniform rate until a temperature is reached at which this rise is halted for a short time, or temperature may even fall, although the temperature of the furnace continues to rise. At this point, the heat is being used to cause a rearrangement of the iron atoms, which in turn cause the formation of a solid solution called austenite. The temperature at which the pearlite is transformed into austenite is called lower critical heating point (around 723°C). The ferrite or cementite get converted into austenite at a higher temperature known as upper critical heating point (which depends upon the carbon content of the steel).

Similarly during uniform cooling, temperature falls at a uniform rate until a temperature is reached at which it starts to cool less rapidly (known as upper critical cooling temperature and is 30°C less than the upper critical heating point). At this point, the austenite starts to break down to produce either ferrite or cementite, according to the carbon content of the steel. This change continues until the lower critical cooling point (695°C) is reached, when any remaining austenite is transformed into pearlite.

The iron-carbon diagram does not tell anything about the state of aggregation of the phases present. In other words, the information like the relative sizes or shapes of the ferrite and cementite cannot be had when both of these are present in a certain temperature region. This information can be had only by the microscopic examination of a polished and etched surface of the metal.

The iron-carbon diagram also helps in determining the phase transformations which occur when a steel specimen of certain composition is cooled slowly from a high temperature. Let us study the case of a steel specimen of composition 0.5% carbon. In the region GSEN the metal is all austenite and will remain in this state until it is cooled and crosses line GS when ferrite will start to form along the grain boundaries of the austenite. The amount of ferrite will increase as the temperature falls from line GS to line PS. At temperature of 723°C all of the remaining austenite will have a carbon content of 0.80% and will transform at constant temperature into alternate plates of ferrite and cementite (this plate like lamellar structure being known as pearlite). At all temperatures below 723°C we still have mixture of ferrite and pearlite.

Next let us examine what changes occur when an eutectoid steel (steel of 0.8% carbon) is cooled. In this case nothing happens till a temperature of 723°C is reached when all of the metal transforms at constant temperature into pearlite. If a steel piece having carbon content of 1.2% be cooled, then cementite rst precipitates along the grain boundaries at temperature corresponding to line SE. This continues until temperature of 723°C is reached when all of the remaining material transforms into pearlite. There is no further change in structure below temperature of 723°C.

The ability of steel to harden depends upon the difference in carbon solubility of austenite and ferrite and the tendency for the excess carbon to precipitate


in the form of cementite when austenite transforms to ferrite. The spacing of the cementite particles depends upon the rate of cooling and their shape. If rate of cooling of steel, while it is crossing 723°C temperature line, is quick then particles of cementite precipitated will be very small and closely spaced, and if it is slow then particles of cementite precipitated will be larger and more widely spaced. The shape of the cementite particles may be of lamellar, spheroidal or acicular type. The lamellar structure known as pearlite consists of alternate plates of cementite and ferrite. Whether the pearlite is coarse or

ne depends upon the relative spacing of the cementite plates. The spheroidal structure consists of roughly spherical globules of cementite in a matrix of ferrite. This type of structure is known as spheroidite in which the globules are relatively large. The acicular structure, also sometimes known as Widmanstatten structure consists of a cross- hatched needle-like structure of ferrite needles in very ne pearlite.

What happens when a 0.5% carbon steel specimen is cooled from above 723°C temperature line is shown below.

Spheroidite Pearlite Widmanstatten

structure

More complex changes

occur leading to very

hard structure

Coarse

(slower rate)

Fine

(slow rate)

Very slowly

cooled

Critical rate

of cooling

Very rapidly

cooled

Austenite

Less slowly

cooled

The more complex changes which occur on very rapid cooling of austenite leading to very hard structures can be explained by a time-temperature-transformation (3T) diagram, or S-curves, so called, due to their shape (also known as isothermal transformations).

It has already been pointed out that when steel is heated above line GS, the iron carbide in the iron decomposes ; all of the iron will transform to the γ iron and the carbon will all go into the solution (the resulting structure being known as austenite). When austenite steel is suddenly cooled to a temperature below 723°C and held at that temperature for varying lengths of time then different types of structures of different hardness are formed. The transformation for different temperatures starts and ends at different times. The S-curves or 3T-curves are the plots on semi-log paper which show the time at which the resulting transformation starts and ends when the austenitic steel is quenched quickly to a particular temperature. Fig. 1.17 shows such a curve. It may be noted that if transformation takes place near temperature of 723°C (A1), the resulting structure will be coarse pearlite and steel will be relatively soft. As the transformation temperature is decreased, the pearlite becomes ner and the steel becomes harder. This trend continues till temperature of about 565°C (A0) is reached, below which upto a temperature of 150°C (Ao′) a new structure known as bainite is obtained. It consists of a feathery combination of iron carbide and ferrite. The

structure near the temperature Ao resembles pearlite and that near Ao′ is ner and harder and has a more acicular structure of the Widmanstatten type.

If the transformation temperature is below 150°C then a structure known as martensite is formed. The austenite mainly consists of delta iron and is of face-centred cubic form whereas iron at low temperature is of body-centred cubic form. Thus the ordinary transformation of delta iron to alpha iron upon cooling requires a signi cant readjustment of the iron atoms which requires time to achieve. If material be quenched at very fast rate then metal has no time to rearrange and super-cooled delta iron is formed.

A1

A0

Coarse

Pearlite

Pea

r-

lite

723°C

Fine Pearlite

Coarse Bainite

565

Transformed

Ferrite

Fine

Bainite

Untrans-

formed

Austenite220

150

0

Tra

nsfo

rm

atio

nT

em

pera

tu

re

°C

Ba

i ni t

eM

arten

-

si t

e

Time, Sec.

Martensite

A′0

1 10 102

103

104

105

106

107

0 100 200 300 400 500 600 700

Brinell Hardness

Number of fully

Transformed Structure

Fig. 1.17. S-Curves, or 3T-Curves.

The super-cooling of iron from austenitic form renders the atoms less mobile and increases the time required for the normal delta to alpha rearrangement to occur and subjects the material to very large internal stresses due to the excess carbon present. The net result is formation of a distorted form of ferrite with tetragonal martensite. The resulting super-saturated solid solution of carbon in body-centred tetragonal distorted ferrite is very hard and brittle and is known as martensite.

Fig. 1.18 shows the formation of ferrite, pearlite and cementite for carbon variation upto 1.2% and the variation of mechanical properties.

0.2 0.4 0.6 0.8 1.0 1.2

30

20

10

Cementite

Elo

nga

tio

n%

300

200

100

Brin

ell

ha

rd

ness

100

75

50

25

0

Ferrite

Pearlite

Pea

rli

te

%T

en

si l

estren

gth

kg/ m

m2

Tensilestrength

Brinell hardness

Elongation

0%

50%

100%

Fig. 1.18. Effect of microstructure on mechanical properties.


1.13.3. Various Phases of Steel and their Description.1. Austenite. Generally austenite is not present in plain

or low alloy steels. The presence of austenite is ensured only when the transformation from γ form to α form is completely suppressed but this is not possible in plain or low alloy steels even if they are very rapidly quenched.

However, in alloy steels containing manganese and nickel, there is always a high percentage of austenite, as the presence of these elements helps in suppressing the transformation from γ to α form. It has been noted that steels containing high percentages of these elements have got appreciable percentage of austenite at room temperature even if the rate of their cooling is quite slow.

Austenite is soft and ductile but at high temperatures it is stronger and less ductile than ferritic steel. Austenite is non-magnetic, more dense than ferrite and has got higher electrical resistance and thermal coef cient of expansion than ferrite.

When austenite is chilled in liquid it is converted into martensite.

2. Martensite. It is formed in carbon steels by fast and continuous cooling of austenite to temperatures 205 to 315°C or even lower than that.

Martensite has tetragonal crystal structure. Hardness of martensite varies from 500 to 1000 Brinell depending upon the carbon content and neness of the structure.

3. Pearlite. It is composed of alternate layers of ferrite and cementite in the ratio of 87 to 13 by weight. The formation of pearlite takes place by the slow cooling of austenite along the line PSK in Fig. 1.15. Pearlite is the eutectoid structure of two phases in iron carbon alloys.

4. Spheroidite. It is produced by slow cooling of hypereutectoid austenite (steel containing more than 0.83% carbon) or by reheating (tempering) martensite in the range 650 to 705°C.

Spheroidite is the structure in which cementite takes the form of rounded particles or spheroids instead of plates.

Spheroidite is softer and more ductile than pearlite but not so much machinable as pearlite.

5. Troostite. If martensite is reheated (tempered) in case of plain steels between temperatures 205 to 395°C, troostite is formed. Troostite is softer and more ductile than spheroidite. Troostite consists of submicroscopic particles of cementite in ferrite.

6. Sorbite. If troostite present in plain carbon steel is heated to temperature range of 595 to 395°C, it changes into a structure called sorbite. In sorbite, cementite is in granular form. Sorbite is softer and more ductile than troostite.

Classi cation of SteelsSteel is divided into three classes, namely carbon

steels, alloy steels and stainless steels. These are discussed below.

1.14. CARBON STEELSCarbon steel is an alloy of iron and carbon with varying quantities of phosphorus and sulphur. To this alloy is added a deoxidiser to remove or minimise the last traces of oxygen. Manganese is added to such an alloy to neutralise sulphur, either alone or in combination with silicon or other deoxidisers.

In carbon steel the maximum content of the following elements does not exceed the limits given against each :

Manganese ... 1.65%Silicon ... 0.60%Copper ... 0.60%The elements which are speci ed and are added into

carbon steels are carbon, manganese, phosphorus, sulphur and silicon. The effect of these elements in carbon steel is given below :

(i) Carbon. Carbon content is very important in determining the properties of steel. The tensile strength of steel increases with increase in carbon content up to 0.83% and beyond this it drops quickly. Hardness increases as the carbon content increases. Ductility and weldability decrease with the increase in carbon content.

(ii) Manganese. Tensile strength and hardness increase with the increase in manganese content. Weldability decreases by increase in manganese. Manganese content in steels varies from 0.2 to 0.8%.

(iii) Phosphorus. Tensile strength and hardness increase with increase in phosphorus content. The phosphorus content in steels varies from 0.005 to 0.12% and the maximum content permitted is 0.4%. In low phosphorus steels, phosphorus is dissolved in matrix and in others it appears as phosphide precipitate.

(iv) Sulphur. Sulphur in steel lowers the toughness and transverse ductility. Sulphur imparts brittleness to chips removed in machining operations. The maximum permitted content of sulphur in steel is 0.055%.

(v) Silicon. It is the principal deoxidiser used in carbon steel. Presence of silicon in steel promotes increase of grain size and deep hardening properties. Its addition is very useful in making steel adaptable for case carburising.

Presence of silicon varies from traces in some cases to 0.1 to 0.35% in other cases.

(vi) Copper. Though it is not an essential constituent of carbon steel yet it is added upto 0.25% to increase the resistance to atmospheric corrosion.

Other elements such as nickel, molybdenum, vana-dium, cobalt etc. are present in carbon steels only in residual quantities derived from scraps.


Table Cont...

Uses of Carbon Steel with Respect to their Carbon Content

Carbon content Various Uses

0.05 to 0.07 For the manufacture of highly ductile wires.

0.07 to 0.15 Such a steel is called rimmed steel and is useful in the manufacture of sheet, strip, rod and wire when high surface nish is required. Typical uses are the manufacture of body and fender stock, panels, deep drawing strip steel for lamps, hood, sectors, pawls clutch and transmission covers, oil pans, etc.Such steels have low tensile strength and should not be used when high strength is desired. As these steels are ferritic in structure, their machinability is very poor. So they should not be used for nuts, screws etc. However, the presence of manganese in such steels improves their machinability and hardness.

0.08 to 0.18 Such kind of steel is useful for the manufacture of ship plates, seamless plates, boiler and weldable boiler tubes.

0.15 to 0.20 These are useful for the manufacture of wrist pins, camshafts, drag links, clutch ngers, sheet and strip for fan blades and welded tubings.Such steels have low strength.

0.2 to 0.30 Their machinability is quite good. Such steels are used for the manufacture of small forgings, crank pins, gears, valves, crank shafts, railway axles, cross heads, connecting rods, rims for turbine gears, armature shaft and sh plates.

0.35 to 0.45 These have got good machinability and have got deep hardening properties. These are useful in the manufacture of axles, special duty shafts, connecting rods, small and medium forgings, cold upset wires and rods, machinery steel spring clips, solid turbine rotors, rotor and gear shafts, armature for turbogenerators, key stock, shift and brake levers, forks and anchor bolts.

0.45 to 0.55 This steel is useful for manufacturing articles which have to be subjected to shocks and heavy reversals of stress. So it is used for the manufacture of railway coach axles, crank pins for heavy machines, large size forgings such as crank shafts, stator ring gears, axles, spline shafts and hard drawn wire for tempered springs.

0.60 to 0.70 Such kind of steel is employed for the production of articles such as drop-forging dies, die blocks, bolt heading dies, plate punches, set screws, self-tapping screws, soap rings, valve springs, cushion springs, clutch springs, lock washers, springs, clips, clutch discs, thrust washers etc.

0.7 to 0.8 These are used for making cold chisels, pick axes, wrenches, jaws for vices, shear blades, hack saws, pneumatic drill bits, wheel for railway service, wire for structural work, automatic clutch discs, lower sections and plough beams etc.

0.8 to 0.9 Various types of articles manufactured out of such steels are : railway rails, plough shapes, rock drills, circular saws, machine chisels, punches and dies, lock pins, clutch discs, leaf springs, music wire, mover knives etc.

0.9 to 1.00 Such steels are used for the manufacture of punches and dies, springs, balls, keys, pins, leaf and coil springs, harrow and seed discs.


1.15. ALLOY STEELSA steel is called alloy steel when the alloying elements such as silicon, manganese and copper exceed the limits given below :

Manganese ... 1.65%Silicon ... 0.6%Copper ... 0.6%In addition to these elements an alloy steel may

contain aluminium, boron, chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium, etc. to get the desired effect. A wide range of alloy steels have been developed to meet varying service conditions, to reduce heat treatment problems, demand for higher strength, improved corrosion resistance, and special properties. Alloy steels are not suitable for forming. They are usually forged for parts like gears, bearings, crankshafts, connecting rods, etc. where good strength and toughness are required. By induction hardening, their surface becomes hard and can take high compressive load and core remains tough.

Effect of Alloying Elements on the Properties of Steel

(a) General. It has already been seen that both ferrite and cementite will be present in the carbon steel in equilibrium at room temperature ; the ferrite being a soft constituent and cementite a hard one. Further it is the shape and distribution of the carbides in the iron (and not the mere presence of cementite) that causes steel to be hard. It has also been seen that it is possible to dissolve all the carbide in the austenite by heating steel in the region GSEN and then obtain the desired size and distribution of carbides in the ferrite by suitable cooling through the cooling range (i.e. proper heat-treatment). If carbon content in steel is equal to 0.8% (eutectoid composition), its slow cooling through the critical temperature produces a structure known as pearlite having alternate plates of ferrite and cementite (because both of these constituents are rejected simultaneously in this case). If carbon is less

than 0.8% (hypoeutectoid composition) then rst free ferrite is rejected till the composition of the remaining austenite reaches 0.80% carbon when the simultaneous rejection of both ferrite and carbide will again occur, thus resulting in a structure consisting of areas of free ferrite and areas of pearlite. When carbon content is more than 0.8% (hypereutectoid composition) and such a steel is cooled slowly through the critical range than cementite is thrown out at the austenite grain boundaries, forming a cementite network until the austenite containing 0.8% carbon at which time pearlite is again formed ; the resulting structure thus being composed of areas of pearlite surrounded by a thin carbide network. If the cooling rate is high then the spacing between the pearlite leamellae becomes smaller and there is greater dispersion of carbide which prevents slip in the iron crystals and makes the steel hard. When the cooling rate is very high (quenching), the carbon does not have suf cient time to separate out in the form of carbide, and the austenite transforms to a highly stressed structure supersaturated with carbon called martensite which is very hard and brittle. When this martensite is heated to some temperature below the critical, the carbide precipitates out in the form of small spheroids (the size of which depends directly on the temperature) and the resulting structure becomes ductile and hardness is lowered.

Although the characteristic behaviour of carbon steel is obliterated by addition of alloying elements (which are added to modify the properties of carbon steel to an appreciable extent) but they still owe their distinctive characteristics to the carbon contained. Any steel always contains only two constituents, i.e. ferrite and carbide. The only way the alloying elements can affect the properties of steel is to change the dispersion of carbide in the ferrite, change the properties of the ferrite, or change the properties of the carbide. With plain carbon steels it is not possible to attain uniform hardness throughout the cross-section of large specimen even with critical quenching mediums. The hardenability of plain carbon steels can be increased by addition of alloying elements

Carbon content Various Uses

1.00 to 1.1 Such kinds of steels are employed for manufacturing railway springs, machine tools, mandrels, springs, taps, etc.

1.1 to 1.2 Taps, tools, thread metal dies, twist drills, knives etc.

1.2 to 1.3 For manufacturing les.

1.3 to 1.5 Dies for wire drawing, paper knives and tools for turning chilled iron.

1.5 to 1.6 Saws for cutting steel and dies for wire drawing.


without resorting to drastic quenching operations which may otherwise lead to cracking and warping.

Elements such as molybdenum, tungsten, and vanadium are present as carbides in the austenite and prevent the agglomeration of carbides in tempered martensite. The presence of these stable carbide-forming elements enables higher tempering temperatures to be used without sacri cing strength. This permits these alloys to have a greater ductility for a given strength than plain carbon steels.

The presence of alloying elements like phosphorus, silicon, manganese, nickel, molybdenum, tungsten, and chromium in the solid solution of ferrite increases the strength of metal. Aluminium in uences the austenite grain size. Martensite formed from a negrained austenite has considerably greater resistance to shock than when formed from a coarse-grained austenite. The oxides formed by deoxidation of the steel by different elements apparently prevent grain growth about the critical temperature over a considerable temperature range. The presence of nely scattered carbides in the austenite appears to have a similar effect on the austenite grain size, so the elements forming such stable carbides will also contribute to the formation of a ne grained austenite.

The reasons for alloying various elements in the steel are given below:

(i) To delay the rate at which austenite is trans-formed to pearlite upon quenching to allow suf cient time for thick sections to be hardened throughout (without the use of a quench that is so drastic as to induce damaging cracks in the steel).The metals contributing this effect are Mn, Cr, Mo, W, Ni, Si.

(ii) To provide higher hardness at high temperature (Mo, Cr, V, W).

(iii) To provide higher strength at elevated tem-peratures (Mn, Ni, Si, C, Cr, Mo).

(iv) To inhibit grain growth in austenite during heat treatment (V, Al).

(v) To provide corrosion resistance, (Cr, Ni).(vi) To provide additional hard abrasive particles

to improve wear resistance (V, Mo, Cr, W).(vii) To combine with oxygen to prevent blowholes

(Si, Al, Ti).(viii) To combine with sulphur which otherwise

causes brittleness (Mn).(ix) To improve machining properties (S, P, Pb).

In general W, Mo, V and Cr achieve above properties by forming carbides that are insoluble in ferrite ; Si, Mn, Ni, Co and Cr by going into solution in the ferrite ; Si, Al, Ti, Mn, S, P, Pb help in achieving these properties by forming inclusions which are insoluble in ferrite.

(b) Effect of individual elements(i) Nickel. It is soluble in all proportions in both alpha

and gamma forms of iron. It strengthens and toughens the ferrite phase. It imparts elasticity, hardness, and fatigue

resistance to steel. It lowers the transformation points and eutectoid composition. It lessens distortion in quenching and improves corrosion resistance.

If carbon content is low, then about 3% nickel is suf cient to make steel tough. If steel is to be tough and respond to oil-quenching, then upto 5% nickel is used. If nickel quantity is too much then martensite will form too readily during quenching, and the steel will be too brittle and hard. Case-hardening steels containing nickel do not suffer grain growth during carburising and do not always require re ning treatment. Certain stainless and heat-resisting steels are produced by adding about 8% nickel with chromium, because these two elements prevent the breakdown of austenite during cooling to room temperature. Nickel is often included as alloying element in heavy forgings and high-strength structural steels that harden by air cooling rather than by quenching in oil.

(ii) Chromium. It forms a complex series of chromium carbides in steel thus improving the depth to which a metal may be hardened and increasing its resistance to abrasion and wear. These carbides are very hard. Chromium improves wear and cutting ability. It raises A3transformation and lowers the carbon content of eutectoid composition.

Combination of nickel and chromium is used to improve mechanical properties of steel.

Chromium counteracts the tendency of nickel to graphitise steel by stabilising the iron carbide and as such both Cr and Ni are found together in steels. Chromium tends to promote coarse-gain structure and increases the dif culty of heat treatment, and this is counteracted by nickel which re nes the grain size.

Corrosion-resisting and heat resisting steels contain very high percentage of chromium.

(iii) Molybdenum. It can form solid solution in the ferrite phase of steels. It can also form complex carbides with certain ratios of carbon to molybdenum. It raises the A3 transformation point. Molybdenum also joins with carbon and promotes hardenability.

Steels containing chromium and nickel suffer from temper brittleness (i.e. they become brittle if held at a temperature between 250—500°C and as such their shock resistance becomes poor at these temperatures. This trouble can be overcome by adding about 0.25% molybdenum. Addition of molybdenum also hampers grain growth in steel at high temperature, making the steel ner grained and tougher.

(iv) Manganese. It is an alloying element when its quantity present is more than the required quantity for deoxidation purposes. It forms manganese carbide. It is soluble in alpha and gamma iron. It increases the depth of hardness. Steels containing 1 to 1.5% carbon and 11 to 14% manganese are resistant to wear, work harden easily, and are resistant to abrasion under shock. It counteracts the effects of sulphur.

(v) Silicon. It is considered an alloying element when its quantity exceeds the quantity required for deoxidation. Silicon is soluble in ferrite component. When it is used up to 2.5%, it increases the strength without decreasing ductility.


(vi) Vanadium. It forms carbides with steels. It gives strength and toughness to steel. It improves the hardening quality of steel. It raises the transformation points and lowers the carbon content of eutectoids. It is used for providing a ne-grained structure over a broad range of temperature.

(vii) Boron. Boron is added in quantities varying from 0.0005 to 0.001%. It improves hardness and mechanical properties of steel. It also improves rolling qualities of steel.

(viii) Aluminium. It is used as effective deoxidiser. Its addition controls grain growth. In nitriding steels it is used from 0.9 to 1.5% for surface hardening due to the formation of stable aluminium nitride.

(ix) Tungsten. It increases cutting hardness and magnetic retentivity. It also causes the re nement of grains. It raises the softening temperature to about 600°C. 18% tungsten is used in high speed steel, a cutting tool material.

(x) Copper. It is added (0.2 to 0.5%) to increase atmospheric corrosion resistance and also as strengthening agent.

(xi) Phosphorus. In low carbon steels, it improves machinability.

Table below shows uses of various nickel chromium low alloy-steels

Nickel per cent

Carbon per cent Various uses

0.4 to 0.6 0.1 to 0.2 Pinion gears, rear axle diff-erentials, case hardened parts.

0.1 0.4 Forming materials for auto-mobile axles.

1.5 0.15 Engine bolts, rivets etc.2.0 0.2 Ship castings, castings sub-

jected to shock and fatigue, large frame castings, steel mill machinery, boiler plates for locomotive, and re box plates and tubes.

2.75 0.15 Locomotive forgings, piston rods, side rods, main rods and axles.

3.25 to 3.75 0.15 Spring clips, piston pins, king-pins, roller bearing races, universal joints.

3.25 to 3.75 0.40 Drive shafts and heavy splined shafting.

3.25 to 3.75 0.50 Connecting rods. Tubes of this kind of steel are used for making bearing races, collars, valve seats, cylinder liners, stressed rings etc.

4.5 to 5.0 0.50 Transmission gears on tru-cks and heavy apparatus.

According to BIS, ve types of commonly used steel castings are designated as under :

(a) Unalloyed steel castings—CS followed by mini-mum tensile strength in kg/mm2.

(b) Unalloyed special and steel castings like high magnetic permeability—CSM followed by minimum tensile strength in kg/mm2.

(c) Alloy steel castings—CS followed by important alloying contents and their amount indicated in percen-tages after the minimum tensile strength value.

(d) Heat resistant steel castings—CSH.(e) Corrosion resistance steel castings—CSC.Both (d) and (e) are followed by minimum tensile

strength and alloying elements contents.

1.16. STAINLESS STEELSStainless steel is iron base alloy that has a great resistance to corrosion. It is observed that a thin, transparent, and very tough lm forms on the surface of stainless steel which is inert or passive and does not react with many corrosive materials. Within a temperature range of 235°C to 980°C, it exhibits strength, toughness and corrosion resistance superior to other metals. It is thus ideally suited for handling and storage of liquid helium, hydrogen, nitrogen and oxygen that exist at cryogenic temperature. The property of corrosion resistance is obtained by adding chromium only or by adding chromium and nickel together. Stainless steel is manufactured in electric furnaces.

Mechanism of the corrosion resistance. Stainless steels are very slightly oxidisable. Slight oxidation forms very thin lm of oxide and this oxide lm acts as a protective coating and in this way further corrosion is stopped. This protective lm of oxide is so thin that the colour and beauty of the basic material is not affected.

Classi cation of Stainless Steels. Stainless steels can be classi ed into the following groups :

(i) Austenitic Stainless Steels. These contain at least 24% chromium and nickel combined and the percentage of each of them alone is not less than 8%. These steels may contain other elements for particular purposes. These are non-hardenable and non-magnetic.

(ii) Martensitic Stainless Steels. These are alloys of iron, carbon and chromium. The difference between the percentage of chromium and seventeen times the percentage of carbon is less than 12.5%. These are also called chromium steels, and are hardenable and magnetic.

(iii) Ferritic Stainless Steels. These are alloys of iron, carbon and steel. In these the difference between the percentage of chromium and seventeen times the percentage of carbon is greater than 12.5%. These are magnetic and non-hardenable.

(iv) Low Chromium Stainless Steels. These contain 4 to 6% of chromium. These have oxidation-resistant properties at high temperatures.

Use of Rare Earths in the Manufacture of Stainless Steels. Rare earths are used to manufacture stainless steel. Such a steel has the plasticity of deformation at rolling and forming temperatures. Ordinary stainless steels are dif cult, rather impossible to roll because they are hot short, that is, they are brittle at high temperature.


Table below shows the in uence of addition of metals in iron for the formation of alloy steels on their properties and the use of resultant alloy steels

Metal In uence on properties Uses

Manganese Hot shortness is removed. Austenite is stabilised. Resistance to abrasion is increased.

Alloy is used for grinding wheels, steering spindles and rails.

Nickel Fine grains are pro-duced. Coef cient of expansion is lowered and resistance to corro-sion is increased.

It is used for balance wheels.

Chromium Tensile strength is increased. Depth hardening is increased. Resistance to corrosion is increased.

It is used for surgical instruments, cutlery, crank shafts and connecting rods.

Nickel and chromium

Increase in tensile strength, and increase in resistance to corrosion take place.

Stainless steels are formed.

Vanadium Resistance to abrasion is increased. Reversible stresses are produced, tensile strength is increased.

Piston rods, axles, crank pins and heavy locomotive forgings are made.

Tungsten Cutting hardness is increased. Re nement of grain structure takes place. Magnetic retentivity is increased.

Cutting tools are produced from this alloy. Also permanent magnets are formed.

Molybdenum Phases are stabilised and thus cutting hardness is increased, hardness at high temperature is increased.

High speed tools are formed.

1.17. COMMERCIAL STEELSSteel has got a variety of applications for engineering purposes due to the wide range of physical properties obtainable by changes in carbon content and heat treatment. First we shall study about the properties and uses of carbon steels.

Low-carbon steel

(0.05 to 0.25% C)

Structural steel

(0.20 to 0.35% C)

Machinery steel

(0.30 to 0.55% C)

Tool steel

(0.60 to 1.30% C)

Carbon steel

Low-carbon Steels. These are used where only moderate strength is required together with considerable plasticity. Steels with carbon content between 0.05 to 0.10% are used for sheet, strip, tubing, wire nails etc. ; steels with carbon content between 0.10 to 0.20% are used for rivets, screws and parts to be case-hardened. Sheets can be produced by hot-rolling or by cold-rolling process. Cold-rolled sheets have a better surface nish, and improved mechanical properties. Cold rolling also permits the rolling of thinner gauge material than hot rolling. Sheets for deep-drawing applications are made dead soft in order to have a maximum amount of plasticity; for better nish these should have a relatively ne grain size. By the small additions of columbium or vanadium, high-strength hot-rolled, cold-rolled and galvanised sheets can be made. Sheets are available in coil form and are extensively used for welding into tubes for construction of furniture, automobiles, refrigerators etc.

Structural Steels. These are available in the form of plates, different sections, and bars in the hot-rolled condition. A uniform strength over a range of section thickness is provided by varying the amount of carbon, manganese and silicon. These have high yield point (3300 kg/cm2) and are suitable for both welded and rivetted construction.

Boron Steels. These steels achieved special importance in times of alloy shortages. It is found that as little as 0.005% of boron increases hardenability of steels with 0.15 to 0.60% carbon, but increasing the boron content above 0.005% has adverse effect on hot workability. Boron sheets can replace such critical alloying elements as nickel, chromium, molybdenum and manganese and when properly heat treated, possess physical properties comparable to the alloy grades they replace. Additional advantages of the use of boron in steels are decrease in susceptibility to aking, formation of less adherent scale, greater softness in the unhardened condition, and better machinability.

Free-cutting Steels. These have high sulphur content present in the form of manganese sulphide inclusions causing the chips to break short on machining. Manganese and phosphorus harden and embrittle the steel which also contributes towards free machining. Lead (0.20 to 0.35%) is sometimes added to steel to further improve the machinability.

Forging Steels. These contain carbon between 0.30 and 0.40%, and are used for axles, bolts, pins, connecting rods, crank shafts etc. These steels are readily forged and after heat-treatment, develop considerably higher mechanical properties than low carbon steels.

Tool Steels. The selection of proper tool steel depends on the purpose or operation it is supposed to perform


i.e. cutting, shearing, forming, drawing, extruding, rolling etc. Each of these operations requires in the tool steel a particular physical property or a combination of such metallurgical characteristics as hardness, strength, toughness, stability, wear resistance and resistance to heat softening. Other important factors requiring due consideration in selection of proper toal steel are hardenability, permissible distortion, surface carburisation during heat-treatment and machinability. Tool steels have been identi ed and classi ed by SAE and AISI into six major groups, based upon quenching methods, applications, special characteristics and use in speci c industries. These six classes are water-hardening, shock resisting, cold-work, high-speed and special-purpose tool steels.

Water-hardening tool steels (0.70 to 1.30% carbon) are widely used because of their low cost, good toughness and excellent machinability. These are shallow-hardening steels unsuitable for non-deforming applications because of high warpage, and possess poor resistance to softening at elevated temperatures. These are used for les, twist drills, shear knives, chisels, hammers and forging dies.

Shock-resisting tool steels contain alloy combinations of chromium-tungsten, silicon-molybdenum, or silicon-manganese. These have good hardenability with outstanding toughness and wearing qualities. Their disadvantage is the tendency to distort easily, which can be minimised by oil quenching. The most common type has 0.6% carbon, and tungesten, chromium, or vanadium.

Cold-work tool steels are further classi ed as oil-hardening ; medium-alloy air hardening ; and high-carbon, high chromium. These possess high wear resistance and hardenability, develop little distortion, but at best are only average in toughness and in resistance to heat softening. Machinability varies from good in the oil-hardening grade to poor in the high-carbon, high-chromium steels.

Air hardening steels are used for larger sizes of tools and dies. These are air hardened in large and intricate sections with little distortion in hardening and easier to machine.

Oil hardening non-deforming die steels have a substantially reduced alloy content. These have wide range of used for all types of medium life tools and dies, easy to machine and harden uniformly.

Hot work tool steels (either chromium based or tungsten based) possess ne non-deforming, hardenability, toughness and resistance to heat softening characteristics, with fair machinability and wear resistance. These are used in blanking, forming, extrusion and casting dies, hot-blanking dies, hot punching dies, forging and die-casting dies, where temperature may rise to 540°C.

High speed steels possess all properties except toughness. These are either tungsten or molybdenum-base types. Cobalt is added sometimes to improve the cutting qualities in roughening operations. These retain considerable hardness at a red heat. They are used in metal cutting upto 593°C without softening below Rc 60.

Special-purpose tool steels are comprised of the low-carbon, low-alloy, carbon-tungsten, mould and other miscellaneous types.

Spring Steels. For manufacture of small springs, steel is often supplied in a form that requires no heat treatment except perhaps a low-temperature annealing to relieve forming strains. For small helical springs, previously treated wire is supplied in any of the following three forms :

(i) Music wire. It is given a special heat-treatment called patenting and then cold-drawn to develop a high yield strength.

(ii) Hard-drawn wire. It is of lower quality than music wire and is made of lower grade material and is seldom patented.

(iii) Oil-tempered wire. It is quenched and tempered.

Depending on the application of the spring and the severity of the forming operation, the wire usually has a Brinell hardness of 350—400.

Steel for both helical and flat springs, which is hardened and tempered after forming, is usually supplied in an annealed condition. For small springs, plain carbon steel is satisfactory ; whereas alloy steel (chrome-vanadium, or silicon-manganese steel) is used for large springs. This is in order to obtain a uniform structure throughout the cross-section. It is especially important for springs in whose case the surface of the steel is free from all defects.

Stainless Steels. Certain alloys of iron and chromium known as stainless steel are highly resistant to corrosion and oxidation at high temperatures and maintain considerable strength at these temperatures. As already indicated, stainless steels may be classi ed according to their micro-structure into three categories :

(1) Martensitic. (Hardenable alloys containing upto 16% Cr and 0.7%C which are martensitic when quenched). These are very hard and possess strain-resisting properties and, therefore, are used for utensils, surgical and dental instruments, springs for high temperature operation, ball valves and seats etc. The proper hardening range depends on composition and size, but in general the higher the quenching temperature, the harder the article. Oil quenching is preferable, but with thin and intricate shapes, which might warp on quenching, satisfactory hardening is obtained by cooling in air. Tempering around 500°C does not lower the tensile strength, and in this condition the steel shows remarkable resistance to weathering, to attack by fruits and vegetable acids, ammonia and other corrosive agents to which cutlery may be subjected.

(2) Ferritic (Low carbon, non-hardenable alloys containing 12 to 27% Cr). These have very low carbon content and possess considerable ductility, ability to be worked hot or cold, excellent corrosion resistance and are relatively inexpensive. These can be hardened to a great extent by cold working, and are best suited for forming and medium-deep drawing operations. These are used extensively for kitchen equipment, dairy machinery, heat


exchangers, boiler tubing, screws, bolts, nuts, interior decorative work, automobile trimmings and for chemical equipment to resist nitric acid corrosion. The chromium content is increased to 25—30% for resisting oxidising conditions at high temperatures. Such alloys are usefull for all types of furnace parts not subjected to high stress.

(3) Austenitic (Chromium-Nickel alloys). These contain 16—26% chromium and 6 to 22% nickel. Carbon is minimum. The addition of substantial quantities of nickel to high chromium alloys lowers down the A3 temperature and thus austenite exists at room temperature ; cold working introduces some excellent properties. These are highly resistant to many acids (even hot or cold nitric acid) ; strong and scale less than any of the plain chromium alloys. These are very useful for parts subjected to severe stress at elevated temperatures. Tungsten and molybdenum are added to increase the strength at elevated temperatures ; and silicon and aluminium to improve the resistance to scaling ; and selenium and sulphur are added to improve the machinability. These find uses in food processing, dairy industry, textile industry, pharmaceuticals.

These are not highly resistant to hot sulphurous gases and are sometimes subject to embrittlement and intergranular corrosion which may be overcome only by adding titanium and columbium. Normal corrosion resistance can be restored by heating the steel above 930°C and cooling rapidly.

Other two types of stainless steels found in common use are :

(i) Extra low-carbon stainless steel.(ii) Precipitation-hardenable stainless steel.(i) Extra low-carbon stainless steel. 18—8 austenitic

steels have been developed having carbon content as low as 0.03%. These have higher intergranular corrosion resistance. The intergranular corrosion in stainless steel occurs because of attack on chromium carbides precipitated in the grain boundaries. Use of low carbon improves corrosion resistance by decreasing the potential amount of such carbide precipitation.

(ii) Precipitation-hardenable stainless steel. These can attain a strength and hardness formerly achievable only by cold working. In 18—8 chromium nickel steel, the effective age-hardening elements viz, titanium, aluminium, copper, molybdenum, columbium and tantalum in various combinations are added. The heat treating temperatures employed for precipitation hardening are low enough to eliminate or minimise the dangers of distortion, cracking and decarburisation inherent in quench-hardening grades. One type contains 17% Cr, 7% Ni, 1% Al, and a small amount of titanium and is martensitic at room temperature and is aged at 482°C. These are available in coils, sheets strips, plates, forgings, billets, bars, rods etc. Final machining operations can be performed before heat-treatment if allowance is made for slight growth that occurs. These can be classi ed into three types—martensitic, semi- austenitic and austenitic.

Special property Alloys. Iron-nickel alloys are used extensively in the electrical industry owing to their

exceptional magnetic properties. Alloys containing 20 to 30% nickel are non-magnetic and are used to some extent for non-magnetic parts in electrical machinery. Alloys having a high permeability and low hysteresis loss have a composition between 55 and 80% Ni, (Permalloy). Perminvar (45% Ni, 20% Co) has a constant permeability over a range of ux densities.

Another important group of iron-nickel base alloys are those with low coefficients of expansion. Invarcontaining 36% Ni has an exceedingly low coef cient of linear expansion.

Elinvar (32% Ni with small percentages of Cr, W, Mn, Si and C) not only has a low coef cient of expansion but also has a constant modulus of elasticity over the temperature range of 0—40°C and is thus useful in hair springs for watches, and springs for other precision instruments.

Platinite (a 46% nickel alloy) has the same thermal coef cient as platinum.

Electrical sheet steels are alloys of iron and silicon with carbon, manganese, phosphorus and sulphur kept as low as possible. The silicon increases resistivity of iron and greatly decreases the hysteresis loss. Silicon-alloys are used in almost all magnetic circuits where alternating current is used. As silicon makes steel brittle, it is limited to 4% in structures subjected to vibrations, as in the case of motor armature, and is kept at 5% for transformers.

Austenitic Manganese Steel. It is also known as Had eld’s manganese steel and is a non-magnetic alloy containing around 12% Mn and 1% carbon. It is relatively soft but work hardens on the surface when subjected to severe abrasion and is thus extremely useful in crushing machinery, for railroad crossings, tractor shoes etc. As cast, this alloy is party martensitic and, therefore, hard and brittle. By quenching from a high temperature of 1040°C, a homogeneous austenite is retained and the alloy has high toughness, strength and ductility characteristics of austenitic steels.

Fabrication Characteristics of Ferrous Metals: The ability of a metal to be formed, cast, welded, or machined is called its fabricating characteristic.

Machinability refers to the ease with which a metal may be machined, i.e., the forces acting on the cutting tool are low, chips are easily broken up, good nish produced, longer tool life between two sharpenings. The machinability of metals is improved by uniform microstructure, small and undistorted grains, spheroidal structure in high carbon steels, lamellar structure in low-and medium-carbon steels. It is also improved by hot working of medium- and high-carbon steels, cold working of low-carbon steels and heat treatments (annealing, normalising, tempering. The addition of small amounts of lead, manganese, sulphur and phosphorus and absence of abrasive inclusions (Al2O3) also improve the machinability of metals.

Ferrite being too soft does not produce good shearing action. Lead (0.15 to 0.35%), or manganese sulphides help break up the ferrite structure. Leaded steels are known as free-machining steels and can be used at 50% higher


cutting speeds than corresponding plain carbon steels. Small amounts of carbon improve the machinability of plain carbon steels upto around 180 BHN, but beyond this further carbon increases hardness and decreases machinability.

Alloy AISI B 1112 is considered to be most machinable and forms basis for 100% machinability.

Steel castings have the same machinability as comparable wrought metals. However the skin of castings has oxide scale which is detrimental to machining and should be removed by pressure blasting before machining.

Malleable iron is considered one of the most readily machined ferrous metal because of its uniform structure and the nodular form of tempered carbon. It has a machinability rating of 120%.

Formability of a metal is a direct function of ductility which is dependent on the crystal structure. Metals having face-centered cubic crystals can be easily formed because of number of slip planes. The grain size, amount of working of metal, addition of alloying elements, annealing and normalising, also help improve formability of metal. Small group sizes are preferred for shallow drawing of copper, and relatively large grains for heavy drawing on the thicker gauges. Hot working of metals reduces the size of the crystals and distorts the grains (and produces the ductility). The amount of distortion is more in cold working and thus poor ductility and poor formability. Ductility can be restored by heating such metals to recrystallisation temperature. Most alloying elements in a pure metal reduce its ductility and hence formability. The atoms of alloying element either replace the atoms of pure metal or nd place in between the atoms of pure metal and thus reduce the number of slip planes. Low carbon steels have good forming qualities because there are less carbon and alloys to interfere with slip planes.

Weldability of a metal refers to the ease with which it can be welded and the quality of the weld obtained. It is affected by heating and cooling effects on the metal, oxidation of base metal, gas vaporisation and solubility. It is generally observed that the deposited weld metal picks up carbon or other alloys and impurities from the parent metal, making the weld hard and brittle, resulting in formation of cracks on cooling. Such metals which oxidise rapidly interfere with the welding process. Normally oxides have higher welding point than base metal which affects owability. Oxides may get entrapped in the weld metal, resulting in porosity, reduced strength, and brittleness. Gases formed in welding of some metals may become entrapped causing porosity and deteriorating the desired property of base metal. Plain carbon steel is the most weldable of all metals. Medium- and high-carbon steels will harden if allowed to cool rapidly just after welding. Preheating and post heating help to remove the brittle micro- structures and thus improve quality of weld. Extra high carbon steels (C—1.0 to 1.7%) and tool steels are not welded but usually joined by brazing with a low-temperature silver alloy.

Castability of a metal is in uenced by solidi cation rate, shrinkage, gas porosity, and hot strength.

1.18. MARKET FORMS OF SUPPLY OF PIG IRON AND CARBON STEEL

Pig Iron. It is available in the following grades :(i) Foundry grade (coke) High manganese in four grades

(for general purposes) Low manganese in four grades(ii) Basic grade coke High manganese

(for steel making purposes) Low manganese(iii) Standard basic.(iv) Low silicon basic.

Carbon Steel. Carbon steel products are available in the following standard market forms :

(i) Semis. (ii) Sections. (iii) Rolled products, (iv) Flat products, (v) Track Materials, (vi) Special Items.

Semis. These are available in the form of ingots, slabs, blooms (150 mm × 150 mm to 350 mm × 350 mm), billets (50 mm2 to 125 mm2).

Sections. These are available in the form of equal angles {35 × 35 × 5 mm (2.6 kg/m weight) to 150 × 150 × 20 (44.1 kg/m weight)} unequal angles, medium channels {41 × 32 mm (4.79 kg/m weight) Tee channels) to 400 × 100 mm (49.4 kg/m)}, telegraphic channels, Medium beams {100 × 70 mm (11.5 kg/m) to 600 × 210 mm (122.6 kg/m)} and wide ange beams.

Rolled Products. These are available in the following forms:

Bars, rounds {12 mm dia (0.9 kg/m weight) to 100 mm dia (61.7 kg/m weight)}, ribbed torsteel bars {0.6 mm (0.2 kg/m) to 50 mm (15.4 kg/m)}, ats {45 × 6 mm (2.1 kg/m weight) to 250 × 25 mm (49.2 kg/m weight)}, squares (16 mm (2.0 kg/m weight) to 110 mm (91.2 kg/m weight)}.

Flat Products. These are available in the following forms:

Wide and heavy plates, medium plates, hot-rolled sheets and coils, high silicon sheets, skelp-hot-rolled.

Track Materials. These are available in the following forms :

Crossing sleeper bars, shplate bars, bearing plate bars, sh plates, light rails, crane rails.

Special Items. These are available in the following forms:Tin plates—Hot dipped and electrolytic. These can be

light plates, sheared heavy plates (rimming or semi-killed quality), sheared heavy plates (killed quality), unsheared heavy plates (rimming or semi-skilled), unsheared heavy plates (killed and special quality steels) ; large diameter pipes—ERW ; pressed steel sleeper ; wheels, axles and wheel sets ; galvanised sheets—cold rolled ; chequered plates.

1.19. MARKET FORMS OF SUPPLY OF SPECIAL STEELS AND ALLOY STEELS

Special steel and alloy steel products are marketed in any of the following three forms :

(i) Forged Products, (ii) Rolled Products, (iii) Sheet Mill Products.

Forged products are supplied in the form of :


(a) Bars, which may be square, round or at in cross-section. Square bar can be had in sizes from 30 to 420 mm and 6 m length. Round bars can be had in diameter from 30 to 535 mm and 6 m length. Flats can be had in width from 30 to 650 mm and 6 m length.

(b) Die blocks. Maximum sizes for die blocks are : Thickness : 600 mm, width : 750 mm, length : 1500 mm. Max. cross-sectional area : 230,000 sq. mm.

(c) Rings. These are available in sizes : outside diameter : 150 to 1250 mm, inside dia 1200 mm max., thickness : 13 to 440 mm.

(d) Discs. These are available in sizes : Minimum : 13 mm thick × 150 mm dia and max. 110 mm thick × 300 mm dia.

Rolled products are supplied either as hot rolled round cornered square billets (dia from 40 to 195 mm) or as hot rolled rounds (dia from 22 to 125 mm).

Sheet mill products are supplied either as hot rolled sheets/plates (these may be supplied in annealed and pickled condition) (thickness range : 1.6 to 12 mm and widths 100 mm upto 5 mm thickness and 1250 mm over 5 mm, standard lengths 2000 mm and 2500 mm), or as cold rolled sheets (which are annealed, pickled, skinpassed, stretched, levelled and resquared (thickness range : 0.8 to 3.25 mm, standard widths : 1000 mm and standard lengths : 2000 mm).

1.20. IS SPECIFICATIONSThose interested in knowing about the Indian Standards on steel and steel products may refer the following IS speci cations:

IS : 2004—General purpose steel forging.IS : 2611—High temperature service steel forgings.IS : 2644—High tensile steel castings.IS : 2707—Surface hardening steel castings.IS : 2708—1.5% Manganese alloy steel castings.IS : 2856—Fusion welding quality steel castings.IS : 3038—High temperature and pressure steel

castings.IS : 3261—Steel forgings for ship building.IS : 3444—Alloy steel and nickel based steel castings.IS : 4367—General industrial use alloy and tool steel

forgings.IS : 4491—High magnetic permeability steel castings.IS : 4896—10% chromium steel castings.IS : 4898—Case carburising steel castings.IS : 4899—Ferritic and austenitic steel castings for

low temperature service.IS : 7806—Ferritic and austenitic steel castings for

high temperature service.IS : 7899—Low alloy steel castings for pressure service.

1.21. BUREAU OF INDIAN STANDARD (BIS) CODE FOR DESIGNATION OF STEEL

Accordings to Bureau of Indian Standards, steels can be designated either based on letter symbols {IS : 1962 (Part I)—1974} or based on numerals {IS : 1962 (Part II)}.

Minimum number of symbols are recommended to be used in designating any steel.

Under letter symbols of designating carbon and low alloy steels could be designated on the basis of (a) Mechanical properties, and (b) Chemical composition. The basis of mechanical property is used where the major criterion for selection of steel is its tensile strength or yield stress. Symbol Fe is used where the basis is tensile strength and Fe E where the basis is yield stress. These symbols are followed with minimum tensile strength (yield stress) value in N/mm2, gure 00 being used if these values are not guaranteed. The next letter in the code designation is the chemical symbols for such elements whose presence characterises the steel. Finally letters are indicated at fourth place for special characteristics and at fth place for application, if necessary.

The special characteristics at fourth place could be for(a) method of deoxidationR for rimming (or semi-killed) steelK for killed steel(b) Steel quality. It is designated by symbols Q1

to Q5 depending on whether it is non-ageing, free from akes, controlled grain size, controlled inclusion, and

guaranteed internal homogeneity respectively.(c) Degree of purity. It gives the maximum

content of phosphorus and sulphur in ladle analysis. It is designated by letter ‘P’ followed by 100 times the maximum percentage of phosphorus and sulphur, if they are equal. Thus P 25 means the phosphorus and sulphur content are 0.025 and 0.025% respectively. If maximum contents of phosphorus and sulphur are not same, then degree of purity is designated by SP followed by 100 times the maximum sulphur rounded off to the nearest integer and 100 times the phosphorus content rounded off to the nearest integer.

(d) Weldability guarantee. It is represented by symbol W for fusion weldable and Wl for the steel weldable by resistance welding but not fusion weldable.

(e) Resistance to brittle fracture. Depending on the resistance to brittle fracture (as per results of charpy V-notch specimens), steels could be designated by symbols B, B0, B2, or B4.

(f) Surface condition. For forged or rolled steel, no symbol is used. Other symbols used are S1 for deseamed or scarfed, S2 for descaled, S3 for pickled, S4 for shot or sand blasted, S5 for skinned or peeled, S6 for bright drawn or cold rolled and S7 for ground.

(g) Formability (for sheet only). It is designated by D1 for drawing quality, D2 for deep drawing quality, and D3 for extra deep drawing quality.

(h) Surface nish (for sheet only). It is designated by 14 grades from F 1 for general purpose nish to F 14 for direct annealed nish. Other symbols are F 2 (full

nish), F 3 (exposed), F 4 unexposed, F 5 (Matt nish), F 6 (Bright nish), F 7 (plating nish), F 8 (unpolished

nish), F 9 (polished nish), F 10 (polished and coloured blue), F 11 (polished and coloured yellow), F 12 (Mirror

nish), F 13 (vitreous enamel nish).


(i) Treatment. Symbols T 1 to T 14 are used to indicate the treatment given to steel. These are T1 (shot peened), T 2 (Hard drawn), T 3 (Normalised), T 4 (controlled rolled), T 5 (Annealed), T 6 (Patented), T 7 (solution treated), T 8 (solution treated and aged) T 9 (controlled cooled), T 10 (Bright annealed), T 11 (spherodised) T 12 (stress-relieved) T 13 (case hardened), and T 14 (Hardened and tempered).

(j) Elevated temperature properties. Letter H is used where elevated temperature properties are guaranteed, and letter L for guaranteed cryogenic quality.

On the basis of chemical composition, steels are designated as follows :

(a) Unalloyed steel.35 C 12 G

100 times the average % age of carbon (0.35% in this case)

for unalloyed steel

10 times the average % age of Mn (1.2% in this case)

Special characteristic(guaranteed hardenability)

(Note : Various symbols are actually written without gap, i.e., as 35 C 12 G—Gaps are shown for clarity only).

(b) Unalloyed tool steel.75 T 10

100 times the average % age of carbon (0.75% in this case)

for tool steel 10 t imes the average % age of manganese content (1% in this case)

(c) Unalloyed free cutting steel.30 C 12

100 times the average % age of carbon

for unalloyed free cutting carbon

10 times the average % of Mn

Pb 15 KSymbol of element that makes steel free cutting. It could be S, Se or Te also

100 times the % age of element Pb

Special characteristic symbol

(d) Low and Medium alloy steels (total alloying elements not exceeding 10%).

40 Ni 8 Cr 8 V 2 G100 times the average % age of carbon

Chemical symbols for alloying elements followed by their average % age content multiplied by a factor, (rounded to nearest integer) arranged in decreasing order of contents

Special characteristics (guaranteed harden-ability in this case)

The multiplying factors for various alloying elements are:

4 for Cr, Co, Ni, Mn, Si and W10 for Al, Be, V, Pb, Cu, Nb, Ti, Ta, Zr and Mo100 for P, S, N.(Symbol Mn is included if its content > 1%)(e) High alloy steels (where total alloying

elements are more than 10%).X 12

for high alloy steel carbon 100 times the % age followed by their characteristic content

Cr 18 Ni 12 HChemical symbols for alloying elements followed by their average %age content, rounded nearest integer

Symbols for special characteristic (elevated temperature properties in this case).

(f) Alloy tool steels. Same method as for (e), except that X will be substituted by T for low alloy and medium alloy tool steels, and by XT for high alloy tool steels.

(g) Free cutting steels. Same method as for low and medium, and high alloy steels as at (d) and (e) above except that the %age of S, Se, Te, and Zr will be designated by their symbols followed by 100 times their content.

Schedules for wrought steels for general engineering purposes are given in IS : 1570 in seven parts as are under :

Part 1. Steels specified by tensile and/or yield properties.

Part 2. Carbon steels (unalloyed steels).Section. 1. Wrought products (other than wires) with

speci ed chemical composition and related properties.Section 2. Carbon steel wires with related properties.Part 3. Carbon and carbon manganese free cutting

steels.Part 4. Alloy steels (constructional and spring

steels) with speci ed chemical composition and related mechanical properties.

Part 5. Stainless and heat resisting steels.Part 6. Alloy tool steels.Part 7. Steels for elevated temperature services

(creep resisting steels).Designation of Steel Castings and ForgingsFive types of steel castings are designated as below

by Bureau of Indian standards(i) Unalloyed steel castings are designated by letter

CS followed by minimum tensile strength in MPa (N/mm2).(ii) Unalloyed special steel castings (high magnetic

permeability) are designated by letters CSM followed by the minimum tensile strength in MPa (N/mm2).

(iii) Alloy steel castings are designated by letters CS followed by the minimum tensile strength in MPa and the chemical name and percentage of important alloying elements.


(iv) Heat resistant steel castings are designated by letters CSH followed by minimum tensile strength in MPa and chemical name and percentage of important alloying elements.

(v) Corrosion resistance steel castings are designated by letters CSC followed by the minimum tensile strength and chemical name and percentage of important alloying elements.

American Iron and Steel Institute (A.I.S.I.) Method of Classi cation of Steels: The composition of A.I.S.I. steels is identi ed by using a numerical index. A capital letter pre x is used to indicate the steel making process. The letter B indicates an acid Bessemer Carbon steel ; C, a basic open-hearth carbon steel ; and E, an electric furnace alloy steel. A series of four numericals designates the composition ; the rst two indicate the alloy type, and the last two indicate, as far as feasible, the average carbon content in “points”, or hundredths of 1 per cent. Some examples or rst two letters are : 10 for carbon steels, 11 and 12 for free-cutting steel, 13 for Manganese steels, 31 for Nickel-chromium steels, 40 for Molybdenum steels, 41 for Chromium-molybdenum steels, 43 for Nickel-Chromium-Molybdenum steels, 44 and 45 Molybdenum steels, 46 and 48 for Nickel-molybdenum steels etc. Thus C—1040 is a carbon steel with a carbon range of 0.37 to 0.44 made in the basic open hearth and E—2512 is a 5% nickel steel with 0.09 to 0.14 carbon made in the electric furnace.

Society of Automotive Engineers (SAE) Method of Coding Steels: It comprises, in general, a series of four-digit numbers, the rst digit of which indicates the type of steel as follows :Carbon steels 1 Chromium steels 5Nickel steels 2 Chromium-vanadium steels 6Nickel-chromium steels 3 Tungsten steels 7Molybdenum steels 4 Silicon-manganese steels 8

For simple alloys the second digit generally indicates the approximate percentage of the predominant alloying element. The last two digits usually indicate the average carbon content in points, i.e., hundredths of 1%.

Thus 2440 indicates a nickel steel of approximately 4% Ni and 0.40% C (0.35% to 0.45%) ; 71,360 indicates a tungsten steel containing about 13% tungsten (12—15%) and 0.60% C (0.50—0.70%).

The pre x ‘X’ is used to designate variations in the range of manganese, sulphur and chromium. The pre x ‘T’ is used to differentiate 1300 series of manganese steels of different manganese range, identi ed by the same number without the pre x.

Thus the various types of steels are speci ed by chemical composition only, since the physical properties can be varied by heat treatment.

American Society for Testing Materials (ASTM), American Society of Mechanical engineers (ASME), Aerospace Materials Specifications (AMS) have also published their metal speci cations.

1.22. STEELS AS PER BISIndian standards specification IS : 1570 (on Indian Standard Wrought Steels) have standardised the plain carbon and alloy steels under the following heads :

(a) Steels speci ed by tensile properties but without detailed chemical composition ; and

(b) Steels speci ed by chemical composition and these are further sub-divided as :

(i) Carbon steels.(ii) Carbon and carbon manganese free cutting steels.(iii) Alloy steels other than stainless and heat

resisting steels.(iv) High alloy steels (stainless and heat resisting

steels).(v) Carbon and alloy tool steels.IS : 1871 provides a commentary on IS : 1570 and it

regroups the steels given in IS : 1570 as follows :(a) Steel speci ed by tensile properties but without

detailed chemical composition.(b) Carbon and low alloy steels with speci ed chemical

composition and related mechanical properties.(c) Carbon and carbon-manganese free cutting steels.(d) Hardened and tempered steels.(e) Case hardening steels (flame and induction

hardening, case carburising, cyaniding, carbo-nitriding).(f) Creep resisting steels.(g) High alloy steels (stainless and heat resisting

steels including valve steels).(h) Carbon and alloy tool steels.IS : 1870 provides comparison of Indian and Overseas

standards for wrought steels for general engineering purposes in order to enable to select the equivalent steels in Indian Standards. The following eight equivalents are compared :

BS —British StandardSAE —Society of Automotive EngineersAISI —American Iron and Steel InstituteASM —American Society of MetalsASTM —American Society of Testing MaterialsDIN —German StandardsJIS —Japanese StandardsGOST —Russian Standards.Designation of Steel Castings and ForgingsFive types of steel castings are designated as below

by Bureau of Indian Standards :(i) Unalloyed steel castings are designated by

letter CS followed by minimum tensile strength in MPa (N/mm2).

(ii) Unalloyed special steel castings (high magnetic permeability) are designated by letters CSM followed by the minimum tensile strength in MPa (N/mm2).


(iii) Alloy steel castings are designated by letters CS followed by the minimum tensile strength in MPa and the chemical name and percentage of important alloying elements.

(iv) Heat resistant steel castings are designated by letters CSH followed by minimum tensile strength in MPa and chemical name and percentage of important alloying elements.

(v) Corrosion resistance steel castings are designated by letters CSC followed by the minimum tensile strength and chemical name and percentage of important alloying elements.

1.23. NON-FERROUS METALSUnlike mild steel, non-ferrous metals do not in general display the discontinuity of yield point in the stress-strain curve. Annealed alloys may pass through a period of more rapid extension but cold-worked metals pass imperceptibly from the proportional relationship to the condition where plastic ow has occurred, and the whole curve is smooth. For such materials the yield strength is usually found by determining the load necessary to produce a speci ed total elongation (usually 0.5%).

An important method of increasing the strength and hardness of non-ferrous metals is by cold working in which the grains ow by a process involving the slip of blocks of atoms over each other, along de nite crystallographic planes. Many alloys (those of aluminium, copper, nickel, magnesium etc.) can be hardened and strengthened by heat-treatment (Precipitation hardening) consisting of two-step process. First the alloy is given a solution heat-treatment followed by rapid quenching, and then a precipitation or ageing treatment is given to cause separation of second phase from solid solution and hardening. These alloys after a solution treatment are comparatively soft and consist of homogeneous grains of solid solution generally indistinguishable microscopically from a pure metal. Rapid cooling after solution treatment retains the supersaturated solution at room temperature, and if the alloy be subsequently reheated to a suitable temperature, ne particles of a new phase are formed and in time will grow to a microscopically resolvable size. At some stage in this precipitation process, the hardness, the tensile strength, and particularly the yield strength will be considerably increased. If the reheating treatment is carried out for tool long a time, the alloy will overage and soften. Therefore, the temperature and time, for both solution—and precipitation—heat treatments must be closely controlled to obtain the best results.

1.24. COPPERCopper is a very important metal in industry as it has great corrosion resistance property. It has got good strength which is maintained at moderate temperatures. It is very ductile and can be worked into complex shapes. It can be very easily welded, soldered and rivetted. It has got very high heat conductivity and electrical conductivity.

Mechanical PropertiesAnnealed Cold worked

1. Tensile strength kg/cm2 ... 2000—2250 3000—45002. Hardness Brinell ... 45—55 80—1003. Elongation per cent on

50 mm ... 50—60 5—204. Modulus of Elasticity ... 0.95 to 1.2 0.95 to 1.2

× 103 MPa × 103 MPaDuctility can be increased at the expense of hardness

and strength by annealing. Copper and its alloys can be easily joined by soldering, brazing and welding.

Re ning of Copper. Blister copper, as obtained after roasting and converting the copper ore, contains about 98.5% copper and the rest 1.5% is made up of nickel, iron, selenium, tellurium, lead, arsenic, antimony, bismuth, sulphur, precious metals, etc. Such a copper cannot be used in industry. So it is further re ned by the processes in the sequence given below :

1. Fire Re ning, in order to produce purer and homogeneous anodes.

2. Electrolytic Refining, for refining precious metals and removing the impurities.

3. Second Firing, for adjusting the physical properties of electrolytic copper for use in industry.

After electrolytic re ning operation, cathodes may be used directly for making alloys, but if copper is to be rolled to fabricated forms, it is melted and cast into wires, bars, cakes or billets. After the second ring operation, the correctly re ned castings solidify with an approximately-level surface, the gas evolved during solidification balancing the shrinkage that would otherwise occur. This is known as tough-pitch copper and it has a density of 8.4 to 8.7 g/cc when cast, 8.89 to 8.92 when worked and annealed.

Types of Copper. Following are the different types of copper used in industry :

(i) High Conductivity (H.C.) Copper. This type of copper has as high purity as 99.9% copper (silver being counted as copper). It is largely used for electrical purposes. The presence of even traces of certain impurities (particularly phosphorus, arsenic, iron, titanium and silicon) decreases the conductivity of copper considerably.

(ii) Best Select Copper (B.S.). It is re ned copper containing small quantities of various other impurities which prevent its being classi ed as H.C. copper. It is widely used in industry.

(iii) Arsenical Copper. It contains up to 0.5% of arsenic. The conductivity of arsenical copper is quite high though lower than H.C. copper. Arsenic gives increased strength to the copper at ordinary and moderate temperatures. Upon annealing arsenic raises the softening temperature by about 100°C. Arsenical copper has greater resistance to oxidation at moderate temperature than H.C. copper.

(iv) Deoxidised copper. Deoxidised copper, arsenical or otherwise, is obtained by removing oxygen from copper by the addition of deoxidants to the molten copper. Phosphorus is commonly used as a deoxidant, but decreases the conductivity. Oxygen free copper of high


conductivity and density is now-a-days made by casting without contact with air. This is more ductile tough pitch copper and is immune to embrittlement by hot reducing gases unless it has been allowed to absorb oxygen during fabrication.

The presence of oxygen in copper is considered desirable in making the copper slightly harder. Before the commercial adoption of electrolytic re ning process, oxygen was used in neutralising the effect of certain impurities. Oxygen is harmful if the copper is to be welded or otherwise heated in a reducing atmosphere, as it causes embrittlement of copper and renders it useless.

Uses of Copper. Copper is used in huge quantity for making copper wire. It is used for making alloys such as brass. On account of its high resistance to corrosion, it is used in the form of copper sheets in chemical work, and food and brewing plants.

Copper can be cold rolled extensively upto 870°C and beyond it hot-worked. Cold rolling increases the hardness and strength. Copper wire above 0.10 mm diameter is commonly made by drawing from a hot-rolled rod without annealing but smaller sizes involve intermediate anneals. Copper shapes of electrical switch parts are made by extrusion, brushes and commutator sections by rolling and drawing.

Copper containing small amounts of silver or antimony retains the properties attained by cold working to a higher temperature than pure copper (about 320°C compared with about 205°C). This is useful where comparatively high temperatures are to be withstood, as in soldering or enamelling operations.

Addition of 0.5% arsenic to copper allows it to be used at temperature of upto 300°C without loss of strength. Addition of 0.08% silver prevents it from softening during soldering and addition of 0.5% tellurium produces free-cutting characteristics.

Copper alloys nd extensive applications on account of their heat and electrical conductivity, good cold and hot-working properties ; machinability, and corrosion resistance. Commercially pure copper is best-suited where high thermal or electrical conductivity is desirable. Where strength combined with high conductivity is the consideration than alloys containing cadmium or other elements are used. Brass is the cheapest copper alloy. It contains high zinc content and is widely used unless high corrosion resistance under stress or the special mechanical properties of other alloys are required. Brass with 30—35% zinc is best-suited for deep drawing and forming operations. Leaded brass is used when much machining is to be done, particularly for automatic screw machine work. For high elastic strength, the tin bronzes are used. For corrosion resistance, alloys of copper with aluminium or silicon or nickel are most suited.

IS : 2378—1978 speci es the designation and other characteristics of copper and its alloys. According to it copper is designed by letters Cu followed by letters CATH, ETP, FRHC, or DPH (indicating the important characteristics) ; these being for cathode copper, electrolytic tough pitch copper, ne re ned high conductivity copper

and phosphorised high residual phosphorus non-arsenical grade copper respectively.

Copper alloys are designated by symbol Cu followed by symbols for next most signi cant elements and their percentage in the decreasing order. Sometimes a letter for method of casting is placed before the designation and letter for surface nish at the end.

1.25. LEADLead is obtained from its ores, by concentration, oatation, and reduction in blast furnaces. The crude lead is puri ed by washing with molten zinc (Parkes process), and the resulting lead is cast into pigs.

Properties of Lead. Lead is a soft and weak metal. Its tensile strength is about 150 kg/cm2. It is very malleable and ductile. It is very heavy and resists corrosion. It has a high density, low melting point and high boiling point. It can be easily melted (melting point 327°C), cast, rolled and extruded. It is highly malleable and pliable. So it is easily handled during fabrication and installation. It has low strength and due to that its ductility is also low. It has got low elastic limit, high coef cient of thermal expansion and has get very high anti-frictional properties. It is a good insulator against nuclear radiation. Impurities present in lead are very small but have profound effect chemically. Based on the content of impurities lead can be classi ed into the following groups :

1. Corroding Lead. It is pure and is called corroding lead as white lead is prepared out of it.

2. Chemical Lead. Such a lead contains 0.04 to 0.8% copper, 0.002 to 0.02% silver and less than 0.005% bismuth. Presence of these impurities in lead makes lead resistant to corrosion, particularly to sulphuric acid. Such a lead is very commonly used. Its copper content confers added stiffness to it.

3. Tellurium Lead. Small quantities of tellurium in lead (between 0.5 and 0.005 per cent) will make lead have ner grain size, higher tensile strength and greater fatigue strength than ordinary lead. Tellurium lead has greater resistance to corrosion than pure lead.

Precisely tellurium will have the following effects on lead:

(i) Grain is re ned in a remarkable way.(ii) Temperature of crystallisation is raised

appreciably.(iii) Work toughening properties are imparted.(iv) Tensile strength is doubled.(v) Resistance to fatigue increases there times at

ordinary temperature and four times at 100°C when compared with lead without tellurium.

Tellurium lead is many times resistant to corrosion. It has much greater resistance to the solvent action of water than ordinary lead. Tellurium lead toughens when strained, and strain is evenly distributed. So tellurium lead is resistant to hydraulic bursting.

4. Antimonial Lead. Presence of antimony improves mechanical properties of lead. Antimonial lead has got better mechanical properties than chemical or tellurium


lead but at high temperature this difference decreases. With 6.7% antimony it is used for storage battery plates.

Creep or long-time tensile strength of lead is quite low.Uses of Lead

1. Lead is used in the manufacture of a number of chemicals. For example, it is used for lining in the tank, when ground bauxite is treated with sulphuric acid for manufacturing alum, etc. The heating coil is also made of lead.

2. Lead resists corrosion due to brine solutions, salt sprays and sea coast atmospheres. So lead is used for waste pipes conveying sea water aboard ships, for lining refrigerator roofs and for lining large aquariums.

3. In the manufacture of dyes. The equipment where halogenation, sulphonation, hydrolysis, oxidation, esteri cation, reduction, extraction and condensation, etc., has to take place, is lined with lead.

4. It is resistant to corrosion by sulphuric acid upto 9.5% concentration of sulphuric acid. So lead chambers are used in chamber process of manufacturing sulphuric acid. Lead lining is also used in tanks where pickling of steel and the manufacture of nitroglycerine, titanium dioxide and ethers is carried out.

5. Lead can be used in contact with sodium hydroxide up to 30% concentration at 25°C and upto 10% at 90°C. So it is used in petroleum re ning where sulphuric acid treatment is followed by caustic wash.

6. Lead can resist wet sulphur dioxide gas. So it is used in lead covered anodes and for the lining of electrostatic precipitators used to remove sulphuric acid mist from sulphur dioxide gas.

In paper and pulp industry lead pipes are used to cool sulphur dioxide gas, for bleaching with hydrogen peroxide or zinc hydrosulphite, and for taking away the discharge liquors from the pulp digestors.

7. Lead can be safely used in various processes where it comes in contact with the following chemicals :

(a) Solvents such as alcohols. acetone, trichloro-ethylene, etc.

(b) Acids such as sulphuric acid, chromic acid, hydro uoric acid, etc.

(c) Alkalies such as ammonium hydroxide, sodium hydroxide etc.

8. Lead having density as high as 11.3 is used in X-ray protection and is also used as a protection against deadly rays from nuclear ssion and radioactive isotopes.

Due to high density and ease of casting it is used as a counter-weight in various equipments and in keels of ships.

9. Due to its softness it is used as impression lead for reproducing halftone plates and electrotypes, etc. Due to the softness and low melting point of lead, it is readily extruded and pipes can be formed.

10. Lead is used under machines and buildings for reducing vibrations. For such cases it is generally used in conjunction with asbestos. 25 mm thickness of asbestos is covered with 3 mm thick 6% antimonial lead.

11. Lead can be easily rolled into any shape so it is used for making ideal gasket material because of its pliability and low creep strength.

12. Lead sheets are used for ooring in chemical plants, acid works in the explosive manufacturing plants. Lead is also used for covering laboratory benches.

13. Lead is used in battery plates.14. In paint industry lead is used as oxide of basic

carbonate as pigment.15. It is introduced into alloys to produce free-cutting

characteristics.Bureau of Indian standards have published following

standards, relating to lead :IS : 25—Antifriction bearing alloys.IS : 404—Lead pipes.IS : 405—Lead sheets and strips.IS : 1654—Antimony lead alloys.IS : 8475—Antifriction bearing alloys for heavy duty.

1.26. ALUMINIUMFirst stage in the production of aluminium is the production of alumina by chemical re nement of bauxite. Aluminium is then produced by the electrolysis of alumina dissolved in a bath of molten cryolite. It is available in the market as wrought and cast products in the form of ingots or notched bars for remelting. It is possible to obtain over 99.97% pure aluminium commercially.Properties of AluminiumPhysical. Aluminium is a silver white metal. Its outstanding properties are lightness (its density is one-third that of iron) good electrical and thermal conductivity. It is a good re ector of light and a good radiator of energy. It is non-magnetic. It is resistant to atmospheric attack. Oxide lm that is set up upon exposure to air insulates it against continued attack. It has good tensile strength in the form of alloys. Due to its ductility it can be easily worked. By itself it is very weak and ductile and melts at 660°C. It’s tensile strength is 600 kg/cm2 but it can be increased by cold working.

Aluminium is generally 99.9% pure as obtained by Hall-Heroult process and impurities of iron and silicon present from alloy with aluminium. Pure aluminium is silvery white in colour but commercial aluminium due to impurities has got a bluish tinge. It is used mainly as the base of reasonably strong and light alloys.

Chemical Properties. It is resistant to atmosphere due to the formation of a protective oxide lm. This oxide

lm is very thin, less than 0.02 micron (µm) is thickness but is impervious and highly protective. On heating, this

lm increases in thickness.Heat of combination of aluminium with oxygen is very

high. Finely divided powered aluminium burns in air.Anodic Coating. By anode treatment of aluminium

oxide, lm of suf cient thickness resistant to abrasion can be formed in certain electrolytes which are acidic in character. Anodic coatings are amorphous in nature and they have got rm adherence to the metal surface.


Electrolyte used generally is 15% sulphuric acid. Thickness of oxide lm is between 0.002 to 0.02 mm depending upon the quantity of current employed. Chromic acid and oxalic acid are also used a electrolyte.

As the anode coating is minutely porous, it should be made non-absorptive to prevent staining etc. The sealing of pores is done by dipping the anodised surface in hot water. Such sealed surfaces are not stained by coffee or other coloured liquids.

Anodised surfaces may be impregnated with corrosion inhibitor by treating with chromate solutions. Chromate gets absorbed in coating and protects the coating from any corrosive attack. Coatings which are sealed with chromate solutions form an excellent paint base and are used very much for protecting aircraft.

Anodic coatings can be coloured by impregnation with organic dyes and with mineral pigments. Coatings coloured by organic dyes are permanent indoors but not outdoors in sunlight because they fade in sunlight. Coatings which are impregnated with mineral pigments do not fade even in sunlight.

Oxide coatings can also be formed on aluminium by chemical treatment. Such coatings are inferior in thickness, hardness and abrasion resistance as compared to anodic coating : A hot solution of sodium carbonate and potassium or sodium dichromate will produce a greyish green oxide coating.

Electro-brightening or Electro-polishing.Brightening is carried out by anodic treatment in uoboric acid as electrolyte. Such brightened surface has as high re ectivity as 90%. Such electrobrightened surfaces can be protected by anode oxide coating.

Electroplating. Aluminium can be electroplated with other metals by electrolysis. The surface is rst dipped in sodium zincate solution.

Uses and Engineering Applications of Aluminium1. Chemical and Food Industries. Aluminium

is resistant to many mineral and organic acids, salt solutions, organic compounds, sulphur and many other substances. Aluminium is available in different fabricated forms and it can be assembled and nished by different processes. Due to all these reasons it is used for fabricating equipment for chemical and food processing industries. For these very reasons it is used for making cooking utensils. Cookers and steam jacketed kettles, etc. are produced from aluminium.

2. Metallurgical Industry. Aluminium is used in the metallurgy of iron and steel as it is a powerful deoxidizer and reduces the dissolved and combined oxygen content of molten steel.

Metallic aluminium is also used to reduce oxides of metals such as iron, chromium, vanadium and molybdenum.

Aluminium is a fine alloying metal in ferrous metallurgy, in steel for nitriding and in iron alloys where certain electrical, magnetic and oxidation resistant properties are desired.

3. Structural Applications. Due to its light weight and high tensile strength it is used for the construction of aeroplanes, buses, tracks, trains and ships.

Resistance of aluminium to weather makes it possible to use aluminium in architecture for constructing such parts as roo ng, sheathing, windows. etc. Star rails and furniture, etc. are also made out of aluminium.

4. Electrical Industry. It is used in the manufacture of cables. In cables, steel wire core is surrounded by aluminium conductors that carry the current. The strength of the core and the light weight of the cable permit long spans.

Aluminium bus bars are used for the distribution of power in and around factories.

Induction motors are produced with cast aluminium windings. The rotor is made with laminated steel core and the aluminium winding is cast in slots-extending through the laminations.

Aluminium conductors are used in the rotors of high speed turbine generators.

5. Brewery Industry. In brewery industry, aluminium is used extensively as it does not react chemically with the beverage and also does not alter the taste of the beverage. For example, beer is produced, stored and is shipped in aluminium tanks.

6. Cryogenic Applications. Cryogenic means science of low temperature. In modern times there is a great worth of missile and rocket industry. Cryogenic applications have close connection with missiles, rockets and space navigation. Aluminium alloys have the unusual property of remaining ductile and resistant to stock loading at extremely low temperatures. As the temperature decreases, their tensile and yield strengths improve. So they have got many advantages when used as a material of construction for cryogenic equipment. For example, aluminium is used in liquid fuelled missiles and rockets as storage material for liquid oxygen and forms the integral part of the missile or rocket.

7. Process equipments in plastics, rubber, rayons, synthetic-resins and petroleum industry are usually made of aluminium.

IS : 617 deals with the speci cations, designation, characteristic, and uses of aluminium and aluminium alloy ingots and castings for general engineering purposes, and the similar information for wrought aluminium and its alloys is given in IS : 737.

According to IS : 617, all aluminium and its alloys ingots are designated by letter ‘A’ followed by numbers from 0 to 24 depending on the variety of ingot ; for example number ‘0’ is for aluminium ingots of 99% aluminium used for re-melting, ‘1’ for aluminium alloy ingots, ‘2’ for aluminium alloy ingot mainly used for pressure die casting, ‘4’ for sand casting ‘5’ for gravity casting, ‘6’ for special die casting, etc.

Aluminium castings are designated in same way with a last letter M, P, W, or WP depending on the method of casting and treatment ; letter M being used for aluminium alloy castings, letter P if casting is precipitation treated, W if solution treated, WP if fully heat treated.

According to IS : 836, wrought aluminium is designated by letter I, followed by letters A, B and C as there are three grades A, B and C. Aluminium alloys for


normal duty are designated by letter N followed with number 2 to 8, and medium strength aluminium alloys by letter H followed by numbers 9, 11, 12, 14, 15, 18, 19, 20 and 30.

1.27. NICKELUses of Nickel. It is used (i) for manufacturing chromium-nickel stainless steels and nickel-chromium molybdenum steels.

(ii) for electroplating on base metals. It is used as an under plating before chromium plating to give protection against corrosion.

(iii) for minting coins.Properties of Nickel. Nickel is a hard, lustrous,

white metal. It fuses at 1484°C. It can take up high polish and is stable in dry air. However, when exposed to dampness, it tarnishes. It is not attacked by alkalies. Cast nickel contains carbon which makes it non-malleable. Its electrical conductivity is less than iron. It is magnetic and is more resistant to corrosion and to loss of strength due to heating in comparison to iron.

The ease with which nickel can be cast, machined, spun, drawn into wire, forged, welded, brazed soft and silver-soldered, makes it as good as mild steel. Nickel is used in large quantities due to its high resistance to even highly corrosive solutions. The following are its physical and mechanical properties :

Physical PropertiesSpeci c gravity ... 8.9Melting point ... 1458°CTensile strength ... 3700 kg/cm2

Thermal conductivity at 100°C(cal/cm3/sec/°C) ... 0.145Thermal conductivity at 290°C ... 0.128Speci c heat mean 0 to 100°C(cal/gm/°C) ... 0.1147Co-ef cient of linear expansionbetween 25 and 100°C ... 1.33 × 10–6

Its protection is a complicated affair due to dif culty in separating copper from it.

1.28. ZINCZinc is a weak metal (tensile strength of 1550 kg/cm2). It resists corrosion due to formation of a dense layer of corrosion product which insulates it against continued corrosion.

Uses of Zinc. Zinc is used as a protective coating for steel. Zinc is applied on steel by hot dipping (the method is called galvanising) or by electroplating (this method is called electro-galvanising). It is used in the form of rolled sheets for roo ng and battery containers, and as a lining for transportation cases, because it can be made water and air-tight and is proof against insects, etc.

Underground lines can be protected against corrosion by connecting them with insulated wires to zinc anodes that are buried near-by.

Bureau of Indian Standards have published following speci cation on zinc material.

IS : 209—1966—General for zinc.IS : 713—1966—Alloy ingots for die castings.IS : 742—1966—Alloy die castings.IS : 2258—1967—Rolled sheet and strip.

1.29. TINProperties of Tin. Tin has speci c gravity of 7.285 and has got melting point of 232°C. It is a soft metal having very low tensile strength (130 kg/cm2). When cold it is quite brittle and it cracks when it is bent. It is malleable at about 100°C. At this temperature it can be rolled into sheets or drawn into pipes. It resists corrosion due to water and most of the organic acids.

Uses. (i) As it is not corroded by water and organic acid, it is used for lining copper and iron tanks and also cooking vessels. It is plated on iron sheets.

(ii) It is used to form very useful alloys such as solder, bell metal, bearing alloys etc.

1.30. MOLYBDENUMUses of Molybdenum in Industry

1. Lubrication. Molybdenum disulphide dispersed in greases and oils is used for industrial and automotive lubrication. Molybdenum disulphide in volatile carriers forms dry coating which acts as lubricating agent.

2. Corrosion Inhibitor. Sodium molybdate acts as corrosion inhibitor especially on aluminium surfaces.

3. Catalysts. Cobalt molybdate and various other molybdenum salts aid hydrogen treatment of petroleum stocks for desulphurisation. Phosphomolybdates promote oxidation.

4. Coloration. Molybdenum compounds form pigments and dyestuffs.

5. Agriculture. Molybdenum in the form of sodium molybdate or molybdic oxide, is used as fertiliser.

6. Protective Coating. Zinc or calcium molybdate is used as an inhibitory pigment for protective coating for metals exposed to corrosive atmospheres.

1.31. MAGNESIUMIt is a very light metal which melts at 651°C. It has tensile strength of 1000 kg/cm2 and is rather brittle. It can be hot-worked easily but requires to be cold-worked carefully. It has poor resistance to corrosion when air is humid and contains traces of salt. Corrosion resistance can be improved by priming and painting. It is used as the base of light alloys.

1.32. NEW METALSMany formerly rare and/or new metals have recently

been used as alloying elements to improve the properties of the more common metal. The manufacture and use of these new metals is increasing at a very rapid rate. Some of these are described below.


Titanium. It is becoming an important structural material. Commercially pure titanium melts at about 1660°C. It has good weight to strength ratio, high temperature properties and it is very corrosion resistant. It is about 67% heavier than aluminium, and about 40% lighter than stainless steels. It retains its strength very well upto 540°C.

Titanium-carbon alloys (with carbon varying from 0.015 to 1.1%) are nding wide acceptance. With increase in carbon content, these become more brittle but also become more corrosion resistant. Maximum tensile strength of 7.5 MPa is reached with 0.4% carbon. However, 0.04% carbon alloy has a strength of about 6150 kg/cm. Titanium alloys using tungsten and carbon have a tensile-strength of about 8.7 MPa.

These ne wide application in aircraft industry as titanium-manganese alloys with 8% Mm.

The extraction of titanium from its ore is dif cult, because of the great af nity of titanium for oxygen.

Zirconium. It is a rare metal which is found in nature combined with silicon as zirconium silicate (ZrSiO4). It melts at 1852°C whereas Zirconium oxide melts at 2700°C and is used as lining for high temperature furnaces. It is dif cult to separate from its ore. It is silvery-white in colour and reasonably strong.

Zirconium oxidises easily. Its strength is affected by oxygen, nitrogen and hydrogen with which it combines. It is used as air alloying element in alloy steels.

Zirconium alloys with tin, nickel, Cr and iron have improved corrosion resistance and higher strength than unalloyed Zirconium. It is also used as a nuclear engineering material.

Beryllium. It is a rare metal which is expensive to produce. It is light metal having density of 1.845 gm/cc. (lighter than aluminium). It is often used as an alloying element with other metals (like copper and nickel to increase elasticity and strength characteristics). It is a hard, steel-grey metal that melts at about 1285°C. It does not react to any marked degree with neutrons which pass through it.

It has high thermal and electrical conductivity and high heat absorption rate. It has good strength at high temperatures.

Some beryllium copper alloys are very strong and are used to replace forged steel tools in places when an explosive atmosphere may be present since these alloys are non-sparking. Because of its low density and high modulus of elasticity, beryllium also nds use in high speed aircrafts and rockets.

Beryllium is mixed with magnesium to reduce its tendency to burn during melting and casting.

This metal and its compounds are reported to have dangerously poisonous properties and as such special precautions are needed in working with it.

Halnium. This is similar to zirconium. Its melting point is 2120°C. It is heavy and slightly stronger than beryllium. Because of its high strength and corrosion resistance, capacity to absorb neutrons and free radiation

attack, it is used as a control-rod material for pressurised water cooled reactors.

Niobium. This is a silvery white metal which is extremely ductile and soft. Its melting point is 2468°C. It is slightly weaker than iron. Since its ductility is affected by small amounts of oxygen and carbon, hot working and heating in air should be avoided. It is used in nuclear engineering applications and for gas-turbine blades.

1.33. ALLOYSAlloy is a mixture with metallic properties and is composed of two or more elements, of which at least one is a metal.

In course of development of metallography it has been found that some alloys are de nite compounds, some are solid solutions of one metal in another, some are mere mechanical mixtures and others are combinations of these conditions.

Microscopic and X-ray studies of alloys together with spectroscopic analysis have helped metallurgists to carry out systematic study and development of useful alloys.

A metal can be changed with respect to its physical and chemical properties such as hardness, tensile strength and resistance to corrosion, by simply adding a small quantity of one or more elements into it. This basic fact has given great impetus to the development of useful alloys continuously.

Metals employed for making Alloys. Important metals used for making alloys are copper, lead, zinc, nickel, aluminium, chromium and tungsten. To some extent, antinomy and bismuth are also added.

Precautions necessary for making Alloys. The following precautions are essential for making alloys :

1. When two elements which are to be mixed have got widely different melting points or when one of the elements is easily oxidised or volatilised, then the minor element is rst mixed into the major element in known excess quantity. This mixture whose composition is known is then added into the required quantity of the major element. For example, if any alloy has to be formed by adding small quantities of silicon to copper, at rst a known excess quantity of silicon is added into copper. This mixture is then mixed with the required quantity of copper.

2. In preparing alloys, it is essential that oxidation be prevented or minimised as far as possible. This is done either by covering the metals with ne charcoal or by using electric induction furnaces for melting where furnace conditions can be accurately controlled.

3. The least fusible metal is melted rst and the more fusible is added afterwards.

4. Heaviest metal is added last of all in order to prevent its settling at the bottom.

5. Suf cient stirring with a graphite, wooden or iron rod is necessary before casting.

6. For casting of an alloy, suitable moulds are necessary. If moulds are made of metal, surface should be coated with a mould wash for preventing the casting from sticking to the mould surface, otherwise the smoothness of


the surface of the cast will be spoiled. Mould wash should be such that it does not give off any gas.

7. Metal when poured into another metal should not be at very high temperature. Generally it is added at a temperature about 100°C above the melting point of the alloy.

Theory of Alloys. Theory of alloys can be viewed and studied from two aspects. These two aspects are atomic structure and phase aspects. Both these aspects are described below.

1. Atomic Structure. Atoms in an alloy are arranged in a periodic manner in a crystal space. Different alloys have different crystal structures, and in crystal structure of different alloys the distances between atom centres in the lattice are also different. The differences in properties of different alloys can be related to the difference in their crystal structure. For example, metals and alloys which have body-centred cubic-crystal structure (such as copper and silver) are more ductile but not so strong as the metals and alloys which have face-centred cubic crystal structure (such as iron and tungsten). Alloys having hexagonal and other non-cubic crystal structure have not so good a malleability as metals and alloys having cubic crystal structure.

2. Phase aspect of Alloys. A phase in an alloy or metal is the region having the same crystal structure, composition and interatomic distances. A solid metal or alloy consists of one or more phases. The distribution of phases in an alloy determines the properties of an alloy. For example, in age-hardened alloys, it is the ne dispersion of the second phase throughout the alloy that causes strengthening of the alloy. However, when the second phase is distributed as coarse particles by annealing, the strength of the alloy is reduced.

Modes of Formation of Alloys. Following are the modes of formation of alloys :

1. Solubility Alloys. When different metals are made to alloy, either they dissolve into each other completely or partially or may not dissolve at all and remain insoluble. Good alloys are formed only when the component metals dissolve into each other. For example, copper and zinc are completely soluble in liquid state but partially soluble in solid state. An alloy containing 10% zinc and 90% copper consists of a single phase, face-centred cubic solid solution. Some of the copper atoms in the crystal structure are replaced or substituted at random by zinc element. This type of solid solution is called substitutional. All metallic solutions are of this type.

Iron and lead are insoluble in both phases, liquid and solid but yet free machining lead steel containing 0.2% lead is made by dispersing lead through the modern steel when ingot is being prepared. Lead remains in suspension while the ingot is freezing quickly. Lead is present as solid particles of lead not as solid solution.

Following are the rules which govern the solubility of the substitutional type alloys :

(i) Complete solubility occurs if the components have the same crystal structure. Example of complete solubility is the alloy of copper and nickel ; both the elements have got face centred cubic structures.

(ii) The more is the difference between atomic size (radius), the less is the solid solubility. Complete solubility occurs only when the sizes differ up to 15%.

(iii) Metals having the same valency show more solid solubility than metals having different valencies. Also metals of lower valency tend to dissolve a metal of high valency to a greater extent.

(iv) If the metals are placed farther apart in electro-chemical series the tendency to form solid solution is greater.

2. Compound formation. Metals react together to form compounds. Following kinds of compounds are commonly found :

(i) Valence compounds. Normal rules of valency are followed in their formation.

(ii) Electron compounds. Electron compounds are phases with wide range of homogeneity. Their crystal structure is determined by the number of valence electrons in the alloying elements. So they are called electron compounds. The electron compounds may have the following three electron atom ratios–3 : 2 (beta brass or beta manganese structure), 21 : 13 (gamma brass structure), and 7 : 4 (epsilon brass hexagonal structure). These are the ratios at which electron compounds occur. Examples of beta brass type are CuZn, AuZn, AgZn, FeAl, NiAl, CoAl, Cu3A1, Cu3 Sn. In all these the electron-atom ratio is 3 : 2.

3. Miscellaneous compounds. Among the miscellaneous compounds are included extremely hard metallic carbides, nitrides and borides. In these compounds, small atoms of carbon, nitrogen and boron t interestitially between the metal atoms in the structure.

Examples are Fe3C (cementite), a very hard carbide used for making edges of razor blades and tools and W2C with 13% cobalt to form heavy duty tungsten carbide are used for cutting tools.

4. Order-disorder. In solid solutions, atoms of different metals are distributed at random on the lattice points. In compounds, atoms of each kind occupy assigned lattice points. There is another class of solid solutions that change to compounds called superlattice compounds. In these superlattice compounds, atoms go from random places to assigned places. The superlattices are like ionic compounds. Examples are dental alloys (mainly CuAu) which are solid solutions when cooled rapidly but they form superlattice structure when cooled slowly.

Types of Alloys. Classi cation of alloys can be done in the following four ways :

1. Classification based on metallurgical structure. Alloys are classi ed according to the fact whether they consist of single phase or of two or more phases. For example, monel metal (2/3 Ni and 1/3 Sn), some brasses (70% Cu, 30% Zn), transformer iron (96% Fe, 4% Si) are single-phase alloys. Annealed steels (phases of ferrite and carbide), Muntz metal (60% Cu and 40% Zn) are two-phase alloys.

2. Classi cation based on principal metal in the Alloy. Alloys are classi ed according to the principal metal contained in them, for example aluminium alloys,


magnesium alloys. These alloys when distinguished by principal metals, have certain distinguishing characteristics too. For example, Al alloys and Mg alloys have low sp. gravity (2.8 for Al alloys and 1.8 for Mg alloys), and as strong as steel. Copper alloys are characteristic for their corrosion resistance and good pliability. Lead alloys and tin alloys are corrosion-resistant and heat-resistant and are used for bearings and solders. Nickel alloys are notable for their strength.

3. Classi cation based on method of Fabrication.Different alloys are used for different types of fabrications. So they are classi ed according to the type of fabrication for which they are used. For example, there are copper casting alloys and copper wrought alloys. Casting alloys contain 5% each of tin, zinc and lead for getting pressure tightness and easy machinability. Wrought copper alloys contain 5 to 40% of zinc.

4. Classi cation based on the application of alloys. Alloys are made for different purposes and thus these may be classi ed accordingly. For example, solder alloys containing tin and lead (tin 40 to 60% and lead 60 to 40%), and bearing metal alloys, etc.

Properties of Alloys1. Thermal and electrical conductivities of a solid

solution are less than those of the pure metals. According to Mathiessen’s rule when small quantities of an alloying element are added in solid solution to metal, in increase in resistance does not depend upon the temperature.

For mixtures of insoluble phases, the thermal and electric-resistances follow the law of mixtures.

2. Density is increased by heavier metal in solid solution and is decreased by a lighter metal. In case of interstitial solid solution, there is little effect on density by the added element.

3. Specific heat and co-efficient of thermal expansion are governed by the law of mixtures.

4. Melting Point of a metal is converted into a range by the addition of an alloying element. The melting point range of alloy can be higher or lower than the melting point of metal. The greater is the difference in valencies between the metals of alloy, the wider is the melting range.

5. Boiling Point is also converted into a range by the addition of alloying elements.

Treatment of Alloys. Alloys have to pass through one or more of the following processes before they are converted into nished products :

1. Melting. Alloys may be melted and in the molten state, hardeners, deoxidisers, etc. may be added, or the molten metal may be superheated. Lead, aluminium and magnesium alloys are melted in iron pots. Bronze is melted in graphite crucibles. Nickel and steel are melted in refractory lined furnaces. Magnesium alloys are melted with ux which surrounds the entire melt.

2. Casting. Castings are made by gravity is sand or metal moulds. Cast iron and bronze are cast in green sand moulds. Magnesium alloys are cast in dry sand mould as molten magnesium reacts with water. Centrifugal force is employed for casting into spinning steel moulds. Pressure is used to force liquid alloys into die casting moulds.

3. Sintering. Sintering is employed for blending metallic powders which have been pressed into shape. Either an alloy powder or a mixture of powders of alloying metals is used. In sintering, diffusion takes place. This diffusion homogenises the mixture of powders and the

nished product obtained is uniform in composition. For example, for the formation of Alnico magnets, the pressed mixture of iron, nickel, and aluminium powders is sintered upto the melting point of the alloy to ensure good diffusion.

4. Hot working. For making plates, rods and structural shapes, hot rolling is employed. Hot forging is carried out for complicated shapes. Extrusion is adopted for making rods and structural shapes from aluminium, magnesium and copper.

Lower limiting temperature for hot working is the re-crystallisation temperature or a bit higher than that. The maximum hot working temperature is generally below the solidus. Some alloys between certain ranges of temperature are brittle and cannot be hot worked. For example, monel metal cannot be hot worked in the range 650 to 870°C.

5. Cold working. Cold working is carried out below recrystallisation temperature. By cold working, the metal is strengthened. The other advantage of cold working is the convenience of operation as it is impossible to hot work thin metal for foil rolling and wire drawing.

6. Surface treatments. There are numerous surface treatments to which alloys can be subjected. Carburising in which carbon steel is heated up to 900°C in a carburising material is an example of surface treatment. Nitriding is another example of surface treatment, in which alloy steel (C = 0.30%, Cr = 1.3%, Al = 1.3%, Mo = 0.2%) is subjected to an atmosphere of dissociated ammonia for 48 to 96 hours at a temperature of 550°C. A surface coating of aluminium nitride is formed. This coating raises the hardness to above 1000 Brinell.

Other surface treatments are colouring, chromising, and siliconising in which layers about 1.0 mm thick containing 25% Al, 20% Cr and 14% Si are produced on the surface of mild and other steels. Colorised steels are used for service at high temperatures to 815°C in an atmosphere containing sulphur. Chromised and siliconised steels are used where heat and corrosion resistance are required.

7. Joining. Finished materials like machines, pressure vessels, and big structures, etc., can be fabricated by the process of fusion welding. In fusion welding alloys are welded together by bringing them to molten form with or without the addition of ller metal.

In brazing, the ller metal is non-ferrous metal or alloy whose melting point is more than 540°C but less than that of the metals or alloys to be joined.

8. Heat treatment. Heat treatments of following types are commonly used:

(i) Annealing. When an alloy is cold worked there is increase in tensile strength, hardness, diffraction line width, electric resistivity, coef cient of expansion, and electrode potential, but there is a decrease in ductility, cold workability, density and resistance to notches. When


temperature of alloy is raised, all the properties tend to return to normal values. This heating of the alloy for softening is called annealing.

(ii) Precipitation hardening. It is a heat treatment that increases the strength of an alloy by virtue of the appearance and growth of a second phase in a supersaturated solid solution.

When 4% copper alloy is slowly cooled from 254°C, crystals of theta phase appear when the temperature falls below the saturation curve. These theta crystals increase in size and the copper content of the solid solution decreases during continued slow cooling to room temperature. This appearance of crystals of theta phase is called precipitation hardening.

(iii) Martensite hardening. It is carried out by the suppression of a eutectoid phase transformation by rapid cooling or quenching. In this way there is a formation of hard unstable phase.

For example, heat treatment of steel consists in the formation of martensite which is a hard unstable phase. Martensite is formed by quenching austenite (solid solution of carbon in face-centred cubic iron) from the temperature above the iron-iron carbide eutectoid at 720°C.

Metal Alloys: Commercially pure metals generally do not have suf cient strength for many engineering applications. Most of metals when alloyed with various elements, change their structures and properties. Important concepts in alloying of metals are the solubility of the alloying elements in base metals and the phases that are present at various ranges of temperature and composition. Phase diagrams show the relationships among temperature, composition, and the phase present in a particular alloy system.

A lot of research is going on for narrowing the hardenability bonds of alloys for better property control in heat treating of parts.

1.34. ALLOYS OF COPPERCopper-Zinc Alloys-Brasses. These are very important alloys having high resistance to corrosion, easily machinable and acting as good bearing materials. The percentage of zinc varies from 5 to 45% in these alloys. However, commonly used alloys contain 57% to 70% of copper. Besides zinc, a little quantity of metals such as tin, manganese, aluminium, iron and lead are also added to give special properties to these alloys.

Brasses have greater strength than copper metal and by adding the appropriate quantities of constituent metal, mechanical properties such as ductility, strength, machinability, etc., can be changed according to the desired limits. For example, ductility of alloy increases by addition of zinc upto 37% but after it the ductility falls sharply. Machining properties can be improved considerably by including small amounts of lead. Leaded brass bars are frequently used to produce highly loaded components on automatic screw machines, because by addition of lead, tensile strength remains same, machinability improves, but shock resistance is lowered. Season cracking may

occur with high zinc brasses but rarely with 15% zinc or below. Cracking occurs spontaneously on exposure to atmospheric corrosion in brass objects with high residual tensile stresses at the surface. It may be prevented by avoiding the induction of internal macrostresses or by removing such stresses by relief annealing at 250 to 280°C without softening the work. It should be noted that alloys susceptible to spontaneous season cracking, even if free from internal strains, will crack when exposed to corrosive conditions under high service stresses. Following types of brasses are very common.

(i) 60% Copper and 40% Zinc (Muntz metal). It has high tensile strength and less ductility and is suitable for hot working by rolling, stamping or extruding, and used for certain marine ttings and pump parts. This alloy containing small quantities of other metals to modify the properties according to requirements is very useful for castings, hot stampings or extrusion and machining. So it is used for ttings, valves and other such parts.

Mn and Fe are added to increase tensile strength and resistance to salt corrosion. With addition of Mn, it is known as manganese bronze (typical composition : Cu–65%, Zn–35% and Mn–5%, and it can be rolled, drawn and cast ; and is used for under water shafts and heavy duty bearings and gears.

(ii) 70% Copper and 30% Zinc. It is strong, highly ductile alloy. It is very much useful for cold working. So it is largely used for making tubes, sheets and cold press work.

(iii) 70% Copper, 29% Zinc and 1% Tin. (Admirality Brass). It has high corrosion and abrasion resistance. So such a brass is used for marine work, and steam condenser tubes.

(iv) 76% Copper, 22% Zinc and 2% Aluminium. Such an alloy is used for condenser tubes in marine and other installations as it is highly resistant to corrosion and impingement attack of sea water.

(v) 95% Cu, 5% Tin. Used for worms, gears, pump bodies and bushes.

(vi) 5% to 20% Zinc alloys. (Known as red brass). These nd application because of freedom from season cracking due to their red colour. These have desirable melting point in brazing operation. Used for plumbing of pipe end connections, rivets, hardware etc.

(vii) Delta metal (Cu–55%, Zn–41%, Pb–2%, Fe–2%). It is used for parts of marine engines, screw properllers, and in chemical, hydraulic and mining plants.

Brass is used for the manufacture of evaporators in sugar and salt industries as brass has got high corrosion resistance and heat conductivity. The addition of small quantities of tin to brass (as shown above) increases the corrosion and abrasion resistance of brass. Addition of small quantity of aluminium to brass (as shown above) increases corrosion-resistance and resistance to impingement attack by sea water. Addition of lead to brass renders it free cutting and remarkably machinable. Addition of 0.75 to 1.25% tin improves the corrosion resistance.


Dezinci cation takes place in brasses containing high percentage of zinc and brass containing 85% or less of copper. Dezinci cation if visible by the formation of spongy areas of copper in the form of layers or plugs. Brass for springs should be rolled as hard as consistent with the requirements of subsequent forming operations. For articles requiring sharp bends, or for deep-drawing operations, annealed brass must be used. All the brasses may be hot worked if they are free from lead, particularly those alloys containing less than 60% or less of copper. Extruded sections of many copper alloys are made in a wide variety of shapes.

Copper-Tin Alloys-Bronzes. Tin content varies from 2 to 12% but generally bronzes having 5% tin and 10% tin are commonly used. Bronzes containing 5% tin are used in wrought form and those containing 10% tin are used in the cast condition. Addition of tin in bronze makes it harder and stronger alloy than brass. Bronze is formed by casting (brass is formed by working). As the tin content of simple bronzes oxidises quickly, when the metal is hot, resulting in brittleness ; it is usual to add various deoxidisers like Zn or phosphorus. Phosphorus in small quantities (max. 4.0%) is added to bronzes and such bronzes are called phosphor bronzes. Addition of phosphorus improves the casting qualities and hardens the bronzes and increases the resilience. Phosphor bronzes are used for bearings, worm wheels, rods, sheets.

Bronzes can be readily cast and machined. So these are widely used for the manufacture of pumps, valves,

anges, etc., where resistance to corrosion is required.If zinc (as deoxidiser) is added to bronzes, gun metal

is formed. A common composition of gun metal is 83% copper, 10% tin and 2% zinc. Gun metals are very useful for foundry work. Gun metal being highly resistant to corrosion, is used for marine ttings, valves and ttings for water and general service and other castings requiring good mechanical and corrosion-resisting properties.

Small quantities of other metals such as nickel and lead are also added to the copper-tin alloys in order to modify their mechanical or corrosion-resisting properties to make them useful for some special purposes. Engineer’s bronzes containing 88% Cu, 10% tin, and 2% Zn is used for engine parts, steam ttings and hydraulic machinery.

The aluminium bronzes with 5–8% aluminium nd application because of their high strength and corrosion-resistance, and sometimes because of their golden colour. If aluminium content exceeds 10% then the bronze becomes very plastic when hot and has exceptionally high strength, particularly after heat treatment.

Bell metal is formed by adding 75–80% tin and such an alloy is used for making bells and gongs.

A number of alloys are made with silicon as the primary alloying agent and using appreciable amount of zinc, iron, tin, or manganese. These alloys are as corrosion-resistant as copper and possess excellent hot workability with high strength. These alloys are extensively fabricated by arc or acetylene welding like tanks and vessels for hot-water storage and chemical processing.

Copper-Aluminium Alloys-Aluminium Bronzes.These alloys contain 5 to 11% of aluminium. The common alloys contain about 10% of aluminium. These have got high tensile strength, as high as 5.6 MPa when they are in cast state. By heat treatment tensile strength can be increased to over 6.4 MPa. The heat treatment consists in heating the alloy to 850°C, quenching in water, reheating to about 650°C and then allowing it to cool slowly.

Small quantities of other metals such as iron, manganese and nickel are also added to aluminium bronzes in order to improve certain mechanical properties.

Aluminium bronzes are highly corrosion-resistant due to the formation of protective lm of aluminium oxide. These are resistant to action of acids also.

These make good castings when properly cast. These have got hot forging properties and mechanical strength. So these alloys are used for valves, pumps and for other parts where corrosive liquids act.

These alloys are resistant to oxidation at high temperatures.

Copper-Nickel Alloys. Even when small percentages of nickel are added to copper there is a de nite affect on the mechanical and physical properties.

The following are the effects produced in mechanical properties of copper when nickel is added to it :

(i) Copper-nickel alloys have high tensile strength. With increase in nickel content, there is increase in tensile strength up to 60 to 70% of nickel. However, it has been found that if the alloy is properly annealed, there is a very little difference in the tensile strength of alloys containing 50 to 80% nickel.

(ii) Copper-nickel alloys have high ductility and malleability. Alloy which has highest ductility contains 80% nickel and 20% copper. These may be worked extensively without annealing.

(iii) Such alloys retain their strength even at high temperature. The alloy which has maximum strength at high temperature contains approximately 70% nickel. This alloy has limiting creep stress of 3200 kg/cm2 at a temperature of 400°C. This alloy has high resistance to shock also, having an impact value of 16 kg-m at 500°C.

(iv) Copper-nickel alloys show great resistance to corrosion. With increase in nickel-content, the resistance to corrosion increases. Such alloys resist the attack of alkalies like sodium hydroxide, sea water and also organic acids, and hydrochloric acid. However, these are prone to attack of nitric and sulphuric acids. These are used for condenser tubes for most severe service.

(v) Copper-nickel alloys are quite resistant to impingement corrosion called eorrosion. These being white in colour nd applications as base for most silver-plated ware.

The following are some of the copper-nickel alloys. Copper-nickel alloys with 2, 20, 30, 45 and 68% nickel are commonly used.

Alloy containing about 2% of nickel is used for manufacture of wire for electrical purposes. This alloy


has trade names such as Constantan, Eureka, Ferry, etc. This alloy has high speci c resistance and has almost zero temperature coef cient. So such an alloy is used for making resistors for speed and voltage regulators. This element is also used in thermocouple pyrometers in conjunction with copper. Copper-nickel alloy, to which 5 to 6% of zinc is added is used for making condenser tubes as by doing so resistance to corrosion increases.

Copper-nickel alloys are used for heat exchanger tubes in oil and other processing industries.

A common alloy known as copper-nickel containing 75% Cu and 25% Ni is used for coinage, casing of ri e bullets and condenser tubes.

Copper-Silicon Alloys. Copper-silicon alloys have corrosion resisting properties of copper but physical properties and structural qualities are comparable with those of mild steel.

In copper-silicon alloys, the thermal and electrical conductivities are very much less than copper. But their strength is comparable to mild steel. Welded joints are highly resistant to corrosion. Copper-silicon alloys are used to manufacture bolts, screws, tie rods, etc. because of their high resistance to corrosion.

Copper-Beryllium Alloys. These are expensive but their strength and elastic properties are far superior to other materials. These are ideal for springs, diaphragms and the like where high fatigue resistance under some-what corrosion conditions is needed. Cobalt provides a substitute for beryllium and produces a cheaper alloy of only slightly inferior properties.

Copper-Uranium Alloys. This is used principally for electrodes in resistance welding machines. It has high conductivity of the order of 80% of that of copper and does not soften on prolonged heating at temperatures around 450°C. This alloy is also used in the strip form for electrical switch parts because of its high softening temperature.

Fabricating Characteristics of Copper Alloys Machinability Ratings of Various Copper Alloys.

Copper alloys are generally machined at as high cutting speed as possible with a light feed and moderate depth of cut. Sand cast alloys are either sand blasted or pickled in acid to remove abrasive surface scale or nished by high-speed tool with low speed and coarse feed to remove hard scale.

Free cutting brass (61.5% Cu, 35.25% Zinc and 3.25% Pb) is the most machinable non-ferrous alloy. If it is given a machinability rating of 100% then other brasses and bronzes have machinability ratings as under :

Leaded commercial bronze—90%Leaded naval brass, and Selenium Copper—80%Admirality brass, leaded phosphor bronze, and leaded

nickel silver—50%Yellow brass and tin bronze—40%Manganese bronze—30%Formability. The forming qualities are dependent

upon the composition of alloy, grain size, and temper. Alloys with more than 63% copper are easily press worked but with less than 63% copper, the beta phase which is

brittle produces surface defects in the form of fracture or waviness. Pure copper is very ductile and gets work hardened less rapidly than brass or bronze. Copper alloys with ne grains (0.01 mm diameter grain size) are very smooth after forming operation and can be buffed to a high luster. As grain size increases, the ductility also increases. For deep drawing and heavy drawing, bigger grain size (0.05 to 0.12 mm) is preferred.

Weldability. Copper can be welded by gas, metal arc, carbon arc or gas-shielded arc. It is also joined by brazing or ultrasonic welding.

Castability. Copper can be cast in sand mould. Copper alloys are also easily cast and handled in foundry.

Designation of copper and its alloys as per IS : 2378

Copper is designated by Cu followed by group of symbols like CATH, ETP, FRHC, DPH for cathode copper, electrolytic tough pitch copper, ne re ned high conductivity copper, and phosphorised high residual phosphorus non-arsenical grade respectively.

Depending on method of casting, following symbols are used :

G – for sand cast, GD – for die cast, GK – for chill cast, GZ – for continuous cast, and no symbol for wrought form.

If nominal or average alloy content is upto 1%, then index number for copper alloys is designated by alloy symbols in the descending order of % age content. Average alloy can be expressed upto one decimal place, the decimal digit being underlined by a bar.

If nominal or average alloy content is more than 1%, then index number for copper alloys is designated by average or nominal alloying content being rounded to the nearest whole number. As above, one decimal digit can be used. Surface nish and heat treatment are designated as

a – annealed, d – cold worked, e – extruded, f – forged, H –full hard, 1/4 H – Quarter hard, 1/2 H – half hard, 3/4 H – three fourth hard, J – bright rolled as drawn, J7 – bright pickled, J8 – bright annealed, M – machined.

1.35. LEAD ALLOYS1. Antimonial-Lead. Its composition varies from 6 to 8% of antimony with balance lead. Antimonial lead is highly resistant to sulphuric acid and many chemical solutions containing sulphuric acid. The hardness and strength of antimonial lead is more than that of lead. It has a high tensile strength of about 470 kg/cm2 and elongation of 22%.

Lead containing 13% antimony, 1% tin, 0.5% arsenic and 1% copper has got good casting properties. The castings obtained are quite strong too.

2. Lead-Tin Alloys. An alloy containing 10 to 25% tin and 90 to 75% lead is used as metallic coating for sheet iron. Such a coating is applied by hot dipping process. Sheet iron which is coated with such an alloy is used for the manufacture of containers. When harder and more resistant coating is required, antimony is also added. Alloys of lead, tin and antimony are used as type metals.

3. Alloys of lead for Cable Industry. Cables require a exible covering and sheathing for protection from


moisture and oil. Bearing lead and 1% antimony lead are used for covering electric power and communication cables due to the following reasons : (i) These are impervious to moisture and oil. (ii) These can be readily extruded. (iii) These have got ample strength. (iv) These are not corroded in all normal atmospheric environments. (v) These are pliable and sheathing can be reeled and unreeled and can be bent around sharp corners.

An alloy containing about 0.40% of calcium can also be used for sheathing cables. Addition of calcium increases strength without affecting ductility and pliability of lead. Fatigue strength is also increased.

Another alloy used for this purpose has composition of 0.15% arsenic, 0.1% tin, and 0.1% bismuth and the balance lead.

1.36. ALLOYS OF ALUMINIUMAlloys of aluminium are light in weight compared to steel, brass, nickel and copper. These resist corrosion and have good electrical and thermal conductivities. These can readily accept a wide range of surface nishes and can be fabricated by all common processes. These lose part of their strength at elevated temperature. At sub zero temperatures, however, their strength increases without loss of ductility and thus these are best suited for low-temperature applications.

Aluminium alloys can be classi ed according to the method of shaping them, viz. wrought products and cast aluminium alloys. These are further classed as to whether they respond to heat-treatment of strengthening-type or not. Based on the sequences of basic treatments used to produce the various tempers, all forms of wrought and cast aluminium and aluminium alloys could be classi ed as:

(i) Fabricated–Products which acquire some temper from shaping processes not having special control over the amount of strain-hardening or thermal treatment.

(ii) Annealed, recrystallised (wrought products only)–Wrought products subjected to softest temper.

(iii) Strain-hardened (wrought products only)–Products whose strength is increased by strain-hardening with or without supplementary thermal treatments to produce partial softening.

(iv) Solution heat-treated–This refers to unstable temper possible to alloys which spontaneously age at room temperature after solution heat treatment.

(v) Thermally treated to produce stable tempers–Products which are thermally treated, with or without supplementary strain-hardening, to produce stable tempers.

Wrought aluminium alloys. Strength greater than that of pure aluminium can be achieved by addition of other elements. Alloys can be strengthened by heat treating in some cases. Alloys which can’t be strengthened by heat treating are known as non-heat-treatable alloys. The initial strength of these alloys depends upon the hardening effects of elements such as Mn, Si, Fe and Mg, singly or in various combinations. These alloys are work-hardenable and further strengthening is achieved by various degrees of cold working. Alloys containing

appreciable amount of magnesium when supplied in strain-hardened tempers are usually given a final elevated-temperature treatment called stabilising.

Heat-treatable alloys show increasing solid solubility in aluminium with increasing temperature and as such it is possible to subject them to thermal treatments which will impart pronounced strengthening. In the rst step (solution heat-treatment), the soluble element is put in solid solution at elevated temperature of about 490°C. Then it is rapidly quenched (usually in water) which momentarily freezes the structure and for a short time renders the alloy very workable. Since alloy in this stage is very soft and ductile, it can be easily cold worked. It is at this stage that some fabricators retain this more workable structure by storing the alloys at below freezing temperature until they are ready to form them. At room or elevated temperatures the alloys are not stable after quenching, however, the precipitation of the constituents from the supersaturated solution begins. After a period of several days at room temperature, termed ageing or room-temperature precipitation, the alloy is considerably stronger. Many alloys approach a stable condition at room temperature, but some alloys (containing Mg and Si, or Mg and Zn) continue to age-harden for long periods of time at room temperature. Thus depending on composition of alloy, some alloys start to become harder and stronger shortly after quenching (age-hardening). An interesting application is with Duralumin alloy containing 4% copper, whose rivets after solution treating are kept in a refrigerator. After riveting, the become stronger due to age hardening at room temperature. Some alloys do not become stronger unless they are heated for a few hours after they have been solution treated. In such cases, by heating for a controlled time at slightly elevated temperature even further strengthening is possible and properties are stabilised (Arti cial or precipitation hardening).

Casting aluminium alloy. These alloys contain large quantities of Si or Mg, to produce a lower melting point alloy. Only a few alloys are suitable for die casting and sand casting. Some of these alloys naturally age after solution treatment, but others have to be given precipitation treatment.

Clad aluminium alloys. Heat treatable alloys containing copper or zinc as major alloying elements are found to be less resistant to corrosion attack than the majority of non-heat and treatable alloys. To increase corrosion resistance in sheet and plate form these are often clad (upto 2.5 to 5% of total thickness on either side) with high purity aluminium, a low magnesium-silicon alloy, or an alloy containing 1% zinc. Cladding not only protects the composite due to its own inherently excellent corrosion resistance but also exerts a galvanic effect which further protects the core metal.

Duraluminium. It is a very important alloy of aluminium. Its composition is as given below :Copper ... 3.5 to 4.5%, Manganese ... 0.4 to 0.7%Magnesium ... 0.4 to 0.7%, Iron or Silicon ... Not more

than 0.7%,Aluminium ... Rest.


This alloy has got high machinability. It can be heat treated to increase its tensile strength. By heat treatment tensile strength of the alloy can be raised up to 4300 kg/cm2

without affecting ductility of the alloy. Duraluminium is as strong as steel but has only about one-third of its weight. So it is used to fabricate parts of aircraft and automobiles.

Aluminium silicon alloys have excellent casting qualities and resistance to corrosion. The alloys are not hot-short and are easy to cast in thin or thick sections, but these are dif cult to machine. Aluminium-magnesium alloys are superior to practically all other aluminium casting alloys with respect to resistance to corrosion and machinability. At the same time these have high mechanical strength and ductility. Aluminium alloys for pressure die-casting must possess considerable uidity and be free from hot-shortness.

Effect of Alloying Elements on AluminiumTable below summarises the effect of various elements

on aluminium:Alloying Element Effects

Iron Impurity in aluminium ores. Small percentages increase the strength and hardness of some alloys and reduce hot-cracking tendencies in castings. Iron reduces pick up tendency in die-casting cavities.

Manganese It is used in combination with iron to improve castability. It alters the nature of the intermetallic compounds. Increases ductility and impact strength and reduces shrinkage.

Silicon Increases uidity in casting and welding alloys and reduces solidi cation and hot cracking tendencies. Improves corrosion resistance. Additions in excess of 13% make the alloy extremely dif cult to machine.

Copper Improves elevated temperature pro-perties and machinability. Increases strength upto about 12%. Higher concentrations cause brittleness.

Magnesium It decreases castability and improves strength by solid solution strengthening. Alloys with over about 6% will precipi-tation harden.

Zinc Lowers castability. High percentage promotes hot cracking and high shrinkage. Percentages over 10% produce tendencies for stress corrosion cracking. Promotes very high strength in combination with other elements.

Designation of Aluminium and its AlloysBIS have brought out IS : 6051 to describe the designation for aluminium and its alloys:

(a) Element Number Designations

Element Group Number

Unalloyed aluminium 1

(Wrought and cast form)

Copper 2

Manganese 3

Silicon 4

Magnesium 5

Magnesium Silicide (Mg2 Si) 6

Zinc 7

Other elements (such as Ni, Ti, Cr, Pb, Bi, etc.)

8

Unassigned 9

(b) Five Digit System(i) Wrought Aluminium Alloys

First Digit — Identi es the major alloying element.

Second Digit — Indicates rounded off mean value of the percentage of the major alloying element except for Group 4 containing silicon, when the digit refers to the mean percentage halved and rounded off ; and for Group 6 containing magnesium silicide, the digit refers to ve times the mean magnesium percentage rounded off.

Third, Fourth and Fifth Digits

— Identify the minor alloying elements in the descending order of their percentage and in the case of same alloy per’centage in the serial order, except for Group 6 containing intermetallic compound (Mg2Si) when the third digit refers to either magnesium or silicon which is in excess of that required for Mg2 Si (i.e. 1.7). For balanced composition, the third digit will be zero. In case of high purity aluminium base alloy, fth digit will be 1.

(ii) Wrought Aluminium (Unalloyed)First Digit – Always 1.Second Digit – Always 9, the unit digit of 99

minimum purity percentage.Third and Indicate the decimal purity Fourth Digits – percentage.Fifth Digit – Indicates variants, if any.


(c) Four Digit System(i) Casting Alloys.

First Digit – Identi es major alloying element.Second Digit – Indicates the mean value percentage

of the major alloying element halved and rounded off.

Third and Fourth Digit

– Identify the minor alloying elements in the order of their decreasing percentage, or in the case of same alloy percentage in the serial order.

(ii) Primary Ingots or Castings (Un-Alloyed)First Digit – Always 1.Second Digit – Always 9.Third Digit – Indicates the decimal purity percentage.Fourth Digit – Indicates variants, if any.

1.37. ALLOYS OF NICKELNickel-Molybdenum Alloys. There are several nickel-molybdenum alloys which are very much resistant to corrosion. These alloys are called Hastelloys A, C, D according to their composition.

(i) Hastelloy A. It is nickel-molybdenum iron alloy. It has high strength and ductility. Its strength does not decrease even at high temperature and it can withstand a load of 11 MPa at 900°C with a creep of one per cent per year.

It is easily forged and rolled into sheet. It can be machined and can form good castings. It can be welded by oxyacetylene or electric arc.

(ii) Hastelloy C. It is nickel-molybdenum-chromium-iron alloy. Its malleability is less than that of Hastelloy A. It cannot be worked hot or cold. It is good for casting but cannot be machined at high speed.

(iii) Hastelloy D. It is cast alloy consisting of nickel, silicon, copper and aluminium. This alloy is strong and tough and has high transverse strength. It is good for casting but cannot be worked either hot or cold. Its machinability is poor.

It can be welded with oxyacetylene or electric arc but addition of a ux makes the operation easy.

Nickel-Copper AlloyMonel Metal. It contains about 60% nickel, 33%

copper and a small percentage of iron and manganese and a very small quantity of silicon and carbon.

Monel Metal can be easily cast, forged, machined, silver soldered, brazed and can be drawn into wire. The alloy can be welded with oxyacetylene and electric arc. In rolled and annealed condition its tensile strength is 465 MPa. At a temperature of 600°C its tensile strength is 285 MPa while that of low carbon steel is 110 MPa.

Monel metal is not affected by atmosphere and sea-water. Monel metal resists the corroding effect of alkalies and acids other than nitric and sulphurous acids.

Monel metal is used for the following purposes :(i) It is used for evaporators and other parts of

chemical plants for the production and recovery of caustic soda, pump impellers used for handling corrosive liquors.

(ii) It is used for pickling plants, drying plants and for pump contact parts where tar and tar products are handled.

(iii) Monel metal lter cloth is used extensively.(iv) It is used for lining autoclaves used for the

manufacture of dye-stuffs.(v) Paints and varnish plants are made of monel.

Contact points of colloid mills are also made of monel metal.

‘K Monel’ Metal. K monel metal can be made by adding aluminium into monel metal. The utility of K monel metal lies in the fact that by heat treatment it can be made hard and strong retaining the corrosion resisting properties of monel metal. This K monel metal can be obtained in four different conditions by simple heat treatment.

(i) Hot rolled and softened.(ii) Hot rolled and softened and thermally hardened.(iii) Cold worked and ready for thermal hardening

after machining or fabrication.(iv) Cold worked and thermally hardened.Their strength, yield point and hardening progressively

increase from (i) to (iv).Uses of K Monel Metal. It is used for applications

where high strength and hardness combined with high corrosion resistance including immunity from rusting is required. For these reasons it is widely used for the following purposes :

(i) As valves and seats in pumps working with oil containing brines and sodium sulphide.

(ii) As valves and seats on starting air bottles for diesel engines.

(iii) For blades in paper-making machinery.(iv) For impulse blades of steam turbines working at

high pressures and high temperatures.(v) Due to its non-magnetic nature, high strength and

corrosion-resistance qualities, it is used for aircraft and radio instruments etc.

Constantan. It is a nickel-copper alloy having high electrical resistance and low temperature coef cient.

Nickel-Silver or German Silver. It is a nickel, copper and zinc alloy. It contains 25% nickel, 60% copper and the rest zinc. It is very resistant to corrosion. It has tensile strength varying from 170 to 140 MPa.

Nickel-Iron Alloys. A continuous series of alloys are obtained by the addition of varying quantities of nickel into iron. These alloys are very important with regard to the following properties :

(i) Magnetic Properties. By adding increasing quantities of nickel in iron, the magnetic property of the alloy obtained gradually goes on decreasing. When the percentage of nickel reaches 28 to 30% the alloy obtained is practically non-magnetic. Iron generally contains a little carbon in it and thus the alloys obtained by addition of different quantities of nickel are high nickel steels and these possess very high mechanical strength.

Such alloys are commonly used for machines and plants which are put to uctuating magnetic elds in order to reduce the energy losses due to electromagnetic in uence because the interference with the magnetic eld


is reduced. Accordingly such alloys are used for making end plates of alternator rotors, switch gear parts and transmission system parts.

When percentage of nickel in iron is increased beyond 28 to 30%, alloys obtained have got high magnetic permeability and low hysteresis losses. The magnetic permeability goes on increasing by the addition of increasing quantities of nickel. It is maximum when nickel content is 78.5%.

(ii) Dilatation properties. With the addition of nickel into iron in different quantities, the thermal co-ef cient of expansion of the alloy so obtained uctuates very much. The co-ef cient of expansion falls rapidly with the progressive addition of nickel above 25% and when the percentage reaches 35, the co-ef cient of expansion becomes practically zero. With the further addition of nickel, the co-ef cient of expansion again increases rst rapidly and then slowly till the co-ef cient becomes equal to that of pure nickel.

By utilising this property of these alloys, an alloy, having practically zero co-ef cient of expansion has been obtained. This alloy is called Invar and is used in making clock pendulums, measuring tapes and thermostats.

Alloys having special co-ef cients of expansion are made and used for making glass metal joints and also for making bead in wires for electric lamps.

Alloys having a wide range of co-ef cient of expansion are made by adding different quantities of nickel into iron.

Nickel-Chromium Alloys(i) By the addition of chromium to nickel, the

resistance to oxidation of the resulting alloy increases. By adding increasing quantities of chromium to nickel, the property of resistance to oxidation goes on increasing progressively up to 20% of chromium. With such a composition (20% chromium, 80% nickel) an alloy is obtained which resists oxidation upon 1000°C. This resistance to oxidation takes place due to the formation of a protective oxide lm over the alloy.

(ii) By addition of increasing quantities of chromium to nickel the electrical resistance of alloy obtained goes on increasing progressively. An alloy containing 20% chromium and 80% nickel has electrical resistance which is eleven times the electrical resistance of nickel.

(iii) Tensile strength also goes on increasing by the addition of increasing quantities of chromium to nickel. Alloy containing 20% chromium and 30% nickel has tensile strength of 800 MPa. At high temperatures, the tensile strength remains high. Its limiting creep stress is far higher than any other non-ferrous alloy.

Uses of Nickel-Chromium Alloys. Due to the above described properties these alloys are used in electrically heated appliances which have to work continuously above 850°C.

In electrically heated muf e furnaces, these alloys are used in the form of wire or tape for heating elements, as sheet or cast plates for the furnace bottom, as cast grids for protecting the heating elements, as tubes for thermocouple sheaths, and as wire for one of the thermo-

couple elements. Nickel-chromium alloys are also used for making heating elements.

A nickel-chromium alloy called nichrome or chromel is used for heating coils for furnaces on account of its high resistance.

Inconel. It is an alloy having 80% nickel, 14% chromium and 6% iron. The alloy has high mechanical properties coupled with corrosion and heat resisting properties.

It can be made into sheet, strip, rod, wire or in cast form. It can be welded, riveted, brazed, pressed and soldered. It is highly resistant to oxidation even at high temperatures up to 900°C.

The alloy is generally used for the manufacture of machinery for food processing industries, especially milk and milk products.

1.38. ZINC ALLOYSThe main zinc alloys are following :

(i) Aluminium-zinc alloys, (ii) Cadmium-zinc alloys, (iii) Copper-zinc alloys (Brass), (iv) Iron-zinc alloys (galvanising), (v) Magnesium-zinc alloys, (vi) Lead-zinc alloys.

Die Casting Alloys of Zinc. In die casting, steel mould is used for casting. The alloy is heated a little above its melting point. Then it is forced into the die by a steel plunger and is kept there under a pressure of 170 kg/cm2

or more until solidi cation takes place. The die being built in two or more parts can be opened.

Composition of Die Casting Alloys. Aluminium is added to improve strength and ductility of the alloy by reducing grain size. It also reduces the attack of molten metal on dies and other parts of casting machine. Magnesium is added in small quantity as it makes the casting permanently stable.

In these alloys it has been found that corrosion is affected in the following manner:

(i) It is very much accelerated by small quantities of lead and cadmium which are found as impurities in zinc.

(ii) It is accelerated by tin.(iii) It is slightly accelerated by nickel and manganese.(iv) Small percentage of copper reduces it.Zinc alloys for die casting are commonly used because

of the following advantages:(i) Low cost per kg and per casting.(ii) Can be cast to close dimensions and thus further

machining etc., is very much minimised.(iii) Casting is done very easily. Temperature

employed is about 400°C.(iv) Cost of die material and die redressing is low.(v) These have got higher strength than other die

casting alloys except brass.(vi) These have suf cient corrosion resistance and

accordingly the cost of additional protection is very low.Slush Casting Alloys of Zinc. By slush casting

process hollow-ware can be produced.


Zinc Aluminium CopperI 94.5% 5.5% ...II 95.0% 4.75% 0.25%In alloy I, aluminium concentration is not allowed to

go very much lower than 5.5% as zinc-aluminium eutectic occurs at 5% aluminium. These alloys meet following requirements of slush casting alloys:

(i) These have fairly low freezing temperature.(ii) Solidi cation occurs over a range of temperature

rather than at one de nite temperature to produce slushy consistency.

Forming and Shearing Alloys. Composition of one such alloy is given below:

Aluminium ... 3.5 to 5%, Copper ... 4%,Magnesium ... 1%, Zinc ... Rest.These alloys are used for making shearing and

forming dies for sheet metal parts especially for aircraft. As they have got low melting points, dies could be remelted and recast if so required.

1.39. ALLOYS OF TIN1. Solder. It is used for joining two metals. It melts at a lower temperature than the metals to be joined. Soft solder consists of tin and lead in various properties. Three commonly used solders are :

1. Tin ... ...50%Lead ... ...50%

2. Tin ... ...40%Lead ... ...60%

3. Tin ... ...67%Lead ... ...33%

Tin-lead solders are called soft solders. Sometimes cadmium and bismuth are partly substituted for tin to make a solder for wetting copper and brass.

Hard solders used for joining copper, brass etc., contain copper and zinc with little tin.

Other solders. Silver solder consists of mainly silver with a little of copper and zinc. Its use is desirable where lower melting point justi es higher cost. Gold solder consists of gold with a little copper and silver.

Solder should not have more than 0.15% copper and a total of less than 0.10% other impurities (except bismuth) with no aluminium.

2. Bearing metal. It is a tin-antimony-copper alloy and is used for bearings. Bearing should be made from a material which is tough and hard. Tin reduces brittleness and increases compression strength. The composition of babbit metal is given below :Tin ... ... 70 to 90%, Antimony ... ... 7 to 24%,Copper ... ... 2 to 24%.

Babbit metal is a general term used for soft-lead or tin-base metals which are used as cast liners in bronze or steel backing. In general, babbit metals are used in preference to the bronzes for higher speeds and uctuating loads but are less proof against abuse. These have excellent embedability and conformability characteristics.

Embedability is the ability of a metal to embed in itself foreign particles, and conformability is the ability to deform plastically to compensate for irregularities in bearing assembly. The tin-base alloys compared to the lead-base alloys have better resistance to corrosion in acidic oils but have considerably higher cost.

For low-speed operation under high pressures such as bridge bearings and expansion plates, hard bronzes containing upon 20% tin are used. These can be used only with a mating surface of hardened steel and under conditions of proper alignment. For lower loads, the tin content may be decreased to 10% or less. Under conditions where alignment and lubrication are to be poor, lead is often added to bronzes. The alloy of 80% copper, 10% tin and 10% lead is most commonly used for general machinery. Alloy of 88% copper, 10% tin and 2% zinc has good casting properties and is used for housings, etc. cast with integral bearings. Chill casting gives a ner structure and is preferable to sand casting for the best bearing properties. Alloy with 70% copper, 16% tin and rest lead is very useful for higher pressures. In internal combustion engines for higher loads, straight copper lead alloys with 21% lead, and balance copper with small amounts of nickel or tin are used. Because of the softness of the alloy, it is applied at a high temperature in a very thin layer (0.50 mm) of a steel backing strip which is subsequently formed to t the journal or backing. Aluminium bronzes containing 8 to 10% aluminium with 3.6% iron are used for wearing surface for heavy duty. These are extensively used for bushings, guides, gears etc., and do not withstand conditions of poor lubrication.

Other bearing alloys. Bearing alloys form a separate class of alloys. There are several types of bearing alloys:

(i) Tin base (Babbit). These are available in several compositions.

(ii) Lead base bearing alloys. These have an average 4% tin. 9% antimony and balance lead. Hardening compound in both lead base and tin base alloys is Sn. Sb. (tin-antimony) which occurs as cubes distributed throughout the soft matrix.

(iii) Cadmium-nickel bearing metals. It contains 1.25 to 3% cadmium and balance nickel. Compound NiCd7 acts as hardening agent. These retain their bearing properties upto higher temperatures. These do not withstand corrosion resistance in acidic oils.

(iv) Silver bearing metals. It contains 4% lead and shows the lowest coef cient of friction against iron having any combination of nickel.

(v) Copper base heavy duty bearing metals. These are generally mixtures of copper with 10 to 30% lead which gives antifrictional qualities to the alloy. A composition of such alloy is 80% copper, 10% silicon and 10% lead.

As the main component in all types of antifriction bearing alloys is tin (Sn), these are designated with letters Sn followed by the symbol of other elements and percentages thereof in the order of importance. For example, SnSb 10 Cu 5 means an antifriction bearing alloy containing 10% Sb, 5% Cu and rest, i.e. 85% Sn. Sometimes


the percentage of Sn is speci ed, but percentage of lead is not given e.g., Sn 6 Sb 15Pb is an alloy containing 6% Sn, 15% Sb and 79% Pb. I.S. : 25 designates the antifriction bearing alloys according to the percentages of tin (Sn) in them and that percentage is called the grade. For example, it speci es following grades, 90, 84, 75, 69, 60, 20, 10, 6, 5, 1 whose designations are Sn 90 Sb 7 Cu 3, Sn 84 Sb 11 Cu 5, Sn 75 Sb 11 Pb, Sn 69, Zn 30, Pb, Sn 60 Sb 11 Pb, Sn 20 Sb 15 Pb, Sn 10 Sb 14 Pb, Sn 6 Sb 15 Pb, Sn 5 Sb 15 Pb, and Sn 1 Sb 15 Pb, respectively. Grades 90 and 84 are used for lining of petrol and diesel engine bearings, cross heads in steam engines and other bearings used at high speeds. Grade 75 is used for repair jobs in mills and marine installations. Grade 69 is used for under-water applications. Grade 60 is used for lining for bearings required for medium speed applications, such as centrifugal pumps, circular saws, electric motors etc. Grades 20 and 10 are used for low speed bearings, such as pulp crushers, concrete mixers and rope conveyors. Grade 5 is used for mill shaftings, railway carriage and wagon bearings ; and grade 1 is used as a thin line overlay on steel strips.

1.40. IMPROVED BEARING MATERIALSFriction results in wastage of lot of energy. Eighty to ninety per cent of mobile parts go out of order due to wear, that is, due to friction. While friction reduces the technical and economic ef ciency, reliability and longevity of machinery, modern trends in the development of technology are leading to the intensive growth of loads, to an increase in the speeds and temperatures at the places of contact of friction parts.

Wear and friction problems can be solved by different methods. One of them is to some extent paradoxical–instead of ideally smooth surfaces, which are hardly attainable or even unattainable at all, a preset relief with microdepressions and gradients is created. A lot of oil pockets are formed. They hold lubricants. Such a micro relief has a favourable effect on the friction properties of the surfaces of parts which come into contact.

A unique anti-friction material–metal- uoroplastic has been developed in Russia. It consists of a steel base, and a thin (0.3 mm) porous bronze layer whose pores are lled with a mixture of uoroplastic and molybdenum disulphide. Bearings from this material operate without lubrication or with low lubrication within a wide temperature range. They are manufactured through simple stamping. The mechanical working through cutting is brought to a minimum. Hence, material itself is used very economically. Actually this is a waste-free process.

New bearings are 10–15 times lighter and 50 per cent shorter in outer diameter than corresponding metallic bearings. If bearings are lighter and smaller, then their metallic environment (shells and adjacent parts) are also much more moderate in mass and size. It should be added that metal- uoroplastic bearings operate well in the event of overloads and have high radiation resistance.

Metal- uoroplastic has brought some new oppor-tunities into the machine-building industry for reducing the size and weight of machinery.

The service life of friction units can be substantially raised also by another method which is quite new. Here lubricants are supplied into the lubrication zone by a relatively weakmagnetic field. Research has proved that such lubricants operate well in the most extreme conditions–at high temperatures, in vacuum etc.

Magnetopowders create a very thin protective lm from an elastic porous lubricant on the surface of parts. This lm is continuously renewed, and the magnetic

eld supplies the lubrication zone with new portions of magnetopowder. Experiments prove the promising nature of this method. If magneto-powder lubrication is used, gear wheels have a service life 10 to 20 times longer than usual.

Considerable reserves in combating wear are concentrated in wear resistant surface layers. As usual, the layer created here is small-several tenths of a millimetre. But in this case, practically new material with speci c physical and mechanical properties can be produced. Such a combination of different properties in a single part is also new for technology. Surface layers can be formed by plasma spraying in vacuum or by ion-diffusion spraying in a glow discharge. Methods of cathode sputtering in a D.C. discharge and in high-frequency discharge, as well as methods of ion deposition and alloying are applicable.

The use of powerful laser radiations for a directional change in the properties of friction surfaces holds great promise. After laser treatment, the friction coef cient decreases essentially, while the micro-hardness of friction surfaces increases. Wear resistant coatings can be built up and the surface layer can be alloyed (that is improved by micro-additives). Besides, in case of layer hardening, there is no need for nal production operations.

Machines can be improved, using advanced structural materials in particular, composite materials in which a polymer or a metal is reinforced by carbon bres, glass

bres, boron bres or organic bres. In this case one cannot only reduce the mass of a machine, increase its strength and longevity, but also obtain some quite new properties of parts and machines due to the unique properties of composite materials.

The use of different polymeric bases, diverse reinforcing bres and their combinations and varying the number of layers and the orientation of bres give scope for designers imaginative activity.

The increases in the speed of machinery, their sophistication and other qualities give rise to some new problems in the machine-building industry, speci cally, combating vibration of machinery, structures and devices. Vibrations involve great losses. Most of the materials discussed here smooth out and damp down undesirable vibration and materials with reduced vibration activity have been designed.

1.41. NEWER MATERIALSShape-Memory Alloys: A typical shape memory alloy has composition of 55% Ni and 45% Ti. These alloys after being plastically deformed at room temperature into various


shapes, return to their original shapes on heating. These have good ductility, corrosion resistance, and electrical conductivity. These find application in connectors, clamps, fasteners and seals. For space application, they are folded at room temperature to occupy less space and upon reaching their destination they are heated to attain their original shapes.

Amorphous Alloys: These alloys do not have a long-range crystalline structure. They have no grain boundaries, and the atoms are randomly and tightly packed. Because their structure resembles that of glasses, these alloys are called metallic glasses. Such a structure was earlier obtained by extremely rapid solidi cation of the molten alloy. Amorphous alloys typically contain iron, nickel, and chromium alloyed with carbon, phosphorus, boron, aluminium and silicon. They are available in the form of wire, ribbon, strip, and powder.

These alloys exhibit excellent corrosion resistance, good ductility, high strength, and very low loss from magnetic hysteresis. Thus it is ideally suited to make magnetic steel cores for transformers, generators, motors, lamp ballasts, magnetic ampli ers, etc.

Polymers: These constitute an important class of materials because of wide ranging mechanical, physical and chemical properties. These have lower density, strength, elastic modulus compared to metals. Plastics are composed of polymer molecules and various additives. Monomers are linked by polymerisation processes to form larger molecules. Polymer structures can be modi ed by various means to impart a wide range of properties to plastics.

Elastomers comprise a large family of amorphous polymers having a low glass transition temperature. They have the characteristic capability to undergo a large elastic deformations without rupture. Synthetic rubbers having wide ranging applications have been developed. Silicones have the highest useful temperature range (upto 315°C).

Ceramics: These are compounds of metallic and non metallic materials. These are characterised by high hardness and compressive strength, high temperature resistance and chemical inertness.

Composite Materials: These have superior mechanical properties and yet are light weight. The reinforcing

bres are usually glass, graphite, boron, etc. Epoxies and polyester commonly serve as a matrix material. Reinforced plastics are being developed rapidly. New developments concern metal-matrix and ceramic-matrix composites and honey comb structures. (Honey comb structure consists of a core of honey comb or other corrugated shapes bonded to two thin outer skins. Ceramic-matrix cutting tools are being developed, made of silicon carbide-reinforced alumina, with greatly improved tool life.

A composite material, as stated above, contains more than one component. The compound materials are incorporated into the composite to take advantage of their attributes, thus obtaining improved material. They become cohesive structures made by physically combining

two or more compatible materials. Fibre reinforced composites are heterogeneous materials prepared by associating and bonding in a single structure of materials possessing different properties. Due to complementary nature, the composite material possesses additional and superior properties. These thus become ideal materials for structural applications requiring high strength-to-weight and stiffness-to-weight ratios. Fibre reinforced materials exhibit anisotropic properties. Glass bres are strong but if notched they fracture readily. By encapsulating them in a polyester resin matrix, they can be protected from damage. Fibres of graphite and boron are also used in composites. Commonly used bres for composite materials are–glass, silica and boron for amorphous structure, ceramic and metallic for single crystals as well as polycrystals, carbon and boron (amorphous) materials for multiphase structure, and organic material for macromolecular structure.

For two-dimensional structural applications such as in plates, walls, shells, cylinders, pipes etc. a planar reinforcement is much more advantageous as compared to the linear reinforcement.

Duplex Composite Components: Components subjected to severe wear and high contact stresses can be made of duplex composite, the composite layer being located on outer or inner surface depending on the requirement. Aluminium composite alloys reinforced by ceramic have been developed and these have relative high strength to weight ratio, high modulus of elasticity and good wear characteristics.

Silicon carbide particles are incorporated into the surface of aluminium alloy heated to its mushy state and pressure is applied to get a good wetting between the aluminium alloy and the silicon carbide particles.

Experiments can be carried out to determine the semi-solid forming conditions. Specimen surrounded by SiC particles is heated upto this temperature for about 45 minutes in order to homogenise the temperature through the specimen. A hydraulic press is used to apply the necessary low pressure for the semi-solid forming process. There is an optimum combination of temperature and pressure values to obtain optimum mechanical properties. In this way, a composite layer of about 2.5 mm width can be formed with uniformly distributed particles having good bond with aluminium matrix, with no separation or porosity at the composite layer/matrix interface.

Surface composite layer has hardness and wear resistance about 1.75 and 10 times those of as received aluminium matrix alloy.

1.42 SUPER MATERIALS(i) Aerogel: Aerogel is known as ‘frozen smoke’

because of its look of solid smoke. Aerogel is actually 99.8% empty space, and it has semitransparent look. This material is a terri c insulator. Thus a shield made from aerogel will take all the heat from a ame surrounding it. Aerogel’s internal structure is so complex that


just 2.5 cm of it could have a surface area as long as a football eld.

(ii) Carbon Nanotubes: These superstructures are bonded together by the greatest power of sp2

bond. The carbon nanotubes are constructed with a length-to-diameter ratio of up to 132,000,000:1- signi cantly larger than for any other material.They are exceptionally strong. Carbon nanotubes are 300 times stronger than steel. 3D chip using carbon nanotubes can store and process massive amounts of data, paving the way for smaller, faster and more energy-ef cient devices. Nanotubes are sheets of 2D graphene formed into nanocylinders, and resistive random-access memory (RRAM) cells. Integration of over one million RRAM cells and two million carbon nanotube eld-effect transistors has resulted in most complex nanoelectronic system ever made.

(iii) Metamaterials: Metamaterials are artificial materials engineered to have properties that may not be found in nature. They are assemblies of multiple individual elements fashioned from conventional microscopic materials, such as metals or plastics, but the materials are usually arranged in periodic patterns. Metamaterials don’t gain their properties from their composition but from their exactly-designed structures. Their precise shape, geometry, size, orientation and arrangement can affect waves of light or sound in an unconventional manner, creating material properties that cannot be achieved using conventional materials.The metamaterials achieve the desired effects by incorporating structural elements of sub-wavelength sizes, i.e., features that are actually smaller than the wavelength of the waves they affect. The primary research into metamaterials investigates materials with a negative refractive index. Negative refractive index materials appear to permit the creation of superlenses, which can have spatial resolution below that of the wavelength.

(iv) Nanodimond: Nanodiamond is produced by compression of graphite. It is much harder than bulk diamond, which makes it the hardest known material, while also being incredibly strong, light and made of the most common element (carbon). It is an amazing heat conductor and has the highest melting point of all materials. Machines built with this material would be lighter, stronger and more powerful.

(v) Amorphous metal: By cooling molten metal quickly before it has time to re-align its particles in a solid shape, we can create amorphous metals that have a disorganized atomic structure. Due to this structure anomaly, they are twice as strong as steel. In addition to their strength, amorphous metals have improved

electricity conductivity.(vi) Transparent alumina: Transparent alumina

is three times as strong as steel while being transparent. There can be so many applications for this material. Buildings could be made using this material and will be transparent and stronger.

(vii) E-textiles: Clothes in the future will be embedded with the tiny E-Textiles that can monitor our health, project videos, make phone calls and bring up information from the internet when we need it.

SOLVED QUESTIONS

Q. 1.1. What is the difference between body centred and face centred lattice structure?

Ans. Body centred cubic lattice structure consists of an atom at each corner and one in the centre of the cube. Thus, each atom has eight other atoms situated next to it. Metallic iron at ordinary room temperature has this structure. Chromium, tungsten and molybdenum also have same structure. (F.C.C.) face-centred cubic lattice, in addition to having an atom at each corner possesses an atom in the centre of each face of the cube also. Each atom is thus surrounded by thirteen atoms. Iron at 910°C has F.C.C. structure. The atoms in fcc structure are more closely packed together.

Fig. 1.19. Body-centred Fig. 1.20. Face-centredCubic Lattice Cubic Lattice

Other metals possessing a similar structure include aluminium, copper nickel, silver and gold. Metals possessing the f.c.c. structure are ductile and excellent conductors of heat and electricity.

Q. 1.2. What do you understand by allotropy of iron?

Ans. Pure iron possesses a remarkable property of allotropy. Some substances exist in two or more crystralline forms: charcoal, graphite and diamonds are all polymorphic forms of carbon. Tin when cooled to low temperature disintegrates from a lustrous metal to a grey powder. Substances which can exist in more than one form are said to be allotropic.

Allotropy is not common amongst metals and the allotropy exhibited by iron confers on it some of its most useful properties. At ordinary temperature and upto 910°C


iron is stable and exists in the body-centred cubic form and is known as alpha iron. From 770°–910°C, iron loses its magnetism and in this condition is referred to as beta (β) iron. From 910°C–1,400°C the metal changes to the face centred cubic structure and is known as gamma iron, and from 1400°C to its melting point it again changes from the f.c.c. gamma iron to b.c.c. delta iron. Thus, the forms alpha and delta have the same space lattice (b.c.c.) while gamma iron has the f.c.c. structure. Hence iron can be considered to posses two allotropic modi cations.

Q. 1.3. What is ‘packing factor’? Find its value for (a) F.C.C., and (b) B.C.C. structures.

Ans. Packing factor is the ratio of atomic volume of atoms in an unit cell to the volume of that unit cell.

aa

a

a

Fig. 1.21 (a) Fig. 1.21 (b)(a) In F.C.C. structure Fig. 1.21 (a), atoms in the face

diagonals touch one another. In the gure they are shown as seperate for the sake of clarity. Thus

4r = a 2 ,where, a = length of unit cell, r = radius of the atom.

Now for F.C.C. structure, Number of atoms in the unit cell = 4.

∴ Volume of atoms in the unit cell

= 4 43

4 43

24

32

× = × × ×

π πr a

= 163

2 216 4 2

33

3π π× ××

=a a

Volume of unit cell = a3,

and Packing factor = π πa a

33

3 2 3 20 74= = . .

(b) In B.C.C. structure [Fig. 1.21 (b)], atoms along the body diagonal touch one another. Therefore,

4r = 3 .For B.C.C. structures, Number of atoms in a unit

cell = 2.∴ Volume of atoms in this unit cell

= 2 43

2 43

34

33

× = × ×

π πr a

= π 3

83a

Volume of unit cell = a3

∴ Packing factor = π 3

80 68

33a a = . .

Q. 1.4. Mention the crystal structures for the following elements :

Iron ; Chromium ; Tungsten ; Nickel and Zinc.Ans. The crystal structure ofIron is bcc (body centered cubic)Chromium is bcc (body centered cubic)Tungsten is bcc (body centered cubic)Nickel is fcc (face-centered cubic)Zinc is cph (close packed hexagonal).Q. 1.5. Discuss the factors, which govern the

selection of a material for a machine component.Ans. Refer Art 1.9.Q. 1.6. What are the principal uses of (i) low

carbon, (ii) Medium carbon and (iii) high carbon steels? Mention also the amount of carbon present in these steels.

Ans. Carbon content in low carbon steel is less than 0.25%, medium carbon steels 0.3 to 0.6% and high carbon steels 0.6 to 1.5%.

For their uses, refer Table under Art. 1.14.Q. 1.7. Sketch and discuss the defects in a lattice

structure of a crystalline material.Ans. Defects in a lattice structure of a crystalline

material are:(i) Point Defect. It occurs when an imperfection

is restricted to the neighbourhood of a lattice point. Fig. 1.22 illustrates the three different types of point defects. In Fig. 1.22 (a) one lattice atom is missing, creating a vacancy. Since an atom vibrates about its lattice position, the tendency of the atom to jump out of its regular position creating a vacancy increases rapidly with temperature. It is possible to increase the vacancy density at a given temperature by rapid cooling or extensive plastic deformation. An atom may occupy an abnormal position as in Fig. 1.22 (b) (Interstitial impurity atom). An interstitial impurity can be caused when an atom possesses large enough thermal energy or when its energy is increased by nuclear bombardment. It is possible that a regular lattice position may be occupied by an atom of a different material (substitutional impurity).

(a) Vacancy (b) Interstitial impurity

(c) Substitutional impurityFig. 1.22. Point defects.


(ii) Line Defect. It occurs when an imperfection extending along a line has a length much larger than the lattice spacing also called dislocation. Two common, simple types of dislocations are shown in Fig. 1.23. Edge dislocation occurs when an extra half-plane of atoms is accommodated by distorting the regular lattice arrangement (as done with the AB half-plane in Fig. 1.23 (a), screw dislocation occurs due to movement of the lattice atoms from their regular ideal positions, Fig. 1.23 (b). The line separating the deformed and the undeformed regions is normally called the dislocation line [line AB in Figs. 1.23 (a) and (b)]. The dislocation density is de ned as the total length of all the dislocation lines per unit volume. Plastic deformation takes place mainly through a movement of dislocations.

D

A

B

C

(a) Edge dislocation

A

B

(b) Screw dislocationFig. 1.23. Edge and screw dislocations.

(iii) Surface Defect. It occurs when an imperfection extends over a surface. Fig. 1.24 below shows a common type of surface defect known as twins, produced due to stressing of metal at low temperature.

Displaced atoms

Twin planes

Fig. 1.24. Twinning mechanism.

Q. 1.8. What is the effect of addition of silicon to cast iron?Ans. It promotes graphite ake formation.Q. 1.9. When a metal/alloy is cold worked, which

properties increase?Ans. Hardness and strength.Q. 1.10. At which temperature Eutectoid reaction occurs?Ans. At 723°C.Q. 1.11. Which phenomenon is responsible for strain

hardening effect when metals are subjected to cold working?Ans. Dislocation mechanism.

Q. 1.12. What is the value of notch sensitivity of cast iron?Ans. Zero.

Q. 1.13. (a) What do you understand by steel?(b) What is the difference between hypo-

eutectoid and hyper-eutectoid steels?(c) How the plain carbon steels are classi ed

depending upon carbon content?Ans. (a) Steel is a malleable alloy of iron and carbon,

usually containing substantial quantities of manganese.(b) Hypo-eutectoid steels are those which contain less

than approximately 0.9% carbon whereas hyper-eutectoid steels contain more than 0.9% carbon.

(c) The plain carbon steels are broadly classi ed as hypo-eutectoid and hyper-eutectoid steels depending upon whether carbon content is less than or more than 0.9% (approximately).

The hypo-eutectoid steels (containing carbon between 0.05 to 0.9%) are classi ed as

(i) low carbon steels (0.05 to 0.3% carbon).(ii) medium carbon steels (0.3 to 0.6% carbon).(iii) high carbon steels (0.6 to 0.9% carbon).Hyper-eutectoid steels are often referred to as cast

steels, tool steels or special high carbon steels.Q. 1.14. (a) Distinguish between plain carbon

steel and alloy steel.(b) How the mechanical properties of an

untreated plain carbon steel are in uenced by increase in the carbon content?

Ans. (a) Plain carbon steel is one which owes its distinctive properties chie y to the carbon it contains.

An alloy steel is one which owes its distinctive properties chie y to some element or elements other than carbon, or jointly to such other elements and carbon. The other elements are deliberately introduced in alloy steel to confer special properties.

(b) The tensile strength of untreated plain carbon steel increases roughly linearly from about 350 MPa at 0.05% carbon to about 900 MPa at 0.9% carbon. The hardness value increases roughly linearly from about HB 100 at 0.05% carbon to about HB 200 at 0.9% carbon. Ductility (measured by an elongation value) decreases with increase in the percentage of carbon. Malleability (measured by reduction in area) decreases with increase in percentage of carbon.


Q. 1.15. (a) What do you understand by killed steel?

(b) What is ‘piping’ in steel and how it can be minimised?

Ans. (a) Steels in which no gas evolution occurs on solidi cation are called killed steels.

(b) When the steel cools in the mould, shrinkage of the steel on solidifying causes ‘piping’ usually in the upper portion of the ingot. This can be minimised by using a large-end-up mould with a refractory ‘hot top’ which supplies molten steel to the main body of the ingot while solidi cation proceeds.

Q. 1.16. Describe the following alloy steels :(a) Case-hardening steels(b) Nitriding steels(c) High-tensile steels(d) Wear-resisting steels(e) Corrosion-resisting steels(f) Heat-resisting steels.Ans. (a) Case hardening steels–These usually contain

less than 0.15% carbon. As carbon content is low, the core does not respond to hardening. By adding about 3% nickel and 0.45% manganese, these do not require further re ning after carburising. Such a steel can be water-quenched and is used for gudgeon pins, camshafts, etc. If percentage of nickel is increased to 5% then oil-quenching can be used for hardening. Such a steel is used for gearbox parts, etc.

(b) Nitriding steels–These contain carbon from 0.4 to 0.5%. Accordingly these respond to direct hardening and a high strength case is developed after hardening and tempering. It may contain chromium between 1.6 to 3%. It is used when a high-strength core and a high hardness case are required. With 3% chromium, surface hardness of about 850 HV can be developed and when 1.1% aluminium is also added, then surface hardness of the order of 1100 HV can be developed.

(c) High-tensile steels–These contain carbon of the order of 0.3 to 0.4%. Addition of 3% nickel and 0.6% manganese makes it strong and tough as nickel produces a ne grain structure. It can then be oil-quenched and is used for crank-shafts, connecting rods etc. A further addition of 0.8% chromium makes steel suitable for highly stressed parts. However it suffers from temper brittleness which can be prevented by specifying the tempering temperature. (It should not be tempered at particular temperature which depends upon composition). Another method to overcome temper brittleness is to add 0.25% molybdenum. With 4.25% nickel and 1.25% chromium, steel becomes suitable for air-hardening and can be used for complex shapes, but it also suffers from temper brittleness. A further addition of 0.45% vanadium and 1% silicon (1.25% molybdenum to remove temper brittleness) makes it suitable for forging dies, die-casting dies and extrusion-press dies.

(d) Wear-resisting steels–These contain carbon from 1 to 1.2%. Addition of 12.5% manganese makes austenite to be retained as a result of heating and quenching. This

is known as ‘Had elds’ austenitic steel which responds to work hardening and becomes harder as a result of rough treatment. Accordingly it is used for track work, dredging equipment and crushing machinery. Steels with 1.4% chromium and 0.45% manganese can be oil-quenched and are very hard and used for roller and ball races.

(e) Corrosion resisting steels–These are classi ed as ferritic stainless steels, martensitic stainless steels, and austenitic stainless steels. Ferritic stainless steels contain low percentage of carbon (0.04%) and as such these can’t be hardened by heat-treatment and can be hardened only by working. To make them corrosion resistant, 14% chromium and 0.45% manganese are added. These are used for spoons, forks etc. Martensitic stainless steels contain about 0.3% carbon, 13% chromium and 0.5% manganese. Due to high carbon, these can be hardened by heat treatment and are used for cutlery and similarly edged tools. Austenitic stainless steels contain 0.1% carbon, 18% chromium and 8% nickel. They can’t be hardened by heat treatment and respond only to work hardening. They are non-magnetic.

(f) Heat resisting steels–These contain carbon varying between 0.15% and 0.3%. To make them corrosion resistant, 12 to 16% chromium is added. While chromium also improves the strength, other elements like carbon, tungsten, molybdenum strengthen the alloy. These also resist oxidation and attack by vapour and gases at high temperature. They also retain their strength at high temperatures and resist creep. Steel used for turbine blades and discs for use upto 600°C contains 0.15%C, 1% Mn, 12% Cr and 1% Ni. Some of the heat resisting steels respond to heat treatment and some can be hardened only by working.

Q. 1.17. What are the differences between hot-rolled steels (HRS) and cold rolled steels (CRS)?

Ans. Hot-rolled steels contain an oxide scale over their surface. It is formed during the heating process. Accordingly dimensional tolerances are not tight. In cold-rolled steels, the scale is removed before cold rolling process and thus the surface nish is very good in this case. Thus its dimensions are held within closer tolerances. Low-carbon cold rolled steels are easier to machine.

Q. 1.18. Arrange the various types of cast irons in decreasing order of their machinability?

Ans. The decreasing order for machinability of various cast irons is : malleable, gray iron ( ake graphite), ductile iron, grey iron (pearlitic).

Q. 1.19. Malleable iron is considered one of the most readily machined ferrous material. Give reasons.

Ans. It has uniform structure and the nodular form of the tempered carbon.

Q. 1.20. What do you understand by vacuum casting and when it is used?

Ans. It has been observed that hydrogen (even as low as 5 ppm) causes internal akes in large steel sections, and by pouring the liquid steel in vacuum chamber, it is possible to reduce hydrogen content to 1 ppm. Such


vacuum castings are used extensively for large forgings such as electrical rotors.

Sometimes to reduce oxygen content to 0.001%, carbon deoxidation is carried out by removal of oxygen as carbon monoxide. This treatment substantially decreases the number of non-metallic inclusions in the steel and is used for bearings and other high-quality steels.

Q. 1.21. Write a brief note about weldability of various types of cast irons.

Ans. Grey cast iron can be welded with oxyacetylene gas or with electric arc with special precautions (preheating in case of gas welding and post heating in arc welding) to avoid cracking in welding area. Welding of gray iron is usually restricted for repair work. Braze welding with bronze or nickel copper, is however, used for fabrication also.

Malleable irons are not considered weldable. It is repaired by brazing, preferably silver brazing.

Ductile cast iron needs special considerations in welding. It can be welded by a carbon arc process and other fusion processes. For best results ux-cored electrodes having 60% nickel and 40% iron are used.

Q. 1.22. (a) What is the difference between sheet and strip?

(b) What are the characteristics of cold rolled sheets?

(c) What do you understand, by (i) orange-peel effect, (ii) stretcher strains or Luders’ lines?

(d) De ne (i) temper rolling, and (ii) aging in steel.

Ans. (a) The difference between sheet and strip is based on width and is arbitrary.

(b) Cold working produces a better surface nish, improves the mechanical properties, and permits the rolling of thinner gauge material than hot rolling.

(c) (i) Sheets for deep-drawing applications must be dead soft to have maximum amount of plasticity. These must also have a relatively ne grain size, because a large grain size causes a rough nish, an “orangepeel” effect, on the deep drawn components.

(ii) Usually low-carbon steel has sharp yield point characteristic which results in sudden local elongation in sheet during forming, which results in strain markings called stretcher strains of Luders’ lines. This characteristic must be eliminated.

(d) (i) The sharp yield point characteristic of low carbon steel can be eliminated by cold rolling (resulting in 1% reduction of thickness), known as temper rolling, followed by alternate bending and reverse bending in a roller leveller.

(ii) An important phenomenon of the temper-rolled low-carbon sheets is the return of the sharp yield point after a period of time, known as aging in steel.

Q. 1.23. What are the important characteristics of various tool steels and what is the criterion for their selection?

Ans. Since plain carbon steels are cheapest and easiest to fabricate ; rst attempt should be to select the

material from this category. The next choice should be for oil-or air-hardening steels which have better wear resistance than plain carbon steel but toughness is not good. These offer maximum safety in hardening and minimum dimensional change after heat treatment. These are preferred for dies with adjacent thin and thick sections, sharp corners, or numerous holes. These are, however, not suited for elevated temperature use. For red hardness, and wear plus shock resistance, hot-work steel must be used. For withstanding cold battering conditions the shock-resisting steels are preferred. For red hardness and high abrasion resistance with some shock resistance, high speed steel is the choice.

Q. 1.24. What do you understand by music wire? What for it is used?

Ans. Music wire is a previously treated steel wire used for small helical springs. It requires no heat-treatment except a low-temperature anneal to relieve the forming strains. Such a wire is given a special heat treatment called patenting and then cold-rolled to develop a high yield strength.

Q. 1.25. What is the composition of following alloys? What are their special characteristics and what for these are used?

(i) Permalloy, (ii) Invar, (iii) Elinvar, (iv) Platinite,(v) Dumet wire.

Ans. (i) Permalloy has 78.5% nickel. It has a high permeability and low hysteresis loss.

(ii) Invar contains 36% nickel and has an exceedingly low co-ef cient of linear expansion. Since its expansion is proportional to the temperature within limits of atmospheric temperature change, it is used for secondary standards of length.

(iii) Elinvar contains 32% nickel with small percentage of Cr, W, Mn, Si and C. In addition to a low coef cient of expansion, it also has a constant modulus of elasticity over the temperature range of 0 to 38°C and is thus used for hair springs for watches and springs for other precision instruments.

(iv) Platinite contains 46% nickel. It has the same thermal co-ef cient of expansion as platinum and thus is used as a substitute for platinum.

(v) Dumet wire contains 42% nickel. It is covered with copper to prevent degassing at the seal and is used to replace platinum as the “seal in” wire in incandescent lamps and vacuum tubes.

Q. 1.26. What do you understand by a free-cutting steel? What elements are usually added to make a steel free cutting, and how they make the steel free- cutting?

Ans. A free-cutting steel is one in which an element like lead or sulphur is deliberately added to promote rapid machining.

When sulphur is added to steel, it forms a brittle constituent with manganese, known as manganese sulphide, which being brittle, allows chip cracks to propagate and breaks chips into easily handled lengths.


The lead does not chemically combine with the other elements. It gets distributed throughout the mass as minute droplets, and thus chip cracks are propagated easily and the chips come out in easily handled lengths.

Q. 1.27. (a) Which two elements are alloyed to form a brass?

(b) What are the differences (i) in composition,(ii) in uses, of an ‘alpha’ and an ‘alpha-beta’ brass?

Ans. (a) A brass is essentially an alloy having major constituents as copper and zinc. Minor proportions of elements such as lead and tin may be included to promote special properties.

(b) An alpha brass is an alloy in which zinc does not exceed by 38%. This brass is ductile and, therefore, used for cold-rolled sheets, wire, tube and cold pressing. Cartridge brass contains 70% copper, 30% zinc and is a very suitable material for deep-drawing cartridge cases.

An alpha-beta brass is an alloy of copper and zinc containing zinc between about 38% and 47%. The appearance of the beta constituent is associated with increased strength at the expense of ductility. An alpha-beta brass does not lend itself to cold-working, but is readily hot-worked by rolling, extrusion and hot-pressing. It is readily machined, the machinability being even more improved by the addition of lead. A typical alpha-beta brass is Muntz metal (60% copper, 40% zinc), which is used for the production of low pressure water ttings by hot-pressing.

Q. 1.28. (a) What is the main difference between a brass and a bronze?

(b) State the composition, and two common uses of a bronze commonly used in engineering.

(c) What is the general effect of adding a small proportion of (i) phosphorus, (ii) lead, to a bronze?

Ans. (a) A brass is an alloy of copper and zinc, the major constituent being copper.

A bronze is an alloy of copper and tin, the major constituent being copper.

(b) A bronze commonly used in engineering and commonly known as ‘eighty- ves’, ‘three ves’ consists of 85% of copper, with 5% each of tin, zinc and lead. Because of its improved machining qualities, it has replaced the traditionally used admiralty gun metal.

It is used for low pressure pipe ttings and small pump castings where reasonable corrosion resistance is desired.

(c) (i) If phosphorus is added in very small quantities, often only a trace, improves uidity of casting. If the phosphorus ranges from about 0.05 to 0.25% as in a phosphor-bronze, it forms cuboids which resist wear and can carry heavy loads. It thus becomes an excellent bearing material.

(ii) Lead is distributed throughout a bronze as globules. It allows chip cracks to propagate easily and hence the addition of lead improves machining qualities.

Q. 1.29. State, with reasons, suitable material for the manufacture of each of the following, giving the approximate composition :

(a) a brass for deep pressed containers ; (b) a brass for small machined bolts ; (c) a bronze for the impeller of a sea-water pump.

Ans. (a) The suitable brass for deep pressed containers is the admiralty brass containing copper and zinc in the proportion of 7 to 3, with 1% of tin.

The addition of tin to the basic cartridge brass (70% copper, 30% zinc), gives improved resistance to corrosion, renders the material less prone to season cracking, and could possibly lessen the number of interstage annealings. This material is very ductile.

(b) The brass used for small machined bolts is free cutting brass with composition : 61.5% copper, 35.5% zinc, and 3% lead.

This material is basically a leaded Muntz metal, the addition of lead imparts the highest machinability to the brass.

(c) The suitable bronze for the impeller of sea water pump is admiralty gun metal having composition of 85% copper, 13% tin, and 2% zinc.

It produces sound casting and has excellent resistance to salt-water corrosion. Machinability could be improved by addition of lead, but this has to be balanced against a lower resistance to corrosion.

Q. 1.30. What do you understand by season cracking in brasses and how it can be prevented?

Ans. Season cracking is spontaneous cracking which occurs on exposure to atmospheric corrosion in brass objects with high residual tensile stresses at the surface. It occurs with high-zinc brasses but rarely with 15 per cent zinc or below. Alloys susceptible to spontaneous season cracking will crack when exposed to corrosive conditions under high service stress, even if they are free from internal strains. Season cracking can be prevented by avoiding the production of internal microstresses or by removing such stresses by relief annealing at 245 to 275°C without softening the work.

Alloys containing more than 80% copper are highly resistant to season cracking.

Q. 1.31. Discuss the effect of temperature on zinc and zinc alloys.

Ans. Temperature has pronounced effect on the properties of zinc and zinc alloys. The creep resistance, in particular, decreases rapidly with increasing temperature. Ductility and general fabricating characteristics increase with temperature. Drawing and forming operation should never be attempted below 20°C. More severe operations can be performed readily at temperatures above 50°C. Zinc alloys become somewhat brittle below 0°C, depending on the particular composition, but recover their normal properties on reaching room temperature again.

Q. 1.32. What are Hastelloys?Ans. Hastelloys are nickel molybdenum alloys, very

much resistant to corrosion. These alloys are called Hastelloys A, C, D according to their composition.

(i) Hastelloy A. It is nickel-moybdenum iron alloy. It has high strength and ductility. Its strength does not


decrease even at high temperature and it can withstand a load of 100 MPa at 900°C with a creep of one per cent per year.

It is easily forged and rolled into sheet. It can be machined and can form good castings. It can be welded by oxyacetylene or electric arc.

Hastelloy A can resist the attack of even concentrated hydrochloric acid at boiling point. It is resistant to concentrated sulphuric acid upto 70°C. It resists the attack of acetic, and formic acids also but not of nitric acid. Alkalies do not attack it. Salt solutions also have no effect on it.

(ii) Hastelloy C. It is nickel-molybdenum-chromium-iron alloy. Its malleability is less than that of Hastelloy A. It cannot be worked hot or cold. It is good for casting but cannot be machined at high speed. Hastelloy C is resistant to strong oxidising agents such as nitric acid, free chlorine and acid solutions of salt such as cupric and ferric. In this respect it is an improvement over Hastelloy A.

(iii) Hastelloy D. It is cast alloy consisting of nickel, silicon, copper and aluminium. This alloy is strong and tough and has high transverse strength with good de ection. It is good for casting but cannot be worked either hot or cold. Its machinability is poor. It can be welded with oxyacetylene or electric arc but addition of a

ux makes the operation easy. It is resistant to all acids excepting nitric acid.

Q. 1.33. Match parts A and B in connection with the structures of steel.

A B

1. Pure iron that has a body-centred cubic lattice structure.

(a) ferrite

2. Pure iron which has a face-centred-cubic lattice structure.

(b) cementite

3. α-iron with a small amount of carbon in solution, ferromagnetic.

(c) hyper-eutectoid steels

4. γ-iron with carbon in solution ; non-ferromagnetic.

(d) hypo-eutectoid steels

5. A compound of iron and Fe3C. (e) martensite

6. Eutectoid structure composed of alternate layers of ferrite and cementite.

(f) α-iron

7. Steels having a carbon content less than 0.8%

(g) austenite

8. Steels with a carbon content above 0.8%

(h) pearlite

9. A hardened structure produced by quenching a steel from above the A3 point.

(i) γ-iron

Ans. 1. (f) 2. (i) 3. (a) 4. (g) 5. (b) 6. (h) 7. (d) 8. (c)9. (e).

Q. 1.34. Match parts A and B relating to effect of various alloying elements on steel.

A B

1. Imparts hardness and wear resistance. The red hardness is signi cantly improved and strength imparted at high temperature.

(a) Silicon

2. Contributes to red hardness, sustains hardness during tempering and increases hardenability.

(b) Manganese

3. Increases toughness and impact strength and also improves corrosion resistance.

(c) Molybdenum

4. Acts as deoxidiser and impro-ves magnetic properties when present in large percen-tage.

(d) Titanium

5. Produces fine grain size, increases hardenability and imparts hardness and wear resistance.

(e) Boron

6. Increases austenitic harden-ability, reduces marstensitic hardness in chromium steels, and increases strength while retaining ductility.

(f) Cobalt

7. Increase hardenability, and reduce ductility and welda-bility.

(g) Phosphorus

8. Promotes deoxidisation and nitriding, and restricts grain growth.

(h) Copper

9. Increases hardenability and strength in low carbon and low alloy steels and improves machinability and corrosion resistance.

(i) Nickel

10. Increases hardenability signi-ficantly, increases strength, toughness, red hardness and hot strength when used with Cr, Mn and V. Also enhances corrosion and abrasion resis-tance.

(j) Tungsten

11. Increases corrosion resistance and counteracts brittleness from sulphur.

(k) Aluminium

12. Increases hardenability, and impairs impact strength sligh-tly.

(l) Vanadium

Ans. 1. (j) 2. (f) 3. (i) 4. (a) 5. (l) 6. (d) 7. (b) 8. (k) 9.(g) 10. (c) 11. (h) 12. (e).


Q. 1.35. Match parts A and B is relation to Engineering Materials

A De nition B Word1. The occurrence of

two or more crystal structures at the temperature same chemical composition.

(a) Glass transition

2. Materials exhibiting ionic or covalent bonds or both.

(b) Solid solution

3. The temperature at which the rate of contraction of a cooling non-crystalline material changes to a lower value.

(c) Modulus of rupture

4. Characterised by the lack of a regular arrangement or order in a collection of atoms in space.

(d) Peritectoid

5. A solution formed when the addition of one or more new elements still results in a single-phase structure.

(e) Ceramic materials

6. The relationship among time, strain and temperature that can lead to permanent deformation in materials such as polymer.

(f) Plastic deformation

7. A measure of the tensile stress required to cause failure in the bending of a beam of a brittle material like ceramic.

(g) Cleavage

8. The permanent displacement of atoms from a given starting position, as by slip or twinning.

(h) Visco-elasticity

9. Fracture along speci c crystallographic planes, as characterised by failure in ceramic materials.

(i) Allotropy

10. Transforming from two solid phases to a third solid phase upon cooling.

(j) Non-crystalline

Ans. 1. (i), 2. (e), 3. (a), 4. (j), 5. (b), 6. (h), 7. (c), 8.(f), 9. (g), 10. (d).

Q. 1.36. (a) Describe the steps usually considered in the process of material selection.

(b) How material selection is done using decision theory?

Ans. (a) Like any other aspect of engineering design, material selection is also equally important aspect. Material of part depends on the requirements of the part, number of parts needed, and manufacturing process to be adopted to make the part. The material selection primarily depends upon its properties and several other factors like availability, economic aspects, etc.

As regards properties of materials, all the following properties need to be considered, viz.

(i) chemical (characteristics that relate to the structure of a material and its formation from other elements),

(ii) physical (characteristics that are determined by nature, like electrical, magnetic, thermal, etc.),

(iii) mechanical (strength, ductility, toughness, rigidity and behaviour under application of force).

(iv) dimensional (relating to available size, shape, microtopography and tolerances),

(v) technological (machinability, weldability, castability, forgability, bendability, malleability, ductility, hardenability, etc. all relating to manufacturing process).

It is essential to establish the performance requirements that the material must meet ; then select the appropriate materials for evaluation, and nally select the material that best meets the performance requirements and economic constraints.

It is best to consider the alternatives from a wide gambut of materials, viz. ferrous materials (cast iron, cast steel, mild steel, alloy steel, carbon steel, tool steel etc.), non-ferrous materials and their alloys, plastics, ceramics, elastomers, composite materials, etc.

Having selected the material from consideration of properties, next step is to check availability, i.e. whether material is readily available, lead time for procurement, quantity required, nature of supplies, whether any special method required to manufacture.

The nal step is to examine economics, i.e. the cost should be minimum. Cost of a material consists of the initial cost, processing cost and maintenance cost.

(b) The problem of material selection by using ‘Decision theory’, can be solved as under. Suppose we have materials m1, m2 and m3 and these are subjected to environmental situations denoted by θ1, θ2, θ3 and θ4. For example m1 could be steel, m2 aluminium and m3 bre reinforced plastic (FRP). The environmental factors, θ1represent salt water, θ2 represent acidic environment and θ3 represent shock loading etc. First of all, a loss table is drawn up as shown below.

MaterialNature

θ1 θ2 θ3 θ4

m1 1 4 10 12m2 3 2 4 7m3 5 4 3 2


Probability of each state of nature is determined by past experience, say θ1 = .1, θ2 = .3, θ3 = .4, θ4 = .2.

Expected loss functions as a consequence of selecting materials m1, m2 and m3 are :

E (a1) = .1 (1) + .3 (4) + .4 (10) + .2 (12) = 7.7E (a2) = .1 (3) + .3 (2) + .4 (4) + .2 (7) = 3.9E (a3) = .1 (5) + .3 (4) + .4 (3) + .2 (2) = 3.3Thus the material a3 i.e., FRP results in minimum

loss and should be selected by the designer.Q. 1.37. What are identi cation tests for metals?Ans. Identi cation tests like appearance, sound,

spark, weight, magnetic, bend and ling enable to gain a useful insight into many of the properties of the metals.

Appearance Test. Appearance comprises of the lustre, colour and any sorts of lines and marks appearing on a metal to be identi ed in its normal condition.

Sound Test. A ringing sound is a speci c characteristic of metals which is absenting in non-metals. Different metals produce different types of sound when struck by a hammer or dropped on a hard oor.

Spark Test. If a piece of particular metal or alloy is held gently against a running grinding wheel, a speci ed pattern of sparking is produced which helps in its identi cation.

Weight Test. Different metals have different weight per unit volume (density).

Magnetic Test. This type of test can be used for differentiating magnetic metals such as iron, nickel, cobalt and their alloys from other metals and alloys with the help of a magnet and magnetic chuck.

Bend Test. If a strip or bar of metal is gripped in a vice and hammered backwards and forwards until fracture occurs, provides a great deal of information about toughness, ductility and brittleness.

Filing Test. This test is based upon the basis of the hardness of the metal. It is possible to test the hardness of a material by trying to mark it with a small smooth/rough le. The depth of the scratch or mark produced by a le depends upon the hardness of the mark or scratch.

Q. 1.38. Show that in the steel containing 0.6% carbon, ferrite is about 7 times thicker than cementite.

Ans. For 0.6% C steel,

% pearlite = 0 6 0 0020 8 0 002

100 75 6. .. .

. %−−

× =

and % ferrite = 0 8 0 6

0 8 0 002100 24 4. .

. .. %−

−× =

Since 0.6% C steel is essentially a hypoeutectoid steel, no cementite can be present in addition to pearlite.

∴ % cementite = 0Pearlite is a mixture of cementite and ferrite.

Therefore to say that a steel contains pearlite and ferrite means the steel contains a mixture of ferrite and cementite.

% ferrite = 6 67 0 6

6 67 0 002100 91. .

. .%−

−× =

% cementite = 0 06 0 0026 67 0 002

100 9. .. .

%−−

× = .

Comparing the relative amounts of pearlite and ferrite with the equivalent relative amounts of ferrite and cementite, it is easy to visualise that

(91 – 24.4) = 66.6% of ferrite combines with 9% cementite to form pearlite.

Assuming the densities of ferrite and cementite to be same, a rough idea of relative volume of two phases can be obtained.

∴ Relative volume of ferrite to cementite in pearlite

= 66 66

9.

∼ 7Thus, the layer of ferrite in pearlite is seven times

thicker than the other constituent i.e., cementite.Q. 1.39. Mark the Plane with Miller indice (11–0) in a

cubic structure.Ans. For a plane with miller indices (11–0), the

intercepts on x, y and z axis will be 11

11

10

, , and i.e., 1, –1 and a.

The shaded plane in Fig. 1.25 below has miller indice (11–0)

z

x

y

– 1

+ 1

Fig. 1.25Q. 1.40. What is equilibrium diagram?Ans. For an alloy with different compositions of the

constituent elements, an equilibrium (phase) diagram is drawn showing the phase at various compositions of elements at different temperatures. With such diagrams, the temperature levels at the beginning and end of melting for alloys of various compositions can be determined. Further the structure of alloys for various compositions under equilibrium condition (i.e., suf cient time being allowed for the changes of phases to take place) during cooling and heating can also be established.

Q. 1.41. What is the difference between distortion and dilatation?

Ans Change in shape of solid materials is distortion and change in volume is dilatation.

Q. 1.42. What is the ductile SG iron? What is the purpose of adding magnesium or cerium in molten metal? Why is its tensile strength more than the tensile strength of grey cast iron? When the metal is cooled fast from molten state, what type of microstructure is obtained? Make an approximate sketch of microstructure indicating the microconstituents.

Ans. Ductile SG iron is also known as ductile cast iron, nodular cast iron, spheroidal graphite iron and SG iron. S.G. iron is an engineering material distinguished by its high strength, toughness and ductility, combined


with excellent casting properties and good machinability. It has a high modulus of elasticity and good resistance to corrosion and wear. While most varieties of cast iron are brittle, ductile iron is much more ductile and elastic, due to its nodular graphite inclusions.

Graphite is present in SG cast iron as small, round, and well-distributed particles. While weakening effect is small such cast irons have higher ductility. This form of graphite can be achieved by adding elemental magnesium or cerium or a combination of the two elements to molten cast iron. Magnesium is added in quantities of 0.07 to 0.10% followed by the addition of ferro-silicon to promote graphitisation. During solidi cation, magnesium helps in the distribution of graphite throughout the metal.

The ductile iron components are produced by casting process, wherein better control of component shape can be achieved compared to drop forging. Thus, many a components such as crank shafts and connecting rods manufactured usually by drop forging is increasingly being replaced by ductile iron castings.

Q. 1.43. Write a brief note on martensite.Ans. When a steel part is cooled suddenly, the

breakdown of austenite is suppressed. The new phase that forms is martensite in which all the dissolved carbon is held in form of body centered tetragonal structure. Martensite is only metastable phase and may be regarded as intermediate transition product since its structure is broken down by tempering. It is extremely hard and brittle and has a characteristic acicular appearance when examined under microscope under high magni cation. In the steels upto eutectoid composition, the martensite formed by this drastic quenching operation contains all the carbon that was contained in austenite. In higher eutectoid steels, however, some carbon is converted into carbide particles also.

The hardness of martensite is dependent upon the percentage of carbon present in structure.

Hardness of plain carbon steels increases rapidly until the eutectoid composition is reached. Hardest martensite is formed at eutectoid composition while hardness remains at this level in hypereutectoid steels. The slight increase in hardness of hypereutectoid steel is due to formation of carbide particles which are hard and brittle.

A rough microstructure of martensite is shown in Fig. 1.26.

Fig. 1.26.Q. 1.44. What is Bauschinger’s effect? Explain on stress

strain diagram how yield strength in compression is reduced than yield strength in tension?

Ans. Bauschinger effect is the directionality of straining, i.e., if strain hardening in a material takes place due to application of stress in a particular direction it will be easier to deform in the opposite direction. It can

also be said that if a material has a yield point stress Y in tension and is continued to deform from tension to compression in the same cycle it will show a yield point in the compression at Y′, where Y′ is less than Y. This effect is mainly due to the fact that lower stress is required to reverse the direction of slip on a certain slip plane than to continue slip in the original direction.

In Fig. 1.27 OYA is the stress-strain curve of a certain ductile material in tension. The yield occurs at Y. If the same material were tested in compression the yield would occur at Y′. Let another specimen of the same material be loaded in tension up to point A which is beyond yield point Y. The specimen is then unloaded so that it follows the path AD. A compressive stress is applied from the point D on the specimen. The plastic ow will begin at the point E. The stress corresponding to point E is appreciably below that corresponding to point Y. This decrease in the yield stress is the Bauschinger effect.

If the loading cycle is completed by further loading in compression to point F, then unloading and reloading in tension, the Bauschinger effect will result in closure of the stress strain loop at C. This is known as mechanical hysteresis loop. The area enclosed in the loop is the loss of energy due to Bauschinger effect.

+�

– �

– � + �

F

Y

E'

Y'

O D

A

E

C

Fig. 1.27.Q. 1.45. What is atomic packing factor (APF) and

coordination number? What are their values for simple cubic structure, B.C.C. structure and F.C.C. structure?

Ans. Atomic packing factor (APF) is de ned as the volume of unit cell covered by the atoms. The coordination number is de ned as the no. of atoms that are in the physical proximity of any atom.

For simple cubic structure, APF = 0.52 and coordination number of simple cubic is 6.

For body centred cubic (B.C.C.) structure, APF = 0.68 and coordination number is 8.

For face centred cubic (F.C.C.) structure, APF = 0.74 and coordination number is 12

Q. 1.46. What is Hardenability curve and what is Jominy distance.

Ans. Hardenability is dependent upon chemical composition of steel alloy. Addition of Ni, Cr, and Mo will slow down the transformation to other phases and allow more martensite to be formed. Thus alloy steels are hardened easily in comparison to plain carbon.


Distance from tip

Hard

ness 50% pearlite + 50% martensite

Jp

Jominy distance

Quenching

at one end

Fig. 1.28.

If austenite sample (a long beam) is quenched on one side then different cooling rates will be experienced by the sample from the tip to the other end. Martensite hardness will be maximum at quenched end. Gradually the percentage of martensite will decrease and percentage of pearlite will increase. Upon plotting the graph between hardness and distance from the tip the resulting curve is called hardenability curve. On this curve, the distance upto which there is mixture of 50% pearlite and 50% martensite is called Jominy distance.

Q. 1.47. Draw Iron-Carbon diagram, showing A1, A2, A3, ACN, A1,2,3 lines and phases like g = Austenite, a = Ferrite, d = Delta iron and CM = Cementite.

Ans. Refer Fig. 1.29.

Pearlite

and

ferrite

Cementite, pearlite

and transformed

ledeburite

Austentite

ledeburite

and

cementite

Austenite

to pearlite

Primary

austenite

begins

to solidify

CM begins

to solidify

Temperature

°C

Cementite

and

ledeburite

Magnetic

point

Peritectic

point

0.008%

Magnetic change of Fe C3

0.50 0.83% 1% 2.1% 3% 4.3% 5% 6.67%

ACM

Eutectic

pointL + Fe C3

Fe C3

Austenite in

liquid

� + L

�

��

�

� + L

A E B

A1�

��

A2A3

A1,2,3

A0

Cast ironSteel

Hypo-eutectoid Hyper eutectoid Hypo-eutectic Hyper-eutectic

0.025

� + Fe C3

� + Fe C3

Austenite solid solution

of carbon in gamma iron

Eutectoid

point

Pearlite and Cementite

1535

1492

1400

1150

910

760

725

210

Liquid (L)

Fig. 1.29.Q. 1.48. What is the difference between eutectic and

eutectoid reaction?Ans. In eutectic reaction, liquid on cooling converts

to two solid phases. For example, pearlite at 1450° on cooling converts to gamma iron and cementite.

In eutectoid reaction, a solid phase converts into two different solids. For example, austenite at 725°C on cooling converts to ferrite and cementite.

Q. 1.49. What is peritectic reaction?Ans. Peritectic reaction appears at 1493°C and at

0.18%C where a mixture of liquid and solid (delta iron)

converts into another solid phase (gamma iron) upon cooling.

Q. 1.50. What are composite materials and how they are classi ed?

Ans. Composite materials are manufactured by the combination of 2 or more materials metal, polymer and ceramic. The strength of composite can be even higher than the pure matrix and pure bres. The composite materials are generally classi ed according to the type of matrix and the orientation of bres. When an organic polymer e.g. epoxy phenolic resin etc. is used as matrix material, the composite is called polymer composite.


Glass bres reinforced plastic (GFRP): It has high strength but low stiffness. It is used in sports equipments,

lament rocket motor cases, pressure vessels, aircraft components etc.

Metal matrix composites: In these composites, there is a matrix of any ductile material e.g., Al, Mg, Ti etc., These composites have high speci c strength, creep resistance and dimensional stability.

Q. 1.51. De ne the terms: (a) Solid solution, (b) interstitial solid solution, (c) substitutional solid solution, (d) diffusion, (e) phase

Ans. (a) In solid solution, solute atoms are dissolved in solvent atoms. If a solution is allowed to freeze without separating the constituents, a solid solution would result. In a solid solution, the materials are present only as a mixture but not as chemical compounds.

(b) In an interstitial solid solution, the solute atom is positioned in the empty space between the adjacent atoms formed by the solvent atoms. For it to happen the solvent atom should be much larger compared to the solute atom. Also, the extent of solubility depends on the difference in the atomic sizes.

(c) In the substitutional solid solutions, the solute atoms would replace the solvent atoms. For this, both the atoms should be similar in size and also in nature.

(d) Diffusion is the process of movement of atoms from one location of higher concentration to another of lower concentration or to a vacant place. Diffusion of atoms would be faster at high temperatures and in liquid phase. It is also a time-dependent phenomenon as the atoms have to physically travel from one site to the other. In an alloy system, the component metals may combine within a certain temperature range to form two homogeneous coexisting portions. Each of these portions (phases) may have different compositions and consequently different properties.

(e) A phase may be de ned as any part of a chemical system that possesses distinctive physical characteristics. An alloy may consist of one phase or a combination of different phases. When an alloy is transformed from liquid to solid state, i.e., several solid phases may be formed and nonhomogeneity may appear.

Q. 1.52. What are super alloys? Name few super alloys. Give composition of Nimonic alloy.

Ans. A superalloy is a high performance alloy that exhibits excellent mechanical strength, resistance to thermal creep deformation, good surface stability and resistance to corrosion/oxidation. The crystal structure is typically FCC austenitic. These alloys are referred to as iron-base, cobalt-base or nickel base super alloys. Nickel-based super alloys are most common and these contain alloying elements, including Cr, Al, Tl, Mo, W, Nb, Ta and Co. They develop high temperature strength through solid solution strengthening.

Names of few important super alloys are:Haynes alloys, Hastalloy, Incoloy, Inconel, Rene 41,

Nimonic, Supertherm WaspaloyThe primary application of superalloys is in turbine

engines, both aerospace and marine, nuclear, chemical

and photochemical industries. Nimonic alloys consist of more than 50% of Ni and 20% Cr with additives like Ti and Al. These are Ni-based high-temperature low creep super alloys and nd application in gas turbine components and high performance reciprocating I.C engines.

Q. 1.53. What happens at eutectic composition? Explain with an example.

Ans. At eutectic composition of alloys, the melting point is lowest (lower than melting point of each constituent). As an example, the m.p. of solder (62% of tin and 38% lead) is 183°C whereas m.p. of lead is 327°C and of tin is 232°C.

Q. 1.54. What is atomic packing factor (APF) of coordination number? What is their values for simple cup structure. BCC structure, and FCC structure?

Ans. Atomic packing factor (APF) is de ned as the volume of unit cell covered by the atoms. The coordination number is de ned as the no. of atoms that are in the physical proximity of any atom. Let us elaborate on cubic crystal structure.

For simple cubic structure,APF = 0.52 and,

and coordination number of simple cubic is = 6.For body centred cubic (BCC) structures,

APF = 0.68,and coordination number is 8.For face centred cubic (FCC) structures,

APF = 0.74,and coordination no. is 12Q. 1.55. Discuss the effect of manganese and nickel as

alloying elements in steels and cast iron.Ans. For effect of Mn and Ni in steel, refer Art 1.16.1.For effect of Mn and Ni on cast iron, refer para before

Art. 1.11.3.Q. 1.56. Distinguish between white and nodular cast iron.Ans. Refer Art. 1.11.1.Q. 1.57. (a) Give the %ages of carbon in (i) low carbon

steels, (ii) medium carbon steels, (iii) high carbon steels.(b) Give the %age of the alloying elements in

(i) low alloy steels and (ii) high alloy steels.(c) All cast irons contain at least six elements. Name these

elements.(d) When will you recommend carburising of steel? Give

salient features of the process.(e) Give the composition and use of (i) Bronze, (ii)

Duralumin.Ans. (a) Carbon in low carbon steel : < 0.25%, medium

0.3 to 0.6%,high carbon steel : 0.6 to 1.5%

(b) Refer Art. 1.15. (c) Refer Art. 1.11.(d) Refer Art. 2.12.1.(e) Refer Art. 1.34 for (i) and 1.36 for (ii).


PROVIDE SINGLE WORD FOR FOLLOWING STATEMENTS

1. The characteristics of a material that are displayed when a force is applied to the material.

2. The stress at which a material exhibits a speci ed deviation from proportionality of stress and strain.

3. The stress developed at the outer bre when a material is loaded as a simply supported beam and de ected to a certain value of strain.

4. The serviceability factor related with the mechanical properties like impact strength, notch sensitivity.

5. The mechanical properties like bend radius, percentage elongation and percentage reduction in area are related with ......

6. When one is interested in attribute of durability of a material, he must look for properties like hardness, wear resistance and ......

7. Modulus of elasticity and exural modulus refer to ...... of the material.

8. The material characteristics that relate to the structure of a material and its formation from other elements.

9. The structure of a polished and etched metal as revealed by microscope.

10. The ordered, repeating arrangement of atoms or molecules in a material.

11. The ability of a material to resist deterioration by chemical or electrochemical reaction with its environment.

12. The properties like thermal conductivity, refractive index, speci c heat, dielectric strength fall under the category of ......

13. The temperature at which ferromagnetic materials can no longer be magnetised by outside forces.

14. The ratio of the velocity of light in vacuum to its velocity in another material.

15. The temperature at which a polymer under a speci ed load shows a speci ed amount of de ection.

16. The highest voltage an insulating material of a speci ed thickness can withstand for a speci ed time without occurrence of electrical breakdown through its bulk.

17. A hard and brittle alloy of iron, carbon (between 1.8 to 3%), and silicon.

18. Material used for machine housings and frames due to its high compression strength and good vibration absorption properties.

19. Cast iron which contains carbon as free carbon in the form of graphite.

20. Cast iron in which the carbon is present in the combined form as iron carbide.

21. Cast iron which contains graphite in the form of small spheres/nodules.

22. The free carbon in grey cast iron acts as an excellent ...... during machining and offers little frictional resistance.

23. Black heart, white heart, and pearlitic cast irons fall under the category of ......

24. Which cast iron has good ductility as cast?25. Spheroidal graphite iron having tensile strength of

900 N/mm2 and elongation of 2% is designated as ......

26. A mechanical mixture of pig iron and distributed silicate slag.

27. The material used for chains, crane hooks, bolts subjected to shock loads etc. because it is malleable, ductile and tough.

28. Which alloying element in cast iron improves machinability, hardness, strength and corrosion resistance?

29. Which alloying element in cast iron checks the formation of graphite and promotes the formation of carbides?

30. Which alloying element in cast iron improves strength and wear resistance?

31. The resistance to corrosion of steel is increased by adding ......

32. In order to eliminate blow holes in castings, ...... is added to steel.

33. Addition of boron to steel improves ......34. Red hardness is imparted to steel by adding ......35. Which alloying element in steel removes oxygen in

steel making and facilitates rolling and forging?36. Which alloying element in steel promotes tenacious

oxide lm to aid atmospheric corrosion resistance?37. Machinability of steel is improved by adding ......38. Simple alloys of iron and carbon with no other

alloying element.39. A group of steels with carbon content varying upto

about 1% and total alloy content below 5%.40. A solid formed by the combination of metallic and

non-metallic elements.Ans. to Single Word Questions

1. Mechanical property 2. Yield strength 3. Flexural strength 4. Toughness5. Formability 6. Fatigue strength7. Rigidity 8. Chemical properties 9. Microstructure 10. Crystal structure11. Corrosion resistance 12. Physical properties13. Curie point 14. Refractive index 15. Heat distortion temperature16. Dielectric strength17. Cast iron 18. Cast iron19. Grey cast iron 20. White cast iron21. Spheroidal or nodular graphite iron22. Lubricant 23. Malleable cast iron 24. Spheroidal graphite 25. SG 900/226. Wrought iron 27. Wrought iron28. Nickel 29. Chromium30. Molybdenum 31. Chromium32. Phosphorus 33. Rolling qualities34. Tungsten 35. Silicon36. Copper 37. Lead38. Carbon steels 39. Alloy steels40. Ceramic.


TRUE OF FALSE QUESTIONSState whether following statements are true (T) or false (F)?

1. A reversible change in an atomic structure of the metal with a corresponding change in the properties of steel, is called allotropic.

2. Steel containing 0.8% carbon has only one critical point.

3. Alpha iron exists at below 768°C, gamma iron in 900-1400°C, and delta iron in range of 1400-1530°C.

4. Steel having combination of 6.67% carbon and 93.33% iron, is called cementite.

5. If steel is slowly cooled in furnace, the structure obtained is called pearlite.

6. If steel is quenched in oil during transformation, the structure obtained is called troostite.

7. The effective number of lattice points in the unit cell of simple cubic, body centered cubic, and face centered cubic space lattices, respectively, are 1, 2, 4.

8. The main purpose of spheroidising treatment is to improve machinability of high carbon steels.

9. During normalizing process of steel, the specimen is heated above the upper critical temperature and cooled in still air.

10. Charpy test is done to measure toughness, knoop test for microhardness and cupping test to measure formability of metal.

11. The crystal structure of austenite is face centered cubic.

12. If a particular Fe-C alloy contains less than 0.83% carbon, it is called hypereutectoid steel.

13. The material property which depends only on the basic crystal structure is fracture strength.

14. Ability of a material to resist deformation due to stress is called stiffness.

15. Materials exhibiting time bound behaviour are called visco elastic and visco elastic behaviour is common in rubber.

16. Crystalline solids are usually built up of a number of crystals which may be similar or of widely varying sizes and metallic or non-metallic.

17. In non-crystalline materials (called amorphous), the internal structure is not based on a regular repetition pattern.

18. In crystallography the structure implies the arrangement and disposition of atoms within a crystal.

19. Most ductile lattice is body centered cubic space lattice.

20. Alpha iron, gamma iron and delta iron represent the allotropic form of iron.

21. Moh’s scale is used in connection with hardness of materials.

22. Weld decay is the phenomenon found with nonferrous materials.

23. Carbon in iron is an example of substitutional solution.

24. Duralumin contains 94% aluminium, 4% copper and 0.5% Mn, Mg, Si and Fe.

25. Sub-zero treatment of steel is used to reduce the retained austenite in hardened steel.

26. The structure which have the highest packing of atoms are hexagonal close packed lattice.

27. Dislocations in materials are example of line defect.28. Atomic Packing factor is fraction of volume applied

by spherical atoms as compared to the total available volume of the structure.

29. Atomic packing tactor of simple cubic structure, BCC structure, FCC structure and HCP structures, respectively is 0.5, 0.68, 0.74, and 0.74.

30. Properties of solid crystalline depends on the basic crystal structure of the solid e.g. fcc is more ductile while hep is less ductile and bcc is usually harder.

31. Vacancy defect appears due to missing of atom from the lattice.

32. When foreign atom occupies the interstitial site, the defect is called interstitial defect.

33. If regular atom is replaced by another foreign atom, the defect is called substitutional defect.

34. When atom in the lattice point goes and occupies interstitial void of other atom, then it is called Frenkel defect.

35. In the combination of cation and anion if there is a vacancy defect, it is called Schottky defect.

36. If a plane intersects the co-ordinate axes at x = 23

,

y = 13

, z = 12

, then Miller index of this plane is 364.

37. Eddy currents can be minimised by laminating the iron core, i.e., building it up with thin sheets covered by a thin coating of insulating varnish.

38. Superconductivity is phenomenon in which the resistance of certain metals, alloys and compounds drop to zero at a critical temperature near absolute zero.

39. Wrought iron has the least percentage of carbon 0.12 to 0.2 percent.

40. Cast iron produced from white cast iron by annealing process is malleable cast iron and that produced by adding magnessium to molten cast iron is nodular cast iron.

41. If carbon present in cast iron is mostly in the combined state, it is called white cast iron and if it is present in free state, it is called grey cast iron.

42. Eutectoid composition of carbon steel at room temperature is called pearlite.

43. Age hardening and precipitation hardening process is applicable only for those alloys that exist as a two-phase material at the room temperature and can be heated up to a single phase.

44. If precipitation takes place at the room temperature, a longer time is necessary for the completion of precipitation, this process is called age hardening.


45. If precipitation rate is increased by quenching the specimen to a temperature higher than the room temperature, then process is called precipitation hardening.

SOLUTIONSAll statements except 13, 14, 19, 22 and 23 are True.

Answers for False statements are as under:13. It is fatigue strength.14. It is called toughness.19. It is face centered cubic space lattice.22. It is found with stainless steel.23. It is interstitial solid solution.

MULTIPLE CHOICE QUESTIONS1. The lattice energy of ionic compound increase

(a) with the increasing size of the ions(b) with the decreasing size of the ions and with

increasing number of bonding electrons(c) with the decreasing number of bonding

electrons.(d) none of above

2. Burger’s vector of an edge dislocation is(a) perpendicular to the dislocation line(b) parallel to dislocation line(c) at any angle with dislocation line except 90°(d) at any angle with dislocation line including 0

and 90°.3. Yield point in mild steel occurs because

(a) carbon atoms form Fe3C with iron(b) jog formation in dislocation line is promoted by

presence of carbon(c) carbon atoms occrpy the vacancy line just

below the dislocation and anchor dislocation by reducing its energy

(d) all of above4. Choose the wrong statement.

When two ions are brought closer they experience a net force(a) The net force is zero when bond is formed(b) The net force is sum of force of attraction and

force of repulsion(c) The force of repulsion is a short range force

while force of attraction is a long range force.(d) The net force is minimum (negative) when the

bond is formed.5. The force of attraction between two ions depends

upon(a) atomic numbers of atoms whose ions form the

bond and the charge of an electron(b) number of electrons removed from cation and

the number added to anion and the charge of an electron

(c) only charge of electron(d) only number of electrons removed from cations.

6. The equilibrium distance between ions in an ionic bond is(a) slightly greater than the sum of radii of the two

ions (b) slightly less than the sum of radii of the two

ions(c) difference between the diameters of the two ions(d) Sum of the radii of the two ions.

7. Match the lists I and II using code.List I List II(Bonds) (Characteristics)1. Ionic A. Directionality and strength2. Covalent B. Ductility and strength3. Metallic C. Low strength4. Dipole D. Strength and high melting

point Codes:

1 2 3 4(a) B A C D(b) D B A C(c) D A B C(d) A C D B

8. Mixed ionic-covalent bonds are found in(a) semi conductor(b) goodelectricity conductor(c) high strength metals(d) heat insulator

9. Choose the wrong statement(a) Carbon atoms in low carbon steel (0.2% C)

anchor the dislocation lines by migrating just below the line of dislocations. This causes mild steel to yield.

(b) Breaking dislocation away from carbon atmosphere requires stress higher than the stress required for moving the dislocation after their bonds with anchoring carbon atoms have been broken.

(c) Dislocations once torn away from anchoring atmosphere of carbon atoms can never be anchored again by the carbon atoms. Hence the yield point never reappears.

(d) Yield point in low carbon steel reappears if steel is unloaded after yielding and allowed to rest for several hours.

10. Strain ageing in mild steel causes(a) yield point of occur when it is loaded once(b) yield point to recur in alternate loading beyond

yielding and unloading(c) material to behave in brittle manner(d) material to lose its ultimate tensile strength

11. Blue brittlenessk in mild steel is caused between 230°C and 370°C because(a) mild steel strain hardens(b) mild steel strain ages(c) dislocations do not move in this temperature

range(d) modulus of elasticity increases.


12. The yield strength of a yielding material will increase if(a) grains are ne(b) grains are coares(c) ne and coarse grains are present at the same

time(d) if grains have very irregular shape.

13. The precipitation hardening in Al-Cu alloy is because of(a) cluster formation of Cu atoms in Al matrix, thus

impeding dislocation slide(b) Cu atoms surround dislocation in aluminium

matrix(c) Cu atoms annihilate vacancies(d) Cu forms a chemical compound which is

uniformly distributed in Al matrix.14. If a metal/semiconductor carries a current ‘I’ and is

placed in a. transvers magnetic eld ‘B’ an electric eld ‘E’ is induced in a direction perpendicular to

both I and B. This effect is known as(a) Hall effect (b) Silsbee effect(c) Meissner effect(d) Wiedemann Franz effect.

15. A semiconductor material doped with impurity atoms is called(a) dielectric material (b) nonpolar material(c) intrensic semiconductor(d) extrinsic material

16. The temperature dependence conductivity in intrinsic semiconductor is nearly(a) constant (b) exponential(c) hyperbolic (d) parabolic

17. When tetravalent impurity (Ga or B) is added and in Si and Ge, the semiconductor is called(a) p-type extrinsic (b) n-type extrinsic(c) nitrinsic (d) superconductor.

18. Match lists I and II using code given below:I (Change) II (Temperature)

A. Loss of ferromagnetism 1. 1148°Cof ferrite

B. Change of magnetic 2. 770°Cproperties of cementite

C. Eutectoid 3. 210°Ctransformation

D. Eutectic 4. 723°Ctransformation

Codes:A B C D

(a) 3 2 4 1(b) 3 2 1 4(c) 1 3 2 4(d) 2 3 4 1

19. Match lists I and II using code given below: I (Allotropic form of Fe) II (Characteristic)

A. a 1. b.c.c.B. d 2. Non-magneticC. g 3. MagneticD. b 4. f.c.c.

Codes:A B C D

(a) 3 1 4 2(b) 1 2 4 3(c) 3 4 2 1(d) 2 3 1 4

20. Which phase in steel is metastable and hence is not encountered if equilibrium cooling takes place(a) ferrite (b) martensite(c) cementite (d) austenite

21. In which of the following phases of steel cementite is in lamellar form?(a) Pearlite (b) Martensite(c) Ferrite (d) Bainite

22. In which of the following phases of steel cementite is in lamellar form?(a) Bainite (b) Martensite(c) Pearlite (d) Ferrite

23. The pearlite content in plain carbon steel(a) increases with increasing carbon content upto

1.2%(b) decreases as carbon content increases(c) increases with carbon content upto 0.8% and

then decreases(d) remains constant upto carbon content of 0.8%.

24. The complete transformation of austenite takes place during cooling from liquid state(a) just below 723°C (b) at 723°C(c) just above 723°C (d) at 910°C.

25. When FCC iron is cooled, it’s crystals change to BCC.(i) no. of atoms/unit cell decrease from 4 to 2

(ii) Atomic packing factor reduces from 0.74 to 0.68

(iii) Atomic radius increases from a a2

43

4to

(iv) increase in volumeState which statement is true

(a) (i), (ii) and (iv) (b) (i), (ii) and (iii)(c) (ii), (iii) and (iv) (d) (i), (ii), (iii) and (iv)

26. Atomic packing factor is highest for elements like silver: Al, Au, Pt etc. Their cubic crystal structure is, (a) simple cube (b) BCC(c) FCC (d) Diamond cube.

27. The alloying element in steel which increases hardness but does not sacri ce ductility is(a) Cr (b) Mo(c) V (d) Ni.


28. Choose the wrong statement(a) Free cutting steel cannot be forged(b) Free cutting steel contains 0.33% S.(c) The alloying elements in steel that improve its

machinability are Pb, P, S(d) The martenste is harder than ferrite because

Ferrite is b.c.c. wheares martensite is distorted b.c.c. with carbon on the edge

29. With increasing carbon percentage in steel which group of properties is not affected beyond 0.8% C?(a) UTS and impact strength(b) UTS and % elongation (c) Hardness and UTS(d) % elongation and impact strength.

30. In which range of carbon content the greatest change in hardness of marten site occurs?(a) 0.2 to 0.4 (b) 0.4 to 0.6(c) 0.6 to 0.8 (d) 0.8 to 1.0.

31. For permanent magnetic material important alloying elements in steel are(a) W, Cr, V (b) W, Cr, Co(c) Si, W, Cr (d) Mn, W and Cr.

32. For transformer cores the steels must contain(a) W and V (b) V and Cr(c) carbon and Si (d) Si and Co.

33. No. of atoms per unit cell is maximum for elements like Ge, Si, C. Their cubic crystal structure is(a) simple cube (b) BCC(c) FCC (d) Diamond, cube

34. The no. of atoms per unit area of a crystal plane is known as(a) atomic packing factor (b) planar density(c) inter-planar spacing(d) coordination number.

35. Consider following1. When all the atoms at

the lattice points are identical

(A) Miller in dices

2. No. of nearest and equidistant atoms w.r.t. any other atom in a unit cell

(B) Bravais Lattice

3. The spacing between a given plane and the other parallel plane passing through the origin

(C) Coordination number

4. Styles to designate the plane and directions in the unit cells and crystals

(D) Inter planar spacing

Which of the following provides right matching?(a) 1—B, 2—C, 3-D, 4—A(b) 1—C, 2—B, 3—D, 4—A(c) 1—B, 2—C, 3—A, 4—D(d) 1—B, 2—D, 3—C, 4—A

36. Aluminium bronze is well known for its(a) golden colour and strength(b) golden colour and corrosion resistance(c) strength and corrosion resistance(d) corrosion resistance and formability.

37. The alloy of copper which can be used against fatigue, wear and corrosion is(a) phosphor bronze (b) beryllium bronze(c) aluminium bronze (d) cartridge brass.

38. Materials like titanates of barium and lead, zirconate, ammonium dihydrogen phosphate beong to category(a) piezoelectric (b) ferromagnets(c) electron restriction (d) hysteresis

39. An alloy of Co, Cr, W and C used for cutting metals at high speed and temperature is(a) cemented carbide (b) cermet(c) stellite (d) ceramic.

40. Resistance to creep is improved by(a) solidifying alloys directionally(b) allowing grains to grow coarse(c) precipitating uniformly dispersed coares

particles in the metal matrix(d) allowing hard particles to precipitate along

grain boundaries.41. A state of material in which it has zero resistivity

is called(a) supercritical material (b) superconductivity(c) supersensitivity(d) superdiamegnetic material

42. The group of dielectric materials which exhibit spontaneous polarization are called(a) super-conductive materials(b) non-polar materials (c) ferro-electrics(d) polar molecular

43. Molybdenum and tungsten are high melting point elements but cannot be used against creep because they(a) are weak in tension(b) get oxidized easily at elevated temperature(c) from weak unstable structure at elevated

temperature(d) are brittle at room temperature.

44. Pick up false statement about hard and soft magnetic materials(a) Magnetic energy stored is high in hard magnetic

materials and low (nil) in case of soft magnetic materials.

(b) Soft magnetic materials are easy to be both magnetised and demagnetised but hard magnetic materials retain their magnetism

(c) Hysterisis loss and eddy current loss is high in soft, and low in hard magnetic materials

(d) Coercive force and retentivity are large in hard megnetic materials and less in case of soft magnetic materials.


45. Which of following is hard magnetic material(a) Cu-Ni-Fe alloy (b) Ferrites(c) metallic glasses (d) Fe-Si alloys

46. Match parts A and B pertainity to magnetic materials

A B1. diamagnetic

materials(A) ferrites

2. ferromagnetic materials

(B) salts of transition elements

3. ferrimagnetic materials

(C) transition metals

4. paramagnetic material

(D) superconductors

5. antiferro magnetic materials

(E) alkali metals

(a) 1—D, 2—C, 3—A, 4—E, 5—B(b) 1—D, 2—C, 3—E, 4—A, 5—B(c) 1—C, 2—D, 3—A, 4—E, 5—B(d) 1—D, 2—C, 3—A, 4—B, 5—E

47. Match parts A and B in relation to units of magnetic properties

A B1. Magnetic ux

diversity(A) A/m

2. Magnetic pernuability

(B) Wb/m2

3. Magnetic lled strength

(C) J/m3

4. Hysteresis loss (D) Wb/Am (Henry/m)

(a) 1—B, 2—D, 3—A, 4—C(b) 1—D, 2—B, 3—A, 4—C (c) 1—B, 2—D, 3—C, 4—A(d) 1—D, 2—B, 3—C, 4—A

48. Chemical bonds are formed due to binding forces between atoms and molecules and these are either primary or secondary bonds. Which of following is not primary bond?(a) dipole (b) metallic(c) ionic (d) covalent

49. A crystal is composed of(a) electrons (b) atoms or ions(c) ions or molecules (d) lattices

50. Secondary bonds are formed due to inter-molecular attraction forces between molecules. Which is not secondary bond?(a) hydrogen (b) dipole(c) dispersion (d) covalent

51. A crystal structure is(a) a regular linear pattern of atoms over a long

range(b) a planar arangement of atoms repeated

in nitely in the solid

(c) a regular three dimensional pattern of atoms or ions in space

(d) a regular three dimensional pattern of molecules in space.

52. In a b.c.c. unit cell one central atom is in contact with(a) four identical atoms(b) six identical atoms(c) eight identical atoms(d) tweleve identical atoms.

53. Atoms bond primarily to reduce their potential energy and(a) lose stability (b) gain stability(c) gain bond energy (d) lose bond energy

54. In a f.c.c. unit cell a central atom is surrounded by(a) twelve identical atoms(b) eight identical atoms(c) six identical atoms(d) four identical atoms.

55. No. of atoms/unit cell is maximum for diamond cube crystal structure. Its value is(a) 2 (b) 4(c) 8 (d) 12

56. In a unit cell of aluminium atoms occupy(a) 74% of volume of the cube(b) 80% of volume of the cube(c) 85% of volume of the cube(d) 90% of volume of the cube.

57. An element having atomic radius of r has a cubic structure (bcc or fcc). The density of element is(a) const × Atomic mass × coordination number/r3

(b) const × Atomic mass × packing factor/r3

(c) const × Avogadro’s number × atomic mass/r3

(d) const × Atomic mass/r3.58. Atomic packing factor is maximum for fcc crystal.

Its value is(a) 0.68 (b) 0.74(c) 0.81 (d) 0.92

59. A solid solution is(a) alloy of two metals(b) alloy of two non metals(c) alloy of metals and non matel(d) alloy of two metals or a metal and a non metal.

60. In an interstitial solid solution(a) a metallic atom occupies the interstices between

non-metallic atoms(b) a solute atoms occupies the interstices between

the atoms of solvent(c) a non metallic atoms occupies the regular atomic

site(d) an impurity atoms reacts with solvent atom to

form a compound.


61. For formation of substitutional solid solution between two elements which statement is incorrect?(a) The valency of two elements must be different(b) The atomic sizes of two elements must not differ

by more than 15%(c) The two elements must have same crystalline

structure(d) The electronegativity of two elements must not

differ appreciably.62. A vacancy can exist

(a) in a solid solution only when a solute atom does not ll the interstitail

(b) in a crystal when atom is missing from its regular site

(c) in an ionic bonded solid if a cation exists alone(d) if temperature is very high.

63. Vacancies in a crystalline solid(a) form only at the time of nucleation(b) multiply only when solid is subjected to stress(c) can form and multiply at any temperature due

to diffusion process(d) are formed due to impurities.

64. Energy that retards crystallization is(a) activation energy (b) kinetic energy(c) vibration energy (d) free surface energy.

65. Substitutional diffusion becomes possible if(a) atoms are signi cantly different in size(b) vacancies are present in a substitutional solid

solution(c) the temperature of solid solution is suf ciently

high(d) the activation energy for vacancy formation is

low.66. Three elastic properties are measured as stress

from stress-strain diagram(A) Yield point (B) Elastic limit(C) Proportional limitArrange them in ascending order(a) ABC (b) CBA(c) BCA (d) ACB.

67. Which of the following is the measure of ductility?(a) Ultimate tensile strength(b) Yield strength(c) Percent elongation(d) Modulus of toughness.

68. Proof stress corresponds to(a) lower yield point (b) higher yield point(c) elastic limit (d) a speci ed strain.

69. The yield strength of a steel is increased from 400 to 600 MPa, which one of the following will increase?(a) Percent elongation(b) Ultimate tensile strength(c) Modulus of toughness(d) Modulus of resilience.

70. In a tension specimen the elongation at the time of fracture is(a) localized in the region of necking(b) uniformly distributed over the gauge length(c) localized near the ends(d) localized in the centre of the length.

71. The cup and cone fracture in a tension specimen is due to(a) shearing of planes at 45° to the axis(b) tearing of metal at the centre of the cross-section(c) sliding of atomic planes perpendicular to and

at 45° to the axis(d) tearing of metal in the centre followed by

shearing of planes at 45° to the axis.72. Which is not correct statement

(a) True stress is great than engineering stress(b) True stress is based upon actual area of cross

section which is always less than original area of cross section.

(c) Engineering stress is based upon original area of cross section

(d) Engineering stress is greater than true stress.73. Increasing strain rate in a tension test

(a) increases yield strength and tensile strenght(b) increases yield strength but decrease tensile

strength(c) increases tensile strength but decreases yield

strength(d) increases percent elongation but decreases

tensile strength.74. Which combination of alloying elements increases

ductility transition temperature?(a) C and O2 (b) C and Mn(c) O2 and Mn (d) Si and Mn.

75. Which phenomenon is not used in the measurement of hardness?(a) scratch (b) indentation(c) wear (d) fracture.

76. The Brinell hardness is(a) a dimensionless number(b) a number that represents N/mm(c) a number having dimensions of force/area(d) a number having dimensions of length.

77. In which hardness the area of indented surface is not measured(a) brinell (b) rockwell(c) knoop (d) vickers.

78. Pick us wrong statement(a) shore sceleroscope measures rebound(b) mhos scale is concerned with scratch(c) rockwell C hardness is measured by diamond

cone(d) rockwell B hardness is measured on basis of

fracture.


79. Which hardness method can measure hardness of a grain?(a) rockwell F (b) vickers(c) knoop (d) shore.

80. Which one of the following is not the correct reason for fatigue crack to initiate from the surface?(a) the surface is free of dislocations.(b) the stress is greater on surface than at any other

at any other point inside.(c) the surface is more likely to carry a stress

concentration.(d) the surface is exposed to corrosion.

81. Which method will not increase fatigue strength?(a) surface hardening (b) shot peening(c) grain coarsening (d) grain re ning.

82. Shot peening improves fatigue strength by way of(a) re ning the grain(b) removing stress concentration from the surface(c) polishing the surface(d) inducing residual compressive stress on the

surface.83. Hardenability is the

(a) depth of penetration in Rockwell test(b) ability to resist abrasion(c) property which determines the depth of the

hardened zone(d) ability to withstand shocks.

84. Diamagnetic materials(a) are non-magnetic(b) can be magnetised in one direction only(c) are magnetised in direction opposite to that of

applied load(d) can be magnetised in any direction.

85. Gibb’s phase rule is given by(a) F = C + P (b) F = C + P – 2(c) F = C – P – 2 (d) F = C – P + 2.where, F = number of degree of freedom

C = number of components, and P = number of phases.

86. Polyesters belong to the group of(a) thermoplastic plastics(b) thermosetting plastics (c) phenolics (d) PVC.

87. A semi-conductor material has following number of electrons in the outermost orbit(a) 2 (b) 4(c) 5 (d) 6.

88. Dielectric strength of a material is the(a) capacity to withstand biaxial stresses(b) magnetic property(c) capacity to withstand high voltage(d) capacity to resist ow of current.

89. Cementite in the form of lamellar pearlite appears as follows under microscope(a) dark (b) white(c) light (d) nger print.

90. Cementite in white cast iron appears as follows under microscope(a) dark (b) white(c) light (d) nger print.

91. Ferrite appears as follows under microscope(a) dark (b) white(c) light (d) nger print.

92. Pearlite appears as follows under microscope(a) dark (b) white(c) light (d) nger print.

93. The basic ingredient of cemented carbide is(a) aluminium oxide (b) vanadium(c) ceramics (d) tungsten oxide.

94. Stellite is a non-ferrous cast alloy composed of(a) cobalt, chromium and tungsten(b) tungsten, chromium and vanadium(c) tungsten, molybdenum and cobalt(d) molybdenum, vanadium and cobalt

95. Materials exhibiting time bound behaviour are known as(a) visco elastic (b) anelastic(c) isentropic (d) resilient.

96. Visco elastic behaviour is common in(a) rubber (b) plastics(c) crystalline materials(d) non-crystalline materials.

97. Diamond’s weight is expressed in terms of carats. One carat is equal to(a) 1 mg (b) 20 mg(c) 200 mg (d) 350 mg.

98. The degradation of plastics is accelerated by(a) high ambients (b) dampness(c) corrosive atmosphere(d) ultravoilet radiation.

99. Which of the following metals can be easily drawn into wire?(a) tin (b) copper(c) lead (d) zinc.

100. Following element is added to molten cast iron to obtain nodular cast iron(a) Cr (b) Mn(c) Cu (d) Mg

101. Silicon when added to copper increases its(a) machinability(b) brittleness(c) electrical conductivity(d) hardness and strength

102. Which of the following is an amorphous material?(a) mica (b) lead(c) rubber (d) glass.


103. Following etching solution is used for medium and high carbon steel, pearlitic steel, and cast iron(a) nital - 2% HNO3 in ethyl alcohol(b) picral-5% picric acid and ethyl alcohol(c) 1% hydro uoric acid in water(d) 50% NH2OH and 50% water.

104. Duralumin Alloy contains aluminium and copper in the ratio of

%A1 %Cu(a) 94 4(b) 90 8(c) 88 10(d) 86 12

105. Quartz is a(a) ferroelectric material(b) ferromagnetic material (c) piezoelectric material(d) diamagnetic material

106. To reduce the consumption of synthetic resins, the ingredient added is(a) accelerator (b) elastomer(c) modi er (d) ller.

107. Match List I (materials) with List II (applications) and select the correct answer using the codes given below the Lists:

List I List IIA. Engineering

ceramics1. Bearings

B. Fibre reinforced plastics

2. Control rods in nuclear reactors.

C. Synthetic carbon 3. Aerospace industryD. Boron 4. Electrical insulator

Codes:A B C D

(a) 1 2 3 4 (b) 1 4 3 2(c) 2 3 1 4(d) 4 3 1 2

108. Match List I with List II and select the correct answer, using the codes given below the lists:

List I(Heat treatment)

List II (Effect on the

properties)A. Annealing 1. Re ned grain

structureB. Nitriding 2. Improves the hardness

of the whole masC. Martempering 3. Increases surface

hardnessD. Normalising 4. Improves ductility.Codes:

A B C D(a) 4 3 2 1(b) 1 3 4 2(c) 4 2 1 3(d) 2 1 3 4

109. Match List I with List II and select the correct answer using the codes given below the lists:

List I(Name of Material)

List II (% Carbon

Range)A. Hypo-eutectoid steel 1. 4.3-6.67B. Hyper-eutectoid steel 2. 2.0-4.3C. Hypo-eutectic cast iron 3. 0.8-2.0D. Hyper-eutectic cast iron 4. 0.008-0.8.

Codes:A B C D

(a) 4 3 2 1(b) 1 3 2 4(c) 4 1 2 3(d) 1 2 3 4

110. Which one of the following sets of constituents is expected in equilibrium cooling of a hypereutectiod steel from austenitic state?(a) Ferrite and pearlite.(b) Cementite and pearlite.(c) Ferrite and bainite.(d) Cementite and martensite.

111. Match List I with List II and select the correct answer using the codes given below the lists:

List I(Alloy)

List II (Use)

A. Low carbon steel 1. Bearing.B. Had eld manganese

steel2. Thermocouple

C. Constantan 3. Wire nailsD. Babbit alloy 4. Bulldozer blades

Codes:A B C D

(a) 1 2 3 4(b) 3 4 1 2(c) 3 2 1 4(d) 3 4 2 1

112. Addition of magnesium to cast iron increases its(a) hardness(b) ductility and strength in tension(c) corrosion resistance(d) creep strength.

113. Match List-I (Alloying element in steel) with List-II (property conferred on steel by the element) and select the correct answer using the codes given below the lists:

List I List IIA. Low carbon steel 1. BearingA. Nickel 1. Corrosion resistanceB. Chromium 2. Magnetic

permeabilityC. Tungsten 3. Heat resistanceD. Silicon. 4. Hardenability


Codes:A B C D

(a) 4 1 3 2(b) 4 1 2 3(c) 1 4 3 2(d) 1 4 2 3

114. Match List-I (Alloys) with List-II (Applications) and select the correct answer using the codes given below the lists:

List I List IIA. Chromel 1. Journal bearingB. Babbit alloy 2. Milling cutterC. Nimonic alloy 3. Thermocouple wireD. High speed steel 4. Gas turbine blades

Codes:A B C D

(a) 3 1 4 2(b) 3 4 1 2(c) 2 4 1 3(d) 2 1 4 3

115. Pick up false statement. Addition of carbon in steel(a) hardens and strengthens it.(b) makes it more dif cult to weld without cracking.(c) makes it more brittle and more difficult to

machine.(d) increases melting point of steel and makes it

dif cult to heat-treat.116. The temperature at which the phase changes occur

in a metal is called(a) recrystallisation temperature(b) phase transformation temperature(c) transition temperature(d) critical temperature.

117. The following material is used for imitating(a) silicon bronze (b) babbitt alloy(c) duralumin (d) aluminium bronze.

118. Consider the following statements:Fibre reinforced plastics are1. made of thermosetting resins and glass bre.2. made of thermoplastic resins and glass bre.3. anisotropic.4. isotropicOf these statements:(a) 1 and 4 are correct (b) 1 and 3 are correct(c) 2 and 3 are correct (d) 2 and 4 are correct.

119. Which one of the following materials is used for car tyres?(a) styrene-butadine rubber (SBR)(b) butyl rubber (c) nitrile rubber(d) ebonite.

120. Ceramic materials are:(a) used when load is high(b) basically crystalline oxides or metals(c) inorganic compounds of metallic and non

metallic elements(d) good conductors of electricity.

121. Which of the following is not ceramic?(a) alumina (b) BN and BaTiO3

(c) Indium tin oxide (d) Diamond.122. Composite materials are:

(a) made with strong bres embedded in weaker and softer matrix to obtain strength better than strength of matrix.

(b) made with strong bres embedded in weaker and softer matrix to obtain strength better than strength of both matrix and ller.

(c) made mainly to improve temperature resistance(d) used for improved optical properties.

123. Which of the following does not fall under categories of electronic material?(a) Intrinsic semiconductor like Si, Ge(b) non-linear conductor like ZnO(c) dielectric like A12O3, ZrO2(d) piezo electric material like zirconate, barium

titanale

124. The term

Average drift velocity at whichcharge carries move

Voltage ddifference between 2 pointsdistance separating them

is

called(a) mobility of carriers(b) electric eld intensity(c) electrical resistivity(d) current density

125. Semiconductors are materials having conductivity between(a) l0–12W cm–1 to 103 W cm–1

(b) 10–19 to l0–14

(c) 10–15 to 10–17

(d) none of above126. The structure of a polymer is shown below. This

polymer nds special application inF

C

F

F

C

F

(a) packaging (b) adhesives(c) bearings (d) fertilizers.

127. Consider the following statements:Addition of silicon to cast iron1. promotes graphite nodule formation.2. promotes graphite ake formation.3. increases the uidity of the molten metal.4. improves the ductility of cast iron.Of these statements(a) 1 and 4 are correct (b) 2 and 3 are correct(c) 1 and 3 are correct (d) 3 and 4 are correct


128. Killed steels(a) have minimum purity level(b) have almost zero percentage of phosphorus and

sulphur(c) are produced by LD process(d) are free from oxygen.

129. Carburized machine components have high endurance limit because carburization(a) raises the yield point of the material(b) produces a better surface nish(c) introduces a compressive layer on the surface(d) suppresses any stress concentration produced

in the component.130. Decreasing grain size in a polycryatalline material

(a) increases yield strength and corrosion(b) decreases yield strength and corrosion

resistance(c) decreases yield strength but increases corrosion

resistance(d) increases yield strength but decreases corrosion

resistance.131. Composites may be de ned as

(a) Combination of two or more materials to form a new material system with enhanced material properties.

(b) Artificially produced multiphase materials (reinforcement + matrix)

(c) A combination of 2 or more materials differing in form or composition on a macroscale. Constitutes do not dissolve or merge into each other, although they act in concert.

(d) all are true132. Factors in creating composites are

(a) Matrix material and reinforcement material(b) properties of new material controlled by

concentration of matrix and reinforcement material, their size, shape, distribution and orientation.

(c) all of above(d) none of above

For Questions 133 to 136, two Statements A and B are given. Put your answer as (a) if S-A is true and S-B is false. (b) if S-A is ase and S-B is true, (c) if both S-A and S-B are true and (d) if both S-A and S-B are false.133. S–‘A’ Thermoplastics have long chains of

polymerization which are cross linked.S–‘B’ Thermosets are amorphous and thermoplastic may be either amorphous or crystalline.

134. S–A Both Freukel’s defect and Schottky’s defect occur in ionic solids. While former is mainly interstitial type of defect, latter is mainly vacancy type of defectS–B In substitutional imperfection, a foreign atom

replaces/substitutes a parent atom at the lattice site in the crystal. In interstitial defect, an extra atom may be lodged with the crystal structure.

135. S–A Diamond is extremely hard due to strong interatomic covalent bonds between carbon atoms.S–B Graphite has excellent lubricating properties due to weak van der Walls type of bond between the layers of hexagonally arranged carbon atoms.

136. S–A Isostatic pressing is the process by which small particles of a material are bonded together by solid state diffusion.S–B In sintering process the ceramic powder is loaded into a exible, air tight container and hydraulic pressure is applied which compacts the powder uniformly in all directions.

137. Total area under the stress-strain curve is indication of(a) ductility of material (b) brittleness(c) toughness (d) impact strength

138. Which property of material is signified by the amount of energy absorbed by a material at the time of fracture under impact loading. (a) hardness (b) toughness(c) ductility (d) creep

139. The measure of the amount of plastic deformation a material can undergo under tensile forces without fracture signi es following property of material(a) brittleness (b) ductility(c) toughness (d) hardness

140. Wood is a natural composite consisting of which of the following?(a) Lignin bers in collagen matrix(b) Lignin bers in apatite matrix(c) Cellulose bers in apatite matrix(d) Cellulose bers in lignin matrix

141. Which one of the following is the correct ascending order of packing density for the given crystal structures of metals?(a) Simple cubic–Face central cubic–Body centred

cubic(b) Body centred cubic–Simple cubic–Face centred

cubic(c) Simple cubic–Body centred cubic–Face centred cubic(d) Body centred cubic–Face centred cubic–Simple

cubic

142. Vibration damping in machinery is best achieved by means of base structures made of which one of the following material?(a) Low carbon steel (b) Nodular iron(c) Grey cast iron (d) White cast iron


143. For a Rhombohedral space lattice, which one of the following is correct?(a) a = b = g = 90° (b) a = b = g ≠ 90°(c) a = g = 90° ≠ b (d) a ≠ b ≠ g ≠ 90°

144. Which of following is not correct statement?(a) Screw dislocation is a surface imperfection,

which separates crystals of different orientations in a polycrystalline aggregate

(b) Malleability is the property by which a metal or alloy can be plastically deformed by applying compressive stress

(c) Manganese and Molybdenum determine(s) the maximum attainable hardness in steel

(d) Coef cient of expansion is practically nil in invar.

145. Which of following statements is not correct?(a) Shear strength of ceramics is low.(b) Austenitic stainless steels are hardened and

strengthened by cold working and cannot be quenched and tempered.

(c) Lever Rule can be applied to determine relative amounts of phases present in an alloy at any temperature.

(d) The valence electron structures contribute to the primary bonding between the atoms to form aggregates.

146. Which one of the following is correct for ‘Climb’?(a) Dislocation moves parallel to the slip plane(b) Dislocation moves perpendicular to the slip

plane(c) Sliding of one plane of atoms over the other

plane.(d) Dislocation moves from a slip plane to another

slip plane.147. Movement of block of atoms along certain crystallo-

graphic plane/direction is called(a) slip (b) twinning(c) glide (d) climb

148. Which one of the following defects is ‘Schottky defect’?(a) Vacancy defect (b) Compositional defect(c) Interstitial defect (d) Surface defect

149. Which one of the following elements is an austenitic stabilizer?(a) Chromium (b) Tungsten(c) Nickel (d) Molybdenum

150. Which one of the following elements is a ferritic stabilizer?(a) Nickel (b) Manganese(c) Copper (d) Chromium

151. Which one of the following cast irons consists of carbon in rosette form?(a) White cast iron (b) Gray cast iron(c) Malleable cast iron (d) Nodular cast iron

152. Nano composite materials are highly preferable in design consideration for their(a) high resistance to crack propagation(b) vibration resistance(c) impact resistance(d) high resilience

153. Which of the following composites are ‘dispersion-strengthened composites’?(a) Particulate composites(b) Laminar composites(c) Fiber reinforced composites(d) Short- ber discontinuous composites

154. Why are Babbitt alloys used for bearing material?(a) They have excellent embeddability(b) They are relatively stronger than other bearing

materials(c) They do not lose strength with increase in

temperature.(d) They have high fatigue strength

155. The material property which depends only on the basic crystal structure is(a) fatigue strength (b) work hardening(c) fracture strength (d) elastic constant

156. Pick up wrong statement(a) Cast Iron has higher compressive strength

compared to that of steel.(b) Precipitation hardening of nonferrous alloys

involves solution heat treatment followed by precipitation heat treatment.

(c) The pattern known as Widmanstatten structure is encountered in tempering

(d) There must exist at least one double bond in the monomer for long chain polymerization

157. Match List–I (Fe–Fe3C Phase Diagram Charac-teristic) with List–II (Phase) and select the correct answer using the codes given below the Lists:

List–I

A. Alpha (a) ironB. Iron carbide having crystal latticeC. BCC pure allotrope of iron is stable between

1388°C and its melting point at 1535°CList–II

1. d iron2. Eutectic with 3 iron atoms and 1 carbon atom3. Ferrite stable upto 910°C4. Cementite

Codes:A B C

(a) 4 2 3(b) 3 4 1(c) 4 2 1(d) 3 1 2


158. Which one of the following pairs is not correctly matched?

Space lattice Relation between ‘r’ and atomic radius ‘r’ edge element ‘a’.

(a) Simple cubic : a2 = 4r2

structure(b) Body-centred : 3a2 = 16r2

cubic structure(c) Triclinic : 2a2 = 3r2

(d) Face-centred cubic structure : a2 = 8r2

159. What is the planar density of (100) plane in FCC (face-centred cubic) crystal with unit cell side ‘a’equal to?

(a) 1 4842

.a

(b) 22a

(c) 12a

(d) 22a

160. Pick up wrong statement(a) The notch sensitivity of cast iron component is

high(b) Cast iron does not have yield point.(c) Austenitic stainless steel contains 18%

chromium and 8% nickel. Since it retains its austenitic structure at room temperature, it is called austenitic stainless steel.

(d) Chromium present in the steel improves its corrosion resistance by forming a thin lm of chromium oxide on the surface.

161. Match List-I (Alloying element in steel) with List-II (Property conferred on steel by the element) and select the correct answer using the codes given below the Lists:

List-I List-IIA. Nickel 1. Corrosion resistanceB. Chromium 2. Magnetic permeabilityC. Tungsten 3. Heat resistanceD. Silicon 4. HardenabilityCodes:

A B C D(a) 4 1 3 2(b) 4 1 2 3(c) 1 4 3 2(d) 1 4 2 3

162. The alloying element in aluminium which increases uidity and strength is

(a) Silicon (b) Copper(c) Zinc (d) Tin.

163. Match List–I (Phase diagram) with List–II (Characteristic) and select the correct answer using the codes given below the lists:

List–I List–IIA. Isomorphous system1. One liquid decomposes

into another liquid and solid

B. Eutectic system 2. One liquid and another solid combine to form a new solid

C. Peritectic system 3. Two metals are comp-letely soluble in liquid state and completely insoluble in solid state

D. Monotectic system 4. Two metals, soluble in solid and liquid state

Codes:A B C D

(a) 2 3 4 1(b) 4 1 2 3(c) 2 1 4 3(d) 4 3 2 1

164. Strength of a material can be de ned as resistance offered to(a) indentation (b) external force(c) impact forces(d) sudden application of impact forces

165. Ductility of a material can be de ned as the (a) resistance offered to compressive load(b) ratio of elongation of the material at fracture

during the tensile test to the original length(c) resistance offered to shear forces(d) resistance offered to sudden application of

impact forces.166. Fine grain size during the solidi cation of a metal

is achieved by(a) lower nucleation rate(b) higher nucleation rate with lower growth rate(c) higher nucleation rate with higher growth rate(d) lower growth rate

167. Ferrite is the (a) ductile iron (b) wrought iron(c) pure iron with very low carbon(d) inter-metallic compound iron carbide

168. Grey cast iron is(a) iron with gray colour(b) all the carbon is in combined form(c) iron where part of the carbon is in graphite form(d) graphite present is in spherical form

169. Given below are two statementsStatement ‘A’ Chromium as an alloying element in alloy steels is used principally to improve the corrosion and oxidation resistanceStatement ‘B’ Tungsten as an alloying element in alloy steels is used principally to improve mechanical properties at elevated temperaturesIn above question, state whether(a) Statement A is right (b) Statement B is right(c) Both statements are O.K.(d) none of statements is true


170. Nitriding process is used to increase surface hardness for(a) low and medium carbon steels(b) alloy steels(c) high-carbon steels(d) all of above

171. Which pair is not correctly matched?(a) Alloy of copper and zinc – brass(b) Alloy of copper with all elements except zinc –

bronze(c) Alloy of Mn and Mg with aluminium – Y-alloy(d) Alloy of 70% of lead and 30% tin – soft solder

172. Which one of the following sets of constituents is expected in equilibrium cooling of a hypereutectoid steel from austenitic state?(a) Ferrite and pearlite(b) Cementite and pearlite(c) Ferrite and bainite(d) Cementite and austenite

173. Match List-I (Alloy) with List-II (Use) and select the correct answer using the codes give below the lists:

List-I List-IIA. Low carbon steel 1. BearingB. Had eld manganese steel 2. ThermocoupleC. Constantan 3. Wire nailsD. Babbitt alloy 4. Bulldozer blades

Codes:A B C D

(a) 1 2 3 4(b) 3 4 1 2(c) 3 2 1 4(d) 3 4 2 1

174. Addition of magnesium to cast iron increases it.(a) Hardness(b) Ductility and strength in tension(c) Corrosion resistance(d) Creep strength.

175. Tin base white metals are used where the bearings are subjected to(a) Large surface wear(b) Elevated temperature(c) Vibration and shock load(d) high pressure and load,

176. Which is not correctly matched(a) Reduced oxidation resistance — Si(b) Improves mechinability — P(c) Forms abrasion resisting particles — Mo(d) Contributes to red hardness — Co

177. Extrusion nozzle is manufactured from(a) while C.I. (b) nodular C.I.(c) malleable C.I. (d) grey C.I.

178. In which process, two or more chemically different manomers are polymerised to form a crosslink polymer with a by product like water/ammonia?(a) condensation polymerisation(b) linear polymerisation(c) addition polymerisation(d) co-polymerisation

179. Structure of a polymer is(a) cubic (b) rhombic(c) long chain(d) closed packed hexagonal.

180. Which is not a ceramic?(a) pyrosil (b) porcelain(c) alumina (d) whisker

181. Phenol formaldehyde is(a) thermoplastic polymer (b) elastomer(c) rubber (d) thermoset polymer

182. Examine 3 statements below and answerS.A. The elutectoid steel contains 0.80% carbon and elutectic steel contain 4.3% carbon.S.B. Eutectoid transformation (gamma iron to alpha iron and Fe3C) takes place at 723°CS.C. Eutectic transformation (liquid iron to gamma iron and Fe3C) takes place at 1130°C

(a) Statements S-A and S-B are correct(b) S-A and S-C are correct(c) S-B and S-C are correct(d) All are correct

183. Consider the three statements below and select correct choice.S-A On reduction of free energy of a parent phase, a driving force leads to crystallisationS-B During nucleation, the process of formation of the rst stable tiny particles starts.S-C After onset of nucleation, the size of particles increases and this process is called grain growth(a) S-A and S-B are correct(b) S-B and S-C are correct(c) S-A and S-C are correct(d) S-A, S-B and S-C are correct

184. Consider following statementsS.A. Maximum amount of carbon that can be alloyed with iron is 6.67%S.B. Alloys upto 0.8% carbon are termed steelsS.C. Alloys from 2% to 6.67% carbon are called cast irons.(a) S-A and S-B are correct(b) S-B and S-C are correct(c) S-A and S-C are correct(d) All are correct.


185 Which pair is not correctly matched about strengthening mechanism of metals?(a) Alloys are always stronger than pure metals

– Alloying.(b) Strength of ne grain structure material is

more than of coarse grain structured material – grain re nement.

(c) Bauschinger effect – work hardening(d) A mixture of liquid of solid convert to solid

phase on cooling – Eutectoid reaction186. A material is known as allotropic or polytropic if

it(a) has a xed structure under all conditions(b) exists in several crystal forms at different

temperatures(c) responds to heat treatment(d) has its atoms distributed in a random pattern.

187. A reversible change in the atomic structure of steel with corresponding change in the properties is known as (a) atomic change (b) allotropic change(c) solidus change (c) molecular change

188. In grey cast iron, carbon is present in the form of (a) free carbon (b) cementite(c) spheroids (d) akes

189. Iron is(a) paramagnetic (b) ferromagnetic(c) ferroelectric (d) dielectric

190. Diamagnetic materials(a) are non-magnetic(b) can be magnetised in one direction only(c) are magnetised in direction opposite to that

of applied load(d) can be magnetised in any direction

191. The imperfection in the crystal structure of a metal is called (a) dislocation (b) void(c) impurity (d) defect

192. Satellite is a non-ferrous cast alloy containing (a) cobalt, chromium and tungsten(b) tungsten, chromium and vanadium(c) tungsten, molybdenum and cobalt(d) molybdenum, vanadium and cobalt

193. A plane having intercepts x = 3, y = 2 and z = 1, has Miller indices(a) (3 2 1) (b) ( / / )3 14 2 14 1 14(c) (⅓ –½ 1) (d) [2 3 6]

194. Dielectric strength of a material is the (a) capacity to withstand biaxial stresses(b) magnetic property(c) capacity to withstand high voltage(d) capacity to resist ow of current.

195. For molybdenum, which has a BCC lattice structure, the number of atoms per unit cells is(a) 1 (b) 2(c) 4 (d) 6

196. The Miller indices of the diagonal plane of a cube are(a) [110] (b) [111](c) [100] (d) [000]

197. The temperature at which a ferromagnetic material becomes paramagnetic is called(a) critical temperature(b) inversion temperature(c) Curie temperature(d) Debye temperature

198. The ‘tunnel effect refers to the (a) migration of conducting electrons in a magnetic

eld(b) use of lasers in surveying(c) ability of electrons to exist on both sides of a

large energy barrier(d) existence of cathode rays in a vaccum

199. Which of the following is a copper free alloy?(a) brass (b) phosphor bronze(c) invar (d) muntz metal

200. Metal with HCP structure is(a) silver (b) iron(c) magnesium (d) aluminium

201. A plane intersects the coordinate axes at x y z= = =2

313

12

, , . What is the Miller index of

this plane?(a) [931] (b) [432](c) [423] (d) [364]

202. The lattice parameter of gold (FCC lattice) having atomic radius of 0.144 nm is(a) 0.14 nm (b) 0.407 nm(c) 0.333 nm (d) 0.576 nm

ANSWERS1. (b) 2. (a) 3. (c) 4. (d) 5. (b) 6. (d)7. (c) 8. (a) 9. (c) 10. (b) 11. (b) 12. (a)

13. (a) 14. (a) 15. (d) 16. (b) 17. (a) 18. (d)19. (a) 20. (b) 21. (d) 22. (c) 23. (c) 24. (a)25. (d) 26. (c) 27. (d) 28. (d) 29. (a) 30. (a)31. (b) 32. (c) 33. (d) 34. (b) 35. (a) 36. (a)37. (b) 38. (a) 39. (c) 40. (a) 41. (b) 42. (c)43. (b) 44. (c) 45. (a) 46. (a) 47. (a) 48. (a)49. (b) 50. (d) 51. (c) 52. (c) 53. (b) 54. (a)55. (c) 56. (a) 57. (b) 58. (b) 59. (d) 60. (b)61. (a) 62. (b) 63. (c) 64. (d) 65. (b) 66. (b)67. (c) 68. (d) 69. (d) 70. (a) 71. (d) 72. (d)73. (a) 74. (a) 75. (d) 76. (c) 77. (b) 78. (d)79. (c) 80. (a) 81. (c) 82. (d) 83. (c) 84. (c)85. (d) 86. (b) 87. (b) 88. (c) 89. (a) 90. (c)91. (c) 92. (d) 93. (d) 94. (a) 95. (a) 96. (d)97. (c) 98. (d) 99. (b) 100. (d) 101. (d) 102. (d)

103. (a) 104. (a) 105. (c) 106. (d) 107. (d) 108. (a)109. (a) 110. (b) 111. (d) 112. (b) 113. (a) 114. (a)115. (d) 116. (d) 117. (d) 118. (c) 119. (a) 120. (c)121. (c) 122. (a) 123. (d) 124. (a) 125. (a) 126. (c)


127. (b) 128. (d) 129. (c) 130. (d) 131. (d) 132. (c)133. (b) 134. (c) 135. (c) 136. (d) 137. (c) 138. (b)139. (c) 140. (d) 141. (c) 142. (c) 143. (b) 144. (a)145. (a) 146. (b) 147. (a) 148. (a) 149. (c) 150. (d)151. (d) 152. (b) 153. (a) 154. (a) 155. (c) 156. (d)157. (b) 158. (c) 159. (b) 160. (a) 161. (a) 162. (a)163. (d) 164. (b) 165. (b) 166. (b) 167. (c) 168. (c)169. (c) 170. (b) 171. (d) 172. (b) 173. (d) 174. (b)175. (a) 176. (a) 177. (a) 178. (a) 179. (c) 180. (a)181. (d) 182. (d) 183. (d) 184. (c) 185. (d) 186. (b)187. (b) 188. (d) 189. (b) 190. (c) 191. (a) 192. (a)193. (d) 194. (c) 195. (b) 196. (a) 197. (c) 198. (c)199. (c) 200. (c) 201. (d) 202. (b)

MATCH THE TWO PART QUESTIONS1. Match Part-A with Part-B

Part A Part BHardness test Uses indenter

1. Brinell hardness test (a) s p h e r o - c o n i c a l diamond of 120° angle

2. Vicker’s hardness test (b) sphere made of steel or carbide

3. Rockwell (c) square-base pyramid diamond

Ans. 1—b, 2—c, 3—a2. Match List-A (Name of Material) with List-B

(% Carbon Range)List-A List-B

1. Hypo-eutectoid steel (a) 4.3 – 6.672. Hyper-eutectoid steel (b) 2.0 – 4.33. Hypo-eutectic cast iron (c) 0.8 – 2.04. Hyper-eutectic cast iron (d) 0.008 – 0.8

Ans. 1—d, 2—c, 3—b, 4—a3. Match parts A and B

Part A Part B1. Fibre reinforced plactics (a) automobile tyre2. acrylics (b) aircraft3. phenolics (c) lenses4. Styrene butadine rubber (d) electric switch

coverAns. 1—b, 2—c, 3—d, 4—a

4. Match parts A and B relating to point defects in metals.

Part APoint Defects

Part BName of Defect

1. (a) Substitutional defect

2. Interstitial

void

(b) Interstitial defect

3. (c) Frenkel defect

4. (d) Vacancy defect

Ans. 1—d, 2—b, 3—a, 4—c5. Match parts A and B.

Part A Part B1. Strong electrostatic

attraction between cation and anion resulting in permanent bond

(a) Covalent bond

2. Sharing of one or more electrons from the adjacent atoms

(b) van der Waal bond

3. Materials with one, two or three valency electrons and the electrons not bound to any particular atom in the solid and drift through out the entire metal

(c) Metallic bond

4. When atoms are brought close to each other there is a separation of +ve and –ve charges and a weak attractive force develops

(d) Ionic bond

Ans. 1—d, 2—a, 3—c, 4—b

UNSOLVED QUESTIONS1. Describe how separate grains are formed when

a metal is solidi ed. What two characteristics of metal determine the number of grains that will form? Explain how columnar growth and dendritic segregation can occur?

2. How the impurities in the metals affect their properties? How the various types of grain structure and the characteristics such as cold work, slip, strain hardening, recrystallisation, annealing, grain growth, hot working, fracture and creep affect the behaviour of the impure metals (pure metals containing small percentage of impurities)?

3. What are various allotropic forms of iron? What takes place in iron when it changes from one allotropic form to another? How can this allotropic change be produced?

4. What is recrystallisation?5. What are the methods of re ning and forming

metals from their ores?6. What is an alloy and how the alloying elements

arrange themselves with the base metal in an alloy?7. What is the difference between a eutectic point and

a eutectoid point?


8. Explain coring and why it is objectionable and how it can be eliminated?

9. What do you understand by grain growth? How it can be overcome by alloying?

10. Why does a metal with a large grain size form more easily than that with a small grain size? What are the disadvantages of a large grain size?

11. What do you understand by the terms ferrite, austenite, cementite, pearlite, martensite, ledeburite and how are these obtained?

12. Describe precipitation hardening process.13. Carbon appears in different forms in various kinds

of cast iron. What are the various forms and how can they be produced as desired?

14. What is the difference between rimmed ingot and killed ingot? What products might be made from each?

15. Which element produces air-hardening and what is the advantage of this property?

16. Why ferritic stainless, steels, and austenitic stainless steels do not respond to quench-hardening? What are these steels awkward to machining?

17. How does carbon effect hardenability, strength, ductility, machinability and weldability of steel?

18. How does the addition of following elements to steel effect its properties : molybdenum, silicon, chromium, nickel, tungesten?

19. Though alloy steels possess so many advantages, why is it so that all steels are not alloyed?

20. What kind of cast iron would be used for (i) wear resistance, (ii) impact resistance, (iii) corrosion resistance, (iv) good machinability, (v) high hardness?

21. What is the difference between acid and basic furnace? What impurities are removed in the basic open hearth furnace, and how does the acid open hearth compare in re ning ability?

22. What are the methods of making wrought iron? When is wrought iron used instead of steel, even though steel is often less expensive?

23. There are hundreds of varieties of plain carbon and alloy steels used commercially. The various grades are usually distinguished by chemical and physical speci cations. What is the method followed by Indian Standards Institution to designate various types of steels?

24. What is the difference between grey cast iron, white or chilled cast iron, malleable cast iron, and alloyed cast iron? How each is produced and what are the applications of each?

25. Which stainless steel would you recommend for (a) very high strength, (b) hardness and strength, (c) good weldability, (d) good forming qualities, (e) corrosive environment, (f) good machinability.

26. How do copper, aluminium, magnesium, zinc and their alloys compare with plain low-carbon steel in respect of the following properties :(i) magnetic properties, (ii) electrical and thermal conductivity, (iii) melting point, (iv) tensile strength, (v) modulus of elasticity, (vi) cost of metal?

27. Why composite materials are able to obtain very high strength even at very high temperatures?

28. What characteristic properties of the non-ferrous metals dictate and justify their use inspite of their high cost?

29. What are the three grades of commercial copper and how do they differ in composition? Which of these grades will you use when strength is the criterion and a cold drawn wire is to be (i) soldered, (ii) gas welded?

30. What are the various types of brasses? How are these corroded? What are the two characteristic kinds of corrosion and what types of brasses are most susceptible to each?

31. Which brasses are best adapted to cold working and which to hot working? Under what conditions can cartridge brass containing 30% zinc be both hot and cold worked successfully?

32. What characteristics of aluminium make it resistant to corrosion? How does anodisation increase the corrosion resistance of aluminium? Do soldered aluminium joints have good resistance to corrosion?

33. What are the advantages and disadvantages of zinc alloy die castings?

34. Many nickel alloys have been developed to satisfy at least one speci c requirement? What are these requirements?

35. What special precautions should be observed in designing magnesium castings and what special practices must be used in foundries?

36. Two alloying elements are commonly mixed with aluminium, what are these and how do they affect the properties of aluminium?

37. What are the industrial uses of tin, lead and their alloys? What are the non-precipitation and precipitation hardening aluminium alloys?

38. Name three good metallic conductors of elasticity and three relatively poor. What non-metallic conductors are important in engineering?

39. Explain “skin effect” and “proximity effect”, and describe how they affect the resistance of a conductor?

40. De ne dielectric strength and dielectric constant.41. Explain the dangers which may be encountered

when two materials of different dielectric constant are used in series to form an insulator?

42. How do diamagnetic, paramagnetic and ferro-magnetic materials react when exposed to a magnetic eld?


43. What are the important characteristics of electromagnet materials? What are electromagnet materials?

44. Name three characteristics of a metal surface which might cause corrosion to be localised.

45. Which of the chromium steels is most used for (i) a hard, wear resistant surface, (ii) high temperature oxidation, (iii) maximum corrosion resistance?

46. What three general methods of approach may be employed to lengthen the life or reduce the corrosion of a metal part?

47. Describe how separate grains are formed when a metal is solidi ed. What two characteristics of metal determine the number of grains that will form? Explain how columnar growth and dendritic segregation can occur?

48. What are various allotropic forms of iron? What takes place in iron when it changes from one allotropic form to another? How can this allotropic change be produced?

49. What is recrystallisation?50. What are the methods of re ning and forming

metals from their ores?51. What is an alloy and how the alloying elements

arrange themselves with the base metal in an alloy?52. What is the difference between a eutectic point and

a eutectoid point?53. Explain coring and why it is objectionable and how

it can be eliminated?54. What do you understand by grain growth? How it

can be overcome by alloying?55. Why does a metal with a large grain size form more

easily than that with a small grain size? What are the disadvantages of a large grain size?

56. What do you understand by the terms ferrite, austenite, cementite, pearlite, martensite, ledeburite and how are these obtained?

57. Describe precipitation hardening process.58. Carbon appears in different forms in various kinds

of cast iron. What are the various forms and how can they be produced as desired?

59. What is the difference between rimmed ingot and killed ingot? what products might be made from each?

60. Which element produces air-hardening and what is the advantage of this property?

61. Why ferritic stainless, steels, and austenitic stainless steels do not respond to quench-hardening? What are these steels awkward to machining?

62. How does carbon effect hardenability, strength, ductility, machinability and weldability of steel?

63. How does the addition of following elements to steel effect its properties : molybdenum, silicon, chromium, nickel, tungesten?

64. Though alloy steels possess so many advantages, why is it so that all steels are not alloyed?

65. What kind of cast iron would be used for (i) wear resistance, (ii) impact resistance, (iii) corrosion resistance, (iv) good machinability, (v) high hardness?

66. What is the difference between acid and basic furnace? What impurities are removed in the basic open hearth furnace, and how does the acid open hearth compare in re ning ability?

67. What are the methods of making wrought iron? When is wrought iron used instead of steel, even though steel is often less expensive?

68. There are hundreds of varieties of plain carbon and alloy steels used commercially. The various grades are usually distinguished by chemical and physical speci cations. What is the method followed by Indian Standards Institution to designate various types of steels?

69. What is the difference between grey cast iron, white or chilled cast iron, malleable cast iron, and alloyed cast iron? How each is produced and what are the applications of each?

70. Which stainless steel would you recommend for (a) very high strength, (b) Hardness and strength, (c) good weldability, (d) good forming qualities, (e) corrosive environment, (f) good machinability.

71. How do copper, aluminium, magnesium, zinc and their alloys compare with plain low-carbon steel in respect of the following properties :(i) magnetic properties, (ii) electrical and thermal conductivity, (iii) melting point, (iv) tensile strength, (v) modulus of elasticity, (vi) cost of metal?

72. Why composite materials are able to obtain very high strength even at very high temperatures.

73. What characteristic properties of the non-ferrous metals dictate and justify their use inspite of their high cost?

74. What are the three grades of commercial copper and how do they differ in composition? Which of these grades will you use when strength is the criterion and a cold drawn wire is to be (i) soldered, (ii) gas welded?

75. What are the various types of brasses? How are these corroded? What are the two characteristic kinds of corrosion and what types of brasses are most susceptible to each?

76. Which brasses are best adapted to cold working and which to hot working? Under what conditions can cartridge brass containing 30% zinc be both hot and cold worked successfully?

77. What characteristics of aluminium make it resistant to corrosion? How does anodisation increase the corrosion resistance of aluminium? Do soldered aluminium joints have good resistance to corrosion?

78. What are the advantages and disadvantages of zinc alloy die castings?

79. Many nickel alloys have been developed to satisfy at least one speci c requirement? What are these requirements?


80. What special precautions should be observed in designing magnesium castings and what special practices must be used in foundries?

81. Two alloying elements are commonly mixed with aluminium, what are these and how do they affect the properties of aluminium?

82. What are the industrial uses of tin, lead and their alloys? What are the non-precipitation and precipitation hardening aluminium alloys?

83. Name three good metallic conductors of elasticity and three relatively poor. What non-metallic conductors are important in engineering?

84. Explain “skin effect” and “proximity effect”, and describe how they affect the resistance of a conductor?

85. De ne dielectric strength and dielectric constant.

86. Explain the dangers which may be encountered when two materials of different dielectric constant are used in series to form an insulator?

87. How do diamagnetic , paramagnetic and ferromagnetic materials react when exposed to a magnetic eld.

88. What are the important characteristics of electromagnet materials? What are electromagnet materials ?

89. Name three characteristics of a metal surface which might cause corrosion to be localised.

90. Which of the chromium steels is most used for (i) a hard, wear resistant surface, (ii) high temperature oxidation, (iii) maximum corrosion resistance.

91. What three general methods of approach may be employed to lengthen the life or reduce the corrosion of a metal part?

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