basics of material science

7/28/2019 Basics of Material Science

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Introduction

Historical Perspective

Materials are so important in the development of civilization that we associate ages with

them. In the origin of human life on earth, the Stone Age, people used only naturalmaterials, like stone, clay, skins, and wood. When people found copper and how to make

it harder by alloying, the Bronze Age started about 3000 BC. The use of iron and steel, a

stronger material that gave advantage in wars started at about 1200 BC. The next big stepwas the discovery of a cheap process to make steel around 1850, which enabled the

railroads and the building of the modern infrastructure of the industrial world.

Materials Science and Engineering

Understanding of how materials behave like they do, and why they differ in properties

was only possible with the atomistic understanding allowed by quantum mechanics, that

first explained atoms and then solids starting in the 1930s. The combination of physics,chemistry, and the focus on the relationship between the properties of a material and its

microstructure is the domain of Materials Science. The development of this science

allowed designing materials and provided a knowledge base for the engineering

applications (Materials Engineering).


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Why Study Materials Science and Engineering?

To be able to select a material for a given use based on considerations of cost

and performance.

To understand the limits of materials and the change of their properties with use.

To be able to create a new material that will have some desirable properties.

All engineering disciplines need to know about materials. Even the most "immaterial",

like software or system engineering depend on the development of new materials, whichin turn alter the economics, like software-hardware trade-offs. Increasing applications of

system engineering are in materials manufacturing (industrial engineering) and complex

environmental systems.

Classification of Materials

Like many other things, materials are classified in groups, so that our brain can handle

the complexity. One could classify them according to structure, or properties, or use. Theone that we will use is according to the way the atoms are bound together:

Metals: valence electrons are detached from atoms, and spread in an 'electron sea' that"glues" the ions together. Metals are usually strong, conduct electricity and heat well and

are opaque to light (shiny if polished). Examples: aluminum, steel, brass, gold.Mixtures of metals are called ALLOYS.

Semiconductors: the bonding is covalent (electrons are shared between atoms). Theirelectrical properties depend extremely strongly on minute proportions of contaminants.

They are opaque to visible light but transparent to the infrared. Examples: Si, Ge, GaAs.

Ceramics: atoms behave mostly like either positive or negative ions, and are bound byCoulomb forces between them. They are usually combinations of metals orsemiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Non-

Metallic, inorganic materials. Originally considered to be oxides; from the Greek

Keramos for burnt earth, traditional ceramics were clays.

Modern ceramics can be carbides, borides etc., or can be based on non-metallic elementssuch as carbon. Certain ceramics can be amorphous/non-crystalline; they are termed

glasses

Examples: glass, porcelain, many minerals.

Polymers: From the Greek, poly- many; meros- part. Polymers are organic materialsconsisting of chains of molecules containing carbon. Polymers are bound by covalentforces and also by weak van der Waals forces, and usually based on H, C and other non-

metallic elements. They decompose at moderate temperatures (100 400 C), and are

lightweight. Other properties vary greatly.Examples: plastics (nylon, teflon, polyester) and rubber.


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Other categories are not based on bonding. A particular microstructure identifiescomposites,made of different materials in intimate contact (example: fiberglass,concrete, wood) to achieve specific properties. Biomaterialscan be any type of materialthat is biocompatible and used, for instance, to replace human body parts.

Advanced Materials

Materials used in "High-Tec" applications, usually designed for maximum performance,

and normally expensive. Examples are titanium alloys for supersonic airplanes, magneticalloys for computer disks, special ceramics for the heat shield of the space shuttle, etc.

Modern Material's Needs

Engine efficiency increases at high temperatures: requires high temperaturestructural materials

Use of nuclear energy requires solving problem with residues, or advances innuclear waste processing.

Hypersonic flight requires materials that are light, strong and resist hightemperatures.

Optical communications require optical fibers that absorb light negligibly. Civil construction materials for unbreakable windows. Structures: materials that are strong like metals and resist corrosion like plastics.


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Mechanical properties of material

Elasticity is the property of deformed solid to come to the original dimension uponunloading.

Stress = Load/Area (P/A)Strain = L / LYoungs modulus of Elasticity (E) = Stress/Strain

Stiffness is directly proportional to E

Plasticity is the property of a material by virtue of which it may be permanentlydeformed when it has been subjected to force.

Toughness is the ability of material to absorb energy during plastic deformation up tofracture i.e., ability to withstand shock load

Resilience is capacity of material to absorb energy when it is elastically deformed andthen upon unloading, to have this energy recovered.

Modulus of resilienceis the strain energy per unit volume required to stress amaterial from an unloaded state up to the point of yielding.

Tensile strength(Kg/cm2) is the ratio of maximum load to original cross sectional area

Yield strength is the point where stretch or elongation takes place suddenly withoutincrease in stress.

Impact strength is the capacity of material to resist energy before it fracture. It takes intoconsideration both toughness and strength.

Ductility is the capacity of material to undergo deformation under tension with rupturesEg., wire drawing

Malleability is the capacity of material to withstand deformation under compressionwithout rupture. Eg., Forging and rolling

Both ductility and malleability are the elastic property of the material.

Brittleness is the tendency to fracture without appreciable deformation.

Hardness is the resistance of material to plastic deformation by indentation


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Micro structure of materials

Introduction:The majority of engineering materials, many ceramics, plastics and all metals are

crystalline in structure. The type of their crystal structure bears significantly on the

physical properties of these materials.

Crystal structure:A regular and repetitious pattern in which atoms or group of atoms (i.e

molecules) of a crystalline material arrange themselves is known as acrystal structure.

In other words, a crystal structure is one in which a unit of structure called the

unit cell repeats at regular intervals in three dimension.

Space lattice:The locations of atoms and their particular arrangement in a given crystal are

described by means of a space lattice.It can be considered as an infinite array of points in space, so arranged that it

divides space into equal volumes with no space excluded.

Crystal systems:One of seven categories (cubic, hexagonal, tetragonal, trigonal, orthorhombic,

monoclinic, and triclinic) into which a crystal may be classified according to the shape of

the unit cell of its Bravais lattice, or according to the dominant symmetry elements of itscrystal class.

Cubic Structure:Three equal axes are at right angles.

Tetragonal Structure:Three axes are at right angles, two of these axes are equal while third

one is different

Orthorhombic Structure:Three unequal axes are at right angles.

Rhombohedral Structure:Threeequal axes are equally included but at an angle other than a rightangle.


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Hexagonal Structure:Three equal axes are in one plane at 120 to each other, and a fourth

axis normal to this plane. The interval along the fourth axis is unique.

Monoclinic Structure:There are three unequal axes. One of these axes is at right angles to the

other two axes, but the other two axes are not at right angles.

Triclinic Structure:

Three unequal axes are unequally inclined and none being at rightangles.

Crystal structures Metallic Elements

Body Centered Cubic Structure (BCC)In this type of crystal structure, the unit cell has one atom

at each corner of the cube and one at body centre of thecube.

Face centered cubic Structure (FCC)The unit cell has an atom at each corner of the cube and in

addition has one atom at the centre of each face.

Hexagonal close packed structure (HCP)The unit cell has one atom at each of twelve corners of the

hexagonal prism, one atom at the centre of the two hexagonal

faces and three atoms symmetrically arranged in the body ofthe cell.


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Miller IndicesThe Simple and universal notation for the designation of crystallographic planes anddirections of crystal has been names as miller indices.

Rules for Miller Indices:

Determine the intercepts of the face along the crystallographic axes, in terms of unit celldimensions.

Take the reciprocals Clear fractions Reduce to lowest terms

For example, if the x-, y-, and z- intercepts are 2, 1, and 3, the Miller indices arecalculated as:

Take reciprocals: 1/2, 1/1, 1/3 Clear fractions (multiply by 6): 3, 6, 2 Reduce to lowest terms (already there)

Thus, the Miller indices are 3,6,2. If a plane is parallel to an axis, its intercept is atinfinity and its Miller index is zero. A generic Miller index is denoted by (hkl).

If a plane has negative intercept, the negative number is denoted by a bar above the

number. Never alter negative numbers. For example, do not divide -1, -1, -1 by -1 to get1,1,1. This implies symmetry that the crystal may not have!

Some General Principles If a Miller index is zero, the plane is parallel to that axis. The smaller a Miller index, the more nearly parallel the plane is to the axis. The larger a Miller index, the more nearly perpendicular a plane is to that axis. Multiplying or dividing a Miller index by a constant has no effect on the

orientation of the plane Miller indices are almost always small.

Why Miller Indices? Using reciprocals spares us the complication of infinite intercepts. Formulas involving Miller indices are very similar to related formulas from

analytical geometry. Specifying dimensions in unit cell terms means that the same label can be applied

to any face with a similar stacking pattern, regardless of the crystal class of the

crystal. Face 111 always steps the same way regardless of crystal system.


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Deformation and Tensile properties

Elastic deformationWhen the stress is removed, the material returns to the dimension it had before the load

was applied. Valid for small strains (except the case of rubbers).

Deformation is reversible, non permanent

Plastic deformationWhen the stress is removed, the material does not return to its previous dimension butthere is a permanent, irreversible deformation.

In tensile tests, if the deformation is elastic, the stress-strain relationship is called Hooke'slaw:

=E That is, E is the slope of the stress-strain curve. E isYoung's modulusormodulus ofelasticity.

Tension Test and Stress-Strain diagram


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Tensile Properties

Yield point. If the stress is too large, the strain deviates from being proportional to the

stress. The point at which this happens is the yield pointbecause there the material yields,deforming permanently (plastically).

Yield stress. Hooke's law is not valid beyond the yield point. The stress at the yield pointis called yield stress, and is an important measure of the mechanical properties ofmaterials. In practice, the yield stress is chosen as that causing a permanent strain of

0.002

The yield stress measures the resistance to plastic deformation.

The reason for plastic deformation, in normal materials, is not that the atomic bond is

stretched beyond repair, but the motion of dislocations, which involves breaking andreforming bonds.

Plastic deformation is caused by the motion of dislocations.


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Tensile strength. When stress continues in the plastic regime, the stress-strain passesthrough a maximum, called the tensile strength (TS) , and then falls as the material startsto develop a neckand it finally breaks at the fracture point .

For structural applications, the yield stress is usually a more important property than the

tensile strength, since once the it is passed, the structure has deformed beyond acceptablelimits.

Ductility. The ability to deform before braking. It is the opposite ofbrittleness. Ductilitycan be given either as percent maximum elongation max or maximum area reduction.%EL =maxx 100 %%AR =(A0- Af)/A0These are measured after fracture (repositioning the two pieces back together).

Resilience.Capacity to absorb energy elastically. The energy per unit volume is the areaunder the strain-stress curve in the elastic region.

Toughness. Ability to absorb energy up to fracture. The energy per unit volume is thetotal area under the strain-stress curve. It is measured by an impact test.


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Dislocations and Strengthening Mechanisms

Basic Concept of dislocation

Dislocations can be edge dislocations, screw dislocationsand exist in combination of thetwo. Their motion (slip) occurs by sequential bond breaking and bond reforming. The

number of dislocations per unit volume is the dislocation density, in a plane they aremeasured per unit area.

Characteristics of DislocationsThere is strain around a dislocation which influences how they interact with other

dislocations, impurities, etc. There is compressionnear the extra plane (higher atomicdensity) and tension following the dislocation line.

Dislocations interact among themselves. When they are in the same plane, they repel if

they have the same sign and annihilate if they have opposite signs (leaving behind aperfect crystal). In general, when dislocations are close and their strain fields add to a

larger value, they repel, because being close increases the potential energy (it takes

energy to strain a region of the material).

The number of dislocations increases dramatically during plastic deformation.

Dislocations spawn from existing dislocations, and from defects, grain boundaries and

surface irregularities.

Plastic DeformationSlip directions vary from crystal to crystal. When plastic deformation occurs in a grain, it

will be constrained by its neighbors, which may be less favorably oriented. As a result,polycrystalline metals are stronger than single crystals (the exception is the perfectsingle crystal, as in whiskers.)

Mechanisms of Strengthening in Metals

General principles. Ability to deform plastically depends on ability of dislocations to

move. Strengthening consists in hindering dislocation motion. We discuss the methods of

grain-size reduction, solid-solution alloying and strain hardening. These are forsinglephase metals. We discuss others when treating alloys. Ordinarily, strengthening

reduces ductility.

Strengthening by Grain Size ReductionThis is based on the fact that it is difficult for a dislocation to pass into another grain,

especially if it is very misaligned. Atomic disorder at the boundary causes discontinuity

in slip planes. For high-angle grain boundaries, stress at end of slip plane may triggernew dislocations in adjacent grains. Small angle grain boundaries are not effective in

blocking dislocations.


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The finer the grains, the larger the area of grain boundaries that impedes dislocationmotion. Grain-size reduction usually improves toughness as well. Grain size can becontrolled by the rate of solidification and by plastic deformation.

Solid-Solution Strengthening

Adding another element that goes into interstitial or substitutional positions in a solutionincreases strength. The impurity atoms cause lattice strain which can "anchor"

dislocations. This occurs when the strain caused by the alloying element compensates

that of the dislocation, thus achieving a state of low potential energy. It costs strainenergy for the dislocation to move away from this state (which is like a potential well).

The scarcity of energy at low temperatures is why slip is hindered.

Pure metals are almost always softer than their alloys.

Strain HardeningDuctile metals become stronger when they are deformed plastically at temperatures wellbelow the melting point (cold working). (This is different from hot working is the

shaping of materials at high temperatures where large deformation is possible.) Strainhardening (work hardening) is the reason for the elastic recovery. The reason for strain

hardening is that the dislocation density increases with plastic deformation (cold work)due to multiplication. The average distance between dislocations then decreases and

dislocations start blocking the motion of each one.

The measure of strain hardening is the percent cold work (%CW), given by the relativereduction of the original area, A0 to the final value Ad :

%CW =100 (A0Ad)/A0


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Recovery, recrystallization and Grain Growth

Plastic deformation causes 1) change in grain size, 2) strain hardening, 3) increase in the

dislocation density. Restoration to the state before cold-work is done by heating through

two processes: recovery and recrystallization. These may be followed by grain growth.

RecoveryHeatingincreased diffusionenhanced dislocation motionrelieves internal strainenergy and reduces the number of dislocation. The electrical and thermal conductivity arerestored to the values existing before cold working.

RecrystallizationStrained grains of cold-worked metal are replaced, upon heating, by more regularly

spaced grains. This occurs through short-range diffusion enabled by the high temperature.

Since recrystallization occurs by diffusion, the important parameters are both temperatureand time.

Recrystallization temperature: is that at which the process is complete in one hour. It istypically 1/3 to 1/2 of the melting temperature. It falls as the %CW is increased. Below a"critical deformation", recrystallization does not occur.

Grain GrowthThe growth of grain size with temperature can occur in all polycrystalline materials. It

occurs by migration of atoms at grain boundaries by diffusion, thus grain growth is faster

at higher temperatures. The "driving force" is the reduction of energy, which isproportional to the total area. Big grains grow at the expense of the small ones.


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Failure

Fundamentals of Fracture

Fracture is a form of failure where the material separates in pieces due to stress, at

temperatures below the melting point. The fracture is termed ductile or brittle dependingon whether the elongation is large or small.

Steps in fracture (response to stress):

Track formation

Track propagation

Ductile vs. brittle fracture

Ductile BrittleDeformation Extensive LittleTrack propagation Slow, needs stress Fast

Type of materials Most metals (not too cold) Ceramics, ice, cold metalsWarning Permanent elongation NoneStrain energy Higher LowerFractured surface Rough SmootherNecking Yes No

Ductile Fracture

Stages of ductile fracture

Initial necking

Small cavity formation (microvoids)

Void growth (ellipsoid) by coalescence into a crack

Fast crack propagation around neck. Shear strain at 45o

Final shear fracture (cup and cone)

The interior surface is fibrous, irregular, which signify plastic deformation.

Brittle Fracture

There is no appreciable deformation, and crack propagation is very fast. In most brittle

materials, crack propagation (by bond breaking) is along specific crystallographic planes

(cleavageplanes). This type of fracture is transgranular (through grains) producing grainytexture (or faceted texture) when cleavage direction changes from grain to grain. In some

materials, fracture is intergranular.


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Impact Fracture:Impact fractures can best be described as a flute or strip of material that was cleanly

sheared from a projectile point. The most common type of impact fracture starts at the tip

of a point and runs down one blade edge possibly reaching the shoulder of a point. Somepoints were reworked into a useable point after having been damaged by an impact

fracture.

Normalized tests, like the Charpoy and Izod tests measure the impact energyrequired tofracture a notched specimen with a hammer mounted on a pendulum. The energy is

measured by the change in potential energy (height) of the pendulum. This energy is

called notch toughness.

Ductile brittle transition:Ductile to brittle transition occurs in materials when the temperature is dropped below atransition temperature. Alloying usually increases the ductile-brittle transition

temperature, for ceramics, this type of transition occurs at much higher temperatures thanfor metals.

Fatigue:Fatigue is the catastrophic failure due to dynamic (fluctuating) stresses. It can happen in

bridges, airplanes, machine components, etc. The characteristics are: long period of cyclic strain the most usual (90%) of metallic failures (happens also in ceramics and polymers) is brittle-like even in ductile metals, with little plastic deformation it occurs in stages involving the initiation and propagation of cracks.

Cyclic StressesThese are characterized by maximum, minimumand mean stress, the stress amplitude,and the stress ratio.

Crack Initiation and PropagationStages in fatigue failure:

I. crack initiation at high stress points (stress raisers)II. propagation (incremental in each cycle)

III. final failure by fractureNfinal = Ninitiation + Npropagation

Stage I - propagation slow along crystallographic planes of high shear stress flat and featureless fatigue surface

Stage II - propagation

Crack propagates by repetitive plastic blunting and sharpening of the crack tip.


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CreepCreep is the time-varying plastic deformation of a material stressed at high temperatures.Examples: turbine blades, steam generators. Keys are the time dependence of the strain

and the high temperature.

Generalized Creep BehaviorAt a constant stress, the strain increases initially fast with time (primary or transientdeformation), then increases more slowly in the secondary region at a steady rate (creep

rate). Finally the strain increases fast and leads to failure in the tertiary region.

Characteristics: Creep rate: d/dt Time to failure.

Stress and Temperature Effects:Both temperature and the level of the applied stress influence the creep characteristics.

The results of creep rupture tests are most commonly presented as the logarithm of stressversus the logarithm of rupture lifetime.

Creep becomes more pronounced at higher temperatures. There is essentially no creep at

temperatures below 40% of the melting point.

Creep increases at higher applied stresses.

The behavior can be characterized by the following expression, where K, nand Qc areconstants for a given material:

d/dt= Kn exp(-Qc/RT)


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Hardness tests

Tests based on indentation or penetration

Brinell hardness test for determining hardness of metallic materials to check the

quality level of product or uniformity of samples of a metal or uniformity of the results ofsome treatment on the metal. Is not recommended for extremely hard metals

Lead 4-8 BHN, Mile Steel 80-105 BHN, Nickel Chrome Steel 175-300 BHN

Rockwell hardness test in situations similar to Brinell test. Its use is preferred when aquick and direct reading is desirable. Its also applicable to the testing of materials havinghardness beyond the scope of Brinell test.

Vickers hardness test when very accurate results of harness regarding metals neededsuch as research work. It is slow compared to Rockwell test. It may be used to determine

hardness of metals varying in thickness from thin sheets to large sections.


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Classification of Metals

Classification of metal alloys

Ferrous Non Ferrous

Steel

%C is up to 2%Cast iron

%C is above 2%

Grey cast iron

White cast iron

Malleable iron

Spheroidal Graphite cast iron (SG Iron)Al loy SteelCarbon Steel

Low carbon steel (mild steel) 0.08 to 0.3% C

Medium carbon steel - .03 to 0.55 % C

High carbon steel 0.55 to 1.5% C

Stainless steel

Tool Steel

Classification of metal alloys

Ferrous Non Ferrous

Steel

%C is up to 2%Cast iron

%C is above 2%

Grey cast iron

White cast iron

Malleable iron

Spheroidal Graphite cast iron (SG Iron)Al loy SteelCarbon Steel

Low carbon steel (mild steel) 0.08 to 0.3% C

Medium carbon steel - .03 to 0.55 % C

High carbon steel 0.55 to 1.5% C

Stainless steel

Tool Steel

Cast IronCast iron is a general term applied to a wide range of iron carbon alloys. Their carbon

content is such as to cause some liquid of eutectic composition (Ledeburite) to solidify.The max. Carbon content is therefore about 2%, while the practical max. is about 4.5%.

Cast iron should not be thought of as a metal having single element. It rather has at least

6 elements. These are Iron, Carbon, Silicon, manganese, Phosphorus & Sulphur. AlloyCast Iron has still other elements, which have important effects on its physical properties.

Its melting point is approximately 1150C to 1250C. It is lower than that for steel.Consequently, it can be more rapidly and cheaply melted. It has the ability to take good

casting impression.

Grey cast iron (% C, 3 3.75%)Its name is derived from grayish appearance of its fracture because of presence of

sufficient graphitic carbon. Graphite flakes act as stress raisers at their sharp tips makingthe grey iron brittle in tension. Cooling rate has its effect on the size of pearlite andgraphite, and slow cooling produces coarse pearlite and graphite.

Grey Cast Iron is machinable but its machinability varies with variation of micro

constituents. Annealing improves its machinability. It can be welded by gas or electricarc welding. It possess good castability as it has low shrinkage during solidification.Applications: Ingot Moulds, Automobile cylinders and pistons, Machine castings watermain pipes etc.


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White cast iron (% C, 1.7-2.3%)This iron possess all its carbon as Iron carbide and it is this constituent which

gives white appearance to the fracture of this iron. It is difficult to machine especially

when with large percentage of Iron carbide. It has good castability but poor weldability.

To produce white iron graphitising elements (like Silicon) are modified and the

cooling rate is considerably increased (chilling).Applications: Ploughshares, Chilled rolls, balls, Stamp shoes, dies, Wearing plates andmanufacture of malleable iron.

Malleable ironMalleable iron castings are produced by proper annealing of white cast iron

castings. This iron has a yield point of about 2300 kg/cm2, percentage elongation about10, considerable shock resistance, good machinability, but practically no weldability.Applications: Automobile wheel hubs, differential housings, Brake drums, pedals,levers, pipe fittings, small parts of textile and agricultural machinery, gas and oil burners.

Spheroidal graphite cast ironIt is obtained by first desulphurising completely molten grey iron and then addingMagnesium in the form of nickel magnesium alloy to the ladle before casting.Magnesium causes graphite of grey iron to take a spheroidal shape. It transforms the

brittle cast iron into a tough strong material with good castability.

S.G. iron with a pearlitic matrix may be heat treated in a manner similar to steel.Its use enables reduced section thickness and consequently results I reduced weight.

Toughness and ductility can be obtain in castings, which are too thick for malleabilising.Applications: Cast crank shafts, Agricultural and marine castings, heavy machineryframes, hand tools, gas and water pipes.

Carbon Steels

Low-carbon steelsby definition, contains less carbon than other steels and are inherentlyeasier to cold-form due to their soft and ductile nature.

When strength is not a major concern, low-carbon steels are good choices because they

are easy to handle (draw, bend, punch, swage, etc.) and fairly inexpensive. Surfacehardness can be improved by a process called carburizing which involves heating the

alloys in a carbon-rich atmosphere.Medium-carbon steelscan be heat treated to have a good balance of ductility andstrength. These steels are typically used in large parts, forgings, and machined

components.

Bolts, rods, crankshafts, and tubings in the automotive industry and Axles, gears, andcomponents that require higher hardness and wear resistance are generally made Medium

carbon steelHigh-carbon steelsare extremely strong yet more brittle. They offer better responses toheat treatment and longer service life than medium-carbon steels.

High-carbon steels typically have high wear resistance due to their superior surface

hardness.


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Alloy Steels

Alloy steels comprise a wide variety of steels which have compositions that exceed the

limitations of C, Mn, Ni, Mo, Cr, Va, Si, and B which have been set for carbon steels.

However, steels containing more than 3.99% chromium are classified differently asstainless and tool steels.

Alloy steels are always killed, but can use unique deoxidization or melting processes for

specific applications. Alloy steels are generally more responsive to heat and mechanicaltreatments than carbon steels.

ClassificationStandard Alloy SteelsH-SteelsHSLA (High strength Low allow steel)

Stainless Steels

Stainless steels are high-alloy steels that have superior corrosion resistance than othersteels because they contain large amounts of chromium. Stainless steels can contain

anywhere from 4-30 percent chromium, however most contain around 10 percent.

Stainless steels can be divided into three basic groups based on their crystalline structure:austenitic, ferritic, and martensitic. Another group of stainless steels known as

precipitation-hardened steels are a combination of austenitic and martensitic steels.

Ferritic grades: Ferritic stainless steels are magnetic non heat-treatable steelsthat contain chromium but not nickel. They have good heat and corrosion resistance, inparticular sea water, and good resistance to stress-corrosion cracking. Their mechanical

properties are not as strong as the austenitic grades, however they have better decorative

appeal.Martensitic grades: Martensitic grades are magnetic and can be heat-treated by

quenching or tempering. They contain chromium but usually contain no nickel, except for

2 grades. Martensitic steels are not as corrosive resistant as austenitic or ferritic grades,

but their hardness levels are among the highest of the all the stainless steels.Austenitic grades: Austenitic stainless steels are non-magnetic non heat-treatable

steels that are usually annealed and cold worked. Some austenitic steels tend to become

slightly magnetic after cold working. Austenitic steels have excellent corrosion and heatresistance with good mechanical properties over a wide range of temperatures. There are

two subclasses of austenitic stainless steels: chromium-nickel and chromium-manganese-

low nickel steels. Chromium-nickel steels are the most general widely used steels and arealso known as 18-8(Cr-Ni) steels. The chromium nickel ratio can be modified to improve

formability; carbon content can be reduced to improve intergranular corrosion resistance.

Molybdenum can be added to improve corrosion resistance; additionally the Cr-Nicontent can be increased.


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Tool Steels

Tool steels are steels that are primarily used to make tools used in manufacturing

processes as well as for machining metals, woods, and plastics. Tool steels are generally

ingot-cast wrought products, and must be able to withstand high specific loads as well as

be stable at elevated temperatures.

High-Speed Tool Steels: High-speed alloys include all molybdenum andtungsten alloys. High-speed tools steels can be hardenend to 62-67 HRC and canmaintain this hardness in service temperatures as high as 540 C (1004F), making them

very useful in high-speed machinery. Typical applications are end mills, drills, lathe

tools, planar tools, punches, reamers, routers, taps, saws, broaches, chasers, and hobs.

Hot-work Tool Steels: Hot-work tool steels include all chromium, tungsten, andmolybdenum alloys. They are typically used for forging, die casting, heading, piercing,trim, extrusion, and hot-shear and punching blades.

Cold-work Tool Steels: Cold-work tool steels include all high-chromium,medium-alloy air-hardening, water hardening, and oil hardening alloys. Typicalapplications include cold working operations such as stamping dies, draw dies,

burnishing tools, coining tools, and shear blades.

Shock-Resistant Tool Steels: Cold-work tool steels include all class S alloys.They are among the toughest of the tool steels, and are typically used for screw driver

blades; shear blades, chisels, knockout pins, punches, and riveting tools.

Mold Steels: Mold steels include all low-carbon and one medium-carbon class Ptool steels. They are typically used for compression and injection molds for plastics, and

die-casting dies.

Special-Purpose Tool Steels: Special-Purpose Tool Steels include all low-alloyclass L Tool steels. They are usually quenched, which makes them relatively tough and

easily machinable. They are typically used for arbors, punches, taps, wrenches, drills, and

brake-forming dies.

Water-Hardening Tool Steels: Water-Hardening Tool steels include all class Wtool steels, and while they do not retain hardness well at elevated temperatures, they dohave high resistance to surface wear. Typical applications include blanking dies, files,

drills, taps, countersinks, reamers, jewelry dies, and cold-striking dies


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Nonferrous alloys

Alloys of cobalt Stellite (chromium, tungsten, carbon) Talonite

Alloys of nickel Mu-metal (iron) Monel metal (copper, iron, manganese) Nichrome (chromium, iron)

Alloys of copper Brass (zinc) Prince's metal (zinc) Gilding metal (zinc) Bronze (tin, aluminium or any other element) Phosphor bronze (tin and phosphorus) Bell metal (tin) Beryllium copper (beryllium) Cupronickel (nickel) Nickel silver (nickel) Billon (silver) Nordic gold (aluminium, zinc, tin)

Alloys of silver Sterling silver (copper)

Alloys of tin Pewter (lead, copper) Solder (lead)

Alloys of gold Electrum (silver, copper)

Alloys of mercury Amalgam

Alloys of lead Solder Type metal

Alloys of bismuth Wood's metal


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Ceramic

Ceramics offer a high temperature range. However, ceramics are not very strong.

To compensate for their lack of strength ceramics are usually combined with some other

material to form a ceramic composite.

Industry of Ceramic Segments

Structural clay productsBrick, sewer pipe, roofing tile, clay floor and wall tile (i.e.,quarry tile), flue linings

Whitewares Dinnerware, floor and wall tile, sanitaryware, electrical porcelain,decorative ceramics

Refractories Brick and monolithic products are used in iron and steel, non-ferrousmetals, glass, cements, ceramics, energy conversion, petroleum, and chemicals industries

GlassesFlat glass (windows), container glass (bottles), pressed and blown glass(dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical fibers)

AbrasivesNatural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fusedalumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure

blasting of materials

CementsUsed to produce concrete roads, bridges, buildings, dams, and the like

Polymers

The description of stress-strain behavior is similar to that of metals, but a very important

consideration for polymers is that the mechanical properties depend on the strain rate,temperature, and environmental conditions.

The stress-strain behavior can be brittle, plastic and highly elastic (elastomeric or

rubberlike).Tensile modulus (modulus) and tensile strengths are orders of magnitude smaller than

those of metals, but elongation can be up to 1000 % in some cases. The tensile strength is

defined at the fracture point and can be lower than the yield strength. Mechanicalproperties change dramatically with temperature, going from glass-like brittle behavior at

low temperatures (like in the liquid-nitrogen demonstration) to a rubber-like behavior athigh temperatures.

Thermoplastic and Thermosetting PolymersThermoplasticpolymers (thermoplasts) soften reversibly when heated (harden whencooled back)


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Thermosettingpolymers (thermosets) harden permanently when heated, as cross-linkinghinder bending and rotations. Thermosets are harder, more dimensionally stable, andmore brittle than thermoplasts.

CompositesParticle-reinforced compositesThese are the cheapest and most widely used. They fall in two categories depending onthe size of the particles:

large-particle composites, which act by restraining the movement of the matrix,if well bonded.

dispersion-strengthened composites, containing 10-100 nm particles, similar to

what was discussed under precipitation hardening. The matrix bears the major portion ofthe applied load and the small particles hinder dislocation motion, limiting plastic

deformation.

Large-Particle CompositesProperties are a combination of those of the components. The rule of mixturespredictsthat an upper limit of the elastic modulus of the composite is given in terms of the elastic

moduli of the matrix (Em) and the particulate (Ep) phases by:Ec=EmVm+EpVp

where Vm and Vp are the volume fraction of the two phases. A lower bound is given by:Ec=EmEp / (EpVm+EmVp)

ConcreteThe most common large-particle composite is concrete, made of a cement matrix that

bonds particles of different size (gravel and sand.) Cement was already known to theEgyptians and the Greek. Romans made cement by mixing lime (CaO) with volcanic ice.

In its general from, cement is a fine mixture of lime, alumina, silica, and water. Portlandcement is a fine powder of chalk, clay and lime-bearing minerals fired to 1500 o C

(calcinated). It forms a paste when dissolved in water. It sets into a solid in minutes and

hardens slowly (takes 4 months for full strength). Properties depend on how well it is

mixed, and the amount of water: too little - incomplete bonding, too much excessiveporosity.

The advantage of cement is that it can be poured in place, it hardens at room temperature

and even under water, and it is very cheap. The disadvantages are that it is weak andbrittle, and that water in the pores can produce crack when it freezes in cold weather.

Concrete is cement strengthened by adding particulates. The use of different size (stoneand sand) allows better packing factor than when using particles of similar size.Concrete is improved by making the pores smaller (using finer powder, adding polymeric

lubricants, and applying pressure during hardening.

Reinforced concreteis obtained by adding steel rods, wires, mesh. Steel has theadvantage of a similar thermal expansion coefficient, so there is reduced danger of

cracking due to thermal stresses. Pre-stressed concreteis obtained by applying tensile


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stress to the steel rods while the cement is setting and hardening. When the tensile stress

is removed, the concrete is left under compressive stress, enabling it to sustain tensileloads without fracturing. Pre-stressed concrete shapes are usually prefabricated. A

common use is in railroad or highway bridges.

Cermetsare composites of ceramic particles (strong, brittle) in a metal matrix (soft,ductile) that enhances toughness. For instance, tungsten carbide or titanium carbideceramics in Co or Ni. They are used for cutting tools for hardened steels.

Reinforced rubber is obtained by strengthening with 20-50 nm carbon-black particles.Used in auto tires.

Dispersion-Strengthened CompositesUse of very hard, small particles to strengthen metals and metal alloys. The effect is like

precipitation hardening but not so strong. Particles like oxides do not react so thestrengthening action is retained at high temperatures.

Fiber-reinforced compositesIn many applications, like in aircraft parts, there is a need for high strength per unitweight (specific strength). This can be achieved by composites consisting of a lowdensity

(and soft) matrix reinforced with stiff fibers.

The strength depends on the fiber length and its orientation with respect to the stressdirection.

The efficiency of load transfer between matrix and fiber depends on the interfacial bond.

Structural CompositesLargest and most diverse use of composites due to ease of fabrication, low cost and goodproperties.

Glass-fiber reinforced composites (GFRC) are strong, corrosion resistant and lightweight,

but not very stiff and cannot be used at high temperatures. Applications include auto andboat bodies, aircraft components

Carbon-fiber reinforced composites (CFRC) use carbon fibers, which have the highest

specific module (module divided by weight). CFRC are strong, inert, allow high

temperature use. Applications include fishing rods, golf clubs, aircraft components.Kevlar, and aremid-fiber composite can be used as textile fibers. Applications include

bullet-proof vests, tires, brake and clutch linings.

WoodThis is one of the oldest and the most widely used structural material. It is a composite of

strong and flexible cellulose fibers (linear polymer) surrounded and held together by amatrix of lignin and other polymers. The properties are anisotropic and vary widely

among types of wood. Wood is ten times stronger in the axial direction than in the radial

or tangential directions.


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Objectives of Heat Treatments

Heat Treatment is the controlled heating and cooling of metals to alter their physical and

mechanical properties without changing the product shape. Heat treatment is sometimes

done inadvertently due to manufacturing processes that either heat or cool the metal such

as welding or forming.

Heat Treatment is often associated with increasing the strength of material, but it can also

be used to alter certain manufacturability objectives such as improve machining, improveformability, restore ductility after a cold working operation. Thus it is a very enabling

manufacturing process that can not only help other manufacturing process, but can alsoimprove product performance by increasing strength or other desirable characteristics.

Steels are particularly suitable for heat treatment, since they respond well to heattreatment and the commercial use of steels exceeds that of any other material. Steels are

heat treated for one of the following reasons:

1. Softening2. Hardening3. Material Modification

Softening: Softening is done to reduce strength or hardness, remove residual stresses,improve toughnesss, restore ductility, refine grain size or change the electromagnetic

properties of the steel.Restoring ductility or removing residual stresses is a necessary operation when a

large amount of cold working is to be performed, such as in a cold-rolling operation or

wiredrawing. The principal ways by which steel is softened are

Annealing full Process, spheroidizing, normalizingTempering austempering, martempering

Hardening: Hardening of steels is done to increase the strength and wear properties. Oneof the pre-requisites for hardening is sufficient carbon and alloy content. If there is

sufficient Carbon content then the steel can be directly hardened. Otherwise the surface

of the part has to be Carbon enriched using some diffusion treatment hardeningtechniques. Three types of hardening process are

Diffusion TreatmentsDirect HardeningSelective Hardening

Material Modification: Heat treatment is used to modify properties of materials inaddition to hardening and softening. These processes modify the behavior of the steels in

a beneficial manner to maximize service life, e.g., stress relieving, or strength properties,

e.g., cryogenic treatment, or some other desirable properties, e.g., spring aging.


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Softening Annealing

Full Annealing

Full annealing is the process of slowly raising the temperature about 50 C (90 F) above

the Austenitic temperature line A3 or line ACM in the case of Hypoeutectoid steels(steels with < 0.77% Carbon) and 50 C (90 F) into the Austenite-Cementite region in

the case of Hypereutectoid steels (steels with > 0.77% Carbon).

It is held at this temperature for sufficient time for all the material to transform into

Austenite or Austenite-Cementite as the case may be. It is then slowly cooled at the rate

of about 20 C/hr (36 F/hr) in a furnace to about 50 C (90 F) into the Ferrite-Cementiterange. At this point, it can be cooled in room temperature air with natural convection.

The grain structure has coarse Pearlite with ferrite or Cementite (depending on whetherhypo or hyper eutectoid). The steel becomes soft and ductile.


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Normalizing

Normalizing is the process of raising the temperature to over 60 C (108 F), above line

A3 or line ACM fully into the Austenite range. It is held at this temperature to fully

convert the structure into Austenite, and then removed form the furnace and cooled at

room temperature under natural convection. This results in a grain structure of finePearlite with excess of Ferrite or Cementite. The resulting material is soft; the degree of

softness depends on the actual ambient conditions of cooling. This process is

considerably cheaper than full annealing since there is not the added cost of controlledfurnace cooling.

The main difference between full annealing and normalizing is that fully annealed partsare uniform in softness (and machinablilty) throughout the entire part; since the entire

part is exposed to the controlled furnace cooling. In the case of the normalized part,

depending on the part geometry, the cooling is non-uniform resulting in non-uniformmaterial properties across the part. This may not be desirable if further machining is

desired, since it makes the machining job somewhat unpredictable. In such a case it isbetter to do full annealing.

Process Annealing

Process Annealing is used to treat work-hardened parts made out of low-Carbon steels ( 0.6%) that

will be machined or cold formed subsequently. This is done by one of the following

ways:

1. Heat the part to a temperature just below the Ferrite-Austenite line, line A1 or below

the Austenite-Cementite line, essentially below the 727 C (1340 F) line. Hold the

temperature for a prolonged time and follow by fairly slow cooling. Or2. Cycle multiple times between temperatures slightly above and slightly below the 727

C (1340 F) line, say for example between 700 and 750 C (1292 - 1382 F), and slow

cool. Or3. For tool and alloy steels heat to 750 to 800 C (1382-1472 F) and hold for several

hours followed by slow cooling.

All these methods result in a structure in which all the Cementite is in the form of smallglobules (spheroids) dispersed throughout the ferrite matrix. This structure allows for

improved machining in continuous cutting operations such as lathes and screw machines.

Spheroidization also improves resistance to abrasion.

Softening Tampering

Temperingis a process done subsequent to quench hardening. Quench-hardened partsare often too brittle. This brittleness is caused by a predominance of Martensite. This

brittleness is removed by tempering. Tempering results in a desired combination ofhardness, ductility, toughness, strength, and structural stability. Tempering is not to be

confused with tempers on rolled stock-these tempers are an indication of the degree of

cold work performed.


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The mechanism of tempering depends on the steel and the tempering temperature. The

prevalent Martensite is a somewhat unstable structure. When heated, the Carbon atomsdiffuse from Martensite to form a carbide precipitate and the concurrent formation of

Ferrite and Cementite, which is the stable form. Tool steels for example, lose about 2 to 4

points of hardness on the Rockwell C scale. Even though a little strength is sacrificed,

toughness (as measured by impact strength) is increased substantially. Springs and suchparts need to be much tougher these are tempered to a much lower hardness.

Tempering is done immediately after quench hardening. When the steel cools to about 40C (104 F) after quenching, it is ready to be tempered. The part is reheated to a

temperature of 150 to 400 C (302 to 752 F). In this region a softer and tougher structure

Troostite is formed. Alternatively, the steel can be heated to a temperature of 400 to 700C (752 to 1292 F) that results in a softer structure known as Sorbite. This has less

strength than Troostite but more ductility and toughness.

The heating for tempering is best done by immersing the parts in oil, for tempering upto

350 C (662 F) and then heating the oil with the parts to the appropriate temperature.Heating in a bath also ensures that the entire part has the same temperature and will

undergo the same tempering. For temperatures above 350 C (662 F) it is best to use abath of nitrate salts. The salt baths can be heated upto 625 C (1157 F). Regardless of the

bath, gradual heating is important to avoid cracking the steel. After reaching the desired

temperature, the parts are held at that temperature for about 2 hours, then removed fromthe bath and cooled in still air.

Austempering

Austempering is a quenching technique. The part is not quenched through the Martensitetransformation. Instead the material is quenched above the temperature when Martensite

forms MS, around 315 C (600 F). It is held till at this temperature till the entire part

reaches this temperature. As the part is held longer at this temperature, the Austenitetransforms into Bainite. Bainite is tough enough so that further tempering is not

necessary, and the tendency to crack is severely reduced.

Martempering

Martempering is similar to Austempering except that the part is slowly cooled through

the martensite transformation. The structure is martensite, which needs to tempered justas much as martensite that is formed through rapid quenching. The biggest advantage of

Austempering over rapid quenching is that there is less distortion and tendency to crack.


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Hardening - Diffusion Treatments

The Carbon content in the steel determines whether it can be directly hardened. If the

Carbon content is low (less than 0.25% for example) then an alternate means exists to

increase the Carbon content of the surface. The part then can be heat-treated by either

quenching in liquid or cooling in still air depending on the properties desired. Note thatthis method will only allow hardening on the surface, but not in the core, because the

high carbon content is only on the surface. This is sometimes very desirable because it

allows for a hard surface with good wear properties (as on gear teeth), but has a toughcore that will perform well under impact loading.

Carburizing

Carburizing is a process of adding Carbon to the surface. This is done by exposing the

part to a Carbon rich atmosphere at an elevated temperature and allows diffusion totransfer the Carbon atoms into steel. This diffusion will work only if the steel has low

carbon content, because diffusion works on the differential of concentration principle. If,for example the steel had high carbon content to begin with, and is heated in a carbon free

furnace, such as air, the carbon will tend to diffuse out of the steel resulting inDecarburization.

Pack Carburizing: Parts are packed in a high carbon medium such as carbonpowder or cast iron shavings and heated in a furnace for 12 to 72 hours at 900 C (1652F). At this temperature CO gas is produced which is a strong reducing agent. The

reduction reaction occurs on the surface of the steel releasing Carbon, which is then

diffused into the surface due to the high temperature. When enough Carbon is absorbedinside the part (based on experience and theoretical calculations based on diffusion

theory), the parts are removed and can be subject to the normal hardening methods.

The Carbon on the surface is 0.7% to 1.2% depending on process conditions. The

hardness achieved is 60 - 65 RC. The depth of the case ranges from about 0.1 mm (0.004in) upto 1.5 mm (0.060 in). Some of the problems with pack carburizing is that the

process is difficult to control as far as temperature uniformity is concerned, and the

heating is inefficient.Gas Carburizing: Gas Carburizing is conceptually the same as pack carburizing,

except that Carbon Monoxide (CO) gas is supplied to a heated furnace and the reduction

reaction of deposition of carbon takes place on the surface of the part. This processes

overcomes most of the problems of pack carburizing. The temperature diffusion is asgood as it can be with a furnace. The only concern is to safely contain the CO gas. A

variation of gas carburizing is when alcohol is dripped into the furnace and it volatilizes

readily to provide the reducing reaction for the deposition of the carbon.Liquid Carburizing: The steel parts are immersed in a molten carbon rich bath.

In the past, such baths have cyanide (CN) as the main component. However, safety

concerns have led to non-toxic baths that achieve the same result.


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Nitriding

Nitriding is a process of diffusing Nitrogen into the surface of steel. The Nitrogen forms

Nitrides with elements such as Aluminum, Chromium, Molybdenum, and Vanadium. The

parts are heat-treated and tempered before nitriding. The parts are then cleaned and

heated in a furnace in an atmosphere of dissociated Ammonia (containing N and H) for10 to 40 hours at 500-625 C (932 - 1157 F). Nitrogen diffuses into the steel and forms

nitride alloys, and goes to a depth of upto 0.65 mm (0.025 in). The case is very hard and

distortion is low. No further heat treatment is required; in fact, further heat treatment cancrack the hard case. Since the case is thin, surface grinding is not recommended. This can

restrict the use of nitriding to surfaces that require a very smooth finish.

Carbonitriding

Carbonitriding process is most suitable for low carbon and low carbon alloy steels. In thisprocess, both Carbon and Nitrogen are diffused into the surface. The parts are heated in

an atmosphere of hydrocarbon (such as methane or propane) mixed with Ammonia(NH3). The process is a mix of Carburizing and Nitriding.

Carburizing involves high temperatures (around 900 C, 1652 F) and Nitriding involves

much lower temperatures (around 600 C, 1112 F). Carbonitriding is done at

temperatures of 760 - 870 C (1400 - 1598 F), which is higher than the transformationtemperatures of steel that is the region of the face-centered Austenite.

It is then quenched in a natural gas (Oxygen free) atmosphere. This quench is less drasticthan water or oil-thus less distortion. However this process is not suitable for high

precision parts due to the distortions that are inherent. The hardness achieved is similar tocarburizing (60 - 65 RC) but not as high as Nitriding (70 RC). The case depth is from 0.1

to 0.75 mm (0.004 to 0.030 in). The case is rich in Nitrides as well as Martensite.

Tempering is necessary to reduce the brittleness.


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Hardening - Direct Hardening

Hardness is a function of the Carbon content of the steel. Hardening of a steel requires a

change in structure from the body-centered cubic structure found at room temperature to

the face-centered cubic structure found in the Austenitic region. The steel is heated to

Austenitic region. When suddenly quenched, the Martensite is formed. This is a verystrong and brittle structure. When slowly quenched it would form Austenite and Pearlite

which is a partly hard and partly soft structure. When the cooling rate is extremely slow

then it would be mostly Pearlite which is extremely soft.

Hardenability, which is a measure of the depth of full hardness achieved, is related to the

type and amount of alloying elements. Different alloys, which have the same amount ofCarbon content, will achieve the same amount of maximum hardness; however, the depthof full hardness will vary with the different alloys. The reason to alloy steels is not to

increase their strength, but increase their hardenability the ease with which full

hardness can be achieved throughout the material.

Usually when hot steel is quenched, most of the cooling happens at the surface, as does

the hardening. This propagates into the depth of the material. Alloying helps in the

hardening and by determining the right alloy one can achieve the desired properties forthe particular application.

Such alloying also helps in reducing the need for a rapid quench cooling therebyeliminate distortions and potential cracking. In addition, thick sections can be hardened

fully.


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Quench Media

Quenching is the act of rapidly cooling the hot steel to harden the steel.

Water: Quenching can be done by plunging the hot steel in water. The water

adjacent to the hot steel vaporizes, and there is no direct contact of the water with thesteel. This slows down cooling until the bubbles break and allow water contact with the

hot steel. As the water contacts and boils, a great amount of heat is removed from the

steel. With good agitation, bubbles can be prevented from sticking to the steel, andthereby prevent soft spots.

Water is a good rapid quenching medium, provided good agitation is done. However,water is corrosive with steel, and the rapid cooling can sometimes cause distortion or

cracking.

Salt Water: Salt water is a more rapid quench medium than plain water because

the bubbles are broken easily and allow for rapid cooling of the part. However, salt wateris even more corrosive than plain water, and hence must be rinsed off immediately.

Oil: Oil is used when a slower cooling rate is desired. Since oil has a very highboiling point, the transition from start of Martensite formation to the finish is slow and

this reduces the likelihood of cracking. Oil quenching results in fumes, spills, andsometimes a fire hazard.

Polymer quench: Polymer quenches that will produce a cooling rate in betweenwater and oil. The cooling rate can be altered by varying the components in the mixture-

as these are composed of water and some glycol polymers. Polymer quenches are capableof producing repeatable results with less corrosion than water and less of a fire hazard

than oil. But, these repeatable results are possible only with constant monitoring of the

chemistry.

Cryogenic Quench: Cryogenics or deep freezing is done to make sure there is noretained Austenite during quenching. The amount of Martensite formed at quenching is a

function of the lowest temperature encountered. At any given temperature of quenchingthere is a certain amount of Martensite and the balance is untransformed Austenite. This

untransformed Austenite is very brittle and can cause loss of strength or hardness,

dimensional instability, or cracking.

Quenches are usually done to room temperature. Most medium carbon steels and low

alloy steels undergo transformation to 100% Martensite at room temperature. However,high carbon and high alloy steels have retained Austenite at room temperature. To

eliminate retained Austenite, the quench temperature has to be lowered. This is the reason

to use cryogenic quenching.


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Hardening - Selective Hardening

Carbon steels that have minimum carbon content of 0.4%, or alloy steels with a lower

carbon content (hardenable stainless steels with only 0.1% Carbon), can be selectively

hardenened in specific regions by applying heat and quench only to those regions. Parts

that benefit by flame hardening include gear teeth, bushings etc. These techniques arebest suited for medium carbon steels with a carbon content ranging from 0.4 to 0.6%.

Flame Hardening: A high intensity oxy-acetylene flame is applied to the selectiveregion. The temperature is raised high enough to be in the region of Austenite

transformation. The "right" temperature is determined by the operator based on

experience by watching the color of the steel. The overall heat transfer is limited by thetorch and thus the interior never reaches the high temperature. The heated region is

quenched to achieve the desired hardness. Tempering can be done to eliminate

brittleness.

The depth of hardening can be increased by increasing the heating time. As much as 6.3mm (0.25 in) of depth can be achieved. In addition, large parts, which will not normally

fit in a furnace, can be heat-treated.

Induction Hardening: In Induction hardening, the steel part is placed inside a electricalcoil which has alternating current through it. This energizes the steel part and heats it up.Depending on the frequency and amperage, the rate of heating as well as the depth of

heating can be controlled. Hence, this is well suited for surface heat treatment. The

details of heat treatment are similar to flame hardening.


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Economic, Environmental, and Social Issues of Material UsageEconomic Considerations:It is essential for the engineer to know about and understand economic issues simply

because the company/institution for which he or she works must realize a profit from the

products it manufactures. Materials engineering decisions have economic consequenceswith regard to both material and production cost

Environmental and Social considerations:Issues of environmental protection and sustainable development are gaining an increasing

importance in everyday life, and nowhere is this more so than in the field of Materials

Science and Engineering. Almost every aspect of materials usage, from extraction andproduction, through product design and ultimately disposal issues, is now subject to

environmental considerations. Furthermore there are many cases where the developments

of novel environmentally-friendly materials are providing new challenges for materialsscientists and engineers.

The growing awareness of environmental issues has increased the attention focused onthe materials industry. There is a danger that this could give a negative picture,

highlighting examples where materials production and use has led to environmental

problems. In many of these cases, materials, additives and production methods were usedfor very good materials engineering reasons before environmental concerns were

established. The more positive image of materials engineering that can be portrayed is

one where the industry is at the forefront of technical advances - not only to 'deal with

past mistakes', but also to drive sustainable and safe use of materials for the future. Thetopic of 'Environmental Materials' is broad and can touch on some relatively in-depth

aspects of materials structure, chemical and physical properties, processing and design as

well as more general areas such as legislative, economic and social aspects. Interestingtopics within this area can be presented at a range of levels: for instance the sustainable

use of materials in the IT sector can be discussed by 11 year olds with as muchenthusiasm as by post-graduate students - although the latter would be expected to grasp

the chemical details of identifying brominated flame-retardants, whereas the former

would consider much simpler aspects, that are nonetheless important.

Recycling issues:It could easily be argued that steel is a very sustainable material; it is abundant, takesrelatively little energy to extract and is easy to recycle, however people living near a

steelworks would argue against this. It is probably sensible to define such materials as

those that have distinct differences that achieve environmental benefit compared toconventional materials. With this definition, the list would include:

1. Materials of a significantly plant-based nature, including wood, natural fiber

composites, natural polymers.

2. Materials produced using a large proportion of waste material, includingrecycled polymers, composites made from waste mineral powders, and arguably also

much steel and aluminum.


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The most exciting developments in Materials Science are in the realm of functional

materials, and many of these serve an environmentally-beneficial purpose, particularly inthe production of green energy.

These include: Solar-cell materials

Fuel-cell technology Catalytic pollution control

The figure below schematically shows how the disparate areas under the heading of

'environmental materials' can be linked via a life cycle analysis approach.

18.4 Life Cycle Analysis and its use in design:Life Cycle Analysis is essentially a method of considering the entire environmental

impact, energy and resource usage of a material or product. It is often known as a cradle

to-grave' analysis and can encompass the entire lifetime from extraction to end-of-lifedisposal. Life cycle analysis can be an extremely effective way of linking many different

aspects of the environmental impacts of materials usage. The scope of a life cycle

analysis can be adjusted to suit a particular case. For instance it could cover theenvironmental impact of the global aluminium industry or simply that of one single

plastic injection moulding machine. In order to gain most learning benefit from this area,

students would be expected to have a good grasp of the necessary underlying technicalareas, which could be quite complex and so this ideally suits more advanced degree level

students. The most conventional way of approaching a life cycle analysis is to follow a

particular material or product through its lifetime. Therefore the first consideration wouldbe the impact of materials extraction, and then production and manufacture, product use

and finally end-of-life considerations. This approach is followed below. Various aspects,

such as energy usage, economic and legislative issues occur throughout the cycle.

basics of material science

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