engineering materials

Engineering Materials

General considerations: After the general layout of the machine has been determined and the necessary mechanisms chosen or devised, it becomes necessary that the designer select a proper material for each machine member. This involves the consideration of such factors as the engineering properties of the available materials; the weight, size, and shape of the machine member as well as the loads that it must carry; cost of the material; cost of fabricating the machine element form each material, usually with several alternative production procedures possible for each material; and any properties of the material peculiar to the use to which the member will be put. The major engineering properties of materials which usually are of importance to the designer are strength, stiffness, ductility, toughness, resilience, fatigue resistance, shock resistance, wear resistance, creep characteristics, corrosion resistance, hardness, hardenability, machinability, formability, castability, weldability, ability to be surface finished in an acceptable manner, effects of high and low temperatures upon the behavior of the material, visual appearance, frictional properties, and internal vibrational damping properties.

Materials play an important role in the construction and manufacturing of equipment/tools. Right selections of materials add to the economy, working and life of machinery. This selection process includes choosing the material, paying attention to its specific type or grade based on the required properties. Engineers will select a particular grade of material based on its properties such as malleability or tensile strength. Composites comprise two materials, such as a metallic mesh and a resin, the combination of which also depends on the properties required. Materials from which the item is to be manufactured are noted on the engineering drawing using standard material and grade codes. It is important that manufacturers do not interchange materials because the switch may make the products susceptible to failures.

The selection of a material for a machine part or a structural member is one of the most important decisions the designer is called on to make. The decision is usually made before the dimensions of the part are established. After choosing the process of creating the desired geometry and the material (the two cannot be divorced), the designer can proportion the member so that loss of function can be avoided or the chance of loss of function can be held to an acceptable risk.

In the next chapters, methods for estimating stresses and deflections of machine members are presented. These estimates are based on the properties of the material from which the member will be made. For deflections and stability evaluations, for example, the elastic (stiffness) properties of the material are required, and evaluations of stress at a critical location in a machine member require a comparison with the strength of the material at that location in the geometry and condition of use. This strength is a material property found by testing and is adjusted to the geometry and condition of use as necessary.

As important as stress and deflection are in the design of mechanical parts, the selection of a material is not always based on these factors. Many parts carry no loads on them whatever. Parts may be designed merely to fill up space or for aesthetic qualities. Members must frequently be designed to also resist corrosion. Sometimes temperature effects are more important in design than stress and strain. So many other factors besides stress and strain may govern the design of parts that the designer must have the versatility that comes only with a broad background in materials and processes.

The engineering materials are mainly classified as:1. Metals and their alloys, such as iron, steel, copper, aluminum, etc.2. Non-metals, such as glass, rubber, plastic, etc.The metals may be further classified as:a) Ferrous metals and b) Non-ferrous metals.The ferrous metals are those which have the iron as their main constituent, such as cast iron, wrought iron and steel.The non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminum, brass, tin, zinc, etc.

Fig 1.1 Block Diagram of Engineering Materials

The engineering materials are mainly classified as:3. Metals and their alloys, such as iron, steel, copper, aluminum, etc.4. Non-metals, such as glass, rubber, plastic, etc.

The metals may be further classified as:c) Ferrous metals and d) Non-ferrous metals.

The ferrous metals are those which have the iron as their main constituent, such as cast iron, wrought iron and steel.

The non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminum, brass, tin, zinc, etc.

The selection of a proper material, for engineering purposes, is one of the most difficult problems for the designer. The best material is one which serves the desired objective at the minimum cost. The following factors should be considered while selecting the material:

1. Availability of the materials,2. Suitability of the materials for the working conditions in service, and3. The cost of the materials.

The important properties, which determine the utility of the material, are physical, chemical and mechanical properties.

Physical Properties of Metals

The physical properties of the metals include luster, color, size and shape, density, electric and thermal conductivity, and melting point.Some of the important physical properties are:

1. DensityDensity is defined as mass per unit volume for a material. The derived unit usually used by engineers is the kg/m3. Relative density is the density of the material compared with the density of the water at 4C. The formulae of density and relative density are:

2. Electrical ConductivityElectrical conductivity is a measure of how well a material accommodates the movement of an electric charge. It is the ratio of the current density to the electric field strength.

3. Melting temperature of materialThe melting temperatures and the re-crystallization temperatures have a great effect on the materials and the alloys of the materials properties and as a result on its applications.

4. SemiconductorsSemiconductor materials are capable of having their conductors properties changed during manufacture. Examples of semiconductor materials are silicon and germanium. They are used extensively in the electronics industry in the manufacture of solid-state devices such as diodes, thermistors, transistors and integrated circuits.

5. Thermal conductivityThis is the ability of the material to transmit heat energy by conduction.

6. FusibilityThis is the ease with which materials will melt.

7. Reluctance (as magnetic properties)Just as some materials are good or bad conductors of electricity; some materials can be good or bad conductors of magnetism. The resistance of magnetic circuit is referred to as reluctance. The good magnetic conductors have low reluctance and examples are the ferromagnetic materials which get their name from the fact that they are made from iron, steel and associated alloying elements such as cobalt and nickel. All other materials are non-magnetic and offer a high reluctance to the magnetic flux field.

8. Temperature stabilityAny changes in temperature can have very significant effects on the structure and properties of materials. However, there are several effects can appear with changes in temperature such as creep..Mechanical Properties of MetalsThe mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness. We shall now discuss these properties as follows:

1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called stress.

2. Stiffness. It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.

3. Elasticity. It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.

4. Plasticity. It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.

5. Ductility. It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminum, nickel, zinc, tin and lead.

Note : The ductility of a material is commonly measured by means of percentage elongation and percentage reduction in area in a tensile test.

6. Brittleness. It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Cast iron is a brittle material.

7. Malleability. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron, copper and aluminum.

8. Toughness. It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture. This property is desirable in parts subjected to shock and impact loads.

9. Machinability. It is the property of a material which refers to a relative case with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. It may be noted that brass can be easily machined than steel.

10. Resilience. It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for spring materials.

11. Creep. When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines.

12. Fatigue. When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of a progressive crack formation which are usually fine and of microscopic size. This property is considered in designing shafts, connecting rods, springs, gears, etc.

13. Hardness. It is a very important property of the metals and has a wide variety of meanings.It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness is usually expressed in numbers which are dependent on the method of making the test. The hardness of a metal may be determined by the following tests:a) Brinell hardness test,b) Rockwell hardness test,c) Vickers hardness (also called Diamond Pyramid) test, andd) Shore scleroscope.Chemical PropertiesMetals are usually inclined to formcationsthrough electron loss,reacting with oxygen in the air to formoxidesover various timescales (ironrustsover years, while potassiumburns in seconds). Examples:4 Na + O2 2 Na2O (sodium oxide)2 Ca + O2 2 CaO (calcium oxide)4 Al + 3 O2 2 Al2O3(aluminum oxide).

Thetransition metals(such asiron,copper,zinc, andnickel) are slower to oxidize because they formpassivating layerof oxide that protects the interior. Others, like palladium,platinumandgold, do not react with the atmosphere at all.

Some metals form a barrier layer ofoxideon their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades (likealuminum, magnesium, somesteels, andtitanium).

Theoxidesof metals are generallybasic, as opposed to those of nonmetals, which areacidic.

Chemical properties are any of the properties of matter that may only be observed and measured by performing achemical changeor chemical reaction.

1. Reactivity. It refers to therateat which achemical substancetends to undergo achemical reactionin time

2. Toxicity. It is the degree to which something is able to produce illness or damage to an exposed organism.

3. Flammability. It is defined as how easily something will burn or ignite, causing fire or combustion.

4. Oxidation. It is the interaction between oxygen molecules and other substances.

5. Chemical Stability. Tendency of amaterialto resistchangeordecompositiondueto internalreaction, or due to the action of air, heat, light,pressure, etc.

6. Corrosion. It is the gradual destruction of material, usually metals, by chemical reaction with its environment.

The term 'ferrous' comes from a Latin word ferrum, meaning 'containing iron'. Hence, ferrous metals are all those metals that contain iron. Ferrous metals may contain small amounts of other elements such as carbon or nickel, in a specific proportion that are added to achieve the desired properties. All the ferrous metals are generally magnetic and have high tensile strength. Now that you know what ferrous metals are, let us have a look at a ferrous metals list.Most commonly used ferrous metals are Mild Steel, High Speed Steel, Stainless Steel, High Tensile Steel and Cast Iron.

1. Cast IronThe cast iron is obtained by re-melting pig iron with coke and limestone in a furnace known as cupola. It is primarily an alloy of iron and carbon. The carbon contents in cast iron vary from 1.7 per cent to 4.5 per cent. It also contains small amounts of silicon, manganese, phosphorous and sulphur. The carbon in a cast iron is present in either of the following two forms:a) Free carbon or graphite, andb) Combined carbon or cementite.

Since the cast iron is a brittle material, therefore, it cannot be used in those parts of machines which are subjected to shocks. The properties of cast iron which make it a valuable material for engineering purposes are its low cost, good casting characteristics, high compressive strength, wear resistance and excellent machinability. The compressive strength of cast iron is much greater than the tensile strength. Following are the values of ultimate strength of cast iron:

Tensile strength = 100 to 200 MPaCompressive strength = 400 to 1000 MPaShear strength = 120 MPa

Types of Cast Irona. Grey cast iron. It is an ordinary commercial iron having the following compositions:Carbon = 3 to 3.5%; Silicon = 1 to 2.75%; Manganese = 0.40 to 1.0%; Phosphorous = 0.15 to 1% ; Sulphur = 0.02 to 0.15% ; and the remaining is iron.

The grey color is due to the fact that the carbon is present in the form of free graphite. It has a low tensile strength, high compressive strength and no ductility. It can be easily machined. A very good property of grey cast iron is that the free graphite in its structure acts as a lubricant. Due to this reason, it is very suitable for those parts where sliding action is desired. The grey iron castings are widely used for machine tool bodies, automotive cylinder blocks, heads, housings, fly-wheels, pipes and pipe fittings and agricultural implements.

Table 1.1 Grey Iron Casting per IS: 210 1993

According to Indian standard specifications (IS: 210 1993), the grey cast iron is designated by the alphabets FG followed by a figure indicating the minimum tensile strength in MPa or N/mm2. For example, FG 150 means grey cast iron with 150 MPa or N/mm2 as minimum tensile strength. The seven recommended grades of grey cast iron with their tensile strength and Brinell hardness number (B.H.N) are given in Table 1.3.

b. White cast iron. The white cast iron shows a white fracture and has the following approximate compositions:Carbon = 1.75 to 2.3%; Silicon = 0.85 to 1.2%; Manganese = less than 0.4%; Phosphorus = less than 0.2%; Sulphur = less than 0.12%, and the remaining is iron.

The white color is due to fact that it has no graphite and whole of the carbon is in the form of carbide (known as cementite) which is the hardest constituent of iron. The white cast iron has a high tensile strength and a low compressive strength. Since it is hard, therefore, it cannot be machined with ordinary cutting tools but requires grinding as shaping process. The white cast iron may be produced by casting against metal chills or by regulating analysis. The chills are used when a hard, wear resisting surface is desired for such products as for car wheels, rolls for crushing grains and jaw crusher plates.

c. Chilled cast iron. It is a white cast iron produced by quick cooling of molten iron. The quick cooling is generally called chilling and the cast iron so produced is called chilled cast iron. All castings are chilled at their outer skin by contact of the molten iron with the cool sand in the mould. But on most castings, this hardness penetrates to a very small depth (less than 1 mm). Sometimes, a casting is chilled intentionally and sometimes chilled becomes accidently to a considerable depth. The intentional chilling is carried out by putting inserts of iron or steel (chills) into the mould. When the molten metal comes into contact with the chill, its heat is readily conducted away and the hard surface is formed. Chills are used on any faces of a casting which are required to be hard to withstand wear and friction.

d. Mottled cast iron. It is a product in between grey and white cast iron in composition, color and general properties. It is obtained in castings where certain wearing surfaces have been chilled.

e. Malleable cast iron. The malleable iron is a cast iron-carbon alloy which solidifies in the as-cast condition in a graphite free structure, i.e. total carbon content is present in its combined form as cementite (Fe3C).

f. Nodular or spheroidal graphite cast iron. The nodular or spheroidal graphite cast iron is also called ductile cast iron or high strength cast iron. This type of cast iron is obtained by adding small amounts of magnesium (0.1 to 0.8%) to the molten grey iron. The addition of magnesium causes the *graphite to take form of small nodules or spheroids instead of the normal angular flakes. It has high fluidity, castability, tensile strength, toughness, wear resistance, pressure tightness, weldability and machinability. It is generally used for castings requiring shock and impact resistance along with good machinability, such as hydraulic cylinders, cylinder heads, rolls for rolling mill and centrifugally cast products.

2. Alloy Cast IronThe cast irons contain small percentages of other constituents like silicon, manganese, sulphur and phosphorus. These cast irons may be called as plain cast irons. The alloy cast iron is produced by adding alloying elements like nickel, chromium, molybdenum, copper and manganese in sufficient quantities. These alloying elements give more strength and result in improvement of properties. The alloy cast iron has special properties like increased strength, high wear resistance, corrosion resistance or heat resistance. The alloy cast irons are extensively used for gears, automobile parts like cylinders, pistons, piston rings, crank cases, crankshafts, camshafts, sprockets, wheels, pulleys, brake drums and shoes, parts of crushing and grinding machinery etc.

Effect of Impurities on Cast IronWe have discussed in the previous articles that the cast iron contains small percentages of silicon, sulphur, manganese and phosphorous. The effect of these impurities on the cast iron is as follows:1. Silicon. It may be present in cast iron up to 4%. It provides the formation of free graphite which makes the iron soft and easily machinable. It also produces sound castings free from blow-holes, because of its high affinity for oxygen.2. Sulphur. It makes the cast iron hard and brittle. Since too much sulphur gives unsound casting, therefore, it should be kept well below 0.1% for most foundry purposes.3. Manganese. It makes the cast iron white and hard. It is often kept below 0.75%. It helps to exert a controlling influence over the harmful effect of sulphur.4. Phosphorus. It aids fusibility and fluidity in cast iron, but induces brittleness. It is rarely allowed to exceed 1%. Phosphoric irons are useful for casting of intricate design and for many light engineering castings when cheapness is essential.

3. Wrought IronIt is the purest iron which contains at least 99.5% iron but may contain up to 99.9% iron. The typical composition of a wrought iron is Carbon = 0.020%, Silicon = 0.120%, Sulphur = 0.018%, Phosphorus = 0.020%, Slag = 0.070%, and the remaining is iron.

The wrought iron is produced from pig iron by remelting it in the puddling furnace of reverberatory type. The molten metal free from impurities is removed from the furnace as a pasty mass of iron and slag. The balls of this pasty mass, each about 45 to 65 kg are formed.

These balls are then mechanically worked both to squeeze out the slag and to form it into some commercial shape.

The wrought iron is a tough, malleable and ductile material. It cannot stand sudden and excessive shocks. Its ultimate tensile strength is 250 MPa to 500 MPa and the ultimate compressive strength is 300 MPa.

It can be easily forged or welded. It is used for chains, crane hooks, railway couplings, and water and steam pipes.

4. SteelIt is an alloy of iron and carbon, with carbon content up to a maximum of 1.5%. The carbon occurs in the form of iron carbide, because of its ability to increase the hardness and strength of the steel. Other elements e.g. silicon, sulphur, phosphorus and manganese are also present to greater or lesser amount to impart certain desired properties to it. Most of the steel produced now-a-days is plain carbon steel or simply carbon steel.

Plain carbon steels are the most important group of engineering alloys and account for the vast majority of steel produced. Their relatively low cost and wide range of useful properties makes them attractive as engineering materials. Applications for plain carbon steel are countless, with product forms consisting of sheet, strip, plate, bar, wire, and tubular products. Plain carbon steels are members of the family of ferrous alloys, which also includes alloy steels, stainless steels, tool steels, and cast irons.

A carbon steel is defined as a steel which has its properties mainly due to its carbon content and does not contain more than 0.5% of silicon and 1.5% of manganese. The plain carbon steels varying from 0.06% carbon to 1.5% carbon are divided into the following types depending upon the carbon content.

1. Dead mild steel up to 0.15% carbon

2. Low Carbon Steels (0.10 0.25 % C)1. Carbon content up to 0.30 percent or 30 points2. Microstructure consists of ferrite and pearlite constituents.3. Soft and weak, but have outstanding ductility and toughness.4. Machinable, weldable, and the least expensive to produce.5. Applications: automobile body components, structural shapes.6. Yield strength around 275MPa (40,000psi), Tensile strength between 415MPa and 550 MPa (60,000psi and 80,000psi), ductility of 25& EL

3. Medium Carbon Steels (0.25 0.50 % C)1. Carbon content from 0.30 to 0.50 percent or 30 to 50 points1. High strength, wear resistance, toughness.1. Applications: railway wheels and tracks, gears, crankshafts, and other machine parts.

4. High Carbon Steels (0.50 0.70 % C)1. Carbon content from 0.50 to 1.05% or 50 to 105 points2. Hardest, strongest, least ductile carbon steels.3. Wear resistant, capable of holding a sharp cutting edge4. Usually contains chromium, vanadium, tungsten, and molybdenum.5. Utilize as cutting tools, dies for forming and shaping materials.

According to Indian standard [IS: 1762 (Part-I)1974], a new system of designating the steel is recommended. According to this standard, steels are designated on the following two basis:2. On the basis of mechanical properties, and2. On the basis of chemical composition.5. High Strength, Low Alloy Steels (HSLA)1. Contains other alloying elements such as copper, vanadium, nickel and molybdenucombined in concentration high as 10wt% possess high strength.2. Ductile, formable, machinable.3. Corrosion resistant at normal atmosphere.4. Tensile strength in excess of 480MPa (70,000psi).

5. High Strength, Low Alloy Steels (HSLA) Contains other alloying elements such as copper, vanadium, nickel and molybdenum combined in concentration high as 10wt% possess high strength. Ductile, formable, machinable. Corrosion resistant at normal atmosphere. Tensile strength in excess of 480MPa (70,000psi).

Effect of Impurities on Steel

The following are the effects of impurities like silicon, sulphur, manganese and phosphorus on steel.1. Silicon. The amount of silicon in the finished steel usually ranges from 0.05 to 0.30%. Silicon is added in low carbon steels to prevent them from becoming porous. It removes the gases and oxides, prevent blow holes and thereby makes the steel tougher and harder.2. Sulphur. It occurs in steel either as iron sulphide or manganese sulphide. Iron sulphide because of its low melting point produces red shortness, whereas manganese sulphide does not affect so much. Therefore, manganese sulphide is less objectionable in steel than iron sulphide.3. Manganese. It serves as a valuable deoxidizing and purifying agent in steel. Manganese also combines with sulphur and thereby decreases the harmful effect of this element remaining in the steel. When used in ordinary low carbon steels, manganese makes the metal ductile and of good bending qualities. In high speed steels, it is used to toughen the metal and to increase its critical temperature.4. Phosphorus. It makes the steel brittle. It also produces cold shortness in steel. In low carbon steels, it raises the yield point and improves the resistance to atmospheric corrosion. The sum of carbon and phosphorus usually does not exceed 0.25%.

6. Free Cutting SteelsThe free cutting steels contain sulphur and phosphorus. These steels have higher sulphur content than other carbon steels. In general, the carbon content of such steels vary from 0.1 to 0.45 per cent and sulphur from 0.08 to 0.3 per cent. These steels are used where rapid machining is the prime requirement. It may be noted that the presence of sulphur and phosphorus causes long chips in machining to be easily broken and thus prevent clogging of machines. Nowadays, lead is used from 0.05 to 0.2 per cent instead of sulphur, because lead also greatly improves the machinability of steel without the loss of toughness.

7. Alloy SteelAlloy steel may be defined as a steel to which elements other than carbon are added in sufficient amount to produce an improvement in properties. The alloying is done for specific purposes to increase wearing resistance, corrosion resistance and to improve electrical and magnetic properties, which cannot be obtained in plain carbon steels. The chief alloying elements used in steel are nickel, chromium, molybdenum, cobalt, vanadium, manganese, silicon and tungsten. Each of these elements confers certain qualities upon the steel to which it is added. These elements may be used separately or in combination to produce the desired characteristic in steel.

Following are the effects of alloying elements on steel:1. Nickel. It increases the strength and toughness of the steel. These steels contain 2 to 5% nickel and from 0.1 to 0.5% carbon.

2. Chromium. It is used in steels as an alloying element to combine hardness with high strength and high elastic limit. It also imparts corrosion-resisting properties to steel. The most common chrome steels contains from 0.5 to 2% chromium and 0.1 to 1.5% carbon. The chrome steel is used for balls, rollers and races for bearings

3. Tungsten. It prohibits grain growth, increases the depth of hardening of quenched steel and confers the property of remaining hard even when heated to red color. It is usually used in conjunction with other elements. Steel containing 3 to 18% tungsten and 0.2 to 1.5% carbon is used for cutting tools. The principal uses of tungsten steels are for cutting tools, dies, valves, taps and permanent magnets.

4. Vanadium. It aids in obtaining a fine grain structure in tool steel. The addition of a very small amount of vanadium (less than 0.2%) produces a marked increase in tensile strength and elastic limit in low and medium carbon steels without a loss of ductility.

5. Manganese. It improves the strength of the steel in both the hot rolled and heat treated condition. The manganese alloy steels containing over 1.5% manganese with a carbon range of 0.40 to 0.55% are used extensively in gears, axles, shafts and other parts where high strength combined with fair ductility is required.

6. Silicon. The silicon steels behave like nickel steels. These steels have a high elastic limit as compared to ordinary carbon steel. Silicon steels containing from 1 to 2% silicon and 0.1 to 0.4% carbon and other alloying elements are used for electrical machinery, valves in I.C. engines, springs and corrosion resisting materials.

7. Cobalt. It gives red hardness by retention of hard carbides at high temperatures. It tends to decarburize steel during heat-treatment. It increases hardness and strength and also residual magnetism and coercive magnetic force in steel for magnets.

8. Molybdenum. A very small quantity (0.15 to 0.30%) of molybdenum is generally used with chromium and manganese (0.5 to 0.8%) to make molybdenum steel

8. Stainless Steel Highly resistant to corrosion. 11wt% chromium required. Applications: gas turbines, steam boilers, furnaces, aircraft, missiles, nuclear power plant generating unit.It is defined as that steel which when correctly heat treated and finished, resists oxidation and corrosive attack from most corrosive media.

a. Martensitic stainless steel. The chromium steels containing 12 to 14 per cent chromium and 0.12 to 0.35 per cent carbon are the first stainless steels developed. Since these steels possess Martensitic structure, therefore, they are called Martensitic stainless steels. These steels are magnetic and may be hardened by suitable heat treatment and the hardness obtainable depends upon the carbon content. These steels can be easily welded and machined. When formability, softness, etc. are required in fabrication, steel having 0.12 per cent maximum carbon is often used in soft condition. With increasing carbon, it is possible by hardening and tempering to obtain tensile strength in the range of 600 to 900 N/mm2, combined with reasonable toughness and ductility. In this condition, these steels find many useful general applications where mild corrosion resistance is required. Also, with the higher carbon range in the hardened and lightly tempered condition, tensile strength of about 1600 N/mm2 may be developed with lowered ductility.

These steels may be used where the corrosion conditions are not too severe, such as for hydraulic, steam and oil pumps, valves and other engineering components. However, these steels are not suitable for shafts and parts working in contact with non-ferrous metals (i.e. brass, bronze or gun metal bearings) and with graphite packings, because electrolytic corrosion is likely to occur. After hardening and light tempering, these steels develop good cutting properties. Therefore, they are used for cutlery, springs, surgical and dental instruments.

Note: The presence of chromium provides good resistance to scaling up to a temperature of about 750C, but it is not suitable where mechanical strength in the temperature range of 600 to 750C is required. In fact, creep resistance of these steels at this temperature is not superior to that of mild steel. But at temperature below 600C, the strength of these steels is better than that of carbon steels and up to 480C is even better than that of austenitic steels.

b. Ferritic stainless steel. The steels containing greater amount of chromium (from 16 to 18 per cent) and about 0.12 per cent carbon are called ferritic stainless steels. These steels have better corrosion resistant property than martensitic stainless steels. But, such steels have little capacity for hardening by heat treatment. However, in the softened condition, they possess good ductility and are mainly used as sheet or strip for cold forming and pressing operations for purposes where moderate corrosion resistance is required. They may be cold worked or hot worked. They are ferro-magnetic, usually undergo excessive grain growth during prolonged exposure to elevated temperatures, and may develop brittleness after electric arc resistance or gas welding. These steels have lower strength at elevated temperatures than martensitic steels. However, resistance to scaling and corrosion at elevated temperatures are usually better. The machinability is good and they show no tendency to intercrystalline corrosion.

Note: When nickel from 1.5 to 2.5 per cent is added to 16 to 18 per cent chromium steel, it not only makes more resistant to corrosion than martensitic steel but also makes it hardenable by heat treatment. Such steel has good resistance to electrolytic corrosion when in contact with non-ferrous metals and graphite packings. Thus it is widely used for pump shafts, spindles and valves as well as for many other fittings where a good combination of mechanical and corrosion properties are required.

c. Austenitic stainless steel. The steel containing high content of both chromium and nickel are called austenitic stainless steels. There are many variations in chemical composition of these steels, but the most widely used steel contain 18 per cent chromium and 8 per cent nickel with carbon content as low as possible. Such steel is commonly known as 18/8 steel. These steels cannot be hardened by quenching; in fact they are softened by rapid cooling from about 1000C. They are nonmagnetic and possess greatest resistance to corrosion and good mechanical properties at elevated temperature.

These steels are very tough and can be forged and rolled but offer great difficulty in machining. They can be easily welded, but after welding, it is susceptible to corrosive attack in an area adjacent to the weld. This susceptibility to corrosion (called intercrystalline corrosion or weld decay) may be removed by softening after welding by heating to about 1100C and cooling rapidly. These steels are used in the manufacture of pump shafts, rail road car frames and sheathing, screws, nuts and bolts and small springs. Since 18/8 steel provide excellent resistance to attack by many chemicals, therefore, it is extensively used in chemical, food, paper making and dyeing industries.

Note: When increased corrosion resistance properties are required, for some purposes, then molybdenum from 1to 3 percent may be added.

9. High Tensile SteelIt is very strong and very tough ferrous metal and is exclusively used for manufacturing of Gears, shafts, engine parts etc. This is one of the most frequently used ferrous metals in industries because of its strength, hardness and toughness.10. Heat Resisting SteelsThe steels which can resist creep and oxidation at high temperatures and retain sufficient strength are called heat resisting steels.

11. High Speed Tool SteelsThese steels are used for cutting metals at a much higher cutting speed than ordinary carbon tool steels. The carbon steel cutting tools do not retain their sharp cutting edges under heavier loads and higher speeds. This is due to the fact that at high speeds, sufficient heat may be developed during the cutting operation and causes the temperature of the cutting edge of the tool to reach a red heat. This temperature would soften the carbon tool steel and thus the tool will not work efficiently for a longer period. The high speed steels have the valuable property of retaining their hardness even when heated to red heat. Most of the high speed steels contain tungsten as the chief alloying element, but other elements like cobalt, chromium, vanadium, etc. may be present in some proportion.

Following are the different types of high speed steels:

a) High speed steel. This steel, on an average, contains 18 per cent tungsten, 4 per cent chromium and 1 per cent vanadium. It is considered to be one of the best of all purpose tool steels. It is widely used for drills, lathe, planer and shaper tools, milling cutters, reamers, broaches, threading dies, punches, etc.b) Molybdenum high speed steel. This steel, on an average, contains 6 per cent tungsten, 6 per cent molybdenum, 4 per cent chromium and 2 per cent vanadium. It has excellent toughness and cutting ability. The molybdenum high speed steels are better and cheaper than other types of steels. It is particularly used for drilling and tapping operations.c) Super high speed steel. This steel is also called cobalt high speed steel because cobalt is added from 2 to 15 per cent, in order to increase the cutting efficiency especially at high temperatures. This steel, on an average, contains 20 per cent tungsten, 4 per cent chromium, 2 per cent vanadium and 12 per cent cobalt. Since the cost of this steel is more, therefore, it is principally used for heavy cutting operations which impose high pressure and temperatures on the tool.

12. Spring SteelsThe most suitable material for springs are those which can store up the maximum amount of work or energy in a given weight or volume of spring material, without permanent deformation. These steels should have a high elastic limit as well as high deflection value. The spring steel, for aircraft and automobile purposes should possess maximum strength against fatigue effects and shocks. The steels most commonly used for making springs are as follows:

Following are the various heat treatment processes commonly employed in engineering practice:

1. Normalizing. The main objects of normalizing are: To refine the grain structure of the steel to improve machinability, tensile strength and structure of weld. To remove strains caused by cold working processes like hammering, rolling, bending, etc., which makes the metal brittle and unreliable. To remove dislocations caused in the internal structure of the steel due to hot working. To improve certain mechanical and electrical properties.

The process of normalizing consists of heating the steel from 30 to 50C above its upper critical temperature (for hypoeutectoid steels) or Acm line (for hypereutectoid steels). It is held at this temperature for about fifteen minutes and then allowed to cool down in still air.

This process provides a homogeneous structure consisting of ferrite and pearlite for hypereutectoid steels, and pearlite and cementite for hypereutectoid steels. The homogeneous structure provides a higher yield point, ultimate tensile strength and impact strength with lower ductility to steels. The process of normalizing is frequently applied to castings and forgings, etc. The alloy steels may also be normalized but they should be held for two hours at a specified temperature and then cooling in the furnace.

Notes:(a) The upper critical temperature for a steel depends upon its carbon content. It is 900C for pure iron, 860C for steels with 2.2% carbon, 723C for steel with 0.8% carbon and 1130C for steel with 1.8% carbon.

(b) Steel containing 0.8% carbon is known as eutectoid steel, steel containing less than 0.8% carbon is called hypoeutectoid steel and steel containing above 0.8% carbon is called hypereutectoid steel.

2. Annealing. The main objects of annealing are: To soften the steel so that it may be easily machined or cold worked. To refine the grain size and structure to improve mechanical properties like strength and ductility. To relieve internal stresses which may have been caused by hot or cold working or by unequal contraction in casting. To alter electrical, magnetic or other physical properties. To remove gases trapped in the metal during initial casting.

The annealing process is of the following two types:

a. Full annealing. The purpose of full annealing is to soften the metal to refine the grain structure, to relieve the stresses and to remove trapped gases in the metal.

In order to avoid decarburization of the steel during annealing, the steel is packed in a cast iron box containing a mixture of cast iron borings, charcoal, lime, sand or ground mica. The box along with its contents is allowed to cool slowly in the furnace after proper heating has been completed.

The following table shows the approximate temperatures for annealing depending upon the carbon contents in steel.Table 1.2 Annealing temperatures

b. Process annealing. The process annealing is used for relieving the internal stresses previously set up in the metal and for increasing the machinability of the steel. In this process, steel is heated to a temperature below or close to the lower critical temperature, held at this temperature for some time and then cooled slowly. This causes complete recrystallization in steels which have been severely cold worked and a new grain structure is formed. The process annealing is commonly used in the sheet and wire industries.3. Spheroidising. It is another form of annealing in which cementite in the granular form is produced in the structure of steel. This is usually applied to high carbon tool steels which are difficult to machine. The operation consists of heating the steel to a temperature slightly above the lower critical temperature (730 to 770C). It is held at this temperature for some time and then cooled slowly to a temperature of 600C. The rate of cooling is from 25 to 30C per hour. The spheroidising improves the machinability of steels, but lowers the hardness and tensile strength. These steels have better elongation properties than the normally annealed steel.

4. Hardening. The main objects of hardening are:(a) To increase the hardness of the metal so that it can resist wear.(b) To enable it to cut other metals i.e. to make it suitable for cutting tools.

The process of hardening consists of:(i) Heating the metal to a temperature from 30 to 50C above the upper critical point for hypoeutectoid steels and by the same temperature above the lower critical point for hypereutectoid steels.(ii) Keeping the metal at this temperature for a considerable time, depending upon its thickness.(iii) Quenching (cooling suddenly) in a suitable cooling medium like water, oil or brine.

5. Tempering. The steel hardened by rapid quenching is very hard and brittle. It also contains internal stresses which are severe and unequally distributed to cause cracks or even rupture of hardened steel. The tempering (also known as drawing) is, therefore, done for the following reasons: 6. Surface hardening or case hardening. In many engineering applications, it is desirable that a steel being used should have a hardened surface to resist wear and tear. At the same time, it should have soft and tough interior or core so that it is able to absorb any shocks, etc. This is achieved by hardening the surface layers of the article while the rest of it is left as such. This type of treatment is applied to gears, ball bearings, railway wheels, etc.Following are the various surface or case hardening processes by means of which the surface layer is hardened:

(a) CarburizingCarburizing process has evolved with advancements in heat treatment techniques that have improved the hardness and durability of products like carbon steel wire springs and carbon steel forgings.

(b) CyanidingCyaniding is a case hardening with powdered potassium cyanide or potassium ferro-cyanide mixed with potassium bicarbonate substituted for the carbon.

(c) NitridingNitriding is a surface hardening accomplished by heating certain steel alloys immersed for the carbon.

(d) Induction hardeningInduction hardening is a surface hardening technique in which the surface of the metal is heated very quickly, using a no-contact method of induction heating. The alloy is then quenched, producing a marten site transformation at the surface while leaving the underlying metal unchanged. This creates a very hard, wear resistant surface while maintaining the proper toughness in the majority of the object. Crankshaft journals are a good example of an induction hardened surface.

(e) Flame hardeningFlame hardening is used to harden only a portion of a metal. Unlike differential hardening, where the entire piece is heated and then cooled at different rates, in flame hardening, only a portion of the metal is heated before quenching. This is usually easier than differential hardening, but often produces an extremely brittle zone between the heated metal and the unheated metal, as cooling at the edge of this heat affected zone is extremely rapid.

7. Aging (and age hardening) is a change in metal by which its structure recovers from an unstable or metastable condition that has been produced by quenching or cold working. The change in structure, which proceeds as a function of time and temperature, consists in precipitation often submicroscopic. The result is a change of mechanical and physical properties, a process that may be accelerated by using a temperature slightly higher than room temperature.

8. Graphitizing and annealing process causes the combined carbon to transform wholly or in part graphitic or free carbon; it is applied to cast iron, sometimes to high carbon steel.

9. Selective hardening. Many heat treating methods have been developed to alter the properties of only a portion of an object. These tend to consist of either cooling different areas of an alloy at different rates, by quickly heating in a localized area and then quenching, or by thermo-chemical diffusion.

10. Differential hardening. A differentially hardened katana. The bright, wavy line, called the nioi, separates the martensitic edge from the pearlitic back. The inset shows a close-up of the nioi, which is made up of single martensite grains surrounded by pearlite. The wood-grain appearance comes from layers of different composition.

11. Malleablizing is an annealing process whereby combined carbon in white cast iron is transformed wholly or in part to temper carbon. Temper carbon is free (graphitic) carbon in the form of rounded nodules, characteristic forms in graphitizing and malleablizing.

12. Stress relieving (thermal) is the heating of a metal body to a suitable temperature (generally just below the transformation range for steel, say 1100-1200oF) and holding it at that temperature for a suitable time (1 and 3 times for steel) for the purpose of reducing internal residual stresses. The internal stresses may be present because the body has been cast, quenched, normalized, machined, cold-worked, or welded.

13. Quench hardening. Quenching is a form of hardening whereby the metal is subjected to heat at a temperature above the critical point, then quickly immersing it into cold water or other cooling medium. The degree of hardness depends on the amount of carbon present and on the rate of cooling medium as ice water, cool water, oil, hot oil, molten lead, and etc.

There are numerous standard materials specifications. The two most widely used are the American Society of Testing Materials (ASTM), the Society of Automotive Engineers (SAE), and the American Iron and Steel Institute (AISI). The AISI and SAE specification numbers for steel are almost alike except that the AISI uses prefixes B, C, D, and E to indicate the method of manufacturing the carbon grades.

In general, the first two digits of the number represent a type of steel. And the last two digits in four digit numbers invariably give the approximate or average carbon content in points or hundredths of per cent.

The first digit (1), of this designation indicates the major alloying element. The SAE-AISI system then classifies all other alloy steels using the same four digit index as follows:

1 Carbon Steels;5 - Chromium steels;

2 - Nickel steels;6 - Chromium-vanadium steels;

3 - Nickel-chromium steels;7 - Tungsten-chromium steels;

4 - Molybdenum steels;9 - Silicon-manganese steels.

The second digit of the series indicates the concentration of the major element in percentiles (1 equals 1%). The last two digits of the series indicate the carbon concentration to 0.01%.

For Example, SAE 5130 indicates a chromium steel alloy, containing 1% of chromium and 0.30% of carbon.

Additional letters added between the second and third digits includeBwhen boron is added (between 0.0005 and 0.003%) for enhanced hardenability, andLwhen lead is added (between 0.15 and 0.35%) for enhanced machinability. The prefix M is used to designate merchant quality steel (the least restrictive quality descriptor for hot-rolled steel bars used in noncritical parts of structures and machinery). The prefixE(electric-furnace steel) and the suffixH(hardenability requirements) are mainly applicable to alloy steels

The Society of Automotive Engineers (SAE) designates SAE steel grades. These are four digit numbers which represent chemical composition standards for steel specifications. The American Iron and Steel Institute (AISI) originally started a very similar system. Over time they used the same numbers to refer to the same alloy, but the AISI system used a letter prefix to denote the steelmaking process. The prefix "A" denotes alloy basic open-heart. The prefix "B" denoted carbon acid Bessemer. The prefix "C" denoted open-hearth furnace, electric arc furnace or basic oxygen furnace. The prefix "D" denotes carbon acid open-heart while "E" denotes electric arc furnace steel.

If the prefix is omitted, the steel is assumed to be open hearth. (Example: AISI C1050 indicates a plain carbon, basic-open hearth steel that has 0.50 % Carbon content.)Another letter is the hardenability or H-value. (Example: 4340H)

Table 1.3 Major Classification of Steels

SAE DesignationType

1xxxCarbon steels

2xxxNickel steels

3xxxNickel-chromium steels

4xxxMolybdenum steels

5xxxChromium steels

6xxxChromium-vanadium steels

7xxxTungsten steels

8xxxNickel-chromium-vanadium steels

9xxxSilicon-manganese steels

86XXTriple Alloy steels which include Nickel (Ni), Chromium (Cr), and Molybdenum (Mo).These steels exhibit high strength and also high strength to weight ratio, good corrosion resistance.

87XX

93XX

94XX

97XX

98XX

Table 1.4 Classification of Steel According to AISI Standard









AISITypes of SteelAlloy Elements %

10Plain Carbon0.4 Mn

11Free machining0.7 Mn, 0.12 S

13High Manganese1.6 - 1.90 Mn

2Nickel Steels3.5 - 5.0 Ni

3Nickel Chromium1.0 - 3.5 Ni, 0.5 -1.75 Cr

40Molybdenum0.15 - 0.3 Mo

41Chrome-Molybdenum0.80 -1.1 Cr, 0.15 - 0.25 Mo

43Nickel - Chrome - Molybdenum1.65 - 2.0 Ni, 0.4 - 0.9 Cr,0.2 - 0.3 Mo

46Nickel - Molybdenum1.65 Ni, 1.65 Mo

5Chromium0.4 Cr

61Chromium - Vanadium0.5 - 1.1 Cr, 0.1 - 0.15 Va

81Nickel - Chrome - Molybdenum0.2 - 0.4 Ni, 0.3 - 0.55 Cr, 0.08 - 0.15 Mo

86Nickel - Chrome - Molybdenum0.4 - 0.6 Ni, 0.4 - 0.6 Cr, 0.15 - 0.25 Mo

92Silicon1.8 - 2.2 Si

From the table above showing that the first digit indicates the type of steel. Then the second one is the modification in alloys. Lastly, the two digits remainings show the carbon content in percentage.

Prior to 1995 the AISI was also involved, and the standard was designated the AISI/SAE steel grades. The AISI stopped being involved because it never wrote any of the specifications.Table 1.5 Steel Alloy Designation System

Table 1.5 Steel Alloy Designation System
















AISI-SAE DesignationNumberType

Carbon steels

10xxPlain Carbon (Mn. 1.00% max.)

11xxResulfurized

12xxResulfurized and rephosphorized

15xxPlain Carbon (max. Mn. range 1.00-1.65%)

Manganese steels

13xxMn 1.75

Nickel steels

23xxNi 3.50

25xxNi 5.00

Nickel-chromium steels

31xxNi 1.25; Cr 0.65, 0.80

32xxNi 1.75; Cr 1.07

33xxNi 3.50; Cr 1.50, 1.57

34xxNi 3.00; Cr 0.77

Molybdenum steels

40xxMo 0.20, 0.25

44xxMo 0.40, 0.52

Chromium-molybdenum steels

41xxCr 0.50, 0.80, 0.95; Mo 0.12, 0.20, 0.25, 0.30

Nickel-chromium-molybdenum steels

43xxNi 1.82; Cr 0.50, 0.80; Mo 0.25

43BVxxNi 1.82; Cr 0.50; Mo 0.12, 0.25; V 0.03 min.

47xxNi 1.05; Cr 0.45; Mo 0.20, 0.35

81xxNi 0.30; Cr 0.40; Mo 0.12

86xxNi 0.55; Cr 0.50; Mo 0.20

87xxNi 0.55; Cr 0.50; Mo 0.25

88xxNi 0.55; Cr 0.50; Mo 0.35

93xxNi 3.25; Cr 1.20; Mo 0.12

94xxNi 0.45; Cr 0.40; Mo 0.12

97xxNi 1.00; Cr 0.20; Mo 0.20

98xxNi 1.00; Cr 0.80; Mo 0.25

Nickel-molybdenum steels

46xxNi 0.85, 1.82; Mo 0.20, 0.25

48xxNi 3.50; Mo 0.25

Chromium steels

50xxCr 0.27, 0.40, 0.50, 0.65

51xxCr 0.80, 0.87, 0.92, 0.95, 1.00, 1.05

50xxxCr 0.50; C 1.00 min.

51xxxCr 1.02; C 1.00 min.

52xxxCr 1.45; C 1.00 min.

Chromium-vanadium steels

61xxCr 0.60, 0.80, 0.95; V 0.10, 0.15

Tungsten-chromium steels

72xxW 1.75; Cr 0.75

Silicon-manganese steels

92xxSi 1.40, 2.00; Mn 0.65, 0.82, 0.85; Cr 0.00, 0.65

High-strength low-alloy steels

9xxVarious SAE grades

Boron steels

xxBxxB denotes boron steels

Leaded steels

xxLxxL denotes leaded steels

Table 1.6 Type and Description of Steels

TYPEDESCRIPTION

Carbon Steels The first digit is "1" as in10xx, 11xx, and 12xx. The second digit describes processing: "1" isresulfurizedand "2" is resulfurized andrephosphorized.

Manganese Steel The first digit is "1" as in 13xx and is, indeed, a carbon steel. However, sincemanganeseis a normal by-product of carbon steel making the AISI/SAE has decided not to classify it as an alloy steel. The second digit is always "3".

Nickel Steel The first digit is "2" as in 23xx and 25xx. The second digit designates the percentage ofnickelin the steel.

Nickel-Chromium Steel

The first digit is "3" as in 31xx, 32xx, and 33xx, The second digit designates the percentage of nickel andchromiumin the steel.

Molybdenum Steels

The first digit is "4" as in 40xx and 44xx. The second digit designates the percentage ofmolybdenumin the steel.

Chromium Steel

The first digit is "5" as in 51xx and 52xx. The second digit designates the percentage of chromium in the steel.

Chromium-Vanadium Steel The first digit is "6" as in 61xx. The second digit designates the percentage of chromium andvanadiumin the steel.

Tungsten-Chromium Steel

The first digit is "7" as in 72xx. The second digit designates the percentage oftungstenand chromium.

Silicon-Manganese Steel

The first digit is "9" as in 92xx. The second digit designates the percentage ofsiliconand manganese in the steel.

Triple Alloy Steels

These steels contain three alloys. The first digit can be "4", "8", or "9" depending on the predominate alloy. The second digit designates the percentage of the reaming two alloys.

As shown, the AISI / SAE steel designation system gives information about the chemical composition of the steel (alloy type and carbon content). However, in many cases, this is not enough information for the purchasing company to procure the steel. The ASTM specification of fabrication methodology will often be added to the material specifications demonstrated but fabrication methods will not be discussed here. It remains to be seen how the UNS will designate manufacturing specifications.

Following are two examples of UNS designators and how they relate to AISE/SAE designations:

1. Resulfurized carbon steel containing 0.21% carbon would be UNS G11210 or AISI/ASE 1121.

2. Steel alloyed with 20% chromium and vanadium and containing 0.75% carbon would be UNS G62750 or AISI/SAE 6275.

A number system or series to identify carbon and alloy steel had been developed by Society of Automotive Engineers (SAE) and American Iron and Steel Industry (AISI). Steel are names based on their alloying elements and carbon content.

The first 2 numbers refer to the alloying elements present in that steel. First number tells the kind of steel and the second number shows the approximate percent of alloy elements.

The last 2 numbers shows the carbon content in points (100 points equal 1 percent).Table 1.7 Steels and their Respective Numbers

SteelNumberRange of Number

a) Carbon steel

Carbon steel SAE-AISI1XXX

Plain Carbon10XX

Free machining (resulphurized)11XX

Resulphrized, rephosporised12XX

b) Alloy Steel

Manganese13XX

Molybdenum4XXX

C-Mo(0.25% Mo)40XX

CR-Mo(0.70% Cr, 0.15% Mo)41XX

Ni-Cr-Mo(1.8% Ni, 0.65% CR)43XX

Ni-Mo(1.75% Ni)46XX

Ni-Cr(0.45% Ni, 0.2% Mo)47XX

Ni-Mo(3.5% Ni, 0.25% Mo)48XX

Chromium5XXX

0.5% Cr50XX

1.0% Cr51XX5120-5152

1.5% Cr52XX52095-52100

Corrosion-heat resistant514XX(AISI 400 Series)

Chromium-Vanadium6XXX

1% Cr, O.12% V61XX6120-6152

Silicon Manganese

0.85% Mn, 2% Si92XX9255-9262

Triple-alloy Steels

0.55% Ni, 0.50% Cr, 0.20% Mo,86XX8615-8660

0.55% Ni, 0.50% Cr, 0.25% Mo87XX8720-8750

3.25% Ni, 1.2% Cr, 0.12% Mo93XX9310-9317

0.45% Ni, 0.4% Cr, 0.12% Mo94XX9437-9445

0.45% Ni, 0.15% Cr, 0.2% Mo97XX9747-9763

1.00 % Ni, 0.8% Cr, 0.25% Mo98XX9840-9850

Boron (~0.05% Mn)XXBXX

AISI DesignationApplication

AISI 2330Bolts, studs, tubing subjected to torsional stress

AISI 2340Quenched and tempered shafting connecting rods, very highly stresses bolts, forgings

AISI 2350High capacity gears, shafts, heavy ductile machine parts

AISI 3130Shafts, bolts, steering knuckles

AISI 3140Air craft and truck-engine crank-shafts, axels, earth moving equipment

AISI 3150Wear-resisting parts in excavating and farm machinery, gears, forgings

AISI 3240Shafts, highly stressed pins and keys, gears

AISI 3300 seriesFor heavy parts requiring deep penetrating of the heat treatment and high fatigue strength per unit weight

AISI 4063Leaf and coil springs

AISI 4130, 4140Automobile connecting rods and axels, air craft parts and tubing

AISI 4340Crankshafts, axels, gears, landing gear parts

AISI 4640Gears, splined shafts, hand tools miscellaneous heavy duty machine parts

AISI 8630Connecting rods, bolts shapes, air hardens after welding

AISI 8640, 8740Gears, propeller-shafts, knuckles shapes

Table. 1.8 Uses of Alloy Steels

We have already discussed that the non-ferrous metals are those which contain a metal other than iron as their chief constituent. The non-ferrous metals are usually employed in industry due to the following characteristics: Ease of fabrication (casting, rolling, forging, welding and machining), Resistance to corrosion, Electrical and thermal conductivity, and Weight.

1. ALUMINUMIt is white metal produced by electrical processes from its oxide (alumina), which is prepared from a clayey mineral called bauxite. It is a light metal having specific gravity 2.7 and melting point 658C. The tensile strength of the metal varies from 90 MPa to 150 MPa.

Aluminum AlloysThe aluminum may be alloyed with one or more other elements like copper, magnesium, manganese, silicon and nickel. The addition of small quantities of alloying elements converts the soft and weak metal into hard and strong metal, while still retaining its light weight. The main aluminum alloys are discussed below:

a) Duralumin. It is an important and interesting wrought alloy. This alloy possesses maximum tensile strength (up to 400 MPa) after heat treatment and age hardening. After working, if the metal is allowed to age for 3 or 4 days, it will be hardened. This phenomenon is known as age hardening.b) Y-alloy. It is also called copper-aluminum alloy. The addition of copper to pure aluminum increases its strength and machinability. It is mainly used for cast purposes, but it can also be used for forged components like duralumin. Since Y-alloy has better strength (than duralumin) at high temperature, therefore, it is much used in aircraft engines for cylinder heads and pistons.c) Magnalium. It is made by melting the aluminum with 2 to 10% magnesium in a vacuum and then cooling it in a vacuum or under a pressure of 100 to 200 atmospheres. It also contains about 1.75% copper. Due to its light weight and good mechanical properties, it is mainly used for aircraft and automobile components.d) Hindalium. It is an alloy of aluminum and magnesium with a small quantity of chromium. It is the trade name of aluminum alloy produced by Hindustan Aluminum Corporation Ltd, Renukoot (U.P.). It is produced as a rolled product in 16 gauges, mainly for anodized utensil manufacture.

2. COPPERIt is one of the most widely used non-ferrous metals in industry. It is a soft, malleable and ductile material with a reddish-brown appearance. Its specific gravity is 8.9 and melting point is 1083C. The tensile strength varies from 150 MPa to 400 MPa under different conditions. It is a good conductor of electricity. It is largely used in making electric cables and wires for electric machinery and appliances, in electrotyping and electroplating, in making coins and household utensils.

Copper AlloysThe copper alloys are broadly classified into the following two groups:

Copper-zinc alloys (Brass). The most widely used copper-zinc alloy is brass. There are various types of brasses, depending upon the proportions of copper and zinc. This is fundamentally a binary alloy of copper with zinc each 50%. By adding small quantities of other elements, the properties of brass may be greatly changed.

Copper-tin alloys (Bronze). The alloys of copper and tin are usually termed as bronzes. The useful range of composition is 75 to 95% copper and 5 to 25% tin. The metal is comparatively hard, resists surface wear and can be shaped or rolled into wires, rods and sheets very easily. In corrosion resistant properties, bronzes are superior to brasses. Some of the common types of bronzes are as follows:a) Phosphor bronze. When bronze contains phosphorus, it is called phosphor bronze. Phosphorus increases the strength, ductility and soundness of castings. This alloy possesses good wearing qualities and high elasticity. The metal is resistant to salt water corrosion. The composition of the metal varies according to whether it is to be forged, wrought or made into castings. b) Silicon bronze. It contains 96% copper, 3% silicon and 1% manganese or zinc. It has good general corrosion resistance of copper combined with higher strength. It can be cast, rolled, stamped, forged and pressed either hot or cold and it can be welded by all the usual methods.c) Beryllium bronze. It is a copper base alloy containing about 97.75% copper and 2.25% beryllium. It has high yield point, high fatigue limit and excellent cold and hot corrosion resistance. It is particularly suitable material for springs, heavy duty electrical switches, cams and bushings. Since the wear resistance of beryllium copper is five times that of phosphor bronze, therefore, it may be used as a bearing metal in place of phosphor bronze.d) Manganese bronze. It is an alloy of copper, zinc and little percentage of manganese. The usual composition of this bronze is as follows:Copper = 60%, Zinc = 35%, and Manganese = 5%This metal is highly resistant to corrosion. It is harder and stronger than phosphor bronze. It is generally used for bushes, plungers, feed pumps, rods etc. Worm gears are frequently made from this bronze.e) Aluminum bronze. It is an alloy of copper and aluminum. The aluminum bronze with 68% aluminum has valuable cold working properties. The maximum tensile strength of this alloy is 450 MPa with 11% of aluminum. They are most suitable for making components exposed to severe corrosion conditions. When iron is added to these bronzes, the mechanical properties are improved by refining the grain size and improving the ductility.

3. MAGNESIUM ALLOYSMagnesium alloys are approximately two-thirds as heavy as aluminum alloys. The common alloys contain from 4 to 12 per cent of aluminum and from 0.1 to 0.3 per cent of manganese; those with more than 6 per cent of aluminum can be heat-treated and aged to increase the yield strength. The alloys are resistant to atmospheric corrosion if kept dry, but when humidity is high, corrosion proceeds slowly with a powder forming on the surface. In very moist or salty atmosphere, or when rain is trapped on the part, surface roughening is pronounced. In general, all magnesium-alloy parts should be given a protective coating. 4. GUN METALIt is an alloy of copper, tin and zinc. It usually contains 88% copper, 10% tin and 2% zinc. This metal is also known as Admiralty gun metal. The zinc is added to clean the metal and to increase its fluidity.It is not suitable for being worked in the cold state but may be forged when at about 600C. The metal is very strong and resistant to corrosion by water and atmosphere. Originally, it was made for casting guns. It is extensively used for casting boiler fittings, bushes, bearings, glands, etc.

5. LEADIt is a bluish grey metal having specific gravity 11.36 and melting point 326C. It is so soft that it can be cut with a knife. It has no tenacity. It is extensively used for making solders, as a lining for acid tanks, cisterns, water pipes, and as coating for electrical cables.The lead base alloys are employed where a cheap and corrosion resistant material is required. An alloy containing 83% lead, 15% antimony, 1.5% tin and 0.5% copper is used for large bearings subjected to light service.

6. TINIt is brightly shining white metal. It is soft, malleable and ductile.. It is used for making important alloys, fine solder, as a protective coating for iron and steel sheets and for making tin foil used as moisture proof packing.

A tin base alloy containing 88% tin, 8% antimony and 4% copper is called babbit metal. It is a soft material with a low coefficient of friction and has little strength. It is the most common bearing metal used with cast iron boxes where the bearings are subjected to high pressure and load.

7. BEARING METALSThe following are the widely used bearing metals:a) Copper-base alloys, b) Lead-base alloys, c) Tin-base alloys, and d) Cadmium-base alloys8. ZINC BASE ALLOYSThe most of the die castings are produced from zinc base alloys. These alloys can be casted easily with a good finish at fairly low temperatures. They have also considerable strength and are low in cost. The usual alloying elements for zinc are aluminum, copper and magnesium and they are all held in close limits.

9. NICKEL BASE ALLOYSThe nickel base alloys are widely used in engineering industry on account of their high mechanical strength properties, corrosion resistance, etc.

10. TITANIUMThere are three structural types of titanium alloys:a) Alpha Alloys are non-heat treatable and are generally very weld-able. They have low to medium strength, good notch toughness, reasonably good ductility and possess excellent mechanical properties at cryogenic temperaturesb) Alpha-Beta Alloys are heat treatable and most are weldable. Their strength levels are medium to high. Their hot-forming qualities are good, but the high temperature creep strength is not as good as in most alpha alloys.c) Beta or near-beta alloys are readily heat treatable, generally weldable, capable of high strengths and good creep resistance to intermediate temperatures. Excellent formability can be expected of the beta alloys in the solution treated condition. Beta-type alloys have good combinations of properties in sheet, heavy sections, fasteners and spring applications.

11. COBALT-CHROMIUM-TUNGSTEN-MOLYBDENUM WEAR-RESISTANT ALLOYSThese alloys feature a wear resistance which makes them ideal for metal-cutting operations. Their ability to retain hardness even at red-heat temperatures also makes them especially useful for cutting tools.

12. PRECIOUS METALSThese include silver, gold, platinum, palladium, iridium, osmium, rhodium, and ruthenium, and their alloys. These alloys are produced under technical and legal requirements. Gold alloys used for jewelry are described in karats. The karat is the content of gold expressed in twenty-fourths. An 18-karat gold alloy would contain 18/24 gold (75 percent by weight). Other than jewelry, there are many industrial uses for precious metals.

13. TRUE BRASS True Brass sinks or protects bench tops where a large amount of acid is used. Lead-lined pipes are used in systems that carry. This is an alloy of copper and zinc. Additional corrosive chemicals, frequently, lead are used in alloyed elements, such as aluminum, lead, tin, iron, manganese, form to increase its low-tensile strength. Alloyed with or phosphorus, are added to give the alloy specific tin, lead produces a soft solder.

14. BRONZE Brass is a combination of 84% copper and 16% tin and was the best metal available before steel-making techniques were developed. Many complex bronze alloys, containing such elements as zinc, lead, iron, aluminum, silicon, and phosphorus, are now available. Today, the name bronze is applied to any copper-based alloy that looks like bronze. In many cases, there is no real distinction between the composition of bronze and that of brass.

15. COPPER-NICKEL ALLOYS Nickel is used in these alloys to make them strong, tough, and resistant to wear and corrosion. Because of their high resistance to corrosion, copper nickel alloys, containing 70% copper and 30% nickel or 90% copper and 10% nickel, are used for saltwater piping systems. Small storage tanks and hot-water reservoirs are con. You often see zinc used on iron or steel in the form of a protective coating called galvanizing. Zinc is also used in soldering fluxes, die castings, and as an alloy in making brass and bronze.

16. BERYLLIUMBeryllium has one of the highest melting points of the light metals. The modulus of elasticity of beryllium is approximately 1/3 greater than that of steel. It has excellent thermal conductivity, is nonmagnetic and resists attack by concentrated nitric acid.

17. SINTERED MATERIALSCertain materials that cannot be alloyed by melting can be formed into useful products by mixing in powdered form, compressing under high pressure, and bonding by sintering. After the powders are obtained, they are intimately mixed and compressed in hard steel dies under pressures up to 100 tons/ in.2, depending on the materials. The compressed mass usually is weak mechanically and has a density about 0.8 of that of the solid material.

18. METALS FOR HIGH-TEMPERATURE SERVICEMetal parts for nuclear reactors, internal-combustion engines, valves, superheated steam equipment; oil stills, chemical and petroleum processes, and similar service are stressed at temperatures ranging from 200 to 1800oF. Ceramics usually are used for higher operating temperatures. The metals used for such parts must be specially selected from those materials which retain a large percentage of their strength at high temperatures and which do not creep excessively.

19. METALLIC REFRACTORY MATERIALSRefractory materials are materials with melting points above approximately 2550oF. The advent of gas turbines, rockets, glassmaking, electronics, and high-temperature industrial processes has necessitated the development of suitable refractory materials. These materials include metals, oxides, carbides, silicides, borides, intermetallic compounds, and graphite in addition to ceramics.

20. NON-METALLIC MATERIALSThe non-metallic materials are used in engineering practice due to their low density, low cost, flexibility, resistant to heat and electricity. Though there are many non-metallic materials, yet the following are important from the subject point of view.

a. Plastics. The plastics are synthetic materials which are molded into shape under pressure with or without the application of heat. These can also be cast, rolled, extruded, laminated and machined. Following are the two types of plastics:a) Thermosetting plastics, andb) Thermoplastic.

Thermosetting plastics are those which are formed into shape under heat and pressure and results in a permanently hard product.

Thermoplastic materials do not become hard with the application of heat and pressure and no chemical change occurs. They remain soft at elevated temperatures until they are hardened by cooling. These can be remelted repeatedly by successive application of heat. Some of the common thermoplastics are cellulose nitrate (Celluloid), polythene, polyvinyl acetate, polyvinyl chloride (P.V.C.), etc.b. Rubber. It is one of the most important natural plastics. It resists abrasion, heat, strong alkalis and fairly strong acids. Soft rubber is used for electrical insulations. It is also used for power transmission belting, being applied to woven cotton or cotton cords as a base. The hard rubber is used for piping and as lining for pickling tanks.c. Leather. It is very flexible and can withstand considerable wear under suitable conditions. It is extensively used for power transmission belting and as a packing or as washers.d. Ferrodo. It is a trade name given to asbestos lined with lead oxide. It is generally used as a friction lining for clutches and brakes.e. Ceramics. Materials having a high percentage of alumina (Al2O2) or steatite (MgOSiO2) are known as ceramics, particularly if they have been fired. They have high thermal and electric resistivity, good chemical resistance, and relatively high hardness and strength, and can be used at temperatures much higher than the average metals. Aluminum casting alloys are listed in many specifications of various standardizing agencies. These include Federal Specifications, Military Specifications, ASTM Specifications and SAE Specifications, to mention some. The numbering systems used by each differ and are not always correctable. Casting alloys are available from producers who use a numbering system is the one used in the table of aluminum casting alloys which are given further along this section.

A system of four digit numerical designation for wrought aluminum and wrought aluminum alloys are adopted by the Aluminum Association in 1954. This system is used by the commercial producers and is similar to the one used by the SAE; the differences being the addition of two prefix letters.

Aluminum alloys can be categorized into a number of groups based on the particular materials characteristics such as its ability to respond to thermal and mechanical treatment and the primary alloying element added to the aluminum alloy. When we consider the numbering / identification system used for aluminum alloys, the above characteristics are identified. The wrought and cast aluminums have different systems of identification; the wrought having a 4-digit system, and the castings having a 3-digit and 1-decimal place system.

Wrought Alloys Wrought alloys fall into two distinct categories:a) Those which derive their properties from work hardening.b) Those which depend upon solution heat treatment and age hardening.

We shall first consider the 4-digit wrought aluminum alloy identification system.Table 1.9 Wrought Aluminum Alloy Designation System

Alloy SeriesPrincipal Alloying Element

1xxx99.000% Minimum Aluminum

2xxxCopper

3xxxManganese

4xxxSilicon

5xxxMagnesium

6xxxMagnesium and Silicon

7xxxZinc

8xxxOther Elements

9xxxUnused Series

NOTE:The first digit (xxxx) indicates the principal alloying element, which has been added to the aluminum alloy and is often used to describe the aluminum alloy series. (Example:1000 series, 2000 series, 3000 series up to 8000 series.

The second single digit (xxxx), if different from 0, indicates a modification of the specific alloy, and the third and fourth digits (xxxx) are arbitrary numbers given to identify a specific alloy in the series. (Example: In alloy 5183, the number 5 indicates that it is of the magnesium alloy series, the 1 indicates that it is the 1st modification to the original alloy 5083 and the 83 identifies it in the 5xxx series.)

The only exception to this alloy numbering system is with the 1xxx series aluminum alloys (pure aluminums) in which case, the last 2 digits provide the minimum aluminum percentage above 99%. (Example: Alloy 1350 (99.50% minimum aluminum).)

Cast alloysThe cast alloy designation system is based on a 3-digit plus decimal designation (xxx.x).(Example: 356.0)Alloy SeriesPrincipal Alloying Element

1xx.x99.000% minimum Aluminum

2xx.xCopper

3xx.xSilicon Plus Copper and/or Magnesium

4xx.xSilicon

5xx.xMagnesium

6xx.xUnused Series

7xx.xZinc

8xx.xTin

9xx.xOther Elements

NOTE:The first digit (xxx.x) indicates the principal alloying element, which has been added to the aluminum alloy.

The second and third digits (xxx.x) are arbitrary numbers given to identify a specific alloy in the series. The number following the decimal point (xxx.x) indicates whether the alloy is a casting (.0) or an ingot (.1 or .2). A capital letter prefix indicates a modification to a specific alloy.

(Example:Alloy A356.0: the capital A (Axxx.x) indicates a modification of alloy 356.0. The number 3 (A3xx.x) indicates that it is of the silicon plus copper and/or magnesium series. The 56 (Ax56.0) identifies the alloy within the 3xx.x series, and the .0 (Axxx.0) indicates that it is a final shape casting and not an ingot.)

DIFFERENT KINDS OF TESTING

DestructivetestingDestructivetestingis a type oftestingused in manufacturing that ultimately destroys the sample being tested. Used to determine the soundness, safety, and lifespan of products,destructivetestingis often used to test welds, but is probably most well-known as a method to test car safety.Destructivetestingcomes in threeforms: stress, or stability; impact, or safety; and hardness, or resistance, tests.Designed to find weaknesses that are not immediately apparent, destructivetestingis usually much more decisive than non-destructive testing. When dealing with mass-produced items, this form oftestingis also less expensive than other methods because only a small handful of the product will be destroyed. When dealing with other products, however, this method can be expensive.Destructivetests may be conducted on a product at any time in its development, from beginning research to production-ready stages.

DIFFERENT FORMS OF DESTRUCTIVE TESTING

Stress testing.Is a form of testing that is used to determine the stability of a given system or entity. It involves testing beyond normal operational capacity, often to a breaking point, in order to observe the results. Stress testing may have a more specific meaning in certain industries, such asfatigue testingfor materials.

Acrash testis a form ofdestructive testingusually performed in order to ensure safe design standards incrashworthinessand crashfor various modes of transportation or related systems and components

Impact testing is testing an object's ability to resist high-rate loading. An impact test is a test for determining the energy absorbed in fracturing a test piece at high velocity. Most of us think of it as one object striking another object at a relatively high speed.Impact resistance is one of the most important properties for a part designer to consider, and without question, the most difficult to quantify. The impact resistance of a part is, in many applications, a critical measure of service life. More importantly these days, it involves the perplexing problem of product safety and liability.

Ductile vs. BrittleMost real world impacts are biaxial rather than unidirectional.Further complication is offered by the choice of failure modes: ductile or brittle. Brittle materials take little energy to start a crack, little more to propagate it to a shattering climax. Other materials possess ductility to varying degrees. Highly ductile materials fail by puncture in drop weight testing and require a high energy load to initiate and propagate the crack.

Many materials are capable of either ductile or brittle failure, depending upon the type of test and rate and temperature conditions. They possess a ductile/brittle transition that actually shifts according to these variables.

Nick-Break is useful for determining the internal quality of the weld metal. This test reveals various internal defects (if present), such as slag inclusions, gas pockets, lack of fusion, and oxidized or burned metal. To accomplish the nick-break test for checking a butt weld, you must first flame-cut the test specimens from a sample weld (fig. 7-65). Make a saw cut at each edge through the center of the weld. The depth of cut should be about 1/4 inch. Next, place the saw-nicked specimenontwosteel supports, as shown in figure 7-65. Using a heave hammer, break the specimen by striking it in the zone where you made the saw cuts. The weld metal exposed in the break should be completely fused, free from slag inclusions, and contain no gas pockets greater than 1/16 inch across their greatest dimension. There should not be more than six pores or gas pockets per square inch of exposed broken surface of the weld.

What is Hardness?Hardness is the property of a material that enables it to resist plastic deformation, usually by penetration. However, the term hardness may also refer to resistance to bending, scratching, abrasion or cutting.

Rockwell Hardness TestThe Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor loadF0(usually 10 kgf).

The Brinell Hardness TestThe Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation.

Vickers Hardness TestThe Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.

DIFFERENT FORM OF NON-DESTRUCTIVE TESTINGNon-destructive Testing is one part of the function of Quality Control and is complementary to other long established methods. By definition non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service. The technique can be applied on a sampling basis for individual investigation or may be used for 100% checking of material in a production quality control system.

Visual inspectionThe simplest and easiest technique to apply and often called by the generic term 'inspection' on process plant.It is able to detect surface damage and distortion. However, access to the surface is required and the capability relies on the illumination and the eyesight of the inspector.

Thickness measurementThe commonest damage found on process plant is corrosion and so techniques which allow remaining wall thickness to be measured are widely applied.

Defect detectionDefect detection techniques fall into two categories: Those that can only detect defects on or near to the surface of a component (Surface Techniques); Those which can detect both surface and embedded defects (Volumetric Techniques).

UltrasonicsUltrasonics is the use of high frequency sound waves in a similar manner to sonar or radar: sound pulses are reflected from interfaces or discontinuities.In thickness checking the reflections from the wall surfaces are measured. In defect detection reflections from cracks, voids and inclusions are detected and assessed.

Ultrasonics C ScanFor the ultrasound techniques, internal damage is detected by the attenuation or reflection of 1-25MHz ultrasonic waves. The selection of which technique to use (through transmission or pulse echo) depends on the accessibility of the surfaces of the component. The C-scan technique is one of the most useful techniques available. Here, the relative attenuation of ultrasonic waves across a component surface creates a plan view of any damage contained within the component. Some systems require components to be removed from structures and immersed in a water bath which acts as a medium for ultrasonic transmissions. Other systems allow the component to be examined without being removed.

X-rayX-ray methods are based on the attenuation of X-rays. Die penetrants such as diodobutane (DIB) can be used to aid damage visibility in certain composites. This is usually practical if some cracking is evident at the surface, allowing a path for the die to penetrate into the internal damage. Fine detail of damage can often be extracted using this technique, however, in some situations penetrants can potentially act as contaminating agents in the damaged area.

ThermographyMaterials emit an infrared radiation whose intensity is dependent upon its absolute temperature. Use of a thermographic cam

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