Materials Science PDF

CONTENTS CONTENTS 16 n A Textbook of Machine Design C 2 H A P T E R Engineering Materials and their Properties 1. Introduction. 2. Classification of Engineering Materials. 3. Selection of Materials for Engineering Purposes. 4. Physical Proper ties of Metals....

CONTENTS CONTENTS 16 n A Textbook of Machine Design C 2 H A P T E R Engineering Materials and their Properties 1. Introduction. 2. Classification of Engineering Materials. 3. Selection of Materials for Engineering Purposes. 4. Physical Proper ties of Metals. 5. Mechanical Properties of Metals. 6. Ferrous Metals. 7. Cast Iron. 9. Alloy Cast Iron. 10. Effect of Impurities on Cast Iron. 11. Wrought Iron. 12. Steel. 15. Effect of Impurities on Steel. 16. Free Cutting Steels. 17. Alloy Steels. 19. Stainless Steel. 20. Heat Resisting Steels. 2.1 Introduction Introduction 21. Indian Standard Designation of High Alloy Steels (Stainless The knowledge of materials and their properties is of Steel and Heat Resisting great significance for a design engineer. The machine Steel). elements should be made of such a material which has 22. High Speed Tool Steels. properties suitable for the conditions of operation. In 23. Indian Standard Designation addition to this, a design engineer must be familiar with of High Speed Tool Steel. 24. Spring Steels. the effects which the manufacturing processes and heat 25. Heat Treatment of Steels. treatment have on the properties of the materials. In this 26. Non-ferrous Metals. chapter, we shall discuss the commonly used engineering 27. Aluminium. materials and their properties in Machine Design. 28. Aluminium Alloys. 29. Copper. 2.2 Classification of Engineering Materials 30. Copper Alloys. The engineering materials are mainly classified as : 31. Gun Metal. 32. Lead. 1. Metals and their alloys, such as iron, steel, 33. Tin. copper, aluminium, etc. 34. Bearing Metals. 2. Non-metals, such as glass, rubber, plastic, etc. 35. Zinc Base Alloys. 36. Nickel Base Alloys. The metals may be further classified as : 37. Non-metallic Materials. (a) Ferrous metals, and (b) Non-ferrous metals. 16 CONTENTS CONTENTS  Engineering Materials and their Properties n 17 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, aluminium, brass, tin, zinc, etc. 2.3 Selection of Materials for Engineering Purposes The selection of a proper material, for engineering purposes, is one of the most difficult problem for the designer. The best material is one which serve 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 work- ing conditions in service, and A filament of bulb needs a material like tungsten 3. The cost of the materials. which can withstand high temperatures without undergoing deformation. The important properties, which determine the utility of the material are physical, chemical and mechanical properties. We shall now discuss the physical and mechanical properties of the material in the following articles. Aluminium Copper Zinc Iron Lead Valua ble Metals aluable 2.4 Physical Pr Physical oper Proper ties of Metals operties The physical properties of the metals include luster, colour, size and shape, density, electric and thermal conductivity, and melting point. The following table shows the important physical properties of some pure metals. * The word ‘ferrous’ is derived from a latin word ‘ferrum’ which means iron. 18 n A Textbook of Machine Design Table 2.1. Physical pr Physical oper proper ties of metals operties metals.. Metal Density Melting point Thermal Coefficient of conductivity linear expansion at (kg/m3) (°C) (W/m°C) 20°C (µm/m/°C) Aluminium 2700 660 220 23.0 Brass 8450 950 130 16.7 Bronze 8730 1040 67 17.3 Cast iron 7250 1300 54.5 9.0 Copper 8900 1083 393.5 16.7 Lead 11 400 327 33.5 29.1 Monel metal 8600 1350 25.2 14.0 Nickel 8900 1453 63.2 12.8 Silver 10 500 960 420 18.9 Steel 7850 1510 50.2 11.1 Tin 7400 232 67 21.4 Tungsten 19 300 3410 201 4.5 Zinc 7200 419 113 33.0 Cobalt 8850 1490 69.2 12.4 Molybdenum 10 200 2650 13 4.8 Vanadium 6000 1750 — 7.75 2.5 Mechanical Proper Properties of Metals operties The 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 applica- tion 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, aluminium, 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. (Refer Chapter 4, Art. 4.11). * For further details, refer Chapter 4 on Simple Stresses in Machine Parts.  Engineering Materials and their Properties n 19 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 aluminium. 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 Gauge to show the material has absorbed after being stressed upto pressure applied. 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 Ball is forced into easily machined than steel. the surface of the ordinary 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 Screw to position a constant stress at high temperature for a long sample 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 Br inell Tester : Hardness can be defined as the resis- Brinell *fatigue. The failure is caused by means of a tance of a metal to attempts to deform it. This ma- progressive crack formation which are usually chine invented by the Swedish metallurgist Johann August Brinell (1849-1925), measure hardness precisely. 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 * For further details, refer Chapter 6 (Art. 6.3) on Variable Stresses in Machine Parts. 20 n A Textbook of Machine Design 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, and (d) Shore scleroscope. 2.6 Ferrous Metals errous We have already discussed in Art. 2.2 that the ferrous metals are those which have iron as their main constituent. The ferrous metals commonly used in engineering practice are cast iron, wrought iron, steels and alloy steels. The principal raw material for all ferrous metals is pig iron which is obtained by smelting iron ore with coke and limestone, in the blast furnace. The principal iron ores with their metallic contents are shown in the following table : Table 2.2. Pr incipal iriron Principal on ores es.. ores Iron ore Chemical formula Colour Iron content (%) Magnetite Fe2O3 Black 72 Haemetite Fe3O4 Red 70 Limonite FeCO3 Brown 60–65 Siderite Fe2O3 (H2O) Brown 48 2.7 Cast Iron Iron The cast iron is obtained by re-melting pig iron Waste gas used as fuel Iron ore, coke with coke and limestone in a furnace known as cupola. and limestone It is primarily an alloy of iron and carbon. The carbon are loaded into contents in cast iron varies from 1.7 per cent to 4.5 per the furnace 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: 1. Free carbon or graphite, and 2. Combined car- Coke burns to carbon bon or cementite. monoxide Since the cast iron is a brittle material, therefore, which releases it cannot be used in those parts of machines which are the iron from the ore subjected to shocks. The properties of cast iron which make it a valuable material for engineering purposes Slag, or are its low cost, good casting characteristics, high impurities, floats to the top of the Waste gas compressive strength, wear resistance and excellent iron used as fuel machinability. The compressive strength of cast iron is Smelting : Ores consist of non-metallic much greater than the tensile strength. Following are elements like oxygen or sulphur combined the values of ultimate strength of cast iron : with the wanted metal. Iron is separated Tensile strength = 100 to 200 MPa* from the oxygen in its ore heating it with carbon monoxide derived from coke (a Compressive strength = 400 to 1000 MPa form of carbon made from coal). Limestone Shear strength = 120 MPa is added to keep impurities liquid so that the iron can separate from them. * 1MPa = 1MN/m2 = 1 × 106 N/m2 = 1 N/mm2  Engineering Materials and their Properties n 21 2.8 Types of Cast Iron Iron The various types of cast iron in use are discussed as follows : 1. 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. Haematite is an ore of iron. It often The grey colour is due to the fact that the carbon is forms kidney-shaped lumps, These present in the form of *free graphite. It has a low tensile give the ore its nickname of kidney ore. 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 agricul- tural implements. Table 2.3. Grey ir Gre on castings iron castings,, as per IS : 210 – 1993. IS Designation Tensile strength (MPa or N/mm2) Brinell hardness number (B.H.N.) FG 150 150 130 to 180 FG 200 200 160 to 220 FG 220 220 180 to 220 FG 260 260 180 to 230 FG 300 300 180 to 230 FG 350 350 207 to 241 FG 400 400 207 to 270 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 2.3. 2. 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 colour 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. 3. 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 * When filing or machining cast iron makes our hands black, then it shows that free graphite is present in cast iron. 22 n A Textbook of Machine Design 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. 4. Mottled cast iron. It is a product in between grey and white cast iron in composition, colour and general properties. It is obtained in castings where certain wearing surfaces have been chilled. 5. 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). It is ductile and may be bent without breaking or fracturing the section. The tensile strength of the malleable cast iron is usually higher than that of grey cast iron and has excellent machining qualities. It is used for machine parts for which the steel forgings would be too expensive and in which the metal should have a fair degree of accuracy, e.g. hubs of wagon wheels, small fittings for railway rolling stock, brake supports, parts of agricultural machinery, pipe fittings, door hinges, locks etc. In order to obtain a malleable iron castings, it is first cast into moulds of white cast iron. Then by a suitable heat treatment (i.e. annealing), the combined carbon of the white cast iron is separated into nodules of graphite. The following two methods are used for this purpose : 1. Whiteheart process, and 2. Blackheart process. In a whiteheart process, the white iron castings are packed in iron or steel boxes surrounded by a mixture of new and used haematite ore. The boxes are slowly heated to a temperature of 900 to 950°C and maintained at this temperature for several days. During this period, some of the carbon is oxidised out of the castings and the remaining carbon is dispersed in small specks throughout the structure. The heating process is followed by the cooling process which takes several more days. The result of this heat treatment is a casting which is tough and will stand heat treatment without fracture. In a blackheart process, the castings used contain less carbon and sulphur. They are packed in a neutral substance like sand and the reduction of sulphur helps to accelerate the process. The castings are heated to a temperature of 850 to 900°C and maintained at that temperature for 3 to 4 days. The carbon in this process transforms into globules, unlike whiteheart process. The castings produced by this process are more malleable. Notes : (a) According to Indian standard specifications (*IS : 14329 – 1995), the malleable cast iron may be either whiteheart, blackheart or pearlitic, according to the chemical composition, temperature and time cycle of annealing process. (b) The whiteheart malleable cast iron obtained after annealing in a decarburizing atmosphere have a silvery-grey fracture with a heart dark grey to black. The microstructure developed in a section depends upon the size of the section. In castings of small sections, it is mainly ferritic with certain amount of pearlite. In large sections, microstructure varies from the surface to the core as follows : Core and intermediate zone : Pearlite + ferrite + temper carbon Surface zone : Ferrite. The microstructure shall not contain flake graphite. * This standard (IS : 14329-1995) supersedes the previous three standards, i.e. (a) IS : 2107–1977 for white heart malleable iron casting, (b) IS : 2108–1977 for black heart malleable iron casting, and (c) IS : 2640–1977 for pearlitic malleable iron casting.  Engineering Materials and their Properties n 23 Household mixed waste, containing steel (mainly food cans), paper, plastics aluminium and glass Steel objects are carried away on conveyor belt for processing Electromagnet removes iron and steel Powerful fans blow paper into wire receptacles Second conveyor belt made of chains Glass falls through chains and is sorted by hand into three colour-brown, green and clear Magnetized drum holds Plastic waste is carried away aluminium for processing In a modern materials recovery plant, mixed waste (but no organic matter) is passed along a conveyor belt and sorted into reusable materials-steel, aluminium, paper, glass. Such recycling plants are expensive, but will become essential as vital resources become scarce. Note : This picture is given as additional information and is not a direct example of the current chapter. (c) The blackheart malleable cast iron obtained after annealing in an inert atmosphere have a black fracture. The microstructure developed in the castings has a matrix essentially of ferrite with temper carbon and shall not contain flake graphite. (d) The pearlitic malleable cast iron obtained after heat-treatment have a homogeneous matrix essentially of pearlite or other transformation products of austenite. The graphite is present in the form of temper carbon nodules. The microstructure shall not contain flake graphite. (e) According to IS: 14329 – 1995, the whiteheart, blackheart and pearlitic malleable cast irons are designated by the alphabets WM, BM and PM respectively. These designations are followed by a figure indicating the minimum tensile strength in MPa or N/mm2. For example ‘WM 350’ denotes whiteheart malleable cast iron with 350 MPa as minimum tensile strength. The following are the different grades of malleable cast iron : Whiteheart malleable cast iron — WM 350 and WM 400 Blackheart malleable cast iron — BM 300 ; BM 320 and BM 350 Pearlitic malleable cast iron — PM 450 ; PM 500 ; PM 550 ; PM 600 and PM 700 6. 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 24 n A Textbook of Machine Design 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. According to Indian standard specification (IS : 1865-1991), the nodular or spheroidal graphite cast iron is designated by the alphabets ‘SG’ followed by the figures indicating the minimum tensile strength in MPa or N/mm2 and the percentage elongation. For example, SG 400/15 means spheroidal graphite cast iron with 400 MPa as minimum tensile strength and 15 percent elongation. The Indian standard (IS : 1865 – 1991) recommends nine grades of spheroidal graphite cast iron based on mechanical properties measured on separately-cast test samples and six grades based on mechanical properties measured on cast-on sample as given in the Table 2.4. The letter A after the designation of the grade indicates that the properties are obtained on cast- on test samples to distinguish them from those obtained on separately-cast test samples. Table 2.4. Recommended grades of spher oidal gra spheroidal phite cast iriron graphite on as per IS : 1865–1991. Grade Minimum tensile Minimum Brinell hardness Predominant strength (MPa) percentage number (BHN) constituent of matrix elongation SG 900/2 900 2 280 – 360 Bainite or tempered martensite SG 800/2 800 2 245 – 335 Pearlite or tempered structure SG 700/2 700 2 225 – 305 Pearlite SG 600/3 600 3 190 – 270 Ferrite + Pearlite SG 500/7 500 7 160 – 240 Ferrite + Pearlite SG 450/10 450 10 160 – 210 Ferrite SG 400/15 400 15 130 – 180 Ferrite SG 400/18 400 18 130 – 180 Ferrite SG 350/22 350 22 ≤ 150 Ferrite SG 700/2A 700 2 220 – 320 Pearlite SG 600/3A 600 2 180 – 270 Pearlite + Ferrite SG 500/7A 450 7 170 – 240 Pearlite + Ferrite SG 400/15A 390 15 130 – 180 Ferrite SG 400/18A 390 15 130 – 180 Ferrite SG 350/22A 330 18 ≤ 150 Ferrite 2.9 Alloy Cast Ir Alloy on Iron The cast irons as discussed in Art. 2.8 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 * The graphite flakes in cast iron act as discontinuities in the matrix and thus lower its mechanical properties. The sharp corners of the flakes also act as stress raisers. The weakening effect of the graphite can be reduced by changing its form from a flake to a spheroidal form.  Engineering Materials and their Properties n 25 gears, automobile parts like cylinders, pistons, piston rings, crank cases, crankshafts, camshafts, sprock- ets, wheels, pulleys, brake drums and shoes, parts of crushing and grinding machinery etc. 2.10 Effect of Impur Effect ities on Cast Ir Impurities on Iron We 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 are as follows: 1. Silicon. It may be present in cast iron upto 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 Phosphorus is a non-metallic are useful for casting of intricate design and for many light engineering element. It must be stored castings when cheapness is essential. underwater (above), since it catches fire when exposed to 2.11 Wrought Ir Wrought on Iron air, forming a compound. It is the purest iron which contains at least 99.5% iron but may contain upto 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. Polarized light gives false-colour image. Slabs of impure iron Iron is hammered to remove impurities A close look at cast iron Wrought IrIron Wrought on 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, water and steam pipes. 26 n A Textbook of Machine Design Û The ocean floor contains huge amounts of manga- nese (a metal used in steel and industrial processes). The manganese is in the form of round lumps called nodules, mixed with other elements, such as iron and nickel. The nodules are dredged up by ships fitted with hoselines which scrape and suck at the ocean floor. Nodule Suction line Dredging rake Û Nodules look rather like hailstones. The minerals are washed into the sea by erosion of the land. About one-fifth of the nodule is manga- nese. Note : This picture is given as additional information and is not a direct example of the current chapter. 2.12 Steel It 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. 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 or mild steel — 0.15% to 0.45% carbon 3. Medium carbon steel — 0.45% to 0.8% carbon 4. High carbon steel — 0.8% to 1.5% carbon 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 : (a) On the basis of mechanical properties, and (b) On the basis of chemical composition. We shall now discuss, in detail, the designation of steel on the above two basis, in the following pages. 2.13 Steels Designa ted on the Basis of Mechanical Pr Designated oper Proper ties operties These steels are carbon and low alloy steels where the main criterion in the selection and in- spection of steel is the tensile strength or yield stress. According to Indian standard **IS: 1570 (Part–I)- 1978 (Reaffirmed 1993), these steels are designated by a symbol ‘Fe’ or ‘Fe E’ depending on whether * This standard was reaffirmed in 1993 and covers the code designation of wrought steel based on letter symbols. ** The Indian standard IS : 1570-1978 (Reaffirmed 1993) on wrought steels for general engineering purposes has been revised on the basis of experience gained in the production and use of steels. This standard is now available in seven parts.  Engineering Materials and their Properties n 27 the steel has been specified on the basis of minimum tensile strength or yield strength, followed by the figure indicating the minimum tensile strength or yield stress in N/mm2. For example ‘Fe 290’ means a steel having minimum tensile strength of 290 N/mm2 and ‘Fe E 220’ means a steel having yield strength of 220 N/mm2. Table 2.5 shows the tensile and yield properties of standard steels with their uses according to IS : 1570 (Part I)-1978 (Reaffirmed 1993). Table 2.5. Indian standar standard d designa tion of steel accor designation ding to according IS : 1570 (Par artt I)-1978 (Reaf (Par (Reafffir med 1993). irmed Indian standard Tensil Yield stress Minimum Uses as per IS : 1871 (Part I)–1987 designation strength (Minimum) percentage (Reaffirmed 1993) (Minimum) N/mm2 elongation N/mm2 Fe 290 290 170 27 It is used for plain drawn or enamelled parts, tubes for oil well casing, steam, Fe E 220 290 220 27 water and air passage, cycle, motor cycle and automobile tubes, rivet bars and wire. Fe 310 310 180 26 These steels are used for locomotive carriages and car structures, screw stock Fe E 230 310 230 26 and other general engineering purposes. Fe 330 330 200 26 Fe E 250 330 250 26 Fe 360 360 220 25 It is used for chemical pressure vessels and other general engineering purposes. Fe E 270 360 270 25 Fe 410 410 250 23 It is used for bridges and building construction, railway rolling stock, Fe E 310 410 310 23 screw spikes, oil well casing, tube piles, and other general engineering purposes. Fe 490 490 290 21 It is used for mines, forgings for marine engines, sheet piling and machine Fe E 370 490 370 21 parts. Fe 540 540 320 20 It is used for locomotive, carriage, wagon and tramway axles, arches for Fe E 400 540 400 20 mines, bolts, seamless and welded tubes. Fe 620 620 380 15 It is used for tramway axles and seamless tubes. Fe E 460 620 460 15 Fe 690 690 410 12 It is used for locomotive, carriage and wagon wheels and tyres, arches for Fe E 520 690 520 12 mines, seamless oil well casing and drill tubes, and machine parts for heavy loading. Fe 770 770 460 10 It is used for locomotive, carriage and wagon wheels and tyres, and machine Fe E 580 770 580 10 parts for heavy loading. Fe 870 870 520 8 It is used for locomotive, carriage and wagon wheels and tyres. Fe E 650 870 650 8 28 n A Textbook of Machine Design Notes : 1. The steels from grades Fe 290 to Fe 490 are general structural steels and are available in the form of bars, sections, tubes, plates, sheets and strips. 2. The steels of grades Fe 540 and Fe 620 are medium tensile structural steels. 3. The steels of grades Fe 690, Fe 770 and Fe 870 are high tensile steels. 2.14 Steels Designated on the Basis of Chemical Composition According to Indian standard, IS : 1570 (Part II/Sec I)-1979 (Reaffirmed 1991), the carbon steels are designated in the following order : (a) Figure indicating 100 times the average percentage of carbon content, (b) Letter ‘C’, and (c) Figure indicating 10 times the average percentage of manganese content. The figure after multiplying shall be rounded off to the nearest integer. For example 20C8 means a carbon steel containing 0.15 to 0.25 per cent (0.2 per cent on average) carbon and 0.60 to 0.90 per cent (0.75 per cent rounded off to 0.8 per cent on an average) manganese. Table 2.6 shows the Indian standard designation of carbon steel with composition and their uses. Table 2.6. Indian standar standard d designa tion of carbon steel accor designation ding to according IS : 1570 (Par (Par artt II/Sec 1) – 1979 (Reaf fir med (Reaffirmed 1991). Indian standard Composition in percentages Uses as per IS : 1871 (Part II)–1987 designation (Reaffirmed 1993) Carbon (C) Manganese (Mn) 4C2 0.08 Max. 0.04 Max. It is a dead soft steel generally used in electrical industry. 5C4 0.10 Max. 0.50 Max. These steels are used where cold form- 7C4 0.12 Max. 0.50 Max. ability is the primary requirement. In the rimming quality, they are used as sheet, 10C4 0.15 Max. 0.30 – 0.60 strip, rod and wire especially where excellent surface finish or good drawing qualities are required, such as automobile body, and fender stock, hoods, lamps, oil pans and a multiple of deep drawn and formed products. They are also used for cold heading wire and rivets and low carbon wire products. The killed steel is used for forging and heat treating applications. 10C4 0.15 Max. 0.30 – 0.60 The case hardening steels are used for making camshafts, cams, light duty gears, 14C6 0.10 – 0.18 0.40 – 0.70 worms, gudgeon pins, spindles, pawls, ratchets, chain wheels, tappets, etc. 15C4 0.20 Max. 0.30 – 0.60 It is used for lightly stressed parts. The material, although easily machinable, is not designed specifically for rapid cutting, but is suitable where cold web, such as bending and riveting may be necessary.  Engineering Materials and their Properties n 29 Indian standard Composition in percentages Uses as per IS : 1871 (Part II)–1987 designation (Reaffirmed 1993) Carbon (C) Manganese (Mn) 15C8 0.10 – 0.20 0.60 – 0.90 These steels are general purposes steels used for low stressed components. 20C8 0.15 – 0.25 0.60 – 0.90 25C4 0.20 – 0.30 0.30 – 0.60 25C8 0.20 – 0.30 0.60 – 0.90 30C8 0.25 – 0.35 0.60 – 0.90 It is used for making cold formed parts such as shift and brake levers. After suitable case hardening or hardening and tempering, this steel is used for making sprockets, tie rods, shaft fork and rear hub, 2 and 3 wheeler scooter parts such as sprocket, lever, hubs for forks, cams, rocket arms and bushes. Tubes for aircraft, automobile, bicycle and furniture are also made of this steel. 35C4 0.30 – 0.40 0.30 – 0.60 It is used for low stressed parts, automobile tubes and fasteners. 35C8 0.30 – 0.40 0.60 – 0.90 It is used for low stressed parts in machine structures, cycle and motor cycle tubes, fish plates for rails and fasteners. 40C8 0.35 – 0.45 0.60 – 0.90 It is used for crankshafts, shafts, spindles, push rods, automobile axle beams, connecting rods, studs, bolts, lightly stressed gears, chain parts, umbrella ribs, washers, etc. 45C8 0.40 – 0.50 0.60 – 0.90 It is used for spindles of machine tools, bigger gears, bolts, lead screws, feed rods, shafts and rocks. 50C4 0.45 – 0.55 0.30 – 0.60 It is used for keys, crankshafts, cylinders and machine parts requiring moderate wear resistance. In surface hardened condition, it is also suitable for large pitch worms and gears. 50C12 0.45 – 0.55 1.1 – 1.50 It is a rail steel. It is also used for making spike bolts, gear shafts, rocking levers and cylinder liners. 55C4 0.50 – 0.60 0.30 – 0.60 These steels are used for making gears, coil 55C8 0.50 – 0.60 0.60 – 0.90 springs, cylinders, cams, keys, crankshafts, sprockets and machine parts requiring moderate wear resistance for which toughness is not of primary importance. It is also used for cycle and industrial chains, spring, can opener, umbrella ribs, parts of camera and typewriter. 60C4 0.55 – 0.65 0.30 – 0.60 It is used for making clutch springs, hardened screws and nuts, machine tool spindles, couplings, crankshafts, axles and pinions. 65C9 0.60 – 0.70 0.50 – 0.80 It is a high tensile structural steel used for making locomotive carriage and wagon tyres. It is also used for engine valve springs, small washers and thin stamped parts. 30 n A Textbook of Machine Design Indian standard Composition in percentages Uses as per IS : 1871 (Part II)–1987 designation (Reaffirmed 1993) Carbon (C) Manganese (Mn) 70C6 0.65 – 0.75 0.50 – 0.80 It is used for making baffle springs, shock absorbers, springs for seat cushions for road vehicles. It is also used for making rail tyres, unhardened gears and worms, washers, wood working saw, textile and jute machinery parts and clutch plates, etc. 75C6 0.70 – 0.80 0.50 – 0.80 It is used for making light flat springs formed from annealed stock. Because of good wear properties when properly heat treated, it is used for making shear blades, rack teeth, scrappers and cutlivators’ shovels. 80C6 0.75 – 0.85 0.50 – 0.80 These steels are used for making flat and coil springs for automobile and railway 85C6 0.80 – 0.90 0.50 – 0.80 vehicles. It is also used for girder rails. The valve spring wire and music wire are special applications of steel 85 C6. After suitable heat treatment, these steels are also used for making scraper blades, discs and spring tooth harrows. It is also used for clutch parts, wood working saw, band saw and textile and jute machinery parts. 98C6 0.90 – 1.05 0.50 – 0.80 These steels in the oil hardened and tempered condition are used for coil or spiral 113C6 1.05 – 1.20 0.50 – 0.80 springs. It is also used for pen nibs, volute spring, spring cutlery, knitting needle and hacksaw blades. 2.15 Effect of Impur Effect ities on Steel Impurities 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 effect so much. Therefore, manganese sulphide is less objectionable in steel than iron sulphide. 3. Manganese. It serves as a valuable deoxidising 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%.  Engineering Materials and their Properties n 31 2.16 Free Cutting Steels Free The 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. Now a days, 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. According to Indian standard, IS : 1570 (Part III)-1979 (Reaffirmed 1993), carbon and carbon manganese free cutting steels are designated in the following order : 1. Figure indicating 100 times the average percentage of carbon, 2. Letter ‘C’, 3. Figure indicating 10 times the average percentage of manganese, and 4. Symbol ‘S’ followed by the figure indicating the 100 times the average content of sulphur. If instead of sulphur, lead (Pb) is added to make the steel free cutting, then symbol ‘Pb’ may be used. Table 2.7 shows the composition and uses of carbon and carbon-manganese free cutting steels, as per IS : 1570 (Part III)-1979 (Reaffirmed 1993). 2.17 Alloy Steel An alloy 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 confer 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. In this range, nickel contributes great strength and hardness with high elastic limit, good ductility and good resistance to corrosion. An alloy containing 25% nickel possesses maximum toughness and offers the greatest resistance to rusting, corrosion and burning at high temperature. It has proved to be of advantage in the manufacture of boiler tubes, valves for use with superheated steam, valves for I.C. engines and spark plugs for petrol engines. A nickel steel alloy containing 36% of nickel is known as invar. It has nearly zero coefficient of expansion. So it is in great demand for measuring instruments and standards of lengths for everyday use. 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. A nickel chrome steel containing 3.25% nickel, 1.5% chromium and 0.25% carbon is much used for armour plates. Chrome nickel steel is extensively used for motor car crankshafts, axles and gears requiring great strength and hardness. 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 colour. It is usually used in conjuction 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.  Table 2.7. Indian standard designa standard tion of carbon and carbon–mang designation carbon–manganese anese free cutting steels free 32 n according to IS:1570 (P according ar (Par artt III) – 1979 (Reaf (Reafffir med 1993). irmed Indian Composition in percentages standard Uses as per IS : 1871 (Part III)–1987 designation Carbon Silicon Manganese Sulphur Phosphorus (Reaffirmed 1993) (C) (Si) (Mn) (S) (P) Max 10C8S10 0.15 Max. 0.05 – 0.30 0.60 – 0.90 0.08 – 0.13 0.06 It is used for small parts to be cyanided or carbonitrided. 14C14S14 0.10 – 0.18 0.05 – 0.30 1.20 – 1.50 0.1 – 0.18 0.06 It is used for parts where good machinability and finish are important. 25C12S14 0.20 – 0.30 0.25 Max. 1.00 – 1.50 0.10 – 0.18 0.06 It is used for bolts, studs and other heat treated parts of A Textbook of Machine Design small section. It is suitable in either cold drawn, normalised or heat treated condition for moderately stressed parts requiring more strength than mild steel. 40C10S18 0.35 – 0.45 0.25 Max. 0.80 – 1.20 0.14 – 0.22 0.06 It is used for heat treated bolts, engine shafts, connecting rods, miscellaneous gun carriage, and small arms parts not subjected to high stresses and severe wear. 11C10S25 0.08 – 0.15 0.10 Max. 0.80 – 1.20 0.20 – 0.30 0.06 It is used for lightly stressed components not subjected to shock (nuts, studs, etc.) and suitable for production on automatic lathes. It is not recommended for general case hardening work but should be used when ease of machining is the deciding factor. 40C15S12 0.35 – 0.45 0.25 Max. 1.30 – 1.70 0.08 – 0.15 0.06 It is used for heat treated axles, shafts, small crankshafts and other vehicle parts. It is not recommended for forgings in which transverse properties are important.  Engineering Materials and their Properties n 33 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. The chrome-vanadium steel containing about 0.5 to 1.5% chromium, 0.15 to 0.3% vanadium and 0.13 to 1.1% carbon have extremely good tensile strength, elastic limit, This is a fan blade from a jumbo jet engine. On take-off, the stress on the metal is immense, so to endurance limit and ductility. These steels are prevent the fan from flying apart, the blades must frequently used for parts such as springs, shafts, be both light and very strong. Titanium, though gears, pins and many drop forged parts. expensive, is the only suitable metal. 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. The principal uses of manganese steel is in machinery parts subjected to severe wear. These steels are all cast and ground to finish. 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 decarburise 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. These steels possess extra tensile strength and are used for air-plane fuselage and automobile parts. It can replace tungsten in high speed steels. 2.18 Indian Standar Standardd Designation of Lo Designation w and Medium Allo Low Alloyy Steels According to Indian standard, IS : 1762 (Part I)-1974 (Reaffirmed 1993), low and medium alloy steels shall be designated in the following order : 1. Figure indicating 100 times the average percentage carbon. 2. Chemical symbol for alloying elements each followed by the figure for its average percentage content multiplied by a factor as given below : Element Multiplying factor Cr, Co, Ni, Mn, Si and W 4 Al, Be, V, Pb, Cu, Nb, Ti, Ta, Zr and Mo 10 P, S and N 100 For example 40 Cr 4 Mo 2 means alloy steel having average 0.4% carbon, 1% chromium and 0.25% molybdenum. Notes : 1. The figure after multiplying shall be rounded off to the nearest integer. 2. Symbol ‘Mn’ for manganese shall be included in case manganese content is equal to or greater than 1 per cent. 3. The chemical symbols and their figures shall be listed in the designation in the order of decreasing content. Table 2.8 shows the composition and uses of some low and medium alloy steels according to Indian standard IS : 1570-1961 (Reaffirmed 1993).  Table 2.8. Composition and uses of allo y steels accor alloy ding to according IS : 1570-1961 (Reaf med 1993). irmed (Reafffir 34 n Indian Composition in percentages standard Uses as per IS : 1871–1965 designation Carbon Silicon Manganese Nickel Chromium Molybdenum (C) (Si) (Mn) (Ni) (Cr) (Mo) 11Mn2 0.16 Max. 0.10 – 0.35 1.30 – 1.70 – – – It is a notch ductile steel for general purposes. It is also used in making filler rods, colliery cage suspension gear tub, mine car draw gear, couplings and rope sockets. 20Mn2 0.16 – 0.24 0.10 – 0.35 1.30 – 1.70 – – – These are used for welded structures, crankshafts, steering 27Mn2 0.22 – 0.32 0.10 – 0.35 1.30 – 1.70 – – – levers, shafting spindles, etc. A Textbook of Machine Design 37Mn2 0.32 – 0.42 0.10 – 0.35 1.30 – 1.70 – – – It is used for making axles, shafts, crankshafts, connecting rods, etc. 47Mn2 0.42 – 0.52 0.10 – 0.35 1.30 – 1.70 – – – It is used for tram rails and similar other structural purposes. 40Cr1 0.35 – 0.45 0.10 – 0.35 0.60 – 0.09 – 0.90 – 1.20 – It is used for making gears, connecting rods, stub axles, steering arms, wear resistant plates for earth moving and concrete handling equipment, etc. 50Cr1 0.45 – 0.55 0.10 – 0.35 0.60 –.90 – 0.90 – 1.20 – It is spring steel. It is used in a helical automobile front suspension springs. 35Mn2Mo28 0.30 – 0.40 0.10 – 0.35 1.30 – 1.80 – – 0.20 – 0.35 These are used for making general 35Mn2Mo45 0.30 – 0.40 0.10 – 0.35 1.30 – 1.80 – – 0.35 – 0.55 engineering components such as crankshafts, bolts, wheel studs, axle shafts, levers and connecting rods. 40Cr1Mo28 0.35 – 0.45 0.10 – 0.35 0.50 – 0.80 – 0.90 – 1.20 0.20 – 0.35 It is used for making axle shafts, crankshafts, connecting rods, gears, high tensile bolts and studs, propeller shaft joints, etc. Contd.. Indian Composition in percentages standard Uses as per IS : 1871–1965 designation Carbon Silicon Manganese Nickel Chromium Molybdenum (C) (Si) (Mn) (Ni) (Cr) (Mo) 15Cr3Mo55 0.10 – 0.20 0.10 – 0.35 0.40 – 0.70 0.30 Max. 2.90 – 3.40 0.45 – 0.65 These are used for components requiring medium to high tensile 25Cr3Mo55 0.20 – 0.30 0.10 – 0.35 0.40 – 0.70 0.30 Max. 2.90 – 3.40 0.45 – 0.65 properties. In the nitrided condition, it is used for crank-shafts, cylinder liners for aero and automobile engines, gears, and machine parts requiring high surface hardness and wear resistance. 40Ni3 0.35 – 0.45 0.10 – 0.35 0.50 – 0.80 3.20 – 3.60 0.30 Max. – It is used for parts requiring excessively high toughness. In particular, it is used for components working at low temperatures (in refrigerators, compressors, locomotives and aircraft) and for heavy forgings, turbine blades, severely stressed screws, bolts and nuts. 30Ni4Crl 0.26 – 0.34 0.10 – 0.35 0.40 – 0.70 3.90 – 4.30 1.10 – 1.40 – It is used for highly stressed gears and other components requiring high tensile strength of the order of 16 N/mm2 and where minimum distortion in heat treatment is essential. 35NilCr60 0.30 – 0.40 0.10 – 0.35 0.60 – 0.90 1.00 – 1.50 0.45 – 0.75 – It is used in the construction of aircraft and heavy vehicles for crankshafts, connecting rods, gear shafts, chain parts, clutches, flexible shafts for plenary gears, camshafts, etc. 40Ni2CrlMo28 0.35 – 0.45 0.10 – 0.35 0.40 – 70 1.25 – 1.75 0.90 – 1.30 0.20 – 0.35 It is used for high strength machine Engineering Materials and their Properties parts collets, spindles, screws, high tensile bolts and studs, gears, n pinions, axle shafts, tappets, crankshafts, connecting rods, boring bars, arbours, etc. 35 36 n A Textbook of Machine Design 2.19 Stainless Steel It is defined as that steel which when correctly heat treated and finished, resists oxidation and corrosive attack from most corrosive media. The different types of stainless steels are discussed below : 1. 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 Stainless steel was invented in 1913 by British strength of about 1600 N/mm2 may be developed with metallurgist Harry Brearley (1871-1948). He lowered ductility. made a steel containing 13 per cent chromium. The new alloy proved to be highly These steels may be used where the corrosion resistant to corrosion: chromium reacts with conditions are not too severe, such as for hydraulic, oxygen in the air to form a tough, protective film which renews itself if the metal is steam and oil pumps, valves and other engineering scratched. 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 upto a temperature of about 750°C, but it is not suitable where mechanical strength in the temperature range of 600 to 750°C is required. In fact, creep resistance of these steels at this temperature is not superior to that of mild steel. But at temperature below 600°C, the strength of these steels is better than that of carbon steels and upto 480°C is even better than that of austenitic steels. 2. 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  Engineering Materials and their Properties n 37 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 a 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. 3. 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 a 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 1000°C. They are non- magnetic 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 1100°C 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 2 to 3 per cent may be added. 2.20 Heat Resisting Steels The steels which can resist creep and oxidation at high temperatures and retain sufficient strength are called heat resisting steels. A number of heat resisting steels have been developed as discussed below : 1. Low alloy steels. These steels contain 0.5 per cent molybdenum. The main application of these steels are for superheater tubes and pipes in steam plants, where service temperatures are in the range of 400°C to 500°C. 2. Valve steels. The chromium-silicon steels such as silchrome (0.4% C, 8% Cr, 3.5% Si) and Volmax (0.5% C, 8% Cr, 3.5% Si, 0.5% Mo) are used for automobile valves. They possess good resistance to scaling at dull red heat, although their strength at elevated temperatures is relatively low. For aeroplane engines and marine diesel engine valves, 13/13/3 nickel-chromium-tungsten valve steel is usually used. 3. Plain chromium steel. The plain chromium steel consists of (a) Martensitic chromium steel with 12–13% Cr, and (b) Ferritic chromium steels with 18–30% Cr. These steels are very good for oxidation resistance at high temperatures as compared to their strength which is not high at such conditions. The maximum operating temperature for martensitic steels is about 750°C, whereas for ferritic steels it is about 1000 – 1150°C. 4. Austenitic chromium-nickel steels. These steels have good mechanical properties at high temperatures with good scaling resistance. These alloys contain a minimum of 18 per cent chromium and 8 per cent nickel stabilised with titanium or niobium. Other carbide forming elements such as molybdenum or tungsten may also be added in order to improve creep strength. Such alloys are suitable for use upto 1100°C and are used for gas turbine discs and blades.  Table 2.9. Indian standar standardd designa tion of high allo designation y steels (stainless steel and hea alloy esisting steels) heatt rresisting accor ding to IS : 1570 (Par med 1991). 38 according (Par (Reafffir artt V)-1985 (Reaf irmed n Indian Composition in percentages standard Uses as per IS : 1871–1965 designation Carbon Silicon Manganese Nickel Chromium. Molybdenum (C) (Si) (Mn) (Ni) (Cr) (Mo) 30Cr13 0.26 – 0.35 1.0 Max. 1.0 Max. 1.0 Max. 12.0 – 14.0 – It is used for structural parts with high strength and kitchen utensils. 15Cr16Ni2 0.10 – 0.20 1.0 Max. 1.0 Max. 1.5 – 3.0 15.0 – 18.0 – It is used for aircraft fittings, wind shield wiper arms, bolting materials, paper machinery etc. 07Cr18Ni9 0.12 Max. 1.0 Max. 2.0 Max. 8.0 – 10.0 17.0 – 19.0 – It is used for aircraft fire walls and cawlings, A Textbook of Machine Design radar and microwaves antennae, jewellery, household novelties, automotive wheel covers, refrigerator trays, kitchen utensils, railway passenger car bodies, ice making equipment, tubular furniture, screen door and storm window frames, electric switch parts, flexible couplings etc. 04Cr17Ni12 0.08 Max. 1.0 Max. 2.0 Max. 10.5 – 14.0 16.0 – 18.5 2.0 – 3.0 It is used for high temperature chemical Mo2 handling equipment for rayon, rubber and marine industries, photographic developing equipment, pulp handling equipment, steam jacketed kettles, coke plant equipment, food processing equipment, edible oil storage tanks. 45Cr9Si4 0.40 – 0.50 3.25 – 3.75 0.30 – 0.60 0.05 Max. 7.50 – 9.50 – It is used for heat resisting outlet valves in oil engines, lorries and cars. 80Cr20Si2 0.75 – 0.85 1.75 – 2.25 0.20 – 0.60 1.20 – 1.70 19.0 – 21.0 – It is used for highly stressed outlet valves in high speed carburetors and heavy oil Nil engines.  Engineering Materials and their Properties n 39 2.21 Indian Standar Standardd Designa tion of High Allo Designation Alloyy Steels (Stainless Steel and Heat Resisting Steel) According to Indian standard, IS : 1762 (Part I)-1974 (Reaffirmed 1993), the high alloy steels (i.e. stainless steel and heat resisting steel) are designated in the following order: 1. Letter ‘X’. 2. Figure indicating 100 times the percentage of carbon content. 3. Chemical symbol for alloying elements each followed by a figure for its average percentage content rounded off to the nearest integer. 4. Chemical symbol to indicate specially added element to allow the desired properties. For example, X 10 Cr 18 Ni 9 means alloy steel with average carbon 0.10 per cent, chromium 18 per cent and nickel 9 per cent. Table 2.9 shows the composition and uses of some types of the stainless steels and heat resisting steels according to Indian standard IS : 1570 (Part V)-1985 (Reaffirmed 1991). 2.22 High Speed Tool Steels These 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 Gold is found mixed with quartz rock, deep underground. Most met- to red heat. Most of the high als occur in their ores as compounds. Gold is so unreactive that it occurs naturally as pure metal. 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: 1. 18-4-1 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. 2. 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. 3. 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. 40 n A Textbook of Machine Design 2.23 Indian Standard Designa Standard tion of High Speed Tool Steel Designation According to Indian standard, IS : 1762 (Part I)-1974 (Reaffirmed 1993), the high speed tool steels are designated in the following order : 1. Letter ‘XT’. 2. Figure indicating 100 times the percentage of carbon content. 3. Chemical symbol for alloying elements each followed by the figure for its average percentage content rounded off to the nearest integer, and 4. Chemical symbol to indicate specially added element to attain the desired properties. For example, XT 75 W 18 Cr 4 V 1 means a tool steel with average carbon content 0.75 per cent, tungsten 18 per cent, chromium 4 per cent and vanadium 1 per cent. Table 2.10 shows the composition of high speed tool steels as per Indian standard, IS : 7291- 1981 (Reaffirmed 1993). 2.24 Spring Steels The most suitable materia