A Textbook of Production Technology - PDF

62 A Textbook of Production Technology cut, sheared, punched, cold worked, hot worked and weldable. The casting alloys can be machined, ground, welded and brazed. Nickel alloys are more costly than steel and aluminium but are less...

62 A Textbook of Production Technology cut, sheared, punched, cold worked, hot worked and weldable. The casting alloys can be machined, ground, welded and brazed. Nickel alloys are more costly than steel and aluminium but are less costly than refractory metals (W, Mo, Ti etc.) for solving severe temperature strength problems. Also, they display magnetic, magneto-strictive, electrical and thermal properties that are important for particular applications. Nickel alloys which are particularly useful for general industrial purposes are described below : (i) Monel Metal. Monel metals are the most important nickel-copper alloys, having high strength and toughness and excellent corrosion resistance. Monel metal contains 67% Ni, 30% Cu, with small amounts of iron, manganese, silicon and carbon. It is a white, tough, and ductile metal that can be easily machined and welded and can be heat treated. It is used in the form of rod, sheet, wire and welded tubing. It is widely employed for structural and machine parts which must have a very high resistance to corrosion and have high strength at elevated temperatures, for example, steam turbine blades, high temperature valves, impeller of centrifugal pump, and for springs subjected to temperatures above 200°C. (ii) K-Monel. This alloy contains 66% Ni, 29% Cu, 2.57% Al, and small quantities of iron, manganese and carbon. Addition of aluminium increases the strength and hardness above that of Monel and also makes it susceptible to hardening by heat treatment K-monel has the same corrosion- resisting and high temperature properties as Monel and is wholly non-magnetic. (iii) German Silver. An alloy of copper (50%), nickel (20%) and Zinc (30%) is known as german silver. Sometimes, tin and lead are also added. This alloy is hard, white and ductile. It has good mechanical and corrosion resisting properties. German silver is used for making utensils, resistances in electrical work, shop and house fittings and ornamental work of cars. (iv) Invar. Invar is an alloy of iron (70%) with 30% nickel. It has a very low co-efficient of heat expansion, making it useful for measuring instruments, for example, surveying tapes, compensation collars etc. (v) Nichrome. It is an alloy of nickel (80%) with chromium (20%) and is used widely as resistance wire for electrical appliances. (vi) Inconel. Inconel contains 79.5% Ni, 0.2% Cu, 13% Cr, 6.5% iron, and a small amount of silicon and manganese. It has the corrosion resistant properties of Monel but has better resistance to sulphur at higher temperatures. It retains its strength at extremely high temperatures and can be used at 1150°C. Its creep properties are very good. It is non-magnetic at all temperatures above –22°C. (vii) Nimonics. A new type of nickel alloys called ‘‘nimonics’’ are being developed, which by proper heat treatment attain excellent properties for very high temperature service. Their common composition is : Cr (15 to 18%), Co (15 to 18%), 3.5 to 5% Mo, 1.2 to 4% Ti, 1.2 to 5.0% Al, and the remainder Ni. 2.9.8. Alloys for High Temperature Service. Many components in jet and rocket engines, and in nuclear equipments have to with stand temperatures above 1100°C. This has resulted in the development of a number of highly specialised alloys. These alloys have nickel or cobalt (melting point 1495°C) as the base metals. Their yield strength is above 700 N/mm2 and hardness is 250 to 370 BHN at room temperature. Some typical high-temperature alloys are : (i) Inconel. Discussed above (ii) Nimonics. Discussed above (iii) Incoloy 910. Ni (12%), Cr (13%), Ti (2.4%), Mo (6%), C (0.04%), Iron, remainder. Engineering Materials and Heat Treatment 63 (iv) Hastealloy. Ni (45%), Cr (22%), Co (1.5%), W (0.5%), Mo (9%), C (0.15%), Balance Iron. (v) Vitallium. Ni (2.5%), Cr (25%), Co (62%), Iron (1.7%), Mo (5.5%) and C (0.28%). 2.9.9. Titanium Alloys. Titanium is a silvery white metal with melting point of 1670°C and a specific gravity of 4.505. Titanium can be alloyed to give high elevated-temperature combined with low weight and corrosion resistance. Industrial titanium alloys contain vanadium, molybdenum, chromium, manganese, aluminium, tin, iron or other metals, singly or in various combinations. These alloys respond to heat treatment, case-hardening and work hardening techniques. However, due to the great affinity of Ti to oxygen, the high melting point and low fluidity, special skills are needed for its casting. Properties of castings can be greatly improved by HIP (Hot Isostatic Pressing). Because of their corrosion resistance, titanium and its alloys, in the form of sheets and tubes, are extensively used in chemical plants. A combination of high mechanical properties (due to heat treatment) with a low specific weight and excellent corrosion resistance, make the titanium alloys indispensable for critical aircraft components in subsonic and, especially in supersonic aircraft. 2.9.10. Refractory Metals. Refractory metals are those metals which are resistant to heat and difficult to melt. The most important refractory metals are : tungsten (melting point 3410°C), molybdenum (2610°C) and niobium, also called columbium (2470°C). These metals oxidise extremely rapidly, therefore, special (vacuum are or electron beam) melting and casting techniques are required. For the same reason, they must be processed (by metal forming techniques) in vacuum or protective atmosphere. Tungsten is used extensively in the form of wire in incandescent wire lamps. The refractory metals are indispensable in some applications such as rocket motor nozzles. 2.9.11. Metals for Nuclear Energy. In nuclear energy power plants, the various metals used for construction have to with stand very stringent conditions. As a result, some metals, previously considered rare are being widely used in this field. The metals which are used for nuclear engineering purposes are : uranium, plutonium, zirconium, beryllium, niobium and their alloys. They are used as raw fuel materials, moderators, reflectors, fuel elements, fuel canning materials, control elements and pressure vessel materials. 1. Uranium. This metal is found in nature and is the most important metal used for nuclear engineering purposes. Its isotope U235 is used as the nuclear fuel. The metal is radioactive, easily oxidised, has a poor resistance to corrosion and needs to be protected for use as fuel elements by roll cladding it in a thin aluminium or zirconium jacket. Pure uranium is weak and is susceptible to severs irradiation damage and growth in the reactor environment. Addition of some alloying elements such as chromium, molybdenum, plutonium and zirconium make the metal highly suitable for nuclear energy applications. Uranium compounds, such as UO2 as a dispersion in cermets or as ceramic slugs, have been found to give better results. Uranium oxide is highly refractory, shows no phase change in an inert atmosphere, is highly corrosion resistant and possesses a good strength. But it has low thermal shock resistance, poor thermal conductivity and a high co-efficient of expansion. 2. Thorium. Thorium is also available in nature. It is a fertile nuclear fuel. It can be converted into fissile nuclear fuel, U233, by neutron absorption and beta decay. In pure state, the metal is soft and weak. Its mechanical properties are drastically changed by small addition of impurities. Its tensile strength is raised from 140 to 380 N/mm2 by the addition of only 0.2% C. Addition of uranium also increases its strength. But small additions of titanium, zirconium and niobium decrease the strength and hardness of the metal. Like uranium, thorium is also radioactive, but is less susceptible to irradiation damage. 3. Plutonium. This metal does not occur in nature. It is produced from an isotope of uranium, U238, through neutron absorption and subsequent beta decays. The metal is then used as fissile 64 A Textbook of Production Technology nuclear fuel. It is extremely toxic and emits alpha rays. The metal is chemically more reactive than uranium and has a poor resistance to corrosion. It is used as a fissile nuclear fuel in fast breeder reactors and also for making atomic weapons. 4. Zirconium. Zirconium minerals contain 0.5 to 2% hafnium which is a strong absorber of neutrons and, therefore, must be removed. The main use of the metal is for cladding fuel elements and for structural components in water cooled systems. So, it must have increased corrosion resistance. Zircaloy-2 containing 1.5 Zn, 0.1 Fe, 0.5 Ni and 0.1 Cr which provide better corrosion resistance are generally used in water-cooled reactors. Zirconium has a relatively poor resistance to CO2 at elevated temperatures, but this is improved by the addition of 0.5 Cu, 0.5 Mo with an increase of tensile strength to 510 N/mm2 and improved creep resistance at 450°C. This Zirconium is specially useful in gas cooled reactors. 5. Beryllium. It is a light metal with melting point of about 1280°C. The metal is very reactive and forms compounds with furnace atmospheres and refractories. Therefore, vacuum or inert gas is necessary during its melting. The cast metal is usually coarse grained and brittle. Due to this, powder metallurgy methods are employed for its fabrication. The metal is used as a moderator, reflector and neutron source. 6. Niobium. Niobium, Nb, also called as columbium, Cb, is a refractory metal. The metal has good hot strength, ductility and corrosion resistance, especially to liquid sodium coolants. Its oxidation resistance above 400°C is greatly improved by alloying. 2.9.12. Stellite. Stellite is cobalt base metal alloyed with various proportion of chromium (35), cobalt (45) and tungsten (15) with iron, manganese and silicon present only as impurities. The most notable property of these alloys is high red hardness. They have high abrasive resistance, non-tarnishing, corrosion resistance and nonmagnetic properties. These properties make it excellent cutting tool for various machine operations on cast iron and malleable iron and on some steels. Because of the cost of stellite, the larger tools are made of steel with stellite tips welded on it. Coatings of stellite, applied by welding, are used for surface protection against wear and abrasion on such items as oil-well pits, cement-mill grinding rigs, and for some bearing surfaces whose lubrication is impossible or unreliable. The co-efficient of friction on dry metals varies from 0.15 to 0.24 with an average of 0.18. Mechanical properties of various stellite materials are given in table 2.13. Table 2.13. Mechanical Properties of Stellite Material Ultimate strength Brinell Properties and Uses Tension Comp. Tension hardness at 500°C (MN/m2) (MN/m2) (MN/m2) Welded 265.0 1792.0 168.0 512 Not machinable, grind Cast 265.0 2177.0 175.0 600 cast 600.0 1540.0 126.0 402 Forged 938.0 1800.0 164.0 - Can be rolled, forged and punched at 1000°C. Welded 462.0 1350.0 133.0 444 Hard-surfacing weld rod, not used for cutting tools. 2.9.13. Non-metallic Materials. The commonly used non-metallic materials are wood, glass, rubber, leather, carbon and plastics. Wood. Wood has some uses in machine members. It is used where light weight parts subjected to moderate shock loading are required, e.g., in circuit breaker operating rods where non-metallic bearing material is desirable. Engineering Materials and Heat Treatment 65 Glass. It is usually thought of as a weak brittle material. The greatly improved mechanical properties of the newer glass compositions justify their consideration in design problems. Glass parts can be moulded by heat and pressure or by heating and blowing. They can be finished and cut by grinding and can be machined by carbide tools, but the surface will be too rough for ordinary uses. Glass is used for all parts of centrifugal pumps for acids, piping for mechanical processes, pipe fittings, exchangers and lining for tanks and fittings. The tensile strength of glass ranges from 42.0 to 84.0 MN/m2 with small surface scratches reducing the strength by 50 per cent. Compressive strength of glass is over 700 MN/m2. The modulus of elasticity varies from 45 GN/m2 to 70 × 106 GN/m2. Allowable design stresses range from 3.5 to 7.0 MN/m2. Rubber. Rubber and similar synthetic materials such as Neoprene have a variety of application in machinery. Rubber should be protected from high temperature, oil and sunlight. It is an excellent material for seats and diaphragms, for water lubricated bearings, for parts subjected to vibrations (such as vibration mountings, flexible couplings and flexible bearing) and for tubes and hose. In industry, hard rubber is used for electric insulation, switch handles, bearings, etc. Table 2.14 gives the different properties of rubber. Leather. Leather is very flexible and will stand considerable wear under suitable conditions. Modulus of elasticity varies according to load. It is used in belt drives and as a packing or a washers. Table 2.14. Properties of Rubber and Rubber Like Materials Material Form Compressive Tensile Shear Max. Effect of heat Strength Strength Strength temp. for (MN/m2) (MN/m2) (MN/m2) use(°C) Duprene - - 1.4 – 28.0 - 150 Stifens slightly Kore seal Hard - 14.0 – 63.0 - 100 Softens Kore seal Soft - 3.5 – 17.5 - 85 Softens Pilo form Plastic 60.0 – 77.0 28.0 – 35.0 49.0 – 63.0 70 – 120 Softens Rubber Hard 14.0 – 105.0 7.0 – 70.0 63.0 – 105.0 55 – 70 Softens Rubber Soft - 3.5 – 4.2 - 60 – 90 Softens Rubber Linings - – - 85 Softens Carbon. Since long carbon has been used in electrical insulations and it has not been generally used in mechanical insulations. Use of carbon gives low friction losses, low wear rates when operating against metals and is used in chemical handling equipment. Pump rotors, vanes and gears of carbon have been used with good results to replace similar parts of bronze and laminated plastics. Clutch plates and rings of metal and cork have been replaced by carbon parts. The properties of carbon are given in table 2.15. Graphite, a form of carbon, has long been used in bronze bearings as a lubricant, but now bearings made entirely of carbon are being used with operating pressure as high as 7.0 MN/m2 at low speed. Table 2.15. Properties of Carbon Strength MN/m2 Compression 14.0 – 175.0 Tension 2.8 – 26.5 Shear 22.4 Modulus of elasticity 9.31 – 17 GN/m2 66 A Textbook of Production Technology Maximum operating temperature 450°C. Co-efficient of friction Carbon on hard steel, unlubricated 0.18 – 0.3 Well lubricated 0.04 – 0.08 Carbon on glass, unlubricated 0.17 Plastics. These are difficult to define because any acceptable definition usually gives too many exceptions. Common usage has applied the term to a class of materials familiar to everyone in such products as buttons, fountain pens, telephones, dials, knobs, etc. The word plastic was used originally to indicate a material that could be formed or moulded by pressure at moderately elevated temperature. Some of the plastics are available as thin sheets, foams, coatings, petroleum laminates and filaments for weaving. Raw materials are mainly derived from petroleum and agriculture. Plastic moulding is one of the high volume, low cost production methods with products that are replacing those formerly made from wood, glass or metal. Plastic materials when used have the following advantages : 1. Good accuracy, light, smooth surfaces, low thermal conductivity, pleasing appearance, self lubrication characteristics, resistance to corrosion, and good dielectric strength. 2. Compared to metal, plastics have better resistance to shock and vibration and higher abrasion and wear resistance. 3. They can be either transparent or opaque and can be made in desired colour. 4. No finishing is required after removal from the mould. 5. Although the moulds are expensive, but are long lasting and capable of producing many thousands of parts. The use of the most of the plastics is limited due to the following reasons : 1. Low strength and rigidity, low heat resistance, and sometimes low dimensional stability. 2. Small loads at room temperatures will induce a continuous type of creep behaviour. This effect is intensified at higher temperatures. 3. The thermal expansion of plastics usually runs from five to ten times that of metal. 4. Embrittlement with age is another disadvantage. 5. The cost of plastic materials is high, but has been decreasing, while other materials have been increasing in price. Types of Plastics Plastic materials can be broadly classified as (1) Thermoplasts or thermoplastic materials (2) Thermosets or thermosetting materials. Thermoplastics, when moulded, undergo no chemical change. Although rigid at room temperature, heating beyond softening point will cause the material to assume a viscous or liquid state. Thermoplastics are sold under such trade name as Celluloid, Nitron, Pyralin, Fibestos, Nixonite and Tenite. Thermosets, undergo a chemical change when heated. This change is permanent and the material cannot be softened by heating. Thermosets cannot be extruded or calendared. Thermosetting plastics are sold under trade names such as Bakelite, Durite, Textolite, Bakelite uses, Beetle and Plaskon. The chemistry of plastic materials is very involved. Many hundreds of such materials are available and the number is continuously increasing. The most widely used moulding plastics are given in tables 2.16 and 2.17. The table also indicates the common trade names, characteristic properties and typical applications. Engineering Materials and Heat Treatment 67 Table 2.16. Names, Characteristics and Uses of Thermoplastics Chemical Trade Characteristics Typical Applications Classiffication name 1. Callulose Tenite 1 Nonflammable, good dielectric, Machine guards and covers, tool acetate plastacel good toughness, high impact and cutlery handles, toys and Fibestos strength, moulding and fabricating knobs. Brush backs, Jewellery, Lumarith versatility, available in wide range Electrical insulation. of colours, dimensional stability may not be good. 2. Cellulose Celluloid Flammable, but otherwise same Toilet articles, moviefilm. pyralin as cellulose acetate. Oldest Drawing instruments. Piano and Nitron commercial plastic. typewriter keys. Pyroxylin 3. Methyl Lucite Light weight, weather resistant, Aircraft cockpits and windows plexiglass Best of plastics for light piping, outdoor signs Crystalite transmission and edge illumination effects. 4. Polyamide Nylon Low co-efficient of friction. Small moulded gear and bushings, zytel Dimensional stability. Good brush bristles fuel container electrical properties. Not coatings. affected by gasoline or hydraulic fluids. Bearings can operate without lubricant. Abrasion resistance. 5. Polyethylene Alathon Flexible and tough. Excellent Automotive parts, House wares, Polythene dielectric properties. Flammable containers, electrical cable but slow burning. Dimensional jacketing, tubing, Squeeze bottles. variaions difficult to predict. 6. Polystyrene Lystrex Resistant to acids and alkalies. Electrical insulation, containers Styron Good dimensional stability and closures. Instrument panels, Cerex Good electrical properties. knobs, wall tile, radio cabinets. 7. Polytetra- Teflon Low co-efficient of friction. Gaskets, packings, seals fluoro Resistance to chemical attack. bushings, lining for pipe and Ethylene Tough at low temperature. vessels. Non stick surfacing. (PTFE) Bushings can operate without Electrical insulation. lubricant in presence of abrasives. 8. Polyvinyl Vinylite Flame resistant Resistant to Automotive panels, parts for Chloride Q Koroseal chemicals, oils, and solvents. vacuum cleaners and refrigerators, Tygon Abrasion resistant. Sound and floor coverings, pipe fittings, Geen vibration deadening tanks, balls and floats. Table 2.17. Names, Characteristics and Uses of Thermosets Chemical Trade Characteristics Typical Applications Classification name 1. Epoxy Araldite Good toughness. Resistant to Adhesive and coatings, tools and Oxiron acids, alkalies and solvents. dies, filament wound vessels, Excellent adhesion to metal, glass laminates for aircraft, patching and wood. compound for metal and plastics. 2. Melamine- Melmae Good for application requiring Table-ware, electric insulation, formal- Resimeve cycling between wet and dry automotive ignition parts, cutlery 68 A Textbook of Production Technology dehyde conditions. Hard and abrasion handles, jars and bowls. resistant. Good dielectric Does not impart taste to food. 3. Phenol- Bakelite Good dimensional stability Industrial electrical parts. formal- Marblette Excellent insulating qualities. Inert automotive electrical compo- dehyde Durez to most solvents and weak acids. nents, paper impregnated battery Catalin Good strength around inserts. separators. 4. Phenol- Durite Similar to Phenolformaldehyde. Electrical insulation. Mechanical furfural parts. Housings and containers. 5. Alkyd Glyptal Can be made flexible, resilient or Boats, Tanks, Trailer and Tractor (Modified Duraplex rigid. Can resist acids but not components. Ducts, shrouds. polyester) Beckosol alkalies, with glass fibre Vaulting poles. Teglac reinforcement resists salt water Rezly and fungus growth. 2.9.14. Cemented Carbides. These are cutting tool alloys. These are made of a powdery mixture of tungsten and titanium (and also of tantalum) carbides and cobalt which is first compacted and then sintered. Cemented carbides are extremely hard and they retain their hardness at temperatures upto 1000°C. These cutting tool materials will be discussed in detail in chapter 7. PROBLEMS 1. Classify the materials of construction. 2. What is malleable iron ? What advantages has this form over white or grey cast iron ? 3. How does carbon content affect cast iron, wrought iron, and steel with reference to hardness and toughness ? 4. What is the chief reason for the use of alloy steels in machine parts ? 5. Explain the difference between malleable cast iron and grey cast iron. 6. Differentiate between hot and cold working. 7. Under what circumstances alloys of cast iron are used ? 8. Explain in detail the effects of nickel, copper, chromium and molybdenum as alloying elements in cast iron. 9. When the cast steels are preferred over cast iron ? 10. Enlist the properties of 0.2% carbon steel. 11. Name the steel alloys used for high temperature service. 12. List the alloying materials of aluminium. 13. What alloy steel is suitable for springs ? 14. What are the copper alloys ? 15. List the advantages and disadvantages of bronze over brass for industrial uses. 16. What is the composition of red brass and yellow brass and where these are commonly used ? 17. Differentiate between the tin base and lead base alloys. 18. What are the constituents of Muntz metal and monel metal ? 19. What are the constituents of Babbit metal ? 20. What are the constituents of stellite and where it is commonly used ? 21. List the important non-metallic materials of construction. 22. Explain clearly the use of wood and glass in place of steel in machine design. Engineering Materials and Heat Treatment 69 23. Enumerate the advantages and disadvantages of plastic materials over metallic materials. 24. Give the trade names of plastic items used in daily life. 25. Give the materials which are commonly used for the following parts of a steam engine. (a) Cylinder head and cylinder (b) Connecting rod (c) D-slide valve (d ) Crankshaft (e) Piston ( f ) Piston rings 26. What are cupronickels or nickel silver alloys ? 27. Discuss the materials of the following : (a) Rivets used in a boiler (b) gears of a lathe (c) Connecting rod of an I.C. Engine (d ) Valve of a safety valve (e) body of a safety valve ( f ) Steam pipe (g) Water pipe 28. Give two typical applications of each of the following alloys : (a) 70/30 Copper zinc alloy (b) monel metal (c) babbit metal (d ) Copper-aluminium alloy (e) 92/8 Copper lead alloy. 29. What is the difference between cast iron and wrought steel ? 30. Name the four types of cast irons. Indicate the advantages and disadvantages of each. 31. Define the following : (a) hypoeutectoid steel, (b) hypereutectoid steel, (c) eutectoid steel. 32. What are the constituents of the following : austenite, ferrite, cementite, pearlite, and martensite ? 33. Plain carbon steels are divided into three groups. Name these groups and write some typical applications of each. 34. Distinguish between annealing and normalizing. What is the purpose of these processes ? 35. What is meant by tempering of Steel, and why is this process employed? 36. Describe martempering and austempering. 37. Distinguish between hardness and hardenability. 38. How does surface hardening differ from through hardening ? What is its purpose ? Name the various surface hardening methods. 39. What are the advantages of nitriding over carburizing and cyaniding ? 40. Differentiate between age hardening and precipitation hardening. What is artificial aging ? 41. List the three grades of stainless steels. What constituents in stainless steels make them particularly resistant to corrosion ? Write some typical product applications of each. 42. What two methods are used to harden aluminium alloys ? 43. For most atmospheres, aluminium is known to resist corrosion. Explain why. 44. What is the difference between brass and bronze ? 45. List the metals used for high temperature requirements. 46. What is the difference between rimmed steel and killed steel ? 47. What are the two ways in which white cast iron can be obtained ? 48. What is the difference between ‘‘Full annealing’’ and ‘‘Isothermal annealing’’ ? 49. Define critical points in heat treatment of steels. 50. Explain the principle of heat treatment. 51. What is Flame hardening ? 70 A Textbook of Production Technology 52. What is Induction hardening ? 53. Explain Iron-carbon diagram. Also, discuss the various transformations which are taking place at different temperatures. 54. Explain Eutectic and Eutectoid reactions. 55. Explain nitriding process of case-hardening. 56. What information may be obtained from equilibrium diagram ? 57. What information may be obtained from TTT curves. 58. What is Martensite ? How does it appear under microscope ? 59. Compare martempering and austempering. 60. What are the principal advantages of austempering as compared to conventional hardening and temper methods ? 61. Define hardenability. How it can be measured ? Explain the variables affecting hardenability. 62. List various objectives of heat treatment. 63. List various objectives of hardening heat treatment process. 64. What are: Sorbite, Troostite and Bainite ? 65. What are: Process annealing, homogenizing and spheroidising heat treatment processes ? 66. Write on the various quenching media used in hardening process. 67. Explain the various carburising methods. 68. Explain cyaniding process. 69. Explain Carbo-nitriding process. 70. Write on three types of tempering processes. 71. What is stepped quenching ? 72. Discuss the importance of cooling rate in hardening process. 73. What is the ‘‘critical cooling rate’’? 74. What is meant by ‘‘Soaking time’’? 75. Which methods may be used to localize case-hardening? 76. Draw the cooling curve for pure iron and explain the charactersitics. 77. Draw the Iron-Carbon phase diagram. 78. Why is it called as ‘‘Equilibrium diagram’’? 79. Identify the various phases of iron-carbon phase diagram. 80. Explain the above phase diagram. 81. Identify the constituents of steel from the iron-carbon diagram. 82. Explain the effect of slow cooling for various compositions. 83. What are : Hypo-eutectic cast irons, Hyper-eutectic cast iron and eutectic cast irons ? 84. Explain the TTT diagram. 85. Why is it called as Non-equilibrium diagram ? 86. Draw and write on cooling curves for Pure metals and for alloys. 87. What is the major purpose of tempering steel after hardening it ? 88. Why must hardening of steel always precede tempering ? 89. What property is sacrified in order to get benefits of tempering ? 90. Why is steel always tempered after hardening ? 91. Why is steel not tempered without hardening ? Engineering Materials and Heat Treatment 71 92. Apart from effect on hardness, what is the major advantage of an oil-quenching steel ? 93. What is the effect of quenching a water-hardening steel in oil ? 94. What is the effect of quenching an oil-hardening steel in water ? 95. Define an air-hardening steel. 96. What is the objective of case-hardening ? 97. Why should the specimens be heated to as high as 930°C during carburization ? 98. What is the function of the carbonate compounds in the carburizer ? 99. What is the mechanism of carburization ? 100. Why does further heat treatment usually follow carburizing ? 101. What type of steel is best suited to carburizing ? 102. What is the difference between case-hardening and case-carburizing ? 103. Which steel has the highest hardenability ? 104. Why is hardenability an important property ? 105. Why is hardenability a property of steel and not of brass or aluminium ? 106. Define ‘ideal critical diameter’. 107. On a Jominy curve, where is the point which represents the microstructure condition of 50% martensite and 50% softer constituents ? 108. How can relative hardenability be judged directly from two Jominy curves, each of a different steel ? 109. Define recrystallization annealing. 110. What is ‘‘Boronizing’’ ? 111. What is Chapmanizing ? 112. Which one of the following is not a compound ? (a) Chalk (b) Acetylene (c) Calcium (d) Sulphuric acid (Ans. : c) (AMIE, I.Mech.E.) 113. Brittle fracture in a metal can be the result of : (a) High temperature during service (b) Low temperature during service (c) Excessive loading during service (d) Reduction of ductility during service. [Ans. : b] (A.M.I.E., L.U.) 114. How annealing is related to cold working ? 115. Describe the process of Normalizing. (D.U., L.U.) 116. When a steel suffers hot shortness, it is mostly due to the presence of : — (a) S (b) P (c) Si (d) Mn (B.T.E., A.M.I.E., U.P.S.C.) (Ans. : a) 117. When a steel is subjected to a form of heat treatment known as tempering after it has been hardened, the process is one of :— (a) Heating without quenching (b) Heating and quenching (c) Heating and cooling slowly (d) Heating and hammering. (Ans. : c) 118. Discuss the effect of carbon on steel. 119. What are the effects of heat on the steel structure ? 120. Write briefly about the different heat treatments given to steel to obtain desired properties. (L.U.) 72 A Textbook of Production Technology 121. Which one of the following properties is related to C.I. ? (a) Ductility (b) Malleability (c) Plasticity (d) Brittleness (Ans. : d ) 122. Which one of the following metals is used when producing a magnetic field ? (a) Lead (b) Sn (c) Zn (d) Iron (Ans. : d ) 123. What is meant by Stress relieving ? 124. Describe the process of Stress relieving. 125. Is the range of temperature used in the relief of stress : (a) Below the recrystallisation temperature (b) Above the recrystallisation temperature (c) At the recrystallisation temperature (d) At the solidification temperature. [Ans. : b] 126. Write a short note on: Nickel Maraging Steels. These are extra strength steels, having very low C-content (  0.03%). Because of the presence of Ni, these steels can be air cooled from a temperature of 800°C to set soft distortion free martensite. High hardness (HRC = 52-54) and high strength can be produced by aging treatment, that is, keeping this steel at 480°C for 3 hours and then air cooling. The three grades of this steel, widely used, are:- Ni Co Mo Ti Max. stress, N/mm2 1. 18 8.5 3 0.2 1400 2. 18 8 5 0.4 1750 3. 18 9 5 0.6 2000 Advantages:- 1. Ultra high strength, high yield strength, and high temp. strength. 2. High notch toughness. 3. High toughness and ductility. 4. Simple heat treatment and dimensional stability. 5. Properties of good machinability and weldability. 6. Can be nitrided. 7. Low Co-efficient of thermal expansion. Applications:- Best suitable for Al die casting dies, Precision plastic moulds, forging dies, carbide die holders, wear resisting index plates. 127. Describe the following properties of metals : ductility, plasticity, elasticity. 128. When a metal regains its original shape and size while the stress acting upon it is removed, the metal in said to have : (a) Ductility (b) Plasticity (c) Malleability (d) Elasticity (Ans : d) 129. Percentage elongation of a metal undergoing a tensite test is a measure of : (a) Elasticity (b) Plasticity (c) Ductility (d) Malleability (Ans : c) 130. Name some of the common alloying elements present in alloy steels and describe their effects on the properties of these steels. 131. Name the elements usually present in plain Carbon steels and describe their effects on the properties of this steel.  Chapter 3 The Casting Process 3.1. GENERAL Casting is probably one of the most ancient processes of manufacturing metallic components. Also, with few exceptions, it is the first step in the manufacture of metallic components. The process involves the following basic steps : 1. Melting the metal. 2. Pouring it into a previously made mould or cavity which conforms to the shape of the desired component. 3. Allowing the molten metal to cool and solidify in the mould. 4. Removing the solidified component from the mould, cleaning it and subjecting it to further treatment, if necessary. The solidified piece of metal, which is taken out of the mould, is called as ‘‘Casting’’. A plant where the castings are made is called a ‘‘Foundary’’. It is a collection of necessary materials, tools and equipment to produce a casting. The casting process is also called as ‘‘Founding’’. The word ‘‘Foundry’’ is derived from Latin word ‘‘fundere’’ meaning ‘‘melting and pouring’’. Types of Foundries. All the foundries are basically of two types : (i) Jobbing Foundries. These foundries are mostly independently owned. They produce castings on contract, within their capacity. (ii) Captive Foundries. Such foundries are usually a department of a big manufacturing company. They produce castings exclusively for the parent company. Some captive foundries which achieve high production, sell a part of their output. 3.1.1. Advantages of Casting Process 1. Parts (both small and large) of intricate shapes can be produced. 2. Almost all the metals and alloys and some plastics can be cast. 3. A part can be made almost to the finished shape before any machining is done. 4. Good mechanical and service properties. 5. Mechanical and automated casting processes help decrease the cost of castings. 6. The number of castings can vary from very few to several thousands. However, casting imposes severe problems from the points of view of material properties and accuracy. Also, a complicated sequence of operations is required for metal casting. Again, the geometric complexity of the final product may be such that this process is of no use. 3.1.2.Applications There is hardly any machine or equipment which does not have one or more cast components. The list is very long, for example, automobile engine blocks, cylinder blocks of automobile and 73 74 A Textbook of Production Technology airplane engines, pistons and piston rings, Machine tool beds and frames, mill rolls, Wheels and housings of steam and hydraulic turbines, Turbine vanes and aircraft jet engine blades, Water supply and sewer pipes, Sanitary fittings and agricultural parts etc. In machine tools, internal combustion engines, compressors and other machines, the mass of castings may be as great as 70 to 85% of the product's total mass. 3.1.3. Classification of Casting Process The list of the various casting processes is very long. However, there is one convenient way of classifying these processes. It is according to whether the moulds, patterns (used to make mould cavities) and cores (used to produce internal details in a component) are permanent or expendable (disposable). 1. Expendable Mould Casting. In this process, the mould cavity is obtained by consolidating a refractory material (moulding material) around a pattern. The mould has to be broken to take out the casting from the mould cavity. So, such moulds are one casting moulds. The moulding material can be sand or some other refractory material. The main drawback of sand mould casting process is that the dimensional accuracy and surface finish of the castings do not satisfy in many cases the requirements of modern machine building and instrument making industry. However, if the moulding material is used in the form of slurry (Slurry moulding), better surface finish and dimensional accuracy can be obtained. The pattern used in this process can be permanent pattern (which can be used again and again and is made of wood, metal or plastic) or expendable pattern (Full mould process, lost wax method). 2. Permanent Mould Casting. In this process, the mould is used repeatedly and is not destroyed after the solidification of the casting. The moulds are adaptable to the production of tens and thousands of castings. Generally, the process is practical for making parts of small and medium mass from light non-ferrous alloys. The castings produced by this method have smooth surface and increased accuracy of dimensions. Due to the high cost of permanent moulds, the use of this method is limited to mass or quantity production. 3. Semi-permanent Mould Casting. These moulds are prepared from high refractory materials, for example, based on graphite. These moulds are not as durable as permanent moulds. So, these can not be used for mass or quantity production, but for only a few tens of castings. In all the above three methods, the cores used may be permanent (metallic) or expendable (made of core sand or of some other suitable material). An iron foundry may have the following six prominent sections : (a) Moulding and core making. (b) Metal melting. (c) Metal handling and pouring. (d) Knockout. (e) Fettling. (f) Miscellaneous. 3.2. SAND-MOULD CASTING This process accounts for about 80% of the total output of cast products. As mentioned above, the sand moulds are single-casting moulds and are completely destroyed for taking out the casting, after the metal has solidified in the mould cavity. The moulding material is sand, which is mixed with small amounts of other materials (binders and additives) and water to improve the cohesive strength and mouldability of sand. For making the mould, the moulding material will have to be consolidated and contained around the pattern. The metallic container is called as flask. There can be one flask or more than one flasks. The most common design is two flask system. In the assembled position the upper flask is called ‘‘Cope’’ and the bottom one ‘‘Drag’’. In three flask The casting Process 75 system, the central flask is called ‘‘Cheek’’. One flask design is used in `Full mould process' or in `Pit moulding', where it is used as cope, the pit acting as the drag. Depending upon the type of pattern used, the sand mould casting process is of two types : 1. Permanent or Removable pattern process. Here, the pattern is removed from the mould cavity, before the molten metal is poured into the mould cavity. This is the most common sand mould casting process. 2. Expendable or Disposable pattern process. Here, the pattern is not removed from the mould cavity, before the molten metal is poured into the mould cavity. It gets melted and forms a part of the final casting. This process will be discussed later. 3.2.1. Steps in making a casting by the expendable mould, ‘‘removable pattern sand moulding process’’. 1. Make the pattern. The material of the pattern can be : wood, metal or plastic. 2. With the help of patterns, prepare the mould and necessary cores. 3. Clamp the mould properly with cores placed properly in the mould cavity. 4. Melt the metal or alloy to be cast. 5. Pour the molten metal/alloy into the mould cavity. 6. Allow the molten metal to cool and solidify. Remove the casting from the mould. This operation is called ‘‘Shake out’’. 7. Clean and finish the casting. The operation is called as ‘‘fettling’’. 8. Test and inspect the casting. 9. Remove the defects if any and if possible (Salvaging the casting). 10. Stress relieve the casting by heat treatment. 11. Again inspect the casting. 12. The casting is ready for use. 3.2.2. Types of Sand Mould Casting Process Sand moulds can either be made by hand or on moulding machines. Hand moulding is done for piece and small lot production, whereas machine moulding is employed in large lot and mass production. Depending upon the nature of the work place, hand moulding process can be classified as: 1. Bench Moulding. This is done only for small work. 2. Floor Moulding. This process is done on the foundary floor and is employed for medium sized and large castings. 3. Pit Moulding. This method is used for very large castings and is done on the foundary floor. However, a pit dug in the floor acts as the lower flask (drag) and the top flask (cope) is placed over the pit to complete the assembly. The walls of the pit are brick-lined and plastered with loam sand and allowed to dry. Sometimes, for large and tall castings, the bottom of the pit is rammed with a 50 to 80 mm layer of coke to improve the permeability of the mould. Vent pipes are run from this layer to the surface (at the sides) and the coke is covered with blacking sand. In ‘‘Machine Moulding’’, the operations done ordinarily by hand, are done by machine. The operations include : Compacting the sand, rolling the mould over and drawing the pattern from the mould etc. 3.2.3. Types of Sand Moulds As already discussed, a mould is an assembly of two or more flasks (metallic) or bonded refractory particles, with a primary cavity which is a negative of the desired part. It contains secondary cavities (pouring basin, sprue, runners and gates) for pouring and channeling the liquid metal into 76 A Textbook of Production Technology the primary cavity. If necessary, it also has a large cylindrical cavity for storing the molten metal (riser or feeding head) for feeding into the primary cavity to compensate for the shrinkage of molten metal in the primary cavity, on cooling and solidification. According to the material used in their construction, the moulds are of following types : 1. Green Sand Moulds. A green sand mould is composed of a mixture of sand (silica sand, SiO2), clay (which acts as binder) and water. The word ‘‘green’’ is associated with the condition of wetness or freshness and because the mould is left in the damp condition, hence the name ‘‘green sand mould’’. This type of mould is the cheapest and has the advantage that used sand is readily reclaimed. But, the mould being in the damp condition, is weak and can not be stored for a longer period. Hence, such moulds are used for small and medium sized castings. 2. Dry Sand Moulds. Dry sand moulds are basically green sand moulds with two essential differences : the sand used for dry sand moulds contains 1 to 2% cereal flour and 1 to 2% pitch, whereas the sand mixture for green sand moulds may not contain these additives. Also, the prepared moulds are baked in an oven at 110 to 260°C for several hours. The additives increase the hot strength due to evaporation of water as well as by the oxidation and polymerization of the pitch. So, dry sand moulds can be used for large castings. They give better surface finish and also reduce the incidence of the casting defects such as gas holes, blows or porosity that may occur as a result of steam generation in the mould (when the molten metal is poured into the green sand mould cavity). However, due to the greater strength of these moulds, tearing may occur in hot-short materials. 3. Skin - dry Sand Moulds. Here, after the mould is prepared, instead of entirely drying it out, the mould is partially dried around the cavity (to a depth of about 25 mm). This can be done in two ways : (i) About 12.5 mm around the pattern, the proper moulding sand (as described under dry sand moulds) is used, the remaining mould contains ordinary green sand. (ii) The entire mould is made of green sand and then the surface of the cavity is coated with a spray or wash of linseed oil, gelatinized starch or molasses water etc. The advantages and limitations of such moulds are the same as of the dry sand moulds. 4. Loam Sand Moulds. Loam sand consists of fine sand plus finely ground refractories, clay, graphite and fibrous reinforcements. It differs from ordinary moulding sand in that the percentage of clay in it is very high (of the order of 50%). This sand is used in pit moulding process for making moulds for very heavy and large parts (engine bodies, machine tool beds and frames etc.). 5. Cemented - bonded Moulds. Here, the moulding sand contains 10 to 15% of cement as the binder. Such a mould is stronger and harder. Such moulds are made in the pit moulding process and develop their strength by air drying and are used for large steel castings. However, it is very difficult to break away the sand from the casting. 6. CO2 Moulds. The CO2 moulding process is a sand moulding process in which sodium silicate (Na2O. x Si O2), that is, water glass is used as a binder, rather than clay. After the mould is made, CO2 gas is made to flow through the mould, the sand mixture hardens due to the following reaction. Na 2 O. x Si O2 + n H 2 O + CO2  Na 2 CO3 x.Si O2. n (H 2 O) Stiff gel. here, x = 1.6 to 4, most often 2. This reaction is very rapid and takes about 1 minute, which is very much less than the several hours needed to produce a dry sand mould. Such moulds can be used for producing very smooth The casting Process 77 and intricate castings., because the sand mix has a very high flowability to fill up corners and intricate contours. 7. Resin-bonded Sand Moulds. Here, the green sand mixture is mixed with thermosetting resins (polymers) or an oil, such as, linseed oil or soyabean oil. During baking of the mould, the resin or oil oxidises and polymerizes around the sand particles, thus bonding them together. The strength of the polymerized resin is greater than that of pitch used in dry sand moulds. So, the moulds produced are stronger. Such a sand mixture is commonly used for making cores. Baking is usually needed to make strong moulds or cores. Many times, the required strength of moulds is obtained without baking. These moulds are known as ‘‘Furan-no bake’’ moulds, ‘‘oil- no bake’’ moulds etc. Furan is a generic term denoting the basic structure of a class of chemical compounds. The resins used in the ‘‘no-bake’’ systems are compounds of furfuryl alcohols, urea and formaldehyde. A very low water content (less than 1%) is used in the above moulds. The synthetic liquid resin is mixed with sand and the mixture hardens at room temperature. 8. Dry Sand Core Moulds. When the moulding flasks are too large to fit in an oven (for baking) or when it costs too much to dry a large mass of sand, moulds are made from assemblies of sand cores. A sand core is usually prepared from core sand mixtures (discussed later) and is baked at 175 to 230°C for 4 to 24 h, depending upon sand preparation and mass. 9. Cold-box Mould Process. Here, various organic and inorganic binders are blended into sand to bond the grains chemically, imparting greater strengths to the mould. These moulds are dimensionally more accurate than green sand moulds, but are more expensive. 10. Composite Moulds. These moulds are made of two or more different materials, such as shells, plaster, sand with binder and graphite. These moulds combine the advantages of each material. They are used in shell moulding and other casting processes, generally for casting complex shapes, such as turbine impellers. These moulds result in : increased mould strength, improved dimensional accuracy and surface finish of castings and reduced overall costs and processing times. The above mentioned types of moulds are compared in Table 3.1. Table. 3.1. Comparison of Moulds Type of Mould Advantages Disadvantages (a) Green Sand Mould 1. Least expensive 1. Sand control is more critical than in dry sand moulds. 2. Less distortion than in dry sand 2. Erosion of mould is more moulds, because no baking is common in the production of large required. castings. 3. Flasks are ready for reuse in 3. Surface finish and dimensional minimum time accuracy deteriorate as the weight of the casting increases 4. Dimensional accuracy is good across the parting line. 5. Less danger of hot tearing of Casting than in other types of castings. (b) Dry Sand Mould 1. Stronger than green sand 1. Castings are more prone to hot moulds,thus are less prone to tearing. damage in handling. 2. Overall dimensional accuracy is 2. Distortion is greater than for green better than for green sand moulds. sand mould because of baking. 78 A Textbook of Production Technology 3. Surface finish of castings is 3. More flask equipment is needed better, mainly because dry sand to produce the same number of moulds are coated with a wash. finished pieces because processing cycles are longer than for green sand moulds. 4. Production is slower than for green sand moulds. (c) Dry Sand 1. Exceptionally good dimensional 1. Extreme care must be taken in Core Moulds accuracy can be maintained. setting the cores. 2. Dry sand core moulding is adaptable to greens and found- aries that do not have large drying facilities. (d) Furan, oil, 1. Sands are free flowing, therefore, 1. Sands must be used immediately CO2 Moulds ramming is eliminated or reduced. 2. Tensile strengths of moulds are higher than those of conventional moulds. This permits reduction of mould weight and easier handling of large moulds. 3. Moulds can be made without flasks. 4. Production rates are high. 5. Most of these moulds can produce castings to closer tolerances than are obtainable in green sand moulds. Note. 1. More than 85% of all metal castings are poured in sand moulds, the balance are made in ceramic shell or metal moulds. 2. Majority of castings are poured in green sand moulds. 3. In a foundry, upto 90% of the moulding sand can be reprocessed to make new moulds. 3.2.4. Preparing a Sand Mould. The sequence of operations performed in the making of a sand mould is outlined below. For this, a green sand mould and a split pattern have been chosen. The appropriate split pattern is made which is split into two equal parts at the parting plane and joined together with dowel pins. We will use a two flask system, (Fig. 3.1). (A) Hand Moulding Process 1. The drag half of the pattern, that is, the half with dowel holes rather than dowel pins, is placed with the flat parting plane on a flat board called ‘‘Moulding board’’. 2. The drag is placed over the moulding board with the alignment or locating pins downwards. 3. A parting material is dusted over the pattern and the moulding board to facilitate both the removal of the pattern from the mould and the separation of the two mould halves. 4. The drag is filled with moulding sand and it is packed and rammed around the pattern. The ramming is done manually with hands and with hand rammers (wooden or iron). The sand should be properly rammed, that is, neither too hard nor too soft. If it is too soft, the mould will fall apart during handling or during pouring and if it is too hard, gases produced on pouring will not be able to leave it. Pneumatic ramming or mechanical ramming can be used for large moulds. The casting Process 79 Pattern Riser Core Pin half pin Cope Dowel Bottom Pin board Alignment Core pin Parting Drag line print Face board (a) (a) Core Mould cavity Riser Pouring basin Sprue Runner Gate (c) Fig. 3.1. A Green Sand Mould. 5. After the ramming, the excess sand is scrapped off with a straight bar called a ‘‘strike rod’’. 6. Vent holes are pierced in the sand (within 15 to 20 mm of the pattern surface) with ‘‘vent wires’’, for the gases to escape through. 7. A second moulding board is placed on the moulded drag half and clamped if the mould is too heavy to be turned over conveniently by hand. The mould is then turned over 180° and the original moulding board is removed. 8. The cope is mounted onto the drag and the two halves are properly aligned with the help of alignment pins. 9. The cope half of the pattern is properly positioned over the drag half of the pattern with help of dowel pins and dowel holes. 10. For making the sprue and riser, the sprue pin and riser pin are placed approximately 25 mm on either side of the pattern (usually along the parting line passing through the alignment pins). 11. Steps 3 to 6 are repeated. 12. A pouring basin is cut adjacent to the sprue and then the sprue and riser pins are with drawn. 13. The cope is carefully lifted off the temporarily separated from the drag and placed on one side. 14. To take out the split pattern from the drag, ‘‘draw spikes’’ are drawn into the pattern and the pattern is loosened from the sand by rapping them lightly in all directions with a wooden hammer called a ‘‘mallet’’. Then the pattern is lifted off with the help of draw spike. Before with drawing the pattern, the sand around it is moistened with a ‘‘Swab’’ so that the edges of the mould remain firm when the pattern is withdrawn. 15. The gate and runner are cut in the drag or both cope and drag, connecting the mould cavity and the sprue opening. Sometimes, the gate and runner are automatically made 80 A Textbook of Production Technology with the help of extensions on the pattern. If needed, all the cavity edges are repaired. Dirt remaining in the mould cavity is blown off with a stream of air. If cores are to be used, they are properly placed in position in the drag. 16. The mould is now assembled, the cope being carefully placed over the drag so that the locating pins fit into the holes. 17. If the lifting force on the cope due to the hydraulic pressure of the molten metal is greater than the weight of the cope, the cope must either be clamped to the drag or else weights must be placed on the top of the cope. 18. The mould is now ready for pouring. Some terms used above are defined below : (a) Core. It is made of core sand (or of some other suitable material and even of metal) and is used to make holes in the casting. (b) Core Prints. Core prints are the projections on a pattern and are used to make recesses (core seats) in the mould to locate the core. (c) Pouring Basin. It is a reservoir at the top of the sprue (in the cope) that receives the stream of molten metal poured from the ladle. (d) Sprue. A sprue or downgate is a vertical channel that connects the pouring basin with runners and gates. It is made somewhat tapered downward for ease of moulding and more importantly to have a decreasing cross-sectional area corresponding to the increase in velocity of the molten metal as it flows down the sprue hole. This prevents turbulent flow and hence the drawing in of air alongwith the liquid into the mould cavity. (e) Sprue base or Well. It is a reservoir at the bottom end of the sprue. It prevents excessive sand erosion when the molten metal strikes the runner at the sprue base. Also, there is considerable loss of velocity in the well. (f) Runner. The runner is generally a horizontal channel whose functions are to trap slag and connect the sprue base with the gates (ingates), thus allowing the molten metal to enter the mould cavity. (g) Gates. The gates (ingates) are the channels through which the incoming metal directly enters the mould cavity. (h) Risers. The risers or feed heads are a part of the feeding system. These are reservoirs of molten metal that feed the metal in the casting proper as it solidifies, to prevent shrinkage cavities in the casting. Bars and Gaggers. With large cope flasks, added support is normally required to keep the moulding sand from sagging or falling out when the cope is raised to remove the pattern. This support is provided by two elements, namely bars and gaggers. The bars subdivide the large cope flask areas into smaller areas which are able to support themselves, (Fig. 3.2). The bars should extend downward to within 25 to 50 mm of the pattern or parting surface. If added support is needed, gaggers should also be used. A gagger is a L-shaped steel rod (6.35 to 12.5 mm in diameter). Gaggers extend vertically downward from the top of the bar to within 25 to 50 mm of the pattern or parting surface. The gaggers are generally dipped in a clay slurry to improve the bonding of the mould sand to the gagger. Or their surfaces should be rough to help them obtain a better grip in the sand. They are placed in position after a depth of about 25 to 50 mm of rammed sand is placed over the pattern and parting surface. Their lower ends are then pressed into the first layer of moulding sand with their upright portions against bars for support. The function of ‘skim bob’ in the figure is to trap foreign matter and slag in the molten metal, so that these do not enter the mould cavity. The portion of the skim bob in the cope traps lighter impurities and the heavier impurities are trapped is the portion in the drag. The casting Process 81 Fig. 3.2. Bars and Gaggers. Soldiers. In some cases, additional support may be secured by the use of nails or soldiers. Soldiers are wooden or steel pins with rough surfaces. These are placed in the moulding sand, as it is rammed, where reinforcement of the mould is needed. Three Flask Mould. Sometimes a casting has re-entrant sections which make it more convenient to use a three or more flask mould rather than a two flask mould. The flasks between the cope and drag flasks are referred to as ‘‘cheeks’’. Fig. 3.3 illustrates the use of a cheek to cast a wheel having a groove in the rim (for example a V-belt pulley or a rope pulley). Fig. 3.3. Three Flask Mould. (B) Machine Moulding. The hand moulding process discussed above is suitable only when the number of moulds to be made is small. But, when large quantities of castings are to be produced, the moulding is done on a machine. Moulding machines pack the sand and draw the pattern from the mould. The use of a moulding machine results in the following advantages: 1. The time required to make a mould is greatly reduced. This reduces the overall costs. 2. The labour productivity is greatly increased, owing to the mechanization of laborious operations of sand ramming and pattern removal. 82 A Textbook of Production Technology 3. More accurate castings are produced, because the method employs more accurate and less tapered patterns, dispenses with rapping for removing the pattern and secures more exact location of the cope and drag. This results in smaller machining allowances. 4. A higher quality of product is maintained. A moulding machine consists of a large number of interconnected parts and mechanisms, which transmit and guide various motions in order to prepare a mould. According to the method in which the sand is compacted around the pattern to make a mould, the moulding machines are classified as : (i) Squeeze Moulding Machines. (ii) Jolt Machines (iii) Sand Slingers (a) Squeeze Moulding Machines. These machines are operated by compressed 5 6 6 air at a pressure from 5 to 7 atm. A 4 3 schematic diagram of a top squeeze machine 3 7 is shown in Fig. 3.4 (a). The pattern plate with pattern 2 is clamped on the work table 1 and flask 3 is placed on the plate. Then 1 2 1 2 the sand frame 4 is placed on flask 3. The (a) (b) flask and frame are filled with moulding sand from a hopper located above the Fig. 3.4. Squeeze Moulding Machines. machine. Next the table lift mechanism is switched on and the flask together with the sand frame and pattern is lifted up against platen 5 of the stationary squeeze head 6. The platen enters the sand frame and compacts the moulding sand down to the upper edge of the flask (shown by a dash line). After the squeeze, the work table returns to its initial position. The principle of a bottom squeeze machine is shown in Fig. 3.4 (b). The pattern plate 2 with the pattern is clamped on work table 1. Flask 3 is placed on frame 7 of the machine and is filled with sand from a hopper. Next, the squeeze head 6 is brought against the top of the flask and the lift mechanism is switched on. Table 1 with plate 2 and the pattern are pushed up to the lower edge of the flask (shown by the dash line). After this the table returns to the initial position. Limitations of Squeezing 1. Sand density is not uniform. It is maximum near the plate and then falls gradually towards the pattern. Due to this, this method is used for work that can be moulded in shallow flasks. 2. If the pattern contains cavities for the formation of green sand cores, squeezing does not make sand flow into the cavities effectively and get it packed properly. (b) Jolt Machine. In the jolt moulding 2 machine, the pattern and flask are mounted 3 on a mould plate and the flask is filled with 5 5 sand. The entire assembly is raised a small 1 amount by means of an air cylinder and is 1 6 then dropped against a fixed stop. The 6 4 compacting of sand is achieved by the 7 7 decelerating forces acting on it. The working of a jolt moulding 8 9 machine is shown in Fig. 3.5. The table 1 with moulding sand, is lifted by plunger 4 to a definite height (about 5 cm) when compressed Fig. 3.5. Jolt Moulding Machine. The casting Process 83 air is admitted through pipe 5 and channels 6. Next the table drops since the air is released through hole 7. In falling, the table strikes the stationary guiding cylinder 8 and this impact packs the moulding sand in the flask. Springs 9, by cushioning the table blows, reduced noise and prevent destruction of the mechanism and the foundation. About 20 to 50 drops are needed to compact the sand, and the average machine operates at about 200 strokes per minute. The drawback of the method is that the density of the sand in the mould is not uniform. It is greatest in the layers next to the pattern plate and lowest near the top of the mould, because in the course of impact, every upper layer acts on the lower layer. Also, there is high level of noise produced by the jolt machines in operation and there is considerable load on the foundation. (c) Jolt Squeeze Machine. This machine is similar in appearance to a jolt machine. In addition, it has a vertical column rising above and behind the table to which is fastened a rigid support that overhangs the table. Also, the base contains two concentric air cylinders ; the smaller one to provide the jolting and the larger one to squeeze the mould against the overhead support. On this machine, the pattern can be attached to the match plate. The jolt squeeze method is free of the basic limitations of the squeeze method, that is, weak compaction near the bottom of the mould and that of the jolting method-weak compaction near the top of the mould. The method combines jolting with squeezing, which gives the mould a high and uniform density and increased strength and enables the production of accurate castings of high quality. Jolt squeeze machine now find most widespread use in the foundary practice. (d) Jolt Squeeze Strip Machine. The jolt squeeze strip or jolt squeeze vibrate machine is similar to the jolt squeeze machine except that it has an air hose attached to the match plate so that the pattern can be vibrated while it is being with drawn from the mould. In this machine, both the cope and drag halves of the mould are made on one machine. First the drag half of the mould is completed and then the cope half is made with the match plate (with its attached air hose) is assembled between the cope and drag. The operator raises and turns over the mould halves by hand. (e) Jolt Squeeze Roll-over Machine. The jolt squeeze roll over draw-type of moulding machine is used when the size of the mould is too large to be turned over by hand. Only a half of the mould is made at one time on this machine. The mould is compacted by jolting on the table in the foreground to which it is clamped. The table is raised and the mould is rolled over the centre column onto the roller table in back of the column. The pattern is stripped from the mould and returned to the jolting table while the completed mould is rolled onto a conveyer that transports it to the assembly and pouring area. Drawing or Stripping the pattern from the mould halves. There are various methods employed to draw or strip the pattern from the mould half, (Fig. 3.6). In Fig. 3.6 (a), the rammed half mould 2 is raised by stripping fins 4 while the pattern 1 with pattern plate remains on the table 5. In Fig. 3.6 (b), the pattern is drawn through a stripping plate. Pattern 1 with pattern plate 3 is lowered while the moulded flask and the stripping plate 4 remain stationary. This method is used for stripping high patterns. In the stripping plate procedure shown in Fig. 3.6(c), the finished half mould with stripping plate 4 are lifted by pins 6. The pattern drawing principle incorporated in a roll over moulding machine is shown in Fig. 3.6 (d). The moulded flask 2 together with pattern 1 and work table 5 is rotated 180° about their approximate centre of gravity and then pins 6 lift table 5 together with pattern 1 out of the mould. In rock-over pattern draw method, Fig. 3.6 (e), the moulded flask 2 together with pattern 1 and table 5 is swung over by the arm onto the drawing table 7 which is then lowered with the mould away from the pattern. (f) Sand Slingers. In these machines, the sand is thrown out by centrifugal force from a rapidly rotating single bladed impeller and directed over the pattern in the flask. This type of compaction results in a mould having a more uniform density throughout, than does the squeezing 84 A Textbook of Production Technology or jolting method. These machines can fill flasks of any size, but are generally used only in the making of large moulds. They can be efficiently employed in both mass and piece production. These machines operate with a high output ; one sand slinger can fill flasks, packing the sand, at a rate of 60 m3 per hour. The disadvantage of the machine is that it does not draw the pattern or handle the mould in any may. 2 2 1 1 3 4 5 5 4 3 (a) (b) 5 2 4 2 2 6 6 3 5 1 2 5 5 3 (c) (d) 3 2 1 3 2 5 7 (e) Fig. 3.6. (a) to (e). Stripping the Pattern. The principle of operation of the impeller head on the sand slinger is shown in Fig. 3.7. The head consists of housing in which blade rotates rapidly. Moulding sand is fed by a belt conveyer to opening in the end face of the housing where it is picked up by blade and thrown in separate portions at a high speed through outlet down into the flask under the head. 3.3. PATTERNS A pattern is an element used for making cavities in the mould, into which molten metal is poured to produce a casting. It is not an exact replica of the casting desired. There are certain essential differences. It is slightly larger than the desired casting, due to the various allowances (shrinkage allowance, machining allowance etc.) and it may Fig. 3.7. Sand Slinger. The casting Process 85 have several projections or bosses called core prints. It may also have extensions to produce runners and gates during the moulding process. 3.3.1. Pattern Materials. The requirements of a good pattern are : 1. Secure the desired shape and size of the casting. 2. Cheap and readily repairable. 3. Simple in design for ease of manufacture. 4. Light in mass and convenient to handle. 5. Have high strength and long life in order to make as many moulds as required. 6. Retain its dimensions and rigidity during the definite service life 7. Its surface should be smooth and wear resistant. 8. Able to withstand rough handling. The common materials used in pattern making include ‘‘wood, metal, plastic and quick setting compounds. Each material has its own advantages, limitations and field of application. Also, the required accuracy, strength and life of a pattern depend on the quantity of castings to be produced. Based on the above factors, we can choose the pattern material as follows: (i) Piece and short run production. Wood (ii) Large scale and mass production. Metal, being more durable than wood, though costlier. (iii) Batch production. Plastics, for example, epoxy resins and also from gypsum and cement. (a) Wood. The wood used for pattern making should be properly dried and seasoned. It should not contain more than 10% moisture to avoid warping and distortion during subsequent drying. It should be straight grained and free from knots. Advantages 1. Light in weight. 2. Comparatively inexpensive. 3. Good workability. 4. Lends itself to gluing and joining. 5. Holds well varnishes and paints. 6. Can be repaired easily. Limitations 1. Inherently non uniform in structure. 2. Posses poor wear and abrasion resistance. 3. Can not withstand rough handling. 4. Absorbs and gives off moisture, so that it varies in volume, warps and thus changes its mechanical properties. These drawbacks, however, can be remedied by drying and seasoning it and then giving coats of water proof varnishes and paints. The following types of wood are commonly used for pattern making: (i) White Pine. It is the most widely used wood, because of its straight grain and light weight and because it is soft, easy to work and unlikely to warp. (ii) Mahogany. It is harder and more durable than white pine. Can be worked easily if straight grained. It is less likely to warp than some of other woods. 86 A Textbook of Production Technology (iii) Maple, Birch and Cherry. It woods are harder and heavier than white pine. They tend to warp in large sections, so should be used for small patterns only. They should be carefully treated, because, they pick up moisture readily. The other common wood materials are : Teak, Shisham, Kail and Deodar. (b) Metal. A metal pattern can be either cast from a master wooden pattern or may be machined by the usual methods of machining. Metal patterns are usually used in machine moulding. Advantages 1. More durable and accurate in size than wooden patterns. 2. Have a smooth surface 3. Do not deform in storage. 4. Are resistant to wear, abrasion, corrosion and swelling. 5. Can withstand rough handling. Limitations 1. Expensive as compared to wood. 2. Not easily repaired. 3. Heavier than wooden patterns. 4. Ferrous patterns can get rusted. The common metals used for pattern making are : (i) C.I. With fine grain can be used as a pattern material. It has low corrosion resistance unless protected. Heavier and difficult to work. However it is cheaper and more durable than other metals. (ii) Brass. May be easily worked and built up by soldering or brazing. It has a smooth, closed pore structure. It is expensive, therefore, generally used for small cast parts. (iii) Aluminium. It is the best pattern material, because it is easily worked, light in weight and is corrosion resistant. It is, however, subject to shrinkage and wear by abrasive action. (iv) White Metal. It has low shrinkage, can be cast easily, has low melting point, is light in weight and may be built up by soldering. However, it is subject to wear by abra- sive action of sand. (c) Plastics. The use of plastics for pattern material results in following advantages : 1. Facilitates the production process. 2. Makes it more economical in cost and labour. 3. Plastic patterns are highly resistant to corrosion, lighter and stronger than wood pat- terns. 4. Moulding sand sticks less to plastics than to wood. 5. No moisture absorption. 6. Smooth surface of patterns. 7. Strong and dimensionally stable. Various plastics make good materials for the production of patterns. These are the compositions based on epoxy, phenol formaldehyde and polyester resins ; polyacrylates, polyethylene, polyvinylchloride, and others. In most wide use are cold-curing plastics based on epoxy resins and acrylates. Plastic patterns are made by one of the following methods : (i) By injecting a plastic material into a die. The casting Process 87 (ii) Utilizing laminated construction by building up successive layers of resin and glass fibre. (iii) By pouring a plastic material into a plaster mould. Laminated plastic patterns with surface as smooth as glass are more durable than wood patterns. They draw more easily, are not prone to wear and scratches in service, and can be repaired quickly when damaged. A laminated fibre glass core box can resist tremendous pressures and rough treatments in foundry practice. Various combinations of apoxys, often with lay-on fibre glass for added strength, are employed. Metal grains are also added to the resin, to provide added strength to those areas likely to be abused by foundry methods. The polyurethanes exhibit outstanding resistance to damage, both from abrasion and indentation. Gypsum patterns are capable of producing castings with intricate details and to very close tolerances. The two main types of gypsum are soft ‘‘plaster of paris’’ and hard metal casting plaster. However, soft plaster does not have the strength of hard plaster. Gypsum can be easily formed, has plasticity and can be easily repaired. Patterns are also made of a combination of materials used for special purposes. To improve wear qualities and strength, metal inserts are often used, as well as resin-impregnated materials. Practically, all aluminium match plates and cope and drag plates are cast in plaster moulds. This results in smooth pattern surfaces and accurate reproduction of fine details in the pattern. Due to this castings will have close dimensional tolerances. Soft wood patterns : upto 50 pieces of medium size castings. Hard wood patterns : for 50 to 200 castings. Metal patterns with hardened steel wear plates : 200 to 5000 castings. Note. Sometimes, if a pattern is first made in wood and then in some other metal by casting, double shrinkages allowances are provided. 3.3.2. Finishing of Patterns. After the patterns are made, they should be finished by sanding so that tool marks and other irregularities are erased. Then they should be applied with 2 to 3 coats of shellac. Shellac fills up the pores and imparts a smooth finish. The finish of the casting depends on the finish of the pattern. If the pattern is to be preserved for a long period and if a colour scheme is to be used, a good quality enamel paint should be selected to spray or brush paint it. 3.3.3. Pattern Allowances. The difference in the dimensions of the casting and the pattern is due to the various allowances considered while designing a pattern for a casting. These allowances are discussed below : 1. Shrinkage Allowance. Since metal shrinks on solidification and contracts further on cooling to room temperature, linear dimensions of patterns are increased in respect of those of the finished casting to be obtained. This is called the ‘‘shrinkage allowance’’. It is given as mm/m. Typical values of shrinkage allowance for various metals are given below: C.I., Malleable iron = 10 mm/m Brass, Cu, Al = 15 mm/m Steel = 20 mm/m Zinc, Lead = 25 mm/m While laying out a pattern, the dimensions are taken from a Pattern maker’s rule, called ‘‘Shrink scale’’, which is longer than a standard scale by the shrinkage value for the appropriate metal. 2. Machining Allowance. Machining allowance or finish allowance indicates how much larger the rough casting should be over the finished casting to allow sufficient material to insure 88 A Textbook of Production Technology that machining will ‘‘clean up’’ the surfaces. This machining allowance is added to all surfaces that are to be machined. The amount of finish allowance depends on the material of the casting, its size, volume of production, method of moulding, configuration of the casting, the position the wall surface occupies in the mould and during pouring. Machining allowance is larger for hand moulding as compared to machine moulding. The largest allowances are taken for the surfaces located in the cope half of the mould, since they are liable to contamination due to slag. Typical machining allowances for sand casting are given in Table 3.2. The allowances are in mm per side. For internal surfaces such as bores, the allowance is about 0.8 mm greater and is negative. Table 3.2. Typical Machining Allowances for Sand Casting Material Cast Over all length of external surfaces, cm 0 to 30 30 to 60 60 to 105 105 to 150 Al alloys 1.6 3.2 3.0 4.8 Brass, Bronze 1.6 3.2 3.0 4.8 C.I. 2.4 3.2 4.8 6.4 C.S. 3.2 4.8 6.0 9.6 3. Pattern draft or Taper. Pattern draft, also termed ‘‘draw’’, is the taper placed on the pattern surfaces that are parallel to the direction in which the pattern is withdrawn from the mould (that is perpendicular to the parting plane), to allow removal of the pattern without damaging the mould cavity, Fig. 3.8 (a). The draft depends upon the method of moulding, the sand mixture used, the design and economic restrictions imposed on the casting. The common draft is 1° to 3° After applying the draft, the largest cross-section of the pattern will be at the parting line for external surfaces and reverse will be for internal surfaces, (Fig. 3.8 (b).) Fig. 3.8. Pattern Taper. The casting Process 89 4. Corners and Fillets. The intersection of surfaces in castings must be smooth and form no sharp angles. For this, the external and internal corners of patterns are suitably rounded. They are called rounded corners and fillets respectively. Fillets facilitate the removal of the pattern from the mould, prevent the formation of cracks and shrink holes in the casting. The radius of a fillet is 1 1 given as  to (arithmetic mean of the thickness of the two walls that form the angle in the 5 3 pattern). 5. Rapping or Shake Allowance. To take the pattern out of the mould cavity it is slightly rapped to detach it from the mould cavity. Due to this, the cavity in the mould increases slightly. So, the pattern is made slightly smaller. 6. Distortion Allowance. This allowance is considered only for castings of irregular shape which are distorted in the process of cooling because of metal shrinkage. 3.3.4 Types of Patterns. Patterns may be classified as temporary and permanent patterns, depending upon the material used for the pattern. Temporary patterns of soft wood are easily made. However, they soon wear, warp or crack and so have a short life. Hardwood patterns are used more than any other type of patterns. The portions that wear may be protected by sheet metal. Permanent patterns are made of metals (usually Al or brass) or plastics that are easily cast and machined. Patterns may be classified from their utility point of view. The following factors affect the choice of a pattern. (i) Number of castings to be produced. (ii) Size and complexity of the shape and size of casting. (iii) Type of moulding method to be used. The common types of patterns are discussed below : 1. Loose Pattern. A loose pattern is simply a replica of the desired casting. It is slightly larger than the casting (due to the allowances discussed above) and it may have several projections called core prints that the resulting casting does not have. Loose patterns get their name because they are not attached or mounted on a plate or frame. These patterns may be made of wood or metal depending upon the volume of production. The gates, runners and risers are added during moulding. This makes mould slow and labour intensive. Due to this, loose patterns are used when the number of castings to be made is small, say, upto 100. Loose patterns are of two types : (a) One piece or solid pattern (b) Split pattern (a) One Piece or Solid Pattern. This is the simplest type of pattern, exactly like the desired casting. For making a mould, the pattern is accommodated either in cope or drag. The moulding process is quite inconvenient and time consuming. So, such patterns are used for producing a few large castings, for example, stuffing box of steam engine. (b) Split or Parted Pattern. These patterns are split along the parting plane (which may be flat or irregular surface) to facilitate the extraction of the pattern out of the mould before the pouring operation. Moulding with a split pattern has already been explained under Art. 3.2.4. Fig. 3.9 (a) shows a split pattern for casting a bush. The two parts of the pattern are joined together with the help of dowel pins. For a more complex casting, the pattern may be split in more than two parts. 2. Loose Piece Pattern. When a one piece solid pattern has projections or backdrafts which lie above or below the parting plane, it is impossible to with draw it from the mould. With such patterns, the projections are made with the help of loose pieces. A loose piece is attached to the main body of the pattern by a pin or with a dovetail slide. While moulding, sand is rammed securely 90 A Textbook of Production Technology around the loose piece. Then the pins are removed. The sand is then packed and rammed around the total pattern. When the main pattern is drawn, the loose pieces remain in the mould. These are then carefully rapped and drawn as shown in Fig. 3.9 (b). One drawback of loose pieces is that their shifting is possible during ramming. Drawbacks. Another technique to make a mould with a one piece solid pattern (with projections) is the use of drawbacks. A drawback is a portion of the mould, which can be drawn back horizontally in order to allow removal of the pattern. It may be rammed around a rigid support called an ‘‘arbor’’ to facilitate moving it, (Fig. 3.9 (c).) 3. Gated Patterns. A gated pattern is simply one or more loose patterns having attached gates and runners, (Fig. 3.9 (d)). Since the gates and runners are not to be cut by hand, gated patterns reduce the moulding time somewhat. Because of their higher cost, these patterns are used for producing small castings in mass production systems and on moulding machines. 4. Match Plate Pattern. A match plate pattern is a split pattern having the cope and drag portions mounted on opposite sides of a plate (usually metallic), called the ‘‘match plate’’ that conforms to the contour of the parting surface. The gates and runners are also mounted on the match plate, (Fig. 3.9 (e)), so that very little hand work is required. This results in higher productivity. This type of pattern is used for a large number of castings. Several patterns can be mounted on one match plate if the size of the casting is small. The patterns need not all be for the same casting. When match plate patterns are used, the moulding is generally done on a moulding machine. Piston rings of I.C. engines are produced by this process. 5. Cope and Drag Pattern. A cope and drag pattern is a split pattern having the cope and drag portions each mounted on separate match plates. These patterns are used when in the production of large castings, the complete moulds are too heavy and unwieldy to be handled by a single worker. The patterns are accurately located on the plates, so that when the two separately made mould halves are assembled together, the mould cavity is properly formed. For a higher rate of production, each half of the pattern is mounted on a separate moulding machine, one operator working on the cope part of the mould and the other on the drag part of the mould. 6. Sweep Patterns. A sweep is a section or board (wooden) of proper contour that is rotated about one edge to shape mould cavities having shapes of rotational symmetry, (Fig. 3.9 (f)). This type of pattern is used when a casting of large size is to be produced in a short time. A complete pattern is not necessary and would be very expensive for a very large casting