A Textbook of Production Technology PDF

Mechanical Working of Metals 311 (b) Comparison of Spinning and Deep Drawing. Metal spinning is both supplementary and competitive with drawing in presses. Due to its low tooling and equipment costs, spinning is normally used...

Mechanical Working of Metals 311 (b) Comparison of Spinning and Deep Drawing. Metal spinning is both supplementary and competitive with drawing in presses. Due to its low tooling and equipment costs, spinning is normally used for low volume production. But drawing is preferred for mass production (due to high cost of dies and presses) to reduce the production cost of the component. Labour costs are higher for manual spinning than for press work. Also, production rates are much less. However, as the size and complexity of the part increase, the spinning process becomes more competitive. (c) Hot Spinning. Hot spinning of metals is used commercially to dish or form thick circular plates to some shape over a revolving form block. A blank of sheet metal is clamped on centre against a form block or chuck, which is revolved on the spindle of a lathe. A rounded stick or roller is pressed against this revolving piece and moved in a series of sweeps. This displaces the metal in several steps to conform to the shape of the chuck. The pressure may be applied by hand or mechanically. A hot spinning machine is a combination of a vertical press and a large vertical spinning lathe which rotates the blank about a vertical axis. Power actuated rollers do the forming, as the part is rotated. Metal upto 150 mm thick is routinely hot spun into dished pressure vessels and tank shapes. Thinner plates of hard to form metals like Titanium are also shaped by hot spinning. 7. Flow turning. This method is used Mandrel to form axisymmetric, conical, cylindrical, Blank t sin  parabolic and hemispherical shapes. Unlike power spinning, where there is no significant change is section thickness of the work  material, in flow turning, (Fig. 4.105), the metal in plastically deformed and progressively displaced under the compressive forces of a pressure roller. The metal flow is entirely by shear. The thickness of the blank gets reduced to the required amount, resulting in an increase in length. However, the Rolls at Start diameter of the blank remains unchanged. The & thickness of the finished part will be given Finish of Operation as : Fig. 4.105. Flow Turning. For conical parts, thickness of finished part = Thickness of blank × sin  If  is less than 30°, the part is finished in more than one stages with an anneal between the stages. Upto 80% reduction in metal thickness can be achieved. The method is also known as ‘‘Shear forming’’, ‘‘Shear spinning’’ or ‘‘Spin forging’’. The method can also be used to reduce the thickness of the tube or to produce tubes of multiples diameters, (Fig. 4.106). Then the method is called as ‘‘Tube spinning’’. Advantages: 1. Very less or no material wastage 2. Economical, due to lesser tooling and set up costs. 3. Improved metallurgical properties like hardness and tensile strength. 4. Ability to produce heavier flanges or end sections. 5. Possibility of seamless construction. 6. Flexible to adopt to any design changes. Product Applications. Television tube cones, airframes, aircraft engine parts, Hotel utensils, cooking pots, pressure cookers, pans, Lamp shades, reflectors, dishes, missile noses, cream separator bowls, milk separator components, Monobloc Aluminium Alloy milk cans, thin walled seamless 312 A Textbook of Production Technology tubing, pesticide bottles, air craft engine parts, pressure vessel components, V-pulleys and brake drums etc. Blank Feed Roller Mandrel (a) Chuck Finished Workpiece Face Plate Feed Roller Mandrel (b) Finished Workpiece Fig. 4.106. Tube Spinning. 8. Coining and Embossing. Both coining and embossing are cold press working operations in which the starting material is in the form of a blank of sheet metal. The aim of both the operations is to force impressions into the surface or surfaces of the metal. However, whereas coining is a pressing operation, embossing is a forming operation. Upper Die Upper Die Blank Blank Lower die Lower Die Section of Section of embossed piece Coined Piece Fig. 4.107. Coining Fig. 4.108. Embossing. Mechanical Working of Metals 313 (a) Coining. Coining is a special type of closed die forging operation is which the lateral surfaces are restrained (metal is completely confined within a set of dies) resulting in a variable thickness and a well defined imprint of the die faces on the metal (Fig. 4.107). Very large pressures are exerted (1200 to 3000 MPa) to cause the metal to flow to all portions of the die cavity. The metal is caused to flow in directions perpendicular to the compressive force along the die surfaces. Usually the two sides of any coined article bear totally different designs. The depth of impression is never very great, rarely exceeding 0.8 mm due to the difficulty of forcing the metal to flow. Only the metal at or near the surface is deformed. Hard currency is probably the best known product of coining operation. Other applications of coining are the manufacture of insignia, medals, badges, piece of art and household hardware. (b) Embossing. Embossing is a forming or drawing operation for producing a raised or projected design in relief on the surface of the workpiece. The operation uses matching punch and die with the impressions machined into both surfaces, (Fig. 4.108). The process differs from coining process in that the material thickness remains constant. It is actually a combination of shallow drawing and stretching process rather than a squeezing process. The forces needed are much less than in the coining process. The metal flow is in the direction of the applied force. The entire metal thickness is affected. The product applications of the process are : to obtain various rigidity ribs, functional and ornamental recesses and projections and to manufacture name plates, medallions, identification tags and aesthetic designs on thin sheet metal. 4.9. HOBBING, RIVETING AND STAKING (i) Hobbing. Hobbing or hubbing is the process of forming a very smooth, accurately shaped die cavity by pressing a hardened and polished shaped punch (hob) (of tool steel) into a softer metal die block (of mild steel) (Fig. 4.109 (a)). The die cavity is formed slowly and carefully and some times in several stages with annealing between stages. A retaining ring helps to keep the die/ mould from spreading out of shape. The method is used for making dies/moulds for the plastic and die casting industries. The main advantage of the process is that many duplicate cavities can be made with a single hob/punch. (ii) Riveting. To join two parts permanently by riveting, a rivet is put through the drilled or punched hole in the parts and is placed on the anvil. A riveting punch with a hollowed end mashes the stem of the rivet which is headed by a single squeezing action, (Fig. 4.109 (b)). (iii) Staking. Staking is again a method of fastening two parts permanently. It is a substitute to drilling/punching and riveting. In this method, a shaped punch is forced into the top of a projection of one of the parts to be fastened. This indentation action of the punch deforms the metal sufficiently to squeeze it tightly against the second component so that they are firmly locked together, Fig. 4.109 (c). 4.10. SHOT PEENING Shot peening is mainly employed to increase the fatigue strength of work pieces subjected to impact and/or fatigue loads (parts made of steel and non-ferrous alloys), and also for strengthening welds. Typical applications include:- Coil springs, leaf springs, gear wheels and other complex parts. The other functions of shot peening are to prevent the cracking of work pieces in corrosive media and to improve the oil retaining properties of the processed surfaces. The process is based on plastic deformation of the surface layer and consists in subjecting the surface to impacts of a jet of shots. Many overlapping indentations are made, causing localized compressive deformation of the surface. Since bulk of the material is not affected, compressive residual stresses are set up. Since fatigue failure occurs due to tensile stresses, the compressive residual stresses greatly offset any tendency to fatigue failure. The surface also gets slightly hardened and strengthened by shot peening (a cold working process). The shots are made of cast iron, steel, aluminium or glass. Cast 314 A Textbook of Production Technology iron or steel shot is used in peening steel work pieces, and aluminium or glass shot for non ferrous alloys. The depth of the workhardened layer obtained does not usually exceed 2 mm. This depth increases with the diameter of shot (0.4 to 2 mm) and its velocity (60 to 100 m/s), and decreases with an increase in the initial hardness of the workpiece. The efficiency of the process also depends upon the angle between the path of the shot and the surface being peened, and the duration of peening, which is not more than 10 min. Ram Punch Hob Stem Retaining Head Ring Parts to be Mould/Die Riveted (a) Anvil (b) Staking Punch WORK PIECE SHAFT (c) Fig. 4.109. Hobbing, Riveting, Staking. Shot peening is performed in special equipment consisting of a workpiece chamber and a shot blasting device. The most widely used shot blasting devices are : 1. Air - nozzle 2. Centrifugal - wheel type. In the air-nozzle device, the shot is thrown by compressed air at a pressure of 6 to 8 atm. from several nozzles simultaneously. The centrifugal wheel device (airless blast equipment) consists of a work piece chamber with a mechanism for moving the work piece under the jet of shot and a centrifugal wheel. The latter is a rapidly rotating bladed wheel which throws the shot centrifugally in the required direction. This process of shot peening is done on the following types of products :- torsion bars, the tension side of automobile springs, other types of leaf springs, all sorts of shafts and axles (sometimes only in key ways), splines and fillets, gear teeth roots after carburizing and case hardening, oil well drill pipe (inside or outside, or both), and automobile connecting rods and crankshafts etc. Characteristics of the shot peened surfaces are given below : Increase in hardness = 20 to 50% over the original Residual compressive stresses set up in the surface layer = 500 to 800 N/mm2.  Table 4.3 Metal Forming Process—Design Considerations S.No. Process Choice of Complexity of Maximum Minimum Mechanical Materials part size size Properties 1. Drop Forging Medium-many alloy Moderate Large Small even fraction of High are forgeable a kg. 2. Press Forgoing Medium-Best for Limited, but better Moderate : 0.012 to Smaller than drop High non-ferrous alloys than drop forging 0.014 t forging Mechanical Working of Metals 3. Upset Forging Medium Limited to cylindrical Medium 250 Moderate High shapes mm bar about largest 4. Cold Headed Parts. Narrow-Steel and Limite-less than Small-12.5 cm by 1.25 Small : 3 mm dia parts High hightly ductile alloys forgings cm diameter usual maximum 5. Extrusion Restricted-Light Limited-can be Medium 20 to 25 cm Small-1.25mm Good metals, some steels, Complex in dia maximum sections in Al and Mg Cu and Til. Cross-section only 6. Impact Cold Extrusion Narrow-Al, Tin, Mg, Limited-must be Small-15 cm dia in Moderate : 18 mm dia High Lead concentric soft alloys, 10 cm in smallest hard. 7. Roll Forming Narrow-Cold rolled Restricted to thin Large Sections from 3 mm up Good steel; Some Al and sections and uniform stainless steel cross-section 8. Press Wroking Wide-Include all Limited-many design Large Small sections as thin Fair to High workable metals restrictions as 0.075 mm possible 9. Metal Spinning Wide-Many sheet Limited-cylindrical or Large : Upto several m Moderate-6 mm in dia Good metals can be spun concentric shapes is dia in gauges less than 1 mm 315  Table 4.4 Metal Forming Process-Design Considerations (Cont.) 316 S.No. Process Precision and Special Surface Surface Getting into Rate of Remarks Tolerance Structural smoothness Details production output properties 1. Drop Forging  0.25 to 0.75 Grain flow Fair Fair Slow-dies Medium-120 Used for high mm provides require much per hour on strength toughness work small parts 2. Press Forging Medium-Better Same Same Same Same Medium-Slower Greater than drop than drop complexity than forging forging drop forging 3. Upset forging Medium-compara Toughness Medium Medium Slow Medium Best Suited to ble to press Small parts forging 4. Cold Headed  0.25 mm Toughness High Fair Fair-dies Extremely high One of fastest parts common æ 0.05 relatively simple Process mm possible 5. Extrusion  0.125 to 0.5 Grain flow Good Only as part of Moderate-dies High Sometimes used mm improves Contour relatively simple as blanks for properties other processes 6. Cold Impact  0.25 mm Same Good Good Same High-Upto Can eliminate Extrusion 2000/h machining 7. Roll Forming  0.05 to 0.4 mm Cold work Good None Slow-roll takes High Requires large improves long to be made quantities properties 8. Press Working  0.25 mm None High Fair Slow-dies High-Upto Low cost for common require several Several thousand mass production weeks 9. Metal Spinning  0.40 to 1.5 mm Grain flow and Good None Fast-forms can Slow 12 to 30 Except for large Cold work be made quickly per h. piece best in improves short runs properties A Textbook of Production Technology  Table 4.5. Metal Forming Process-Cost Consideration S.No. Process Raw Material Costs Tool and die costs Direct Labour Finishing Costs Scrap loss Optimum lot size Cost Medium 1. Drop Forging Low to Moderate High Medium Medium Moderate Large, > 1000 2. Press Forgoing Low High, usually less Medium-less than Medium Moderate, usually Medium to High Moderate-equal to than drop forging drop forging less than drop drop forging forging 3. Upset Forging Low to Moderate High Medium less than Medium-less than Medium-lowest of Same other forging other forging Forging processes Mechanical Working of Metals process process 4. Cold Headed Parts Low to Medium Low-almost Low Low-practically nil Large, not suited to Moderate-chiefly completely small quantity Steel wire automatic 5. Extrusion Moderate Moderate Moderate Low Low Moderate primarily non-ferrous metals, some alloy steels 6. Impact Extrusion Moderate-Primarily Medium Low-Little skilled Low-often nil Low-most scrap in Wide, from Tin, Lead labour needed blank scrap hundreds to Aluminium thousands. 7. Roll Forming Low to High Moderate Low-cutting done Low High Moderate-mostly low carbon steel sheet 8. Press Forming Low to High Medium Low-cleaning and Low to Moderate Large, > 10,000 Moderatre-ranging from carbon steel to stainless stell 9. Metal Spinning Low to Moderate High-skilled Low Low-only cleaning Low-practically no Low operators needed and trimming scrap 317 318 A Textbook of Production Technology Increase in Fatigue life : Gear wheels = 2.5 times Coil springs = 1.5 to 2 times Leaf springs = 10 to 12 times Roughness of shot peened surface = 3.2 to 0.8 μm Ra. Finish of originally rough-machined surfaces becomes finer and that of finish machined surfaces coarser. Another peening method is with pneumatic hammer. It is shaped like a spherical striker, its action results indents on the surface worked. The process is used for cold working of stressed areas in large parts prior to finish machining. 4.11. CHARACTERISTICS OF METAL FORMING PROCESSES The characteristics of metal forming processes are given in tables 4.3 to 4.5. The greatest extent of automation is typical of 3rd, 4th and 7th methods and to a lesser extent of 1st and 2nd methods. In the die-forging of small parts on forging hammers and presses, upto 1000 parts can be obtained per hour. Open die- forging gives the lowest production rates. The greatest metal utilization factor (0.9) is typical of 3rd method and especially of the 4th method, where this factor is unity. The shortest processing cycle (without cleaning and heat treatment) is provided by the 3rd and 4th methods. PROBLEMS 1. Define the process of Mechanical working of metals. 2. What is the starting material for mechanical working of metals ? 3. Enumerate the advantages of mechanical working of metals over other manufacturing processes. 4. Differentiate between hot working and cold working of metals. Bring out the advantages and disadvantages of each of these techniques. 5. Discuss ‘‘worm working’’ of metals. 6. Define the Rolling process. 7. What is the difference between a bloom and a billet ? 8. What is the difference between plate, sheet, strip and foil ? 9. What is the difference between wire and rod ? 10. Define ‘‘Slab’’. 11. Why is metal cold worked ? 12. Explain what happens to a metal, when it is cold worked. 13. Why a cold worked metal is annealed ? 14. What is angle of bite in rolling ? On what factors does its value depend? 15. Describe and specify the merits and limitations of the different kinds of rolling mills. 16. Sketch and explain the working of ‘‘universal rolling millª and ‘‘Planetary rolling mill’’. 17. What is ‘‘Steckel rolling’’ ? 18. Why are a number of passes required to roll a steel bar ? 19. Describe what occurs in metal when it is rolled. 20. How is cold rolling done ? What benefits are obtained from cold rolling metal ? 21. Why for cold rolling, a four high rolling mill is usually used ? 22. Define forging process. Mechanical Working of Metals 319 23. Give the advantages and drawbacks of forging process. 24. Describe the common types of forging hammers. 25. How the size of a forging hammer is specified ? 26. How are a forging hammer and drop hammer alike and how do they differ ? 27. What is the difference between hammer forging and drop forging ? 28. What is Press forging ? How does it differ from drop forging ? 29. What is upset forging and how it is done ? 30. Discuss the rules which govern the flow of metals in upset forging process. 31. How are the sizes of presses and upset forging machines specified ? 32. What is roll forging and how it is done ? 33. What is rotary forging or swaging and how it is done ? 34. With the help of a suitable sketch, explain the working of ‘‘Board drop hammer’’. 35. Describe the common defects in forgings and write about the causes of each. 36. Describe the various methods for cleaning the forgings. 37. Describe the various methods for heat treatment of steel forgings. 38. What is the purpose of heat treatment of forgings ? 39. Define ‘‘Forgeability’’. On what factors does it depend ? 40. List the advantages and limitations of liquid forging. 41. List the steps of operations in liquid forging process. 42. Write a short note on isothermal forging. 43. Write a short note on Powder metal forging. 44. What is ‘‘No-draft forging’’ ? 45. Write a short note on ‘‘Orbital forging’’. 46. Write a short note on ‘‘Hot isostatic pressing’’. 47. Write short notes on : Incremental forging, Gatorizing and Super-plastic forging. 48. Define extrusion process. 49. Compare extrusion and rolling processes, 50. Describe the common ways of extruding metals. 51. Compare direct extrusion and indirect extrusion. 52. What is Tubular extrusion ? 53. Define ‘‘extrusion ratio’’ and give its common values for various metals. 54. What is impact extrusion ? Give its product applications. 55. What is ‘‘The Hooker method’’ of cold extrusion ? 56. Describe cold hydrostatic extrusion and give its advantages. 57. List the advantages and disadvantages of extrusion process. 58. Compare open and closed die forging. 59. How is seamless tubing pierced ? 60. What are the final operations in producing finished steel tubes ? 61. Describe the three ways of making butt welded pipe. 62. Give the product applications of extruded pipe, pipe made by piercing technique and the welded pipes. 320 A Textbook of Production Technology 63. Describe the operation of hot drawing and cupping of metals. 64. How the oxygen gas cylinders are made by this method ? 65. Describe wire drawing and rod drawing. 66. Define ‘‘Degree of drawing’’. What are its normal values ? 67. Describe Tube drawing. 68. How the hypodermic tubes are produced ? 69. How the metal is prepared for drawing operation ? 70. What is the material of the dies used for drawing operation ? 71. Give the advantages of Sheet metal working processes. 72. What is elastic recovery or spring back ? 73. What are the basic units of a press and what does each do ? 74. How do hydraulic presses compare with mechanical presses ? 75. What are knuckle joint and toggle process and what is the purpose of each ? 76. What are single action, double action and triple action presses ? 77. Compare open and closed frame presses. 78. Sketch a Blanking die and label it properly. Write the function of each part. 79. With the help of a sketch, describe the working of a Cutting Die. 80. Describe a progressive, a combination and a compound die. 81. Describe what happens when sheet metal is sheared ? 82. Write the formula for calculating the cutting force in shearing operation. 83. How can the cutting force be reduced ? 84. Define ‘‘deep drawing process’’. 85. With the help of a neat diagram, describe a ‘‘deep drawing process’’. Give its product applications. 86. What is LDR ? What is its usual range ? 87. What is ‘‘redrawing’’ ? 88. Explain the ‘‘Bending terminology’’ with the help of a suitable sketch. 89. What are the various methods of bending ? Sketch and describe each. 90. What is ‘‘Press brake’’ ? 91. Describe ‘‘Roll forming’’. 92. Describe : ‘‘Rubber press forming’’, ‘‘Rubber hydro forming’’, and ‘‘Hydromechanical forming’’. 93. Describe ‘‘Stretch forming’’. 94. Describe ‘‘Metal spinning’’. Write its product applications. Differentiate between cold and hot metal spinning. 95. What is ‘‘Power spinning’’ ? 96. What is ‘‘Flow turning’’ ? 97. Differentiate between ‘‘coining’’ and ‘‘embossing’’. 98. Compare ‘‘metal spinning’’ with ‘‘deep drawing’’. 99. What is the difference between ‘‘riveting’’ and ‘‘staking’’ ? 100. What does ‘‘hubbing’’ do ? 101. Describe ‘‘shot peening’’. Mechanical Working of Metals 321 102. What is ‘‘cold heading’’ ? What are its uses and advantages ? 103. Name two metals which are hot worked at room temperature. 104. Name six metals or alloys which may be cold worked at rom temperature 105. What are the effects of forging on porosity and segre gation ? 106. What is the effect of forging on grain structure and why is this benefitial to the properties of the metal ? 107. What is the relationship between carbon cantent of a steel and its maximum allowable forging temperature ? 108. What will be the consequences if a steel is forged : (a) at too high a temperature ? (b) at too low a temperature ? 109. Explain the difference between cold working and hot working of metals. (UPSC, DU) 110. How annealing is related to cold working ? 111. What is the effect of hot working on the structure and mechanical properties of metals? 112. Estimate the blanking force to cut a blank of 25 mm wideand 30 mm long from a 1.5 mm thick metal strip, if the ultimate shear stress of the material is 450 N/mm2. Also determine the work done if the percentage penetration is 25% of material thickness. (PTU; Ans.: 74.25kN ; 27.84375 N-m). Hint : — F = P.t.s (eq. 4.1 & 4.2); work done = F × Punch travel Here Punch travel = Percentage penetration = 0.25 × t Lubrication: 1. Rolling : – Hot rolling of ferrous metals is normally done without a lubricant. If need be, graphite can be used. Water-based solutions are used to cool the rolls and to break the scale on the rolled material. For hot rolling of non-ferrous metals, these is a wide choice; a wide variety of compounded oils, emulsions and fatty acids. For cold rolling, the common lubricants are : – water-soluble oils, low-viscosity lubricants, such as mineral oils, emulsions, paraffins and fatty acids. 2. Forging :– Lubricants gretly influence : friction, wear, deforming forces and flow of material in die-cavities. They act as thermal between how workpiece and the relatively cool dies, thereby, slowing the rate of cooling of the workpiece and improving metal flow. Alsol they serve as parting agents which inhibit the forging from sticking to the dies and so help in its release from the dies. For hot forging, the common lubricants are : graphite, MOS2 and sometimes molten glass. For cold forging, these are : mineral oil and soaps. In hot forging, the lubricant is applied to the dies, but in cold forging, it is applied to the workpiece. 3. Extrusion :– For hot extrusino, glass is an excellent lubricant with steels, stainless steels and high temperature metals and alloys. For cold extrusion, lubrication is critical, especially with steels, because of the possibility of sticking (siezure) between the workpiece and the tooling if the lubrication breaks down. Most effective lubricant is : application of a phosphate conversion coating on the workpiece followed by a coating. 113. Explain the difference between elastic and plastic deformation. 114. Which equipment is used for sheet- metal working ?  Chapter 4 Mechanical Working of Metals 4.1. GENERAL Mechanical working processes are based on permanent changes in the shape of a body, that is, on the plastic deformation under the action of external forces. Mechanical working processes include : rolling, forging, extrusion, drawing and press working (sheet metal working). The stresses induced in the part (due to the application of external forces) are greater than the yield strength and less than the fracture strength of the material, except in shearing, piercing and blanking sheet metal working processes, where the stresses induced in the part are equal to or greater than the fracture strength of the material. Mechanical working processes exclude the machining (metal cutting) processes, where, even though external forces are used and the stresses induced in the part are equal to or greater than the fracture strength of the material, the part is obtained by removing the material in the form of chips. In mechanical working processes, no chips are produced. In shearing, piercing and blanking sheet metal working processes, even though the part is obtained by cutting it from the sheet metal, no chips are produced. That is why, these processes are also called ‘chipless manufacturing processes’. In all other mechanical working processes, the size or shape of a part is obtained by deforming and displacing the material under the action of large external forces. Hence these processes are also called as metal working or ‘metal forming processes’. Starting materials for the mechanical working processes are: cast ingots and billets of various cross-sections and weights, cast preforms, powdered-metal bars, bar stock, powdered-metal preforms or blanks (for press working processes). Advantages of Mechanical Working Processes Over Other Manufacturing Processes A high-rate of the output of metals and alloys is one of the mainstay of the present stage of the scientific and technical revolution, and despite the appearance of a great number of new non- metallic materials, mechanical working of metals is certainly to develop at an ever increasing rate in the decades to come. 90% of all metal produced are worked to day by rolling, forging, extrusion, drawing etc. Metal working processes possess certain special advantages as compared to other manufacturing processes. A high effectiveness of mechanical working of metals is due to the following advantages : 1. Higher productivity, as compared to other manufacturing processes. Modern rolling and forging units fabricate hundreds of tonnes of end products per working shift. Hence these processes are high production rate processes and are most suitable for mass production. 212 Mechanical Working of Metals 213 2. The possibility of fabrication items with preset specific physical and mechanical properties and required structure, which, in the final analysis, ensures high-standard quality of fabricated products, one which can not be equalled by any other alternative technique. This is due to the fact that the fibre lines in the material are not interrupted, but are made to follow the (a) Casting contour of the formed part (The strength of a component is maximum parallel to the direction of the fibre lines, and minimum perpendicular to them). In case of casting, the fibre lines get destroyed on melting of the material. A cast component will have the typical cast dendritic structure resulting in very low mechanical properties. Also, the ‘yield’ of casting processes varies between 60 to 70%. A machine component has the draw-backs that during manufacture, the fibre lines are also cut and get interrupted resulting in poor mechanical properties of the component. (b) Machining Also, lot of metal is removed to get the final shape of the component, which goes waste in the form of chips. Again, cut fibres are exposed to atmosphere resulting in corrosion of the machined component. This is best explained by the example of manufacture of a crank shaft. The casting, (Fig. 4.1 (a)) has no grain flow and so has the poorest mechanical properties. In (Fig. 4.1 (b)), the crank shaft has been produced by machining from a bar stock and the fibre of the metal gets cut and interrupted and for this reason, the mechanical (c) Forging properties of this shaft will be poorer than those of the crank shaft made by forging, (Fig. 4.1 (c)), where the fibre Fig. 4.1. Comparison of Grain Flow. of the metal has not been interrupted and continues along the entire length of shaft. 3. It is clear from the above discussion that the formed components will have higher strength, and better corrosion and wear resistance, as compared to casting and machining processes. Also, formed components have high strength-to-weight ratio. 4. Mechanical working of metals involves minimum waste of metal and, therefore, is used on an ever wider scale in various engineering fields, supplanting machining and other methods that involve loss of metal to chips. etc. 5. High dimensional accuracy and surface finish of the products. 6. By controlling the end forming temperature and the degree of deformation, it is possible to impart any strength to the component within the permissible range. This is a unique advantage of metal working processes as compared to other processes. 7. Many an item can not simply be fabricated by any other alternative means. Extra-thin foil, wire, sheet steel and other products, which are indispensable in modern civilization, originated with the advent and development of mechanical working. 8. During mechanical working of metal, the grains of the material get elongated in the direction of metal flow. so, they would be able to offere more resistance to stresses across them. As 214 A Textbook of Production Technology a result, the mechanically, worked metals called wrought products would be able to achieve better mechanical strength in specific orientation, that of the flow direction. However, metal working processes become impractical when the component is very large (requiring very large forming forces and high tonnage machines), or geometrically complex or the material is not suitable for forming operation. Classification of Metal - working Processes. There are many ways of classifying metal working processes : (i) Working Temperature. Depending upon the temperature at which a material is mechanically worked or formed, the metal working process can be classified as : Cold forming, Hot forming and Warm or Semi - hot forming. Uniform Cold forming or cold working Plastic Post Necking can be defined as the plastic Deformation Deformation deforming of metals and alloys Maximum under conditions of temperature and B Load Point strain rate, such that the work- Yield hardening or strain-hardening is not Point Cold Fracture relieved. Theoretically, the working Working C Load temperature for cold working is below the recrystallization A Hot D temperature of the metal/alloy (which n Working Def lastic atio is about one-half the absolute orm melting temperature). However, in E practice cold working is carried out at room temperature or at temperatures less than 0.3 × melting Deformation point of the metal. Fig. 4.2. Load Deformation Curve. Hot working or hot forming can be defined as the plastic deformation of metals and alloys under conditions of temperature and strain rate, such that recovery and recrystallization takes place simultaneously with the deformation. The hot working is carried out above the recrystallization temperature of the material (typically 0.7 to 0.9 times the melting point temperature), and after hot working, a fine refined grained recrystallized structure is obtained. The recrystallisation temperature is discussed in some detail as below : Recrystallisation Temperature: Recrystallisation temperature is very important from the point of view of mechanical working of metals and also regarding process annealing operation. When a metal is heated and deformed under mechanical force, an energy level will be reached when the old gain structure (which is coarse due to previous cold working) starts disintegrating. Simultaneously, an entirely new grain structure (equi-axed, stress free) with reduced grain size starts forming. This phenomenon is known as ‘‘recrystallisation’’ and the temperature at which this phenomenon starts is called ‘‘Recrystallisation temperature’’. It takes some time for this phenomenon to get completed. According to ASM (American Society of Metals), the recrystallisation temperature is defined as ‘‘the approximate minimum temperature at which the complete recrystallisation of a cold worked metal occurs within a specified period of approximately one hour’’. Recrystallisation decreases the strength and raises the ductility of the metal. Factors affecting Recrystallisation Temperature: There are many factors that influence the recrystallisation temperature. These factors are discussed below: Mechanical Working of Metals 215 1. Type of Metal : It is lower for pure metals as compared to alloys. For pure metals, Tcr = 0.3 Tm and For alloys, Tcr  0.5 Tm Where Tm = Melting point of the metal ⎛1 1⎞ In practice, Tcr is taken as equal to ⎜ to ⎟  Tm ⎝3 2⎠ Recrystallisation temperature of lead and Tin is below room temperature and that for Cadmium and Zinc it is room temperature. 2. Extent of prior cold work : Higher the prior cold work, lower will be the recrystallisation temperature. 3. It is a function of time : Recrystallisation temperature is a function of time, because the process involves diffusion, that is, movement and exchange of atoms across grain boundaries. For a constant amount of prior deformation by cold working, the time required for recrystallisation decreases with increasing temperature. 4. Higher the amount of deformation, smaller the grain size becomes during recrystallization. 5. Smaller grain size before cold working, decreases recrystallisation temperature. 6. Increasing the rate of deformation decreases the recrystallization temperature. 7. Presence of second phase particles decreases recrystallisation temperature. The characteristics of cold working and hot working of metals can be understood from the load deformation curve, (Fig. 4.2). To deform the metal plastically for a permanent change in shape of the work-material, the applied load/stress has to be equal to (atleast) or greater than the yield point of the work material (the elastic deformation being completely recoverable, once the load is taken off). The curve OABC represents the cold working, whereas the curve OAD represents the hot working process. The portion AB represents the uniform plastic deformation region, that is the work hardening or strain hardening region (which means that increasing loads will have to the applied to get increasing deformations). At point ‘B’ (maximum load point) local neeking of the material starts and the portion BC represents the post necking deformation, ultimately resulting in fracture at point C. It is clear that in cold working, the force increases with increased deformation, whereas, in hot working, once the forces equals the yield point load, the deformation proceeds at almost constant value of force. (a) Characteristics of Cold - working. It is clear from the Fig. 4.2, that due to work hardening, the strength of the metal increases but its ability to deform further (formability or ductility) decreases Tensile Strength Strength and Hardness Property Property Flow Stress Ductility Ductility % Cold Work Annealing Temp (a) (b) Fig. 4.3. Effects of Cold Working and Annealing. 216 A Textbook of Production Technology with cold working (See Fig. 4.3 (a)). So if the material is excessively deformed, it may fracture before it is formed. To avoid this, large deformations in cold working are obtained in several stages, with intermediate annealing. This will soften the cold worked material and restore its ductility and hence the formability, (Fig. 4.3 (b)). Advantages of Cold-working : 1. Since cold working is done at room temperature or low temperatures, no oxidation and scaling of the work material occurs. This results in reduced material loss. 2. Surface defects are removed. 3. Excellent surface finish which reduces or completely eliminates subsequent machining resulting in enormous saving in material. 4. High dimensional accuracy. 5. Highly suitable for mass production and automation, because of low working temperatures. 6. Thin gauge sheets can be produced by cold working. 7. Heavy work hardening occurs and so the inherent strength of the material is permanently increased. This makes it possible to use inferior materials whose properties are enhanced by work hardening and preferential flow. 8. The physical properties of metals that do not respond to heat treatment can be improved by cold working. Draw backs of Cold-working : 1. At low temperatures, the strength of a metal is very high. So, large forces are needed for deformation. For this, high capacity equipment is required which is costly. 2. The ductility / formability of metals is low at low temperatures. Hence for large deformation cold working requires several stages with interstage annealing, which increases the cost of production. 3. Also, due to limited ductility at room temperature, the complexity of shapes that can be readily produced is limited. 4. Since low reductions are required to attain the required parameters, more sensitive controls are needed in cold working than in hot working. 5. Due to very high forces, tool pressures and power requirements are high too. So, the tooling must be specially designed, which increases the tool cost. 6. Severe stresses are set up in the metal during cold working. This requires stress relieving, which again increases the cost. 7. Due to the above factors, normally, only the ductile metals are cold worked. Materials for Cold-working. In principle, any material can be cold-worked. In practice, however, the choice is limited by the following two factors :- 1. The ability of the tool material to withstand the required pressures for cold -working of a material. Obviously, the tool material must have a mechanical strength greater than that of the material to be cold - worked. Also, from the point of view of economy, the tool (die etc.) must have a reasonable working life, that is, it must be able to withstand the developed working stresses for a reasonable length of time. 2. The economic requirement that the maximum possible deformation of the material should be obtained in a single working operation (single). This will depend upon the cold ductility and cold flowability of the material. Thus, the two principal limitations to cold - working of a material are : the permissible stress placed on the tool material and the ductility of the material to be cold-worked. Mechanical Working of Metals 217 Both cold ductility and cold flowability of a material depend closely on its chemical composition. As for steel, with an increase in the percentage of carbon or alloying constituents, its deformability decreases and the resistance to deformation increases. The maximum limit is usually 0.45% carbon for steels used in cold extrusion and 1.6% carbon for other cold forging operations. Impurities such as S, P, O2 and N2, also impair the cold workability of the steel. For cold working, the micro-structure of the material, also plays an important role. Soft annealing known as spheroidize annealing of steel before cold working, improves its cold workability. The grain size is also an important factor. Large grain is easier to cold work, while the parts made from fine grained material are stronger. A good guiding rule for forging steels used to produce forgings is that the stress on the die should not exceed 2500 N/mm2 and the material must allow at least a 25% deformation in a single step. As a general rule, the requirements for a material to be cold worked are : 1. Yield stress curve of gentle slope. 2. Early yield point. 3. Then great elongation with pronounced necking before fracture. The materials commonly used for cold working include : low and medium carbon steel (0.25 to 0.45% C), low alloy steels, copper and light alloys such as Aluminium, Magnesium, Titanium, and Berrylium. Cold forming is most suitable for axisymmetric components such as shaft components, flanged components, finished gears and bearing races etc. Characteristics of Hot-working. It is clear from Fig. 4.2 that hot working of the metal occurs at an essentially constant load/flow stress, that is, there is no work-hardening of the metal. Advantages of Hot-working : 1. The strength of the metals is low at high temperatures. Hence low tonnage equipments are adequate for hot working. 2. Very large workpieces can be deformed with equipment of reasonable size. 3. Because of high ductiliy at high temperatures and absence of work hardening, large deformations can be undertaken in a single stage, and, complex parts can be fabricated. 4. Interstage annealing and stress relieving are not required. 5. Blow holes and porosities are eliminated by welding action at high temperatures and pressure. 6. Grain size can be controlled to be minimum (Fig. 4.4). This makes the metal tougher. Original large Equi-Axed ROLL Grains Elongated Grains New Grains Grow Completely Recrystallised Small Equi-Axed Grains Nuclei of ROLL New Grains Fig. 4.4. Micro-Structural Changes in a Hot Working Process (Rolling.) 218 A Textbook of Production Technology 7. Inclusions within the metal are broken up and elongated into fibres or threads with definite orientation. This again makes the metal tougher. 8. Segregation may be reduced or eliminated, since hot working promotes diffusion of constituents. Typical hot working temperatures are :- Steels : 1100 to 1260°C Cu and its alloys : 760 to 925°C Magnesium : 315°C Aluminium and its alloys : 370 to 455°C Drawbacks of Hot-working : 1. Due to oxidation and scaling, there is heavy material loss. This also results in poor dimensional accuracy and surface finish. 2. Automation is difficult due to high working temperatures. 3. Thin parts (sheets, wires etc.) can not be produced due to loss of ductility because of high rate of loss of heat. 4. High energy costs to heat the metals to high temperature. 5. Surface decarbonisation in steels, reduces strength and hardness on the surface. 6. Due to high working temperatures, these is a serious problem of surface reactions between the metal and the furnace atmosphere, more so in the case of reactive metals like Ti, calling for inert atmosphere. Note 1. It is clear from the above discussion that hot working is mainly employed to produce large deformations in the material. The final dimensional tolerances accuracy and surface finish are obtained by cold working involving only a small deformation in the material. 2. It should be noted that the difference between cold working and hot working depends only upon the temperature of recrystallization and not on any arbitrary temperature of deformation. Lead, tin and zinc recrystallize rapidly at room temperature (Below 27°C) after large deformations. Hence, the working of these metals at room temperature will constitute their hot working. Similarly working of tungsten at about will be termed its cold working, because it has recrystallization temperature above this value. However, in every day use, cold working means working at room temperature and hot working means working of a preheated material above its recrystallization temperature. Warm working or Semi-hot working. It can be defined as plastic deforming of a metal or alloy under conditions of temperature and strain rate, such that the drawbacks of both cold working and hot working are eliminated and their advantages are combined together. For this, the selection of proper temperature for warm working is very important. This depends upon the following factors : 1. Yield or flow strength of the metal or alloy. 2. Ductility of the material. 3. Dimensional tolerance on the components. 4. Oxidation and scaling losses. The variations of the above properties relative to the working temperature can be studied to arrive at the proper working temperature for warm working. For example, for 0.13% C steel ; the following observations are made : Yield-Strength. In general, yield strength decreases with increase in temperature. However, in the temperature range of 150°C to 350°C and 800°C to 900°C, it increases with, increases in temperature. The first temperature range is called blue brittleness range of steel and in the second Mechanical Working of Metals 219 range, structural changes occur in steel. Both these ranges are brittle ranges and if steel is worked in these temperature ranges, it will fracture. So, the best temperature ranges from the yield strength point of view are 400°C to 750°C and above 900°C. Ductility. In general, the ductility or formability of a material increases with increase in temperature. However, in temperature ranges of 250°C to 350°C and 800°C to 900°C, it decreases with increase in temperature. Thus, from the ductility point of view, the best temperature ranges for the above mentioned steel are : 400°C to 750°C and above 900°. Dimensional tolerance and Scaling and Oxidation Losses. The dimensional tolerances increase rapidly above 700°C. Similarly, scaling and oxidation losses, which are negligible upto 700°C, increases very rapidly above this temperature. From the above discussion, it is clear that the best temperature range for working the above- mentioned steel is 400°C to 700°C. Working of steel within this temperature range is called warm working or Semi-hot working. In general, warm working is done at a temperature of 0.3 Tm to 0.5 Tm, where Tm is the melting point of the material. (ii) Type of Stress applied to the Workpiece. On the basis of the type of stress applied to the workpiece as it is formed into shape, the processes can be classified as : 1. Direct compression-type processes. 2. Tension type processes. 3. Combined stress (tension and compression) type processes. 4. Bending type processes. 5. Shear Stress type processes. Processes in which the material is subjected principally to compressive stress are : forging, rolling, extrusion, coining, spin forging, swaging and kneading etc. The metal flows at right angles to the direction of the applied stress. These processes are also called as ‘‘Squeezing processes’’. Processes involving the use of pure tensile stresses are : Stretch forming, vacuum forming and creep forming. In combined stress type processes, the primary applied forces are frequently tensile with the indirect compressive stresses coming into play due to the reaction of the work piece with the die. The processes include: Drawing processes (wire, rod and tube), deep drawing and embossing etc. Bending involves the application of bending moments to the sheet. The processes include : straight bending or flanging, V-bending, stretch flanging (concave flanging), shrink flanging (convex flanging) and seaming etc. Processes involving shear stresses refer to the application of shear forces of sufficient magnitude to rupture the metal in the plane of shear (chipless forming). The processes include : Piercing, blanking, press shearing and cutting off, notching and nibbling etc. Fig. 4.5 shows typical metal working operations. (iii) Steady or Non-steady State Forming. In steady state processes, the zone in which plastic deformation is enforced, remains fixed in shape and size and does not alter with time. Examples are : drawing processes (wire, rod, tube) and rolling. In non-steady processes, the geometry and zone of plastic deformation continuously change, for example, forging process. Extrusion process is transitory, that is, the deformation is nonsteady at the beginning and end but acquires steady state conditions while the greater part of the billet is extruded. (iv) Primary and Secondary Metal Forming Processes. Primary metal working processes are involved in the initial breakdown of the original material (ingot, bloom, billet) to shapes that are then processed for the final product, for example, rolling, forging, extrusion. 220 A Textbook of Production Technology Container Upper Upper Die Roll Die Product Billet JOB Bottom Roll Lower die Forging Extrusion Rolling Punch Die Blank Pull Die Drawing Deep Drawing (Wire, Rod, Tube) Punch Sheet Sheet p Gri Die Pull Die Pull Stretch Forming Shearing Punch Blank Die Bending Fig. 4.5. Typical Metal Working Processes. Secondary processes are those which take the products of some primary processes and change their geometry and properties to the semi-finished or finished stage, for example, drawing processes, spinning, swaging, coining, embossing, stretching, bending, deep drawing, rubber forming and sheet metal forming etc. (v) Extent of Plastic Deformation Zone. As per this criterion, the metal working processes can be classified as : Bulk deformation processes and Sheet metal working processes. In bulk deformation processes, the thickness (diameter or other major dimension) of the workpiece is substantially reduced (changed). These may be steady state or non-steady state processes, for example, forging, extrusion, rolling, drawing etc. In sheet metal working processes, any change in sheet thickness is fairly limited. But the starting material, that is the sheet, is the product of a bulk deformation process, that is, rolling. (vi) According to Low or High Rate of Strain. Conventional metal working processes and non-conventional (HERF, HVF) processes. (vii) Shape of Workpiece or Finished Part (sheet metal, bar stock etc.) Mechanical Working of Metals 221 4.2. ROLLING Rolling is the process in which the metals and alloys are plastically deformed into semifinished or finished condition, by passing these between circular or contoured rotating cylinders (rolls). the metal is drawn into the opening between the rolls by frictional forces between the metal and the roll surface (see Fig. 4.4). In deforming metal between rolls, the workpiece is subjected to high compressive force from the squeezing action of the rolls. Rolling is done both hot and cold. The starting material is cast ingot, which is broken down by hot rolling into blooms, billets and slabs, which are further hot rolled into plate, sheet, rod, bar, pipe, rails or structural shapes. Cold rolling is usually a finishing process in which products made by hot rolling are given a good surface finish and dimensional accuracy with increased mechanical strength of the material. Thinner gauges are obtained by cold rolling. The main objective in rolling is to decrease the thickness of the metal. Ordinarily, there is negligible increase in width, so that the decrease in thickness results in an increase in length. Rolling is a major and a most widely used mechanical working process. About 75% of steel output is treated in rolling mills and only 25% is consumed for forging extrusion and founding. 4.2.1. Terminology for Rolled Products. The various rolled products are given names according to the dimensions, but the terminology is fairly loose and sharp limits with respects to dimensions can not be made. Bloom. A bloom is the product of the first break down of ingot. It has square or slightly rectangular section, ranging in size from 150 mm × 150 mm to 250 mm × 300 mm. A bloom is used to make structural shapes, that is, I beams, channels etc., by hot rolling. Billet. A reduction of bloom by hot rolling results in a billet. The size of a billet ranges from 50 mm × 50 mm to 125 mm × 125 mm. It is rolled to make rounds, wires and bars. Slab. A slab is a product obtained by hot rolling, either from ingot or from bloom. It has a rectangular cross-section, with thickness = 50 to 150 mm and width = 0.6 to 1.5 m. Slabs are further rolled to get plates, sheets, strips, coil and skelp. Plate. A plate is a finished or semi-finished product with a minimum thickness of 6.35 mm. Its width will be equal to the width of the roll and the length equal to the maximum which can be handled or shipped. Sheet. A sheet is a thin partner of plate vr Top with a maximum thickness of 6.35 mm. A sheet with Roll  01 B/t less than 3 is called a Narrow sheet. For sheet metal working B/t > 3. r A C B Strip. A strip is a narrow sheet and has a v0 h0 h1 V1 maximum width of 600 mm with a maximum C B thickness of 6.35 mm. Since it is normally handled A in coil form, its length can be considerable and is  02 limited only by the manufacturing and handling Bottom Roll facilities. Foil. It is a thin strip with a maximum width of 300 mm and a maximum thickness of 1.5 mm. It is available in coil form. b0 b1 Bar. It is a long, straight, symmetrical piece of uniform cross- section. It may be round, square, or of another configuration. A circular bar is called a rod. Wire. A wire is a thin variety of bar, available in coil form and not normally so identified Fig. 4.6. Schematic Diagram of Rolling Process. over 9.5 mm cross-section. 222 A Textbook of Production Technology Billets are normally further rolled to make rounds, bars and wires. They can also be rolled into flats and sections. These intermediate products are further processed to get final products as shown below. Billets are also hot forged to get final products as shown below:- Flats and Section Engg. Applications, Springs Wires, Rods Wires Rolling Wire Mesh Cold heading Ropes Prestressed Wire Electrodes Cables Chain pin Rolled strips General Purpose Wire Umbrella rib Springs Link Chain Hand Tools Billet Needles Bright Rods Machined Components Cold forgings Hand tools Automobile Components Long High Speed Machinery Industrial Chain Pin Products Engg. Applications Hot Forging Automobile Components Mechanical Working of Metals 223 4.2.2. Mechanism of Rolling. Fig. 4.6 shows a schematic diagram of the rolling process. The metal contacts each of the two rolls along the arc AB, which is known as the arc of contact. This arc corresponds to the central angle , called the ‘‘angle of contact or bite’’. The process of metal rolling is made possible by the friction that occurs between the contact surfaces of the rolls and the part being rolled. At the moment of bite, two forces act on the metal from the side of each roll, normal force P and the tangential force P (Fig. 4.7) where  is the co-efficient of friction between the metal and the roll surfaces. The part would be dragged in if the resultant of horizontal component of the normal force P and tangential force (frictional force) P is directed in that direction. In the limiting case, P sin    P cos    tan  or   tan 1  If  is greater than tan–1 μ, the metal would not enter the space between the rolls automatically, that is, unaided. The maximum permissible angle of bite depends upon the valve of ‘μ’ which in turn depends upon the materials of the rolls and the job being rolled, the 01 roughness of their surfaces, and the rolling temperature and speed. P A In hot rolling, the primary purpose is to reduce the section and hence the maximum possible reduction µp is desired. so, the value of  and hence of μ should be greater. In hot rolling, lubrication is generally not A P necessary. On the other hand, on primary reduction 02 rolling mills such as blooming or rough rolling mills for structural elements, the rolls may sometimes be ‘‘ragged’’ to increase μ. Ragging is the process of making certain fine grooves on the surface of the roll to increase the friction. In cold rolling, the rolling loads Fig. 4.7. Forces During Rolling. are very high, hence should not be much. Besides, cold rolling being a finishing operation, rough rolls will impair the surface of the cold rolled product. Due to this, rolls for cold rolling are ground and lubricants are also used to reduce μ. The usual materials for rolls are : Cast or forged steel. To reduce cast alloyed C.I. can also be used. For superior strength and rigidity, special alloy steels, even though costlier are used. The usual values of biting angle are :  = 3° to 4° for cold rolling of steel and other metals with lubri- cation on well-ground rolls. = 6° to 8° for cold rolling of steel and other metals with lubri- cation on rough rolls. = 18° to 22° for hot rolling steel sheets = 20° to 22° for hot rolling aluminium at 350°C = 28° to 30° for hot rolling steel (blooms and billets) in ragged or well-roughed rolls. During plastic deformation of metals, it is assumed that the volume remains constant, that is, V0 h0 b0 = V1 h1 b1 Since b0  b1 224 A Textbook of Production Technology h0 V1 = V0  h1 Now h1 < h0,  V1 > V0, and so we have V1 > Vr > V0, Vr is the velocity of the roll. Hence, the speed at which the metal is delivered by the rolls, V1 is higher and the metal entrance speed V0 is lower than the peripheral speed of the rolls Vr. At a section C - C in the deformation zone (shown hatched), the velocity of metal will equal the velocity of rolls. This section is called ‘‘neutral or no slip section’’. To the left of neutral section, the deformation Zone is called ‘‘Lagging Zone’’ (V0 < Vr) and to the right of the section, it is called ‘‘Leading Zone’’, (V1 > Vr). We have two definitions, V1 – Vr Vr – V0 Forward slip   100; Backward slip   100 Vr Vr h (i) Absolute draught  h  (h 0  h1 ) mm; relative draught  100 h0 l1 (ii) Absolute elongation,  l  (l1 – l0 ) mm; Co-efficient of elongation = l0 (iii) Absolute spread = (b1 – b0) mm Spread increases with an increase in roll diameter and co- efficient of friction, as well as with a fall in temperature of the metal in the course of hot rolling. Spread is proportional to the draught and depends upon the thickness and width of the job. During cold working, the change in the properties of metals depend upon the percentage of cold work used, which is defined as Original area of cross section – Final area of cross section % of cold work  100 original area of cross section A0 – Af  100 A0 h0b0  h1b1  For cold rolling of sheets, % of cold working   100 h0 b0 h0  h1 ⎛ h ⎞  100  ⎜ 1  1 ⎟  100 (b0  b1 ) h0 ⎜ h ⎟ ⎝ 0 ⎠ 4.2.3. Rolling mills and their Classification. A rolling mill is a complex of machines for deforming metal in rotary rolls and performing auxiliary operations such as transportation of stock to rolls, disposal after rolling, cutting, cooling, melting, piling or coiling etc. A set of rolls in their massive housing is called a ‘‘stand’’. Rolling mills are classified according to the number of rolls in the working stand, the product rolled, arrangement of stands etc. Mechanical Working of Metals 225 1. Classification Based on Number of Rolls in the Stand. According to this criteria, rolling mills are of the following types : (i) two high rolling mill ; (ii) three high rolling mill (iii) four high rolling mill; (iv) multi-roll rolling mill; (v) universal rolling mill (i) Two High Rolling Mill. As the name implies, it has two rolls with a constant direction of rotation about the horizontal axes, (Fig. 4.8(a)). For successive reductions, the stock is returned to the entrance of the rolls by hand carrying or by means of a platform which can be raised to pass the stock over the rolls. (b) (a) Top Top Backup Working Roll Roll Bottom Working Bottom Driven Rotates Roll Backup by Roll Rolls Friction (d) 1. Work Rolls (2) 2. First Intermediate Rolls (4) 3. Drive Rolls (4) 4. Second Intermediate Rolls (2) (c) 5. Bockup Rolls (8) 5 3 4 2 1 Work Sheet (e) (f ) Fig. 4.8. Rolling Mills. The upper roll may be raised or lowered to change the distance between the rolls. This method of successive reductions slows down the process. The faster method is to pass the stock 226 A Textbook of Production Technology through a series of rolls for successive reduction, but this method requires more investment in equipment. The alternative procedure is to use ‘‘two high reversing’’ rolling mill, (Fig. 4.8 (b)), where the direction of rotation of the rolls is reversed after each pass. The rolls are brought closer together after each pass and the bar or plate reciprocates many times between the rolls before the final thickness is obtained. This design is limited by the length that can be rolled and by the inertial forces that must be over come each time a reversal is made. Also, much more power is consumed as compared to simple two high stand. Some time is also lost in reversing the mill. A two high non-reversing mills are the most common for both hot and cold rolling since they are least expensive. Such stands are widely used in mills through which the bar passes only once (continuous mills). Normally, the job movemet is facilitated by providig support rolls at the etry and exit of the rolling mill. Two high reversing mills are often used for the first rolling of an ingot into blooms and slabs. These stands are also employed as roughing stands of plate, universal, rail and structural and other mills. (ii) Three High Rolling Mill. The disadvantages of a two high reversing mill are over come by using a three-high rolling mill in which three rolls with a constant direction of rotation are arranged in a single vertical plane, (Fig. 4.8 (c)). Lifting tables are provided on one or both sides of the stand to raise and lower the bar after each pass. In these mills, the top and bottom rolls are drive rolls and the middle roll rotates by friction. These mills are employed as blooming mills, for billet rolling and finish rolling. These stands also find extensive use in open-train section mills. (iii) Four-High Rolling Mill. For a given width of plate/sheet and reduction per pass, the bending of the rolls will be less as their diameter is increased. However, increasing the roll diameter will increase the arc of contact between the roll and the plate/sheet (see Fig. 4.6). This will result in an increase in roll separating force (due to the reaction of the metal on to the rolls), which is given as, P = mean specific pressure between metal and roll × contact area.  In plate/sheet rolling (especially cold rolling), the diameter of work rolls should be as small as possible to reduce P and power requirements. To avoid bending of work rolls (due to their low strength and rigidity), large diameter back up rolls are installed, Fig. 4.8 (d) resulting in a four-high rolling mill. Four-high rolling mills are used in reversing mills for the hot rolling of armour and other plate, as well as for the hot and cold rolling of sheet steel in continuous mills. Note. The rolling load can be reduced by applying tensile force to the workpiece in the horizontal direction. This will lower the compressive yield strength of the material. Both ‘‘back tension’’ and ‘‘front tension’’ can be applied. (iv) Multiple-Roll Mills. In four-high rolling mill, the diameter of back up rolls can not be greater than 2 to 3 times that of the work rolls. As the diameter of work rolls is decreased more and more to accommodate processes with exceedingly high rolling loads, the size of the back up rolls must also decrease. A point is reached when the back up rolls themselves begin to bend and must be supported, hence the ultimate-design- cluster mill. Stands with a cluster of six (Fig. 4.8 (e)), 12 and 20 rolls are intended to manufacture strips down to 0.001 mm thick and upto 2000 mm wide. In 12 or 20 rolls stands, the very small diameter of working rolls (10 to 30 mm) allows no coupling of driving spindles, and the work rolls are idle running. They are supported by a row of driving rolls which, in turn, are supported by a row of back up rolls. The work rolls are thus rotated by friction against the driving intermediate back up rolls. Mechanical Working of Metals 227 Sendzimir mill. This mill is a modification of the cluster mill with Horizontal Rolls 20 rolls which is very well adapted to rolling thin sheet or foil from high- + Strength alloys (e.g. Stainless Steel). Only the outer rolls are driven. The remaining rolls get the motion due to friction, from the back up rolls, + (Fig. 4.8 (f)). High C-steel sheets of Vertical 5 to 50 μm can be made to an Rolls accuracy of 1 to 5 μm. This type of mill has been set up in Salem Fig. 4.9. Universal Rolling Mill. Stainless Steel Complex in Tamil Nadu. (v) Universal Rolling Mill. In this rolling mill, the metal is reduced by both horizontal and vertical rolls, (Fig. 4.9). The vertical rolls roll the edges of the bar even and smooth. Vertical rolls are mounted either on one side (front or back) or on both sides fo the horizontal roll stand. The horizontal rolls may be either two-, three-, or four-high arrangement. These mills are used for rolling wide strip, sheets, plates and slabs that require rolled edges and also for rolling of beams and H- sections. Planetary Rolling Mill. This mill consists of a pair of heavy backing rolls surrounded by a large number of small planetary rolls, Fig. 4.10. The main feature of this mill is that it hot reduces a slab to coiled strip in a single pass. Each pair of planetary rolls gives an almost constant reduction to the slab as it sweeps out a circular path between the slab and the backing roll. The total reduction is the sum of a series of such small reductions following each other in rapid succession. The feed rolls push the slab through a guide into planetary rolls. On its exit side, a 2- or 4-high planishing mill is installed to improve the surface finish and is followed by a coiler. Planetary Backing Rolls Roll + + + + + + Feed Rolls Planetary Planishing Mill Mill Fig. 4.10. Planetary Rolling Mill. Steckel rolling. In all the above methods, the rolls are powered and the job is pulled in due to the frictional force between the metal and the roll surface. In steckel rolling, the strip is drawn through idler rollers by front tension only, the torque on the rolls being zero. 2. Classification Based on the Products Rolled. According to the products rolled, the rolling mills may be classified as follows : (i) Blooming and slabbing mills. These are heavy mills with rolls from 800 to 1400 mm in diameter. They are designed to roll ingots (2 to 25 tonnes in weight) into blooms and 228 A Textbook of Production Technology slabs. Since blooming mills and slabbing mills are quite similar, these are called as primary mills. (ii) Billet mills. These mills have rolls from 450 to 850 mm in diameter and are designed to further reduce blooms into billets. (iii) Rail and Structural mills. These mills have rolls from 750 to 800 mm in diameter and are used mainly to produce railroad rails, beams, channels and other heavy structural shapes. (iv) Section mills have rolls from 250 to 750 mm in diameter, depending upon shape and section to be rolled. (v) Rod mills have rolls about 250 mm in diameter and are used to produce wire rod. (vi) Sheet and Plate mills have barrel lengths ranging from 800 to 5000 mm for hot rolling and from 300 to 2800 mm for cold rolling. (vii) Seamless Tube mills produce seamless tubes as discussed later. (viii) Tyre and Wheel mills are used to manufacture rail-road wheels and tyres. 3. Arrangement of Rolling Stands. Single stand mills are used singly or in various floor- layout combinations for rolling metals. (i) Looping Mill. (Fig. 4.11 a). The products are looped from one mill stand to another by arranging the stands in a ‘‘train’’ in which the rolls of one mill stand are driven from the ends of the rolls in the adjacent stand. (ii) Cross-country Mill. , (Fig. 4.11 b). It consists of individual roll stands placed some distance apart so that the workpiece must leave one set of rolls before it enters the next set. To save space and avoid complicating the mill drive mechanism, the roll stands are generally arranged in one or more parallel lines. Transfer and skid tables are used to reverse the direction of travel of the workpiece and to convey it from set of roll stands to another. Stand 1 Stand 2 Stand 1 (Line Abreast Mills) ( a) Looping Mill Stand 2 Stand Stand Stand 1 2 3 Stand 3 ( b) Cross Country Mill (c) In Line Continuous Mill Fig. 4.11. Arrangement of Rolling Stands. (iii) Continuous Mill., (Fig. 4.11 c). It employs a number of individual roll stands ar- ranged one behind the other in a straight line (in tandem). The work pieces continu- ously pass through the various roll stands and emerge from the last roll stand as a finished shape. The rolls of each successive stand must turn faster than those of the Mechanical Working of Metals 229 preceding one by a precise amount so as to accommodate the increased length which accompanies each successive reduction in thickness. All the above three mills require a large capital outlay and are only justified for mass production. Looping and cross country mills require the work piece to be bent or turned between the stands and are, therefore, used for rolling rods, rails or sections. Continuous mills are used for plates, strips or sheets. Looping mills and cross-country mills are also called as ‘‘Line abreast continuous mills’’ and continuous mills are called as ‘‘In- line continuous mills’’. 4.2.4. Roll Pass Design. As already discussed, because of the limitations in the equipment and workability of the metal, rolling is accomplished progressively in many steps. Plate, sheet and strip are rolled between rolls having a smooth, cylindrical, slightly cambered (convex) or concave working surface. Bars, rods and special purpose shapes (I-beams, channels and rails etc.) are rolled between grooved rolls. The shape cut into one roll is called the ‘‘groove’’. The shape formed when the grooves of the mating rolls are matched together is called the ‘‘pass’’. By rolling the metal consecutively through the passes, the initial square or rectangular cross-section of the ingot (bloom or billet) can be gradually changed to obtain a bar of the final required shape. According to their designation, passes fall into the following groups: (1) Breakdown or roll down or roughing passes. (2) Leader passes. (3) Finishing passes. Turn 90° (a) Turn 90° ( b) Turn 90° (c) Turn 90° Turn 45° (d) Fig. 4.12. Breakdown Passes. 230 A Textbook of Production Technology 1 2 100 mm × 100 mm Billet 3 4 5 6 7 8 9 10 Fig. 4.13. Reduction of a Billet to a Rod. 1. Break Down Passes. These passes are intended to reduce the cross-sectional areas. These passes may be of the : rectangular (box), diamond, square and oval shapes, (Fig. 4.12). (i) Box passes. These passes are employed in blooming mills, and in roughing and continuous billet mills. The reduction varies from 10 to 30% and the co-efficient of elongation from 1.1 to 1.25. The stock is tilted through 90° after each pass. (ii) Diamond pa