A Textbook of Production Technology PDF

 Chapter 1 Introduction 1.1. INTRODUCTION The main aim behind advances in engineering and technology has been to raise the standard of living of man and to make his life more comfortable. The major role in this direction has been played by manufacturing s...

 Chapter 1 Introduction 1.1. INTRODUCTION The main aim behind advances in engineering and technology has been to raise the standard of living of man and to make his life more comfortable. The major role in this direction has been played by manufacturing science. Manufacturing is an essential component of any industrialised economy. The word ‘Manufacturing’ is derived from the Latin, manus = hand and factus = made, that is, the literal meaning is ‘‘made by hand’’. However, in the broader sense, the word ‘Manufacturing’ means the making of goods and articles by hand and/or by machinery. Thus ‘Manufacturing engineering’ or ‘‘Production Engineering’’ can be defined as the study of the various processes required to produce parts and to assemble them into machines and mechanisms. Production or Manufacturing is a critical link in the design cycle, which starts with a creative idea and ends with a successful product, Fig. 1.1. 1.2. CLASSIFICATION OF MANUFACTURING PROCESSES When one thinks as to how the various components of machines are produced, many techniques come to the mind, for example, casting, forging, rolling, machining, welding etc. The manufacturing processes are so varied that there is no simple and universally accepted criteria of classifying these. However, all the manufacturing processes may be grouped into the following main categories: 1. Casting Processes. Here, the metal in the molten state is poured into a mould and allowed to solidify into a shape. The mould may be expendable Fig. 1.1. Design Cycle. or permanent. 1 2 A Textbook of Production Technology The examples are : Sand casting, Permanent mould casting, Die casting, Precision investment casting and centrifugal casting etc. 2. Deformation Processes. In these processes, the material is plastically deformed (hot or cold) under the action of an external force, to produce the required shape. No material is removed, but is only displaced and deformed to get the final shape. This category includes metal working/ forming processes such as : forging, rolling, extrusion and drawing etc. and also sheet metal working processes such as deep drawing and bending etc. The unconventional forming processes such as High Energy Rate Forming (HERF) and High Velocity Forming (HVF) methods also fall under this category. 3. Machining Processes. In machining processes, also known as Metal cutting or chip forming processes, material is removed from a work piece to get the final shape of the product. The processes include : Turning, milling, drilling, broaching, shaving, grinding, polishing, lapping, honing, buffing and sawing etc. The modern unconventional machining processes such as ECM, EDM, USM, AJM and LBM etc. are also included in this category. 4. Plastic Materials/Polymers processing methods. Under this category are included the various methods for processing plastic materials/ polymers, for example, shape casting, the various moulding processes (compression moulding, injection moulding, transfer moulding etc.) and thermoforming etc. 5. Powder Metallurgy. The more appropriate name should be ‘‘Particulate Processing Methods’’. Here, the particles of various sizes of metals, ceramics, polymers and glass etc. are pressed to shape and then sintered to get the final product. 6. Joining Processes. Here, two or more components are joined together to produce the required product. The category includes : all the welding processes, brazing, soldering, diffusion bonding, riveting, bolting, adhesive bonding etc. 7. Heat Treatment and Surface Treatment Processes. Heat treatment processes are employed to improve the properties of a work piece. The category includes the processes : Annealing, Normalising, Hardening and Tempering methods. Surface treatment processes include electro-plating and painting etc. 8. Assembly Processes. The assembly process for machines and mechanisms is the part of the manufacturing process concerned with the consecutive joining of the finished parts into assembly units and complete machines, of a quality that meets the manufacturing specifications. It is evident from above that, in general, no component can be produced entirely by one single category of manufacturing processes. For example, the starting material for the forging process is in the form of bar, stock or billet, which are the end-products of rolling process. And, for the rolling process, the starting material is ‘ingot’, which is produced by casting process. Metal cutting processes are normally used to give the final shape and size (surface finish, dimensional accuracy) to the components made by other processes. Accordingly, all the manufacturing processes can be grouped into two main categories :- (i) Primary Manufacturing Processes. These processes are involved in the initial breakdown of the original material to shapes that are then processed for the final product; for example, casting, forging, rolling, extrusion, powder metallurgy and plastic technology. However, in many cases, parts produced by these processes do not need other processes for producing the final shape and size, for example, precision investment casting, die-castings, precision forgings, powder metallurgy parts and parts produced by plastic processing methods. (ii) Secondary Manufacturing Processes. These processes take the products of some primary process and change their geometry and properties to the semifinished or finished stage, e.g., all metal removal processes and the rest of the metal forming processes, that is, drawing, spinning, Introduction 3 swaging, coining, stretching, bending, deep drawing, wire/rod/ tube drawing, sheet metal forming and rubber forming etc. 1.3. VARIOUS KINDS OF PRODUCTION Depending upon the scale of production and the type of components being produced, the production or manufacturing can be classified as : piece or job-lot production; medium lot, moderate lot, or batch production; and mass production. 1. Piece or Job-lot Production. Here, the parts are produced in small quantities to order, which are not periodically repeated. Companies engaged in piece production employ mainly general purpose equipment. Standard cutting tools and universal measuring facilities are used. The principle of complete inter changeability is not complied with. Hence, fitting is resorted to in assembly operations. The labour employed is more skilled. Lot sizes usually vary from 10 to 500 parts per lot, for example, manufacture of aeroplanes and oil field equipment, machine tools, giant hydroturbines, rolling mills and other heavy equipment etc. 2. Medium Production. It is characterized by the manufacture of parts in repeating lots or batches and to order. Mostly, the equipment used is general purpose type, equipped with universal, adjustable and sectional built up jigs, fixtures and tools. This enables the labour input and cost of production to be substantially reduced. Principles of interchangeability are to be strictly complied with. Lot size may range from a few parts to over 300 parts per lot and the yearly production may range from 2500 to 100,000 parts. An example of medium production is printing of books. Other examples are : machine tools, pumps, compressors, stationary I.C. engines etc. 3. Mass Production. It is characterized primarily by an established and stable object of production. The parts are produced in large quantities either intermittently or continuously, but are not dependent upon individual orders. The quantity is usually over 100,000 parts per year. A mass- production enterprise deals with standardized products of limited variety, such as consumer durable and products for industrial use. Other features typical of mass production are : an extensive use of specialized (usually permanently set up) and single purpose machine tools, and mechanization and automation of production processes, with strict compliance of the principle of interchangeability. The latter greatly reduces the time required for assembly operations. Examples of mass production are :- nuts, bolts, screws, washers, pencils, matches, engine blocks, automobiles, bicycles, electric motors, sewing machines and tractors etc. The most advanced form of mass production is ‘Continuous-flow production’, whose main feature is that the time required for each operation of the production line is equal to or a multiple of the set standard time all along the line. This enables work to be done without providing stock piles and in strictly definite intervals of time. Examples include : oil refineries and continuous chemical plants. In mass production type of production, semi-skilled or even unskilled workers are needed to operate the machines. This type of production is capital-intensive and the unit cost is low. 1.4. COMPUTERS IN MANUFACTURING Computers are being increasingly used in the design/production cycle of a part. Computer- aided design (CAD) and computer aided manufacturing (CAM) are performing greater role in manufacturing industry. Computers are being used in every aspect of manufacturing, for example, (i) For the control of production processes and machines with the aid of NC (numerical control), CNC (Computer Numerical Control) and DNC (Direct Numerical Control) (ii) Computer-Aided Process Planning, (CAPP) (iii) Production planning and control (iv) Inventory management and for material management planning (MRP) 4 A Textbook of Production Technology (v) Computer-Aided Quality Control, (CAQC) (vi) Flexible Manufacturing Systems, (FMS) (vii) Industrial Robots (viii) Group Technology, (GT) (ix) Computer Integrated Manufacturing System, (CIMS), and so on. The use of computers in manufacturing has resulted in : improved productivity, quality, equipment utilization, reduced inventory and faster delivery. 1.5. SELECTION OF A MANUFACTURING PROCESS From the above, it is thus clear that a manufacturer has the choice of selecting one of the many manufacturing processes for producing a given component. There are many factors involved in this exercise, but the selection of a suitable process can be made on the basis of the following considerations :- (i) Type and nature of the starting material (ii) Volume of production (iii) Expected Quality and properties of the components. (iv) Technical viability of the process (v) Economy Other factors influencing the choice of a manufacturing process are : Geometrical shape, Toolings, jigs, fixtures and gauges needed, available equipment and delivery date. The selection of material for a component should be based on the processing factors as well as the functional requirements of the component (Refer to Tables 1.1 to 1.3). The cheapest material meeting these requirements should be selected, because higher the alloy content, the higher the cost. Also, standard alloys should be selected. Selection of many different alloys should be avoided to reduce inventory costs. The type of material is also an important factor. Hard and brittle materials cannot be mechanically worked conveniently, whereas these can be cast or machined by many methods. A manufacturing method usually alters the properties of the material. For example, cold working renders a material stronger, harder and brittle (less ductile). A very good surface finish and close tolerances can be obtained by cold working operations as compared to hot working operations. However, they will be needing higher capacity equipment. Size, shape and shape complexity are also important factors. Flat components with thin cross- sections cannot be cast conveniently, whereas complex parts can't be mechanically worked. However, these can be conveniently cast or fabricated from individual parts. Complex shapes can be produced by forging and subsequent machining and finishing operations, and they have a toughness that is far superior to that of casting and P/M parts. Landing gear of a jet liner is manufactured by forging and machining processes. Very thin sections can be producted by cold rolling. The geometrical shape of the component dictates the shape of the starting material and the processing method. For simple shaped components, for example, straight shafts, bolts, rivets etc., the selection of the shape and the starting material and the manufacturing method is easily made. As the complexity of the shape increases, there is a wide choice of the shape of the starting material and the manufacturing process. For example, a small pinion can either be machined from a bar stock or from a precision forged gear blank. The final choice will be dictated by economics. For this, volume of production is an important factor. Larger the volume of production, more economical Introduction 5 it will be to invest in special equipment so as to reduce the cost per unit. Similarly, higher the production rate, economical it will be to select special purpose/automatic equipment. They will also be capable of meeting the delivery dates. Production volume depends upon the product. For example, paper pins, paper clips, nuts, bolts, washers, sparks plugs, ball bearings and ball point pens are produced in very large quantities. Whereas, jet engines for large commercial air craft, diesel engines for locomotives etc. are manufactured in limited quantities. Production volume also plays a very significant role in determining, Economic Order Quantity (EOQ) and Economic Lot or Batch quantity. As for the production rate, processes such as Die Casting, P/M, sheet metal working processes and roll forming etc. are high production-rate processes. Whereas, processes such as sand Casting, conventional machining process, Unconventional machining and forming processes and the various adhesive and diffusion bonding processes are relatively slow processes. The production rates can, however, be increased by automation or by the use of multiple machines. Lead time also greatly influences the choice of a process. It is defined as the time to start production. Process such as rolling, forging, extrusion, die casting, roll forming and various sheet metal working process, require extensive and expensive dies, tools and equipment, resulting in long lead-time. On the other hand, the various conventional machining processes are very flexible and can be adapted to most requirements in a relatively short time. Lead time is also very small in the case of machining centres, Flexible Manufacturing Cells (FMC) and Flexible Manufacturing Systems (FMS). They can respond quickly and effectively to product changes (both in design and quantity). It is clear from Table 1.3 that each manufacturing process has the capability of manufacturing a component within a certain tolerance range and surface quality. As the tolerance decreases and surface quality increases, the cost/unit increases. Therefore the widest tolerances and minimum surface quality that will meet the functional requirements of the component, should be specified. Each manufacturing process has its own limitation of producing complex shapes and maximum and minimum size of the component (Table 1.3). It is not always an easy task to select the best manufacturing process. Sometimes a product can be made by more than one competitive processes. So, in the final selection, cost is a very important factor. The cost of manufacturing a component by these methods should be compared and the optimum process selected. While doing these evaluations, along with the cost of processing the material to the finished product, the other important considerations are : material utilization factor, the effect of processing method on the material properties and the subsequent performance of the component in service. The various manufacturing processes have been compared in Tables 1.1 to 1.4 on the basis of joinability, metal forming, design and cost considerations. Production technology has become much more sophisticated since World War – II, due to technological innovations and advances in computerisation and quality management. The economic implications of new technology for manufacturing companies are complex. Changes in Production Technology, such as the development of CAD/CAM, have allowed the manufacturing of high quality, tailor made goods at low cost. Changes in Production Technology create whole classes of highly skilled positions to operate the machines and co-ordinate more complex organizational structures that originate from the technologies.  6 Table 1.1. Manufacturing Processes : Joinability Considerations Material Welding Brazing Soldering Adhesive bond Adhesive Thread Riveting Arc Oxy-. Resistance [TS, TP, EM]1 bond2 Fastening Acet Cast Iron C R O D O (TS) C, (TP) O C C R Carbon steel R R R R D (TS) C, (TP) O C R R Stainless steel R C R R C (TS) C, (TP) O C R C Aluminium-Magnesium C C C C O (TS) R, (TP) O R C C Copper C C C R R (TS) C, (TP) O C C C Nickel R C R R C (TS) C, (TP) O C R C Titanium C X C D O (TS) O, (TP) X C C X Lead C C X X R R R R R Zinc C C O X R R R R R Thermoplastics R R C X X C O O R Ceramics X O X X X R R C O Leather X X X X X (EM) R, (TS) C R O R Fabric X X X X X (EM) R R O R R : Recommended ; C: Common ; D : Difficult ; O : Seldom used ; X : Not used [TS : Thermoset ; TP : Thermoplastic : EM : Elastomeric1] ; 2(Modified Compression Epoxy) A Textbook of Production Technology  Table 1.2. Manufacturing Processes : Metal forming Considerations Manufacturing Process Irons Low- Heat Alumi- Copper Lead Magne- Nickel Precious Tin Titanium Zinc Introduction C-steel and nium alloy alloy sium alloy Metals alloy alloy alloy alloys corrosion alloy alloy resistant alloy Sand casting A A A A A B A A - B - B Shell mould casting A B B A A - - B - - - - Permanent mould A B - A B B A B - B - B casting Die casting - - - A B A A - - B - A Plaster mould casting - - - A A - - - - - - - Investment casting - A B A A - A B B - - - Centrifugal casting A A A B B - - B - - - - Drop forging B A A B B - B B - - B - Press forging - A A B B - B B - - B - Upset forging - A A B B - B B - - B - Cold headed parts - A B A A B - B B - - - Stampings, Drawing - A B B A - B B B - B B Spinning - A B A A B B A B - B B Screw machining B A B A A - B A B - B B Roll forming - A - B B - B - - - B B Extrusion - B - A A B A B - B B - Powder Metallurgy A A B B A - - B B - B - Electro forming A - - B A B - A A B - B A : Materials most frequently used ; B : Also materials currently used 7  8 Table 1.3. Manufacturing Processes : Design Considerations S. No. Manufact- Material Complexity Maxi- Mini- Mecha- Precision and Special Surface Surface Remarks uring choice of part mum mum nical Tolerance Struc- Smoo- Detail Process  Size Size prope- tural thing rties Prope- rties 1. Sand casting Wide ; Ferrous, Non Considerable Largest 3 mm Fair to ± 0.6 to 0.3 mm per mm Good Poor Poor Usually require some -ferrous Light Metals High Bearing machining structure 2. Shell Mould Wide Moderate Best for 1.5 mm Good ± 0.003 to 0.005 mm per High Good Good Best of low casting casting smaller mm Quality methods parts for cast metals 3. Permanent Restricted : brass Limited Moderate 2.5 mm Fair good ±0.6 mm per mm None Good Good Economical for mass Mould casting bronze aluminium production only 4. Plaster Mould Narrow : Brass, Considerable Moderate 0.75 mm Fair ±0.25 to 0.125 None Good Good Little finishing casting bronze, aluminium required 5. Investment Wide Considerable Moderate 0.75 mm Good ±0.125 mm per mm None Excel- Excel- Best for too casting lent lent complicated parts. 6. Die casting Narrow : Zinc, Considerable Large 0.06 mm Fair to ±0.025 mm per mm None Good Good Most economical aluminium, brass, Good where applicable magnesium 7. Drop Forging Medium Moderate Large Small High ±0.25 to 0.75 mm Tough- Fair Fair Used for high ness strength 8. Press Forging Medium : best for Limited Moderate Small High Medium better than Tough- Fair Fair Greater complexity non ferrous alloys drop forging ness than drop forging. 9. Upset Forging Medium : many Limited Medium Moderate High Medium Tough- Medium Med- Best suited to small ferrous and ness ium parts. non-ferrous alloy 10. Welding Wide Consi- Un- Moderate Variable Medium, not highly Tough- De- None Versatile process. derable limited precise ness pends upon compo- nents A Textbook of Production Technology  Introduction Table 1.4. Manufacturing Processes : Cost Considerations S. Manufacturing Raw Material Cost Tool and Die Costs Direct Labour Cost Finishing Cost Scrap loss No. Process  1. Sand Casting Low to Medium Low High High Moderate : Scrap can be depending on metal remelted. 2. Shell Mould Casting Low to Medium Low to Moderate Low Low Low 3. Permanent Mould Medium Medium Moderate Low to Moderate Low Casting 4. Plaster Mould Casting Medium Medium High Low Low 5. Investment Casting High : best for special Low to Moderate High Low Low : Scrap is remelted. and costly alloys 6. Die Casting Medium High Low to Medium Low Low : Scrap can be remelted. 7. Drop forging Low to Moderate High Medium Medium Moderate 8. Press forging Low to Moderate High Medium Medium Moderate 9. Upset forging Low to Moderate High Medium Medium Medium : Lowest of forging processes. 10. Welding Low to Moderate Low to moderate Medium to High Medium Low : Practically none. 9 10 A Textbook of Production Technology PROBLEMS 1. Discuss the importance of manufacturing science. 2. Define the term “Manufacturing”. 3. Define “Manufacturing Engineering”. 4. Classify the Manufacturing Processes. 5. Discuss the various types of production. 6. Discuss the importance of Computers in Manufacturing. 7. Discuss the various factors for selection of a manufacturing process for a given product. 8. Consider a product and describe how its production volume would affect selection of economical manufacturing process for it. (PTU) 9. Give two broad classes of manufacturing processes. Explain differences in these. (PTU) 10. Give the most important plus point of casting process of manufacturing. 11. What is the most important plus point of Deformation processes of Manufacturing. 12. Write on : Primary process and Secondary processes of manufacturing. 13. Write the attributes of "Piece Production of components. 14. Write the attributes of "Medium Production" or "Batch production". 15. Write the attributes of "Mass production" of components. 16. What is : Continuous Flow Production? 17. List the product applications of : Job lot production, Medium production, Mass production and Continuous- flow production. 18. Define the terms : CAD, CAM, NC, CNC, DNC, CAPP, MRP, GT, FMS and CAQC. 19. What are the uses of Industrial Robots in manufacturing. 20. Which manufacturing process you will recommend for the manufacture of the following components : (a) Pump cylinder (b) Connecting rod of an I.C. engine (c) Porous bearings (d) Tungsten filament of electric Bulb. 21. What is lead time ? How it affects the selection of a manufacturing process ? 22. Name a few high production – rate manufacturing processes. 23. How the size, shape, and shape complexity of the component influence the selection of a manufacturing process ? 24. Manufacturing must be carriedout at the lowest cost consistent with the quality and functionality of the product. Discuss. 25. List the advantages of flow production. 26. Safety in manufacturing is everybody's responsibility. Discuss. 27. Write on the routes taken by Primary Processing Methods. The Primary Manufacturing Methods can take the following routes :- Molten Metal Sand Casting Ingot Casting Hot Rolling Forging Drawing Cold Rolling Extruding  Chapter 2 Engineering Materials and Heat Treatment 2.1. ENGINEERING MATERIALS Materials of construction can be classified into two groups :- 1. Metallic materials 2. Non-metallic materials Metallic materials can be further split into :- (i) Ferrous materials, and (ii) Non-ferrous materials. Ferrous materials consist mainly of iron with comparatively small addition of other materials. Ferrous materials are iron and its alloys such as cast iron, gray cast iron, malleable cast iron, wrought iron, and steels of low carbon content and high carbon content etc. Non-ferrous materials contain little or no iron. The materials include : Aluminium, Magnesium, Copper, Zinc, Tin, Lead, Nickel, Titanium and so on and their alloys. Non-metallic materials included plastics, rubber, leather, carbon, wood, glass etc. 2.2. GENERAL MATERIAL PROPERTY DEFINITIONS 1. Homogeneity. A material that exhibits the same properties throughout is said to be homogeneous. Homogeneity is an ideal state that is not achievable by real materials, particularly metals. However, this variation in properties is so small that calculations for stress and deflection can easily assume that a material is homogeneous throughout. 2. Isotropy. A material that displays the same elastic properties in all loading directions is said to be isotropic. The equations of elasticity and strength of materials are based upon this assumption. 3. Anisotropy is that characteristic of a material which exhibits different property values in different directions with respect to a set of reference axes. 4. Elasticity. It is that property of a material by virtue of which deformations caused by applied load disappear upon the removal of load. 5. Plasticity of a material is its ability to undergo some degree of permanent deformation without rupture. Plastic deformation takes place only after the elastic range has been exceeded. 6. Ductility and Brittleness. Ductility is that property of a material which permits permanent deformation before fracture by stress in tension. Ductility is most commonly measured by means of elongation and reduction in area in the tensile test. Final gauge length – Original guage length % elongation   100 Original gauge length 11 12 A Textbook of Production Technology % reduction in area = Original area – Final area  100 Original area A material is generally classified as ductile if the percentage elongation is more than 5 in a gauge length of 50 mm, and as brittle if the percentage elongation is less than 5%. 7. Stiffness. Stiffness is the resistance of a material to elastic deformation or deflection. A material which suffers only a slight deformation under load has a high degree of stiffness. 8. Hardness is the property of solid bodies where by resistance is offered to plastic deformation and fracture, when two bodies in contact over a small area are pressed together. In metal working, hardness generally implies resistance to penetration. It may, however, include resistance to scratching, abrasion or cutting. 9. Toughness of a material is its ability to withstand the plastic and elastic deformation. It is, in fact, the amount of energy a material can absorb before actual fracture or failure takes place. 10. Malleability of a material is its ability to be flattened into thin sheets without cracking, in the cold state, by pressing, rolling, hammering etc. Copper, aluminium and gold have good malleability. 11. Machinability. This property refers to the relative ease with which a material can be machined or cut. For example, brass can be easily machined as compared to steel. 12. Cold shortness. It is the brittleness that exists in some metals at temperatures below their recrystallization temperature. 13. Red shortness. It is the brittleness of steel and tendency towards cracking at high temperatures caused by the formation of iron sulphide. 14. Damping capacity is the ability of a metal to dissipate the energy of vibratory or cyclic stresses by means of internal friction. Some metals such as lead have a high damping capacity; cast iron also has good damping properties. Steel, however, has poor damping characteristics. 15. Embrittlement. Embrittlement is the loss of ductility of a metal. The loss may be due to physical or chemical changes. 16. Hydrogen embrittlement is the low ductility of a metal caused by the absorption of hydrogen. 17. Cold working. Cold working of metals means the mechanical working at temperatures below the recrystallization temperatures. Normally, it is taken to be working of metals at room temperature. 18. Hot working of metals means the working of metals when these are heated sufficiently (above the recrystallization temperature) to make them plastic and easily worked. 2.3. THE IRON-CARBON PHASE DIAGRAM To understand the Iron Carbon Phase diagram, we need to understand the cooling process of pure metals and of alloys. Cooling curve for a pure metal is shown in Fig. 2.1. It is clear that pure metals have clearly defined freezing (as well as melting point) and solidification/melting takes place at constant temperature. However, alloys solidify/melt over a range of temperature, Fig 2.2. for Ni-Cu alloy. Iron may exist in several allotropic forms in the solid state in accordance with the temperature. The transformation of iron from one allotropic form to another is accompanied by either the evolution of heat (on cooling curve of metal) or absorption of heat (on heating curve of metal). Fig 2.3 illustrates the cooling curve for pure molten iron (its melting point being about 1537°C) plotted in time vs. temperature co-ordinates. The first horizontal step appears on this curve at a temperature of 1537°C. It indicates that the heat is evolved and that iron passes from the liquid to the solid Engineering Materials and Heat Treatment 13 state and the mixture consists of liquid plus delta iron solid solution. Delta iron has a body centred cubic crystal lattice (b.c.c.). The second temperature effect occurs at 1392°C on the iron cooling curve and the delta iron gets transformed into a new allotropic form  - iron which has a face centred cubic lattice (f.c.c.). Next step is at 910°C where -iron is transformed into -iron with b.c.c. lattice. The last, fourth step, is observed at 768°C when -iron is transformed into  -iron with b.c.c. lattice.  -iron acquires pronounced ferro-magnetic properties. Fig. 2.1. Cooling Curve for a Pure Metal. Fig. 2.2. Cooling Curve for an Alloy. Since the space lattice does not change in the alpha-to-beta transformation, -iron must be regarded as a paramagnetic state of  -iron. Again delta iron, which exists in the interval 1537°C to 1392°C, has a space lattice of b.c.c., the same as  -iron. Thus, there are actually two allotropic forms of iron :  -iron and  -iron. They exist as follow : –  Iron (b.c.c) : 1392°C to 1537°C  Iron (f.c.c) : 910°C to 1392°C  Iron (b.c.c.) : 768°C to 910°C  Iron (b.c.c.) : < 768°C Critical Points. On the cooling curve (or on the heating curve), the points where structural changes occur, are known as ‘‘critical points’’. In Fig. 2.3, the critical point of the    14 A Textbook of Production Technology 1600 Liquid Iron 1537 Delta Iron, b.c.c. Temperature °C 1392 Paramagnetic Ar4 Gamma Iron, f.c.c. 910 Ar3 Beta Iron, b.c.c. 768 Ar2 Ferromagnetic Alpha Iron, b.c.c. Time Fig. 2.3. A Cooling Curve for Pure Iron. transformation at 910°C is denoted by Ar3 (in cooling) and by Ac3 (in heating). The critical point of the    transformation at 1392°C is denoted by Ar4 (in cooling) and by Ac4 (in heating). Again, the critical point (768°C) corresponding to the magnetic transformation of  -iron is denoted by Ar2. Letter A denotes arrest, r for cooling and C for heating. The significance of ‘‘arrest’’ lies in the fact that structural changes take place at constant temperature. Absolutly pure iron is very difficult to obtain. But, in this state, it is a soft and very plastic material of not much use in engineering. However, it can be alloyed with may elements. Alloys of iron and carbon are the most widely used in engineering. Iron-Carbon Phase diagram. If a series of time-temperature heating or cooling curves are drawn for steels of different carbon contents and the corresponding critical points plotted on a temperature vs. percent carbon curve, a diagram similar to Fig. 2.4 will be obtained. A magnified view of the major (significant) portion of ‘‘Iron-C Phase diagram’’ is shown in Fig. 2.5 or ‘‘Iron-carbon equilibrium diagram’’. It is known as equilibrium diagram because the state of systems remains constant over an indefinite period of time. It provides a complete picture of phase relations, microstructure, and temperature for the knowledge of heat treatment of steels. In addition, it clearly indicates the division between steel and cast iron. Pure iron is on the left side of the diagram and cementite (Fe3C), containing 6.67% carbon, on the right. Cementite is a Fig. 2.4. Iron-Carbon Phase Diagram. chemical compound of iron and carbon and may form upon rapid cooling of the iron-carbon melt from high temperatures. It is brittle, weak in tension, strong in compression, and is the hardest of Engineering Materials and Heat Treatment 15 any material in the equilibrium diagram. To be strictly correct, this diagram should be called as 1600 1539 °C Delta iron and liquid, 2 phase A 1492 °C Delta H Molten alloy, 1 phase iron J B Deltairon Liquidus line 1400 and austenite N Solidus line Austenite in Cementite 1300 & liquid, 2 phase liquid, 1200 2 phase E Austenite 1130 ° C 1100 (gamma iron) 1 phase 1000 ACM Austenite Temperature ° C G 910 °C lederurite 900 & A3 Eutectoid Austenite A2 cementite point & 800 Ferrite & cementite qustenite 0 S A3 723 ° C 700 P A1 Ferrite 600 pearlite Pearlite Cementite, pearlite 500 & & & ferrite cementite transformed ledeburite 0 0 0.5 1 2 3 4 5 Hypoeutectoid Hypereutectoid Steels Cast irons Fig. 2.5. Iron-Carbon Phase Diagram. iron-iron carbide diagram, because the carbon in equilibrium does not appear as free carbon (graphite) but in the form of Fe3C. Common usage, however, terms it the iron-carbon diagram. It will be observed from Fig. 2.4 that carbon is soluble in  -iron to a maximum of 0.025% at 723°C (Point P) and only to 0.008% at room temperature. The result is an interstitial solid solution with dissolved carbon. Alpha iron is commonly called as ‘‘ferrite’’ or more accurately as ‘‘  -ferrite’’. It is the softest of all materials in the diagram. 16 A Textbook of Production Technology Above critical point A3 (Point G), the substance is known as ‘‘austenite’’, which is a solid solution of carbon in -iron. It has f.c.c. structure. It has a solid solubility of upto 2%C at 1130°C. It is denser than ferrite, ductile at high temperature and possesses good formability. At point A3 ferrite begins to separate from solid solution. As the material is cooled to Ar2 point, it becomes magnetic. On further cooling to Ar1 line, additional ferrite is formed. At the Ar1 line, the remaining austenite is transformed to a new structure called ‘‘pearlite’’. So, Ar1 is also a ‘‘critical point’’. A3 is called the ‘‘upper critical point’’ and A1 the ‘‘lower critical point’’. The name ‘‘pearlite’’ has its origin in the fact that its microstructure resembles that of mother-of-pearl. It is clear that the allotropic change occurs over a range of temperature. The line, Ac1 at which the allotropic change starts on heating is called the ‘‘Lower Critical Temperature line’’. The line Ac3 where the allotropic change is fully completed is called the ‘‘upper critical temperature line’’. The ‘‘Critical range’’ is the zone between these two lines. It is sclear from Fig. 2.5 that the lower critical point is same for all steels, but the upper critical temperature varies according to the carbon content in steel. For steel containing 0.8% C, there is only one critical point, the total transformation taking place at that temperature. As the percentage of carbon increases, the temperature at which the ferrite is first rejected from the austenite falls, until at about 0.80% carbon (point S), no free ferrite is rejected from the austenite. This point is called ‘‘eutectoid’’ and is the lowest point on the diagram at which austenite will disappear. The material formed at this point is 100% ‘‘pearlite’’. Pearlite is a mechanical mixture of ferrite and cementite. A ‘‘eutectoid’’ is ‘‘an isothermal reversible reaction in which a solid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system’’. Eutectoid Reaction : Consider steel with 0.8% C (Point S in Fig. 2.4) being cooled very slowly from a temperature, say 1100°C (in the austenite range), so as to maintain equilibrium. At 723°C, a reaction takes place in which austenite decomposes and is transformed into -ferrite (b.c.c.) and cementite. This reaction is called a Eutectoid reaction. A single solid phase (austenite) decomposes into two other solid phases (ferrite and cementite). The structure of eutectoid steel is called ‘‘Pearlite’’, because it resembles mother of pearl. The micro-structure of Pearlite consists of alternating layers (lamellae) of ferrite and cementite. Therefore, its mechanical properties lie in between that of ferrite (soft and ductile) and cementite (hard and brittle). Pearlite is 87% ferrite and 13% Cementite. Cementite is 93.33% Iron and 6.67% carbon. The Eutectoid reaction can be written as: Austenite 0.8%C, Cooling Ferrite Cementite       + ( ) Heating ( ) (Fe3C) 723°C Austenite Cooling Ferrite Cementite or     + ( 0.8% C) Heating 723°C (0.02% C) (6.67% C) Cooling or Solid 1     Solid 2 + Solid 3 Heating 723°C is called the Eutectoid temperature, and 0.8% C steel is called the Eutectoid composition. In steel with carbon percentage less than 0.8, the microstructure consists of Pearlite phase [Ferrite + Cementite] and a ferrite phase. If C% is greater than 0.8, austenite transforms into Pearlite and Cementite. Engineering Materials and Heat Treatment 17 If the carbon content of steel is greater than the eutectoid, a new line is observed in the iron- carbon diagram, that is line Acm. The line denotes the temperature at which iron carbide is first rejected from the austenite instead of ferrite. It is clear from the figure that the maximum solubility of carbon in  -iron is 2% (Point E). Point C is a ‘‘eutectic point’’ containing 4.3% carbon and consists of a mixture of austenite and cementite known as ‘‘lederburite’’. A eutectic point is a point on the phase diagram at which two constituent metals solidify simultaneously. It has the lowest liquidus temperature (above which only liquid exists), and the liquidus and the solidus (below which only solid exists) coincide, so that it melts and solidifies like a pure metal. Eutectic Reaction. Point C in Fig. 2.5, percentage of carbon being 4.3%. When molten alloy is cooled slowly, it will start cooling and solidifying along the liquidus line. For alloy containing 4.3% C, when it is cooled to 1130°C, a reaction takes place in which molten alloy solidifies into a mixture of two solids (austenite and cementite), known as ‘‘Lederburite’’. All the liquid gets solidified at 1130° C, at the same time. Crystals of austenite containing 2% C and Cementite containing 6.67% C separate from it. These crystals of austenite and cementite form the eutectic mixture known as ‘‘Lederburite’’, point C. Thus, eutectic alloy at point C containing 4.3% Carbon solidifies at the constant temperature of 1130°C with the formation of only Lederburite. Thus, the eutectic reaction is 4.3%C Liquid  1130°C Austenite + Cementite ( ) [Fe3C] Thus, at eutectic point, liquid phase and two solid phases co-exist. Lederburite is  -Fe3 C eutectic matrix. At higher carbon content (> 4.3% C), Fe3C is embedded in the eutectic matrix. As above cementite is 6.67% C and 93.33% Iron. The carbon alloys having less than 2% carbon are called ‘‘steels’’ and those containing over 2% carbon are called cast irons. Steels may further be classified into two groups :- Steels having less than 0.8% carbon are called ‘‘hypo- eutectoid steels’’ and those having more than 0.8% carbon called ‘‘hyper-eutectoid steels’’. Steels with carbon content exactly equal to 0.8% are called ‘‘Eutectoid Steels’’. Similarly Cast Irons with carbon content above 2% and upto 4.3% are called ‘‘Hypo- eutectic’’ Cast Iron and those with carbon content above 4.3% and upto 6.67% are called ‘‘Hyper- eutectic’’ Cast Irons. Cast Irons with carbon content exactly equal to 4.3% are called ‘‘Eutectic’’ Cast Irons. The portion of the iron-carbon phase diagram which pertains to steels and which is important for studying the heat treatment of steels is shown in Fig. 2.6 Fig. 2.6. Iron-Carbon Diagram. Thus, as noted above, the information from Iron Carbon phase diagram is as follows :– 18 A Textbook of Production Technology 1. Temperatures at which the alloys will start melting/solidifying and finish melting/solidifying. 2. Possible phase Change which will occur as a result of alteration in the composition or temperature. 2.3.1. TTT Curves : The Iron-carbon Phase diagram, Fig. 2.4, provides a complete picture of phase changes, corresponding microstructure and temperatures under equilibrium condition. That is, sufficient time is allowed for the reactions to complete. So, time is not a variable in this diagram and hence this also known as Equilibrium diagram. The effects of different cooling rates on the structural changes of steel are not revealed in this diagram. However, in the various heat treatment processes (particularly hardening), the austenite transformation of steel depends upon the cooling rate. The most comprehensive conception of the kinetics involved in the structural changes of austenite may be gained by studying the isothermal transformation of decomposition of supercooled austenite, that is its transformation at constant temperature. Thus, time is also a variable. Hence, the basis for heat treatment of steels is time, temperature Transformation diagram (TTT diagram) or Isothermal transformation (IT) diagram, which because of its shape is also known as C curve or S-curve. To construct these curves, a number of small specimens of steel are heated above the critical points where austenite is stable. These are then rapidly cooled to a number of temperatures and are held at these temperatures for different periods of time until austenite is completely decomposed. At any given time, the percentages of austenite and pearlite are also noted by examining the structure. Then, we get the curves as shown in Fig 2.7. These curves have been obtained experimentally for eutectoid steel (0.8% C), by heating steels above critical points and rapidly cooling them to 700°C, 600°C and 400°C and holding them at these temperature for various lengths of time. The higher the temperature and/or the longer the time, the greater the percentage of austenite transformed to pearlite. The transformation of austenite at constant temperatures does not begin immediately, but only after a certain time interval which is called the ‘‘incubation period’’. Fig. 2.7. Austenite to Pearlite Transformation as a Function of Time and Temperature. The TTT Curve Fig. 2.8, curve is obtained from curves of Fig. 2.7. In such a curve, time is in seconds plotted along the horizontal axis in a logarithmic scale and the temperatures in °C in an ordinary scale along the vertical axis. At 700°C, the austenite decomposition begins at point B1 and Engineering Materials and Heat Treatment 19 ends at point E1. Similarly at temperatures of 600°C and 400°C, the beginning and end points are B3, E3 and B2, E2 respectively. Curves 1 and 2 are then drawn passing through these points indicating the beginning and end of austenite transformation. At near about 700°C, austenite decomposes into pearlite. Sorbite is formed at lower temperature (600°C). Troostite is formed at 500 to 550°C. Acicular troostiteor bainite is formed when the temperature is lowered from 550 to 220°C. At 240°C (point Ms) martensite transformation begins and it ends at point MF (–50°C). These points are not affected by cooling rate, but their positions depend upon the chemical composition of steel. Fig. 2.8. T T T Curve. TTT curves are drawn from non-equilibrium conditions. The information available from TTT curve is : Change of phase with the cooling rate which is used in Hardening process. 2.4. HEAT TREATMENT DEFINITIONS The heat treatment operations can be defined as : heating a metal or alloy to various definite temperatures, holding these for various time durations and cooling at various rates. This combination of controlled heating and cooling determine not only the nature and distribution of micro-constituents (which determine the properties of a metal or alloy), but also the grain size. Thus, the main aim of the heat treatment operations is to control the properties of a metal or alloy through the alteration of the structure of the metal or alloy. The purposes of the various heat treatment operations are as given below : 1. To remove or relieve strains os stresses induced by cold working (drawing, bending etc.) or non-uniform cooling of hot metal (for example welding) : Annealing 2. To increase strength or hardness of the material for improved wear resistance: Hardening. 20 A Textbook of Production Technology 3. To improve machinability : Annealing 4. To soften the material : Annealing 5. To decrease hardness and increase ductility and tougness to withstand high impact. (Tempering) 6. To improve the cutting properties of tools. 7. To change or modify the physical properties of the material such as electrical properties, magnetic properties, corrosion resistance and heat resistance etc. 8. Elimination of H2 gas dissolved during pickling or electro-plating which causes brittleness. The common heat treatment processes mentioned above are defined below : 1. Annealing. Annealing consists of heating the metal to a temperature slightly above the critical temperature and then cooling slowly, usually in the furnace with the heat shut off. Annealing is done to accomplish one or more of the following aims : (i) To produce an even grain structure. (ii) To relieve the internal stresses caused by various manufacturing processes or by previous treatments. (iii) To reduce the hardness and increase the ductility. After annealing, the metal becomes soft, whisch improves machinability. 2. Full annealing. It involves prolonged heating just above the critical temperature to produce globular form of carbide, for example, to improve the machinability. 3. Process annealing. Annealing to restore ductility at intermediate stages of cold reduction is called ‘‘process annealing’’. 4. Stress relieving. It is a heat treatment designed to relieve internal residual stresses induced by cold working, machining, welding, casting, and quenching. It is a subcritical anneal process. The metal is heated to below the transformation range, that is, to about their recrystallization temperatures (equal to about 0.4 × melting point) and cooled slowly in air, after being held at that temperature for 1-3 hours. 5. Normalizing. Normalizing implies the heating the metal to the same temperature as that employed for full annealing and then cooling in air. It is used to remove the effects of any previous heat treatment and to produce uniform grain structure before other heat treatments are applied to develop particular properties in the metal. It is often carried out prior to case-hardening. 6. Quenching. Quenching or rapid cooling from above the critical temperature by immersion in cold water or other cooling medium, is a hardening treatment. It increases the strength of the metal and increases the wear resistance, but makes the metal brittle and of low ductility. 7. Tempering. ‘‘Tempering’’ or ‘‘Drawing’’ consists of reheating below the critical temperature the quenched metal to restore some of the ductility and reduce the brittleness. Increased toughness is obtained at the expense of high strength. 8. Case-hardening. Case-hardening or carburising is a process of hardening the outer portion of the metal by prolonged heating, free from contact with air while packed in carbon in the form of bone char or charcoal. The outer metal absorbs carbon, and, when the hot metal is quenched, this high-carbon steel hardens, whereas the low carbon steel of the core remains soft and ductile. In ‘‘gas carburising’’, the metal is heated in an atmosphere of gas and controlled so that the metal absorbs carbon from the gas but will not be oxidized on the surface. 9. Cyaniding. Cyaniding is case-hardening with powdered potassium cyanide, or potassium ferrocyanide mixed with potassium bichromate, substituted for carbon. For a very thin case, immersion in hot liquid cyanide is sufficient. Cyaniding produces a thin but very hard case in a very short time. Engineering Materials and Heat Treatment 21 10. Nitriding. Nitriding is surface hardening, accomplished by heating certain steel alloys (Nitralloys) while immersed in ammonia fumes. 11. Flame hardening. This is a hardening process by which either selected surface areas or the entire part is thoroughly heated by means of a gas burner with subsequent quenching. 12. Induction hardening. It is hardening of parts which follows induction heating, the latter of which either has heated the case portion or the entire part. 13. Aging, Age hardening or precipitation hardening. This is the process by which the structure of a metal recovers from an unstable condition produced by quenching (quench aging) or by cold working (strain aging). The change in structure is due to the precipitation of one of the constituents from a saturated solid solution and results in a material that is stronger and harder, but usually less ductile. This type of aging takes place slowly at room temperature and is called ‘‘Natural aging’’. The process may be accelerated by a slight increase in temperature and is then called ‘‘Artificial aging’’. The first step in age hardening is ‘‘Solution heat treatment’’. It is the heating of an alloy to a suitable temperature, holding it at that temperature long enough to allow one or more constituents to enter into solid solution and then cooling rapidly enough to hold the constituents in solution. The alloy is left in a supersaturated, unstable state and may subsequently exhibit quench aging. This process of hardening is usually used for non-ferrous metals. 2.5. FERROUS METALS Ferrous metals are composed of iron and carbon, plus a number of other elements, which are present either (1) impurity elements (such as S, P, Mn and Si) carried over from the raw materials or unavoidably introduced during the manufacturing processes, or (2) intentionally added alloying elements. The general classes of ferrous metal products differ from one another principally in the amount of carbon content present in each. Commercially pure iron contains upto 0.01% carbon. Steels contain less than or maximum 2% carbon and cast irons over 2% carbon (some of the literature specifies 1.7% carbon as the difference between steels and cast irons). Pig Iron. All iron and steel products are derived originally from pig iron. Pig iron is the raw material obtained from the chemical reduction of iron ore in a blast furnace. The process of reduction of iron ore to pig iron is known as ‘‘Smelting’’. 2.5.1. Cast Iron. Cast iron is pig iron remelted and thereby refined in a cupola or other form of remelting furnace and poured into suitable moulds. As indicated in Figure 2.2, the carbon content of cast irons is from 2 to 6.67%. Most commercial types of cast iron contains between 2.5 and 4% carbon. It is obvious that, with such a high carbon content, cast iron is very brittle and has low ductility. Hence, cast iron can not be cold worked. However, cast iron flows readily when fluid, it is easily cast into intricate shapes that can be machined after cooling and aging. It is the cheapest of the cast materials. Cast iron without the addition of alloying elements is weak in tension and shear, strong in compression and has low resistance to impact. The damping capacity of cast iron is much greater than that of steel. The properties of cast iron can be varied extensively with the addition of alloying elements and proper heat treatment. The primary types of cast iron are : (1) gray cast iron (2) white cast iron (3) malleable cast iron (4) ductile cast iron (5) chilled cast iron (6) alloy cast iron (7) meehanite cast iron (8) mottled cast Iron 1. Gray Cast Iron. When the molten C.I. is cooled, its final structure will depend upon the form in which the carbon solidifies, which in turn will depend upon the cooling rates as well as composition. The control will be affected by the total C and Si (also P) and their total effect is expressed by the ‘carbon equivalent’, (C.E.) : 22 A Textbook of Production Technology Si % + P % C.E.(%) = C% + 3 At relatively high C.E. and slower cooling rates, the solidified cementite, being unstable, breaks up into austenite and graphite flakes. The process is called as ‘‘graphitization’’. The presence of certain elements, of which Si is most important, promotes graphitization. The graphite flakes give the cast iron the gray appearance when fractured, hence the name ‘‘gray’’ C.I. Gray C.I. is the most widely used of all cast irons. In fact, it is common to speak of gray cast iron just as cast iron. It contains 2.50 to 3.75% C and upto 2.5% Si. Gray cast iron is soft, easily machined and only moderately brittle. Its main advantages are : low cost, low melting point, fluidity and good damping capacity. Another good property it possesses is that the free graphite in its structure acts as a lubricant and when very large machine slides are made of it, a very free-working action is obtained. Product Applications. Due to its low cost, gray C.I. is preferred in all fields where ductility and high strength are not required, for example, weights; frames; motor, gear and pump housings; sanitary wares; pipe fittings and gas and water pipes for underground purposes etc. Due to its high compressive strength and good damping characteristics, gray C.I. is used for machine tool bases and supports for structures. Again, due to its fluidity and excellent wear properties, gray C.I. is used extensively in the manufacture of engine blocks, brake drums, sliding surfaces of machines, gearing, gear housings, piston rings, and so on. Gray C.I. has also been used for the manufacture of engine crankshafts, because of its good damping properties, high torsional shear strength and low notch sensitivity. Other product applications of gray C.I. are : Household appliances, manhole covers, cylinder heads for engines and in rolling mill and general machinery parts. 2. White Cast Iron. At low values of C.E., < 3 (C upto 2.5% and Si < 1.5%) and rapid cooling, the cementite will not have sufficient time to break into graphite and austenite. As a result, the total carbon will be exclusively in the combined form of Iron Carbide, Fe3 C (Cementite). It is a very hard and brittle metal with the entire cross-section having a white microstructure. Due to this, the metal is virtually unmachinable except by grinding and so has very limited applications. Product Applications. The use of white C.I. is limited for wear resistant parts such as grinding balls, liners for ore-crushing mills and cement mixers, extrusion dies and some agricultural machinery. However, it is widely used in the manufacture of wrought iron and for making malleable iron castings. 3. Malleable Cast Iron. Malleable castings are first made from white cast iron and then malleabilized by two methods : ‘‘Black heart method’’ and ‘‘white heart method’’ (i) Black heart Method. In this method, the white iron castings are annealed by heating them for a prolonged period of several days (to a temperature of 850 – 1000°C) in air tight pots filled with inert material such as ferrous silicate scale (or iron oxide) or slag. The action of heat and iron oxide partially removes the carbon and reduces the remainder from combined state to a globular form of free carbon, so that after a slow cooling, a strong soft and somewhat ductile casting is obtained. This method, which is also called ‘‘decomposition’’ is used in U.S.A. and the material is Ferritic malleable iron. (ii) White heart Method. In this method (decarburisation), the castings are placed in pots packed with an oxidising material. The oxygen combines with carbon in the castings, reducing its amount to less than 1%. This method is used in Europe, and the material is pearlitic malleable iron. The difference between gray C.I. and malleable C.I. is the form in which the free carbon occurs. In gray C.I., the free carbon occurs in the form of flat or plate like particles, whereas in malleable C.I., the graphite is in the form of irregularly shaped spherical particles which are much Engineering Materials and Heat Treatment 23 more desirable from a strength point of view than flakes. Malleable C.I. is stronger and tougher, ductile, resistant to impact and easily machinable (due to the presence of graphite). Since the carbon change reaches only to a depth of about 10 mm, this process is not suitable for heavy castings. The application of malleable C.I. is considerably more limited than that of gray C.I., because it is more expensive to produce and better mechanical properties are not required in most cases. The use of malleable cast iron usually involves parts of complex shape that often need considerable machining to meet the specifications, such as, : in automotive and agricultural equipment industries (housings, yokes, wheel hubs), hinges, door-keys, spanners, mountings of all sorts, cranks, levers, thin walled components of sewing machines, textile machines and others, brake pedals in cars, spring hangers and so on. 4. Ductile Cast Iron. Ductile cast iron, which is also called as ‘‘Nodular Cast iron’’ and ‘‘Spheroidal cast iron’’ is of higher grade in comparison to malleable cast iron, because, the carbon is precipitated as spherical nodules of graphite which are more perfect spheres than those found in malleable cast iron. To produce ductile cast iron, the molten metal is first completely desulphurised. Then small amounts of special alloys containing magnesium or cerium are added to the molten iron in the ladle causing it, during solidification, to precipitate graphite as small spherical nodules. Ductile cast iron possesses high fluidity, which permits the casting of intricate shapes with excellent combination of strength and ductility. Ductile cast iron can be produced in thicker pieces than those produced by malleable cast iron. Ductile cast iron is stronger, more ductile, tougher and less porous than gray cast iron. So, it is used in parts where density and pressure tightness is a highly desirable quality. These parts include : hydraulic cylinders, valves, pipes and pipe fittings, cylinder heads for compressors and diesel engines. Ductile cast iron is also used to make rolls for rolling mills, many centrifugally cast parts, pulleys, forming dies, pump housings and, in general, for parts subjected to impact loading or requiring a high elastic modulus. 5. Chilled Cast Iron. Quick cooling is called chilling and the iron so produced is chilled iron. It is made by placing ‘‘metal chills’’ inside the mould but near its surface. The molten metal, when poured into the mould, cools rapidly to produce a hard wear- resistant surface (of 1 to 2 mm thick) consisting of white cast iron. Below this surface the material is gray cast iron. Chilled cast iron can only be machined by grinding and is used in making stamping dies, mill and crushing rolls, railway wheels, car wheels, cam followers and so on. 6. Alloy Cast Iron. Alloying elements are added intentionally to cast irons to overcome certain inherent deficiencies in ordinary cast irons to give the required qualities for special purposes. By controlling the rate of graphitization, these elements develop special capabilities, such as better mechanical properties, improved resistance to heat, corrosion, wear, or brittle fracture. Also, alloying can improve both the castability and machinability properties of cast iron. Common alloying elements are : nickel, copper, chromium, molybdenum, vanadium and boron. Note. The reader should not mistake impurities such as Mn, P, S and Si for alloying elements. The process of mass producing steel, cast iron, non-ferrous metals and so on, are not designed for complete removal of all impurities. Below, we discuss the individual effects of these impurities and alloying elements on cast iron. (i) Effect of impurities on Iron. Sulphur. Sulphur is generally considered harmful in C.I. In gray cast iron, it counteracts the graphitizing effect of silicon, lowers fluidity during pouring, decreases strength and makes the 24 A Textbook of Production Technology metal more brittle. So, it should be kept as low as possible, preferably below 0.1%. Manganese. It encourages the formation of carbide and so, tends to whiten and harden cast iron. But it helps to control the harmful effects of sulphur. It has greater affinity for sulphur than for iron and it combines with sulphur to produce manganese sulphide, which is not objectionable. It is often kept below 0.75%. Phosphorus. Phosphorus increases the flowability of gray cast iron. Phosphoric irons are useful for casting of intricate designs and for many light engineering castings when cheapness is essential. Phosphorus induces brittleness in cast iron and it is rarely allowed to exceed 1.0%. Silicon. It is the important graphitizer for cast irons, which makes the cast irons soft and easily machinable. It also produces sound castings free from blow holes because of its affinity for oxygen. It is present in cast irons upto 2.5%. (ii) Effect of alloying elements on Cast Iron Nickel. Nickel is used in cast irons to refine grain structure, increase strength and toughness and increase resistance to corrosion. It has no effect on ductility. It also acts as a graphitizer but is only half as effective as silicon. It, thus, promotes the machinability of cast irons. In low alloy cast irons, its amount is from 0.25 to 5.0%. This alloy is used in steam and hydraulic machinery, compressors and I.C. engine parts. In heat and corrosion resistant cast irons as much as 35% nickel is used. Chromium. Chromium also refines grain structure and increases strength, hardness and resistance to corrosion. It also increases the wear resistance and heat resistance property of cast iron. Gray cast irons which will be subjected to severe wear conditions, often contain chromium (upto 8%). This alloy is used in pumps of all types. The alloy used in higher resistance parts may contain chromium from 10 to 30%. However, chromium tends to prevent graphitization. Copper. Copper is added to cast iron in amounts upto about 1.0%. It increases fluidity for improved mould filling ability, imparts corrosion resistance, and improves mechanical properties, notably toughness and hardness. Machinability of cast iron is also slightly improved, because copper promotes formation of graphite. Molybdenum. The presence of molybdenum in cast iron produces fine and highly dispersed particles of graphite and good uniform structure. This increases the strength and toughness and improve high-temperature strength of cast iron. Its amount ranges from 0.25 to 1.25%. Molybdenum is frequently used in cast iron in combination with nickel or chromium or nickel and chromium. Vanadium is added to cast iron in amounts of 0.10 to 0.50%. It promotes grain refinement, increases strength and increases resistance to fatigue stresses. However, it tends to reduce graphitization. Boron. Until quite recently, boron received little recognition as an addition to regular gray cast iron. 0.05% boron, 3.5% carbon and 1.0% silicon in cast iron help to increase surface hardness and refine structure. This alloy is used for rolls in rolling mills. 7. Meehanite Cast Iron. Meehanite cast iron, produced under patent protection, is made with the addition of a calcium-silicon alloy. Calcium silicide acts as a graphitizer and produces a fine graphite structure giving a cast of excellent mechanical properties. The basic gray cast iron used to obtain Meehanite iron is low in silicon and moderately low in carbon (about 2.5 to 3%). Various grades of Meehanite are produced to meet special requirements. All Meehanite irons have high strength, toughness, ductility and easy machinability. These irons also respond to heat treatment. Meehanite cast iron is ideally suited for machine tool castings. Engineering Materials and Heat Treatment 25 Table 2.1 gives the chemical composition of main types of cast irons (excluding alloy cast irons) Table 2.1 Metal C Si Mn S P Pig iron 3.00 – 4.00 0.50 – 3.00 0.10 – 1.00 0.02 – 0.10 0.03 – 2.00 Gray cast iron 2.50 – 3.75 1.00 – 2.50 0.40 – 1.00 0.06 – 0.12 0.10 – 1.00 Malleable cast iron 2.20 – 3.60 0.40 – 1.10 0.10 – 0.40 0.03 – 0.30 0.10 – 0.20 White cast iron 1.75 – 2.30 0.85 – 1.20 0.10 – 0.40 0.12 – 0.35 0.05 – 0.20 8. Mottled Cast Iron : It is mixture of grey and white cast irons in which the outer layers have the structure of white cast iron and the core, that of grey cast iron. It is obtained by heating cast iron to red hot with powdered red hematite in an oven. This cast iron possesses increased toughness. 2.5.2. Heat Treatment of Cast Iron. Several types of heat treatments are used to alter or to enhance some properties of cast irons and thus to increase their usefulness. Aging. It is applied to relieve the casting stresses without materially affecting physical properties. It is carried on during 1 to 5 hours in the temperature range of 450°C to 550 °C. Annealing. It is carried out for 1 to 5 hours at 660°C to 870°C, the temperature depending upon the size of the part. It is intended to reduce hardness and to facilitate machining. However, annealing is done at the expense of some strength. Baking. It is applied to castings that have been pickled in acid to remove sand and scale. Pickling makes castings brittle but baking for a few hours at 150°C removes this brittleness. Quenching. Quenching cast iron in oil or water after it has been heated above the critical range increases its hardness and also its brittleness. Drawing or tempering. This is done by reheating the quenched metal to a temperature below the critical temperature. It reduces the brittleness but still leaves an increase of hardness. By such a treatment a Brinell hardness number from 200 to 400 can be attained, the value depending on the quenching and tempering temperature. 2.5.3. Wrought Iron. The word ‘‘Wrought’’ means that the metal possesses sufficient ductility to permit hot and/or cold plastic deformation. Wrought iron is a mixture of pure iron and 1 – 3% slag. It also contains traces of carbon, silicon, manganese, sulphur and phosphorus. It is made in this manner : First of all, all the elements in the iron (C, Si, Mn, S, P) are removed, leaving almost pure iron. The molten slag from the open- hearth furnace is then intentionally added into vessels containing pure iron and thoroughly mixed into it. The final mix is then squeezed in a press to remove excess slag and reduced into billets by a rolling mill. The material will consist of fibres of pure iron separated by thin layers of slag material. These layers of glass like slag material acts as barriers to corrosion which may attempt to penetrate the iron. The billets can be reheated to form bars, tubing, plates, structural shapes, pipe, forgings, bolts and nuts, nails, rivets, chains, crane hooks, railway couplings, barbed wire, boiler tubes, fittings, and so on. Wrought iron is ductile and soft and is most readily forged and forge welded. It can with stand sudden and excessive loads. It can neither be hardened nor tempered like steel. The strength of wrought iron can be increased by alloying, typically with nickel (1.5 – 3.5%). The ultimate strength of wrought iron can be increased by cold working and subsequent aging. A typical chemical composition of wrought iron is as given below :- 26 A Textbook of Production Technology C : 0.02 – 0.08%, Si : 0.10 – 0.20%, Mn : 0.02 – 0.10% S : 0.02 – 0.04%, P : 0.05 – 0.20% 2.5.4. Semi-steel. Semi-steel is not steel, but the name is given to the product made by meltinmg 20 to 40% steel scrap with cast iron in the cupola. The product is a tough, close-grained cast iron. 2.5.5. Steels. Steels are alloys, the essential ingredients of which are iron and carbon (upto 2%). The carbon is distributed throughout the mass of the metal not as elemental or free carbon but as a compound with iron. Steels also contain definite amounts of inevitable impurities which include silicon, manganese, sulphur and phosphorus. Alloying elements are added to these plain carbon steels to produce special purpose steels. Classification of Steels. Steels can be grouped into four main categories : Plain carbon steels, Alloy steels and Special alloy steels, and Cast steels 1. Plain Carbon Steels. Plain carbon steels are those containing only two elements – iron and carbon. Silicon, manganese, sulphur and phosphorus exist as impurities and not as ingredients. These constituents have negligible effect on steels when their extent does not exceed: 0.3 – 0.4% Si, 0.5 – 0.8% Mn, 0.08% P and 0.04% S. Plain carbon steels can be classified according to their carbon content: (a) Low carbon or mild steel 0.05 to 0.30% C (b) Medium carbon steel 0.30 to 0.60% C (c) High carbon steel 0.60 to 1.50% C Sulphur is a harmful impurity in steel. It combines with iron chemically to produce iron sulphide which forms in the grain boundaries. Iron sulphide, because of its low melting point, produces red-shortness in steels, that is, causes brittleness at forging temperatures. Phosphorous is also a harmful impurity in steels, because it causes brittleness. Silicon is a very good deoxidizer. It removes the gases and oxides, prevents blowholes and thereby makes the steel tougher and harder. Manganese also serves as a good deoxidizing and purifying agent. It also combines with sulphur to form manganese sulphide and thereby reduces the harmful effects of sulphur remaining in the steel. When used in ordinary low carbon steels, it makes the metal ductile and of good bending qualities. (a) Low Carbon Steels. Low carbon steel is used extensively to make industrial products and also in the construction industry. The product applications include : pipes, tubes, storage tanks, railroad cars, automobile frames, nuts, bolts, automobile bodies and galvanized sheet steel. These steels are soft, very ductile, easily machined, easily welded by any process. Since the carbon content is low, these steels are unresponsive to heat treatment. Free-cutting Steel. These carbon steels have an increased sulphur content (resulphurized steels) which result in a relatively high manganese sulphide, MnS content of controlled globular shape. This steel has excellent machining properties. Free cutting or free machining steels may contain an insoluble, soft element, primarily lead (0.15 – 0.35%) (leaded steels) instead of higher percentage of sulphur. The drawback of these steels is that they become less ductile and fatigue strength and tensile strength are slightly reduced. The wear of cutting tools (due to reduced ductility) can be reduced with affecting the mechanical properties of the steel by the use of calcium as a deoxidizing agent. When cutting such steels, a complex, low-shear-strength oxide forms on the rake face of the tool. When ‘‘Tellurium’’ is added to a leaded steel, it will further improve the machinability of steels. Its content is 0.03 – 0.05%. This free machining steel is extensively used in automatic screw machines. Engineering Materials and Heat Treatment 27 (b) Medium Carbon Steels. These steels can be hardened and tempered. Thus, these steels can be used for products requiring greater strength and wear resistance. Typical product applications include : forgings, castings, axles, shafts, crank shafts, connecting rods, and any machined part that requires a greater strength than that can be provided by low carbon steels. (c) High Carbon Steels. These steels respond better to heat treatment as compared to medium carbon steels. So, these are used to make products which must have high strength, hardness, and good resistance to wear. This steel is often available in annealed state and the finished product is then heat treated to its proper hardness. Typical product applications include : forgings and a wide variety of tools, such as drills, taps, reamers, dies and hand tools. This steel is also used for making products requiring edges, for example, cutlery, chisels, shear blades, planer tools and so on and also for spring wire, and for cable and wire rope. These steels are not so ductile as the medium carbon steels. In the higher carbon ranges, the extreme hardness is accompanied by excessive brittleness. The higher the carbon content, the more difficult it is to weld these steels. 2. Alloy Steels. A steel is said to be alloyed when its composition incorporates specially introduced alloying elements, absent in plain carbon steels, or when the silicon and/or manganese content exceeds the usual percentage. As already discussed, the alloying elements are added to modify and/or enhance the properties of steel, that is, to achieve one or more of the following aims :- to produce fine grained steel to improve wear resistance, corrosion resistance to improve hardenability and hardness to improve machinability to improve weldability to improve electrical properties to improve physical properties at high temperatures to improve tensile strength, ductility, elastic properties etc. The most frequently employed elements for alloying steel are : Cr, Ni, Mn, Si, Mo, Va, W, Cu and Al, and to a lesser extent cobalt, beryllium, Titanium and boron. Often combinations of alloying elements are used. Effects of Alloying elements in Steel Chromium. The addition of chromium results in the formation of various carbides of chromium which are very hard, yet the resulting steel is more ductile than a steel of the same hardness produced by a simple increase in carbon content. Chromium also refines the grain structure so that these two combined effects result in both increased toughness and increased hardness. The addition of chromium increases the critical range of temperatures and raises the strength at high temperature. Alloy of chromium resists abrasion and wear. Chromium also increases resistance to corrosion and oxidation. The amount of chromium may be from a fraction of a percent to about 30%. Nickel. It also increases the critical range of temperature. Nickel is soluble in ferrite and does not form carbides or oxides, and thus increases the strength without decreasing the ductility, Case-hardening of nickel steel results in a better core than can be obtained with plain carbon steels. Chromium is frequently used with nickel to obtain the toughness and ductility provided by the nickel and the wear resistance and hardness contributed by the chromium. The amount of nickel may be upto 50%. Manganese. It is added to all steels as a deoxidizing and desulphurizing agent, but if the sulphur content is low and the manganese content is high (over one per cent), then it is classified as a manganese alloy. It lowers the critical range of temperatures. It increases the time required for 28 A Textbook of Production Technology transformation, so that oil quenching becomes practicable. The percentage of manganese varies from 0.4 to 2.0 and 11 to 14%. Silicon. It is added to all steels as a deoxidizing agent. When added to very-low-carbon steels it produces a brittle and a high magnetic permeability. The principal use of silicon is with other alloying elements, such as manganese, chromium, and vanadium, to stabilize the carbides. The usual amount of silicon is upto 0.8%. However in heat resisting stainless steel it can be upto 3%. Molybdenum. It acts very much like chromium but is more powerful in action. It also increases the depth of hardening after heat- treatment. Molybdenum finds its greatest use when combined with other alloying elements such as nickel, chromium or both. Nickel molybdenum and nickel- chromium-molybdenum steels retain the good features of the nickel chromium steels and in addition have better machining qualities. Molybdenum increases the critical range of temperature. Except for carbon, it has the greatest hardening effect and results in the retention of a great deal of toughness. It varies from 0.20 to 0.70%. Vanadium. It is used to toughen and strengthen the steel to reduce the grain size and to act as a cleaner and degasifer. It has the desirable effect of increasing the life of tools, springs and other members subjected to high temperatures. As vanadium has a very strong tendency to form carbides, hence it is used only in small amounts, 0.2 to 0.5% in alloy carbon tool steels and 1 to 5% in high speed steels. Tungsten. It is widely used in tools steel because the tool maintains its hardness even at red heat. Tungsten produces a fine dense structure and adds both toughness and hardness. Its effect is similar to molybdenum except that greater quantities must be added. The amount of tungsten in steel can vary from 0.4 to 22%. Cobalt. Cobalt is commonly used in high speed to increase the hot hardness so that the cutting tools can be used at a higher cutting speeds and temperatures and still retain their hardness and a sharp cutting edge. Its content ranges from 5 to 12%. Copper. Copper lowers the critical temperatures. Copper is added to steel (from 0.15 to 0.30%) only to improve its resistance to atmospheric corrosion. When more than 0.75% copper is added, steel can be precipitation hardened. Aluminium. Aluminium deoxidizes efficiently, restricts grain growth and is the alloying element in nitriding steel. Its content ranges from 1 to 5%. Sulphur. It is an undesirable impurity in steel because its forms iron sulphide, which can result in cracking. However, in the presence of proper amount of Mn, it forms Mn S which improves the machinability of steels. Its content may very from 0.06 – 0.30%. Boron. Boron (not exceeding 0.003%) is very effective in increasing the hardenability of low and medium carbon steels. It has no effect on tensile strength of steel. Classification of alloy steels. Alloy steels may be classified according to—their chemical composition, structural class and purpose. (a) Classification according to chemical composition (i) Three component steels, containing one alloying element in addition to Fe and C. (ii) Four component steels, containing two alloying elements, and so on. (b) Classification according to structural class (i) Pearlitic. This class of steel is obtained when the amount of alloying elements is relatively small (upto 5%). It has good machinability and its mechanical properties are considerably improved by heat treatment. (ii) Martensitic. The amount of alloying elements is greater than 5%. These steels have a very high hardness and present difficulties in machining. Engineering Materials and Heat Treatment 29 (iii) Austenitic. A very large percentage (10 to 30%) of certain alloying elements (Ni, Mn or Co) enables the austenite structure to be retained in steel at room temperature. This class includes : stainless steels, nonmagnetic steels and heat-resistant steels. (iv) Ferritic. This class contains a large amount of alloying elements (e.g. Cr, W or Si) but has a low carbon content. These steels do not respond to hardening. In the annealed condition, their structure comprises alloyed ferrite and a small amount of cementite. (v) Carbidic or ledeburitic. These steels contain considerable amounts of carbon and carbide-forming elements, Cr, W, Mn, Ti, Nb and Zr. (c) Classification according to purpose Alloy steels can be classified as :- Structural steels, tool steels, and special alloy steels. Structural steels are used to make machine components, structural components and structures. Structural steels may be either of straight carbon or alloy types. Their carbon content does not usually exceed 0.5 or 0.6%. (i) Alloy Structural Steels. Alloy structural steels are divided into three groups depending upon the total content of alloying elements : low-alloy : upto 2% medium-alloy : 2 to 5% high-alloy : 5% After suitable heat treatment alloy structural steels acquire higher mechanical properties than carbon structural steels. This is due to the higher hardenability and, consequently, more uniform p