Forging and rolling are the most common examples of direct compression type ... Rolling, forging and extrusion .... Cracks and blow holes are welded up.
CHAPTER
8
Forming Processes 8.1
INTRODUCTION
In a forming process metal is deformed to attain specific shape. Forming is defined as changing the shape of the material without actually removing any part of it. Food containers, vessels and heating ducts are well known examples of objects that are formed. The deformation in forming process is plastic. If deformation of metal occurs at temperature lower than its recrystallization temperature then the process is called cold forming (Ref. Chapter 4). This method has become very popular with manufactures of sheet products. The most versatile cold forming methods, that find very common applications are bending and drawing. These manufacturing operation produced long continuous products, such as plates, sheets, tubings and bars with various cross-sections. Rolling, extrusion and drawing processes are capable of making complex sections of metallic materials. The overall usefulness of metals and their importance in modern technology are due largely to the ease by which they can be formed into useful shapes. Nearly all metal products undergo metal deformation at some stage of their manufacturing if they are not cast. By rolling, cast ingots and slabs are reduced in size and converted into basic forms such as sheets, rods and plates. These forms then undergo further deformation to produce wire, forging, extrusion, sheets, etc. The forming processes are divided into categories depending upon the workpiece. Following is the classification: 1. 2. 3. 4. 5.
Direct compression-type processes. Indirect compression-type processes. Tension-type processes. Bending processes. Shearing processes.
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Forging and rolling are the most common examples of direct compression type process in which force applied to the surface of metal work piece causes metal to flow right angle to its direction. Fig. 8.1 shows rolling.
Fig. 8.1
Rolling
In indirect compression, the force of compression develops between the work piece and the die. The directly applied force may be tension (as in wire drawing) or compression (as in extrusion and deep drawing). Fig. 8.2 shows these operations. In these processes the high compression force is developed in atleast one principal direction.
Fig. 8.2
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In stretch forming a metal sheet wrapped round the contour of a die, is pulled by tensile force applied at two edges. Wrinkle free surface is produced and force on die is less. The process called stretch forming is shown in Fig. 8.3.
Fig. 8.3
Bending may look like deep drawing but keeping the edges free of force using punch to make only line contact with the work piece (Fig. 8.4) exerts a bending moment on the sheet.
Fig. 8.4
Shearing is performed by punch to cause rupture of plate all along the line of cut. The shearing force applied has sufficient magnitude to achieve cutting as shown in Fig. 8.5.
Fig. 8.5
All the processes described above achieve the objective of changed shape through plastic deformation of metals. The plastic deformation also improves properties of metal by altering the distribution of microconstituents, refining the grains and by inducing strain hardening. The process usually starts from an ingot or billet which are changed into simple shapes like sheets, plates or bar through primary mechanical working. The final finished shapes are obtained through secondary mechanical working. The former process is also known as
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processing operation while latter as fabrication. The ingot and billets are normally obtained from solidification of melt and hence contain several dendritic structures and boundaries. These are sites of weakness and often are the cause of brittle fracture. Under the compressive force these dendrites and boundaries break and between the fragments the sound and ductile matrix flows giving homogeneity to entire structure. Rolling, forging and extrusion are very effective in breaking brittle structure in billet and ingot.
8.2
EFFECT OF TEMPERATURE ON FORMING PROCESS
Forming processes may be performed at ambient temperature or metal may be heated before application of forces. The plastic deformation will be correlated with the movement of line defects (You will learn about line defects later). However, it can be said here that the plastic deformation may cause displacement of these defects in such a way that their energy is high. The defects however, undergo redistribution in positions of lower equilibrium energy. This redistribution of defects is termed recovery. In hot working process such temperatures and strain rates are maintained that deformation and recovery take place simultaneously. Contrarily, the recovery does not take place during cold working. Since recovery keeps on rearranging line defects, in hot working large deformations are possible. In hot working the deformation occurs at almost constant stress and hence energy required for deformation is much less than that in cold working. In cold working the strain hardening keeps on strengthening the metal and hence increasing stress is required to be applied as the deformation proceeds. The total deformation in cold working is much less (of course greater deformation may cause stress equal to ultimate tensile strength leading to fracture). It is possible to restore the capacity of metal for plastic deformation by process annealing which is process in which metal between two cold working operations is heated and cooled slowly to redistribute line defects. It may be realized that the hot working temperatures will be different for different materials, mainly depending upon their melting points. Lead and tin, for example, are hot worked at room temperature as the recovery in these metal takes place at room temperature. On the other hand working tungsten at temperature of 1100°C is still cold working while this is a hot working temperature of steel.
8.3
HOT WORKING
Hot working imparts ductility and toughness to the worked metal. Several defects of ingots and billets obtained directly from solidified metals (cast metals) are removed. Some of the defects of as cast ingots are dendritic grains which form columns, blow holes, non- homogeneous distribution of defects and impurities. Hot working removes these defects as already pointed out. However, hot working has the greatest disadvantage of allowing reaction between surface of metal and oxygen of atmosphere. All metals suffer from this defect and particularly in case of steels the oxides get rolled over in the surface making surface finish still more
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difficult. Further the hot rolling cannot produce closer tolerance and larger finishing allowances need be provided. Cold working on the other hand can provide much better finish, closer tolerance and has no difficulty of surface oxidation. Yet another problem with hot working is that surface gets cooled at much faster rate and the inner bulk of material cools very slowly. This results in finer grain formation in the upper layer while coarse grains inside. It is found that a cold worked metal will result in better homogeneous grains if annealed and normalized properly. The higher limit of hot working is normally about 40ºC below the melting point so that if a lower melting point material is present in segregated region it will not melt to liquid state. A very thin layer of molten metal may cause the metal to crumble. This condition is known as hot shortness. The lower temperature of hot working is determined by recrystallization or recovery to occur conveniently. Whole strain hardening effect must be eliminated during the deformation only. If deformation is large the recrystallization temperature is lowered. Hence the lower limit of temperature of hot working may be further reduced if large deformation is effected. Most hot working operations are done in several stages or passes. The temperature in intermediate pass is kept well above the minimum hot working temperature so that energy in deforming metal may be saved. The working temperature in last pass is just above the minimum hot working temperature. It is just enough to ensure that grain growth during cooling is negligible.
8.4
COLD WORKING
Cold working of a metal is carried out below its recrystallization temperature. In this process metal is forced into new shape at room temperature. During cold working no recovery takes place and strain hardening keeps on increasing strength and reducing ductility. Excessive increase in strength and reduction of ductility may cause the fracture before desired shape is achieved. To eliminate this possibility cold working is divided between steps and between two steps the metal is annealed. In certain cases the process annealing may be costly (such as reactive metals needing controlled atmosphere). However, such increase in cost may be tolerated in return of a well homogenized and stress relieved product. In case of cold working the final pass may be designed not only to achieve desired shape but also the desired strength through strain hardening. The final cold reduction following the last annealing is decided depending upon what strength is desired in the product. The cold working condition achieved finally is described as temper. Thus the final temper can be anneal temper when metal is soft because final treatment was annealing. The other achievable conditions are quarter-hard half-hard, three quarter hard and full hard. Spring temper is the final cold worked state to result in high strength and low ductility.
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8.5
DIFFERENCE BETWEEN HOT WORKING AND COLD WORKING
S.No.
Hot Working
Cold Working
1.
It is carried out above the crystallization It is carried out below the recrystallization temperature, so deformation of metal and temperature so no recovery takes place recovery take place simultaneously. during deformation.
2.
Due to recrystallization, no hardening of The metal gets hardened because working metal take place. is done below recrystallization temperature.
3.
Mechanical properties of metal such as Cold working decreases the elongation, elongation, reduction of area and toughness reduction of area as well as toughness. are improved.
4.
No internal stresses are set up in the metal. Internal stresses are developed in cold working processes.
5.
Hardness, fatigue strength, yeild point, Hardness, fatigue strength, yield point, resistance to corrosion are not-affected by this strength are improved. It decreases the resistance to corrosion. process.
6.
Cracks and blow holes are welded up.
Possibility of new crack formation and its propagation is increased.
7.
Scaling is minimum.
No oxidation take place on the surface.
8.
Surface finish is poor.
Better surface finish is obtained.
9.
Hot working promotes uniformity of Cold working is economical for ductile material, resulting in homogeneous material. structure.
10.
Less powerful equipment is needed.
8.6
More powerful equipment is needed.
ROLLING
Metal sheets or plates are produced by hot rolling processes in which workpiece is squeezed between two or more rotating cylinders. The workpiece is pulled into the space between the rollers by friction between sheets and rollers. Very large heated ingots undergo large deformation in hot rolling. Cold rolling is used for finishing operation or for thinner sheets. The rolling equipment or machine is called rolling mill. When two rolls only are used to squeeze the metal sheet in between it is called two high rolls. 3, 4, and 6 high using 3, 4 and 6 rolls respectively are also used as shown in Fig. 8.6. The two high mill has the obvious disadvantage that for further squeezing the sheet has to be brought back to the entrance. Reversible two high arrangement is better suited for further rolling the job. Three high mill as shown in Fig. 8.6 (c) takes the workpiece in as it exits. The power is supplied to the middle roller and others rotate by friction. The small diameter rollers help decrease the power as in Fig. 8.6 (d). It is four high mill. The cluster mill at (e) is good for very thin sheets. Fig. 8.7 shows continuous strip rolling.
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Fig. 8.6
Fig. 8.7
Typical arrangements of rolls for rolling mills. (a) Two-high pull-over; (b) two-high, reversing; (c) three-high; (d) four-high; (e) cluster.
Schematic drawing of strip rolling on a four-stand continuous mill
Ring rolling is a process of rolling the rings. The thickness of ring is reduced by squeezing between two rolls while edges are finished by two conical rolls (Fig. 8.8). In ring rolling the cylindrical roll working on outside of the ring is larger and is power driven while inner roll of smaller diameter simply presses against internal surface. Ring, gear blanks, rims and heavy wheels are produced by this method. A number of small rollers are attached to the surface of large rolls just like rolling elements of a roller bearing. These small rolls acting like planets to large rolls are used to press the material passing between two planetary rolls. The configuration is used for the purpose of transmitting rolling pressure more effectively to the work piece. As compared to
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thickness reduction of 2 : 1 in normal rolls, a thickness reduction of 25 : 1 is obtainable in planetary rolls. However, movement of rolls on two sides need to be synchronised that two smaller rolls are always opposite to each other.
Fig. 8.8
Ring rolling
Normally rolling is a hot working process. Materials commonly hot rolled are aluminium, copper, magnesium and their alloys and also steels of many grades. Typical parts produced by hot rolling are rear axle of cars, gear shift lever, leaf springs and aluminium propellers. In hot rolling process the recrystallization resulting in refinement of grains occur as the temperature in the final stage drops to recrystallization temperature. However, the surface is not well finished. Cold rolling is done to obtain better finish and close tolerances as well as grain elongation along length which helps improve mechanical strength as shown in Fig. 8.9. (Also ref : Sec. 4.8 and 4.9, Fig. 4.13). Apart from specific parts (mentioned above) rolling is used to produce various sections with appropriate grooves in rolls. Round bars, half round bars, flat bars and bars with such sections as angles, channel, T and I are produced by hot rolling. Thin strips are produced by cold rolling as shown in Fig. 8.9(b).
(a) Hot rolling
(b) Cold rolling Fig. 8.9
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Fig. 8.10 shows some typical roll-pass sequences used in the production of various structured shapes (Sections).
Fig. 8.10
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8.7 EXTRUSION A process in which metal is compressed between a moving punch and fixed die to a stress level higher than its elastic limit so that the metal may flow out of an opening is called extrusion as shown in Fig. 8.11. A tooth paste coming out of the mouth of the tube is a good example of process of extrusion and a very similar situation is shown in Fig. 8.12 (a) in which a punch is pressing the metal out of the opening in the die fixed at the end of the container. The opening can take various positions and shapes, e.g.; opening may be on the side of container. A piston like punch forcing metal in its own direction, effects forward extrusion. If the end is closed and metal is compressed by a hollow punch then the metal is extruded through the hole of the punch in backward extrusion as shown Fig. 8.12(b). The process of extrusion may be performed on hot or cold metal. Hollow tubes are extruded by passing a mandrel through hollow punch and then pressing the workpiece in the forward extrusion or by force a solid punch of diameter smaller than that of the container to effect backward extrusion, as shown in Fig. 8.12 (c) and (d).
Direct extrusion 1. Extruded metal 2. Die backer 3. Die 4. Billet 5. Dummy block 6. Pressing stem 7. Container liner 8. Container body Fig. 8.11
Process of Extrusion
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Fig. 8.12
Types of extrusion Process
Aluminium heated to a temperature of about 80ºC is extruded in different complex sections used for structural purposes. In hot extrusion wear and tear of die is a problem. Dies are made from heat resistant steels or tungsten carbide. Cold extrusion process performed on softer materials is basically similar to hot extrusion process described earlier. Combined forging and extrusion is used to produce stepped parts like poppet valve. The stem of the valve is extruded through a die while head is forged by the punch. The metal is worked hot (Fig. 8.13).
Fig. 8.13
Combined forging and extrusion
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Extrusion process is used to manufacture rods, tubes, a variety of circular, square, rectangular, hexagonal and other shapes both in the solid or hollow form as shown in Fig. 8.14.
Fig. 8.14
Extruded sections of (a) solid, (b) semihollow, and (c) hollow configuration
In short the extrusion provides advantages of (i) faster process, (ii) good tolerance, (iii) mechanical properties better than in rolled products, (iv) good surface finish, and (v) possibility of extruding complex sections.
8.7.1 Defects Defects in extrusion may arise either due to process or due to the geometry of the forming pass. The possible defects are: Fracture (internal and external), sinking or surface inclusions (which could be on raw side or container side). 10-15% wastage is often found in extrusion. The defects are attributed to one or all of the following: 1. Defective billet 2. Unsuitable tooling 3. Processing technique However, it is found that correct design of tooling can eliminate all defects. If we look at the total process extrusion involves distinguisible steps, as outlined below: (i) Preparation of billet (ii) Heating (iii) Lubricating (iv) Providing pilot hole (v) Reheating and lubricating (vi) Extruding (vii) Removing the lubricant (viii) Straightening whenever required. It may not be necessary in case of some materials.
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The billets are prepared by machining or sometimes ground. A surface finish of 7.5 µm is often provided on billet which increases the cost of extrusion. The billet is preheated to reduce the of process but the finished part needs descaling. The higher heating cost may be acceptable if heating does not cause scaling. Induction furnace heating or heating in gas fired furnace upto 800ºC and heating in a salt both avoid scaling to a great extent. For lubrication graphite has been used between surfaces with relative motion. In case of stainless steel it has given good results. Lately glass by means of pads, fibres or pressed powder is in use with good results.
8.8
FORGING
Forging is hammering or pressing of metal into a useful or desired shape. The machine forging came into practice in second half of nineteenth century while earlier only blacksmith’s hand forging was normal practice. The forging today is a versatile manufacturing process applicable to a bolt and a turbine rotor equally efficiently. Forging is normally done hot by using a forging hammer or dropping hammer. Such hammers give rapid blows to heated workpiece surface. Forging press subjects the workpiece to compressive force at slow speed. Impact forging causes maximum force to be absorbed in the surface only because energy of the blow reduces immediately after the strike. On the contrary in press forging the force increases as pressing continues and maximum value is attained just before the pressure is released. Press forging results in deformation that extends deep into the material.
8.8.1 Classification of Forging Forging is a bulk deformation process in which the mass of final product is almost equal to that of raw material. Big and small size products can be shaped by forging with an advantage of control of grain direction resulting into favorable toughness. Forging can be performed hot or cold with large pieces being subjected to hot process for the reason of reducing force required. Forging consists in heating the work piece and squeezing the piece under pressure between dies or beating it with hammer having a die at its end and placing the workpiece on another die or flat surface called anvil. If the end of the hammer and surface of the anvil both are flat the process is called open die forging as shown in Fig. 8.15. Dies with open ends but v-groove or half round groove are also included under class of open dies. Forging with open dies is also known as flat die, hand or smith forging or upsetting. This method is economical but only limited number of parts can be forged. Shafts and gear blocks are good examples. The resultant shape of an upset part depends upon the frictional effects between dies and faces of the billet. In efficiently lubricated process the decrease in height will be accompanied by increase in diameter, very negligible barrelling effect (i.e., larger diameter in the middle of the height of the forged part is seen). In case of poorly lubricated or no lubricated surface the barelling effect is significant. This effect depands upon original ratio of height to diameter and final reduction in height.
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Accurate dimensional finish is not obtained in open forging and machining is used for finishing. This machining results in wastage of some material and additional cost.
Fig. 8.15
Open die forging
Closed die forging or impression die forging is the process in which work piece is completely enclosed between dies. The raw material is placed between the upper and lower dies having shaped cavities and struck with blows from top until metal fills the cavities of dies completely. This is fundamental requirement of close die forging. Additionally it is required that the structure of the material should remain sound. By the nature of operation it is impossible to obtain finished product in a single stroke (strike or compression) hence several stages are involved. Smaller pieces are easily formed by this process whereas large pieces can be forged by flat die forging. In closed die forging extra metal is often provided at start and when forged product is finished extra metal flows into recess provided in the dies. Fig. 8.16 shows view through closed die forging. The recess is called gutter while the extra metal protruding from the edges of the finished forging is called fin or flash. The fin is removed in a subsequent operation called trimming. Powder forming in closed dies compacts the preformed powder product. The general shape is given to preform by compacting metal powder cold. This preform is sintered in a furnace in controlled atmosphere for sometimes and removed in hot condition to the closed dies where it is compressed to required shape. No waste is generated as exact amount required to produce the shape is used. The product from powder forging is superior to that obtained from sintering (A Powder Metallurgy Technique).
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Fig. 8.17
8.8.2
Sectional view through closed-die forging
Forging Operations
Several steps are distinguished in forging operations. Forging operations in general include the following: (i) Flattening: This operation is simply pressing a work piece between two flat dies, thus decreasing dimension in the direction of force and increasing them in the transverse direction (Fig. 8.15). The operation is also called upsetting. If the purpose is to elongate the material in transverse direction the process is also referred to as cogging. (ii) Edging: Edging is performed by trapping material between two cup shaped dies whereby metal flows from edges to inside, thus creating bulge in the middle. This is also known as gathering as metal is gathered in the middle (Fig. 8.17).
Fig. 8.17
Edging
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(iii) Fullering: It is opposite of gathering in which metal is displaced away from center, thus reducing thickness in the middle and increasing length (Fig. 8.18).
Fig. 8.18
Fullering
(iv) Drawing. If the thickness is reduced continuously at different sections along the length (Fig. 8.19) it is called drawing. The operation uses convex dies. If the concave dies are used to produce bar of smaller diameter the process is called swaging.
Fig. 8.19
Drawing
(v) Wire Drawing: A rod is pulled through a hardened hole (a die), cold or hot, wire of smaller diameter is produced. The process is called wire drawing. The die is often a tapered hole (Fig. 8.20).
Fig. 8.20
Wire drawing
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(vi) Bending: Dies are so shaped that they introduce directional change in the work piece rather than the size of section. The operation is used effecting directional changes symmetrical on two sides such as in crank shaft. Before bending, operations like fullering; edging and drawing would have been imparted as in Fig. 8.21.
Fig. 8.21
Bending
Piercing and the Punching are other two forging operations shown in Fig. 8.22.
Fig. 8.22
(vii) Blocking: The workpiece that has passed through several steps of forging is given finishing touches during blocking to obtain final shape complete with corners, holes and fillets, etc. blocking may be the final step in some cases where the part may be put to use but often it is followed by post forging operations described here. (i) Trimming is removing fins or flashes. It is done by shearing. (ii) Planishing is performed to remove burrs left after trimming. It is done by closely fitting polished dies shaped to fit the final product to exact dimensions or by rolling hot or cold. (iii) Coining: The planished workpiece is pressed in well finished dies to obtain closer tolerance or to align various areas of the workpiece.
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8.8.3
Equipment for Forging
Forging would normally require large force though hand forging was the predecessor. Machine forging is done either by hammer or press. Both have two dies. One supports and other strikes. Dies can be flat or shaped. The important difference is that while greatest force in hammer forging acts at the moment of strike the force in press forging increases from the moment of contact till a maximum is reached. In hammer forging a weight is allowed to fall from a height on the workpiece. The hammer is lifted up in three different ways, giving three names to equipment. Drop Hammer: Hammer at the end of a board is lifted by friction wheels run by a separate motor. At certain height one wheel separates and allows the hammer to fall. The weight that falls could be as high as 16500 kgf. Small one could be 220 kgf. Drop hammers are identified by their weight and are used for small forgings weighing a few kgs. Drop hammer also known as board hammer can make 70 strokes in a minute (Fig. 8.23(a)). Air Lift Hammer: This hammer or ram can be lifted by air pressure acting on the bottom of piston at the end of the hammer. Its lift can be adjusted whereby varying amount of energy is delivered at the job. The weight varies between 1000-22000 kgf. Steam Hammer: This hammer is used for very heavy forging as weight of hammer varies from 2200 to 110000 kgf. The end of the ram has a piston and cylinder. Steam pressure at the bottom raises the piston. During its fall the steam may act on piston top to increase force on the workpiece as shown is Fig. 8.23 (b).
Fig. 8.23(a)
Board hammer
Fig. 8.23(b)
Steam hammer
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Helve and Trip Hammers: These are used for small forging force and are often used for planishing and coining. Helve hammer is attached at one free end of an arm which moves on a hinge in the middle and other end is pushed up or down by motor, [Fig. 8.24 (a)]. The trip hammer is attached to crank pin of a crank shaft [Fig. 8.24 (b)].
Fig. 8.24
High energy rate forging machines in which ram moves at about ten times the speed of other forging machines have specially been developed for upsetting operations and making gear blanks. In many of these machines the lower die or anvil also moves upwards as the hammer descends. Such movement cancels the heavy impact and eliminates need for very heavy base. The ram is lifted by hydraulic pressure and held in uppermost position by a lock in a cylinder in which an inert gas enters to trigger downward motion. Forging presses apply heavy forces on dies in which are set workpieces which are squeezed to fill into cavities. Mechanical press operates through a crank shaft (like trip hammer) or gears. Hydraulic presses operate by hydraulic pressure acting on pistons. The mechanical press produces force in the range of 90 to 900 tonnes while hydraulic press can generate force upto 220 kilo tonnes.
8.8.4
Heating
The material to be forged is heated before it is placed on the anvil. While some of the machines use induction or electrical resistance method of heating the bulk of the operation is performed on a directly or indirectly heated workpieces. The indirect heating is used in case of materials whose surface may be oxidized if contacted with fire. The furnaces may have batch or continuous operation. In a batch operation a number of pieces are heated together in the furnace and loading and unloading is done through a
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single door. The heated material is quickly removed to forge hammer. In continuous heating process the material enters at one end and leaves at the other and then is carried to the anvil of machine. Muffle and radiant tube furnaces are indirect heating furnaces. The preheat temperature in hot forging is selected as high as possible to obtain to take the advantage of low flow stress. However, this advantage is accompanied by cost of heating, cost of handling and oxidized and decarburized surfaces. On the other hand cold working which results in good surface finish is accompanied by high tooling cost. However, the cold forging causes the strain hardening which helps improve final strength and there is no wastage of material. To have advantages of both types of forging warm forging with preheating at about 800ºC in case of low carbon steel is preferred. This reduces cost from hot forging slightly. Alternatively warm forging may be performed on low carbon steels which are previously quenched and tempered. The warm forging will make the material stronger and ductile.
8.9
BENDING
Hundreds of shapes can be produced by process of bending which is a process of straining sheet, plate, bar, rod, wire, tube, etc. along a straight axis. A few of the shapes that are produced by bending are shown in Fig. 8.25. Softer metals are easy to blend and can be bent over 180°. Thicker and harder metals are bent to larger radius of bend and smaller angle. Maintaining direction of bend along grains and bend radius equal to thickness can avoid cracking of metal on tension side. Sharper bend (smaller radius of bend) may cause cracking. Bending closer to edge is to be avoided. Recommended distance of bend from edge is 1.5 times the thickness plus radius of bend. Overbending is the bending in terms of radius in excess of that required. This is done to take care of spring back of bent part. Spring back is elastic recovery of deformation, i.e., after deforming forces are removed the part recovers the deformation partly. It may be a small fraction of total deformation required. For example if a sheet is to be finally bent to an angle of 90° in cold process, it should actually be over bent by 1° or 2°. (See Sec. 4.3.1, plasticity). Brake bending, Press-brake bending and roll bending are three different bending methods. Brake bending, shown in Fig. 8.26 is performed on bending brakes. Bending wing, a swinging lever applies the force of bending which may be very large if thickness is large. Bending wing may be operated by hand or power. Metal sheets for heating ducts or larger containers are bent by brake bending. Sheets are bent by forcing them into precisely mated dies of hardened tool steel of suitable lengths. The punches are pressed into sheets supported over dies as shown in Fig. 8.27. Three types of bending actions are shown in Fig. 8.27. In air bending punch has an acute angle between 30° to 60°C and a gap between the bottom most point of the sheet and lowest point of die is always left. The angle of bend is controlled by travel of punch and in a single pair of die and punch several angles of bend may be obtained by controlling punch travel. It becomes convenient to incorporate allowance for spring back.
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Fig. 8.25
Shapes produced by bending
Fig. 8.26
Bending brake action
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Bottom bending is the process in which the sheet is in contact over entire active length of punch and die and is sort of compressed between them. It will require about three to five times pressing force than air bending but results in accurate angular tolerance. However, one apparent drawback is that the setup would need a set of die and punch for an angle of bend. It is a disadvantage of this method that the final dimensions will be affected by die inaccuracy and deflection of frame.
Fig. 8.27
Rigid press brake dies (a) Air bending, (b) Bottom bending, (c) Three point bending, (d) Wiping die action.
Three point bending is different than bottom bending in that besides two points of supports in the die the third supporting point is provided by adjustable platform within the die. Whereas the angular accuracy of bottom bending is not obtainable from three point bending, the flexibility of obtaining several angles of bend from a single pair of punch and die exists. An oil cushion in the ram further distributes the force uniformly and helps avoid error due to frame deflection. Fig. 8.27 (d) illustrates a wiping die action in which the workpiece is bent against a die by a descending punch. Workpiece is pressed between the die and a pressure pad. The bent shapes required in practice are several and each one will need a particular set of punch and die. Some of them are shown in Fig. 8.28. All these are rigid. The use of a rigid die is avoided if workpiece is supported by a flexible pad instead of rigid die. Metal is bent to acquire shape of punch while flexible pad deforms to accommodate the bend (see Fig. 8.29). To avoid undesirable marks on finished surfaces the surfaces of punch and dies should be kept smooth.
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(a) Flattening, (b) Radius forming, (c) Channel forming, (d) Offset forming, (e) Joggle, (f) Lockseam, (g) Acute angle V bend. Fig. 8.28 Rigid punch and die configuration
Fig. 8.29
Flexible press brake die
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8.9.1 Roll Bending Roll bending may produce curved shape in general and circular or cylindrical shape in particular. Four basic roll bending operations are recognized as illustrated in Fig. 8.30. A pyramid machine has plate bent between two fixed rolls supported from below while the third one rolls over the top of the plate. The drawback of this kind of machine is that pinching or prebending is not possible. The leading and trailing ends of the plate being bent must conform to the curvature of bend. As shown in Fig. 8.30 (a) pinching is holding the end of plate between two rolls and effecting bending of the clamped portion by the third roller to correct curvature.
(a) Prebending, (b) Pyramid machine, (c) Three roll single pinch machine, (d) Three roll double pinch machine, (e) Four roll pinch machine. Fig. 8.30
Roll bending Machine arrangements
If not done then the unbent edge of the plate has to be cut off and for long workpiece would represent a big loss of material. If end pinching is done in a separate machine then labour cost for handling of material will increase the cost of production. In three roll single pinch machine the plate is pinched between a fixed top roll and a movable bottom roll. The radius of bend is controlled by a third adjustable roll. Prebending is the major advantage over pyramid machine. This machine can prebend only one edge and for bending the other end the workpiece has to be taken out and inserted again resulting into increased material handling cost. A profitable variation of this machine is to fix the top roll and have two adjustable bottom rolls in a three roll double pinch configuration. Thus both ends can be prebent in a single insertion by altering the clamping and bending actions. However, this machine is not well suited to NC-CNC because of absence of fixed reference point. An alternative best suited to NC-CNC control is four roll double pinch machine which can prebend both leading and trailing ends of the workpiece. The geometry of the machine is basically same as single pinch machine with addition of a movable side roll. This arrangement, of course is more complex and costlier.
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Bend radius in bent tube is measured from centre of curvature to the axis of the tube and is expressed in multiple of tube diameter. For bending, a tube may be held at one end in a die of required radius and die rotated while the tube is supported by a wiper block. Some set up has the fixed die and rotating die block which moves along the outside of the tube. Fig. 8.31 shows the arrangement.
Fig. 8.31
Tube bending dies. When die rotates tube is supported against stationary wiper block. When die is stationary pressure die rotates against the tube.
Wheel shaped rolls are used to bend narrow bar sections (Fig. 8.32).
Fig. 8.32
Wheel bending for bars of L section
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8.9.2 Roll Forming Sheet metal, often in form of coiled strip or short sheet is bent in form of a complex crosssection by passing through a series of roll sets. Each set is a pair of concave and convex matching rolls and sheet passes through them acquiring the shape of recess between the concave and convex pair. A typical section may need passing through 5 to 15 sets (see Fig. 8.33). A simple shape like angle section can be produced just in two passes. Welded tubing can be produced by this process by passing metal strip through proper shaped roll pairs and welding along the seam.
Fig. 8.33
Sequence of roll forming
8.10 DRAWING Drawing is converting sheet metal into thin walled seamless hollow shape. If depth of hollow is small it is shallow drawing. In deep drawing the depth is comparable or even much larger than the section of hollow. In shallow drawing deformation is less severe with moderate movement of material. This process is also called pressing or stamping and is used to produce trays, pans, automobile trim and hub caps. Each drawing operation begins with a blank of sheet material whose surface area is close to that of finished part with slight excess for holding. Extra metal is trimmed. In ideal drawing process the thickness would not decrease (Fig. 8.34).
Fig. 8.34
Shallow drawing
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In deep drawing operation a blank is placed over a die block, also called drawing die or drawing. Another die called punch is allowed to descend upon the blank which presses it through the hole of die block. Cylindrical vessels are commonly made by this process. The thin sheet acquires the shape of punch on the inside and that of die on the outside which are usually same with the difference in dimensions equal to thickness of the sheet. Wrinkling of edges in case of thin metal is unavoidable because metal cramps at the edges. The thicker sheet may resist wrinkles. Thin blanks require holding down rings. The holding down ring first descends, holds the blank under force and then punch descends. This is a double action drawing operation and is used for deep drawing. On the other hand shallow drawing operation does not require holding down ring and is known as single action drawing. If a second punch moves up the bottom of the drawn part the operation is triple action drawing. The lower punch will impart special shape to the bottom. (Fig. 8.35). The drawn parts are mostly circular but square sections can also be produced.
Fig. 8.35
(a) Single action deep drawing, (b) Double action deep drawing, (c) Triple action deep drawing
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Drawn parts may be redrawn by pressing through smaller dies in standard redrawing (i.e., in the same direction as previous drawing) or reverse drawing Fig. 8.36. Inverting the drawn part on die block and pressing the punch upon workpiece and drawing it through the hole in redrawing often helps remove the wrinkles.
Fig. 8.36
Redrawing
Flexible die forming is much the same process as flexible press brake. In this process a flexible pad (rubber) replaces the drawing die. The sheet blank is pressed against flexible pad and acquires the shape of the punch. Hydroform process is a variation of flexible forming in which rubber pad is pressed against a sheet blank placed on the solid die block. The rubber pad is pressed by a fluid sack when inflated (Fig. 8.37).
Fig. 8.37
Rubber forming
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Dies producing a complete form in single stroke of the punch are simple or conventional. The die shape need not be simple. In certain operation progressive dies are used in which several stations in series perform different operations with workpiece passing from one to another. The operation that are performed may be blanking, notching, piercing, forming and cutting. The production rate in progressive dies is high though initial investment is also high. In transfer die method the part is transferred from one die station to another. Particularly a complex shaped part may not be adaptable to progressive dies.
Fig. 8.38
(a) Electrohydraulic forming, (b) Electro spark forming
Explosive forming is a type of high energy rate forming in which an explosive is detonated against a sheet or plate stock to deform it against inside of a die. The speed of deformation is upto 100 m/s whereas in conventional forming this speed ranges between 0.5 and 10 m/s. Shock waves created in water by explosion are used for deforming a sheet in electrohydraulic forming. If deforming force is obtained by combustion of gaseous fuel through an electric spark the process is electro-spark forming (Fig. 8.38).
8.11 WIRE DRAWING Wire drawing is an operation in which a round rod or wire is typically reduced in crosssectional diameter by pulling it through a die under predetermined condition. The major variables in wire drawing are similar to those in extrusion, that is reduction in cross-sectional area, die angle, friction along the die-workpiece interfaces and speed. To satisfy the requirements one must pull through the die, in one operation. The long length of wire is possible at a high speed. It requires high quality raw material and efficient machines and dies as shown in Fig. 8.39. During this process, the rod is drawn through a series of successively smaller dies until it conforms to predetermined size. During the drawing operation, plastic deformation takes place and the material becomes hard.
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(a) Wire drawing arrangement
(b) Continuous wire drawing Fig. 8.39
8.11.1 Operation The drawing of the wire starts with a rod or coil of hot rolled steel which is 0.8 to 1.6 mm larger than the final size. Ideally, the maximum reduction in cross-sectional area per pass is 63 per cent. The hot rolled steel is descaled, pickled in acid, washed in water and coated with lime and other lubricants. In a multiple-die machine the wire is pulled from a spool, passes through the first die, winds two or three turns on the first block, from which it automatically uncoils and passes to the next die, this way the process of wire drawing proceeds.
8.12 METAL SPINNING Metal spinning is a metal working process by which a disc or tube of metal is rotated at high speed and formed into a axisymmetric parts over a mandrel with tools or rollers. This process is some what similar to forming clay on a potter’s wheel. There are three basic types
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of spinning process: Conventional (or manual), shear, and tube spinning. The equipment used in these process is similar to a lathe as well as CNC lathe, but with special features as shown in Fig. 8.40. Metal spinning is a process in which the deformation stresses are localized to a small region. The material is made to flow and move over the rotating mandrel with the help of spinning tool mounted on a saddle and moving along the profile of the mandrel. The process has been in use for a long time to produce axisymmetric shapes and tubler products. The commercial applications of spinning are rocket nose cones, cookware, gas cylinders, brass instrument bells and public waste receptacles.
(a) Standard spinning
(b) Shear spinning Fig. 8.40
8.12.1 Metal Spinning by Hand Metal Spinning is a process by which circles of metal are shaped over mandrels (also called forms) while mounted on a spinning lathe by the application of levered force with various tools. It is performed rotating at high speeds on a manual spinning lathe. The flat metal disc is spun against the mandrel and a series of sweeping motions then evenly transforms the disc around the mandrel into the desired shape.
8.12.2 Safety Considerations When spinning metal by hand, care must be taken not to touch the spinning metal with one’s hands until the metal edge has been “turned over” rolled to a rounded edge so that the bare edge of the metal stock is protected. This is mentioned specifically because wood turners are accustomed to touching the spinning wood in the lathes (once it reaches relative smoothness) to monitor their progress. This practice is very dangerous in metal spinning. Lexan/Clear plastic lathe shields and guards are recommended.
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8.12.3 Metal Spinning Tools The basic hand metal spinning tool is called a Spoon, though many other tools (be they commercially produced, ad hoc, or improvised) can be used to effect varied results. Spinning tools can be made of hardened steel for spinning aluminium or solid brass or for spinning stainless steel/mild steel.
8.12.4 Mandrels The mandrel/chuck can be made from wood, steel alloys, or synthetic materials. The choice of material is dictated by the hardness of the material to be spun and by how many times the tool is expected to be used.
8.12.5 Cut-off Tools Cutting of the metal is done by hand held cutters, often foot long hollow bars with tool steel shaped/sharpened files attached. This is dangerous and should only be done by skilled trademen. In CNC applications, traditional carbide or tool steel cut-off tools are used.
8.12.6 Rotating Tools Some metal spinning tools are allowed to spin on bearings during the forming process. This reduces friction and heating of the tool, extending tool life and improving surface finish. Rotating tools may also be coated with thin film of ceramic to prolong life. Rotating tools are commonly used during CNC metal spinning operations. Commercially, rollers mounted on the end of levers are generally used to form the material down to the mandrel in both hand spinning and CNC metal spinning. Rollers vary in diameter and thickness depending on the intended use. The wider the roller the smoother the surface of the spinning, the thinner rollers can be used to form smaller radii.
8. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
QUESTIONS
Describe basic forging operations. Differentiate between hot and cold working bringing out the differences in finished products. Describe different types of rolling mills. What products are produced from ring rolling? How is the process performed? Distinguish between forward and backward extrusion. How do open forging and close forging differ? Describe hammers used in forging. What products are produced by shallow and deep drawing processes? Describe the processes. Describe spinning process and tools used for spinning. What possible defects could be present in extruded part? What are the reasons for these defects. What steps are taken to avoid these defects.
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11. What is barelling in part which is open forged? How can it be avoided/reduced? On what factor the barelling depends? 12. State why material to be forged should be heated. 13. Sketch shapes produced by bending and describe basic bending operations. 14. Describe the operation of tube bending with sketch. 15. What is deep drawing? Describe different deep drawing operations? 16. In figure below which process is being performed? Identify 1, 2 and 3 and comment upon condition of 2. What can be done to reduce bulging in 2?
17. What does the figure given below present? Describe the effect of small planetary rolls on 1 and state what condition should they follow, what is the expected ratio of h1/h2 with planetary rollers and without them?
18. In figure of Q. 16 a cylinder of steel of length h0 = 60 mm and diameter d0 = 50 mm is compressed in forging. The cylinder bulges in the middle. For a load of 1000 kN, calculate the true stress and diameter in the middle of length. (d = 52.3 mm, σ′ = – 465 N/mm2)
