Forging is a process in which the work piece is shaped by compressive forces applied through various dies and tools. Advantages of Forging 1. Directional strength: Forging produces predictable and uniform grain size and flow characteristics. These qualities translate into superior metallurgical and mechanical qualities, and deliver increased directional strength in the final part. 2. Structural strength: Forging also provides a degree of structural integrity that is unmatched by other metalworking processes. It eliminates internal voids and gas pockets that can weaken metal parts. Predictable structural integrity reduces part inspection requirements, simplifies heat treating and machining, and ensures optimum -part performance under field-load conditions. 3. Variety of sizes: Open die forged part weights can run from a single pound to over 400,000 pounds. 4. Variety of shapes: Shape design is just as versatile, ranging from simple bar, shaft and ring configurations to specialized shapes. 5. Metallurgical spectrum: Forgings can be produced from literally all ferrous and non-ferrous metals. 6. Material savings: Forging can measurably reduce material costs since it requires less starting metal to produce many part shapes. 7. Machining economies: Forging can also yield machining, lead time and tool life advantages. 8. Reduced rejection rules: By providing weld-free parts produced with cleaner forging quality material and yielding improved structural integrity, forging can virtually eliminate rejections. Limitations of Forging 1. The forged parts often need to be machined before use. 2. Tooling for complicated geometry may be expensive and require multiple passes on the same work piece. 3. The rapid oxidation of metal surfaces at high temperature results in scaling which wears the dies. 4. Initial cost of dies and maintenance cost is high. Applications of Forging Typical parts made by forging are crankshafts and connecting rods for engines, turbine disks, gears, wheels, bolt heads, hand tools, and many types o f structural components for machinery and transportation equipment. Forging vs Casting The forging process is superior to casting in that the parts formed have denser microstructures, more defined grain patterns, and less porosity, making such parts much stronger than a casting. To a large degree this is due to the formation of a grain structure which is elongated in the direction of deformation, causing the macrostructure shown below. The metal flow during forging can be visualized by the fibrous morphology of the microstructure. Thus, forging builds in a natural advantageous anisotropy with high mechanical properties in the plane of maximum strain. Furthermore, during the forging deformation, the work piece will often undergo recrystallization, thus developing a fine-grained microstructure and eliminating the cast dendritic structure that has inherently poor properties. If the work piece is hot forged, then some of the segregation resulting from solidification will be eliminated. All of these factors often result in improved mechanical properties for forged parts compared to castings or machined components.
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Classification Classification of forging Open die forging Impression die forging Closed die forging Hot forging Cold forging Forging
Warm forging Hand forging / smith forging Machine forging Press forging
OPEN DIE FORGING Open die forging, also referred to as hand, smith, hammer, and flat-die forging, can be distinguished from most other types of deformation processes in that it provides discontinuous material flow as opposed to continuous flow. All forgeable metals can be forged in open dies. Forgings are made by this process when: The forging is too large to be produced in closed dies. The required mechanical properties of the worked metal that can be developed by opendie forging cannot be obtained by other deformation processes. The quantity required is too small to justify the cost of closed dies . The delivery date is too close to permit the fabrication of dies for closed-die forging. The size of a forging that can be produced in open dies is limited only by the capacity of the equipment available for heating, handling, and forging. Items such as marine propeller shafts, which may be several meters in diameter and as long as 23 m (75 ft), are forged by open-die methods. Similarly, forgings no more than a few inches in maximum dimension are also produced in open dies. An open-die forging may weigh as little as a few kilograms or as much as 600 tons. Highly skilled hammer and press operators, with the use of various auxiliary tools, can produce relatively complex shapes in open dies. However, the forging of complex shapes is time consuming and expensive, and such forgings are produced only under unusual circumstances. Generally, most open-die forgings can be grouped into four categories: cylindrical, upset or pancake forgings, hollow and contour-type forgings. Because the length of the hammer ram stroke and the magnitude of the force must be controllable over a wide range throughout the forging cycle, gravity-drop hammers and most mechanical presses are not suitable for open-die forging. Power forging hammers (air or steam driven) and hydraulic presses are most commonly used for the production of open die forgings that weigh up to 5 tons. Larger forgings are usually made in hydraulic presses. Most open-die forgings are produced in a pair of flat dies--one attached to the hammer or to the press ram, and the other to the anvil. Swage dies (curved), V-dies, V-die and flat-die JO/VJCET
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combinations, FM (free from Mannesmann Effect) dies and FML (free from Mannesmann Effect with low load) dies are also used. The Mannesmann Effect refers to a tensile stress state as a result of compressive stresses in a perpendicular orientation.
Barrelling: It is caused primarily by frictional forces at the die-work piece interfaces that oppose the outward flow of the materials at these interfaces. This can be minimized by lubricants or ultrasonic vibrations of the platens. Barrelling can also occur in upsetting hot work pieces pie ces between cold dies. The material at and near the interfaces cool rapidly, while the rest of the work piece remains relatively hot. Consequently, the central portion of the work piece expands laterally to a greater extent than its ends. Barrelling from thermal effects can be reduced or eliminated by using heated dies. Thermal barrier like glass cloth at die-work piece interface also reduces barrelling.
Open die forging practice: Stock for smaller open-die forgings is usually prepared by cold sawing to a computed length. Large open-die forgings are commonly forged from ingots. The work piece is heated to proper forging temperature carefully in order to minimize decarburization and to avoid cracking due to rapid heating. Flat dies are usually not preheated. Swage or V-dies, if they have become completely cold (as from a weekend shutdown), are sometimes warmed, particularly for hammer operations. Die heating or warming can be accomplished by closing the dies on slabs of heated steel. Lubrication is sometimes used for the upsetting operation in order to eliminate the dead zone (undeformed material) directly under the dies. This is especially critical for materials that cannot be refined through phase transformation, such as austenitic stainless steels, aluminum alloys, and nickel-base alloys. Descaling of the work piece is done by busting and blowoff. Best practice includes the use of compressed air to blow away the scale as it breaks off. High-pressure water is also sometimes used to loosen scale, especially at hard-to-reach locations. Failure to remove the scale causes it to be forged in, resulting in pits and pockets on the forged surfaces. The total amount of scale formed in open-die forging is usually greater than in closed-die closed-die forging because the hot metal is exposed to the atmosphere for a longer time. Metal loss through scaling usually ranges from 3 to 5%. Hammer and press practice vary considerably from one opendie shop to another. Hand forging Sometimes called smithy or blacksmithing, hand forging is the simplest form of forging. The metal to be forged is first heated to red heat in the fire of a forge, and then is beaten into shape on a metal anvil with sledges or hammers. JO/VJCET
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Hand Forging Tools and Equipments: Besides a furnace, hand forging needs anvil, swage block, blacksmith hammers, tongs, swages, fullers and flatters for the various forging operations. Hand forging is used for making simple shapes such as chains, hooks, shackles, and agriculture equipment and tools. 1. Anvil: An anvil acts as a supporting device which is capable of withstanding heavy blows rendered to the job. It is made of cast steel, wrought iron or mild steel provided with a hardened top. The horn or beak is used in bending the metal or forming curved shapes. At the top of the anvil, are a square and a round hole. The square hole (Hardie hole) is used for holding square shank, shaped tools like bottom fullers, swages etc. The round hole is used for admitting the ends of the punches or drifts. This hole is also used for bending round bars of different curvatures. The anvil is supported either on an iron base or on a wooden block. 2. Swage Block It is generally made of cast iron and has round, square, rectangular and half round grooves. These grooves are used either for holding bars while bending or providing support in punching holes. The swage block is supported on a cast iron stand. 3. Hammers: These are basically of two types — (i) Hand hammer, and (ii) Sledge hammer The hand hammers are generally ball peen hammers. These have a slightly convex striking face and generally weigh from 1 to 1.5 kg. The sledge hammers are usually 3 to 4 times heavier than the hand hammers. These are used when heavy blows are needed in forging. Sledge hammers are further classified as cross peen, straight peen and double faced type.
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4. Tongs These are used for holding the jobs in position during forging. Tongs are made of mild steel and are usually named according to the inner shape of the jaws.
5. Swages These are used in pairs i.e., top and bottom parts. Swages are used to reduce and finish the job to the exact size and shape. During swaging the work piece is rotated between swages, which are hammered to produce smooth round surface.
6. Fullers These are used for necking down a work piece. It spreads the metal and reduces the thickness of the work piece. Fullers are made of tool steel and are made in pairs. The top fuller is held by a handle while the bottom fuller is fitted into the hardie hole of the anvil.
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7. Flatter These are used for finishing flat surfaces and are made with a perfect flat face. It provides smoothness and accuracy to the work piece. Flatter is made of tool steel. It is struck by hammer on the head.
Smith Forging Operations In general, six basic types of forging operations exist(i) Upsetting: A piece of metal, called the work, is upset when it is struck along the longest dimension (for example, the end of a rod or bar), which shortens and thickens it. (ii) Swaging: It is accomplished by hammering the metal stock while it is held on the anvil within any one of various concave tools called swages. (iii) Bending: It is accomplished either by hammering the work around a form or by leveraging it against a supporting fulcrum (iv) Welding: In forge welding of iron, a flux such as borax is first applied to the heated metal to remove any oxides from the surfaces of the two pieces, and the pieces are then joined by hammering them together at high temperature. A welded joint of this kind, when properly made, is entirely homogeneous and is as strong and uniform, as the parent metal. (v) Punching: To punch small holes, the work is supported on a ring shaped piece of metal above the anvil, and a punch of the proper shape is driven through the work by hammer blows. (vi) Cutting: Larger holes are cut out, and portions of the work are cut off with heavy, sharp chisels similar to cold chisels which are used to cut cold metal. Combinations of several of these operations can produce forgings of a wide variety of shapes. Power forging It is used to produce large number of identical forgings. Machines which work on forgings by blow are called hammers and those which work by pressure are called presses.
Forging force in open die forging The forging force F in an open die forging operation on a solid cylindrical piece can be estimated from the formula
Where yf ’ is the flow stress of the material, µ is the co-efficient of friction and r and h are the radius and height of the work piece respectively.
IMPRESSION-DIE FORGING In impression-die forging, the work piece acquires the shape of the die cavities while being forged between two shaped dies. The impression for the forging can be entirely in one die or can be divided between the top and bottom dies. The forging stock, generally round or square bar, is cut to length to provide the volume of metal needed to fill the die cavities, in addition to an JO/VJCET
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allowance for flash. In impression die forging, the blank is sometimes preformed by fullering or edging to distribute the material into various regions of the blank. Fullering distributes material away from an area whereas edging gather material into a localized area. The preformed blank is then formed into a rough shape by block forging. The final operation is the finishing of the forging in the impression dies. Some of the material flows outward and form a flash. The thin flash cools rapidly, and, because of its frictional resistance, it subjects the material in the die cavity to high pressures, thereby encouraging the filling of the die cavity. The flash is usually removed by a trimming operation.
With the use of closed dies, complex shapes and heavy reductions can be made in hot metal within closer dimensional tolerances. Impression die / closed-die forgings are usually designed to require minimal subsequent machining. The process is adaptable to low-volume or high-volume production. In addition to producing final, or nearly final, metal shapes, impression die / closed-die forging allows control of grain flow direction, and it often improves mechanical properties in the longitudinal direction of the work piece. In closed-die forging, a material must satisfy two basic requirements. First, the material strength (or flow stress) must be low so that die pressures are kept within the capabilities of practical die materials and constructions, and, second, the forgeability of the material must allow the required amount of deformation without failure. Impression die forging is practiced both in the hot and cold state using hammers as well as presses.
CLOSED-DIE FORGING / FLASHLESS FORGING In closed die or flashless forging, flash does not form and the work piece completely fills the die cavity. Accurate control of the volume of material and proper die design are essential in order to obtain a closed die forging of the desired dimensions and tolerances. Undersize blanks prevent the complete filling of the die cavity. Oversize blanks generate excessive pressures and may cause dies to fail prematurely or to jam.
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Forging force in impression impression die forging The forging force F in an impression die forging operation can be estimated from the formula Where yf ’ is the flow stress of the material at the forging temperature, A is the projected area of the forging including the flash, k is a multiplying factor. Range of k values Simple shape without flash 3-5 Simple shape with flash 5-8 Complex shapes with flash 8 - 12
Precision forging In precision forging, special dies produce parts having greater accuracies than those from impression die forging and requiring much less machining. The process requires a higher capacity equipment, because of the greater forces required to obtain fine details of the part. Precision forging requires special and more complex dies, precise control of the billet ’s volume and shape, accurate positioning of the billet in the die cavity and hence higher investment. However less material is wasted and much less subsequent machining required. Typical precision forged products are gears, connecting rods, housings and turbine blades. Aluminium and magnesium alloys are suitable for precision forging because of Relatively low forging loads and temperature they require Little die wear and Good surface finish Steels and titanium can also be precision forged. Coining: Coining is a closed die forging process used in minting coins, medallions and jewellery. In order to produce fine details the pressures required can be as high as five or six times the strength of the material. Lubricants cannot be applied in coining because they may prevent the full reproduction of die surface details.
Cogging: The process is also called drawing out. It is basically a die forging operation in which the thickness of a bar is reduced by successive forging steps at specific intervals. Blacksmiths perform this operation with a hammer and an anvil.
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Heading: It is an upsetting operation, usually performed at the end of a round rod in order to produce a larger cross-section. Typical examples are the heads of bolts, screws, rivets, nails and other fasteners. Heading processes can be carried out cold, warm or hot. They are performed on machines called headers which are usually highly automated. Production rates are hundreds of pieces per minute for small parts. An important aspect of heading is the tendency for the bar to buckle if its unsupported length to diameter ratio is too high. This ratio is usually limited to less than 3:1.
Roll forging: The cross-section of a bar is reduced or shaped by passing it through a pair of rolls with shaped grooves. The process is used to produce tapered shafts and leaf springs, tableknives and hand tools. It may also be used as a preliminary forming operation to be followed by other forging processes.
Skew rolling: It is a process typically used for making ball bearings. Round wire or rod is fed into the roll gap and roughly spherical blanks are formed continually by the action of rotating rolls. Another method for forming near spherical blanks for ball bearings is to shear pieces from a round bar and then to upset them in ball headers. The balls are laterground and polished in special machinery.
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Rotary swaging/swaging/radial forging: It is a process for reducing the cross-sectional area or otherwise changing the shape of bars, tubes, or wires by repeated radial blows with two or more dies. The work is elongated as the cross-sectional area is reduced. Most swaged work pieces are round, the simplest being formed by reduction in diameter. However, swaging can also produce straight and compound tapers, can produce contours on the inside diameter of tubing, and can change round to square or other shapes.
Figure (a) and (b) show a rotary swaging process. A solid rod or tube is subjected to radial impact forces by a set of reciprocating dies. The die movements are obtained by means of a set of rollers in a cage. The work piece is stationary and dies rotate, striking the work piece at rates as high as 20 strokes per second. Figure (c) shows a die-closing swaging machine. Die movements are obtained through reciprocating motion of wedges. The dies can be opened wider than in rotary swagers and hence can accommodate large diameter or variable diameter parts.
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In tube swaging, the internal diameter and/or the thickness of the tube can be controlled with or without the use of internal mandrels. Figure above shows various contours formed inside tubes by swaging action. Applications of swaging include Screw driver blades and soldering tips Rifling in gun barrels Assembly of fittings over cables and wire Pointing – tapering the tip of a cylindrical part Sizing – finalizing the dimensions of a part
Isothermal Forging: In the isothermal forging process, the dies are maintained at the same temperature as the forging stock. This eliminates the die chill completely and maintains the stock at a constant temperature throughout the forging cycle. The process permits the use of extremely slow strain rates, thus taking advantage of the strain rate sensitivity of flow stress for certain alloys. The process is capable of producing net shape forgings that are ready to use without machining or near-net shape forgings that require minimal secondary machining. The dies for isothermal forging are usually made of nickel or molybdenum alloys. Hot die forging: Die temperatures are significantly higher than those used in conventional hotforging processes. This has the advantage of reducing die chill and results in a process capable of producing near-net and/or net shape parts. This processing technique is primarily used for manufacturing airframe structures and jet-engine components made of titanium and nickel-base alloys, but they have also been used in steel transmission gears and other components. FORGING DIE DESIGN The forging die design should consider Strength and ductility of the work piece material Material sensitivity to deformation rate and temperature. Frictional characteristics of the work piece and die material Shape and complexity of the work piece Preshaping: The most important rule in die design is the fact that the part will flow in the direction of least resistance. So the work piece should be preformed so that it properly fills the die cavity. Preform design is the most difficult and critical step in forging design. Proper preform design assures defect-free flow, complete die fill, and minimum flash loss. General considerations for preform design are • Area of each cross section = area in the finished cross section + flash. • Concave radii of the preform > radii on the final forg ing part. • Cross section of the preform should be higher and narrower than the final cross section, so as to accentuate upsetting flow and minimise extrusion flow. Die design features
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of the forging. The shape and location of the parting line determine die cost, draft requirements, grain flow, and trimming procedures. In most forgings, the parting line is at the largest cross section of the part, because it is easier to spread metal by forging action than to force it into deep die impressions. If the largest cross section coincides with a flat side of a forging, forging, there is an advantage in locating the parting line along the edges of the flat section, thus placing the entire impression in one die half. Die costs can be reduced, because one die is simply a flat surface. Also, mismatch between upper and lower dies cannot occur, and forging flash can be trimmed readily. Because part of the metal flow is toward the parting line during forging, the location of the parting line affects the grain flow characteristics of a forged piece. For more complex shapes, the parting line may not lie in a single plane. The dies are then designed in such a way that they lock during engagement, in order to avoid sidethrust, balance forces and maintain die alignment during forging. f orging. Gutter and land: After sufficiently constraining lateral flow to ensure proper die filling, the flash material is allowed to flow into the gutter so that the extra flash does not increase the forging load. A general guidance for flash clearance between dies is 3% of the maximum thickness of the forging. The length of the land is usually two to five times the flash thickness. The gutter and land design is important because flash serves two purposes:
• Acts as a ‘safety valve’ for excess metal. • Builds up high pressure to en sure that the metal fills all recesses of the die cavity. Draft angles, corner and fillet radii: Draft, or taper, is added to straight sidewalls of a forging to permit easier removal from the die impression. The draft used in die impressions normally varies from 3 to 5° for external walls of the forging. Internal draft angles range from 7 to 10°. Internal draft angles are made larger to prevent the forging from sticking in the die as a result of natural shrinkage of the metal as it cools. Selection of proper radii for corners and fillets is important to ensure smooth flow of the metal into the die cavity and to improve die life. Small radii are generally undesirable because of their adverse effect on metal flow and their tendency to wear rapidly. Small fillet radii also can cause fatigue cracking of the dies. These radii should be as large as can be permitted by forging design.
Die inserts: Dies may be assembled with die inserts, particularly for complex shapes. This reduces the cost of making several similar dies. The inserts can be made of stronger and harder materials, and they can be changed easily in the case of wear or failure in a particular section of die.
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Die materials General requirements of die materials are Strength and toughness at elevated temperatures Hardenability and ability to harden uniformly Resistance to mechanical and thermal shock Wear resistance, particularly abrasive wear resistance, because of scales present in hot forging Selection of proper die material depends on Die size Composition and properties of the work piece Complexity of the shape Forging temperature Type of forging operation Cost Production volume Common die materials are tool and die steels containing chromium, nickel, molybdenum and vanadium. Die manufacturing: Dies are made from die blocks, which are forged from castings, heat treated and then machined and finished to desired shape and surface finish. Most commonly, dies are machined from forged die blocks by processes such as milling, turning, grinding, electrochemical machining and other non-conventional machining processes. Die steels are heat treated for improved hardness, wear resistance and strength. Die failure: The three basic causes of premature die failure are overloading of the die, abrasive action, and overheating. An overloaded die wears rapidly and may break. Overloading can be avoided by careful selection of die steel and hardness, use of blocks and inserts of adequate size, proper application of working pressures, proper die design to ensure correct metal flow, and proper seating of the dies in the hammer or press. Abrasion is particularly severe if the design of the forging is complex, or if the metal forged has a high hot strength, or if there is scale on the work metal. Abrasion effects can be minimized by good die design, careful selection of die composition and hardness, and a forging technique that includes proper heating, any necessary descaling, and correct die lubrication. As a die becomes hotter, its resistance to wear decreases. Overheating causes most of the premature die wear that occurs in forging. Overheating is likely to occur in areas of the die impression that project into the cavity. In addition, overheating may result from continuous production. If an internal die-cooling system that is adequate to prevent overheating cannot be provided economically, dies, or portions of dies, which are susceptible to overheating should be constructed of steels with high heat resistance.
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Die life: It depends on Die material and hardness Work metal composition Forging temperature Condition of the work metal at forging surfaces Type of equipment used Work piece design Lubrication in forging: Lubrication serves various purposes in rolling process. Lubricants influence friction and wear; consequently they affect the forces required and the flow of metal in die cavities. Lubricant acts as a thermal barrier between the hot work piece and the relatively cool dies, slowing the rate of cooling of work piece and improving metal flow. Lubricant serve as a parting agent which inhibits the forging from sticking to the dies For hot forging, the various lubricants used are graphite, molybdenum disulfide, glass etc. For cold forging, mineral oils and soaps are common lubricants. In hot forging, the lubricant is usually applied directly to the dies. In cold forging, it is applied to the work piece. The method of application and the uniformity of lubricant thickness on blank are important to product quality. Forgeability Forgeability is defined as the capability of a material to undergo forging easily without cracking. It is a combination of the following characteristics: characteristics: The flow stress The ability to fill a die The degree of deformation that can be carried out without failure (due to surface or internal cracking)
Although in general the forgeability of metals increases with increasing temperature, for certain metals there is a maximum temperature above which some undesirable phenomena occur, such as fast grain growth or melting of a phase. Fine grain metals have better forgeability. Metals with insoluble inclusions tend to be brittle and have low forgeability. JO/VJCET
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Two popular tests for determining the forgeability of materials are the upset test and the hot twist test. Upset test : Cylindrical specimens are upset in steps until they start cracking radially or circuferentially. The greater the reduction of height prior to cracking, greater is the forgeability. Upsetting tests can be performed at various temperatures and strain rates. An optimal range for these parameters can be then specified for forging a particular material. Hot twist test : A round bar is heated in a tubular furnace and then twisted continuously in same direction. The number of twist turns to failure is a relative measure of forgeability. Testing can be carried out in a range of temperatures and strain rates to determine the best conditions for practical forging. Hammers and presses for forging Forging machines can be classified according to their principle of operation. Hammers and high-energy-rate forging machines deform the work piece by the kinetic energy of the hammer ram; they are therefore classed as energy restricted machines. The ability of mechanical presses to deform the work material is determined by the length of the press stroke and the available force at various stroke positions. Mechanical presses are therefore classified as stroke restricted machines. Hydraulic presses are termed force-restricted machines because their ability to deform the material depends on the maximum force rating of the press. Although they are similar in construction to mechanical and hydraulic presses, screw-type presses are classified as energy-restricted machines. Hammers: Hammers are the least expensive and most flexible type of forging equipment in the variety of forging operations they can perform. Hammers are capable of developing large forces and have short die contact times. The main components of a hammer are a ram, frame assembly, anvil, and anvil cap. The anvil is directly connected to the frame assembly, the upper die is attached to the ram, and lower die is attached to the anvil cap. In operation, the work piece is placed on the lower die. The ram moves downward, exerting a force on the anvil and causing the work piece to deform. Forging hammers can be classified according to the method used to drive the ram downward.
Gravity-drop hammers consist of an anvil or base, supporting columns that contain the ram guides, and a device that returns the ram to its starting position. The energy that deforms the work piece is derived from the downward drop of the ram; the height of the fall and the weight of the ram determine the force of the blow.
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Board drop hammer: In the board-drop hammer, the ram is lifted by one or more boards keyed to it and passing between two friction rolls at the top of the hammer. The boards are rolled upward and are then mechanically released, permitting the ram to drop from the desired height. Power for lifting the ram is supplied by one or more motors. Air or steam lift hammers: The ram in the air-lift hammer is raised by air or steam power. Stroke-control dogs, preset on a rocker and actuated by the ram, control power to the ram cylinder. Stroke control dogs can be reset on the rocker to adjust stroke length. Electro-hydraulic Gravity-Drop Hammers: In this type of hammer, the ram is lifted with oil pressure against an air cushion. The compressed air slows the upstroke of the ram and contributes to its acceleration during the down stroke blow. Therefore, the electro-hydraulic drop hammer also has a minor power hammer action.
Power-Drop Hammers In a power-drop hammer, the ram is accelerated during the down stroke by air, steam, or hydraulic pressure. This equipment is used almost exclusively for closed-die (impression-die) forging.
Forging Presses: Unlike the blow of a forging hammer, a press blow is more of a squeeze than an impact and is delivered by uniform stroke length. The character of the blow in a forging press resembles that of an upsetting machine, thus combining some features of hammers and upsetters. Mechanical Presses All mechanical presses employ flywheel energy, which is transferred to the work piece by a network of gears, cranks, eccentrics, or levers. Driven by an electric motor and controlled by means of an air clutch, mechanical presses have a full eccentric type of drive shaft that imparts a constant-length stroke stroke to a vertically operating ram. ram. Various mechanisms are used used to translate the rotary motion of the eccentric shaft into linear motion to move the ram. The ram carries the top moving die while the bottom stationary die is clamped to the main frame. The ram stroke is shorter than that of a forging hammer or a hydraulic press. Ram speed is greatest at the centre of the stroke, but force is greatest at the bottom of the stroke. The capacities of these forging presses are rated on the maximum force they can apply and range from about 2.7 to 142 MN. MN . Compared to hammer forging, mechanical press forging results in accurate close-tolerance parts. Mechanical presses permit automatic feed and transfer mechanisms to feed, pick up, and move the part from one die to the next, and they have higher production rates than forging hammers (stroke rates vary from 30 to 100 strokes per minute). JO/VJCET
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Because the dies used with mechanical presses are subject to squeezing forces instead of impact forces, harder die materials can be used in order to extend die life. Dies can also be less massive in mechanical press forging. One limitation of mechanical presses is their high initial cost-approximately three times as much as forging hammers that can do the same amount of work. Because the force of the stroke cannot be varied, mechanical presses are also not capable of performing as many preliminary operations as hammers.
Hydraulic presses : Hydraulic presses are used for both open- and closed-die forging. The ram of a hydraulic press is driven by hydraulic cylinders and pistons, which are part of a high-pressure hydraulic or hydro-pneumatic system. After a rapid approach speed, the ram (with upper die attached) moves at a slow speed while exerting a squeezing force on the work metal. Pressing speeds can be accurately controlled to permit control of metal-flow velocities; this is particularly advantageous in producing close-tolerance forgings. The principal advantages of hydraulic presses include : Pressure can be changed as desired at any point in the stroke by adjusting the pressure control valve. Deformation rate can be controlled or varied during the stroke if required. This is especially important when forging metals that are susceptible to rupture at high deformation rates. Split dies can be used to make parts with such features as offset flanges, projections, and back draft, which would be difficult or impossible to incorporate into hammer forgings. When excessive heat transfer from the hot work piece to the dies is not a problem or can be eliminated, the gentle squeezing action of a hydraulic press results in lower maintenance costs and increased die life because of less shock as compared to other types of forging equipment Maximum press force can be limited to protect tooling. Some of the disadvantages of hydraulic presses are: The initial cost of a hydraulic press is higher than that of an equivalent mechanical press. The action of a hydraulic press is slower than that of a mechanical press. The slower action of a hydraulic press increases contact time between the dies and the work piece. When forging materials at high temperatures (such as nickel-base alloys and titanium alloys), this result in shortened die life because of heat transfer from the hot work metal to the dies. Selection criteria for hammers/presses hammers/presses Force or energy requirements Size, shape and complexity of the forging Strength of the work piece material Sensitivity of the material to rate of deformation Production rate Dimensional accuracy JO/VJCET
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Operating skills required Noise level Initial and maintenance costs Heating in forging There are wide variations in the forging temperature ranges for various materials. These differences have resulted in a wide variety of heating equipment. Various types of furnaces used are Blacksmiths’ forge Continuous and batch type furnaces Box type furnaces Muffle type furnaces Rotary hearth furnaces Electric resistance furnaces High frequency induction furnaces Regardless of the heating method used, temperature and atmospheric conditions within the heating unit must be controlled to ensure that the forgings subsequently produced will develop the optimal microstructure and properties. There are three categories of heating associated with forging – heating of work piece, heating of dies and heat treatment of finished forgings.
Forging Defects Incomplete die filling: In this some section of the die cavity are not completely filled by the flowing metal. The causes of this defect are improper design of the forging die or improper forging techniques. Die misalignment Incomplete forging penetration: If the deformation during forging is limited to the surface layers, as when light rapid hammer blows are used, the dendritic ingot structure will not be broken down at the interior of the forging. To minimize incomplete penetration, forgings of large cross section are usually made on a forging press. Forging laps: If there is an insufficient volume of material to fill the die cavity, the web may buckle during forging and develop laps.
Cracks: Surface cracks can occur as a result of excessive working of the surface at too low temperature or as a result of hot shortness. Hot shortness is a term used to describe grain boundary separation that can create a rough surface condition in hot forged steels or nickel alloys. Hot shortness may lead to surface cracks. A high sulpher concentration during forging may lead to hot shortness in steel and nickel. Cracking at the flash of closed die forgings is another surface defect, since the crack generally penetrates into the body of the forging when the flash is trimmed off. This type of cracking is more prevalent when the flash is thinner in relation to the original thickness of the metal. Flash cracking can be avoided by increasing the flash
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thickness or by relocating the flash to a less critical region of the forging. It can be avoided also by hot trimming or stress relieving the forging prior to cold trimming.
Internal cracks can develop during the upsetting of a cylinder or a round as a result of circumferential tensile stresses. Proper design of dies however can minimize this type of cracking. In order to minimize circumferential stresses, it is usual practice to concave dies. Internal cracking is less prevalent in closed die forging because of the lateral compressive stresses.
Pitted surface: This is primarily caused because of improper cleaning of the stock used for forging. The oxide and scales at the forging surface stick on the dies leaving a pitted surface on the finished forging. End grains: There are situations in which the flow lines reach a surface perpendicularly, exposing the grain boundaries directly to the environment. This condition is known as end grains. In service, they can be attacked by the environment, develop a rough surface, and act as stress raisers. Cold shut / fold: Cold shuts are forging defects that arise from the partial separation of some hot metal from the main body of the forging. The defects are formed when the partly separated metal, in the course of the forging cycle, is folded back against, and forged into, the main body of the forging. An oxide film, formed on the underside of the fold, creates a barrier that prevents satisfactory welding of the fold with the parent metal, thus accounting for the defect.
In hot upsetting, the displacement of too much metal in a single p ass is a common cause of cold shuts. When the size or shape of the upset is such that these defects occur, one or more stock-gathering passes must be added to the forging cycle in advance of the finishing pass. Too small die radius also leads to cold shuts and hence should be avoided. Non-destructive testing of forgings Same as that for casting and welding. Economy of forging Total cost of forging includes tool and die costs, set up cost and material cost. Tool and die costs depend up on the complexity of the forging. All other costs other than material costs decreases as the number of forgings increases. JO/VJCET
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The graph below compares the cost of forging process with other manufacturing processes for a typical product.
Drawing Drawing is a cold working process in which the cross-sectional area and/or the shape of a rod, bar, tube, or wire is reduced by pulling through a die . Drawing allows excellent surface finishes and closely controlled dimensions to be obtained in long products that have constant cross sections. In drawing, a previously rolled, extruded, or fabricated product with a solid or hollow cross section is pulled through a die at exit speeds as high as several thousand feet per minute. The die geometry determines the final dimensions, the cross-sectional area of the drawn product, and the reduction in area. The deformation is accomplished by a combination of of tensile and compressive stresses that are created by the pulling force at the exit from the die and by the die configuration.
Rod or wire drawing: Both rods and wires are circular in cross-section. Rods are larger in crosssection compared to wires. Drawn rods are used for shafts, spindles, small pistons and as the raw material for fasteners such as rivets, bolts and screws. Drawn wire products are used for electrical and electronic wiring, cables, tension loaded structural members, welding electrodes, springs, paper clips, bicycle spokes and stringed musical instruments.
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Wire drawing is an operation to produce wire of various sizes within certain specific tolerances. The process involves reducing the diameter of rods or wires by pulling them through a series of wire drawing dies with each successive die having smaller bore diameter than the one preceding it. During drawing, indirect compression causes deformation of the work piece. That is, the tension applied to the emerging product causes compression against the die face and deforms the material. To avoid deformation after the wire has emerged from the die, the maximum drawing force that can be applied is limited by the yield strength of the emerging product. In practice, the drawing stress is limited to about 60% of the flow stress of the emerging product, which limits reductions to about 35% in most cases. The rate at which small wire-type products can be drawn is very high - up to 50m/s. Heavier Sections that cannot be wound into a roll must be drawn straight and line speeds are usually slower - about 1 m/s. The final wire size is reached as the wire passes through the last die in the series. Annealing may occasionally be necessary after a number of drawing passes before the drawing operation is continued.
A wire drawing die as shown below is a tool that consist of a highly polished, shaped hole through which wire is drawn to reduce its diameter. The choice of die material, viz., natural or synthetic single crystal diamond, polycrystalline diamond, carbide etc. depends on the material of the wire to be drawn and the operating parameters.
In the drawing process, cleaned and coated coils of rod or wire are first placed on a payoff tray, stand, or reel; this permit free unwinding of the stock. The leading end of the rod or wire, after JO/VJCET
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being pointed, is then inserted through the drawing die and seized by a gripper attached to a powered cylindrical block or capstan. On so-called dry machines, the die is mounted in an adapter within a box. This die box contains grease, dry soap, oil, or other lubricants through which the stock must pass before reaching the die. Dry drawing: The lubricant is chosen only for its tribological attributes. The lubricant is usually a dry soap powder, placed in a die box and picked up by the wire surface upon its passage through the box. This technique is used for steel wire larger than 0.5 to 1 mm diameter, for which the relatively rough surface produced is acceptable. Wet drawing: The lubricant is chosen both for its tribological attributes and for its cooling power, and it can be either oil-base or aqueous. It can be applied to the die inlet, the wire, or the entire machine can be submerged in a bath. This wet-drawing practice is typical of all nonferrous metals metals and of steel wires less than 0.5 to 1 mm diameter.
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Mechanics of rod/wire drawing
The most important parameters determining the drawing stress, σd, required are: the friction coefficient µ; the approach angle α which is typically between 6 ⁰ and 10⁰ and the reduction ratio. Using these parameters σd can be estimated from
is the flow stress
Tube Drawing
Drawing of flat strips The dies in flat strip drawing are wedge shaped and there is little or no change in the width of strip during drawing. Thus drawing of flat strips can be analysed by plane strain condition. Flat drawing is the fundamental deformation mechanism in ironing. Defects in Wire Drawing Defects occur in wire drawing because of ploughing by hard particles and local breakdown of the lubricating film. Some common defects are: Bulge formation: This occurs in front of the die due to low reduction and high die angle. Internal cracks (Centre burst or centre-cracking): The tendency of cracking increases with increasing die angle, with decreasing reduction per pass, with friction and with the presence of inclusions in the material. Seams: Seams: These appear as longitudinal scratches or folds in the material. Such defects can open up during subsequent forming operations by upsetting, heading, thread rolling or bending of the rod or wire. Surface defects: Various types of surface defects can also result due to improper selection of process parameters and lubrication. JO/VJCET
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