The North American forging industry continues to grow in technical prowess. OEMs are realizing that nothing beats forgings for strength and reliability. Advances in forging technology have expanded the range of shapes, sizes, and properties available in forged products to meet an increasing variety of design and performance requirements. Forgings are regularly specified where strength, reliability, economy, and resistance to shock and fatigue are vital considerations. Forged materials offer the desired degree of high or low temperature performance, ductility, hardness, and machinability.
In automotive and truck applications, forged components are commonly found at points of shock and stress. Cars and trucks may contain more than 250 forgings, most of which are produced from carbon or alloy steel. Forged engine and powertrain components include connecting rods, crankshafts, transmission shafts and gears, differential gears, drive shafts, clutch hubs, and universal joint yokes and crosses. Forged camshafts, pinions, gears, and rocker arms offer ease of selective hardening as well as strength. Wheel spindles, kingpins, axle beams and shafts, torsion bars, ball studs, idler arms, pitman arms, steering arms, and linkages for passenger cars, buses, and trucks typify applications requiring extra strength and toughness.
High strength-to-weight ratio and structural reliability improve performance, range, and payload capabilities of aircraft. That's why ferrous and nonferrous forgings are used in helicopters, piston-engine planes, commercial jets, and supersonic military aircraft. Many aircraft are "designed around" forgings, and contain more than 450 structural forgings as well as hundreds of forged engine parts. Forged parts include bulkheads, wing roots and spars, hinges, engine mounts, brackets, beams, shafts, bellcranks, landing-gear cylinders and struts, wheels, brake carriers and discs, and arresting hooks. In jet turbine engines, iron-based, nickel-base, and cobalt-base superalloys are forged into buckets, blades, couplings, discs, manifolds, rings, chambers, wheels, and shafts--all requiring uniformly high-yield tensile and creep rupture strengths, plus good ductility at temperatures ranging between 1,000 and 2,000°F. Forgings of stainless steels, maraging steels, titanium, and aluminum find similar applications at lower temperatures. Forged missile components of titanium, columbium, super alloys, and refractory materials provide unduplicated mechanical and physical properties under severe service conditions. Aluminum structural beams for boosters, titanium motor cases, and nuclear-engine reactor shields and inflatable satellite launch canisters of magnesium are used in the space shuttle program.
Strength, toughness, machinability, and economy account for the use of ferrous forgings in off-highway and heavy construction equipment, and in mining machinery. In addition to engine and transmission parts, forgings are used for gears, sprockets, levers, shafts, spindles, ball joints, wheel hubs, rollers, yokes, axle beams, bearing holders, and links. Farm implements, in addition to engine and transmission components, utilize key forgings ranging from gears, shafts, levers, and spindles to tie-rod ends, spike harrow teeth, and cultivator shanks.
Forged components are found in virtually every implement of defense, from rifle triggers to nuclear submarine drive shafts. Heavy tanks contain more than 550 separate forgings; armored personnel carriers employ more than 250. The majority of 155-mm, 75-mm, and 3-in. shells as well as mortar projectiles contain at least two forged components.
For valves and fittings, the mechanical properties of forgings and their freedom from porosity are especially suited to high-pressure applications. Corrosion and heat-resistant materials are used for flanges, valve bodies and stems, tees, elbows, reducers, saddles, and other fittings. Oilfield applications include rock cutter bits, drilling hardware, and high-pressure valves and fittings.
Stationary and shipboard internal combustion engines include forged crankshafts, connecting rods, rod34920-34911528 caps, camshafts, rocker arms, valves, gears, shafts, levers, and linkages. Outboard motors, motorcycles, and power saws offer examples of the intensive use of forgings in smaller engines. Industrial equipment industries use forgings in materials handling systems, conveyors, chain-hoist assemblies, and lift trucks.
"Forged" is the mark of quality in hand tools and hardware. Pliers, hammers, sledges, wrenches, and garden implements, as well as wire-rope clips and sockets, hooks, turnbuckles, and eye bolts are common examples. Strength, resistance to impact and fatigue, and excellent appearance are reasons why forgings have been the standard of quality since the earliest of times. The same is true of surgical instruments. Special hardware for electrical transmission and distribution lines is subject to high stresses and corrosion. For strength and dependability, forgings are used for parts such as pedestal caps, suspension clamps, sockets, and brackets.
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Since the dawn of mankind, metalworking has assured strength, toughness, reliability, and the highest quality in a variety of products. Today, these advantages of forged components assume greater importance as operating temperatures, loads, and stresses increase.
Forged components make possible designs that accommodate the highest loads and stresses. Recent advances in forging technology have greatly increased the range of properties available in forgings.
Economically, forged products are attractive because of their inherent superior reliability, improved tolerance capabilities, and the higher efficiency with which forgings can be machined and further processed by automated methods.
The degree of structural reliability achieved in a forging is unexcelled by any other metalworking process. There are no internal gas pockets or voids that could cause unexpected failure under stress or impact. Often, the forging process assists in improving chemical segregation of the forging stock by moving centerline material to various locations throughout the forging.
To the designer, the structural integrity of forgings means safety factors based on material that will respond predictably to its environment without costly special processing to correct for internal defects.
To the production employee, the structural reliability of forgings means reduced inspection requirements, uniform response to heat treatment, and consistent machinability, all contributing to faster production rates and lower costs.
Directional Strength is Key
Directional strength is a direct result of the forging 34920-34911528.07process. In the forging process, controlled deformation (usually at elevated temperatures) results in greater metallurgical soundness and improved mechanical properties of the material. In most cases, forging stock has been pre-worked to remove porosity resulting from the solidification process. This produces directional alignment (or "grain flow") for important directional properties in strength, ductility, and resistance to impact and fatigue.These properties are deliberately oriented in directions requiring maximum strength. Working the material achieves recrystallization and grain refinement that yields the maximum strength potential of the material with the minimum property variation, piece-to-peace.
Properly developed grain flow in forgings closely follows the outline of the component. In contrast, bar stock and plate have unidirectional grain flow; any changes in contour will cut flow lines, exposing grain ends, and render the material more liable to fatigue and more sensitive to stress corrosion.
Designers and materials engineers are recognizing the increasing importance of resistance to impact and fatigue as a portion of total component reliability. With the use of proper materials and heat treatments, if required, improved impact strength of forged components is achievable.
The resulting higher strength-to-weight ratio can be used to reduce section thickness in part designs without jeopardizing performance characteristics of safety. Weight reduction, even in parts produced from less expensive materials, can amount to a considerable cost savings over the life of a product run.
The consistency of material from one forging to the next, and between separate quantities of forgings is extremely high. Forged parts are made through a controlled sequence of production steps rather than random flow of material into the desired shape.
Uniformity of composition and structure piece-to-piece, lot-to-lot, assure reproducible response to heat treatment, minimum variation in machinability, and consistent property levels of finished parts.
Dimensional characteristics are remarkably stable. Successive forgings are produced from the same die impression, and because die impressions exert control over all contours of the forged part, the possibility of transfer distortion is eliminated.
For cryogenic applications, forgings have the necessary toughness, high strength-to-weight ratios, and freedom from ductile-brittle transition problems.
Forgings are produced economically in an extremely broad range of sizes. With the increased use of special punching, piercing, shearing, trimming, and coining operations, there have been substantial increases in the range of economical forging shapes and the feasibility of improved precision. However, parts with small holes, internal passages, re-entrant pockets, and severe draft limitations usually require more elaborate forging tooling and more complex processing, and are therefore usually more economical in larger sizes.
Sizing Up the Competition
|Forging versus||Forging Advantages When Using A Similar Alloy|
Forgings are superior to metal parts produced by other methods in their compatibility with other manufacturing processes.
Forging Spans the Metallurgical Spectrum
||Combines low density with good strength-to-weight ratio|
||Usually employed at service temperatures lower than 500°F but certain alloys provide short-time service to 700°F.|
|Copper, Brass, Bronze||
||Important for applications requiring corrosion resistance.|
|Low-Carbon and Low-Alloy Steels||
||Comprise the greatest volume of forgings produced for service applications up to 900°F.|
||Various automotive and truck applications including crankshafts, connecting rods, yokes, pistons, suspension and steering components, spindles, hubs, and trunio|
|Used in transportation, mining, industrial and agricultural equipment, as well as high-stress applications in missiles and aircraft.||
||Used in pressure vessels, steam turbines, and many other applications in the chemical, food processing, petroleum, and hospital services industries. Used for high-stress service at temperatures up to 1,250°F and low-stress service to 1,800°F and higher. Nickel-Base|
||Service in the 1,200-1,800°F range. Structural shapes, turbine components, and fittings and valves.|
||Used primarily in the temperature services to 1,000°F. Configurations nearly identical to steel parts are forgeable and 40% lighter in weight. Aircraft-engine components and structurals, ship components, and valves and fittings in transportation and chemical industries.|
||High-temperature applications involving advanced chemical, electrical, and nuclear propulsion systems and flight vehicles.|
||Used primarily in nuclear, structural, and heat-sink applications.|
||Produced in relatively limited quantities and used almost exclusively in nuclear applications.|
Forging--metal shaping by plastic deformation--spans a myriad of equipment and techniques. Knowing the various forging operations and the characteristic metal flow each produces is key to understanding forging design.
Hammer and Press Forging
Generally, forged components are shaped either by a hammer or press. Forging on the hammer is carried out in a succession of die impressions using repeated blows. The quality of the forging, and the economy and productivity of the hammer process depend upon the tooling and the skill of the operator. The advent of programmable hammers has resulted on less operator dependency and improved process consistency. In a press, the stock is usually hit only once in each die impression, and the design of each impression becomes more important while operator skill is less critical.
Fig. 1. Compression between narrow dies.
Open Die Forging Open die forging with hammers and presses is a modern-day extension of the pre-industrial metalsmith working with a hammer at his anvil.
In open die forging, the workpiece is not completely confined as it is being shaped by the dies. The open die process is commonly associated with large parts such as shafts, sleeves and disks, but part weights can range from 5 to 500,000 lb.
Most open die forgings are produced on flat dies. Round swaging dies and V dies also are used in pairs or with a flat die. Operations performed on open die presses include:
Fig. 2. Roll forging.
As the forging workpiece is hammered or pressed, it is repeatedly manipulated between the dies until it reaches final forged dimensions. Because the process is inexact and requires considerable skill of the forging master, substantial workpiece stock allowances are retained to accommodate forging irregularities. The forged part is rough machined and then finish machined to final dimensions. The increasing use of press and hammer controls is making open die forging, and all forging processes for that matter, more automated.
In open die forging, metals are worked above their recrystallization temperatures. Because the process requires repeated changes in workpiece positioning, the workpiece cools during open die forging below its hot-working or recrystallization temperature. It then must be reheated before forging can continue. For example, a steel shaft 2 ft in diameter and 24 ft long may require four to six heats before final forged dimensions are reached.
In open die forging of steel, a rule of thumb says that 50 lb of falling weight is required for each square inch of stock cross-section.
Fig. 3. Roll forging using speciality shaped rolls.
Compression between flat dies, or upsetting,is an open die forging process whereby an oblong workpiece is placed on end on a lower die and its height reduced by the downward movement of the top die. Friction between end faces of the workpiece and dies prevents the free lateral spread of the metal, resulting in a typical barrel shape. Contact with the cool die surface chills the end faces of the metal, increasing its resistance to deformation and enhancing barreling.
Upsetting between parallel flat dies is limited to deformation symmetrical around a vertical axis. If preferential elongation is desired, compression between narrow dies (Fig. 1) is ideal. Frictional forces in the ax ial direction of the bar are smaller than in the perpendicular direction, and material flow is mostly axial.
A narrower die elongates better, but a too-narrow die will cut metal instead of elongate. The direction of material flow can also be influenced by using dies with specially shaped surfaces.
Compression between narrow dies is discontinuous since many strokes must be executed while the workpiece is moved in an axial direction. This task can be made continuous by roll forging (Fig. 2). Note the resemblance between Fig. 1 and Fig. 2. he width of the die is now represented by the length of the arc of contact. The elongation achieved depends on the length of this contact arc.
Fig. 4. Impression die forging
Larger rolls cause greater lateral spread and less elongation because of the greater frictional difference in the arc of contact, whereas smaller rolls elongate more. Lateral spread can be reduced and elongation promoted by using specially shaped rolls (Fig. 3).
The properties of roll-forged components are very satisfactory. In most cases, there is no flash and the fiber structure is very favorable and continuous in all sections. The rolls perform a certain amount of descaling, making the surface of the product smooth and free of scale pockets.
Impression Die Forging
In the most basic example of impression die forging, which accounts for the majority of forging production, two dies are brought together and the workpiece undergoes plastic deformation until its enlarged sides touch the die side walls (Fig. 4). Then, some material begins to flow outside the die impression, forming flash. The flash cools rapidly and presents increased resistance to deformation, effectively becoming a part of the tool. This builds pressure inside the bulk of the workpiece, aiding material flow into unfilled impressions.
Impression die forgings may be produced on a horizontal forging machine (upsetter) in a process referred to as upsetting. In upsetting, stock is held between a fixed and moving die while a horizontal ram provides the pressure to forge the stock (Fig .5). After each ramstroke, the multiple-impression dies can open to permit transfer of stock from one cavity to another.
Fig. 5. Upsetting.
A form of impression die forging, closed die forging does not depend on flash formation to achieve complete filling of the die. Material is deformed in a cavity that allows little or no escape of excess material, thus placing greater demands on die design.
For impression die forging, forging dies become more important, and operator skill level is less critical in press forging operations. The press forging sequence is usually block and finish, sometimes with a preform, pierce, or trim operation. The piece is usually hit only once in each die cavity.
The Precision Forging Advantage
Precision forging normally means close-to-final form or close-tolerance forging. It is not a special technology, but a refinement of existing techniques to a point where the forged part can be used parts2cmykwith little or no subsequent machining. Improvements cover not only the forging method itself but also preheating, descaling, lubrication, and temperature control practices.
The decision to apply precision forging techniques depends on the relative economics of additional operations and tooling vs. elimination of machining. Because of higher tooling and development costs, precision forging is usually limited to extremely high-quality applications.
Stages in the Ring Rolling Process
Ring rolling has evolved from an art into a strictly controlled engineering process. Seamless rolled rings are produced on a variety of equipment. All give the same product--a seamless section with circumferential grain orientation. These rings generally have tangential strength and ductility, and often are less expensive to manufacture than similar closed die forgings. In sum, the ring rolling process offers homogeneous circumferential grain flow, ease of manufacture, and versatility in material, size, mass, and geometry.
In the ring rolling process, a preform is heated to forging temperature and placed over the idler (internal) roll of the rolling machine. Pressure is applied to the wall by the main (external) roll as the ring rotates. The cross-sectional area is reduced as the inner and outer diameters are expanded. Equipment can be fully automated from billet heating through post-forge handling. Advanced ring rolling equipment can roll contours in both the inner and outer diameter of the ring, allowing for excellent weight reductions, material savings, and reduced machining cost.
There is an infinite variety of sizes into which rings can be rolled, ranging from rollerbearing sleeves to rings of 25 ft in diameter with face heights of more than 80 in. Various profiles may be rolled by suitably shaping the drive and idling rolls.
Extrusion In extrusion (Fig. 6), the workpiece is placed in a container and compressed until pressure inside the metal reaches flowstress levels. The workpiece completely fills the container and additional pressure causes it to travel through an orifice and form the extruded product.
Extrusion can be forward (direct) or backward (reverse), depending on the direction of motion between ram and extruded product. Extruded product can be solid or hollow. Tube extrusion is typical of forward extrusion of hollow shapes, and backward extrusion is used for mass production of containers.
Fig. 6. a-Foward extrusion; b-backward extrusion; c-tube extrusion; d-container extrusion.
Piercing s closely related to reverse extrusion but distinguished by greater movement of the punch relative to movement of the workpiece material.
Secondary Processes Besides the primary forging processes, secondary operations often are employed. Drawing through a die is a convenient way to eliminate forged draft (Fig. 7a). The mode of deformation is tangential compression. The diameter of the drawing ring can be slightly smaller than the outer diameter of the preforged shell to control or reduce wall thickness and increase the height of the shell in a drawing or ironing operation (Fig. 7b).
Bending can be performed on the finished forging or at any stage during its production.
Because forging stock may assume complex shapes, it is rare that only a single die impression is needed. Preforming the forging stock--by bending or rolling it, or by working it in a preliminary die--may be more desirable. Gains in productivity, die life, and forging quality often outweigh the fact that preforming adds an operation and attendant costs. Forging in one final die impression may be practical for extremely small part runs.
Since bending of larger parts requires a machine of long stroke, special mechanical or hydraulic presses are often necessary. Simple shapes can be bent in one operation, but more complex contours take successive steps. If complex shapes are to be formed in a single operation, the tool must contain moving elements.
Special Techniques After deformation, forged parts may undergo further metalworking. Flash is removed, punched holes may be needed, and improved surface finish or closer dimensional accuracy may be desired.
Trimming-- Flash is trimmed before the forging is ready for shipping. Occasionally, especially with crack-sensitive alloys, this may be done by grinding, milling, sawing, or flame cutting.
Coining-- Coining and ironing are essentially sizing operations with pressure applied to critical surfaces to improve tolerances, smoothen surfaces, or eliminate draft.
Coining is usually done on surfaces parallel to the parting line, while ironing is typified by the forcing of a cup-shaped component through a ring to size on outer diameter. Little metal flow is involved in either operation and flash is not formed.
Swaging-- This operation is related to the open die forging process whereby the stock is drawn out between flat, narrow dies. But instead of the stock, the hammer is rotated to produce multiple blows, sometimes as high as 2,000 per minute. It is a useful method of primary working, although in industrial production its role is normally that of finishing. Swaging can be stopped at any point in the length of stock and is often used for pointing tube and bar ends and for producing stepped columns and shafts of declining diameter.
Fig. 8. Hot extrusion of a valve body.
Hot Extrusion-- Extrusion is most suitable for forming parts of drastically changing cross section and is, therefore, a direct competitor to continuous upsetting and the horizontal forging machine. In Fig. 8, a bar section of car efully controlled volume is heated, descaled, and placed into the die. Under pressure of the closely fitting punch (Fig. 8a), the material first fills the cavity, then part of it is extruded into a long stem. At the end of the stroke (Fig. 8b), a valve body is obtained that needs only grinding of the seating surfaces.
There are a number of variants of the extrusion process, many of them patented. The slug may be hollow (machined), pierced in a separate operation or in the extrusion process itself. In all instances, the quality of heating, the efficiency of scale removal or prevention, and the effectiveness of lubrications are matters of greatest importance. The variety of shapes produced are numerous. Dimensional accuracy, surface quality, and productivity are high, and a greater degree of deformation can be achieved in a single operation than in any other forging method.
Cold, Warm, and Hot Forging--What's the Difference?
Cold forging involves either impression die forging or true closed die forging with lubricant and circular dies at or near room temperature. Carbon and standard alloy steels are most commonly cold-forged. Parts are generally symmetrical and rarely exceed 25 lb. The primary advantage is the material savings achieved through precision shapes that require little finishing. Completely contained impressions and extrusion-type metal flow yield draftless, close-tolerance components. Production rates are very high with exceptional die life. While cold forging usually improves mechanical properties, the improvement is not useful in many common applications and economic advantages remain the primary interest. Tool design and manufacture are critical.
Warm forging has a number of cost-saving advantages which underscore its increasing use as a manufacturing method. The temperature range for the warm forging of steel runs from above room temperature to below the recrystallization temperature, or from about 800 to 1,800°F. However, the narrower range of from 1,000 to 1,330°F is emerging as the range of perhaps the greatest commercial potential for warm forging. Compared with cold forging, warm forging has the potential advantages of: Reduced tooling loads, reduced press loads, increased steel ductility, elimination of need to anneal prior to forging, and favorable as-forged properties that can eliminate heat treatment.
Hot forging is the plastic deformation of metal at a temperature and strain rate such that recrystallization occurs simultaneously with deformation, thus avoiding strain hardening. For this to occur, high workpiece temperature (matching the metal's recrystallization temperature) must be attained throughout the process. A form of hot forging is isothermal forging, where materials and dies are heated to the same temperature. In nearly all cases, isothermal forging is conducted on superalloys in a vacuum or highly controlled atmosphere to prevent oxidation.
Steel axle spindles are used in semi-trailer axles
Steel tool joints are friction-welded to seamless tubing to make drill pipe for oil and gas applications.
High-pressure steel filter bowls are used in industrial applications to filter hydraulic fluids.
The forging process is used to produce steel projectiles for military use.
Seamless rolled rings are available in an infinite variety of diameters, wall thicknesses, and face heights. These rings are produced from various materials and have many applications.
An example of how large closed die forgings can be, this titanium bulkhead is used in F-22 aircraft.
The strength and durability of forged parts make them ideal for use in hoisting applications.
The forging process can produce a variety of automotive parts including (from left): connecting rods and caps, transmission shift forks and u-joint steering yokes.
This aft dome is used in Titan IV space launch vehicles.
Forged titanium parts such as these are often used in airplane production.
Parts produced via open die forging can be extremely large, reaching 500,000 lb. Examples include this blow-out preventer, used in the natural-gas and oil-exploration industries, and this crankshaft, used in drilling applications.
The closed die forging process is ideal for production of crankshafts including what is believed to be the largest closed die-forged crankshaft, this one weighing more than 2.5 tons.
Open die forging is ideal for producing huge parts that must withstand tremendous forces, such as locomotive crankshafts and connecting rods, shipboard crankshafts and propeller shafts, and oil-platform tension legs.