Forging of Carbon and Alloy Steels
Forging of Carbon and Alloy Steels
Forging of carbon (C) and alloy steels constitutes a metal working process, which has the ability to form the material to the desired component shape, while refining the cast structure of the forging material, healing shrinkage voids, and improving the mechanical properties of the material. The amount of subsequent machining gets also reduced, although this depends on the geometry of the finished part and the forging processes being used.
Cast ingots have been the traditional starting point for forgings, either forging directly from the ingot, or from a bloom or billet which has been hot worked from an ingot. With the wide use of the continuous cast steel, continuous cast product is now normally used as the initial stock. Besides cast ingots, rolled and cast blooms and billets, other starting materials for forgings are plates, bars and rods, and steel castings.
Forging of iron and steel started with the beginning of the Iron Age. At that time hot working by hammering was part of the process for producing wrought iron, and for making products in both wrought iron and steel. The crude smelting furnaces using high-grade iron ore, charcoal, and fluxes produced small quantities of iron that had to be forge welded together by hand to produce useful stock. Initially, this was the main purpose of the forging at that time. It is generally acknowledged that the beginning of the industrial revolution started in earnest the forging of the steels. Despite (or perhaps because of) this long history, the forging of steels is an intuitive, empirical process.
Forgings are normally classified in several ways, beginning with the general classifications of open die forging and closed die forging. They are also classified in terms of the close-to-finish factor, or the amount of stock (cover) which is to be removed from the forging by machining to satisfy the dimensional and detail requirements of the finished part. Finally, forgings are further classified in terms of the forging equipment required for their manufacture, such as, hammer upset forgings, ring-rolled forgings, and multiple-ram press forgings. Of the various classifications, those based on the close-to-finish factor are most closely related to the inherent properties of the forging, such as strength and resistance to stress corrosion. In general, the type of forging which needs the least machining to satisfy finished part requirements has the best properties.
As per ASTM specification A 788, steel forging is the product of a substantially compressive plastic working operation that consolidates the material and produces the desired shape. The plastic working may be performed by a hammer, press forging machine, or ring rolling machine and must deform the material to produce an essentially wrought structure. Hot rolling operations may be used to produce blooms or billets for reforging. Forgings may be subdivided into the following three classes on the basis of their forging temperatures.
- Hot-worked forgings – Forgings produced by working at temperatures above the recrystallization temperature for the material.
- Hot-cold-worked forgings – Forgings worked at elevated temperatures slightly below the recrystallization temperature to increase mechanical strength. Hot-cold worked forgings may be made from material previously hot worked by forging or rolling. A hot-cold-worked forging may be made in one continuous operation wherein the material is first hot worked and then cold worked by control of the finishing temperature. Because of differences in manufacture hot-rolled, or hot and cold finished bars (semi-finished or finished), billets or blooms are not considered to be forgings.
- Cold-worked forgings – Forgings produced by plastic working well below the temperature range at which recrystallization of the material occurs. Cold-worked forgings must be made from material previously hot worked by forging or rolling.
The wrought product forms for steel include plate, shapes, bar, sheet, strip, tubes, pipes, extrusions, and forgings. Generally, extrusions are included with forgings, but the definition of a forging excludes rolled plate and bars. This is because forgings, besides conforming approximately to the finished shape of the required component, are not expected to exhibit the traits of laminar inclusions through thickness weakness sometimes associated with hot rolled plate, or the central unsoundness sometimes associated with hot rolled bar.
Carbon and alloy steels are normally by far the most forged materials, and are readily forged into a wide variety of shapes using hot- forging, warm- forging, or cold-forging processes and standard equipment. Selection of forging temperatures for carbon and alloy steels is based on carbon content, alloy composition, the temperature range for optimum plasticity, and the amount of reduction needed to forge the work piece. Of these factors, carbon content has the most influence on upper-limit forging temperatures.
Despite the large number of available compositions, all of the materials in this category show essentially similar forging characteristics. Exceptions to this are steels containing free-machining additives such as sulphides since these materials are more difficult to forge than are non-free machining grades. Normally, the hot forgeability of carbon and alloy steels improves as deformation rate increases. The improvement in workability has primarily been due to the increased heat of deformation generated at high deformation rates.
Justification for selecting forging in preference to other, sometimes more economical, methods of producing useful shapes is based on several considerations. Mechanical properties in wrought materials are maximized in the direction of major metal flow during working. For complex shapes, only forging affords the opportunity to direct metal flow parallel to major applied service loads and to control, within limits, the refinement of the original structures of the forging materials. Refinement of microstructure is a function of the temperature, the direction, and the magnitude of reduction from the forging material to the forged shape. Maximizing the structural integrity of the material permits refinement of design configuration, which in turn permits reduction of weight. Adequate control of metal flow to optimize properties in complex forging configurations generally needs one or more upsetting operations prior to die forging and can require hollow forging or back extrusion to avoid flash formation at die parting lines.
Because of the functions which the steel forgings are intended to fulfill, forging designs often include large heat-treated section sizes and can be of irregular shape, so that significant stresses can be applied in service in all the three principal axes namely (i) longitudinal, (ii) transverse, and (iii) short transverse. By careful selection of the starting material size and forging steps, it is possible for a forging to show favourable properties in all three directions. In other instances, for example, in an upset disk forging, favourable mechanical properties can be achieved in a radial direction around the full circumference, something which is possible in a disk which has been simply cut from a rolled plate.
Hot forging behaviour
The hot forging of carbon and alloy steels into intricate shapes is rarely limited by forgeability aspects with the exception of the free-machining grades. Section thickness, shape complexity, and forging size are limited mostly by the cooling which occurs when the heated work piece comes into contact with the cold dies. For this reason equipment which has relatively short die contact times, such as hammers, is often preferred for forging intricate shapes in steel.
Forgeability – Forgeability is the relative ability of steel to flow under compressive loading without fracturing. Except for resulphurized and rephosphorized grades, most carbon and low alloy steels are usually considered to have good forgeability. Differences in forging behaviour among the various grades of steel are small enough to seldom affect the selection of the steel by forging behaviour. However, the choice of resulphurized or rephosphorized steel for a forging is usually justified only if the forging is to be extensively machined since one of the main reasons for considering production by forging is the avoidance of subsequent machining operations, this situation is uncommon.
One common means of measuring the forgeability of steels is the hot-twist test. As the name implies, this test involves twisting of heated bar samples to fracture at a number of different temperatures selected to cover the possible hot working temperature range of the test material. The numbers of twists to fracture, as well as the torque needed to maintain twisting at a constant rate, are reported. The temperature, at which the number of twists is the greatest, if such a maximum exists, is assumed to be the optimal hot working temperature of the test material. Forgeabilities of several carbon steels as determined by hot twist testing are shown in Fig 1. Various other tests as given below are used to evaluate the forgeability of steels.
- The wedge-forging test – In this test a wedge-shape specimen is forged between flat dies and the vertical deformation which causes cracking is established.
- The side-pressing test – This test consists of compressing a cylindrical bar sample between flat, parallel dies with the axis of the cylinder parallel to the dies. The ends of the cylinder are unconstrained, and forgeability is measured by the amount of deformation obtained before cracking.
- The upset test – In this test, a cylinder is compressed between flat dies and the surface strains at fracture at the equator of the cylinder are measured.
- The notched-bar upset test – This test is similar to the upset test except that axial notches are machined into the test sample to introduce high local stress levels. These higher stresses can be more indicative of the stresses experienced during actual forging operations than those produced in the standard upset test.
- The hot tensile test – This test frequently uses a special test apparatus to vary both strain rates and temperatures over a wide range.
Effect of strain rate on forgeability – The forgeability of steels generally increases with increasing strain rate. This effect has been shown for low-carbon steel in hot-twist testing (Fig1), where the number of twists to failure increases with increasing twisting rate. It is believed that this improvement in forgeability at higher strain rates is due to the increased heat of deformation produced at higher strain rates. Excessive temperature increases from heat of deformation, however, can lead to incipient melting, which can lower forgeability and mechanical properties.
Fig 1 Influence of deformation rate and forgeabilities of various carbon steels
Flow stress and forging pressure – Flow stresses and forging pressures can be obtained from torque curves generated in hot-twist tests or from hot compression or tension testing. The data from these curves show that the relative forging pressure requirements for this group of alloys do not vary widely at normal hot forging temperatures. Considerably greater pressures are needed for the more highly alloyed material, and this alloy material also shows a more significant increase in forging pressure with increasing reduction.
Effect of strain rate on forging pressure – Forging pressures needed for a given steel increase with increasing strain rate. Studies of low carbon steel indicate that the influence of strain rate is more pronounced at higher forging temperatures. This effect is shown in Fig 2, which gives stress-strain curves for low carbon steel forged at various temperatures and strain rates. Similar effects have been observed in alloy steels.
Fig 2 Forging pressure for low carbon steel upset at various temperatures and two strain rates
Selection of steels for forging
Carbon and alloy steel ingots, blooms, billets, and slabs for forging are hot rolled or cast to approximate cross sectional dimensions, and hence, straightness, camber, twist, and flatness tolerances do not apply. Semi-finished steel products for forging are produced to either specified piece weights or specified lengths.
Surface conditioning – Semi-finished steel products for forging can be conditioned by scarfing, chipping, or grinding to remove or minimize surface imperfections. However, it is to be kept in mind that, regardless of surface conditioning, the product is still likely to contain some surface imperfections.
Weight tolerances – The tolerances for billets, blooms, and slabs are frequently +/- 5 % for individual pieces or for lots weighing less than 18 tons. Lots weighing more than that are often subject to weight tolerances of +/- 2.5 %.
Cutting – Semi-finished steel products for forging are generally cut to length by hot shearing. Depending on the steel composition, hot sawing or flame cutting can also be used.
Quality – Quality as the term is applied to semi-finished steel products for forging, is dependent on many different factors, including the degree of internal soundness, relative uniformity of chemical composition, and relative freedom from surface imperfections.
Forging quality semi-finished steel is used in hot forging applications which can involve subsequent heat treatment or machining operations. Such applications need relatively close control of chemical composition and of steel production.
Selection of steel for a forged component is an integral part of the design process, and acceptable performance is dependent on this choice. A careful understanding of the end use of the finished part serves to define the required mechanical properties, surface finish requirements, tolerance to non-metallic inclusions, and the attendant inspection methods and criteria.
Steels of forging quality are produced to a wide range of chemical compositions. With each of the melting and rolling practices, a level of testing and evaluation of quality is done. If there is a necessity, then one or more special quality restrictions can be specified for the steel such as the level of the occurrence of non-metallic inclusions. Occasionally, for higher reliability applications, it is necessary that the steel is subjected to vacuum arc remelting or electroslag remelting process.
The use of micro-alloyed steels has evolved in recent years such applications as automotive crankshafts. These steels typically have small additions (0.05 % to 0.1 %) of vanadium or niobium and can achieve acceptable properties in the non-heat-treated (as forged) condition. Consequently, these alloys retain the advantages of the forging process while being economically competitive with castings because of the elimination of the heat-treat cycle.
Design requirements – The selection of steel for a forged part normally needs some compromise between opposing factors, for example, strength versus toughness, stress-corrosion resistance versus weight, manufacturing cost versus useful load-carrying ability, production cost versus maintenance cost, and the cost of the steel raw material versus the total manufacturing cost of the forging. Material selection also involves consideration of melting practices, forming methods, machining operations, heat treating procedures, and deterioration of properties with time in service, as well as the conventional mechanical and chemical properties of the steel to be forged.
An efficient forging design achieves maximum performance from the minimum amount of material consistent with the loads to be applied, producibility, and desired life expectancy. To match the steel to its design component, the steel is first appraised for strength and toughness and then qualified for stability to temperature and environment. Optimum steels are then analyzed for producibility and finally for economy.
Failure analysis is a useful data source for matching the properties of steels to requirements. Failure of a component can occur during operation within the design stress range. One cause of premature failure is lack of proper orientation of a critical design stress with the preferred grain flow of a forging. Unpredicted failure can also occur because of the deterioration of properties with time and service. For example, stress corrosion cracking, which results from sustained tensile stress, can occur even in a typical ambient atmosphere. Under these conditions, failure is most likely to occur at locations in the forging which coincide with exposed end grain. Failure analyses can uncover other causes of premature failure, such as excessive grain growth, inclusions of non-metallic impurities, grain flow folding from improper forging practice, lack of a wrought metallurgical structure, and the inadvertent production of stress raisers by machining to an overly sharp fillet or by poor fit in assembly.
Effects of forging on properties
The shaping of a complex configuration from a carbon or alloy steel bar or billet needs first that the steel be ‘arranged’ into a suitable starting shape (preformed) and then that it be caused to flow into the final part configuration. This rearrangement of the metal has little effect on hardness and strength of the steel, but certain mechanical properties (such as ductility, impact strength, and fatigue strength) are enhanced. This improvement in properties (Fig 2) take place because forging (i) breaks up segregation, heals porosity, and aids homogenization, (ii) produces a fibrous grain structure which enhances mechanical properties parallel to the grain flow, and (iii) reduces as-cast grain size.
Typical improvements in ductility and impact strength of heat-treated steels as a function of forging reduction are shown in Fig 3. These data illustrate that maximum improvement in each case occurs in the direction of maximum elongation. Toughness and ductility reach maximums after a certain amount of reduction, after which further reduction is of little value.
Fig 3 Effect of forging ratios on the mechanical properties
Tab 1 gives the typical longitudinal mechanical properties of low carbon and medium carbon steel forgings in the annealed, normalized, and quenched and tempered conditions. As can be expected, strength increases with increasing carbon content, while ductility decreases. It is to be recognized that closed-die forgings for the most part are made from wrought billets which have received considerable prior working. Open-die forgings, however, can be made from either wrought billets or as-cast product.
|Tab 1 Longitudinal properties of carbon steel forgings at four carbon contents|
|Sl. No.||Carbon content||Ultimate tensile strength||Yield strength, 0.2 % offset||Elongation||Reduction in area||Fatigue strength *||Hardness|
|Oil quenched and tempered at 595 deg C|
|* Rotating beam test at 10,000,000 endurance limit|
Metal flows in various directions during the closed-die forging. As an example, in the forging of a rib and web shape such as an air frame component, nearly all metal flow is in the transverse direction. Such transverse flow improves ductility in that direction with little or no reduction in longitudinal ductility. Transverse ductility can conceivably equal or surpass longitudinal ductility if forging reductions are large enough and if metal flow is primarily in the transverse direction. Similar effects are observed in the upsetting of wrought billets. In this case, however, the original longitudinal axis of the material is shortened by upsetting, and lateral displacement of metal is in the radial direction. When upset reductions exceed around 50 %, ductility in the radial direction usually exceeds that in the axial direction (Fig 4).
Fig 4 Typical influence of upset reduction on axial and radial ductility of forged steels
Wrought structure and ductility – Another aspect of material control ensures that the final forging has undergone sufficient plastic deformation to achieve the wrought structure necessary for development of the mechanical properties on which the design has been based. Although some plastic deformation is achieved during the breakdown of a cast product into a forging billet, far more is imparted during the closed-die forging process. Material control for high strength forgings can need determination of the mechanical properties of the forging billet, as well as those of the forging.
A measure of ductility or toughness is determined by measuring the reduction in area obtained in transverse tension test samples. When corresponding tests are made of transverse and longitudinal samples taken from forgings heat treated to the same strength level, it is possible to compare the mechanical properties of billet stock and forgings and to estimate the proportion of the final wrought metallurgical structure contributed by each.
Ductility and the amount of forging reduction – A principal objective of material control is to ensure that optimum mechanical properties are achieved in the finished forging. The amount of reduction achieved in forging has a marked effect on ductility, as shown in Fig 4, which compares ductility in the cast ingot, the wrought (rolled) bar or billet, and the forging. The curves in Fig 4(a) indicate that when a wrought bar or billet is flat forged in a die, an increase in forging reduction does not affect longitudinal ductility, but does result in a gradual increase in transverse ductility. When a similar bar or billet is upset forged in a die, an increase in forging reduction results in a gradual decrease in axial ductility and a gradual increase in radial ductility.
The ductility of cast ingots varies with chemical compositions, melting practice, and ingot size. The ductility of steel ingots of the same alloy composition also varies, depending on whether they are poured from air-melted or vacuum arc remelted steel. When starting with a large ingot of a particular alloy, it is sometimes practical to roll portions of the ingot to various billet or bar sizes with varying amounts of forging reduction. The minimum amount of reduction is not standard, but is seldom less than 2:1 (ratio of ingot section area to billet section area). Reduction of steel ingot to billet is normally much greater than 2:1. In contrast, some heat-resisting alloy forgings are forged directly from a cast ingot.
Frequently, it is not feasible to prepare billets for forgings which are so large that they need the entire weight of an ingot. The amount of forging reduction represented by wrought metallurgical structures is best controlled by observation and testing of macro-etch and tension test samples taken from completed forgings. These samples permit exploration of critical areas and, generally, of the entire forging. They are selected from the longitudinal, long-transverse, and short-transverse grain directions, as required. Etch tests permit visual observation of grain flow. Mechanical tests correlate strength and toughness with grain flow.
Grain flow – Macro-etching permits direct observation of grain direction and contour and also serves to detect folds, laps, and re-entrant flow. By macro-etching suitable samples, grain flow can be examined in the longitudinal, long-transverse, and short-transverse directions. Macro-etching also permits evaluation of complete sections, end-to-end and side-to-side, and a review of uniformity of macro- grain size. Fig 2 shows grain flow in a representative forged part.
Grain size and micro-constituents – Metallographic examination, using a microscope, is best suited for examining questionable areas revealed by macro-etching, for measuring grain size, and for determining the nature and amount of micro-constituents.
Fatigue strength – Fatigue tests are used in material control under the conditions and for the purposes such as (i) laboratory testing of small samples for the development or qualification of material, (ii) laboratory testing of complete components or sub-assemblies for design development, and (iii) surveillance of components or assemblies in the field to ensure their continuing reliability in service.
The laboratory fatigue testing of small samples for the qualification or development of material is done by standard methods. Test samples are taken either from mill products or from closed-die forgings, as required. Standard samples are small enough to permit selection from many locations within a forging and to correlate with various directions of grain flow. Testing is generally done at room temperature in air, although testing at higher or lower temperatures and in special atmospheres is feasible.
The application of small-scale laboratory fatigue testing to the analysis of components or assemblies introduces additional variables. One is the effect of surface condition. The curves in Fig 5(a) show that the fatigue strength of steel samples varies markedly, depending on whether the surface is polished, machined, hot rolled, or as-forged. The steel tested is a wrought low alloy steel heat treated to 269 HB to 285 HB, equivalent to a tensile strength of 876 MPa and yield strength of 696 MPa. Sample preparation needed that the samples be machined and polished after heat treatment and that rolling or forging precede heat treatment. For a fatigue life of 106 cycles, the fatigue limit is 395 MPa for the ground samples, 315 MPa for the machined samples, 205 MPa for the as-rolled samples, and only 150 MPa for the as-forged specimens.
The curves in Fig. 5(b) apply to steels with tensile strength ranging from 345 MPa to 2,070 MPa and are approximations from several tests. Sample preparation for as-forged or decarburized samples at the 965 MPa tensile strength level include steels rough machined from bar stock, heated to around 900 deg C in a gas-fired muffle furnace for 20 minutes to 30 minutes, very lightly swaged from an original 7.47 mm diameter to a final diameter of 7.16 mm, and air cooled. Heat treatment consisted of austenitizing in a salt bath at around 830 deg C for 45 minutes, oil quenching, tempering in air for 1 hour at around 620 deg C, and water quenching. Forging and heat treating produced a surface decarburized to a depth of around 0.064 mm. These samples have shown fatigue strength, at 106 cycles, of around 310 MPa, compared with 470 MPa for samples which are not forged but are machined or polished and free of decarburization. Decarburization lowers the strength levels obtained by heat treatment. Laboratory control of surface condition is difficult to duplicate in the quantity production of the forged components. Hence, the fatigue strength of full-size components varies over a wider range than that of small samples, because of variations in surface condition.
Fig 5 Effect of surface condition on fatigue limit
Fracture toughness – The brittle fracture of forgings and other components as the result of crack propagation at stress levels considerably below the yield strength of the steel has led to widespread study of fracture characteristics and methods of evaluating fracture toughness. Results of these studies are of major importance for material control, especially with respect to the development of tests for evaluating fracture toughness on which standards for material control can be based.
In the area of laboratory tests and analytical techniques, major emphasis has been placed on the development of dependable methods for evaluating the strength of steels which contain cracks or crack like defects. Specifically, interest has centered on methods for determining plane-strain fracture toughness. Forged components are evaluated by testing small samples removed from selected locations on the forging which are representative of the various grain directions.
One test procedure comprises the bend testing of the notched and fatigue-cracked samples in a neutral environment. The objective of this test is to get a lower limiting value of fracture toughness which can be used to estimate the relationship between stress and defect size in a metal under service conditions in which high constraint is expected. In the test procedure referred to, a test sample with a chevron notch is suitably pre-cracked in fatigue. It is then tested in a bend test fixture provided with support rolls which rotate and move apart slightly to permit rolling contact and virtually eliminate the friction effect. The sample is subjected to three-point bending, and the imposed load versus displacement change across the notch is recorded on an autographic recorder. Fracture toughness is rated by a calculated parameter, the critical stress intensity.
End-grain exposure – Lowered resistance to stress-corrosion cracking in the long-transverse and short-transverse directions is related to the end-grain exposure. A long, narrow test sample sectioned so that the grain is parallel to the longitudinal axis of the sample has no exposed end grain, except at the extreme ends, which are not subjected to the loading. In contrast, a corresponding sample cut in the transverse direction has end-grain exposure at all points along its length. End grain is especially pronounced in the short-transverse direction on die forgings designed with a flash line. Consequently, forged components designed to reduce or eliminate end grain have better resistance to stress-corrosion cracking.
Residual stress – The sustained tensile stress at the surface of a forging which contributes to stress-corrosion cracking is the total of applied and residual stresses. When the residual stress constitutes a significant percentage of the total stress, it is to be reduced or eliminated. Common sources of residual tensile stresses include quenching, machining, and poor fit in assembly. Each can be suitably modified to reduce or eliminate tensile stresses, especially those present in an exposed surface. As an example, drastic quenching places the surface of a heat-treatable alloy in a state of compression and the core in a state of tension. Furthermore, the compressed surface can be entirely removed during rough machining, exposing the tension-stressed core material. This hazard can be avoided by quenching after, rather than before, rough machining. In some applications, a surface in tension is placed in compression by shot peening.
Hydrogen-stress cracking occurs without corrosion. Hence, its initiation is not confined to exterior surfaces in contact with a corrosive medium. It can start at any suitable nucleus, such as an inclusion or void, as well as at a surface notch or other irregularity. Hydrogen-stress cracking at the interior is described as hydrogen embrittlement or hydrogen flaking. Hydrogen-stress cracking has been observed, studied, and brought under control in most high-strength steels. The modern practice of vacuum melting can reduce residual hydrogen to negligible amounts. A hydrogen content of 3 ppm to 6 ppm in air-melted steel can be readily lowered to 0.6 ppm to 1 ppm by vacuum arc remelting. Provided that the initial hydrogen content of the steel is acceptably low, material control procedures are to ensure that hydrogen pickup is avoided in all subsequent processing, including forging, heat treating, hot salt bath descaling, pickling, and plating. During forging, steels develop a surface scale and a decarburized surface layer, both of which are subsequently removed by grit blasting and machining. Unless the steel is acid pickled, there is no possibility of hydrogen pickup.
Many of the critical parts made from steel forgings are protected by a coating of cadmium. Steel parts heat treated to strength levels higher than 1,655 MPa are especially sensitive to hydrogen pickup, in case they are coated with cadmium, the coating is deposited in vacuum. Parts heat treated to strength levels lower than 1,655 MPa can be cadmium plated electrolytically, provided that a titanium-containing plating bath is used and the parts are subsequently baked at around 190 deg C for 12 hours.
Mechanical properties – A major advantage of shaping metal parts by rolling, forging, or extrusion stems from the opportunities such processes offer the designer with respect to the control of grain flow. The strength of these and similar wrought products is almost always greatest in the longitudinal direction (or equivalent) of grain flow, and the maximum load-carrying ability in the finished part is achieved by providing a grain-flow pattern parallel to the direction of the major applied service loads when, in addition, sound, dense, good-quality metal of sufficiently fine grain size has been produced throughout.
Grain flow and anisotropy – Steel which is rolled, forged, or extruded develops and retains a fiber like grain structure aligned in the principal direction of working. This characteristic becomes visible on external and sectional surfaces of wrought products when the surfaces are suitably prepared and etched. The fibers are the result of elongation of the micro-structural constituents of the steel in the direction of working. Hence, the phrase ‘direction of grain flow’ is normally used to describe the dominant direction of these fibers within wrought metal products.
In wrought steel, the direction of grain flow is also evidenced by measurements of mechanical properties. Strength and ductility are almost always greater in the direction parallel to that of working. The characteristic of showing different strength and ductility values with respect to the direction of working is referred to as mechanical anisotropy and is exploited in the design of wrought products. Although the best properties in wrought steels are most frequently the longitudinal (or equivalent), properties in other directions can yet be superior to those in products not wrought, that is, in cast ingots or in forging stock taken from a lightly worked ingot.
Rectangular sections show anisotropy among all the three principal directions i.e. longitudinal, long transverse, and short transverse. A design which employs a rectangular section involves the properties in all these directions, not just the longitudinal. Hence, the longitudinal, long-transverse, and short-transverse service loads of rectangular sections are analyzed separately.
Anisotropy in high strength steel – Although all wrought steels are mechanically anisotropic, the effects of anisotropy on mechanical properties vary among different metals and alloys. For example, a vacuum-melted steel of a given composition is generally less mechanically anisotropic than a conventionally killed, air-melted steel of the same composition. Response to etching to reveal the grain flow characteristic of anisotropy also varies. Steels with poor corrosion resistance are readily etched, while those with good corrosion resistance need more corrosive etchants and extended etching times to reveal grain flow. In general, fatigue properties are markedly affected by the relation of flow-line direction to direction of stresses from applied loads. When flow lines are perpendicular to load stresses, a stress-raising effect is produced.
For many years, oil-graphite mixtures have normally being used as lubricants for forging carbon and alloy steels. Recent advances in lubricant technology, however, have resulted in new types of lubricants, including water/graphite mixtures and water-base synthetic lubricants. Each of the normally used lubricants has advantages as well as limitations (Tab 2) which is required to be balanced against process requirements.
|Tab 2 Advantages and limitations of the main lubricants used for hot forging of steels|
|Sl. No.||Type of lubricant||Advantages||Limitations|
|1||Water-base micro-graphite||Eliminates smoke and fire; provides die cooling; is easily extended with water||Must be applied by spraying for best results|
|2||Water-base synthetic||Eliminates smoke and fire; is cleaner than oils or water-base graphite; aids die cooling; is easily diluted, and needs no agitation after initial mixing; reduces clogging of spray equipment; does not transfer dark pigment to part||Must be sprayed; lacks the lubricity of graphite for severe forging operations|
|3||Oil-base graphite||Fluid film lends itself to either spray or swab application; has good performance over a wide temperature range (upto 540 deg C).||Generates smoke, fire, and noxious odours; explosive nature may shorten die life; has potentially serious health and safety implications for workers|
Selection criteria – Lubricant selection for forging is based on several factors, including forging temperature, die temperature, forging equipment, method of lubricant application, complexity of the part being forged and environmental and safety considerations. At normal hot-forging temperatures for carbon and alloy steels, water-base graphite lubricants are used almost exclusively, although some hammer shops still employ oil-base graphite.
The most common warm-forming temperature range for carbon and alloy steels is 540 deg C to 870 deg C. Because of the severity of forging conditions at these temperatures, billet coatings are often used in conjunction with die lubricants. The billet coatings used include graphite in a fluid carrier or water-base coatings used in conjunction with phosphate conversion coating of the work piece. For still lower forging temperatures (less than around 400 deg C, molybdenum disulphide has a greater load-carrying capacity than does graphite. Molybdenum disulphide can either be applied in solid form or dispersed in a fluid carrier.
Heat treatment of carbon and alloy steel forgings
Normally steel forgings are specified based upon one of four man conditions namely (i) as forged with no further thermal processing, (ii) heat treated for machinability, (iii) heat treated for final mechanical / physical properties, or (iv) special heat treatment to enhance dimensional stability, particularly in more complex part configurations.
As forged with no further thermal processing – Although the vast majority of steel forgings are heat treated before use, a large tonnage of low carbon steel (0.1 % to 0.25 % C) is used in the as-forged condition. In such forgings, machinability is good, and little is gained in terms of strength by heat treatment. In fact, a number of widely used specifications permit this economic option. It is also interesting to note that, compared to the properties produced by normalizing, strength and machinability are slightly better, which is most likely attributable to the fact that grain size is somewhat coarser than in the normalized condition.
Heat treated for machinability – When a finished machined component is to be produced from a roughly dimensioned forging, machinability becomes a vital consideration to optimize tool life, increase productivity, or both. The specification or forging drawing can specify the heat treatment. However, when specifications give only maximum hardness or micro-structural specifications, the most economical and effective thermal cycle is to be selected. Available heat treatments include full anneal, spheroidize anneal, sub-critical anneal, normalize, or normalize and temper. The heat treatment chosen depends on the steel composition and the machine operations to be performed. Some steel grades are inherently soft while others become quite hard in cooling from the finishing temperature after hot forging. Some type of annealing is usually required or specified to improve machinability.
Heat treated for final mechanical / physical properties – Normalizing or normalizing and tempering can produce the needed minimum hardness and minimum ultimate tensile strength. However, for most steels, a hardening (austenitize) and quenching (in oil, water, or some other medium, depending on section size and hardenability) cycle is employed, followed by tempering to produce the proper hardness, strength, ductility, and impact properties. For steel forgings to be heat treated above the 1,035 MPa strength level and having section size variations, it is general practice to normalize before austenitizing to produce a uniform grain size and minimize internal residual stresses. In some instances, it is normal practice to use the heat for forging as the austenitizing cycle and to quench at the forge unit. The forging is then tempered to complete the heat treat cycle. Although there are obvious limitations to this procedure, definite economies are possible when the procedure is applicable (usually for symmetrical shapes of carbon steels which need little final machining).
Special heat treatment to enhance dimensional stability – Special heat treatments, particularly in more complex part configurations, are sometimes used to control dimensional distortion, relieve residual stresses before or after machining operations, avoid quench cracking, or prevent thermal shock or surface (case) hardening. Although most of the heat-treating cycles can apply, very specific treatments can be needed. Such treatments normally apply to complex forging configurations with adjacent differences in section thickness, or to very high hardenability steels and alloys. When stability of critically dimensioned finished parts permits only light machining of the forging after heat treatment to final properties, special treatments are available, including mar-quenching (mar-tempering), stress relieving, and multiple tempering.
Many applications, such as crankshafts, camshafts, gears, forged rolls, rings, certain bearings, and other machinery components, need increased surface hardness for wear resistance. The important surfaces are normally hardened after machining by flame or induction hardening, carburizing, carbo-nitriding, or nitriding. These processes are listed in the approximate order of increasing cost and decreasing maximum temperature. The latter consideration is important in that dimensional distortion normally decreases with decreasing temperature. This is particularly true of nitriding, which is usually performed below the tempering temperature for the steel used in the forging.
Micro-alloyed forging steels
Micro-alloying (the use of small amounts of elements such as vanadium and niobium to strengthen steels) has been in practice since the 1960s to control the micro-structure and properties of low carbon steels. Most of the early developments have been related to plate and sheet products in which micro-alloy precipitation, controlled rolling, and modern steelmaking technology combined to increase strength significantly relative to that of low carbon steels.
The application of micro-alloying technology to forging steels has lagged behind that of flat-rolled products because of the different property requirements and thermo-mechanical processing of forging steels. Forging steels are normally used in applications in which high strength, fatigue resistance, and wear resistance are needed. These requirements are most often filled by medium carbon steels. Thus, the development of micro-alloyed forging steels has been based around the grades containing 0.3 % to 0.5 % C.
The driving force behind the development of micro-alloyed forging steels has been the need to reduce the production costs. This is accomplished in these materials by means of a simplified thermo-mechanical treatment (that is, a controlled cooling following hot forging) which achieves the desired properties without the separate quenching and tempering treatments required by conventional carbon and alloy steels. In Fig 6, the processing sequence for conventional (quenched and tempered) steels is compared with the micro-alloyed steel-forging process.
Fig 6 Micro-alloyed steels and forgings
Effects of micro-alloying elements
Carbon – Most of the micro-alloyed steels developed for forging have carbon contents ranging from 0.3 % to 0.5 %, which is high enough to form a large amount of pearlite. The pearlite is responsible for substantial strengthening. This level of carbon also decreases the solubility of the micro-alloying constituents in austenite.
Niobium, vanadium, and titanium – Formation of carbo-nitride precipitates is the other major strengthening mechanism of micro-alloyed forging steels. Vanadium, in amounts ranging from 0.05 % to 0.2 %, is the most common micro-alloying addition used in forging steels. Niobium and titanium enhance strength and toughness by providing control of austenite grain size. Frequently niobium is used in combination with vanadium to achieve the benefits of austenite grain size control (from niobium) and carbo-nitride precipitation (from vanadium).
Manganese – Manganese is used in relatively large amounts (1.4 % to 1.5 %) in many micro-alloyed forging steels. It tends to reduce the cementite plate thickness while maintaining the inter-lamellar spacing of pearlite developed. Hence, high manganese levels require lower carbon contents to retain the large amounts of pearlite required for high hardness. Manganese also provides substantial solid solution strengthening, enhances the solubility of vanadium carbonitrides, and lowers the solvus temperature for these phases.
Silicon – The silicon content of most commercial micro-alloyed forging steels is around 0.3 %. Some grades contain upto 0.7 %. Higher silicon contents are associated with significantly higher toughness, apparently because of an increased amount of ferrite relative to that formed in ferrite-pearlite steels with lower silicon contents.
Sulphur – Many micro-alloyed forging steels, particularly those needed for use in automotive forgings in which machinability is critical, have relatively high sulphur contents. The higher sulphur contents contribute to their machinability, which is comparable to that of quenched and tempered steels.
Aluminum and nitrogen – As in hardenable fine-grain steels, aluminum is important for austenite grain size control in micro-alloyed steels. The mechanism of aluminum grain size control is the formation of aluminum nitride particles. It has been shown that nitrogen is the major interstitial component of vanadium carbo-nitride. For this reason, moderate to high nitrogen contents are needed in vanadium containing micro-alloyed steels to promote effective precipitate strengthening.
The concept of grain size control has been used for many years in the production of flat rolled products. Particularly in plate rolling, the ability to increase austenite recrystallization temperature using small niobium additions is well known. The process used to produce these steels is usually referred to as controlled rolling. The benefits of austenite grain size control are not, of course, limited to flat rolled products. Although the higher finishing temperatures needed for rolling of bars limit the usefulness of this approach to micro-structural control, finishing temperatures for micro-alloyed bar steels is nonetheless to be controlled.
It has been shown that, although strength is not significantly affected by finishing temperature, toughness of vanadium-containing micro-alloyed steels decreases with increasing finishing temperature. This effect is shown in Fig 6, which compares Charpy V-notch impact strength for a micro-alloyed steel finished at three temperatures. This detrimental effect of a high finishing temperature on impact toughness also carries over to forging operations, that is, the lower the finish temperature in forging, the higher the resulting toughness, and vice versa. After extensive testing, it has been shown that the finishing temperature for forging if reduced to near 1000 deg C, results in impact properties equal to or better than those of hot rolled bar. It is also shown that rapid induction preheating is beneficial for micro-alloyed forging steels, and that cost savings of 10 % (for standard micro-alloyed forgings) to 20 % (for resulphurized grades) are possible.
Lower finishing temperatures, however, take their toll in terms of higher required forging pressures (and thus higher machine capacities needed) and increased die wear. The improved toughness resulting from lower finishing temperatures, as well as any cost savings which can be achieved as a result of the elimination of heat treatment, is to be weighed against the cost increases caused by these factors.
Micro-alloyed cold heading steels -Steels used in the production of high-strength fasteners by cold heading have been earlier produced from quenched and tempered alloy steels. To obtain sufficient strength with adequate ductility needed six processing steps. Recent developments have led to the use of micro-alloyed niobium-boron steels which need no heat treatment. These steels make use of niobium and boron additions to develop bainitic structures with high work-hardening rates. In most cases they use the deformation of cold heading to achieve the required strength levels without heat treatment.