of central bursts or chevrons is nearly always restricted to isolated lots of material and usually to only a small rcentage of the pieces extruded in any particular production run a change in deformation-zone geometry is usually sufficient to eliminate the problem. The conservative design approach is to ensure that no hydrostatic tension develops. Often, however, the part or tooling design cannot be changed sufficiently to eliminate hydrostatic tension. If the level of hydrostatic tension can be kept below a critical level, bursting can likely be avoided. This may be accomplished by a change in lubricant, die profile, deformation level, or process rate Internal bursts may also occur where the material is weak. For example, with ingot imperfections(such as pipe, porosity, segregation, or inclusions), tensile stresses can be sufficiently high to tear the material apart internally, particularly if the forging temperature is too high(Ref 12 ). Similarly, if the metal contains low-melting phases resulting from segregation, these phases may rupture during forging. Ingot pipe, unhealed center conditions,or voids associated with melt-related discontinuities may also induce center bursts if reduction rates are too severe or temperatures are incorrect during working. The conversion practice to bar or billet must impart sufficient homogenization or healing to produce a product with sound center conditions. An example of an unsound condition that did not heal is shown in Fig. 27 Thefileisdownloadedfromwww.bzfxw.com
of central bursts or chevrons is nearly always restricted to isolated lots of material and usually to only a small percentage of the pieces extruded in any particular production run. A change in deformation-zone geometry is usually sufficient to eliminate the problem. The conservative design approach is to ensure that no hydrostatic tension develops. Often, however, the part or tooling design cannot be changed sufficiently to eliminate hydrostatic tension. If the level of hydrostatic tension can be kept below a critical level, bursting can likely be avoided. This may be accomplished by a change in lubricant, die profile, temperature, deformation level, or process rate. Internal bursts may also occur where the material is weak. For example, with ingot imperfections (such as pipe, porosity, segregation, or inclusions), tensile stresses can be sufficiently high to tear the material apart internally, particularly if the forging temperature is too high (Ref 12). Similarly, if the metal contains low-melting phases resulting from segregation, these phases may rupture during forging. Ingot pipe, unhealed center conditions, or voids associated with melt-related discontinuities may also induce center bursts if reduction rates are too severe or temperatures are incorrect during working. The conversion practice to bar or billet must impart sufficient homogenization or healing to produce a product with sound center conditions. An example of an unsound condition that did not heal is shown in Fig. 27. The file is downloaded from www.bzfxw.com
Fig. 27 Section through a heat-resistant alloy forging showing a central discontinuity that resulted from insufficient homogenization during conversion. Step machining was used to reveal the location of the rupture; original diameter is at right. Macroetching and ultrasonic inspection methods are the most widely used for identifying regions of unsoundness. Bursts usually display a distinct pattern of cracks and do not show spongy areas, thus distinguishing them from pipes. Bursts are readily detected by macroetching. Figure 28 shows a large burst that occurred during the forging of an electroslag remelted ingot. The cause was traced to a weak solidification plane near the bottom of the ingot combined with higher forging temperatures than normal Fig. 28 Cross section of a forged bar showing a forging burst. The burst is located approximately at the centerline of the workpiece arrow indicates the direction of working. Grain Flow and Anisotropy Grain flow refers to the directional pattern of crystals and second-phase particles as they become aligned in the direction of greatest metal flow during the breakdown of cast ingots and the subsequent working of wrought forms. The grain flow consists of a combination of The alignment of nonmetallic inclusions, voids, and chemical segregation Together, these can produce the familiar fibering pattern of wrought products. Both types of preferred orientations can cause anisotropy in the mechanical and physical properties of a metal. 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. The occurrence and severity of fibering varies with such factors as composition, extent of chemical segregation, and the amount of work or reduction the workpiece receives. Furthermore, this condition is a consequence of processing and may not be adequately considered during the design of a particular product Anisotropy is most easily identified by differences in ductility, toughness, and fatigue resistance for longitudinal, transverse, and short-transverse directions. Grain flow also can be used effectively to produce a better product. If the grain flow can be aligned in the direction of maximum tension in the part during service, superior performance is achieved. The resistance to crack initiation and propagation also is highest in the direction normal to the grain flow, because the grain boundaries and elongated inclusions act to blunt advancing cracks. If the tension is normal to the fiber direction, the fiber enhances crack propagation Figure 29 shows grain-flow patterns for a channel section forged with different parting-line locations. Because the grain-flow pattern breaks out to the part surface at the parting line, any stress during service that is normal to the parting line acts perpendicular to the weakest planes. These planes are the easiest to fracture along, for example, the short-transverse equivalent position. Because a channel is usually stressed in a direction normal to
Fig. 27 Section through a heat-resistant alloy forging showing a central discontinuity that resulted from insufficient homogenization during conversion. Step machining was used to reveal the location of the rupture; original diameter is at right. Macroetching and ultrasonic inspection methods are the most widely used for identifying regions of unsoundness. Bursts usually display a distinct pattern of cracks and do not show spongy areas, thus distinguishing them from pipes. Bursts are readily detected by macroetching. Figure 28 shows a large burst that occurred during the forging of an electroslag remelted ingot. The cause was traced to a weak solidification plane near the bottom of the ingot combined with higher forging temperatures than normal. Fig. 28 Cross section of a forged bar showing a forging burst. The burst is located approximately at the centerline of the workpiece. Arrow indicates the direction of working. Grain Flow and Anisotropy Grain flow refers to the directional pattern of crystals and second-phase particles as they become aligned in the direction of greatest metal flow during the breakdown of cast ingots and the subsequent working of wrought forms. The grain flow consists of a combination of: · The crystallographic reorientation of the grains during severe deformation · The alignment of nonmetallic inclusions, voids, and chemical segregation Together, these can produce the familiar fibering pattern of wrought products. Both types of preferred orientations can cause anisotropy in the mechanical and physical properties of a metal. 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. The occurrence and severity of fibering varies with such factors as composition, extent of chemical segregation, and the amount of work or reduction the workpiece receives. Furthermore, this condition is a consequence of processing and may not be adequately considered during the design of a particular product. Anisotropy is most easily identified by differences in ductility, toughness, and fatigue resistance for longitudinal, transverse, and short-transverse directions. Grain flow also can be used effectively to produce a better product. If the grain flow can be aligned in the direction of maximum tension in the part during service, superior performance is achieved. The resistance to crack initiation and propagation also is highest in the direction normal to the grain flow, because the grain boundaries and elongated inclusions act to blunt advancing cracks. If the tension is normal to the fiber direction, the fiber enhances crack propagation. Figure 29 shows grain-flow patterns for a channel section forged with different parting-line locations. Because the grain-flow pattern breaks out to the part surface at the parting line, any stress during service that is normal to the parting line acts perpendicular to the weakest planes. These planes are the easiest to fracture along, for example, the short-transverse equivalent position. Because a channel is usually stressed in a direction normal to
it(flexing the channel legs to open or close), the grain-flow pattern in Fig. 29(b) is least detrimental. The parting line of forging dies should be located so as to minimize disruption to the grain-flow lines May have Metal Grain structure unfilled flow lines is ruptured at section Parting line (a) Metal Most economical as all of the impression is in one die This parting line should not be above the center of the bottom web Metal Parting at the ends flow lines of ribs results in good grain structure Fig. 29 Parting-line location and its influence on grain-flow pattern in a channel section forging. (a) Parting lines resulting in metal-flow patterns that cause forging defects. (b) Parting lines resulting in smooth flow lines at stressed sections Although grain flow can be used to advantage, anisotropy of fracturing in wrought products is extremely widespread, occurring in both ductile and brittle materials and including some that are essentially pure metals Probably the most dramatic evidence for the effects of mechanical fibering are those observed during fracture Thefileisdownloadedfromwww.bzfxw.com
it (flexing the channel legs to open or close), the grain-flow pattern in Fig. 29(b) is least detrimental. The parting line of forging dies should be located so as to minimize disruption to the grain-flow lines. Fig. 29 Parting-line location and its influence on grain-flow pattern in a channel section forging. (a) Parting lines resulting in metal-flow patterns that cause forging defects. (b) Parting lines resulting in smooth flow lines at stressed sections Although grain flow can be used to advantage, anisotropy of fracturing in wrought products is extremely widespread, occurring in both ductile and brittle materials and including some that are essentially pure metals. Probably the most dramatic evidence for the effects of mechanical fibering are those observed during fracture, The file is downloaded from www.bzfxw.com
either in service or during laboratory testing or process deformation. When the stress direction is perpendicular to the lay of the stringer, the fracture direction that is parallel to the lay of the stringer results in a woody appearance to the fracture( e.g, see Fig. 21 in the"Atlas of fractographs"in Fractography, Volume 12 of ASM Handbook Thermal Effects and heat treatment Improper heat treatment can result in failure to attain the desired microstructure in the metal and therefore the desired levels of mechanical or physical properties. Such deficiencies can cause failures in service Overheating. As alloys are heated above their recrystallization temperatures, grain growth occurs. As the temperature increases, so does grain growth, becoming quite rapid and resulting in large grains, often accompanied by many undesirable characteristics. The impairment that usually accompanies large grains is caused not only by the size of the grains but also by the more continuous films that are formed on larger grains by grain-boundary impurities, such as preferential precipitates and evolved gases. Finer grains, on the other hand, present a greater amount of total grain-boundary area over which the impurities may be distributed The detrimental effects of overheating depend on the temperature and the time of exposure as well as on the chemical composition of the alloy. For example, a short exposure of a high-speed tool steel to temperatures at approximately 1250C (2280F)is required to dissolve the carbides, but prolonged heating at that temperature causes grain growth and loss of mechanical properties. The damage caused by overheating is particularly significant in the high-carbon and medium-carbon steels, in which both strength and ductility are affected One of the most conspicuous indications that a metal has been overheated is the coarse-grain fracture surface that results. Usually, examination with a stereoscopic microscope shows the characteristic faceted surface of an intergranular fracture. Microscopically, the large grain size resulting from overheating is quite evident and can be measured and compared to a similar metal with a normal grain size. In addition to large grain size, fine oxide particles are often found dispersed throughout the grains, particularly near the surface. A Widmanstatten structure is often associated with coarse grains in an overheated steel forging with a controlled rate of coolin (neither extremely fast nor extremely slow) Burning is a term applied when a metal is grossly overheated and permanent irreversible damage to the structure occurs as a result of intergranular penetration of oxidizing gas or incipient melting. The micrograph in Fig. 30(b) shows burning at the grain boundaries in a specimen of copper C1 1000(electrolytic tough-pitch copper)heated to 1065C (1950F. Copper oxide migrated to grain boundaries, forming a continuous network that severely reduced strength and ductility
either in service or during laboratory testing or process deformation. When the stress direction is perpendicular to the lay of the stringer, the fracture direction that is parallel to the lay of the stringer results in a woody appearance to the fracture (e.g., see Fig. 21 in the “Atlas of Fractographs” in Fractography, Volume 12 of ASM Handbook.) Thermal Effects and Heat Treatment Improper heat treatment can result in failure to attain the desired microstructure in the metal and therefore the desired levels of mechanical or physical properties. Such deficiencies can cause failures in service. Overheating. As alloys are heated above their recrystallization temperatures, grain growth occurs. As the temperature increases, so does grain growth, becoming quite rapid and resulting in large grains, often accompanied by many undesirable characteristics. The impairment that usually accompanies large grains is caused not only by the size of the grains but also by the more continuous films that are formed on larger grains by grain-boundary impurities, such as preferential precipitates and evolved gases. Finer grains, on the other hand, present a greater amount of total grain-boundary area over which the impurities may be distributed. The detrimental effects of overheating depend on the temperature and the time of exposure as well as on the chemical composition of the alloy. For example, a short exposure of a high-speed tool steel to temperatures at approximately 1250 °C (2280 °F) is required to dissolve the carbides, but prolonged heating at that temperature causes grain growth and loss of mechanical properties. The damage caused by overheating is particularly significant in the high-carbon and medium-carbon steels, in which both strength and ductility are affected. One of the most conspicuous indications that a metal has been overheated is the coarse-grain fracture surface that results. Usually, examination with a stereoscopic microscope shows the characteristic faceted surface of an intergranular fracture. Microscopically, the large grain size resulting from overheating is quite evident and can be measured and compared to a similar metal with a normal grain size. In addition to large grain size, fine oxide particles are often found dispersed throughout the grains, particularly near the surface. A Widmanstätten structure is often associated with coarse grains in an overheated steel forging with a controlled rate of cooling (neither extremely fast nor extremely slow). Burning is a term applied when a metal is grossly overheated and permanent irreversible damage to the structure occurs as a result of intergranular penetration of oxidizing gas or incipient melting. The micrograph in Fig. 30(b) shows burning at the grain boundaries in a specimen of copper C11000 (electrolytic tough-pitch copper) heated to 1065 °C (1950 °F). Copper oxide migrated to grain boundaries, forming a continuous network that severely reduced strength and ductility
A (a) (b) Fig. 30 Micrographs showing the effects of overheating and burning on microstructures of copper forgings.(a)Overheated copper C10200 forging showing oxides (black particles). The forging was heated to 1025C(1875F) (b) Burning (black outlines)at grain boundaries of a copper C11000 forging heated to1065°C(1950°k In steels, burning may manifest itself with the formation of extremely large grains and incipient melting at the grain boundaries. Melting is particularly evident where segregation has occurred, with the marked formation of low-melting phases in the interdendritic regions and at the grain boundaries. Burning cannot be readily detected by visual examination. However, a metallographic examination of the structure shows the enlarged grains and the pronounced grain-boundary network The typical causes of burning include excessive furnace temperatures; welding and flame-cutting operations that are not properly controlled; and operations involving the direct heating of a workpiece surface, such as flame hardening, induction hardening, shrink fitting, and so on, that are inadequately controlled. Occasionally burning occurs in adequately controlled furnaces simply because the flame is allowed to impinge on the metal surface, causing localized overheating. Another source of overheating, though less common, ca conversion of mechanical energy into heat. If there are segregated areas near the center of the billet, and if the initial forging temperature is close to the melting point of the segregated regions, the additional heat supplied by the transformation of work into heat can cause localized burning during the forging operation. Burned material cannot be salvaged and should be scrapped, because the metallurgical changes that have occurred are irreversible Quench cracks are often the cause of failure in steel forgings. These cracks may be obvious and may prompt the removal of a component during production, or they may be obscured during manufacture and may be present in the shipped component. Quench cracks in steel result from stresses produced by the volume increase accompanying the austenite-to-martensite transformation. When a steel forging is quenched, untempered(hard and brittle) martensite is formed at the outer surfaces first As cooling continues, the austenite-transformation reaction progresses toward the center of the workpiece accompanied by a volumetric expansion that can place the center portions of the part in tension. When the workpiece cannot adjust to the strains produced by this process, cracking generally occurs. Such cracking may take place during or after the quench, because the transformation product is hard and brittle before tempering Thefileisdownloadedfromwww.bzfxw.com
Fig. 30 Micrographs showing the effects of overheating and burning on microstructures of copper forgings. (a) Overheated copper C10200 forging showing oxides (black particles). The forging was heated to 1025 °C (1875 °F). (b) Burning (black outlines) at grain boundaries of a copper C11000 forging heated to 1065 °C (1950 °F) In steels, burning may manifest itself with the formation of extremely large grains and incipient melting at the grain boundaries. Melting is particularly evident where segregation has occurred, with the marked formation of low-melting phases in the interdendritic regions and at the grain boundaries. Burning cannot be readily detected by visual examination. However, a metallographic examination of the structure shows the enlarged grains and the pronounced grain-boundary network. The typical causes of burning include excessive furnace temperatures; welding and flame-cutting operations that are not properly controlled; and operations involving the direct heating of a workpiece surface, such as flame hardening, induction hardening, shrink fitting, and so on, that are inadequately controlled. Occasionally, burning occurs in adequately controlled furnaces simply because the flame is allowed to impinge on the metal surface, causing localized overheating. Another source of overheating, though less common, can be the conversion of mechanical energy into heat. If there are segregated areas near the center of the billet, and if the initial forging temperature is close to the melting point of the segregated regions, the additional heat supplied by the transformation of work into heat can cause localized burning during the forging operation. Burned material cannot be salvaged and should be scrapped, because the metallurgical changes that have occurred are irreversible. Quench cracks are often the cause of failure in steel forgings. These cracks may be obvious and may prompt the removal of a component during production, or they may be obscured during manufacture and may be present in the shipped component. Quench cracks in steel result from stresses produced by the volume increase accompanying the austenite-to-martensite transformation. When a steel forging is quenched, untempered (hard and brittle) martensite is formed at the outer surfaces first. As cooling continues, the austenite-transformation reaction progresses toward the center of the workpiece, accompanied by a volumetric expansion that can place the center portions of the part in tension. When the workpiece cannot adjust to the strains produced by this process, cracking generally occurs. Such cracking may take place during or after the quench, because the transformation product is hard and brittle before tempering. The file is downloaded from www.bzfxw.com