This susceptibility to cracking is increased by the presence of stress raisers, such as sharp fillets, tool marks or other notches, massive inclusions, or voids. Quench cracks in forgings may also occur near the trim line of a closed-die forging, where some localized burning may have occurred. Other factors that affect quench cracking are the hardenability of the steel, the rate of cooling, and elapsed time between quenching and tempering. In general, an increase in any of these factors increases the likelihood of quench cracking Quench cracks have several recognizable features. Macroscopically, they are usually straight and extend from crack may have scale present if the part was tempered after quenching. The cracks are open to the surface and ay be detected by magnetic-particle, ultrasonic, or eddy-current inspection. Microscopically, quench cracks are invariably intergranular and may be free of decarburization Carburization and Decarburization. Although surface treatment by carburization is commonly used to improve the wear resistance and overall strength characteristics of many forged steel components, particularly axles, the accidental alteration of the surface structure during heat treatment can produce disastrous results. Heating a metal surface that is contaminated with oil or carbon or heating in a carbon-rich atmosphere can produce a surface with a high carbon content. The mechanical properties of a carburized surface layer are different from those of the core, and cracking problems can arise, because a carburized surface was not considered in the original design. The opposite condition(decarburization) can occur by heating in an oxidizing atmosphere such as air. The result in this case is a surface layer with a lower carbon content than that of the core Reference cited in this section 12. V.N. Whittacker, Nondestr: Test, Oct 1971, p 320 Workability The term workability refers to the ability to deform and shape metals by bulk deformation without fracture. This ability to deform without fracture is highly variable, and the limits of plastic deformation for a given material are greatly affected by the prevailing state of stress, strain rate, and temperature in the metalworking process Deformation is also variable among different alloys and conditions, and thus, the design or specification of metalworking operations is a basic factor during both the conceptual and detailed stages of design and materials selection Most metals and alloys can be worked over a wide range of temperatures, and hot working requires less force for plastic deformation. However, dimensional control of the workpiece often can be difficult in hot working operations, because the metal contracts nonuniformly during cooling. This particular problem is not encountered in the cold working temperature range, thus, cold working has advantages, even though workability is less than that of hot working. The term cold forming can have broad meaning, depending largely on the product forms to which it is applied. Cold forming is frequently defined as forming that occurs below the recrystallization temperature of the specific metal being deformed, and it usually begins at or near room temperature. Depending mainly on the severity of the forming, the temperature of the work metal may increase as much as several hundred degrees Fahrenheit during the deformation process Bulk working is commonly in the hot working regime, where metallic materials are more workable(Fig. 31) (Ref 13). In hot working(typically carried out at temperatures in excess of -60% of the absolute melting point of the metal), individual grains are deformed; the deformed grains then immediately begin to recrystallize, that is, nucleate new stress-free grains that are roughly equiaxed in morphology. The hotter the metal, the more plastic it behaves, and the more easily it deforms. At excessively high temperatures, however, grain growth incipient melting, phase transformation, and changes in composition can occur, which may cause degradation of the properties in the forging. At lower hot working temperatures, the metal is more difficult to work, yet the resultant grain size may be finer, and the product may have better mechanical properties
This susceptibility to cracking is increased by the presence of stress raisers, such as sharp fillets, tool marks or other notches, massive inclusions, or voids. Quench cracks in forgings may also occur near the trim line of a closed-die forging, where some localized burning may have occurred. Other factors that affect quench cracking are the hardenability of the steel, the rate of cooling, and elapsed time between quenching and tempering. In general, an increase in any of these factors increases the likelihood of quench cracking. Quench cracks have several recognizable features. Macroscopically, they are usually straight and extend from the surface (often initiating from a fillet or tool mark) toward the center of the component. The margins of the crack may have scale present if the part was tempered after quenching. The cracks are open to the surface and may be detected by magnetic-particle, ultrasonic, or eddy-current inspection. Microscopically, quench cracks are invariably intergranular and may be free of decarburization. Carburization and Decarburization. Although surface treatment by carburization is commonly used to improve the wear resistance and overall strength characteristics of many forged steel components, particularly axles, the accidental alteration of the surface structure during heat treatment can produce disastrous results. Heating a metal surface that is contaminated with oil or carbon or heating in a carbon-rich atmosphere can produce a surface with a high carbon content. The mechanical properties of a carburized surface layer are different from those of the core, and cracking problems can arise, because a carburized surface was not considered in the original design. The opposite condition (decarburization) can occur by heating in an oxidizing atmosphere such as air. The result in this case is a surface layer with a lower carbon content than that of the core. Reference cited in this section 12. V.N. Whittacker, Nondestr. Test., Oct 1971, p 320 Workability The term workability refers to the ability to deform and shape metals by bulk deformation without fracture. This ability to deform without fracture is highly variable, and the limits of plastic deformation for a given material are greatly affected by the prevailing state of stress, strain rate, and temperature in the metalworking process. Deformation is also variable among different alloys and conditions, and thus, the design or specification of metalworking operations is a basic factor during both the conceptual and detailed stages of design and materials selection. Most metals and alloys can be worked over a wide range of temperatures, and hot working requires less force for plastic deformation. However, dimensional control of the workpiece often can be difficult in hot working operations, because the metal contracts nonuniformly during cooling. This particular problem is not encountered in the cold working temperature range; thus, cold working has advantages, even though workability is less than that of hot working. The term cold forming can have broad meaning, depending largely on the product forms to which it is applied. Cold forming is frequently defined as forming that occurs below the recrystallization temperature of the specific metal being deformed, and it usually begins at or near room temperature. Depending mainly on the severity of the forming, the temperature of the work metal may increase as much as several hundred degrees Fahrenheit during the deformation process. Bulk working is commonly in the hot working regime, where metallic materials are more workable (Fig. 31) (Ref 13). In hot working (typically carried out at temperatures in excess of ~60% of the absolute melting point of the metal), individual grains are deformed; the deformed grains then immediately begin to recrystallize, that is, nucleate new stress-free grains that are roughly equiaxed in morphology. The hotter the metal, the more plastic it behaves, and the more easily it deforms. At excessively high temperatures, however, grain growth, incipient melting, phase transformation, and changes in composition can occur, which may cause degradation of the properties in the forging. At lower hot working temperatures, the metal is more difficult to work, yet the resultant grain size may be finer, and the product may have better mechanical properties
Warm Hot working worKing 兰o3g50s9 0.25MP 0.5 MP 0.75MP MPMP Absolute temperature Fig. 31 Relative workability of coarse-grained cast materials and wrought-and-recrystallized metals as a function of temperature The melting point (or solidus)is denoted as MP(cast)or MPw(wrought). Warm working is conducted in the range between 25 and 60% of the absolute melting temperature Paradoxically, the workability of metals in the warm forging regime is considerably lower than that of cold forging(Fig. 31), due to reduced metal ductility in the warm working regime. For this reason, warm working is generally reserved for bulk working operations that require small deformations and for applications in which a decrease in the workpiece temperature offers an energy savings in terms of reduced heating costs. If working temperature is reduced still further below the recrystallization temperature (i.e, in the cold working regime), the deformed grains do not break up and form new grains but remain deformed and highly stressed. When the metal is in this condition, it can behave in a brittle manner, and cracking may occu Workability is a measure of the ability of a metal to endure deformation without cracking for bulk working processes such as forging, rolling, extrusion, and bending of thick sections. Typically, fracture in bulk deformation processing occurs as ductile fracture, rarely as brittle fracture. However, depending on temperature and strain rate, the details of the ductile fracture mechanism vary. At temperatures below approximately one- half the melting point of a given material(below the hot working region), ductile fracture usually occurs by dimple fracture. Cavitation or ductile rupture is the more dominant mode of ductile fracture when working emperatures are higher than one-half the melting point of a given material Ductility as measured by the tensile test is a good way to compare the inherent resistance of different materials or different microstructures for the same material. However, ductility alone is insufficient when judging fracture susceptibility of a deformation process, because fracture depends on the conditions of both the local strain and stress. Compressive stresses superimposed on tensile or shear stresses by the deformation process can have a significant influence on closing small cavities or limiting their growth and thus enhancing workability Therefore, workability is not a property similar to tensile reduction of area at fracture. Workability depends not Thefileisdownloadedfromwww.bzfxw.com
Fig. 31 Relative workability of coarse-grained cast materials and wrought-and-recrystallized metals as a function of temperature. The melting point (or solidus) is denoted as MPc(cast) or MPw(wrought). Warm working is conducted in the range between 25 and 60% of the absolute melting temperature. Paradoxically, the workability of metals in the warm forging regime is considerably lower than that of cold forging (Fig. 31), due to reduced metal ductility in the warm working regime. For this reason, warm working is generally reserved for bulk working operations that require small deformations and for applications in which a decrease in the workpiece temperature offers an energy savings in terms of reduced heating costs. If working temperature is reduced still further below the recrystallization temperature (i.e., in the cold working regime), the deformed grains do not break up and form new grains but remain deformed and highly stressed. When the metal is in this condition, it can behave in a brittle manner, and cracking may occur. Workability is a measure of the ability of a metal to endure deformation without cracking for bulk working processes such as forging, rolling, extrusion, and bending of thick sections. Typically, fracture in bulk deformation processing occurs as ductile fracture, rarely as brittle fracture. However, depending on temperature and strain rate, the details of the ductile fracture mechanism vary. At temperatures below approximately onehalf the melting point of a given material (below the hot working region), ductile fracture usually occurs by dimple fracture. Cavitation or ductile rupture is the more dominant mode of ductile fracture when working temperatures are higher than one-half the melting point of a given material. Ductility as measured by the tensile test is a good way to compare the inherent resistance of different materials or different microstructures for the same material. However, ductility alone is insufficient when judging fracture susceptibility of a deformation process, because fracture depends on the conditions of both the local strain and stress. Compressive stresses superimposed on tensile or shear stresses by the deformation process can have a significant influence on closing small cavities or limiting their growth and thus enhancing workability. Therefore, workability is not a property similar to tensile reduction of area at fracture. Workability depends not The file is downloaded from www.bzfxw.com
only on material characteristics but also on process variables, such as strain, strain rate, temperature, and stress state Flow Localization. Workability problems can arise when metal deformation is localized to a narrow zone. This cults in a region of different structures and properties that can be the site of failure in service. Localization of deformation can also be so severe that it leads to failure in the deformation process. In either mode, the presence of flow localization needs to be recognized Flow localization is commonly caused by the formation of a dead-metal zone between the workpiece and the tooling. This can arise from poor lubrication at the workpiece-tool interface. When the workpiece is constrained from sliding at the interface, it barrels, and the friction-hill pressure distribution is created over the interfac The inhomogeneity of deformation throughout the cross section leads to a dead zone at the tool interface and a region of intense shear deformation. A similar situation can arise when the processing tools are cooler than the workpiece; in this case, heat is extracted at the tools. Consequently, the flow stress of the metal near the interface is higher because of the lower temperature However, flow localization may occur during hot working in the absence of frictional or chilling effects. In this case, localization results from flow softening(negative strain hardening). Flow softening arises during hot working as a result of structural instabilities, such as adiabatic heating, generation of a softer texture during deformation, grain coarsening, or spheroidization. Flow softening has been correlated with materials properties (Ref 14) by the parameter (y-1) (Eq1) for upset compression and (Eq2) te sensitivity, m, is the differential relation ofo如 ormalized flow-softening rate, and where the strain- d log o/d log In alpha-beta titanium alloys and other materials that exhibit a strong tendency toward flow localization, this phenomenon is likely to occur when the parameter a is greater than 5. Figure 32(Ref 15)shows a crack that initiated in a shear band during the high-energy-rate forging of a complex austenitic stainless steel A 20 mm 0.25mm
only on material characteristics but also on process variables, such as strain, strain rate, temperature, and stress state. Flow Localization. Workability problems can arise when metal deformation is localized to a narrow zone. This results in a region of different structures and properties that can be the site of failure in service. Localization of deformation can also be so severe that it leads to failure in the deformation process. In either mode, the presence of flow localization needs to be recognized. Flow localization is commonly caused by the formation of a dead-metal zone between the workpiece and the tooling. This can arise from poor lubrication at the workpiece-tool interface. When the workpiece is constrained from sliding at the interface, it barrels, and the friction-hill pressure distribution is created over the interface. The inhomogeneity of deformation throughout the cross section leads to a dead zone at the tool interface and a region of intense shear deformation. A similar situation can arise when the processing tools are cooler than the workpiece; in this case, heat is extracted at the tools. Consequently, the flow stress of the metal near the interface is higher because of the lower temperature. However, flow localization may occur during hot working in the absence of frictional or chilling effects. In this case, localization results from flow softening (negative strain hardening). Flow softening arises during hot working as a result of structural instabilities, such as adiabatic heating, generation of a softer texture during deformation, grain coarsening, or spheroidization. Flow softening has been correlated with materials properties (Ref 14) by the parameter: ( 1) m g a - = - (Eq 1) for upset compression and: m g a - = (Eq 2) for plane-strain compression, where γ = (1/σ)dσ/dε is the normalized flow-softening rate, and where the strainrate sensitivity, m, is the differential relation of stress and strain rate: m = d log σ/d log ε In alpha-beta titanium alloys and other materials that exhibit a strong tendency toward flow localization, this phenomenon is likely to occur when the parameter α is greater than 5. Figure 32 (Ref 15) shows a crack that initiated in a shear band during the high-energy-rate forging of a complex austenitic stainless steel
Fig. 32 Austenitic stainless steel high-energy-rate forged extrusion. Forging temperature: 815C(1500 °F);65% reduction in area;ε=1.4×10°s.(a)Ⅴ iew of extrusion showing spiral cracks.(b) Optical micrograph showing the microstructure at the tip of one of the cracks in the extrusion(area A). Note that the crack initiated in a macroscopic shear band that formed first at the lead end of the extrusion. Etchant: oxalic acid Metallurgical Considerations. Workability problems depend greatly on grain size and grain structure. When the grain size is large relative to the overall size of the workpiece, as in conventionally cast ingot structures, workability is lower, because cracks may initiate and propagate easily along the grain boundaries. Moreover, with cast structures, impurities are frequently segregated to the center and top or to the surface of the ingot creating regions of low workability. Because chemical elements are not distributed uniformly on either a micro or a macroscopic scale, the temperature range over which an ingot structure can be worked is rather limited Typically, cast structures must be hot worked. The melting point of an alloy in the as-cast condition is usually lower than that of the same alloy in the fine-grain, recrystallized condition because of chemical inhomogeneities and the presence of low-melting-point compounds that frequently occur at grain boundaries Deformation at temperatures too close to the melting point of these compounds may lead to grain-boundary cracking when the heat developed by deformation increases the workpiece temperature and produces local melter ng This fracture mode is called hot shortness. This type of fracture can be prevented by using a sufficiently low deformation rate that allows the heat developed by deformation to be dissipated by the tooling, by using lower king temperatures, or by subjecting the workpiece to a homogenization heat treatment prior to hot working The relationship between the workability of cast and wrought structures and temperatures is shown in Fig. 31 The intermediate temperature region of low ductility shown in Fig. 31 is found in many metallurgical systems (Ref 16). This occurs at a temperature that is sufficiently high for grain-boundary sliding to initiate grain boundary cracking but not so high that the cracks are sealed off from propagation by a dynam recrystallization process The relationship between workability and temperature for various metallurgical systems is summarized in Fi 33(Ref 17). Generally, pure metals and single-phase alloys exhibit the best workability, except when grain growth occurs at high temperatures. Alloys that contain low-melting-point phases(such as r'-strengthened nickel-base superalloys) tend to be difficult to deform and have a limited range of working temperature. In eneral. as the solute co ontent of the alloy increases, the possibility of forming low-melting-point phases increases, while the temperature for precipitation of second phases increases. The net result is a decreased region for good forgeability(Fig. 34) Thefileisdownloadedfromwww.bzfxw.com
Fig. 32 Austenitic stainless steel high-energy-rate forged extrusion. Forging temperature: 815 °C (1500 °F); 65% reduction in area; ε= 1.4 × 103 s-1 . (a) View of extrusion showing spiral cracks. (b) Optical micrograph showing the microstructure at the tip of one of the cracks in the extrusion (area A). Note that the crack initiated in a macroscopic shear band that formed first at the lead end of the extrusion. Etchant: oxalic acid Metallurgical Considerations. Workability problems depend greatly on grain size and grain structure. When the grain size is large relative to the overall size of the workpiece, as in conventionally cast ingot structures, workability is lower, because cracks may initiate and propagate easily along the grain boundaries. Moreover, with cast structures, impurities are frequently segregated to the center and top or to the surface of the ingot, creating regions of low workability. Because chemical elements are not distributed uniformly on either a microor a macroscopic scale, the temperature range over which an ingot structure can be worked is rather limited. Typically, cast structures must be hot worked. The melting point of an alloy in the as-cast condition is usually lower than that of the same alloy in the fine-grain, recrystallized condition because of chemical inhomogeneities and the presence of low-melting-point compounds that frequently occur at grain boundaries. Deformation at temperatures too close to the melting point of these compounds may lead to grain-boundary cracking when the heat developed by deformation increases the workpiece temperature and produces local melting. This fracture mode is called hot shortness. This type of fracture can be prevented by using a sufficiently low deformation rate that allows the heat developed by deformation to be dissipated by the tooling, by using lower working temperatures, or by subjecting the workpiece to a homogenization heat treatment prior to hot working. The relationship between the workability of cast and wrought structures and temperatures is shown in Fig. 31. The intermediate temperature region of low ductility shown in Fig. 31 is found in many metallurgical systems (Ref 16). This occurs at a temperature that is sufficiently high for grain-boundary sliding to initiate grainboundary cracking but not so high that the cracks are sealed off from propagation by a dynamic recrystallization process. The relationship between workability and temperature for various metallurgical systems is summarized in Fig. 33 (Ref 17). Generally, pure metals and single-phase alloys exhibit the best workability, except when grain growth occurs at high temperatures. Alloys that contain low-melting-point phases (such as γ′-strengthened nickel-base superalloys) tend to be difficult to deform and have a limited range of working temperature. In general, as the solute content of the alloy increases, the possibility of forming low-melting-point phases increases, while the temperature for precipitation of second phases increases. The net result is a decreased region for good forgeability (Fig. 34). The file is downloaded from www.bzfxw.com
L. Pure metals and single phase alloys Alun Niobium alloys ll. Pure metals and single-phase alloys ng rapid grain Beryllium T Magnesium alloys Tungsten alloys All-beta titanium alloys Ill. Alloys containing elements that form insoluble compound Resulfurized steel Stainless steel containing selenium V. Alloys containing elements that form soluble compounds Molybdenum alloys containing oxides TA Stainless steel containing soluble carbides or nitrides e V. Alloys forming ductile second phase Single- phase Twophase on heatin High-chromium stainless steels VI. Alloys forming low-melting second phase on heating Iron containing sulfur Magnesium alloys containing zinc T VIl. Alloys forming ductile second phase Carbon and low-alloy steels Alpha-beta and alpha-titanium alloys Twophase Vill. Alloys forming brittle second phase on cooling Superalloys Precipitation-hardenable stainless Increasing temperature Fig. 33 Typical workability behavior exhibited by different alloy systems. TM, absolute melting temperature Source: Ref 17
Fig. 33 Typical workability behavior exhibited by different alloy systems. TM, absolute melting temperature. Source: Ref 17