Failures related to metalworking Introduction WROUGHT FORMS are produced by a wide variety of metal working operations that can be roughly divided into bulk-working operations and sheet-forming operations(Ref 1). The general distinction here is that bulk working imposes material flow in all directions, while sheet-forming operations are typically limited to two- dimensional deformation. Metalworking operations are also classified as either primary metalworking(where mill forms such as bar, plate, tube, sheet, and wire are worked from ingot or other cast forms)or secondary metalworking(where mill products are further formed into finished products by hot forging, cold forging, drawing, extrusion, straightening, sizing, etc. ) These metal working operations have a two-fold purpose. First, they are obviously designed to produce parts with the desired configuration. Secondly, metal working can develop a final shape with internal soundness and improved mechanical properties by Improved internal quality due to compressive deformation Grain refinement Uniform grain structure Elimination of casting porosity and breakup of macrosegregation patterns Beneficial grain-flow pattern for improved part performance Improved toughness and/or fatigue resistance due to grain flow and fibering Burnished surface and controlled surface quality However, the beneficial factors of deformation processing can become a potential problem if the process is not carefully understood. Potential problems of the deformation process also include fracture-related problems: for example, internal bursts or chevron cracks, cracks on free surface cracks on die-contacted surfaces Metal-flow-related problems: for example, end grain and poor surface performance; inhomogeneous grain size; shear bands and locally weakened structures; cold shuts, folds, and laps; flow-through defects Control, materials selection, and use problems: for example, underfill, part distortion, and poor dimensional control; tool overload and breakage excessive tool wear; high initial investment due to equipment cost; poor material use and high scrap loss The movement of metal during these processes, whether performed at room temperature or at elevated temperatures, makes them common sources of surface discontinuities, such as laps, seams, and cold shuts Oxides, slivers or chips of the base material, or foreign material also can be embedded into the surface during working. These surface imperfections produce a notch of unknown severity that acts as a stress raiser, which may adversely affect strength under load Subsurface and core discontinuities may also occur. Subsurface flaws often(but not always)originate from the as-cast ingot due to shrinkage, voids, and porosity that form during solidification. For example, this is shown schematically in Fig. 1 for rolled bar that contains ingot porosity and pipe imperfections(discussed later in the fabulon,"Imperfections from the Ingot" ). These imperfections can also serve as sites for crack initiation during frication or in service Thefileisdownloadedfromwww.bzfxw.com
Failures Related to Metalworking Introduction WROUGHT FORMS are produced by a wide variety of metalworking operations that can be roughly divided into bulk-working operations and sheet-forming operations (Ref 1). The general distinction here is that bulk working imposes material flow in all directions, while sheet-forming operations are typically limited to twodimensional deformation. Metalworking operations are also classified as either primary metalworking (where mill forms such as bar, plate, tube, sheet, and wire are worked from ingot or other cast forms) or secondary metalworking (where mill products are further formed into finished products by hot forging, cold forging, drawing, extrusion, straightening, sizing, etc.). These metalworking operations have a two-fold purpose. First, they are obviously designed to produce parts with the desired configuration. Secondly, metalworking can develop a final shape with internal soundness and improved mechanical properties by: · Improved internal quality due to compressive deformation · Grain refinement · Uniform grain structure · Elimination of casting porosity and breakup of macrosegregation patterns · Beneficial grain-flow pattern for improved part performance · Improved toughness and/or fatigue resistance due to grain flow and fibering · Burnished surface and controlled surface quality However, the beneficial factors of deformation processing can become a potential problem if the process is not carefully understood. Potential problems of the deformation process also include: · Fracture-related problems: for example, internal bursts or chevron cracks, cracks on free surfaces, cracks on die-contacted surfaces · Metal-flow-related problems: for example, end grain and poor surface performance; inhomogeneous grain size; shear bands and locally weakened structures; cold shuts, folds, and laps; flow-through defects · Control, materials selection, and use problems: for example, underfill, part distortion, and poor dimensional control; tool overload and breakage; excessive tool wear; high initial investment due to equipment cost; poor material use and high scrap loss The movement of metal during these processes, whether performed at room temperature or at elevated temperatures, makes them common sources of surface discontinuities, such as laps, seams, and cold shuts. Oxides, slivers or chips of the base material, or foreign material also can be embedded into the surface during working. These surface imperfections produce a notch of unknown severity that acts as a stress raiser, which may adversely affect strength under load. Subsurface and core discontinuities may also occur. Subsurface flaws often (but not always) originate from the as-cast ingot due to shrinkage, voids, and porosity that form during solidification. For example, this is shown schematically in Fig. 1 for rolled bar that contains ingot porosity and pipe imperfections (discussed later in the section, “Imperfections from the Ingot”). These imperfections can also serve as sites for crack initiation during fabrication or in service. The file is downloaded from www.bzfxw.com
Porosity C. INDICATES USED FOR ROLLING BARS BELOW got A ot B PI -Bar rolled from ingot A-Bar rolled from ingot B- Fig. 1 Longitudinal sections of two types of ingots showing typical pipe and porosity. When the ingots are rolled into bars, these flaws become elongated throughout the center of the bars Metalforming and fabrication of wrought forms also involve other manufacturing operations, such as electroplating, heat treatment, machining, or welding. These operations may also introduce possible defects (Table 1)that may be considered in conjunction with possible defects from metal working. Failures can also occur from a complex series of manufacturing factors. For example, the level of residual hoop and bending stresses can occur in a tube produced by drawing, heat treating, and straightening operations. By varying the severity of these operations, it is possible to produce tubes with very low residual stresses or with very high residual stresses that are near the yield strength of the metal. In other words, parts are made by a rather complex series of operations, which thus requires a broad understanding not only in the context of failure analysis but also in the organizational traditions for failure prevention Table 1 Defects that may result from postforming processes rocess Possible defects Electroplatin Hydrogen embrittlement galvanic corrosion Heat treatment Excessive grain growth, burning of grain boundaries, brittle structure, carburization, decarburization, quench crack Surface hardening, nitriding, Excessive case thickness microcracks. embrittled material at stress carburizing anodic hard coating raisers Machining Tool marks grinding cracks Welding Weld-metal defects, hydrogen-induced cracking, inclusions, improper tructure The primary purpose of this article is to describe general root causes of failure that are associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be
Fig. 1 Longitudinal sections of two types of ingots showing typical pipe and porosity. When the ingots are rolled into bars, these flaws become elongated throughout the center of the bars. Metalforming and fabrication of wrought forms also involve other manufacturing operations, such as electroplating, heat treatment, machining, or welding. These operations may also introduce possible defects (Table 1) that may be considered in conjunction with possible defects from metalworking. Failures can also occur from a complex series of manufacturing factors. For example, the level of residual hoop and bending stresses can occur in a tube produced by drawing, heat treating, and straightening operations. By varying the severity of these operations, it is possible to produce tubes with very low residual stresses or with very high residual stresses that are near the yield strength of the metal. In other words, parts are made by a rather complex series of operations, which thus requires a broad understanding not only in the context of failure analysis but also in the organizational traditions for failure prevention. Table 1 Defects that may result from postforming processes Process Possible defects Electroplating Hydrogen embrittlement, galvanic corrosion Heat treatment Excessive grain growth, burning of grain boundaries, brittle structure, carburization, decarburization, quench cracks Electrolytic cleaning Pitting Surface hardening, nitriding, carburizing, anodic hard coating Excessive case thickness, microcracks, embrittled material at stress raisers Machining Tool marks, grinding cracks Welding Weld-metal defects, hydrogen-induced cracking, inclusions, improper structure The primary purpose of this article is to describe general root causes of failure that are associated with wrought metals and metalworking. This includes a brief review of the discontinuities or imperfections that may be
common sources of failure-inducing defects in bulk working of wrought products. This article does not addres powder metallurgy(P/M). A good review of powder metallurgy with coverage relevant to failure analysis of P/M forms is in Ref 2 ferences cited in this section I.J. Schey, Manufacturing Processes and Their Selection, Materials Selection and Design, Vol. 20, ASM Handbook, ASM International, 1997, p 687-704 ease III and W. West, Fundamentals of powder Metallurgy, Metal Powder Industries Federation 2002 Failures Related to Metalworking Imperfections in wrought forms Various terms are used to describe surface and subsurface imperfections in wrought product. The meaning of terms can vary by location and industry, but Fig. 2 is a schematic illustration of some terms used to describe flaws in rolled bar stock. This schematic is not a complete summary of possible imperfections, for example, die scratches of cold-worked product are not included. However, the schematic of Fig. 2 shows typical terms for surface and subsurface flaws that may occur in wrought products. For example, inclusions(Fig. 2a)and seams (Fig. 2h)are commonly used terms to describe imperfections in wrought form, as discussed in more detail in this section. These imperfections, whether at or the below the surface, can adversely affect performance of part by creating a notch of unknown severity and serve as a crack-initiation site during fabrication or in service Corrosion and wear damage can also be assisted by discontinuities, especially at the surface. These flaws may occur from the melting practices and solidification of ingot, the primary or secondary working of the material or the metallurgical characteristic of a particular alloy system Shver on A-A (b) structure)(c) Seams (1) Chevron g 2 Ten different types of flaws that may be found in rolled bars.(a)Inclusions.( b) Laminations from spatter(entrapped splashes during the pouring.(c) slivers.(d)Scabs are caused by splashing liquid metal in the mold.(e) Pits and blisters caused by gaseous pockets in the ingot. (n Embedded scale fror excessive scaling during prior heating operations. (g) cracks with little or no oxide present on their edges when the metal cools in the mold, setting up highly stressed areas.(h)Seams that develop from elongated trapped-gas pockets or from cracks during working. g Laps when excessive material is squeezed out d turned back into the material. ( k) Chevron or internal bursts. See text for additional discussions. It must be clearly recognized that manufactured materials al ways have some imperfections or discontinuities that can be acceptable, if they do not interfere with the utility or service of a part. Laps, seams, bursts, hot tears, and thermal cracks are typically considered to be manufacturing defects. However, whether these manufacturing defects are a contributing factor in an in-service failure is a separate question that would need to be confirmed during a failure analysis. a discontinuity or flaw only becomes a service defect when it interferes Thefileisdownloadedfromwww.bzfxw.com
common sources of failure-inducing defects in bulk working of wrought products. This article does not address powder metallurgy (P/M). A good review of powder metallurgy with coverage relevant to failure analysis of P/M forms is in Ref 2. References cited in this section 1. J. Schey, Manufacturing Processes and Their Selection, Materials Selection and Design, Vol. 20, ASM Handbook, ASM International, 1997, p 687–704 2. L. Pease III and W. West, Fundamentals of Powder Metallurgy, Metal Powder Industries Federation, 2002 Failures Related to Metalworking Imperfections in Wrought Forms Various terms are used to describe surface and subsurface imperfections in wrought product. The meaning of terms can vary by location and industry, but Fig. 2 is a schematic illustration of some terms used to describe flaws in rolled bar stock. This schematic is not a complete summary of possible imperfections; for example, die scratches of cold-worked product are not included. However, the schematic of Fig. 2 shows typical terms for surface and subsurface flaws that may occur in wrought products. For example, inclusions (Fig. 2a) and seams (Fig. 2h) are commonly used terms to describe imperfections in wrought form, as discussed in more detail in this section. These imperfections, whether at or the below the surface, can adversely affect performance of a part by creating a notch of unknown severity and serve as a crack-initiation site during fabrication or in service. Corrosion and wear damage can also be assisted by discontinuities, especially at the surface. These flaws may occur from the melting practices and solidification of ingot, the primary or secondary working of the material, or the metallurgical characteristic of a particular alloy system. Fig. 2 Ten different types of flaws that may be found in rolled bars. (a) Inclusions. (b) Laminations from spatter (entrapped splashes) during the pouring. (c) Slivers. (d) Scabs are caused by splashing liquid metal in the mold. (e) Pits and blisters caused by gaseous pockets in the ingot. (f) Embedded scale from excessive scaling during prior heating operations. (g) Cracks with little or no oxide present on their edges when the metal cools in the mold, setting up highly stressed areas. (h) Seams that develop from elongated trapped-gas pockets or from cracks during working. (j) Laps when excessive material is squeezed out and turned back into the material. (k) Chevron or internal bursts. See text for additional discussions. It must be clearly recognized that manufactured materials always have some imperfections or discontinuities that can be acceptable, if they do not interfere with the utility or service of a part. Laps, seams, bursts, hot tears, and thermal cracks are typically considered to be manufacturing defects. However, whether these manufacturing defects are a contributing factor in an in-service failure is a separate question that would need to be confirmed during a failure analysis. A discontinuity or flaw only becomes a service defect when it interferes The file is downloaded from www.bzfxw.com
with the intended function and expected life of a part. This distinction is important in failure analysis, because a discontinuity or imperfection may be present, even though the failure is attributed to a different root cause The distinction between a manufacturing imperfection and a manufacturing flaw is thus critical in determination of root cause. Manufactured components typically contain geometric and material imperfections, but whether the imperfection caused failure and could therefore be a defect should be determined in many situations by analysis. In a cylindrical section under axial load, for example, imperfections along the centerline or at the surface are the most likely locations of crack initiation. In the absence of stress raisers at the surface crack initiation from manufacturing imperfections is most likely along the centerline of an unnotched bar under tensile loading. In this case, if cracking initiates in another location, it has to do so because the local stress (residual and/or applied) was higher than along the centerline, although there may be exceptions. Some exceptions include the random distribution of fine quench cracks in steels at locations where martensite formed after quenching or the cracking of divorced cementite in the grain boundaries of low-carbon steel after cold Similar arguments can be used to predict initiation sites for various kinds of bending loading and torsion loading. For example, for a three-point loaded beam, cracking is expected to initiate at the location of maximum bending moment. If it is not at that location, the implication is that a geometric or material imperfection has moved the location of the local maximum stress. Another example is rolling-contact fatigue, where maximu stress develops below the surface and is thus expected to cause subsurface crack initiation. Additionally, residual stresses may be distributed in the component as a result of prior mechanical/thermal processing. In this way, the significance of a material imperfection must be carefully evaluated in terms of stresses created by applied loads and part configuration Imperfections from the ingot Many flaws in wrought products can be traced back to the pouring and solidification of hot metal in molds during production of ingot. Except for forged powder metal components, the starting material in bulk working is a slab, ingot, billet, and so forth produced by casting into stationary molds or by continuous casting techniques. Primary deformation processes, such as hot rolling, tube piercing, extrusion, and open-die forging, are then used for converting the cast structure. Many large open-die forgings are forged directly from ingots, while most closed-die and upset forgings are produced from billets, bar stock, or a preform that has received some previous mechanical working. The product may be suitable for immediate application, but in many cases, it serves as the starting material for another(so-called secondary) deformation process, such as drawing, hot forging, cold forging, and sheet metal working The major problems associated with melting and casting practice are the development of porosity and a condition known as scabs. Porosity is developed in cast ingots when they solidify and is of two types: pipe and blow holes. Factors that work against obtaining a perfect homogeneous product include The fast shrinkage as the molten metals cools(roughly 5% in volume for steel he gaseous products that are trapped by the solidifying metal as they try to escape from the liquid and semisolid meta Small crevices in the mold walls, which cause the metal to tear during the stripping operation Spatter during pouring, which produces globs of metal frozen on the mold walls because of the great difference in temperature of the mold surfaces and the liquid metal Scabs are caused by improper ingot pouring, in which metal is splashed against the side of the mold wall. The plashed material, or scab, tends to stick to the wall and become oxidized. Scabs usually show up only after rolling or working and, as can be expected, give poor surface finish Ingot imperfections can seriously affect the performance and reliability of wrought products, and the types of the imperfections that can be traced to the original ingot product include Chemical segregation Ingot pipe, porosity, and centerline shrinkage High hydrogen content
with the intended function and expected life of a part. This distinction is important in failure analysis, because a discontinuity or imperfection may be present, even though the failure is attributed to a different root cause. The distinction between a manufacturing imperfection and a manufacturing flaw is thus critical in determination of root cause. Manufactured components typically contain geometric and material imperfections, but whether the imperfection caused failure and could therefore be a defect should be determined in many situations by analysis. In a cylindrical section under axial load, for example, imperfections along the centerline or at the surface are the most likely locations of crack initiation. In the absence of stress raisers at the surface, crack initiation from manufacturing imperfections is most likely along the centerline of an unnotched bar under tensile loading. In this case, if cracking initiates in another location, it has to do so because the local stress (residual and/or applied) was higher than along the centerline, although there may be exceptions. Some exceptions include the random distribution of fine quench cracks in steels at locations where martensite formed after quenching or the cracking of divorced cementite in the grain boundaries of low-carbon steel after cold working. Similar arguments can be used to predict initiation sites for various kinds of bending loading and torsion loading. For example, for a three-point loaded beam, cracking is expected to initiate at the location of maximum bending moment. If it is not at that location, the implication is that a geometric or material imperfection has moved the location of the local maximum stress. Another example is rolling-contact fatigue, where maximum stress develops below the surface and is thus expected to cause subsurface crack initiation. Additionally, residual stresses may be distributed in the component as a result of prior mechanical/thermal processing. In this way, the significance of a material imperfection must be carefully evaluated in terms of stresses created by applied loads and part configuration. Imperfections from the Ingot Many flaws in wrought products can be traced back to the pouring and solidification of hot metal in molds during production of ingot. Except for forged powder metal components, the starting material in bulk working is a slab, ingot, billet, and so forth produced by casting into stationary molds or by continuous casting techniques. Primary deformation processes, such as hot rolling, tube piercing, extrusion, and open-die forging, are then used for converting the cast structure. Many large open-die forgings are forged directly from ingots, while most closed-die and upset forgings are produced from billets, bar stock, or a preform that has received some previous mechanical working. The product may be suitable for immediate application, but in many cases, it serves as the starting material for another (so-called secondary) deformation process, such as drawing, hot forging, cold forging, and sheet metalworking. The major problems associated with melting and casting practice are the development of porosity and a condition known as scabs. Porosity is developed in cast ingots when they solidify and is of two types: pipe and blow holes. Factors that work against obtaining a perfect homogeneous product include: · The fast shrinkage as the molten metals cools (roughly 5% in volume for steel) · The gaseous products that are trapped by the solidifying metal as they try to escape from the liquid and semisolid metal · Small crevices in the mold walls, which cause the metal to tear during the stripping operation · Spatter during pouring, which produces globs of metal frozen on the mold walls because of the great difference in temperature of the mold surfaces and the liquid metal Scabs are caused by improper ingot pouring, in which metal is splashed against the side of the mold wall. The splashed material, or scab, tends to stick to the wall and become oxidized. Scabs usually show up only after rolling or working and, as can be expected, give poor surface finish. Ingot imperfections can seriously affect the performance and reliability of wrought products, and the types of the imperfections that can be traced to the original ingot product include: · Chemical segregation · Ingot pipe, porosity, and centerline shrinkage · High hydrogen content
Nonmetallic inclusions Unmelted electrodes and shelf Cracks, laminations, seams, pits, blisters, and scabs The conversion practice must impart sufficient homogenization or healing to produce a product with sound center conditions. Ingot pipe, unhealed center conditions, or voids are melt-related discontinuities, but their occurrence in forgings is often a function of reduction ratio. Macroetching and ultrasonic inspection methods are the most widely used for identifying regions of unsoundness. Preliminary reduction of ingots or billets can also introduce flaws(some of which are similar to flaws that may occur during subsequent bulk working; se the"Forging Imperfections"). This includes internal bursts and various kinds of surface flaws Internal bursts occur where the work metal is weak, possibly from pipe, porosity, segregation, or inclusions The tensile stresses can be sufficiently high to tear the material apart internally, particularly if the hot working temperature is too high. Such internal tears are known as forging bursts or ruptures. Similarly, if the metal contains low-melting phases resulting from segregation, these phases may cause bursts during hot working of the ingot or billet. Bursts can also occur during subsequent bulk-working operations Laps appear as linear defects caused by the folding over of hot metal at the surface. These folds are worked into the surface but are not metallurgically bonded(welded) because of the oxide present between the surfaces. This creates a sharp discontinuity. Seams are a surface defect that also appears as a linear discontinuity. They occur from a crack, a heavy cluster of nonmetallic inclusions, or a deep lap (a lap that intersects the surface at a large angle). a seam can also result from a defect in the ingot surface, such as a hole, that becomes oxidized and is prevented from healing during working. In this case, the hole simply stretches out during forging or rolling, producing a linear cracklike seam in the workpiece surface Slivers are loose or torn pieces of steel rolled into the surface. Rolled-in scale is scale formed durin Ferrite fingers are surface cracks that have been welded shut but still contain the oxides and decarburization Fins and overfills are protrusions formed by incorrect reduction during hot working Underfills are the result of incomplete working of the section during reduction Rolled-in scale Chemical Segregation The elements in a cast alloy are seldom distributed uniformly. Even unalloyed metals contain random amounts of various types of impurities in the form of tramp elements or dissolved gases; these impurities are also seldom distributed uniformly. Therefore, the composition of the metal or alloy varies from location to location Unfortunately, such variation in chemical composition can often be significant and produce deleterious material conditions. This deviation from the mean composition at a particular location in a cast or wrought product is an imperfection termed segregation Chemical segregation originates in alloys during the solidification stage. Such deviations from the nominal composition are due to convection currents in the liquid, gravity effects, and redistribution of the solute during the formation of dendrites. Solute rejection at the solid-liquid interface during dendrite formation typically occurs during solidification, and thus a compositional gradient typically exists from the cores of dendrites to the interdendritic regions, with the latter enriched in alloying elements (solute)and low-melting contaminants Dendrite arms also are generally lower in impurities, such as sulfur and phosphorus in steel, than the interdendritic regions. Consequently, the dendrite arms are stronger and, on working, do not deform and flow as readily as the matrix in which they are incorporated Microsegregation characterizes concentrations of elements in interdendritic regions that range in size from a few to several hundred microns. By contrast, macrosegregation is the gradient difference, measurable on a macroscale, in alloying elements from the surface to the center of an ingot or casting. Macrosegregation becomes more pronounced with increasing section size Microsegregation, particularly within secondary arm branches, can be eliminated by homogenization. However, macrosegregation is harder to eliminate, because Thefileisdownloadedfromwww.bzfxw.com
· Nonmetallic inclusions · Unmelted electrodes and shelf · Cracks, laminations, seams, pits, blisters, and scabs The conversion practice must impart sufficient homogenization or healing to produce a product with sound center conditions. Ingot pipe, unhealed center conditions, or voids are melt-related discontinuities, but their occurrence in forgings is often a function of reduction ratio. Macroetching and ultrasonic inspection methods are the most widely used for identifying regions of unsoundness. Preliminary reduction of ingots or billets can also introduce flaws (some of which are similar to flaws that may occur during subsequent bulk working; see the “Forging Imperfections”). This includes internal bursts and various kinds of surface flaws. Internal bursts occur where the work metal is weak, possibly from pipe, porosity, segregation, or inclusions. The tensile stresses can be sufficiently high to tear the material apart internally, particularly if the hot working temperature is too high. Such internal tears are known as forging bursts or ruptures. Similarly, if the metal contains low-melting phases resulting from segregation, these phases may cause bursts during hot working of the ingot or billet. Bursts can also occur during subsequent bulk-working operations. Surface flaws from preliminary reduction may include: · Laps appear as linear defects caused by the folding over of hot metal at the surface. These folds are worked into the surface but are not metallurgically bonded (welded) because of the oxide present between the surfaces. This creates a sharp discontinuity. · Seams are a surface defect that also appears as a linear discontinuity. They occur from a crack, a heavy cluster of nonmetallic inclusions, or a deep lap (a lap that intersects the surface at a large angle). A seam can also result from a defect in the ingot surface, such as a hole, that becomes oxidized and is prevented from healing during working. In this case, the hole simply stretches out during forging or rolling, producing a linear cracklike seam in the workpiece surface. · Slivers are loose or torn pieces of steel rolled into the surface. Rolled-in scale is scale formed during rolling. · Ferrite fingers are surface cracks that have been welded shut but still contain the oxides and decarburization. · Fins and overfills are protrusions formed by incorrect reduction during hot working. · Underfills are the result of incomplete working of the section during reduction. · Rolled-in scale Chemical Segregation The elements in a cast alloy are seldom distributed uniformly. Even unalloyed metals contain random amounts of various types of impurities in the form of tramp elements or dissolved gases; these impurities are also seldom distributed uniformly. Therefore, the composition of the metal or alloy varies from location to location. Unfortunately, such variation in chemical composition can often be significant and produce deleterious material conditions. This deviation from the mean composition at a particular location in a cast or wrought product is an imperfection termed segregation. Chemical segregation originates in alloys during the solidification stage. Such deviations from the nominal composition are due to convection currents in the liquid, gravity effects, and redistribution of the solute during the formation of dendrites. Solute rejection at the solid-liquid interface during dendrite formation typically occurs during solidification, and thus a compositional gradient typically exists from the cores of dendrites to the interdendritic regions, with the latter enriched in alloying elements (solute) and low-melting contaminants. Dendrite arms also are generally lower in impurities, such as sulfur and phosphorus in steel, than the interdendritic regions. Consequently, the dendrite arms are stronger and, on working, do not deform and flow as readily as the matrix in which they are incorporated. Microsegregation characterizes concentrations of elements in interdendritic regions that range in size from a few to several hundred microns. By contrast, macrosegregation is the gradient difference, measurable on a macroscale, in alloying elements from the surface to the center of an ingot or casting. Macrosegregation becomes more pronounced with increasing section size. Microsegregation, particularly within secondary arm branches, can be eliminated by homogenization. However, macrosegregation is harder to eliminate, because The file is downloaded from www.bzfxw.com
complete homogenization would require longer times than are economically acceptable under production conditions. Therefore, in very large sections, gross differences in alloy concentration sometimes persist and are arried into the final product One function of hot working is to break up the cast(dendritic) structure and promote chemical homogeneity, and a minimum amount of cross-sectional reduction is usually required from the cast ingot to the billet. Hot working can partially correct the results of segregation by recrystallizing or breaking up the grain structure to promote a more homogeneous substructure. Initial working first causes flow in the weaker matrix (interdendritic) regions and tends to reorient the stronger dendrites in the direction of working. With increased mechanical working, the dendrites deform and fracture, thus becoming increasingly elongated a certain degree of alloy segregation occurs in all wrought products, and hot working can alleviate of the inhomogeneity. However, if the ingot is badly segregated, hot working just tends to alter the of the segregation region into a banded structure. Figure 3 shows banding from a carbon-rich centerline condition in a hot-rolled 104 1 steel. Figure 4 shows an extreme example of banding in a hot-rolled plain carbon steel (1022) in which alternate layers of ferrite and pearlite have formed along the rolling direction. The relationship between increasing percentages of reduction by hot rolling and the intensity of banding in type 430 stainless steel is demonstrated by Fig. 5 Fig 3 Longitudinal section through a hot-rolled 1041 steel bar showing a carbon-rich centerline (dark horizontal bands) that resulted from segregation in the ingot. Picral. 3x. Courtesy of j.R. Kilpatrick
complete homogenization would require longer times than are economically acceptable under production conditions. Therefore, in very large sections, gross differences in alloy concentration sometimes persist and are carried into the final product. One function of hot working is to break up the cast (dendritic) structure and promote chemical homogeneity, and a minimum amount of cross-sectional reduction is usually required from the cast ingot to the billet. Hot working can partially correct the results of segregation by recrystallizing or breaking up the grain structure to promote a more homogeneous substructure. Initial working first causes flow in the weaker matrix (interdendritic) regions and tends to reorient the stronger dendrites in the direction of working. With increased mechanical working, the dendrites deform and fracture, thus becoming increasingly elongated. A certain degree of alloy segregation occurs in all wrought products, and hot working can alleviate some of the inhomogeneity. However, if the ingot is badly segregated, hot working just tends to alter the shape of the segregation region into a banded structure. Figure 3 shows banding from a carbon-rich centerline condition in a hot-rolled 1041 steel. Figure 4 shows an extreme example of banding in a hot-rolled plain carbon steel (1022) in which alternate layers of ferrite and pearlite have formed along the rolling direction. The relationship between increasing percentages of reduction by hot rolling and the intensity of banding in type 430 stainless steel is demonstrated by Fig. 5. Fig. 3 Longitudinal section through a hot-rolled 1041 steel bar showing a carbon-rich centerline (dark horizontal bands) that resulted from segregation in the ingot. Picral. 3×. Courtesy of J.R. Kilpatrick
A 82一 Fig 4 Hot-rolled 1022 steel showing severe banding. Bands of pearlite(dark) and ferrite were caused by segregation of carbon and other elements during solidification and later decomposition of austenite. Nital. 250x. Courtesy of J.R. Kilpatrick Thefileisdownloadedfromwww.bzfxw.com
Fig. 4 Hot-rolled 1022 steel showing severe banding. Bands of pearlite (dark) and ferrite were caused by segregation of carbon and other elements during solidification and later decomposition of austenite. Nital. 250×. Courtesy of J.R. Kilpatrick The file is downloaded from www.bzfxw.com
a (b) (c) Fig 5 Type 430 stainless steel hot rolled to various percentages of reduction showing development of a banded structure consisting of alternate layers of ferrite (light) and martensite (dark) as the amount of hot work is increased. (a)63% reduction.(b)81%reduction.(c)94% reduction. 55 mL 35% HCl, 1 to 2 g potassium metabisulfite, 275 mL H2O (Beraha's tint reagent No. 2). 500x Depending on the kind and degree of segregation that develops during solidification, some degree of banding carries over to the wrought form. If banding is severe, it can lead to discontinuities that cause premature fail hg For example, Fig. 6 shows the fatigue fracture of a carburized and hardened steel roller. Banded alloy segregation in the metal used for the rollers resulted in heavy, banded retained austenite, particularly in the carburized case, after heat treatment When the roller was subjected to service loads, the delayed transformation of the retained austenite to martensite caused microcracks near the case -core interface. These internal microcracks nucleated a fatigue fracture that progressed around the circumference of the roller, following the interface between case and core
Fig. 5 Type 430 stainless steel hot rolled to various percentages of reduction showing development of a banded structure consisting of alternate layers of ferrite (light) and martensite (dark) as the amount of hot work is increased. (a) 63% reduction. (b) 81% reduction. (c) 94% reduction. 55 mL 35% HCl, 1 to 2 g potassium metabisulfite, 275 mL H2O (Beraha's tint reagent No. 2). 500× Depending on the kind and degree of segregation that develops during solidification, some degree of banding carries over to the wrought form. If banding is severe, it can lead to discontinuities that cause premature failure. For example, Fig. 6 shows the fatigue fracture of a carburized and hardened steel roller. Banded alloy segregation in the metal used for the rollers resulted in heavy, banded retained austenite, particularly in the carburized case, after heat treatment. When the roller was subjected to service loads, the delayed transformation of the retained austenite to martensite caused microcracks near the case-core interface. These internal microcracks nucleated a fatigue fracture that progressed around the circumference of the roller, following the interface between case and core
ig. 6 Fracture surface of a carburized and hardened steel roller. As a result of banded alloy segregation circumferential fatigue fracture initiated at a subsurface origin near the case -core interface (arrow) Excessive segregation also can have an adverse effect on subsequent fabrication and heat treatment. In heat treatable alloys, variations in composition can produce unexpected responses to heat treatments, which result hard or soft spots, quench cracks, or other flaws. Excessive segregation that leads to significant variations hardness can lead to premature failure and extreme difficulties during cold working or forming. In this case one of the simplest and most effective tests for incoming material is a simple standard upset test. The details of such a test can be worked out between the supplier and the cold forger The methods to reveal the presence of segregation may depend on the alloy and expected impurities Macroetching is commonly used, and the American Society for Testing and Materials(AsTM) has established a graded series(ASTM E 381) of macroetching for center segregation in steel product. Segregations are revealed by differences in the severity of the etchant attack; segregations at the center may appear as a pipe or may be grouped in some fairly regular form about the center, depending on the shape of the ingot and the mechanical work done on it. Segregation as revealed by macroetching does not al ways indicate defective metal A polished specimen should also be examined under the microscope to determine whether the revealed segregation is metallic or a concentration of nonmetallic impurities Sulfur Print Test. The microscopic identification of segregation may be supplemented by chemical or other means of testing. For regions with expected regions of sulfide sulfur-rich segregation, the sulfur print test(Ref 3)can be used. An example of a failure of a steel I-beam with high levels of carbon, sulfur, and phosphorus segregation in the middle of the section is given in Ref 4. The beam was lying flat on the ground near the seacoast under normal weather conditions. It was flame cut into two sections, as required for construction, and approximately -h after cutting, a violent sound was heard. The shorter section of the cut beam had split catastrophically into two portions along the entire length and approximately through the middle of the web Various samples were taken from both the broken and unbroken sections of the beam for analysis(chemistry metallography, macroetching, impact testing, tensile testing) and sulfur printing. Sulfur prints taken at various locations indicated segregation of sulfides within a central zone approximately 2 mm(0.08 in ) wide in the thickness direction of the web that extended throughout the length of the beam. The breadth of the segregation zone varied from 60 mm(2.4 in. ) at the end face of the unfractured section of the I-beam to almost the total idth of the web in most of the fractured section. Sulfide segregation was not found in the flanges of the beam Failures similar to the one investigated have occasionally occurred in structural beams in the shop under no load, and a contributing factor was the presence of residual stresses in the material. Flame cutting caused a quality of the beam, resulted in failure. The failure of the I-beam was probably caused by segregation of carban change in the distribution of the residual stresses, which, aided by low fracture toughness due to the po sulfur, and phosphorus within its web section, which resulted in decreased notch sensitivity and low fracture toughness with respect to crack propagation through the web. The detailed investigation(Ref 4) revealed segregation of high levels of carbon, sulfur, and phosphorus in the middle of the web and high residual stresses attributed to rolling during fabrication Thefileisdownloadedfromwww.bzfxw.com
Fig. 6 Fracture surface of a carburized and hardened steel roller. As a result of banded alloy segregation, circumferential fatigue fracture initiated at a subsurface origin near the case-core interface (arrow). Excessive segregation also can have an adverse effect on subsequent fabrication and heat treatment. In heat treatable alloys, variations in composition can produce unexpected responses to heat treatments, which result in hard or soft spots, quench cracks, or other flaws. Excessive segregation that leads to significant variations in hardness can lead to premature failure and extreme difficulties during cold working or forming. In this case, one of the simplest and most effective tests for incoming material is a simple standard upset test. The details of such a test can be worked out between the supplier and the cold forger. The methods to reveal the presence of segregation may depend on the alloy and expected impurities. Macroetching is commonly used, and the American Society for Testing and Materials (ASTM) has established a graded series (ASTM E 381) of macroetchings for center segregation in steel product. Segregations are revealed by differences in the severity of the etchant attack; segregations at the center may appear as a pipe or may be grouped in some fairly regular form about the center, depending on the shape of the ingot and the mechanical work done on it. Segregation as revealed by macroetching does not always indicate defective metal. A polished specimen should also be examined under the microscope to determine whether the revealed segregation is metallic or a concentration of nonmetallic impurities. Sulfur Print Test. The microscopic identification of segregation may be supplemented by chemical or other means of testing. For regions with expected regions of sulfide sulfur-rich segregation, the sulfur print test (Ref 3) can be used. An example of a failure of a steel I-beam with high levels of carbon, sulfur, and phosphorus segregation in the middle of the section is given in Ref 4. The beam was lying flat on the ground near the seacoast under normal weather conditions. It was flame cut into two sections, as required for construction, and approximately 1 2 h after cutting, a violent sound was heard. The shorter section of the cut beam had split catastrophically into two portions along the entire length and approximately through the middle of the web. Various samples were taken from both the broken and unbroken sections of the beam for analysis (chemistry, metallography, macroetching, impact testing, tensile testing) and sulfur printing. Sulfur prints taken at various locations indicated segregation of sulfides within a central zone approximately 2 mm (0.08 in.) wide in the thickness direction of the web that extended throughout the length of the beam. The breadth of the segregation zone varied from 60 mm (2.4 in.) at the end face of the unfractured section of the I-beam to almost the total width of the web in most of the fractured section. Sulfide segregation was not found in the flanges of the beam. Failures similar to the one investigated have occasionally occurred in structural beams in the shop under no load, and a contributing factor was the presence of residual stresses in the material. Flame cutting caused a change in the distribution of the residual stresses, which, aided by low fracture toughness due to the poor quality of the beam, resulted in failure. The failure of the I-beam was probably caused by segregation of carbon, sulfur, and phosphorus within its web section, which resulted in decreased notch sensitivity and low fracture toughness with respect to crack propagation through the web. The detailed investigation (Ref 4) revealed segregation of high levels of carbon, sulfur, and phosphorus in the middle of the web and high residual stresses attributed to rolling during fabrication. The file is downloaded from www.bzfxw.com
Example 1: Fracture of a Forging Die Caused by Segregation(Ref 5). A cross-recessed die of D5 tool steel fractured in service. The die face was subjected to shear and tensile stresses as a result of the forging pressures from the material being worked. Figure 7(a) illustrates the fractured die D5 tool steel a oSs 850 oEoaaeEo6 Fracture 750 (a) (b) (c) Fig. 7 A D5 tool steel forging die that failed in service because of segregation.(a)Hardness traverse correlated with the microstructure of the die. (b) Section through one arm of the cross on the recessed die face showing a severely segregated(banded) structure. Etched with 5% nital. (c)Micrograph of the segregated area. Etched with 5% nital 200x Investigation. A longitudinal section was taken through the die to include one arm of the cross on the recessed die face. The specimen was polished and examined in the unetched condition. Examination revealed the presence of numerous slag stringers The polished specimen was then etched with 5% nital. A marked banded structure was evident even macroscopically(Fig. 7b). Microscopic examination revealed that the pattern was due to severe chemical segregation or banding( Fig. 7c)
Example 1: Fracture of a Forging Die Caused by Segregation (Ref 5). A cross-recessed die of D5 tool steel fractured in service. The die face was subjected to shear and tensile stresses as a result of the forging pressures from the material being worked. Figure 7(a) illustrates the fractured die. Fig. 7 A D5 tool steel forging die that failed in service because of segregation. (a) Hardness traverse correlated with the microstructure of the die. (b) Section through one arm of the cross on the recessed die face showing a severely segregated (banded) structure. Etched with 5% nital. (c) Micrograph of the segregated area. Etched with 5% nital. 200× Investigation. A longitudinal section was taken through the die to include one arm of the cross on the recessed die face. The specimen was polished and examined in the unetched condition. Examination revealed the presence of numerous slag stringers. The polished specimen was then etched with 5% nital. A marked banded structure was evident even macroscopically (Fig. 7b). Microscopic examination revealed that the pattern was due to severe chemical segregation or banding (Fig. 7c)
