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