Fig 18 Macrograph of failed bolt Figure 19 shows a SEM view of the fracture initiation region. The white inclusions were nonmetallic particles that were high in calcium. Various spectra and x-ray maps were collected. The x-ray maps illustrated the presence of calcium in the white spots. These spots were thus interpreted to be slag inclusions in the steel Areas containing the inclusions were cleaned with a soft brush several times to ascertain that the x-ray indications were not debris that collected on the fracture surface during storage and handling after the failure The gold-colored areas of the bolt were found to have been cadmium plated, followed by a chromate treatment The black-coated surfaces contained molybdenum and sulfur, indicating a Mos2 treatment. Specimen composition, as determined by optical emission spectrometry, was in agreement with nominal specifications for AISI E4340 steel 81525K15:“2:的mC g. 19 Scanning electron microscope micrographs showing segregation of inclusions along the edge where fracture apears to have initiated Thefileisdownloadedfromwww.bzfxw.com
Fig. 18 Macrograph of failed bolt Figure 19 shows a SEM view of the fracture initiation region. The white inclusions were nonmetallic particles that were high in calcium. Various spectra and x-ray maps were collected. The x-ray maps illustrated the presence of calcium in the white spots. These spots were thus interpreted to be slag inclusions in the steel. Areas containing the inclusions were cleaned with a soft brush several times to ascertain that the x-ray indications were not debris that collected on the fracture surface during storage and handling after the failure. The gold-colored areas of the bolt were found to have been cadmium plated, followed by a chromate treatment. The black-coated surfaces contained molybdenum and sulfur, indicating a MoS2 treatment. Specimen composition, as determined by optical emission spectrometry, was in agreement with nominal specifications for AISI E4340 steel. Fig. 19 Scanning electron microscope micrographs showing segregation of inclusions along the edge where fracture apears to have initiated. The file is downloaded from www.bzfxw.com
Conclusion and Remedial Action. It was initially suspected that hydrogen embrittlement caused crack initiation It was concluded, however, that the failure was caused by a high concentration of nonmetallic inclusions Inspection of other bolts from the same shipment was recommended Other lngot-Related Imperfections aminations in wrought products may occur from various types of discontinuities introduced during the ingot stage. As previously noted, pipe can form into laminations in rolled products( Fig. 9). Laminations may also form spatter (entrapped splashes) during the pouring of the molten metal into the ingot mold. These imperfections are elongated during rolling or other working and are usually subsurface, as shown in Fig. 2(b) for bar stock. Figure 2(b)illustrates a lamellar structure opened up by a chipping tool Slivers are most often caused by a rough mold surface, overheating prior to rolling, or abrasion during rolling Very often, slivers are found with seams. Slivers usually have raised edges, as shown in Fig. 2(c 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. The metal first freezes to the wall of the mold, then becomes attached to the ingot, and finally becomes embedded in the surface of the wrought product(e.g, Fig. 2d). Scabs usually show up only after rolling and, as can be expected, give poor surface finish. Scabs thus bear some similarity to laminations Pits and Blisters. Gaseous pockets in the ingot often become pits or blisters on the surface or slightly below the surface of bar products. Other pits may be caused by overpickling to remove scale or rust. Pits and blisters are both illustrated in Fig. 2(e). Embedded scale may result from the rolling or drawing of bars that have become excessively scaled during prior heating operations. The pattern illustrated in Fig. 2(f) is typical Cracks and seams are often confused with each other. Cracks with little or no oxide present on their edges may occur when the metal cools in the mold, setting up highly stressed areas. Seams develop from these cracks during rolling as the reheated outer skin of the billet becomes heavily oxidized, transforms into scale, and flakes off the part during further rolling operations. Cracks also result from highly stressed planes in cold-drawn bars or from improper quenching during heat treatment. Cracks created from these latter two causes show no evidence of oxidized surfaces. A typical crack in a bar is shown in Fig. 2(g) Unmelted electrodes and shelf are two other of ingot flaws that can impair forgeability. Unmelted electrodes are caused by chunks of electrodes eroded away during consumable melting and dropping down into the molten material as a solid. Shelf is a condition resulting from uneven solidification or cooling rates at the ingot surfaces. The consumable-melting operation has occasionally been continued to a point where a portion of the stinger rod is melted into the ingot, which may be undesirable, because the composition of the nger rod may differ from that of the alloy being melted To prevent this occurrence, one practice is to weld a wire to the stinger rod bend the wire down in tension, and weld the other end of the wire to the surface of the electrode a few inches below the junction of the stinger rod and the electrode. When the electrode has been consumed to where the wire is attached to it, the wire is released and springs out against the side of the crucible thus serving as an alarm to stop the melting. a disadvantage of this practice is that the wire may become detached and contaminate the melt Unmelted pieces of electrode or shelf conditions appear infrequently in vacuum-melted alloys. Either of these conditions can seriously degrade forgeability. Macroetching or another appropriate type of surface inspection of the machined forging or the billet is the most effective method of detecting unmelted electrodes or shelf conditions References cited in this section 3. G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984(out of print), reprinted 1999 ASM International 4. R.K. Dayal, J B Gnanamoorthy, and P Rodriguez, Failure of an I-Beam, Handbook of Case Histories in Failure Analysis, Vol 1, ASM International, 1992, p 378-381 5. E. Kauczor, Prakt. Metallogr. Vol 8, July 1971, p 443-446
Conclusion and Remedial Action. It was initially suspected that hydrogen embrittlement caused crack initiation. It was concluded, however, that the failure was caused by a high concentration of nonmetallic inclusions. Inspection of other bolts from the same shipment was recommended. Other Ingot-Related Imperfections Laminations in wrought products may occur from various types of discontinuities introduced during the ingot stage. As previously noted, pipe can form into laminations in rolled products (Fig. 9). Laminations may also form spatter (entrapped splashes) during the pouring of the molten metal into the ingot mold. These imperfections are elongated during rolling or other working and are usually subsurface, as shown in Fig. 2(b) for bar stock. Figure 2(b) illustrates a lamellar structure opened up by a chipping tool. Slivers are most often caused by a rough mold surface, overheating prior to rolling, or abrasion during rolling. Very often, slivers are found with seams. Slivers usually have raised edges, as shown in Fig. 2(c). 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. The metal first freezes to the wall of the mold, then becomes attached to the ingot, and finally becomes embedded in the surface of the wrought product (e.g., Fig. 2d). Scabs usually show up only after rolling and, as can be expected, give poor surface finish. Scabs thus bear some similarity to laminations. Pits and Blisters. Gaseous pockets in the ingot often become pits or blisters on the surface or slightly below the surface of bar products. Other pits may be caused by overpickling to remove scale or rust. Pits and blisters are both illustrated in Fig. 2(e). Embedded scale may result from the rolling or drawing of bars that have become excessively scaled during prior heating operations. The pattern illustrated in Fig. 2(f) is typical. Cracks and seams are often confused with each other. Cracks with little or no oxide present on their edges may occur when the metal cools in the mold, setting up highly stressed areas. Seams develop from these cracks during rolling as the reheated outer skin of the billet becomes heavily oxidized, transforms into scale, and flakes off the part during further rolling operations. Cracks also result from highly stressed planes in cold-drawn bars or from improper quenching during heat treatment. Cracks created from these latter two causes show no evidence of oxidized surfaces. A typical crack in a bar is shown in Fig. 2(g). Unmelted electrodes and shelf are two other types of ingot flaws that can impair forgeability. Unmelted electrodes are caused by chunks of electrodes being eroded away during consumable melting and dropping down into the molten material as a solid. Shelf is a condition resulting from uneven solidification or cooling rates at the ingot surfaces. The consumable-melting operation has occasionally been continued to a point where a portion of the stinger rod is melted into the ingot, which may be undesirable, because the composition of the stinger rod may differ from that of the alloy being melted. To prevent this occurrence, one practice is to weld a wire to the stinger rod, bend the wire down in tension, and weld the other end of the wire to the surface of the electrode a few inches below the junction of the stinger rod and the electrode. When the electrode has been consumed to where the wire is attached to it, the wire is released and springs out against the side of the crucible, thus serving as an alarm to stop the melting. A disadvantage of this practice is that the wire may become detached and contaminate the melt. Unmelted pieces of electrode or shelf conditions appear infrequently in vacuum-melted alloys. Either of these conditions can seriously degrade forgeability. Macroetching or another appropriate type of surface inspection of the machined forging or the billet is the most effective method of detecting unmelted electrodes or shelf conditions. References cited in this section 3. G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984 (out of print), reprinted 1999 ASM International 4. R.K. Dayal, J.B. Gnanamoorthy, and P. Rodriguez, Failure of an I-Beam, Handbook of Case Histories in Failure Analysis, Vol 1, ASM International, 1992, p 378–381 5. E. Kauczor, Prakt. Metallogr. Vol 8, July 1971, p 443–446
6. G. Vander Voort, Embrittlement of Steels, Properties and Selection: Iron, Steels, and High- Performance Alloys, Vol 1, ASM Handbook, ASM International, 1990, p 689-736 7. P.A. Thornton, J. Mater. Sci., Vol 6, 1971, p 347 8. D V. Edmonds and C J. Beevers, J. Mater:. Sci., Vol 3, 1968, p 457 9. A.D. Wilson, Calcium Treatment of Plate Steels and Its Effect on Fatigue and Toughness Properties Proc. 1lth Annual Offshore Technology Conference, OTC 3465, May 1979, p939-948 10. A D. Wilson, Effect of Calcium Treatment on Inclusions in Constructional Steels, Met. Prog., Vol 121 (No.5), April982,p41-46 11 M D Chaudhari, Failure Analysis of a Helicopter Main Rotor Bolt, Handbook of Case Histories in Failure Analysis, Vol 2, ASM International, 1993, p 392 Forging Imperfections In many cases, the internal and surface imperfections that occur during forging are the same as, or at least similar to, those that may occur during hot working of the ingots or billets, as briefly introduced in the previous section. The inspection of forgings and the types of flaws for specific alloy forgings( steel, nickel, aluminum and titanium alloys) are described in more detail in the article"Nondestructive Inspection of Forgings"in Nondestructive Evaluation and Quality Control, Volume 17 of ASM Handbook. a brief review for nonferrous alloys follows Nickel-base heat-resistant alloys: Most of the flaws found in forgings of heat-resistant alloys can be categorized as those related to scrap selection, melting, or primary conversion to bar or billet or those that occur during forging or heat treatment. Nickel-base superalloys are highly susceptible to surface contamination during heating for forging. Fuel oils containing sulfur induce a grain-boundary attack, which causes subsequent rupturing during forging. Paint or marking crayons with high levels of similar contaminants cause similar areas of grain-boundary contamination Aluminum alloys: The internal discontinuities that occur in aluminum alloy forgings are ruptures, cracks, inclusions, segregation, and occasionally porosity. Ruptures and cracks are associated with temperature control during preheating or forging or with excessive reduction during a single forging operation. Cracks can also occur in stock that has been excessively reduced in one operation Inclusions segregation,and porosity result from forging stock that contains these types of discontinuities Titanium alloys: Discontinuities that are most likely to occur in titanium alloy forgings are usually carried over in the bar or billet. Typical discontinuities in titanium alloy forgings are alpha-stabilized voids, macrostructural defects, unsealed center conditions, clean voids, and forging imperfections Alpha-stabilized voids are among the most common discontinuities found in forgings of titanium alloys Three principal macrodefects are commonly found in ingot, forged billet, or other semifinished product forms. These include high-aluminum defects(type II defects), high-interstitial defects(type I defects or low-density interstitial defects), and beta flecks The most common internal imperfections found in steel forgings are pipe, segregation, nonmetallic inclusions, and stringers. Internal flaws caused by forgings also include cracks or tears, which may result either from forging with too light a hammer or from continuing forging after the metal has cooled down below a safe forging temperature. Other flaws in steel forgings that can be produced by improper die design or maintenance are internal cracks and splits. If the material is moved abnormally during forging, these flaws may be formed without any evidence on the surface of the forging. Bursts, as described in the preceding section, may also occur a number of surface flaws also can be produced by the forging operation. These flaws are often caused by the movement of metal over or on another surface without actual welding or fusing of the surfaces. The most common surface flaws in steel forgings are seams, laps, and slivers. Laps and seams are surface discontinuities Thefileisdownloadedfromwww.bzfxw.com
6. G. Vander Voort, Embrittlement of Steels, Properties and Selection: Iron, Steels, and HighPerformance Alloys, Vol 1, ASM Handbook, ASM International, 1990, p 689–736 7. P.A. Thornton, J. Mater. Sci., Vol 6, 1971, p 347 8. D.V. Edmonds and C.J. Beevers, J. Mater. Sci., Vol 3, 1968, p 457 9. A.D. Wilson, Calcium Treatment of Plate Steels and Its Effect on Fatigue and Toughness Properties, Proc. 11th Annual Offshore Technology Conference, OTC 3465, May 1979, p 939–948 10. A.D. Wilson, Effect of Calcium Treatment on Inclusions in Constructional Steels, Met. Prog., Vol 121 (No. 5), April 1982, p 41–46 11. M.D. Chaudhari, Failure Analysis of a Helicopter Main Rotor Bolt, Handbook of Case Histories in Failure Analysis, Vol 2, ASM International, 1993, p 392 Forging Imperfections In many cases, the internal and surface imperfections that occur during forging are the same as, or at least similar to, those that may occur during hot working of the ingots or billets, as briefly introduced in the previous section. The inspection of forgings and the types of flaws for specific alloy forgings (steel, nickel, aluminum, and titanium alloys) are described in more detail in the article “Nondestructive Inspection of Forgings” in Nondestructive Evaluation and Quality Control, Volume 17 of ASM Handbook. A brief review for nonferrous alloys follows: · Nickel-base heat-resistant alloys: Most of the flaws found in forgings of heat-resistant alloys can be categorized as those related to scrap selection, melting, or primary conversion to bar or billet or those that occur during forging or heat treatment. Nickel-base superalloys are highly susceptible to surface contamination during heating for forging. Fuel oils containing sulfur induce a grain-boundary attack, which causes subsequent rupturing during forging. Paint or marking crayons with high levels of similar contaminants cause similar areas of grain-boundary contamination. · Aluminum alloys: The internal discontinuities that occur in aluminum alloy forgings are ruptures, cracks, inclusions, segregation, and occasionally porosity. Ruptures and cracks are associated with temperature control during preheating or forging or with excessive reduction during a single forging operation. Cracks can also occur in stock that has been excessively reduced in one operation. Inclusions, segregation, and porosity result from forging stock that contains these types of discontinuities. · Titanium alloys: Discontinuities that are most likely to occur in titanium alloy forgings are usually carried over in the bar or billet. Typical discontinuities in titanium alloy forgings are alpha-stabilized voids, macrostructural defects, unsealed center conditions, clean voids, and forging imperfections. Alpha-stabilized voids are among the most common discontinuities found in forgings of titanium alloys. Three principal macrodefects are commonly found in ingot, forged billet, or other semifinished product forms. These include high-aluminum defects (type II defects), high-interstitial defects (type I defects or low-density interstitial defects), and beta flecks. The most common internal imperfections found in steel forgings are pipe, segregation, nonmetallic inclusions, and stringers. Internal flaws caused by forgings also include cracks or tears, which may result either from forging with too light a hammer or from continuing forging after the metal has cooled down below a safe forging temperature. Other flaws in steel forgings that can be produced by improper die design or maintenance are internal cracks and splits. If the material is moved abnormally during forging, these flaws may be formed without any evidence on the surface of the forging. Bursts, as described in the preceding section, may also occur. A number of surface flaws also can be produced by the forging operation. These flaws are often caused by the movement of metal over or on another surface without actual welding or fusing of the surfaces. The most common surface flaws in steel forgings are seams, laps, and slivers. Laps and seams are surface discontinuities The file is downloaded from www.bzfxw.com
that are caused by folding over of metal without fusion. They are usually filled with scale and, on steel components, are enclosed by a layer of decarburized metal. Laps, seams, and other surface defects formed during manufacture of the blanks can also lead to major problems during cold forming of the actual parts. Once any surface defect is exposed to the atmosphere, its surfaces become oxidized, providing an initiation point fo failure during future operation Other surface flaws include rolled-in scale. ferrite fingers fins. overfills, and underfills Surface flaws weaken forgings and can usually be eliminated by correct die design, proper heating, and correct sequencing and ositioning of the workpieces in the dies. When such cracks form early in the steel manufacturing process, the surfaces of such breaks can become completely decarburized, forming what is known as a ferrite finger. Even if the original crack about which they formed is then removed by conditioning, the remaining ferrite is extremely low in tensile strength and often splits during the metal movement necessary during cold forming Cold shuts often occur in closed-die forgings. They are junctures of two adjoining surfaces caused by incomplete metal fill and incomplete fusion of the surfaces. Shear cracks often occur in steel for diagonal cracks occurring on the trimmed edges and are caused by shear stresses. Proper design and condition of trimming dies to remove forging flash are required for the prevention of shear cracks Control of heating In addition to flow-related imperfections, proper control of heating in hot forging is necessary to prevent excessive scale, decarburization, overheating, or burning. Excessive scale, in addition to causing excessive metal loss, can result in forgings with pitted surfaces. The pitted surfaces are caused by the scale being hammered into the surface and may result in unacceptable forgings. The development of scale during preheating of ingots, slabs, or blooms is almost inevitable, particularly for steels. Sometimes descaling ale may get rolled into the metal surface and become elongated into streaks during subsequent rolling cessful; operations involving hydraulic sprays or preliminary light passes in rolling operations are not totally successful Severe overheating causes burning, which is the melting of the lower-melting-point constituents. This meltil action severely reduces the mechanical properties of the metal, and the damage is irreparable. Detection and sorting of forgings that have been burned during heating can be extremely difficult. Another defect related to heating practice is blistering, which is a raised spot on the surface caused by expansion of subsurface gas during heating. Blisters may break open and produce a defect that looks similar to a gouge or surface lamination Hot tears in forgings are surface cracks that are often ragged in appearance. They result from rupture of the material during forging and are often caused by the presence of low-melting or brittle phases Thermal cracks occur as a result of nonuniform temperatures in the forging. Quench cracks are one example of uch thermal cracks(see the section"Quench Cracks"in this article). Internal cracks, another type of thermal ck, may occur when forgings are heated too rapidly. These occur as a result of unequal temperatures of the surface relative to the center of the mass, and the resulting differences in the degree of thermal expansion produce tensile stresses near the center. The formation of such cracks depends on both the section size and the thermal conductivity of the material. Large section sizes and poor thermal conductivity promote thermal gradients and favor crack formation Flow-Related Factors Several kinds of forging imperfections are related to metal flow, which is influenced by the workability of the material(discussed later) and the details of component and die design. This section discusses some of the flow- elated imperfections that might constitute a manufacturing flaw during inspection or a potential problem during service. It is also important to keep in mind the effects of design on the overall process. Forging load increases very rapidly as the sharpness of details is increased, and die wear increases directly with die pressure Flash geometry also directly influences the amount of back pressure experienced by the metal in the die cavity These defects described in this section are related to the distribution of metal. They can be avoided by proper die design, preform design, and choice of lubrication system. Strictly speaking, these defects are not fundamental to the workability of the material. However, knowledge of these common forging defects is ecessary for a practical understanding of workability. These are the defects that commonly limit deformation the forging process
that are caused by folding over of metal without fusion. They are usually filled with scale and, on steel components, are enclosed by a layer of decarburized metal. Laps, seams, and other surface defects formed during manufacture of the blanks can also lead to major problems during cold forming of the actual parts. Once any surface defect is exposed to the atmosphere, its surfaces become oxidized, providing an initiation point for failure during future operations. Other surface flaws include rolled-in scale, ferrite fingers, fins, overfills, and underfills. Surface flaws weaken forgings and can usually be eliminated by correct die design, proper heating, and correct sequencing and positioning of the workpieces in the dies. When such cracks form early in the steel manufacturing process, the surfaces of such breaks can become completely decarburized, forming what is known as a ferrite finger. Even if the original crack about which they formed is then removed by conditioning, the remaining ferrite is extremely low in tensile strength and often splits during the metal movement necessary during cold forming. Cold shuts often occur in closed-die forgings. They are junctures of two adjoining surfaces caused by incomplete metal fill and incomplete fusion of the surfaces. Shear cracks often occur in steel forgings; they are diagonal cracks occurring on the trimmed edges and are caused by shear stresses. Proper design and condition of trimming dies to remove forging flash are required for the prevention of shear cracks. Control of Heating In addition to flow-related imperfections, proper control of heating in hot forging is necessary to prevent excessive scale, decarburization, overheating, or burning. Excessive scale, in addition to causing excessive metal loss, can result in forgings with pitted surfaces. The pitted surfaces are caused by the scale being hammered into the surface and may result in unacceptable forgings. The development of scale during preheating of ingots, slabs, or blooms is almost inevitable, particularly for steels. Sometimes descaling operations involving hydraulic sprays or preliminary light passes in rolling operations are not totally successful; scale may get rolled into the metal surface and become elongated into streaks during subsequent rolling. Severe overheating causes burning, which is the melting of the lower-melting-point constituents. This melting action severely reduces the mechanical properties of the metal, and the damage is irreparable. Detection and sorting of forgings that have been burned during heating can be extremely difficult. Another defect related to heating practice is blistering, which is a raised spot on the surface caused by expansion of subsurface gas during heating. Blisters may break open and produce a defect that looks similar to a gouge or surface lamination. Hot tears in forgings are surface cracks that are often ragged in appearance. They result from rupture of the material during forging and are often caused by the presence of low-melting or brittle phases. Thermal cracks occur as a result of nonuniform temperatures in the forging. Quench cracks are one example of such thermal cracks (see the section “Quench Cracks” in this article). Internal cracks, another type of thermal crack, may occur when forgings are heated too rapidly. These occur as a result of unequal temperatures of the surface relative to the center of the mass, and the resulting differences in the degree of thermal expansion produce tensile stresses near the center. The formation of such cracks depends on both the section size and the thermal conductivity of the material. Large section sizes and poor thermal conductivity promote thermal gradients and favor crack formation. Flow-Related Factors Several kinds of forging imperfections are related to metal flow, which is influenced by the workability of the material (discussed later) and the details of component and die design. This section discusses some of the flowrelated imperfections that might constitute a manufacturing flaw during inspection or a potential problem during service. It is also important to keep in mind the effects of design on the overall process. Forging load increases very rapidly as the sharpness of details is increased, and die wear increases directly with die pressure. Flash geometry also directly influences the amount of back pressure experienced by the metal in the die cavity. These defects described in this section are related to the distribution of metal. They can be avoided by proper die design, preform design, and choice of lubrication system. Strictly speaking, these defects are not fundamental to the workability of the material. However, knowledge of these common forging defects is necessary for a practical understanding of workability. These are the defects that commonly limit deformation in the forging process
Most of the defects occur in hot forging, which is most common for impression-die forging. Therefore, defect formation may also involve entrapment of oxides and lubricant. When this occurs, the metal is incapable of rewelding under the high forging pressures; the term cold shut is frequently applied in conjunction with laps, flow-through defects, and so on to describe the flaws generated for underfill are flow related. These include improper fill sequence, insufficient forging pressure, insufficient preheat temperature, lubricant build up in die corners, poor or uneven lubrication, and excessive die chill.An improper fill sequence may result in excessive flash loss, or it may be the result of extraordinary pressure requirements to fill a particular section. Sometimes, venting may eliminate the problem; more often than not, a change in the incoming workpiece shape or a change in the deformation sequence is required Laps and Folds. Laps are surface irregularities that appear as linear defects and are caused by the folding over of hot metal at the surface. These folds are forged into the surface but are not metallurgically bonded( welded) because of the oxide present between the surfaces(Fig. 20). Thus, a discontinuity with a sharp notch is created Fig. 20 Micrograph of a forging lap. Note the included oxide material in the lap. 20X A lap or fold results from an improper progression in fill sequence. Normally, a lap or fold is associated with flow around a die corner, as in the case of an upper rib or lower rib, or with a reversal in metal-flow directio A general rule of thumb is to keep metal moving in the same direction. The die corner radius is a critical tool dimension, and it should be as generous as possible. In progressing through a forging sequence, the die corners should become tighter, so that the workpiece fillets are initially large and progressively become smaller as the forging steps are completed. Figure 21 shows schematically a lap forming as metal flows around a die corner Thefileisdownloadedfromwww.bzfxw.com
Most of the defects occur in hot forging, which is most common for impression-die forging. Therefore, defect formation may also involve entrapment of oxides and lubricant. When this occurs, the metal is incapable of rewelding under the high forging pressures; the term cold shut is frequently applied in conjunction with laps, flow-through defects, and so on to describe the flaws generated. Underfill may not seem like a flow-related defect, but aside from simple insufficient starting mass, the reasons for underfill are flow related. These include improper fill sequence, insufficient forging pressure, insufficient preheat temperature, lubricant build up in die corners, poor or uneven lubrication, and excessive die chill. An improper fill sequence may result in excessive flash loss, or it may be the result of extraordinary pressure requirements to fill a particular section. Sometimes, venting may eliminate the problem; more often than not, a change in the incoming workpiece shape or a change in the deformation sequence is required. Laps and Folds. Laps are surface irregularities that appear as linear defects and are caused by the folding over of hot metal at the surface. These folds are forged into the surface but are not metallurgically bonded (welded) because of the oxide present between the surfaces (Fig. 20). Thus, a discontinuity with a sharp notch is created. Fig. 20 Micrograph of a forging lap. Note the included oxide material in the lap. 20× A lap or fold results from an improper progression in fill sequence. Normally, a lap or fold is associated with flow around a die corner, as in the case of an upper rib or lower rib, or with a reversal in metal-flow direction. A general rule of thumb is to keep metal moving in the same direction. The die corner radius is a critical tool dimension, and it should be as generous as possible. In progressing through a forging sequence, the die corners should become tighter, so that the workpiece fillets are initially large and progressively become smaller as the forging steps are completed. Figure 21 shows schematically a lap forming as metal flows around a die corner. The file is downloaded from www.bzfxw.com