f2 Formation of flash Curve of natural metal flow f2 Reverse flow forming a fold Formed forging defect Fig. 21 Lap formation in a rib-web forging caused by improper radius in the preform die Extrusion-Type Defects. The tail of an extrusion is unusable because of nonuniform flow through the extrusion die. This results in a center-to-surface-velocity gradient, with metal from the workpiece interior moving through the die at a slightly higher velocity than the outer material. The result shows up at the tail of the extrusion as a suck-in or pipe, and, for extrusions, the tail is simply cut off and discarded Alternatively, a follower block of cheaper material may be added so that most of the defect falls in the cheaper material, and less length of the extruded workpiece is lost
Fig. 21 Lap formation in a rib-web forging caused by improper radius in the preform die Extrusion-Type Defects. The tail of an extrusion is unusable because of nonuniform flow through the extrusion die. This results in a center-to-surface-velocity gradient, with metal from the workpiece interior moving through the die at a slightly higher velocity than the outer material. The result shows up at the tail of the extrusion as a suck-in or pipe, and, for extrusions, the tail is simply cut off and discarded. Alternatively, a follower block of cheaper material may be added so that most of the defect falls in the cheaper material, and less length of the extruded workpiece is lost
For forgings that involve forward or backward extrusion to fill a part section, the same situation can develop Metal flow into a rib or hub can result in a suck-in defect, which, in a worst-case scenario, would show up as a fold on the face opposite to the rib, and a best case would be a depression on what otherwise should be a flat surface. One method of eliminating this type of defect is to position more material on the back face initially Another method is to change the rib geometry(aspect ratio and/or angles). If neither of these changes can be accomplished, an extra forging step may be needed to limit the amount of extrusion that is done in any one step Extrusion-type defects are formed when centrally located ribs formed by extrusion-type flow draw too much metal from the main body or web of the forging. a defect similar to a pipe cavity is thus formed(Fig. 22 Methods of minimizing the occurrence of these defects include increasing the thickness of the web or designing the forging with a small rib opposite the larger rib, as shown in Fig. 22 Extrusion defect Small rib added to prevent extrusion defect Fig 22 Extrusion-type defect in(a)centrally located rib and (b) die-design modification used to avoid defect Shear -Related or Flow-Through Defects. Shearing defects are also known as flow-through defects. because they result from excessive metal flow past a filled detail of the part. Flow-through defects form when metal is forced to flow past a recess after the recess has filled or when material in the recess has ceased to defo because of chilling. An example of this is shown in Fig. 23 for a trapped-die forging that has a rib on the top surface. The rib denoted by"is filled early in the forging sequence, and significant mass must flow past the ib in order to fill the inner hub, zone 4. The result can be a complete shearing-off of the rib in the worst case with a lesser case being the formation of a shear-type crack Thefileisdownloadedfromwww.bzfxw.com
For forgings that involve forward or backward extrusion to fill a part section, the same situation can develop. Metal flow into a rib or hub can result in a suck-in defect, which, in a worst-case scenario, would show up as a fold on the face opposite to the rib, and a best case would be a depression on what otherwise should be a flat surface. One method of eliminating this type of defect is to position more material on the back face initially. Another method is to change the rib geometry (aspect ratio and/or angles). If neither of these changes can be accomplished, an extra forging step may be needed to limit the amount of extrusion that is done in any one step. Extrusion-type defects are formed when centrally located ribs formed by extrusion-type flow draw too much metal from the main body or web of the forging. A defect similar to a pipe cavity is thus formed (Fig. 22). Methods of minimizing the occurrence of these defects include increasing the thickness of the web or designing the forging with a small rib opposite the larger rib, as shown in Fig. 22. Fig. 22 Extrusion-type defect in (a) centrally located rib and (b) die-design modification used to avoid defect Shear-Related or Flow-Through Defects. Shearing defects are also known as flow-through defects, because they result from excessive metal flow past a filled detail of the part. Flow-through defects form when metal is forced to flow past a recess after the recess has filled or when material in the recess has ceased to deform because of chilling. An example of this is shown in Fig. 23 for a trapped-die forging that has a rib on the top surface. The rib denoted by “2” is filled early in the forging sequence, and significant mass must flow past the rib in order to fill the inner hub, zone “4.” The result can be a complete shearing-off of the rib in the worst case, with a lesser case being the formation of a shear-type crack. The file is downloaded from www.bzfxw.com
A B(1 Bottompunch Fig 23 Schematic of a flow-through crack at the base of a rib in a trapped-die forging. Excessive metal flow past region 2 causes a shear crack to form at a and propagate toward b Similar to laps in appearance, flow-through defects can be shallow, but they are indicative of an undesirable grain flow pattern or shear band that extends much deeper into the forging. An example is shown in Fig. 24 Flow-through defects can also occur when trapped lubricant forces metal to flow past an impression 0.5cm 然感 500 Fig 24 Flow-through defect in Ti-6Al-4V rib-web structural part
Fig. 23 Schematic of a flow-through crack at the base of a rib in a trapped-die forging. Excessive metal flow past region 2 causes a shear crack to form at A and propagate toward B. Similar to laps in appearance, flow-through defects can be shallow, but they are indicative of an undesirable grain flow pattern or shear band that extends much deeper into the forging. An example is shown in Fig. 24. Flow-through defects can also occur when trapped lubricant forces metal to flow past an impression. Fig. 24 Flow-through defect in Ti-6Al-4V rib-web structural part
Seams are crevices in the surface of the metal that have been closed, but not welded by working the metal Seams result from elongated trapped-gas pockets or from cracks. The surfaces are generally heavily oxidized and decarburized in steels. Depth varies widely, and surface areas sometimes may be welded together in spots Seams may be continuous or intermittent, as indicated in Fig. 2(h). A micrograph of a typical seam is shown in Fig. 25. Oxides in a seam can be the result of exposure to elevated temperatures during processing Fig. 25 Micrograph of a seam in a cross section of a 19 mm(0.75 in. )diameter medium-carbon steel bar showing oxide and decarburization in the seam. 350x Seams seldom penetrate to the core of bar stock. The incomplete removal of seams from forging stock may cause additional cracking in hot forging and quench cracking during heat treatment. Seams are sometimes difficult to detect in an unused fastener, but they are readily apparent after a fastener has been subjected to installation and service stresses Seams have a large number of possible origins, some mechanical and some metallurgical. Seams can develo from cracks caused by working, or seams can develop from an imperfection 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 seam in the workpiece surface Seams also result from trapped-gas pockets, cracks, a heavy cluster of nonmetallic inclusions, or a deep lap. Seams are normally closed tight enough that no actual opening can be visually detected without some nondestructive inspection techniques such as magnetic-particle inspection(see Nondestructive Evaluation and Quality Control, Volume 17 of ASM Handbook) Seams can be difficult to detect, because they may appear as scratches on the forging, or because a machining process may obliterate them. Seams may not become evident until the part has been subjected to installation and service stresses, or when the constraint exerted by the bulk of material is removed from the neighborhood of a seam. Figure 26 is an example of a seam detected by routine magnetic-particle inspection of a hot-rolled 4130 steel bar. No stringer-type inclusions were observed in the region of the flaw, but it did contain a substantial amount of oxide(Fig. 26b) Thefileisdownloadedfromwww.bzfxw.com
Seams are crevices in the surface of the metal that have been closed, but not welded, by working the metal. Seams result from elongated trapped-gas pockets or from cracks. The surfaces are generally heavily oxidized and decarburized in steels. Depth varies widely, and surface areas sometimes may be welded together in spots. Seams may be continuous or intermittent, as indicated in Fig. 2(h). A micrograph of a typical seam is shown in Fig. 25. Oxides in a seam can be the result of exposure to elevated temperatures during processing. Fig. 25 Micrograph of a seam in a cross section of a 19 mm (0.75 in.) diameter medium-carbon steel bar showing oxide and decarburization in the seam. 350× Seams seldom penetrate to the core of bar stock. The incomplete removal of seams from forging stock may cause additional cracking in hot forging and quench cracking during heat treatment. Seams are sometimes difficult to detect in an unused fastener, but they are readily apparent after a fastener has been subjected to installation and service stresses. Seams have a large number of possible origins, some mechanical and some metallurgical. Seams can develop from cracks caused by working, or seams can develop from an imperfection 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 seam in the workpiece surface. Seams also result from trapped-gas pockets, cracks, a heavy cluster of nonmetallic inclusions, or a deep lap. Seams are normally closed tight enough that no actual opening can be visually detected without some nondestructive inspection techniques, such as magnetic-particle inspection (see Nondestructive Evaluation and Quality Control, Volume 17 of ASM Handbook) Seams can be difficult to detect, because they may appear as scratches on the forging, or because a machining process may obliterate them. Seams may not become evident until the part has been subjected to installation and service stresses, or when the constraint exerted by the bulk of material is removed from the neighborhood of a seam. Figure 26 is an example of a seam detected by routine magnetic-particle inspection of a hot-rolled 4130 steel bar. No stringer-type inclusions were observed in the region of the flaw, but it did contain a substantial amount of oxide (Fig. 26b). The file is downloaded from www.bzfxw.com
Fig. 26 Seam in rolled 4130 steel bar(a) Closeup of seam. Note the linear characteristics of this flaw.(b) Micrograph showing cross section of the bar Seam is normal to the surface and filled with oxide 30x Central or Internal Bursts(Chevron Cracking ). Chevrons are internal flaws named for their shape(Fig. 2k).A central burst, or chevron crack, is most commonly associated with extrusion and drawing operations, although it can be generated by forging and rolling processes as well. Central bursts are internal fractures caused by high hydrostatic tension. The severe stresses that build up internally cause transverse subsurface cracks. Some of the factors that can contribute to the formation of chevrons are incorrect die angles; either too great or too small a reduction of cross-sectional area; incomplete annealing of slug material; excessive work hardenability of the slug material; the presence of an excessive amount of seams and other imperfections in the slug materials segregation in a steel slug that results in hard martensitic particles in the center of the slug, which act as barriers to material flow and insufficient die lubrication Internal bursts in rolled and forged metals result from the use of equipment that has insufficient capacity to work the metal throughout its cross section. If the working force is not sufficient, the outer layers of the metal are deformed more than the inside metal, sometimes causing wholly internal, intergranular fissures that can act as initiation sites for further crack propagation during service loads. In forward cold extrusion, the occurrence
Fig. 26 Seam in rolled 4130 steel bar (a) Closeup of seam. Note the linear characteristics of this flaw. (b) Micrograph showing cross section of the bar. Seam is normal to the surface and filled with oxide. 30× Central or Internal Bursts (Chevron Cracking). Chevrons are internal flaws named for their shape (Fig. 2k). A central burst, or chevron crack, is most commonly associated with extrusion and drawing operations, although it can be generated by forging and rolling processes as well. Central bursts are internal fractures caused by high hydrostatic tension. The severe stresses that build up internally cause transverse subsurface cracks. Some of the factors that can contribute to the formation of chevrons are incorrect die angles; either too great or too small a reduction of cross-sectional area; incomplete annealing of slug material; excessive work hardenability of the slug material; the presence of an excessive amount of seams and other imperfections in the slug materials; segregation in a steel slug that results in hard martensitic particles in the center of the slug, which act as barriers to material flow; and insufficient die lubrication. Internal bursts in rolled and forged metals result from the use of equipment that has insufficient capacity to work the metal throughout its cross section. If the working force is not sufficient, the outer layers of the metal are deformed more than the inside metal, sometimes causing wholly internal, intergranular fissures that can act as initiation sites for further crack propagation during service loads. In forward cold extrusion, the occurrence