Hardness measurements were then conducted across the face of the specimen in locations corresponding to the banded and nonbanded regions. These results(Fig. 7a)showed that the segregated region is considerably harder than the neighboring material The reason is that the increased carbon content of the segregated region, together with its higher alloy content, makes the region more responsive to what would have been normal heat treatment for this grade of tool steel The high-hardness material is also subject to microcracking on quenching; microcracks can act as nuclei for subsequent fatigue cracks. Examination of the fracture surface revealed that the fracture originated near the high-stress region of the die face; however, no indications of fatigue marks were found on either a macroscale or a microscale Conclusions. Failure of the die was the result of fracture that originated in an area of abnormally high hardness Ithough fatigue marks were not observed, the fact was that the fracture did not occur in a single cycle but required several cycles to cause failure Example 2: Fatigue Fracture that Originated on the Ground Surface of a Medium-Carbon Steel Forging with a Notch-Sensitive Band Structure. The broken connecting end of a forged medium-carbon steel rod used in an application in which it was subjected to severe low-frequency loading is illustrated in Fig. 8(a); the part shown from one of two identical rods that failed in service by fracture. In each instance fracture extended pletely through the connecting end in two places. The two fractured rods, together with two similar unused forged rods, were examined to determine the mechanism and cause of fracture Medium-cor bon steel 140 Bhn Fractured connecting Rough-ground area(typ Unetched fracture 2x both sides) Fracture (l of 2) surface (d) 80x Nital lOx g.8 Connecting end of forged rod, with banded structure from excessive segregation in the billet (Example 2).(a) Rod end showing locations of fractures at rough-ground areas at the parting line; in view A-A, dashed lines denote a rough-ground area, arrow points to a liquid-penetrant indication of an incipient crack, (b) Fracture surface, with beach marks indicating fracture origin at rough-ground surface.(c) Normal, homogeneous structure of an unused rod examined for comparison; this structure contains equal amounts of ferrite (light) and pearlite (dark).(d) Unsatisfactory structure of the fractured rod, which contains alternating bands of ferrite and pearlite. Thefileisdownloadedfromwww.bzfxw.com
Hardness measurements were then conducted across the face of the specimen in locations corresponding to the banded and nonbanded regions. These results (Fig. 7a) showed that the segregated region is considerably harder than the neighboring material. The reason is that the increased carbon content of the segregated region, together with its higher alloy content, makes the region more responsive to what would have been normal heat treatment for this grade of tool steel. The high-hardness material is also subject to microcracking on quenching; microcracks can act as nuclei for subsequent fatigue cracks. Examination of the fracture surface revealed that the fracture originated near the high-stress region of the die face; however, no indications of fatigue marks were found on either a macroscale or a microscale. Conclusions. Failure of the die was the result of fracture that originated in an area of abnormally high hardness. Although fatigue marks were not observed, the fact was that the fracture did not occur in a single cycle but required several cycles to cause failure. Example 2: Fatigue Fracture that Originated on the Ground Surface of a Medium- Carbon Steel Forging with a Notch-Sensitive Band Structure. The broken connecting end of a forged medium-carbon steel rod used in an application in which it was subjected to severe low-frequency loading is illustrated in Fig. 8(a); the part shown is from one of two identical rods that failed in service by fracture. In each instance, fracture extended completely through the connecting end in two places. The two fractured rods, together with two similar unused forged rods, were examined to determine the mechanism and cause of fracture. Fig. 8 Connecting end of forged rod, with banded structure from excessive segregation in the billet (Example 2). (a) Rod end showing locations of fractures at rough-ground areas at the parting line; in view A-A, dashed lines denote a rough-ground area, arrow points to a liquid-penetrant indication of an incipient crack. (b) Fracture surface, with beach marks indicating fracture origin at rough-ground surface. (c) Normal, homogeneous structure of an unused rod examined for comparison; this structure contains equal amounts of ferrite (light) and pearlite (dark). (d) Unsatisfactory structure of the fractured rod, which contains alternating bands of ferrite and pearlite. The file is downloaded from www.bzfxw.com
Preliminary Examination. The material of the four forged rods was found by spectrographic analysis to be within the normal limits for the specified medium-carbon steel. Except for the fractures on the connecting end of the two failed rods, no significant imperfections or evidence of damage were found by visual examination Surface hardness of the four rods, as measured at various points with a rockwell tester, was equivalent to 140 HB--substantially lower than the specified hardness of 160 to 205 HB The fractures in the two failed rods were in areas of the transition regions that had been rough ground to remove flash along the parting line(see Fig. 8a). The fracture surfaces were fairly flat and were radial with respect to the annular connecting ends Low-magnification examination of the fracture surfaces revealed the presence of beach marks, indicating that the fractures had originated and propagated by fatigue. The location and curvature of the beach marks on the fracture surfaces of the two broken rods(see Fig. 8b)established that the fracture origin in each rod was at the rough-ground surface. The fatigue region of the fracture surface, which was quite smooth, extended approximately halfway through the thickness of the rod end The remainder of the area of the fracture surfaces, corresponding to final, fast fracture, had the typical appearance of brittle fracture, exhibiting little or no evidence of plastic deformation Liquid-Penetrant Examination. The four rods were checked for the presence of imperfections in the general vicinity of rough-ground areas, using liquid-penetrant inspection. This examination revealed an incipient crack 9.5 mm approximately in. )long(see view A-A in Fig. 8)on one of the fractured rods. The crack was apparently a fatigue crack in the initiation stage and was located at a rough-ground area. Liquid-penetrant indications of several other incipient cracks that were smaller in size were also detected on the fractured rods in this examination; several were on rough-ground areas Metallographic Examination. Cross sections for metallographic examination were taken through the annular connecting ends of the two fractured rods and of the two unused rods and were polished and then etched with ital. At a magnification of 80 diameters, the microstructure of the unused rods(Fig. &c) and that of the fractured rods(Fig. 8d)appeared greatly different The two unused rods has a fairly fine-grained, homogeneous structure containing approximately equal amounts of ferrite and pearlite--a normal structure for good-quality medium-carbon steel forgings of this composition that had been properly heat treated after forging The two fractured rods had a banded structure consisting of zones of ferrite (light)and pearlite(dark) Microhardness of the banded region was 140 HB in the light areas and 145 HB in the dark areas(converted from Vickers hardness readings, 100 g load Examination at 320 diameters of sections through the rough-ground areas of the fractured rods established that the incipient cracks found in liquid-penetrant inspection had originated at the surface in the banded region, in areas of ferrite where this constituent had been visibly deformed by grinding Discussion. The loads applied to the annular connecting ends of the rods were apparent\ cumferential complex, consisting of torsional, bending, and axial loads from the forged rods; these loads caused cyclic te stresses and bending stresses in the connecting end The microhardness readings on the ferrite and pearlite bands were not considered to represent true hardness values because of the small size and shallowness of the bands. However, the following observations were considered to be significant with respect to the failures The forged rods that had a normal microstructure(see Fig. 8c)did not fail The forged rods that had a banded microstructure(see Fig 8d)did fail The hardness of all four rods, as measured on the surface using a Rockwell hardness tester, was the ame and was equivalent to 140 HB--substantially below the specified range of 160 to 205 HB The fractures originated in rough-ground areas, and incipient cracks were found in an area of ferrite that had been visibly deformed by grinding Conclusions. The rod-end fractures had originated and propagated by fatigue during continued exposure of the rods to severe cyclic loads in service. The loads on the rods developed tensile and bending stresses in the connecting ends. Contributory factors were
Preliminary Examination. The material of the four forged rods was found by spectrographic analysis to be within the normal limits for the specified medium-carbon steel. Except for the fractures on the connecting ends of the two failed rods, no significant imperfections or evidence of damage were found by visual examination. Surface hardness of the four rods, as measured at various points with a Rockwell tester, was equivalent to 140 HB—substantially lower than the specified hardness of 160 to 205 HB. The fractures in the two failed rods were in areas of the transition regions that had been rough ground to remove flash along the parting line (see Fig. 8a). The fracture surfaces were fairly flat and were radial with respect to the annular connecting ends. Low-magnification examination of the fracture surfaces revealed the presence of beach marks, indicating that the fractures had originated and propagated by fatigue. The location and curvature of the beach marks on the fracture surfaces of the two broken rods (see Fig. 8b) established that the fracture origin in each rod was at the rough-ground surface. The fatigue region of the fracture surface, which was quite smooth, extended approximately halfway through the thickness of the rod end. The remainder of the area of the fracture surfaces, corresponding to final, fast fracture, had the typical appearance of brittle fracture, exhibiting little or no evidence of plastic deformation. Liquid-Penetrant Examination. The four rods were checked for the presence of imperfections in the general vicinity of rough-ground areas, using liquid-penetrant inspection. This examination revealed an incipient crack 9.5 mm approximately ( 3 8 in.) long (see view A-A in Fig. 8) on one of the fractured rods. The crack was apparently a fatigue crack in the initiation stage and was located at a rough-ground area. Liquid-penetrant indications of several other incipient cracks that were smaller in size were also detected on the fractured rods in this examination; several were on rough-ground areas. Metallographic Examination. Cross sections for metallographic examination were taken through the annular connecting ends of the two fractured rods and of the two unused rods and were polished and then etched with nital. At a magnification of 80 diameters, the microstructure of the unused rods (Fig. 8c) and that of the fractured rods (Fig. 8d) appeared greatly different. The two unused rods has a fairly fine-grained, homogeneous structure containing approximately equal amounts of ferrite and pearlite—a normal structure for good-quality medium-carbon steel forgings of this composition that had been properly heat treated after forging. The two fractured rods had a banded structure consisting of zones of ferrite (light) and pearlite (dark). Microhardness of the banded region was 140 HB in the light areas and 145 HB in the dark areas (converted from Vickers hardness readings, 100 g load). Examination at 320 diameters of sections through the rough-ground areas of the fractured rods established that the incipient cracks found in liquid-penetrant inspection had originated at the surface in the banded region, in areas of ferrite where this constituent had been visibly deformed by grinding. Discussion. The loads applied to the annular connecting ends of the rods were apparently complex, consisting of torsional, bending, and axial loads from the forged rods; these loads caused cyclic tensile circumferential stresses and bending stresses in the connecting ends. The microhardness readings on the ferrite and pearlite bands were not considered to represent true hardness values because of the small size and shallowness of the bands. However, the following observations were considered to be significant with respect to the failures: · The forged rods that had a normal microstructure (see Fig. 8c) did not fail. · The forged rods that had a banded microstructure (see Fig. 8d) did fail. · The hardness of all four rods, as measured on the surface using a Rockwell hardness tester, was the same and was equivalent to 140 HB—substantially below the specified range of 160 to 205 HB. · The fractures originated in rough-ground areas, and incipient cracks were found in an area of ferrite that had been visibly deformed by grinding. Conclusions. The rod-end fractures had originated and propagated by fatigue during continued exposure of the rods to severe cyclic loads in service. The loads on the rods developed tensile and bending stresses in the connecting ends. Contributory factors were:
The presence of a notch-sensitive banded structure containing alternately soft(ferrite)and hard The presence of stress raisers produced by rough grinding to remove the forging flash along the highly stressed transition area between the rods and the connecting end Hardness(and, accordingly, strength) below the range specified for the part The banded microstructure of the two fractured rods apparently resulted from severe segregation in the billet from which they were forged Corrective Action It was recommended that Closer control be exercised over the microstructure and hardness of the forgings The connecting end be finished more smoothly in the critical area Consideration be given to increasing the thickness(diametral with respect to the ring) of the connecting end in the transition area Ingot Pipe, Porosity, and Centerline Shrinkage A common imperfection in ingots is pipe, which is an internal shrinkage cavity formed during solidification of ingots. It occurs in the upper central portion of the ingot(Fig. 1) during solidification and contraction of the metal, when there may eventually be insufficient liquid metal to feed the last remaining portions as they contract. A concave cavity thus forms at the top of the ingot because of metal shrinkage during solidification The cavity usually forms in the shape of a cone, hence the term pipe If not completely cropped before subsequent working, pipe may be carried through the various manufacturing processes to the finished product. The pipe becomes elongated and is found in the center of the final product, as shown in ingot B in Fig. 1. The pipe also can form internal lamination in rolled products(Fig 9). These laminations may not be immediately evident following rolling but may become apparent during a subsequent forming operation. The occurrence of laminations is most prevalent in flat-rolled sheet products conventionally cast ingot. Courtesy of V Demski, Teledyne Rodney Metals s of the pipe from the top of a Fig 9 Laminations in rolled steel sheet resulting from insufficient cropping In addition to the primary pipe near the top of the ingot, secondary regions of piping and centerline shrinkage may extend deeper into an ingot( Fig. 10). Primary piping is generally an economic concern, but if it extends sufficiently deep into the ingot body and goes undetected, it can eventually result in a defective product Detection of the pipe can be obscured in some cases, if bridging has occurred Thefileisdownloadedfromwww.bzfxw.com
· The presence of a notch-sensitive banded structure containing alternately soft (ferrite) and hard (pearlite) layers · The presence of stress raisers produced by rough grinding to remove the forging flash along the highly stressed transition area between the rods and the connecting end · Hardness (and, accordingly, strength) below the range specified for the part The banded microstructure of the two fractured rods apparently resulted from severe segregation in the billet from which they were forged. Corrective Action. It was recommended that: · Closer control be exercised over the microstructure and hardness of the forgings · The connecting end be finished more smoothly in the critical area · Consideration be given to increasing the thickness (diametral with respect to the ring) of the connecting end in the transition area Ingot Pipe, Porosity, and Centerline Shrinkage A common imperfection in ingots is pipe, which is an internal shrinkage cavity formed during solidification of ingots. It occurs in the upper central portion of the ingot (Fig. 1) during solidification and contraction of the metal, when there may eventually be insufficient liquid metal to feed the last remaining portions as they contract. A concave cavity thus forms at the top of the ingot because of metal shrinkage during solidification. The cavity usually forms in the shape of a cone, hence the term pipe. If not completely cropped before subsequent working, pipe may be carried through the various manufacturing processes to the finished product. The pipe becomes elongated and is found in the center of the final product, as shown in ingot B in Fig. 1. The pipe also can form internal lamination in rolled products (Fig. 9). These laminations may not be immediately evident following rolling but may become apparent during a subsequent forming operation. The occurrence of laminations is most prevalent in flat-rolled sheet products. Fig. 9 Laminations in rolled steel sheet resulting from insufficient cropping of the pipe from the top of a conventionally cast ingot. Courtesy of V. Demski, Teledyne Rodney Metals In addition to the primary pipe near the top of the ingot, secondary regions of piping and centerline shrinkage may extend deeper into an ingot (Fig. 10). Primary piping is generally an economic concern, but if it extends sufficiently deep into the ingot body and goes undetected, it can eventually result in a defective product. Detection of the pipe can be obscured in some cases, if bridging has occurred. The file is downloaded from www.bzfxw.com
80 mm Fig. 10 Longitudinal section through an ingot showing extensive centerline shrinkage Piping can be minimized by pouring ingots with the big end up, providing risers in the ingot top, and applying sufficient hot-top material (insulating refractories or exothermic materials) immediately after pouring. These techniques extend the time that the metal in the top regions of the ingot remains liquid, thereby minimizing the shrinkage cavity produced in this portion of the ingot. On the other hand, secondary piping and centerline
Fig. 10 Longitudinal section through an ingot showing extensive centerline shrinkage Piping can be minimized by pouring ingots with the big end up, providing risers in the ingot top, and applying sufficient hot-top material (insulating refractories or exothermic materials) immediately after pouring. These techniques extend the time that the metal in the top regions of the ingot remains liquid, thereby minimizing the shrinkage cavity produced in this portion of the ingot. On the other hand, secondary piping and centerline
shrinkage can be very detrimental, because they are harder to detect in the mill and may subsequently produce centerline defects in bar and wrought pr ater proc oducts. Such a material condition may indeed provide the flaw or stress concentrator for a forging burst in some lat cessing operation or for a future product failure Pipe in steel products invariably ciated with segregated impurities, which are deeply attacked by the acid during macroetching. Cavities in the center that are not associated with deeply etched impurities often mistaken for pipe, but such cavities usually can be traced to bursts caused by incorrect processing of the steel during forging or rolling. Pipe should be visible after deep etching; it usually can be distinguished from bursts by the degree of sponginess surrounding the defect. Piped metal usually exhibits considerably more sponginess than burst metal. The micrograph of Fig. 11 is an example of secondary pipe, and the micrograph in Fig. 12 is an example of a cavity that appears to be a pipelike flaw 质, x:.船生, Fig. 11 1038 steel bar, as-forged. Longitudinal section displays secondary pipe (black areas) that was carried along from the original bar stock into the forged piece. Gray areas are pearlite; white areas, ferrite. 2% nital. 50x 100x.(b)600x. Courtesy of Mohan Chaudhari, Columbus Metallurgical Service teel(Example 4).(a) ig. 12 Inclusions and a pipelike cavity in tempered martensite of AIsI E4340 Thefileisdownloadedfromwww.bzfxw.com
shrinkage can be very detrimental, because they are harder to detect in the mill and may subsequently produce centerline defects in bar and wrought products. Such a material condition may indeed provide the flaw or stress concentrator for a forging burst in some later processing operation or for a future product failure. Pipe in steel products invariably is associated with segregated impurities, which are deeply attacked by the acid during macroetching. Cavities in the center that are not associated with deeply etched impurities often are mistaken for pipe, but such cavities usually can be traced to bursts caused by incorrect processing of the steel during forging or rolling. Pipe should be visible after deep etching; it usually can be distinguished from bursts by the degree of sponginess surrounding the defect. Piped metal usually exhibits considerably more sponginess than burst metal. The micrograph of Fig. 11 is an example of secondary pipe, and the micrograph in Fig. 12 is an example of a cavity that appears to be a pipelike flaw. Fig. 11 1038 steel bar, as-forged. Longitudinal section displays secondary pipe (black areas) that was carried along from the original bar stock into the forged piece. Gray areas are pearlite; white areas, ferrite. 2% nital. 50× Fig. 12 Inclusions and a pipelike cavity in tempered martensite of AISI E4340 steel (Example 4). (a) 100×. (b) 600×. Courtesy of Mohan Chaudhari, Columbus Metallurgical Services The file is downloaded from www.bzfxw.com