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)