Example 3: Fatigue Fracture of Alloy Steel Valve Springs Because of Pipe. Two outer valve springs broke during production engine testing and were submitted for laboratory analysis. The springs were from a current production lot and had been made from air-melted 6150 pretempered steel wire. The springs were 50 mm(2 in. in outside diameter and 64 mm(2.5 in. )in free length, had five coils and squared-and-ground ends, and were made of 5.5 mm(0.2 in. )diameter wire. Because both failures were similar, the analysis of only one is discussed Investigation. The spring(Fig. 13a) broke approximately one turn from the end Fracture was nucleated by an apparent longitudinal subsurface defect. Magnetic-particle inspection did not reveal any additional cracks or defects Fig. 13 Valve-spring failure due to residual shrinkage pipe.(a)Macrograph showing fracture, as indicated by arrow. (b) Fracture surface; pipe is indicated by arrow Microscopic examination of transverse sections of the spring adjacent to the fracture surfaces revealed that the defect was a large pocket of nonmetallic inclusions at the origin of the fracture. The inclusions were alumina and silicate particles. Partial decarburization of the steel was evident at the periphery of the pocket of inclusions. The composition and appearance of the inclusions and the presence of the partial decarburization indicated that the inclusions could have been associated with ingot defects. The defect was 0. 8 mm(0.03 in. )in diameter(max), 25 mm(1 in )long, and 1.3 mm(0.05 in ) below the surface( Fig. 13b) The steel had a hardness of 45 to 46 HRC, a microstructure of tempered martensite, and a grain size of AstM 6 or 7, all of which were satisfactory for the application. The fracture surface contained beach marks typical of fatigue and was at a 45 angle to the wire axis, which indicated torsional fracture
Example 3: Fatigue Fracture of Alloy Steel Valve Springs Because of Pipe. Two outer valve springs broke during production engine testing and were submitted for laboratory analysis. The springs were from a current production lot and had been made from air-melted 6150 pretempered steel wire. The springs were 50 mm (2 in.) in outside diameter and 64 mm (2.5 in.) in free length, had five coils and squared-and-ground ends, and were made of 5.5 mm (0.2 in.) diameter wire. Because both failures were similar, the analysis of only one is discussed. Investigation. The spring (Fig. 13a) broke approximately one turn from the end. Fracture was nucleated by an apparent longitudinal subsurface defect. Magnetic-particle inspection did not reveal any additional cracks or defects. Fig. 13 Valve-spring failure due to residual shrinkage pipe. (a) Macrograph showing fracture, as indicated by arrow. (b) Fracture surface; pipe is indicated by arrow. Microscopic examination of transverse sections of the spring adjacent to the fracture surfaces revealed that the defect was a large pocket of nonmetallic inclusions at the origin of the fracture. The inclusions were alumina and silicate particles. Partial decarburization of the steel was evident at the periphery of the pocket of inclusions. The composition and appearance of the inclusions and the presence of the partial decarburization indicated that the inclusions could have been associated with ingot defects. The defect was 0.8 mm (0.03 in.) in diameter (max), 25 mm (1 in.) long, and 1.3 mm (0.05 in.) below the surface (Fig. 13b). The steel had a hardness of 45 to 46 HRC, a microstructure of tempered martensite, and a grain size of ASTM 6 or 7, all of which were satisfactory for the application. The fracture surface contained beach marks typical of fatigue and was at a 45° angle to the wire axis, which indicated torsional fracture
Conclusions. The spring fractured by fatigue, fatigue cracking was nucleated at a subsurface defect that was longitudinal to the wire axis. The stress-raising effect of the defect was responsible for the fracture High Hydrogen Content A major source of hydrogen in certain metals and alloys is the reaction of water vapor with the liquid metal at high temperatures. The water vapor may originate from the charge materials, slag ingredients and alloy additions, refractory linings, ingot molds, or even the atmosphere itself, if steps are not taken to prevent such contamination. The resulting hydrogen goes into solution at elevated temperatures; but as the metal solidifies fter pouring, the solubility of hydrogen decreases, and it becomes entrapped in the metal lattice Hydrogen can be absorbed many places during primary and secondary manufacturing. It can be picked up during melting, teeming, welding, pickling, or electroplating. If hydrogen is absorbed into the base metal, it must be removed by a bake-out heat treatment. Otherwise, severe embrittlement of the base metal may occur especially in steels with hardnesses above approximately 35 HRC. High-strength, highly stressed parts can crack and fracture as a result of hydrogen embrittlement. Failure by hydrogen embrittlement is even more likely to occur if high residual tensile stresses are present The deleterious effects of hydrogen depend on the alloy and the concentration of hydrogen. Hydrogen concentrations in excess of only 1 ppm have been related to the degradation of mechanical properties in high strength steels, especially ductility, impact behavior, and fracture toughness. Hydrogen concentration in excess of approximately 5 ppm has been associated with hydrogen flakes, which are small cracks produced by hydrogen that has diffused to grain boundaries and other preferred sites (such as inclusions or matrix interfaces). Flaking or hairline cracking in steels, particularly in forgings, heavy-section alloy plate steels, and railroad rails, has been an ongoing problem since the beginning of steelmaking and is reviewed in Ref 6. At still higher concentrations of hydrogen in metal, blistering can occur(e.g, see Fig. 18 of"Visual Examination and Light Microscopy " in Fractography, Volume 12 of ASM Handbook) Excessive gas contents in ingots are most serious when oxide inclusions are present. Inclusions provide sites for the agglomeration of gas, and thus molten alloys are often treated to reduce the hydrogen content and alkali element concentration as well as to remove nonmetallic inclusions. For example, vacuum degassing is used to remove dissolved oxygen and hydrogen from steel, thus reducing the number and size of indigenous nonmetallic inclusions and the likelihood of internal fissures or flakes caused when hydrogen content is higher than desired Despite these methods of producing high-quality metal, problems sometimes arise, particularly if there are no methods available for continuously monitoring metal quality. Hydrogen flaking also is a potential problem for carbon tool steels and for some of the medium-carbon low-alloy steels, such as those used as prehardened plastic-mold alloys. Figure 14(a) shows a die made from AISI O1 tool steel that was found to be cracked after heat treatment. When opened(Fig. 14b), these cracks exhibited a coarse, shiny, faceted appearance. The cracks in this part were longitudinally oriented and were confined to the center of the section. Flakes are not observed in the outer region of sections, because there is apparently more time for the hydrogen in the outer region to diffuse to a safe level. Flakes that were close to the surface of the die exhibited temper color, but some of the deeper flakes did not. The temper-colored flakes apparently opened during quenching ig. 14 Die made from AIsI Ol tool steel that was found to be cracked after heat treatment.(a) Longitudinal cracks after the surface was swabbed with 5% nital. (b) One of the cracks opened revealing features typical of hydrogen flakes. 6.5x Nonmetallic Inclusions Thefileisdownloadedfromwww.bzfxw.com
Conclusions. The spring fractured by fatigue; fatigue cracking was nucleated at a subsurface defect that was longitudinal to the wire axis. The stress-raising effect of the defect was responsible for the fracture. High Hydrogen Content A major source of hydrogen in certain metals and alloys is the reaction of water vapor with the liquid metal at high temperatures. The water vapor may originate from the charge materials, slag ingredients and alloy additions, refractory linings, ingot molds, or even the atmosphere itself, if steps are not taken to prevent such contamination. The resulting hydrogen goes into solution at elevated temperatures; but as the metal solidifies after pouring, the solubility of hydrogen decreases, and it becomes entrapped in the metal lattice. Hydrogen can be absorbed many places during primary and secondary manufacturing. It can be picked up during melting, teeming, welding, pickling, or electroplating. If hydrogen is absorbed into the base metal, it must be removed by a bake-out heat treatment. Otherwise, severe embrittlement of the base metal may occur, especially in steels with hardnesses above approximately 35 HRC. High-strength, highly stressed parts can crack and fracture as a result of hydrogen embrittlement. Failure by hydrogen embrittlement is even more likely to occur if high residual tensile stresses are present. The deleterious effects of hydrogen depend on the alloy and the concentration of hydrogen. Hydrogen concentrations in excess of only 1 ppm have been related to the degradation of mechanical properties in highstrength steels, especially ductility, impact behavior, and fracture toughness. Hydrogen concentration in excess of approximately 5 ppm has been associated with hydrogen flakes, which are small cracks produced by hydrogen that has diffused to grain boundaries and other preferred sites (such as inclusions or matrix interfaces). Flaking or hairline cracking in steels, particularly in forgings, heavy-section alloy plate steels, and railroad rails, has been an ongoing problem since the beginning of steelmaking and is reviewed in Ref 6. At still higher concentrations of hydrogen in metal, blistering can occur (e.g., see Fig. 18 of “Visual Examination and Light Microscopy” in Fractography, Volume 12 of ASM Handbook). Excessive gas contents in ingots are most serious when oxide inclusions are present. Inclusions provide sites for the agglomeration of gas, and thus molten alloys are often treated to reduce the hydrogen content and alkali element concentration as well as to remove nonmetallic inclusions. For example, vacuum degassing is used to remove dissolved oxygen and hydrogen from steel, thus reducing the number and size of indigenous nonmetallic inclusions and the likelihood of internal fissures or flakes caused when hydrogen content is higher than desired. Despite these methods of producing high-quality metal, problems sometimes arise, particularly if there are no methods available for continuously monitoring metal quality. Hydrogen flaking also is a potential problem for carbon tool steels and for some of the medium-carbon low-alloy steels, such as those used as prehardened plastic-mold alloys. Figure 14(a) shows a die made from AISI O1 tool steel that was found to be cracked after heat treatment. When opened (Fig. 14b), these cracks exhibited a coarse, shiny, faceted appearance. The cracks in this part were longitudinally oriented and were confined to the center of the section. Flakes are not observed in the outer region of sections, because there is apparently more time for the hydrogen in the outer region to diffuse to a safe level. Flakes that were close to the surface of the die exhibited temper color, but some of the deeper flakes did not. The temper-colored flakes apparently opened during quenching. Fig. 14 Die made from AISI O1 tool steel that was found to be cracked after heat treatment. (a) Longitudinal cracks after the surface was swabbed with 5% nital. (b) One of the cracks opened, revealing features typical of hydrogen flakes. 6.5× Nonmetallic Inclusions The file is downloaded from www.bzfxw.com
Nonmetallic inclusions are an inevitable consequence of commercial alloys and their respective melting or casting practices. Inclusions that originate in the ingot are carried onto wrought products, even though their shapes may be appreciably altered. Furthermore, additional nonmetallic matter, such as oxides, may develop during intermediate hot working stages and also end up in the finished form Two categories of nonmetallic inclusions in metals are Those that are entrapped inadvertently and originate almost exclusively from foreign matter, such as refractory linings, that is occluded in the metal while it is molten or being cast Those that form in the metal because of a change in temperature or compositi Inclusions of the latter type are produced by separation from the metal when it is in either the liquid or the solid state. Some nonmetallic inclusions form in the liquid before solidification; others form during solidification. In steels, for example, aluminates and silicates generally form before solidification, while sulfides form during solidification. Manganese sulfide inclusions frequently form in the interdendritic regions and primary grain boundaries of steel, where the last of the liquid freezes Depending on the alloy, oxides, sulfides, nitrides, or other nonmetallic compounds may occur when solubility limits in the matrix are exceeded. Because these compounds are products of reactions within the metal, they are normal constituents of the metal, and conventional melting practices cannot completely eliminate such inclusions. However, it is desirable to keep the type and amount of inclusions to a minimum, so that the metal is relatively free from those inclusions that may cause unwanted discontinuities. Air-melted alloys commonly contain inclusions. Vacuum- or electroslag-remelted alloys more commonly contain nonmetallic inclusions such as titanium carbonitrides or carbides, when carbon or the hardening element form precipitate during stabilization and aging cycles. Homogenizing cycles prior to conversion or at an early stage of conversion can be done for re-solution of some precipitates, but not all nonmetallic inclusions are affected by homogenization In steels, manganese sulfides can be homogenized Nonmetallic inclusions are unquestionably one of the most common imperfections involved in problems or lilures. Nonmetallic inclusions can easily become stress concentrators because of their discontinuous nature and incompatibility with the surrounding composition. This combination may very well yield flaws of critical size that, under appropriate loading conditions, result in fracture. Inclusions that are very brittle and tend to fracture and fragment during working operations also can be very detrimental. In fact, with certain refractory and slag inclusions, fragmentation frequently occurs when the amount of cross-sectional reduction in the workpiece is large The deleterious nature of nonmetallic inclusions depends on several factors, including chemical composition of the inclusion, volume percentage, shape, orientation, and the mechanical/physical properties of the inclusion as compared to its surrounding matrix(Ref 7). One study has demonstrated that the local constraint of the matrix by hard inclusions during tensile loading can produce severe stress concentrations that depend on the elastic moduli, size, shape, and orientation of the inclusions(Ref 8). For example, inclusions such as mangane sulfide, which are very deformable at hot working temperatures, elongate in the direction of working. This results in an anisotropic material condition, which, depending on its severity, can produce substantial decreases in transverse mechanical properties, such as ductility, fatigue life, and fracture toughness The metal-producing industry attempts to control nonmetallic inclusions, but inclusions cannot be removed ith complete assurance. In demanding applications, inclusion shape-control methods are used to produce inclusions predominately with a spherical shape, which is less of a stress concentrator with the surrounding matrix and less damaging than inclusions with more angular shapes. The principle of inclusion shape control is to reduce the surface energy between the inclusion and the metal--normally through suitable chemistry modification of the metal. Sulfide inclusions, for example, have been shape controlled by the addition of alcium or rare earth metal treatments. These inclusions are not plastic or deformable at typical hot working emperatures and retain their globular shape. Therefore, they are less injurious with respect to ductility, toughness, and fatigue life in finished wrought products With the number and size of inclusions held roughly constant, inclusion shape control reduces the likelihood of crack initiation from an inclusion. This is shown in Fig. 15(Ref 9) for a rolled constructional steel with calcium treatment, which provides longer life under cyclic loading due to inclusion shape modification. The advantage of a spherical inclusion shape is also evident in Charpy V-notch impact data for constructional steels(Fig 16)(Ref 10). Scanning electron microscope (SEM) fractographs also show a pronounced difference in
Nonmetallic inclusions are an inevitable consequence of commercial alloys and their respective melting or casting practices. Inclusions that originate in the ingot are carried onto wrought products, even though their shapes may be appreciably altered. Furthermore, additional nonmetallic matter, such as oxides, may develop during intermediate hot working stages and also end up in the finished form. Two categories of nonmetallic inclusions in metals are: · Those that are entrapped inadvertently and originate almost exclusively from foreign matter, such as refractory linings, that is occluded in the metal while it is molten or being cast · Those that form in the metal because of a change in temperature or composition Inclusions of the latter type are produced by separation from the metal when it is in either the liquid or the solid state. Some nonmetallic inclusions form in the liquid before solidification; others form during solidification. In steels, for example, aluminates and silicates generally form before solidification, while sulfides form during solidification. Manganese sulfide inclusions frequently form in the interdendritic regions and primary grain boundaries of steel, where the last of the liquid freezes. Depending on the alloy, oxides, sulfides, nitrides, or other nonmetallic compounds may occur when solubility limits in the matrix are exceeded. Because these compounds are products of reactions within the metal, they are normal constituents of the metal, and conventional melting practices cannot completely eliminate such inclusions. However, it is desirable to keep the type and amount of inclusions to a minimum, so that the metal is relatively free from those inclusions that may cause unwanted discontinuities. Air-melted alloys commonly contain inclusions. Vacuum- or electroslag-remelted alloys more commonly contain nonmetallic inclusions, such as titanium carbonitrides or carbides, when carbon or the hardening element form precipitate during stabilization and aging cycles. Homogenizing cycles prior to conversion or at an early stage of conversion can be done for re-solution of some precipitates, but not all nonmetallic inclusions are affected by homogenization. In steels, manganese sulfides can be homogenized. Nonmetallic inclusions are unquestionably one of the most common imperfections involved in problems or failures. Nonmetallic inclusions can easily become stress concentrators because of their discontinuous nature and incompatibility with the surrounding composition. This combination may very well yield flaws of critical size that, under appropriate loading conditions, result in fracture. Inclusions that are very brittle and tend to fracture and fragment during working operations also can be very detrimental. In fact, with certain refractory and slag inclusions, fragmentation frequently occurs when the amount of cross-sectional reduction in the workpiece is large. The deleterious nature of nonmetallic inclusions depends on several factors, including chemical composition of the inclusion, volume percentage, shape, orientation, and the mechanical/physical properties of the inclusion as compared to its surrounding matrix (Ref 7). One study has demonstrated that the local constraint of the matrix by hard inclusions during tensile loading can produce severe stress concentrations that depend on the elastic moduli, size, shape, and orientation of the inclusions (Ref 8). For example, inclusions such as manganese sulfide, which are very deformable at hot working temperatures, elongate in the direction of working. This results in an anisotropic material condition, which, depending on its severity, can produce substantial decreases in transverse mechanical properties, such as ductility, fatigue life, and fracture toughness. The metal-producing industry attempts to control nonmetallic inclusions, but inclusions cannot be removed with complete assurance. In demanding applications, inclusion shape-control methods are used to produce inclusions predominately with a spherical shape, which is less of a stress concentrator with the surrounding matrix and less damaging than inclusions with more angular shapes. The principle of inclusion shape control is to reduce the surface energy between the inclusion and the metal—normally through suitable chemistry modification of the metal. Sulfide inclusions, for example, have been shape controlled by the addition of calcium or rare earth metal treatments. These inclusions are not plastic or deformable at typical hot working temperatures and retain their globular shape. Therefore, they are less injurious with respect to ductility, toughness, and fatigue life in finished wrought products. With the number and size of inclusions held roughly constant, inclusion shape control reduces the likelihood of crack initiation from an inclusion. This is shown in Fig. 15 (Ref 9) for a rolled constructional steel with calcium treatment, which provides longer life under cyclic loading due to inclusion shape modification. The advantage of a spherical inclusion shape is also evident in Charpy V-notch impact data for constructional steels (Fig. 16)(Ref 10). Scanning electron microscope (SEM) fractographs also show a pronounced difference in
deformation mode with an inclusion shape modification(Fig. 17). Local ductility increases adjacent to the inclusions when the shape is spherical 64≌ g 375 52 350 · Calcium treated 48 Cycles to failure Fig. 15 Comparison of axial fatigue data for untreated and calcium-treated rolled asTM A516 steel. 51 mm(2 in. ) thick plates tested with alternating stress ratio of o 1. Source: ref Yield strength, 80〓 5 Calcium treated a e80 Conventional 40 0 300 400 500 600 00 9001000 Yield strength, MP Thefileisdownloadedfromwww.bzfxw.com
deformation mode with an inclusion shape modification (Fig. 17). Local ductility increases adjacent to the inclusions when the shape is spherical. Fig. 15 Comparison of axial fatigue data for untreated and calcium-treated rolled ASTM A516 steel. 51 mm (2 in.) thick plates tested with alternating stress ratio of 0.1. Source: Ref 9 The file is downloaded from www.bzfxw.com
Fig. 16 Comparison of Charpy V-notch upper-shelf energies (UsEs) for several grades and thicknesses of untreated and calcium-treated steel. Source: ref 1o Fig. 17 Scanning electron microscope fractographs showing the effect of calcium treatment on the fracture morphology of AsTM A633C steel impact specimens. (a) Untreated steel with type ll manganese inclusions; the fracture is ductile. Courtesy of A D. Wilson, Lukens Steel Compay steel with spherical Macroetching can provide a good indication of cleanliness, but further metallography is desirable if information is needed on the character of nonmetallic inclusions. Nonmetallic inclusions, although they usually appear as pits or pinholes after macroetching, must not be confused with pits that result from etching out metallic segregations or from variations in etching procedure. When nonmetallic inclusions are suspected in highly alloyed steels that may contain metallic segregations, an annealed specimen should be compared with hardened and tempered specimen etched in the same way. If the etching pits are the result of nonmetallic inclusions, they appear similar in the annealed and the hardened specimens. If they are the result of metallic segregation, they differ in appearance, being more prominent in the hardened specimen Example 4: Failure Analysis of a Helicopter Main Rotor Bolt(Ref 11). During assembly, a helicopter main rotor made of AISI E4340 steel failed while being torqued. The torque being applied reached approximately 100N m(900 lbf in. when the pitch horn bolt sheared and shot across the hangar. No other bolts were reported to have failed during assembly The rotor bolt fractured in the black-coated region between the threads and the taper section of the shank. The head, the straight portion of the shank, and the nut were gold colored. The fracture surfaces were first cleaned with soap and water and were then ultrasonically cleaned to remove surface debris. Longitudinal and transverse sections were prepared for optical microscopy. The bolt surface was also inspected for identification of coatings and surface treatments Figure 18 shows the two mating halves of the fracture. The bolt had been heated to obtain a tempered martensitic microstructure. The metallographic specimen showed a significant amount of nonmetallic inclusions(Fig. 12). Figure 12 also shows a pore, probably a pipe defect in the original bar stock
Fig. 16 Comparison of Charpy V-notch upper-shelf energies (USEs) for several grades and thicknesses of untreated and calcium-treated steel. Source: Ref 10 Fig. 17 Scanning electron microscope fractographs showing the effect of calcium treatment on the fracture morphology of ASTM A633C steel impact specimens. (a) Untreated steel with type II manganese sulfide inclusions showing evidence of brittle fracture. (b) Calcium-treated steel with spherical inclusions; the fracture is ductile. Courtesy of A.D. Wilson, Lukens Steel Company Macroetching can provide a good indication of cleanliness, but further metallography is desirable if information is needed on the character of nonmetallic inclusions. Nonmetallic inclusions, although they usually appear as pits or pinholes after macroetching, must not be confused with pits that result from etching out metallic segregations or from variations in etching procedure. When nonmetallic inclusions are suspected in highly alloyed steels that may contain metallic segregations, an annealed specimen should be compared with a hardened and tempered specimen etched in the same way. If the etching pits are the result of nonmetallic inclusions, they appear similar in the annealed and the hardened specimens. If they are the result of metallic segregation, they differ in appearance, being more prominent in the hardened specimen. Example 4: Failure Analysis of a Helicopter Main Rotor Bolt (Ref 11). During assembly, a helicopter main rotor made of AISI E4340 steel failed while being torqued. The torque being applied reached approximately 100 N · m (900 lbf · in.) when the pitch horn bolt sheared and shot across the hangar. No other bolts were reported to have failed during assembly. The rotor bolt fractured in the black-coated region between the threads and the taper section of the shank. The head, the straight portion of the shank, and the nut were gold colored. The fracture surfaces were first cleaned with soap and water and were then ultrasonically cleaned to remove surface debris. Longitudinal and transverse sections were prepared for optical microscopy. The bolt surface was also inspected for identification of coatings and surface treatments. Figure 18 shows the two mating halves of the fracture. The bolt had been heated to obtain a tempered martensitic microstructure. The metallographic specimen showed a significant amount of nonmetallic inclusions (Fig. 12). Figure 12 also shows a pore, probably a pipe defect in the original bar stock