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Failure of cadmium plated maraging steel tension bolt
Engineering Failure Analysis 8 (2001) 263±269
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Failure of cadmium plated maraging steel tension bolt K. Gangadhara Reddy, Abhay K. Jha*, V. Diwakar Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Indian Space Research Organisation, Trivandrum 695 022, India Received 22 February 2000; accepted 2 March 2000
Abstract Cadmium electroplated (EP) high strength steel fasteners are widely used for assembling threaded joints as cadmium gives lubrication and provides excellent corrosion resistance. However, in-house-failure case histories revealed that in some cases delayed failure of cadmium EP high strength steel fasteners occurred. Recently, one such premature failure of a cadmium EP 18 Ni 1800 MPa maraging steel tension bolt occurred at a sustained stress of about 40% of the ultimate tensile stress (UTS), after 11,500 h. For their successful functional application, cadmium EP fasteners require proper and adequate baking treatment after electroplating. This paper highlights details of analysis carried out on the failed bolt to ®nd out the reason for the failure, using standard metallographic and NDT techniques. Remedial measures to minimise, if not totally prevent, such failure are also discussed. 7 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electrodeposition; Embrittlement; Hydrogen embrittlement; Threaded fasteners
1. Introduction Cadmium electroplated (EP) 18 Ni 1800 MPa maraging steel hexagonal head tension bolts of size M14 (Fig. 1a) were used in an assembly of a mechanical system. Two bolts were assembled under precise sustained tension load in the system for their functional and structural quali®cation. The bolts were under a sustained tension load of 40% of ultimate tensile strength (UTS) during the functional test and the structural quali®cation program. One of the bolts failed suddenly after bearing the sustained load for a cumulative period of 11,500 h. Standard metallographic and NDT techniques were used to ®nd out the reason for the failure.
* Corrosponding author. Tel.: +91-471-563-748; fax: +91-471-415-348. E-mail address: [email protected] (A.K. Jha). 1350-6307/01/$ - see front matter 7 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 6 3 0 7 ( 0 0 ) 0 0 0 1 1 - X
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K.G. Reddy et al. / Engineering Failure Analysis 8 (2001) 263±269
2. Material The failed bolt was fabricated out of M250 grade maraging steel and maraged. A schematic of the tension bolt which failed in the assembly and the location where fracture occurred are shown in Fig. 1b. The chemical composition (C 0.023, Ni 19.5, Co 9.0, Mo 4.8, Ti 0.48, Mn 0.18, Al 0.05 wt% and H 1.47 ppm) and mechanical properties (UTS 1765 MPa, 0.2% YS 1725 MPa, %EL 8, %RA 40 and HRc 49) were within acceptable limits. The bolts were given a baking treatment at 1858C for 6 h after Cd plating. 3. Experiments Observation of the failed bolt revealed that it broke into two pieces in the threaded portion. The fracture surface was ¯at and it occurred on the bolt where it was engaged with the nut as in Fig. 1b. On the head-end piece of the bolt, shearing-o of three threads which were inside the nut was noticed. There was no yielding or necking of the bolt. Corrosion was absent. Stereo microscopic observations
Fig. 1. (a) Maraging steel tension bolt. (b) Schematic of failed bolt and location of crack.
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revealed small cracks which initiated at the root of the ®rst few threads, from the fracture end of the head-end piece. X-ray observations revealed the absence of internal defects. Less than 1% austenite (retained) was observed by X-ray diraction (Fig. 2). Specimens were cut along the bolt axis from the head-end piece and thread-end piece, polished, and etched with 5% Nital solution for optical microscopic observations. Fig. 3(a) shows martensitic structure typical of aged M250 maraging steel. Second phase particles such as titanium carbonitrides are present in the microstructure (Fig. 3(b)). Fig. 4 shows a cross-section of the bolt revealing non-uniform thread pro®le and undulations. The fracture surface was ultrasonically cleaned and observed in the SEM. Fracture surfaces revealed quasi cleavage and transgranular cleavage features (Figs. 5 and 6). Figs. 5 and 6(b) also show the presence of second phase particles and cleavage fracture of surrounding grains. Fig. 5 shows a small area of transgranular fracture surrounded by ductile dimples, typical of hydrogen embrittlement (HE) fracture. 4. Discussion The bolt was under a maximum sustained stress of 40% of the UTS (1765 MPa) for over 11,500 h before it failed. The chemical composition and mechanical properties of the material meet the speci®cations, with 1.47 ppm of hydrogen being present. Hardness and mechanical properties indicate material in a maraged condition. X-ray radiography and diraction analysis revealed the absence of internal defects. The microstructure of the failed bolt showed lath martensite (Fig. 3) typical of aged M250 maraging steel. Thread cutting resulted in a non-uniform thread pro®le with undulations and sharp thread root radius. The absence of elongated threads/necking adjacent to the fractured portion ruled out the possibility of over-load fracture.
Fig. 2. X-ray diraction pattern of M-250 maraging steel bolt in aged condition.
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Earlier failure case histories have revealed that in many cases [1±3], delayed failures were observed on Cd EP high strength steel fasteners. In all these cases, the causes were traced to HE due to the EP cadmium process. In the present case, it is inferred that dull Cd electroplating bath was used for the bolts and post plate baking treatment at 1858C for 6 h was given for HE relief. Studies [4] on dull Cd EP M250 maraging steel show that it picks-up about 2.8 ppm of hydrogen during the actual plating process. Along with residual hydrogen present before electroplating, this works out to about 4.27 ppm total hydrogen in the bolt after electroplating. During the subsequent baking process, at 1858C for 6 h, most of the absorbed hydrogen diuses out as the dull Cd plate is porous in nature but some of it diuses into the central core of the steel bolt under the concentration gradient and encounters more and more crystal imperfections and traps (dislocations, SPPs etc.). When a sustained tensile stress is applied to the component, the residual hydrogen defuses from the traps as well as from the solid solution to areas of high stress concentration, such as a notch, SPPs, etc., accumulates there in sucient quantity over a time period and initiates cracks and accelerates their propagation. Birnbaum [5] pointed out that when hydrogen is present in nickel, it undergoes hydrogen related
Fig. 3. Microstructure of maraging steel (a) optical micrograph 200 times (b) scanning electron micrograph.
K.G. Reddy et al. / Engineering Failure Analysis 8 (2001) 263±269
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Fig. 4. Cross section of failed tension bolt, threaded portion, 40 times.
fracture at relatively low phagocytes by increased dislocation mobility and enhanced plastic deformation and the dominant fracture mechanism is failure by a localised ductility which results from hydrogen induced softening of material at crack tip that applies to both transgranular and intergranular fracture. Troiano [6] suggested that hydrogen diuses under the in¯uence of stress gradient to regions of high triaxiality where hydrogen interacts with the metal lattice to reduce its cohesive strength. In the present case, fractographic observations of the head-end and thread-end pieces of the bolt revealed similar fracture features of quasi cleavage (Fig. 6) which con®rms that the failure is due to HE. Cracks originate from SPPs and sharp threads, where atomic hydrogen accumulates at stress concentrators and cleavage of surrounding grains occurr. The small area of intergranular fracture and presence of predominantly quasi cleavage features (Fig. 6b) are attributed to hydrogen enhanced local plasticity due to hydrogen present in the steel, which under sustained load leads to cracking. Though maraging steels are much more resistant to Cd electroplating (hydrogen) embrittlement than the low alloy steels, they are not totally immune to HE [4,7±10]. Thread cutting introduced sharp thread roots and non-uniform thread pro®le which caused stress concentrations to initiate cracks under the
Fig. 5. SEM photomicrograph showing transgranular fracture features.
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in¯uence of hydrogen. Thread rolling gives a smooth and more uniform thread pro®le [11,12]. 18 Ni 1800 MPa maraging steel, electrolytically hydrogenated and baked at 1508C for 6 h, contained about 1.7 ppm hydrogen and failed in less than 10 h at a sustained load of less than 1400 MPa. 18 Ni 1800 MPa maraging steel with 3.4 ppm hydrogen failed within 22 h at 1400 MPa sustained load. To relieve the HE due to electroplating, in most of the studies [4,7±10] and standard speci®cation [7] for Cd electroplating high strength steels, a minimum baking treatment for 23 h was recommended after electroplating for steels with strength level greater than 1500 MPa. The tension bolt baked only at 1858C for 6 h failed after a considerably longer time of over 11,500 h under the sustained tensile stress. However, the short duration 200 h sustained stress tests may not always be fully adequate to assess the long duration sustained life of the components in service. As an alternative, whenever a cadmium EP and baked high strength steel component is subjected to a sustained stress, one may run a parallel control experiment on specimens made from the same material, adopting the same plating and baking procedures. If the applied sustained stress is 25% greater than that applied in service, one may be warned in advance of
Fig. 6. SEM photomicrograph showing (a) Transgranular cleavage. (b) Quasi cleavage features.
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the possible failure of the actual component in service. As an alternative to electroplating, vacuum deposition of cadmium on steels can totally eliminate hydrogen pick-up and hence HE. However, this may cost more. Use of molybdenum disulphide (MoS2) solid lubricant has its own disadvantages in moist environments. 5. Conclusion The maraging steel tension bolt failed due to HE. A baking treatment at 1858C for 6 h after electroplating is not adequate to relieve HE of steels of strength level greater than 1500 MPa. The thread cutting process resulted in stress concentrations initiating cracks under the in¯uence of hydrogen. 6. Recommendations 1. Instead of thread cutting, a thread rolling process should be adopted to minimise stress concentrators. 2. Long duration baking (1958C for 23 h) of components immediately after electroplating is recommendated to minimise HE and increase the sustained load life of the component. 3. Alternatively, vacuum deposition of cadmium should be adopted which totally eliminates HE.
Acknowledgements The authors are grateful to Dr. K.V. Nagarajan, Group Director, MMG and Deputy Director, PCM for encouragement to carry out this analysis. Thanks are due to the Director for his permission to publish this paper. References [1] Reddy KG, Sampath Kumar STG, Arumugham S, Lakshmanan TS. Practical Metallography 1989;26:654. [2] Krishnaswamy P, Sreekumar K, Natarajan A. Practical Metallography 1985;22:412. [3] Reddy KG, Sampath Kumar STG, John KM. Failure of 30NCD16 steel bolts VSSC:MMS:MMG:MTESD:14:89, August 1989. [4] Groeneveld TP, Fletcher EE, Elsea AR. A study of hydrogen embrittlement of various alloy. NASA CR 98448, BMI. Columbus, OH, 1969. p. 9-60. [5] Birnbaum HK. Environment sensitive fracture of metals and alloys. In: Naval Research Workshop, Washington DC, June 3± 4. 1985. p. 105. [6] Troiano AR. Trans American Society for Metals 1960;52:54. [7] Pollock WJ. Hydrogen embrittlement prevention and control. Raymond Louis, editor. ASTM STP 962, Philadelphia, 1988. 68-80. [8] Langstone PF. The eect of composition and tensile strength on the susceptibility of alloy steels to cadmium plating (hydrogen) embrittlement. D. Mat. Report no. 159, British Aerojet, October 1968. [9] 18 percent nickel maraging steels, INCO Databook, INCO Europe 1976 29. [10] Reddy KG, Arumugham S, Lakshmanan TS. J Mat Sci 1992;27:29. [11] Metals handbooks, ASM, vol. 5. 9th ed. 1987. 268. [12] Diwaker V, et al. Sustained load characterisation of M250 maraging steel bolts. VSSC:MMS:MMG:MCD:12:93.
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Failure of cadmium plated maraging steel tension bolt K. Gangadhara Reddy, Abhay K. Jha*, V. Diwakar Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Indian Space Research Organisation, Trivandrum 695 022, India Received 22 February 2000; accepted 2 March 2000
Abstract Cadmium electroplated (EP) high strength steel fasteners are widely used for assembling threaded joints as cadmium gives lubrication and provides excellent corrosion resistance. However, in-house-failure case histories revealed that in some cases delayed failure of cadmium EP high strength steel fasteners occurred. Recently, one such premature failure of a cadmium EP 18 Ni 1800 MPa maraging steel tension bolt occurred at a sustained stress of about 40% of the ultimate tensile stress (UTS), after 11,500 h. For their successful functional application, cadmium EP fasteners require proper and adequate baking treatment after electroplating. This paper highlights details of analysis carried out on the failed bolt to ®nd out the reason for the failure, using standard metallographic and NDT techniques. Remedial measures to minimise, if not totally prevent, such failure are also discussed. 7 2001 Elsevier Science Ltd. All rights reserved. Keywords: Electrodeposition; Embrittlement; Hydrogen embrittlement; Threaded fasteners
1. Introduction Cadmium electroplated (EP) 18 Ni 1800 MPa maraging steel hexagonal head tension bolts of size M14 (Fig. 1a) were used in an assembly of a mechanical system. Two bolts were assembled under precise sustained tension load in the system for their functional and structural quali®cation. The bolts were under a sustained tension load of 40% of ultimate tensile strength (UTS) during the functional test and the structural quali®cation program. One of the bolts failed suddenly after bearing the sustained load for a cumulative period of 11,500 h. Standard metallographic and NDT techniques were used to ®nd out the reason for the failure.
* Corrosponding author. Tel.: +91-471-563-748; fax: +91-471-415-348. E-mail address: [email protected] (A.K. Jha). 1350-6307/01/$ - see front matter 7 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 6 3 0 7 ( 0 0 ) 0 0 0 1 1 - X
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K.G. Reddy et al. / Engineering Failure Analysis 8 (2001) 263±269
2. Material The failed bolt was fabricated out of M250 grade maraging steel and maraged. A schematic of the tension bolt which failed in the assembly and the location where fracture occurred are shown in Fig. 1b. The chemical composition (C 0.023, Ni 19.5, Co 9.0, Mo 4.8, Ti 0.48, Mn 0.18, Al 0.05 wt% and H 1.47 ppm) and mechanical properties (UTS 1765 MPa, 0.2% YS 1725 MPa, %EL 8, %RA 40 and HRc 49) were within acceptable limits. The bolts were given a baking treatment at 1858C for 6 h after Cd plating. 3. Experiments Observation of the failed bolt revealed that it broke into two pieces in the threaded portion. The fracture surface was ¯at and it occurred on the bolt where it was engaged with the nut as in Fig. 1b. On the head-end piece of the bolt, shearing-o of three threads which were inside the nut was noticed. There was no yielding or necking of the bolt. Corrosion was absent. Stereo microscopic observations
Fig. 1. (a) Maraging steel tension bolt. (b) Schematic of failed bolt and location of crack.
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revealed small cracks which initiated at the root of the ®rst few threads, from the fracture end of the head-end piece. X-ray observations revealed the absence of internal defects. Less than 1% austenite (retained) was observed by X-ray diraction (Fig. 2). Specimens were cut along the bolt axis from the head-end piece and thread-end piece, polished, and etched with 5% Nital solution for optical microscopic observations. Fig. 3(a) shows martensitic structure typical of aged M250 maraging steel. Second phase particles such as titanium carbonitrides are present in the microstructure (Fig. 3(b)). Fig. 4 shows a cross-section of the bolt revealing non-uniform thread pro®le and undulations. The fracture surface was ultrasonically cleaned and observed in the SEM. Fracture surfaces revealed quasi cleavage and transgranular cleavage features (Figs. 5 and 6). Figs. 5 and 6(b) also show the presence of second phase particles and cleavage fracture of surrounding grains. Fig. 5 shows a small area of transgranular fracture surrounded by ductile dimples, typical of hydrogen embrittlement (HE) fracture. 4. Discussion The bolt was under a maximum sustained stress of 40% of the UTS (1765 MPa) for over 11,500 h before it failed. The chemical composition and mechanical properties of the material meet the speci®cations, with 1.47 ppm of hydrogen being present. Hardness and mechanical properties indicate material in a maraged condition. X-ray radiography and diraction analysis revealed the absence of internal defects. The microstructure of the failed bolt showed lath martensite (Fig. 3) typical of aged M250 maraging steel. Thread cutting resulted in a non-uniform thread pro®le with undulations and sharp thread root radius. The absence of elongated threads/necking adjacent to the fractured portion ruled out the possibility of over-load fracture.
Fig. 2. X-ray diraction pattern of M-250 maraging steel bolt in aged condition.
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Earlier failure case histories have revealed that in many cases [1±3], delayed failures were observed on Cd EP high strength steel fasteners. In all these cases, the causes were traced to HE due to the EP cadmium process. In the present case, it is inferred that dull Cd electroplating bath was used for the bolts and post plate baking treatment at 1858C for 6 h was given for HE relief. Studies [4] on dull Cd EP M250 maraging steel show that it picks-up about 2.8 ppm of hydrogen during the actual plating process. Along with residual hydrogen present before electroplating, this works out to about 4.27 ppm total hydrogen in the bolt after electroplating. During the subsequent baking process, at 1858C for 6 h, most of the absorbed hydrogen diuses out as the dull Cd plate is porous in nature but some of it diuses into the central core of the steel bolt under the concentration gradient and encounters more and more crystal imperfections and traps (dislocations, SPPs etc.). When a sustained tensile stress is applied to the component, the residual hydrogen defuses from the traps as well as from the solid solution to areas of high stress concentration, such as a notch, SPPs, etc., accumulates there in sucient quantity over a time period and initiates cracks and accelerates their propagation. Birnbaum [5] pointed out that when hydrogen is present in nickel, it undergoes hydrogen related
Fig. 3. Microstructure of maraging steel (a) optical micrograph 200 times (b) scanning electron micrograph.
K.G. Reddy et al. / Engineering Failure Analysis 8 (2001) 263±269
267
Fig. 4. Cross section of failed tension bolt, threaded portion, 40 times.
fracture at relatively low phagocytes by increased dislocation mobility and enhanced plastic deformation and the dominant fracture mechanism is failure by a localised ductility which results from hydrogen induced softening of material at crack tip that applies to both transgranular and intergranular fracture. Troiano [6] suggested that hydrogen diuses under the in¯uence of stress gradient to regions of high triaxiality where hydrogen interacts with the metal lattice to reduce its cohesive strength. In the present case, fractographic observations of the head-end and thread-end pieces of the bolt revealed similar fracture features of quasi cleavage (Fig. 6) which con®rms that the failure is due to HE. Cracks originate from SPPs and sharp threads, where atomic hydrogen accumulates at stress concentrators and cleavage of surrounding grains occurr. The small area of intergranular fracture and presence of predominantly quasi cleavage features (Fig. 6b) are attributed to hydrogen enhanced local plasticity due to hydrogen present in the steel, which under sustained load leads to cracking. Though maraging steels are much more resistant to Cd electroplating (hydrogen) embrittlement than the low alloy steels, they are not totally immune to HE [4,7±10]. Thread cutting introduced sharp thread roots and non-uniform thread pro®le which caused stress concentrations to initiate cracks under the
Fig. 5. SEM photomicrograph showing transgranular fracture features.
268
K.G. Reddy et al. / Engineering Failure Analysis 8 (2001) 263±269
in¯uence of hydrogen. Thread rolling gives a smooth and more uniform thread pro®le [11,12]. 18 Ni 1800 MPa maraging steel, electrolytically hydrogenated and baked at 1508C for 6 h, contained about 1.7 ppm hydrogen and failed in less than 10 h at a sustained load of less than 1400 MPa. 18 Ni 1800 MPa maraging steel with 3.4 ppm hydrogen failed within 22 h at 1400 MPa sustained load. To relieve the HE due to electroplating, in most of the studies [4,7±10] and standard speci®cation [7] for Cd electroplating high strength steels, a minimum baking treatment for 23 h was recommended after electroplating for steels with strength level greater than 1500 MPa. The tension bolt baked only at 1858C for 6 h failed after a considerably longer time of over 11,500 h under the sustained tensile stress. However, the short duration 200 h sustained stress tests may not always be fully adequate to assess the long duration sustained life of the components in service. As an alternative, whenever a cadmium EP and baked high strength steel component is subjected to a sustained stress, one may run a parallel control experiment on specimens made from the same material, adopting the same plating and baking procedures. If the applied sustained stress is 25% greater than that applied in service, one may be warned in advance of
Fig. 6. SEM photomicrograph showing (a) Transgranular cleavage. (b) Quasi cleavage features.
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the possible failure of the actual component in service. As an alternative to electroplating, vacuum deposition of cadmium on steels can totally eliminate hydrogen pick-up and hence HE. However, this may cost more. Use of molybdenum disulphide (MoS2) solid lubricant has its own disadvantages in moist environments. 5. Conclusion The maraging steel tension bolt failed due to HE. A baking treatment at 1858C for 6 h after electroplating is not adequate to relieve HE of steels of strength level greater than 1500 MPa. The thread cutting process resulted in stress concentrations initiating cracks under the in¯uence of hydrogen. 6. Recommendations 1. Instead of thread cutting, a thread rolling process should be adopted to minimise stress concentrators. 2. Long duration baking (1958C for 23 h) of components immediately after electroplating is recommendated to minimise HE and increase the sustained load life of the component. 3. Alternatively, vacuum deposition of cadmium should be adopted which totally eliminates HE.
Acknowledgements The authors are grateful to Dr. K.V. Nagarajan, Group Director, MMG and Deputy Director, PCM for encouragement to carry out this analysis. Thanks are due to the Director for his permission to publish this paper. References [1] Reddy KG, Sampath Kumar STG, Arumugham S, Lakshmanan TS. Practical Metallography 1989;26:654. [2] Krishnaswamy P, Sreekumar K, Natarajan A. Practical Metallography 1985;22:412. [3] Reddy KG, Sampath Kumar STG, John KM. Failure of 30NCD16 steel bolts VSSC:MMS:MMG:MTESD:14:89, August 1989. [4] Groeneveld TP, Fletcher EE, Elsea AR. A study of hydrogen embrittlement of various alloy. NASA CR 98448, BMI. Columbus, OH, 1969. p. 9-60. [5] Birnbaum HK. Environment sensitive fracture of metals and alloys. In: Naval Research Workshop, Washington DC, June 3± 4. 1985. p. 105. [6] Troiano AR. Trans American Society for Metals 1960;52:54. [7] Pollock WJ. Hydrogen embrittlement prevention and control. Raymond Louis, editor. ASTM STP 962, Philadelphia, 1988. 68-80. [8] Langstone PF. The eect of composition and tensile strength on the susceptibility of alloy steels to cadmium plating (hydrogen) embrittlement. D. Mat. Report no. 159, British Aerojet, October 1968. [9] 18 percent nickel maraging steels, INCO Databook, INCO Europe 1976 29. [10] Reddy KG, Arumugham S, Lakshmanan TS. J Mat Sci 1992;27:29. [11] Metals handbooks, ASM, vol. 5. 9th ed. 1987. 268. [12] Diwaker V, et al. Sustained load characterisation of M250 maraging steel bolts. VSSC:MMS:MMG:MCD:12:93.