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Embrittlement of case hardened steel chain link
Engineering Failure Analysis 16 (2009) 2311–2317
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Embrittlement of case hardened steel chain link C.R.F. Azevedo *, D. Magarotto, A.P. Tschiptschin Escola Politécnica da Universidade de São Paulo, Av. Prof. Mello de Morais, n 2463, 05508-030 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 6 March 2009 Accepted 9 March 2009 Available online 17 March 2009 Keywords: Induction case hardening Electrogalvanizing Intergranular cracking Hydrogen stress cracking
a b s t r a c t Various steel chain links presented cracking during their manufacturing process, which includes induction case hardening and electrogalvanizing steps. Fractographic examination of the exposed crack surfaces revealed intergranular cracking with some areas featuring a thin layer of iron oxide, indicating that the cracking took place after the electrogalvanizing step. The location of the cracks coincided with the position of the deepest case hardened layer, suggesting the occurrence of localized overheating during the induction case hardening step. Inductive heating finite element analysis (COSMOS Designstar Software) confirmed that during the case hardening the austenitising temperature reached in the crack region values of approximately 1050 °C. The results indicated that intergranular cracking was caused by hydrogen embrittlement. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction 1045 SAE steel chain links presented cracking during their manufacturing process, which included the following processing steps: plate cutting; plate bending; bore stamping; quenching and tempering (26–32 HRC); induction case hardening and tempering (50–55 HRC and effective depth of hardening, 515 HV1, between 3 and 4 mm), hard particle blasting and electrogalvanizing. According to information, the cracking took place just a ‘‘short time” after the last processing step. Literature [1–5] indicates that both case hardening and electrogalvanizing steps might cause overheating, residual stress and hydrogen charging, so the process control should be very strict to avoid the degradation of mechanical properties. 2. Experimental procedure and results The following exams were used in the present investigation: visual inspection, metallographic and microfractographic characterization, chemical microanalysis, microhardness profile and residual stress measurements. The chain link is shown in Fig. 1, indicating the position of the crack (adjacent to the bore with greater diameter) and the presence of a rather heterogeneous hardened layer depth (much deeper next to the larger bore). Fig. 2 shows the macrostructure of the chain link next to the larger bore (plane parallel to the crack). Besides the plate segregation (vertical darker line), it is possible to observe the intense effect of the induction case hardening, which promoted a steep microstructural gradient from the surface to the core of the chain link. Metallographic characterization was carried out next to the crack region (see Fig. 3a and b), revealing the tempered microstructure of the case hardened region, with a coarse prior austenitic grain size region, and the core region, with a much more refined prior austenitic grain. Microfractography of the exposed crack surfaces (see Fig. 4a and b) was carried out using a PHILIPS-XL-30 SEM microscope and the results are shown in Fig. 5a–d, revealing that the crack propagated by a brittle intergranular mode. EDS micro-
* Corresponding author. Tel.: +55 11 30915916; fax: +55 11 30914308. E-mail address: [email protected] (C.R.F. Azevedo). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.03.010
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Fig. 1. General aspect of the chain link, showing the heterogeneous hardened layer (darker area). The arrow indicates the crack position and the region used to the metallographic exam and microhardness test.
Fig. 2. Macrostructural aspect next to the larger bore caused by the induction case hardening treatment (plane parallel to the crack).
Fig. 3. Microstructural examination. (a) Aspect of the tempered martensitic microstructure next to the induction hardened surface (prior austenite grain size of approximately 100 lm); and (b) aspect of the tempered martensitic microstructure next to the bore, (prior austenite grain size around 10 lm).
C.R.F. Azevedo et al. / Engineering Failure Analysis 16 (2009) 2311–2317
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Fig. 4. Sampling for fractography. (a) Detail of the crack, perpendicular to the hardened surface; and (b) detail of the exposed crack surface, where the dark area corresponds to the crack and the clear area to the fracture, which was imposed in the laboratory to expose the crack surface.
Fig. 5. Fractographic examination (a) and (b) details of the exposed crack surface, indicating intergranular brittle cracking; (c) and (d) details of the fracture surface created in the laboratory, showing quasi-cleavage fracture, typical of tempered martensite.
analysis was carried out on the crack surfaces and the results (see Fig. 6) did not show the presence of zinc, indicating that the crack growth occurred after the electrogalvanizing step. Fig. 7 shows a three-dimensional reconstruction of the intergranular crack surface (using MEX software), showing the presence of large prior austenitic grains next to the hardened surface due to an excessive austenitization temperature. Inductive heating finite element analysis (COSMOS Designstar Software) confirmed that during the case hardening the austenitising temperature can reach values above 1050 °C in the crack region, while in the remaining regions this temperature is around 750 °C (see Fig. 8). Microhardness profile (Fig. 9) was carried and the effective depth of hardening (HV1 515) was determined as 10 mm in the crack regions. This value is much deeper than the product specification (between 3 and 4 mm), confirming once again the hypothesis of overheating during austenitization. Moreover, even areas away from the crack region showed high values, around 6 mm, for the effective depth of hardening (HV1 515), indicating that the control of the case hardening treatment should be optimized. Finally, the present investigation carried out measurements of the residual stress and retained austenite next to the larger bore, using a Rigaku diffractometer (Strainflex MSE – 2 M) with Cr Ka radiation. The Zn layer of the chain link was removed by immersing the entire part in a solution of HCl 20%, neutralizing it in a solution of NaOH 10% and carefully rinsing it off in water. The surfaces for XRD analysis were prepared by Struers Tenupol electropolishing unit, using an electrolyte of 30% HClOy in water at 25 °C and 40–60 V (see results in table 1).
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Fig. 6. EDS microanalysis on the darker region of the exposed crack surface, without the presence of Zinc peaks (see Fig. 5a).
Fig. 7. Three-dimensional reconstruction of the intergranular crack propagation surface (using MEX software), revealing the large austenitic grain size (100 lm) next to the hardened surface.
Fig. 8. COSMOS DesignstarÒ software FEA simulation of the induction heating, showing temperature distribution during the austenitisation (Note temperatures above 900 °C on the region next to the larger bore).
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Fig. 9. Microhardness profile (larger bore region), indicating the value of 10 mm for the effective depth of hardening.
Table 1 Superficial compressive residual stress results by XRDa (1: longitudinal; 2: tangential). Sample
Residual stress (MPa) (%)
Retained austenite (%)
Cracked
1
451 4772
1.2
No cracking
3921 4472
1.0
a
Scan range between 149° and 165°, step width: 0.32° and fixed time: 0.80 s.
3. Discussion The depth of the hardened layer showed a great variation in the effective depth of hardening (HV1 515), as can be observed in Figs. 1, 8 and 9. The results indicated that the case hardening process did not take into account the smaller ‘‘thermal mass” next to the larger bore, causing localized overheating during the austenitisation (see Fig. 8). The local overheating promoted excessive growth of the austenitic grains (see Figs. 3, 5 and 7), which might lead to both grain boundary segregation (embrittlement) and loss in toughness. EDS microanalysis on the crack surface did not show the presence of zinc on the crack surface, indicating that the cracking took place after the electrogalvanizing step (see Fig. 6). Microfractographic examination revealed that the crack propagation took place intergranularly by a brittle mode (see Figs. 5 and 7), while the crack induced in the laboratory took place by a transgranular cleavage mode. These results associated with the existence of a dwell time for the brittle cracking just after the electrogalvanizing step indicates the action of a hydrogen embrittlement mechanism (hydrogen stress cracking). Hydrogen stress cracking (HSC) is characterized by the brittle fracture due to hydrogen exposure (prior to manufacturing or from environmental hydrogen) and it often produces time-delayed (diffusion-controlled) fractures, even without external stress [4–6]. HSC can occur from hydrogen pickup during electrogalvanizing, which can cause high concentrations of atomic or nascent hydrogen to diffuse into the certain regions of the material. Because the Zn layer prevents the hydrogen from leaving the base metal, elevated-temperature baking after plating is required to allow the hydrogen to move to microstructural positions that are less damaging to the mechanical properties (see Fig. 11). HSC occurs most often in high-strength steels, which present hardness higher than 40 HRC or tensile strength higher than 1240 MPa. Heat treated medium-carbon steels have a greatly increased susceptibility to hydrogen embrittlement cracks when hardness is above 43 HRC, such as in the investigated case hardened chain link (approximate hardness value in the case hardened region is above 51 HRC) [4–6]. The results indicate that both case hardening and dehydrogenation treatments should be reviewed in order to avoid further failures. Moreover, the use of high austenitisation temperatures can promote a severe change in the superficial residual stress of tempered microstructures, which can vary from compressive residual stress (mainly due to the contribution of the volumetric expansion, which take places during the austenite ? martensite phase transformation) to tensile residual stress (mainly due to the contribution of the volumetric contraction of the surface during cooling), as shown qualitatively in Fig. 10a. The same effect on the sign of the residual stress can be observed for higher cooling rates (see Fig. 10b) [1–3]. As the presence of tensile residual stress might decrease the susceptibility limits for the hydrogen embrittlement, the investigation carried out measurements of the residual stress by X-ray diffraction to measure the residual stress next to
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Fig. 10. (a) Correlation between the surface austenitisation temperature during the induction case hardening and the value and type of residual stress [1]; and (b) correlation between the maximum cooling rate after the austenitisation during the induction case hardening and the value and type of the residual stress [2].
Fig. 11. Effect of the baking dehydrogenation treatment on the mechanical resistance of high resistance steel. Time to fracture as a function of applied stress and aging (baking) time at 150 °C for notched tensile specimens of AISI 4340 that were hydrogen charged and electroplated with cadmium [5].
the larger bore. The results for both products (cracked and sound chain links) indicated the presence of compressive residual stress of around 450 MPa on the hardened surfaces (see Table 1), suggesting that HSC can take place in regions presenting compressive residual stress. A more detailed measurement mapping of the residual stresses around the larger bore and the use of residual stress FEA might be necessary to confirm this suggestion. 4. Conclusions The intergranular cracking of the link was caused by hydrogen stress cracking. The presence of compressive residual stress of around 450 MPa on the hardened surfaces of the link was not sufficient to prevent the hydrogen stress cracking. Both case hardening and dehydrogenation treatments should be reviewed in order to avoid further failures.
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Acknowledgments The authors would like to thank Tenaris-Confab for the financial support for the modernization of the Laboratório de Caracterização Microstructural Hubertus Colpaert. Additional thanks to Aços Villares Sidenor and Thyssenkrupp Metalúrgica for technical and laboratorial support during the XRD residual stress measurements. References [1] Lee MK, Kim GH, Kim KH, Kim WW. Effects of the surface temperature and cooling rate on the residual stresses in a flame hardening of 12Cr steel. J Mater Process Technol 2006;176(1–3):140–5. [2] Lee MK, Kim GH, Kim KH, Kim WW. Control of surface hardnesses, hardening depths and residual stresses of low carbon 12Cr steel by flame hardening. Surf Coat Technol 2004;184(2–3):239–46. [3] Camarão AC. Um modelo para previsão de tensões residuais em cilindros de aços temperados por indução. Tese de doutorado. Escola Politécnica da Universidade de São Paulo; 1998. [4] ASM Handbook. Hydrogen damage and embrittlement. Failure analysis and prevention. vol. 10, 10th ed. ASM International; 2002. [5] ASM Handbook. Properties and selection: irons, steels, and high-performance alloys. vol. 1, 10th ed. ASM International; 1990. [6] Azevedo CRF. Failure analysis of a crude oil pipeline. Eng Fail Anal 2007;14(6):978–94.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Embrittlement of case hardened steel chain link C.R.F. Azevedo *, D. Magarotto, A.P. Tschiptschin Escola Politécnica da Universidade de São Paulo, Av. Prof. Mello de Morais, n 2463, 05508-030 São Paulo, SP, Brazil
a r t i c l e
i n f o
Article history: Received 6 March 2009 Accepted 9 March 2009 Available online 17 March 2009 Keywords: Induction case hardening Electrogalvanizing Intergranular cracking Hydrogen stress cracking
a b s t r a c t Various steel chain links presented cracking during their manufacturing process, which includes induction case hardening and electrogalvanizing steps. Fractographic examination of the exposed crack surfaces revealed intergranular cracking with some areas featuring a thin layer of iron oxide, indicating that the cracking took place after the electrogalvanizing step. The location of the cracks coincided with the position of the deepest case hardened layer, suggesting the occurrence of localized overheating during the induction case hardening step. Inductive heating finite element analysis (COSMOS Designstar Software) confirmed that during the case hardening the austenitising temperature reached in the crack region values of approximately 1050 °C. The results indicated that intergranular cracking was caused by hydrogen embrittlement. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction 1045 SAE steel chain links presented cracking during their manufacturing process, which included the following processing steps: plate cutting; plate bending; bore stamping; quenching and tempering (26–32 HRC); induction case hardening and tempering (50–55 HRC and effective depth of hardening, 515 HV1, between 3 and 4 mm), hard particle blasting and electrogalvanizing. According to information, the cracking took place just a ‘‘short time” after the last processing step. Literature [1–5] indicates that both case hardening and electrogalvanizing steps might cause overheating, residual stress and hydrogen charging, so the process control should be very strict to avoid the degradation of mechanical properties. 2. Experimental procedure and results The following exams were used in the present investigation: visual inspection, metallographic and microfractographic characterization, chemical microanalysis, microhardness profile and residual stress measurements. The chain link is shown in Fig. 1, indicating the position of the crack (adjacent to the bore with greater diameter) and the presence of a rather heterogeneous hardened layer depth (much deeper next to the larger bore). Fig. 2 shows the macrostructure of the chain link next to the larger bore (plane parallel to the crack). Besides the plate segregation (vertical darker line), it is possible to observe the intense effect of the induction case hardening, which promoted a steep microstructural gradient from the surface to the core of the chain link. Metallographic characterization was carried out next to the crack region (see Fig. 3a and b), revealing the tempered microstructure of the case hardened region, with a coarse prior austenitic grain size region, and the core region, with a much more refined prior austenitic grain. Microfractography of the exposed crack surfaces (see Fig. 4a and b) was carried out using a PHILIPS-XL-30 SEM microscope and the results are shown in Fig. 5a–d, revealing that the crack propagated by a brittle intergranular mode. EDS micro-
* Corresponding author. Tel.: +55 11 30915916; fax: +55 11 30914308. E-mail address: [email protected] (C.R.F. Azevedo). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.03.010
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Fig. 1. General aspect of the chain link, showing the heterogeneous hardened layer (darker area). The arrow indicates the crack position and the region used to the metallographic exam and microhardness test.
Fig. 2. Macrostructural aspect next to the larger bore caused by the induction case hardening treatment (plane parallel to the crack).
Fig. 3. Microstructural examination. (a) Aspect of the tempered martensitic microstructure next to the induction hardened surface (prior austenite grain size of approximately 100 lm); and (b) aspect of the tempered martensitic microstructure next to the bore, (prior austenite grain size around 10 lm).
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Fig. 4. Sampling for fractography. (a) Detail of the crack, perpendicular to the hardened surface; and (b) detail of the exposed crack surface, where the dark area corresponds to the crack and the clear area to the fracture, which was imposed in the laboratory to expose the crack surface.
Fig. 5. Fractographic examination (a) and (b) details of the exposed crack surface, indicating intergranular brittle cracking; (c) and (d) details of the fracture surface created in the laboratory, showing quasi-cleavage fracture, typical of tempered martensite.
analysis was carried out on the crack surfaces and the results (see Fig. 6) did not show the presence of zinc, indicating that the crack growth occurred after the electrogalvanizing step. Fig. 7 shows a three-dimensional reconstruction of the intergranular crack surface (using MEX software), showing the presence of large prior austenitic grains next to the hardened surface due to an excessive austenitization temperature. Inductive heating finite element analysis (COSMOS Designstar Software) confirmed that during the case hardening the austenitising temperature can reach values above 1050 °C in the crack region, while in the remaining regions this temperature is around 750 °C (see Fig. 8). Microhardness profile (Fig. 9) was carried and the effective depth of hardening (HV1 515) was determined as 10 mm in the crack regions. This value is much deeper than the product specification (between 3 and 4 mm), confirming once again the hypothesis of overheating during austenitization. Moreover, even areas away from the crack region showed high values, around 6 mm, for the effective depth of hardening (HV1 515), indicating that the control of the case hardening treatment should be optimized. Finally, the present investigation carried out measurements of the residual stress and retained austenite next to the larger bore, using a Rigaku diffractometer (Strainflex MSE – 2 M) with Cr Ka radiation. The Zn layer of the chain link was removed by immersing the entire part in a solution of HCl 20%, neutralizing it in a solution of NaOH 10% and carefully rinsing it off in water. The surfaces for XRD analysis were prepared by Struers Tenupol electropolishing unit, using an electrolyte of 30% HClOy in water at 25 °C and 40–60 V (see results in table 1).
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Fig. 6. EDS microanalysis on the darker region of the exposed crack surface, without the presence of Zinc peaks (see Fig. 5a).
Fig. 7. Three-dimensional reconstruction of the intergranular crack propagation surface (using MEX software), revealing the large austenitic grain size (100 lm) next to the hardened surface.
Fig. 8. COSMOS DesignstarÒ software FEA simulation of the induction heating, showing temperature distribution during the austenitisation (Note temperatures above 900 °C on the region next to the larger bore).
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Fig. 9. Microhardness profile (larger bore region), indicating the value of 10 mm for the effective depth of hardening.
Table 1 Superficial compressive residual stress results by XRDa (1: longitudinal; 2: tangential). Sample
Residual stress (MPa) (%)
Retained austenite (%)
Cracked
1
451 4772
1.2
No cracking
3921 4472
1.0
a
Scan range between 149° and 165°, step width: 0.32° and fixed time: 0.80 s.
3. Discussion The depth of the hardened layer showed a great variation in the effective depth of hardening (HV1 515), as can be observed in Figs. 1, 8 and 9. The results indicated that the case hardening process did not take into account the smaller ‘‘thermal mass” next to the larger bore, causing localized overheating during the austenitisation (see Fig. 8). The local overheating promoted excessive growth of the austenitic grains (see Figs. 3, 5 and 7), which might lead to both grain boundary segregation (embrittlement) and loss in toughness. EDS microanalysis on the crack surface did not show the presence of zinc on the crack surface, indicating that the cracking took place after the electrogalvanizing step (see Fig. 6). Microfractographic examination revealed that the crack propagation took place intergranularly by a brittle mode (see Figs. 5 and 7), while the crack induced in the laboratory took place by a transgranular cleavage mode. These results associated with the existence of a dwell time for the brittle cracking just after the electrogalvanizing step indicates the action of a hydrogen embrittlement mechanism (hydrogen stress cracking). Hydrogen stress cracking (HSC) is characterized by the brittle fracture due to hydrogen exposure (prior to manufacturing or from environmental hydrogen) and it often produces time-delayed (diffusion-controlled) fractures, even without external stress [4–6]. HSC can occur from hydrogen pickup during electrogalvanizing, which can cause high concentrations of atomic or nascent hydrogen to diffuse into the certain regions of the material. Because the Zn layer prevents the hydrogen from leaving the base metal, elevated-temperature baking after plating is required to allow the hydrogen to move to microstructural positions that are less damaging to the mechanical properties (see Fig. 11). HSC occurs most often in high-strength steels, which present hardness higher than 40 HRC or tensile strength higher than 1240 MPa. Heat treated medium-carbon steels have a greatly increased susceptibility to hydrogen embrittlement cracks when hardness is above 43 HRC, such as in the investigated case hardened chain link (approximate hardness value in the case hardened region is above 51 HRC) [4–6]. The results indicate that both case hardening and dehydrogenation treatments should be reviewed in order to avoid further failures. Moreover, the use of high austenitisation temperatures can promote a severe change in the superficial residual stress of tempered microstructures, which can vary from compressive residual stress (mainly due to the contribution of the volumetric expansion, which take places during the austenite ? martensite phase transformation) to tensile residual stress (mainly due to the contribution of the volumetric contraction of the surface during cooling), as shown qualitatively in Fig. 10a. The same effect on the sign of the residual stress can be observed for higher cooling rates (see Fig. 10b) [1–3]. As the presence of tensile residual stress might decrease the susceptibility limits for the hydrogen embrittlement, the investigation carried out measurements of the residual stress by X-ray diffraction to measure the residual stress next to
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Fig. 10. (a) Correlation between the surface austenitisation temperature during the induction case hardening and the value and type of residual stress [1]; and (b) correlation between the maximum cooling rate after the austenitisation during the induction case hardening and the value and type of the residual stress [2].
Fig. 11. Effect of the baking dehydrogenation treatment on the mechanical resistance of high resistance steel. Time to fracture as a function of applied stress and aging (baking) time at 150 °C for notched tensile specimens of AISI 4340 that were hydrogen charged and electroplated with cadmium [5].
the larger bore. The results for both products (cracked and sound chain links) indicated the presence of compressive residual stress of around 450 MPa on the hardened surfaces (see Table 1), suggesting that HSC can take place in regions presenting compressive residual stress. A more detailed measurement mapping of the residual stresses around the larger bore and the use of residual stress FEA might be necessary to confirm this suggestion. 4. Conclusions The intergranular cracking of the link was caused by hydrogen stress cracking. The presence of compressive residual stress of around 450 MPa on the hardened surfaces of the link was not sufficient to prevent the hydrogen stress cracking. Both case hardening and dehydrogenation treatments should be reviewed in order to avoid further failures.
C.R.F. Azevedo et al. / Engineering Failure Analysis 16 (2009) 2311–2317
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Acknowledgments The authors would like to thank Tenaris-Confab for the financial support for the modernization of the Laboratório de Caracterização Microstructural Hubertus Colpaert. Additional thanks to Aços Villares Sidenor and Thyssenkrupp Metalúrgica for technical and laboratorial support during the XRD residual stress measurements. References [1] Lee MK, Kim GH, Kim KH, Kim WW. Effects of the surface temperature and cooling rate on the residual stresses in a flame hardening of 12Cr steel. J Mater Process Technol 2006;176(1–3):140–5. [2] Lee MK, Kim GH, Kim KH, Kim WW. Control of surface hardnesses, hardening depths and residual stresses of low carbon 12Cr steel by flame hardening. Surf Coat Technol 2004;184(2–3):239–46. [3] Camarão AC. Um modelo para previsão de tensões residuais em cilindros de aços temperados por indução. Tese de doutorado. Escola Politécnica da Universidade de São Paulo; 1998. [4] ASM Handbook. Hydrogen damage and embrittlement. Failure analysis and prevention. vol. 10, 10th ed. ASM International; 2002. [5] ASM Handbook. Properties and selection: irons, steels, and high-performance alloys. vol. 1, 10th ed. ASM International; 1990. [6] Azevedo CRF. Failure analysis of a crude oil pipeline. Eng Fail Anal 2007;14(6):978–94.