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Hydrogen embrittlement of a hard chromium plated cylinder assembly
Engineering Failure Analysis 103 (2019) 259–265
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Hydrogen embrittlement of a hard chromium plated cylinder assembly
T
L.S. Araujo , L.H. de Almeida, D.S. dos Santos ⁎
PEMM/COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Failure analysis Hard chromium plating Hydrogen embrittlement
Plating of high strength steels with a hard chromium layer is widely used in applications where wear and corrosion resistant surfaces are a requisite, with a low coefficient of friction or in the refinishing of worn surfaces in order to restore the original dimensions or to salvage mismachined components. Hydrogen evolution can occur during the processing steps previously to the plating or during the plating itself and this hydrogen can diffuse into the material, with an embrittling effect which is augmented in high strength steel as the 17-4 PH. In the present analysis a cylinder assembly was refinished with a hard chromium layer and a large crack was evidenced after the plating process. This failure was analised by metallographic, fractographic, mechanical and temperature programed desorption analyses, with the results corroboration the hypothesis of hydrogen embrittlement.
1. Introduction Plating of high strength steels with a hard chromium layer is widely used in applications where wear and corrosion resistant surfaces are a requisite, with a low coefficient of friction. Components plated find different applications as engine parts or cutting tools [1,2]. For such applications the layer can vary from 1 to 500 μm and the chromium is electrodeposited on the high strength steel substrate by means of a bath which main components are chromic acid (CrO3) and a sulfate anion catalyst (SO4−2) [1,3,4]. Temperature and current density are controlled, in order to provide a smooth and continuous layer. Another important application is in the refinishing of worn surfaces, restoring the original dimensions or to salvage mismachined components [1,5]. As reported by Gabe [6], additionally to the chromium reaction, during the electrodeposition process there is hydrogen evolution at the cathode from the following reactions:
2H+ + 2e 2H2 O + 2e
H2 or H2 + 2OH
Additional sources of hydrogen can be provided by processes as cleaning, pickling or etching. The effects of hydrogen during electrodeposition can be summarized as follows [6]: - H absorption by the substrate as atomic H, inducing hydrogen embrittlement; - H bubbles in the substract's surface, leading to pores resulting from deposition of the layer around the bubbles; - stirring effect due to the H bubbles evolution, resulting in bubble raft at the solution free surface;
⁎
Corresponding author. E-mail address: [email protected] (L.S. Araujo).
https://doi.org/10.1016/j.engfailanal.2019.04.052 Received 26 October 2018; Received in revised form 18 April 2019; Accepted 25 April 2019 Available online 26 April 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 1. Cylinder assembly. The arrows indicate the extremities of the crack.
For the case of electroplating of high strength steels, the evolution of hydrogen can result in significant embrittlement of the substrate, with the susceptibility increasing with the strength of the steel [1,4,7]. In order to mitigate the H effect, stress relief heat treatments are conducted, prior to the cleaning and plating processes [4]. Furthermore, a baking heat treatment is conducted to diffuse hydrogen away from the material, with parameters adjusted to the specific strength level of the substrate and size of the sample [4]. An example of such treatment for high strength steel is: 177 to 205 °C for 4 h or more [3,4]. 2. Failure analysis In the present case, a cylinder assembly of a 17-4 PH steel was refinished via electrodeposition of a hard chromium layer on the internal surface of the component. During inspection procedure, a large crack was observed, with the direction approximately parallel to the main axis of the cylinder, with a length between 60 and 65 mm, located at a region of the main body of the cylinder assembly free from welds. The crack was apparent at the internal surface as well. No cracks were observed around the welds or threads. Fig. 1 shows the cylinder assembly and the region where the defect is located. The main body of the assembly can be considered a tube with internal diameter of 25.40 mm and wall thickness of 1.55 mm, which is the section with the thinnest wall of the component. 3. Materials and methods Samples were cut for microstructural characterization, fractography, chemical analysis, hydrogen desorption tests and microhardness Vickers measurements. The cutting of samples was made on a low speed diamond saw with alcohol in order to minimize heating of the sample and loss of hydrogen due to diffusion. The samples for microstructural characterization and microhardness measurements were mounted in cold resin, ground with 100 to 2400-grit water cooled SiC paper, polished with diamond paste of 6, 3 and 1 μm and etched by swabbing with Vilella's solution for 80 s. Five microhardness measurements were made only at the substrate with a load of 500gf. The fractography sample was cleaned in an immersion of acetone by ultrasonic agitation. The chemical composition was analyzed by optical emission spectroscopy and the observations of microstructure, hard layer surface and fractography were made via scanning electron microscopy (SEM) in secondary (BSE) and backscattering (BSC) electrons modes and also via energy dispersive spectroscopy (EDS). Estimates of the phases present were performed with Thermodynamic calculatios using Thermocalc software (version 2018b) with the TCFE8 Steels/Fe-alloys database [8]. The amount of hydrogen dissolved in the material was determined by hydrogen desorption in a temperature programmed desorption (TPD) apparatus from 20 to 600 °C and heating rate of 10 °C/min. The approximate dimensions of the sample were 1.5 × 4 × 8 mm. The TPD technique is based on the heating of a sample at a constant rate up to a temperature that assures the complete desorption of the hydrogen dissolved and/or trapped in the sample. During the heating the sample is exposed to a reference gas (a mixture argon-1.5% hydrogen) with a known thermal conductivity. The hydrogen desorbed off the sample induces a change in the thermal conductivity of the gas, which is measured by a detector. To determine the amount of hydrogen trapped, the resulting curve is integrated and compared to the result from the curve for the reference gas, giving the total amount of hydrogen in the sample. 4. Results Based on the chemical analysis of the steel substrate, it was identified as a 17-4 PH stainless steel with a microstructure consisting of fine tempered martensite. Table 1 presents its chemical composition and Fig. 2 shows the microstructure of the substrate. A chemical mapping made by EDS confirmed the chromium layer only at the inner wall of the cylinder assembly and is presented in Fig. 3. The observation of the transversal section of the cylinder revealed that cracks were initiated at the interface between the substrate 260
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Table 1 chemical composition of the substrate (wt%). C
Si
Mn
P
S
Cr
Ni
Mo
Al
Cu
Co
Ti
Nb
V
W
B
Sn
As
Fe
0.0034
0.43
0.54
0.016
0.009
15.63
4.05
0.32
0.014
3.52
0.073
0.024
0.189
0.092
0.035
0.002
0.007
0.022
Bal.
Fig. 2. Microstructure of the substrate (17-4 PH steel), identified as tempered martensite.
Fig. 3. (a): SEM image of the steel substrate and; (b) chromium chemical mapping; (c) iron chemical mapping; 261
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 4. Transversal view of the cylinder assembly, with the main crack and other secondary cracks radiating from the interface between chromium layer and steel substrate end from the main crack.
and hard chromium layer as indicated in Fig. 4. It is interesting to note that close to the origin of the cracks there was an apparent failure of the chromium layer. Based on this observation, SEM images were taken from the surface of the hard chromium layer, revealing several points with a failure with a circular morphology, as indicated in Fig. 5. Such defects were verified in several sites along the hard chromium layer. The microhardness average value on the steel substrate was 419.1 ± 3.2 HV and was consistent with a microstructure of tempered martensite, precipitation hardened [9]. The conversion of this value to Rockwell C scale was based on ASTM E140-12b [10] standard, resulting in a value between 42 and 43 HRC. The fractography revealed considerate evidences of hydrogen embrittlement in the samples, with a predominant intergranular fracture along prior austenite grain boundaries distributed throughout the thickness of the cylinder assembly. Regions of quasicleavage aspect were also evidenced. Such features are characteristic of martensitic samples exposed to a hydrogen environment, as reported in [11,12]. Fig. 6 shows both fracture morphologies. Cracks along the hard chromium layer were observed and, as reported by Lin et al. [13], the cracking of the chromium layer occurs to thickness greater than 0.5 μm and it is related to the formation of chromium hydride, which tends to decompose during electroplating, causing a 15% reduction in volume and consequent tensile stress and cracking. Fig. 7 shows the cracking along the hard chromium layer deposited. The temperature programmed desorption test indicated that the hydrogen desorption main process occurred between 190 and 370 °C, with multiple peaks occurring at 261, 301 and 325 °C. These peaks can be related to the multiple hydrogen traps distributed throughout the microstructure as: dislocations, lath and grain boundaries, Cu-rich precipitates and carbides (NbC and M23C6). However, due to the low carbon concentration on the alloy, the mass fraction of carbides is expected to be low. This is corroborated by the estimates from thermodynamic calculations showing that the mass fraction of M23C6 and MC carbides would be about 0.067 and 0.034%, respectively, which would result in a not relevant amount of trapping sites. Komazaki et al. [14] studied a copper-added ultra-low carbon steel and related the trapping capacity to the iron/matrix/copper particles interface and/or the copper precipitates themselves. Li et al. [15] pointed out that the lath boundaries, prior austenite grain boundaries and the nano-sized copper precipitates act as trapping sites. The thermodynamic calculations indicated that the mass fraction of copper precipitates can reach up to 3.6%. Fig. 8 shows the variation of the mass fraction of precipitates with temperature. These traps present different binding energies, which would reflect on the temperature that hydrogen is desorbed from each microstructural feature, as well as the area under the curve.
Fig. 5. Failure of the hard chromium layer, presenting a circular morphology. 262
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 6. Fracture morphology observed in the main crack: (a) quasi-cleavage aspect; (b) intergranular.
Fig. 7. SEM image of cracks along the hard chromium layer.
Fig. 9 presents the desorption graph with the curve fitting and deconvoluted peaks. The amount of hydrogen trapped in the microstructure of the sample could be determined, because it is proportional to the area under the desorption curve and the weight of the sample. The value calculated of total trapped hydrogen was 3.4 ppm (parts per million). 5. Discussion As high strength steel 17-4 PH is very prone to hydrogen embrittlement, it is critical to minimize hydrogen pick up during processing. As described in the previous section, there are several sources of hydrogen during cleaning, pickling or plating processes and these shall be controlled accordingly. As reported by Louthan [16], the concentration of nascent hydrogen from catholic reactions is very high and hydrogen can be absorbed in the metal. 263
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 8. Variation of the mass fraction of the copper precipitates, MC and M23C6 carbides with temperature, obtained by thermodynamic calculation.
Fig. 9. Graph of temperature programmed desorption of a sample, with multiple peaks related to different microstructural features.
In the same work it is reported that the martensitic structure can be severely degraded by relative low concentrations of hydrogen with the degree of embrittlement increasing with the strength level of the steel. Wanhill et al. [12] informed that the high and ultrahigh strength martensitic steels, with hardness greater than 37 HRC, can be significantly embrittled by hydrogen with solute concentrations as low as 1 ppm. The embrittling effect is augmented if tensile residual stresses are developed in the material, as result from processes as grinding or machining [4,12]. In order to mitigate the embrittlement, if the hardness of the component are above a specified amount, stress relieving heat treatments are indicated before the plating stage and baking heat treatments are indicated after the plating, the latter to promote hydrogen embrittlement relief [1,4,17,18]. It is important to remark that the baking heat treatment shall not be realized a long time after the plating process, as an interaction of hydrogen with residual stresses can occur, promoting irreversible damage to the material. The steel component analyzed presented a hardness value between 42 and 43 HRC. In this case, both stress relieving and baking heat treatment are recommended, in order to avoid potential hydrogen embrittlement [1]. However, based on the measurements of trapped hydrogen in the sample (3.4 ppm), a considerate amount this element was left in the sample, which can indicate severe hydrogen pickup during the processing and that the baking heat treatment was ineffective or did not occur. An interesting feature to consider was the occurrence of the frequent plating failures, with circular shapes, as presented in Fig. 5. As described in [6,19], hydrogen bubbles on the surface of the substrate can lead to the growth of pores, as the plating form around the bubbles, as shown schematically in Fig. 10. This phenomenon, identified as “gas pit” [20], is an indication of intense hydrogen evolution during the plating process and trapping of gas bubbles on the surface, corroboration the hydrogen embrittlement hypothesis. 264
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 10. Schematic representation of the bubble formation influence on the hard chromium layer deposition, forming a “gas pit”.
6. Conclusions Based on the observations made in this analysis the hypothesis of hydrogen embrittlement of the 17-4PH steel substrate could be confirmed based on the following aspects:
• The failure of the steel cylinder assembly associated with the hydrogen embrittlement of the steel, with the cracking originating on the interface between the substrate and plating; • The main source of hydrogen was associated to the hard chromium plating process; • The crack propagation was predominantly intergranular, along prior austenite grain boundaries. However, regions with a quasicleavage morphology were observed as well, both associated with the influence of hydrogen; • The stress relieving and/or baking heat treatment, crucial steps to prevent hydrogen embrittlement in the high strength steel • •
substrate (42–43 HRC), were not conducted properly or were ineffective, as a high amount of trapped hydrogen was measured in the sample (3.4 ppm) by means of a TPD test; The trapping of hydrogen in the microstructure was attributed to the lath boundaries, prior austenite grain boundaries and copper precipitates; H2 bubbles formed during the plating process prevented the formation of hard chromium layer and acted as additional sources of hydrogen to the steel.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
K.R. Newby, Industrial (hard) chromium plating, ASM Handbook - Surface Engineering, ASM International, Materials Park, Ohio, 1994. K. Ranjbar, M. Sababi, Failure assessment of the hard chrome coated rotors in the downhole drilling motors, Eng. Fail. Anal. 20 (2012) 147–155. MIL-STD-1501F, Chromium Plating, Low Embrittlement, Electrodeposition, Standard Practice, Department of Defense, 2011, p. 11. B177/B177M-11 - Standard Guide for Engineering Chromium Electroplating, ASTM International, West Conshohocken, PA, 2011. L.M.I. Raj, S.J.B. Kumaragurubaran, P. Gopal, Analysis of hard chromium coating defects and its prevention methods, Int. J. Eng. Adv. Technol. 2 (2013) 427–432. D.R. Gabe, The role of hydrogen in metal electrodeposition processes, J. Appl. Electrochem. 27 (1997) 908–915. N. Eliaz, A. Shachar, B. Tal, D. Eliezer, Characteristics of hydrogen embrittlement, stress corrosion cracking and tempered martensite embrittlement in highstrength steels, Eng. Fail. Anal. 9 (2002) 167–184. Thermo-Calc Software TCFE9 Steels/Fe-Alloys Database (accessed April, 15th 2019). U.K. Viswanathan, S. Banerjee, R. Krishnan, Effects of aging on the microstructure of 17-4 PH stainless steel, Mater. Sci. Eng. A 104 (1988) 181–189. E140-12b, Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, Scleroscope Hardness, and Leeb Hardness, ASTM International, West Conshohocken, PA, 2012. L.W. Tsay, W.C. Lee, R.K. Shiue, J.K. Wu, Notch tensile properties of laser-surface-annealed 17-4 PH stainless steel in hydrogen-related environments, Corros. Sci. 44 (2002) 2101–2118. R. Wanhill, S. Barter, S. Lynch, D. Gerrard, Prevention of hydrogen embrittlement in high strength steels, with emphasis on reconditioned aircraft components, in: N.S.A.T. Organization (Ed.), RTO-AG-AVT-140 - Corrosion Fatigue and Environmentally Assisted Cracking in Aging Military Vehicles, 2011, p. 434. K.L. Lin, C.J. Hsu, I.M. Hsu, J.T. Chang, Electroplating of Ni-Cr on steel with pulse plating, JMEP 1 (1992) 359–361. S.-I. Komazaki, A. Koyama, T. Misawa, Effect of morphology of copper precipitation particles on hydrogen embrittlement behaviour in Cu-added ultra low carbon steel, Mater. Trans. (9) (2002) 2213–2218. X. Li, J. Zhang, Q. Fu, E. Akiyama, X. Song, S. Shen, Q. Li, Hydrogen embrittlement of high strength steam turbine last stage blade steels: comparison between PH17-4 steel and PH13-8Mo steel, Mater. Sci. Eng. A 742 (2019) 353–363. M.R. Louthan Jr., Hydrogen embrittlement of metals: a primer for the failure analyst, J. Fail. Anal. Prev. 8 (2008) 289–307. B849-02, Standard Specification for Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement, ASTM International, West Conshohocken, PA, 2013. B850-98, Standard Guide for Post-Coating Treatments of Steel for Reducing the Risk of Hydrogen Embrittlement, ASTM International, ASTM International, West Conshohocken, PA, 2009. Technical report: hard chromium plating, Mater. Des. 5 (1984) 126–128. N.V. Mandich, Practical considerations in bright and hard chromium plating—part III, Met. Finish. 97 (1999) 42–45.
265
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Hydrogen embrittlement of a hard chromium plated cylinder assembly
T
L.S. Araujo , L.H. de Almeida, D.S. dos Santos ⁎
PEMM/COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Failure analysis Hard chromium plating Hydrogen embrittlement
Plating of high strength steels with a hard chromium layer is widely used in applications where wear and corrosion resistant surfaces are a requisite, with a low coefficient of friction or in the refinishing of worn surfaces in order to restore the original dimensions or to salvage mismachined components. Hydrogen evolution can occur during the processing steps previously to the plating or during the plating itself and this hydrogen can diffuse into the material, with an embrittling effect which is augmented in high strength steel as the 17-4 PH. In the present analysis a cylinder assembly was refinished with a hard chromium layer and a large crack was evidenced after the plating process. This failure was analised by metallographic, fractographic, mechanical and temperature programed desorption analyses, with the results corroboration the hypothesis of hydrogen embrittlement.
1. Introduction Plating of high strength steels with a hard chromium layer is widely used in applications where wear and corrosion resistant surfaces are a requisite, with a low coefficient of friction. Components plated find different applications as engine parts or cutting tools [1,2]. For such applications the layer can vary from 1 to 500 μm and the chromium is electrodeposited on the high strength steel substrate by means of a bath which main components are chromic acid (CrO3) and a sulfate anion catalyst (SO4−2) [1,3,4]. Temperature and current density are controlled, in order to provide a smooth and continuous layer. Another important application is in the refinishing of worn surfaces, restoring the original dimensions or to salvage mismachined components [1,5]. As reported by Gabe [6], additionally to the chromium reaction, during the electrodeposition process there is hydrogen evolution at the cathode from the following reactions:
2H+ + 2e 2H2 O + 2e
H2 or H2 + 2OH
Additional sources of hydrogen can be provided by processes as cleaning, pickling or etching. The effects of hydrogen during electrodeposition can be summarized as follows [6]: - H absorption by the substrate as atomic H, inducing hydrogen embrittlement; - H bubbles in the substract's surface, leading to pores resulting from deposition of the layer around the bubbles; - stirring effect due to the H bubbles evolution, resulting in bubble raft at the solution free surface;
⁎
Corresponding author. E-mail address: [email protected] (L.S. Araujo).
https://doi.org/10.1016/j.engfailanal.2019.04.052 Received 26 October 2018; Received in revised form 18 April 2019; Accepted 25 April 2019 Available online 26 April 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 1. Cylinder assembly. The arrows indicate the extremities of the crack.
For the case of electroplating of high strength steels, the evolution of hydrogen can result in significant embrittlement of the substrate, with the susceptibility increasing with the strength of the steel [1,4,7]. In order to mitigate the H effect, stress relief heat treatments are conducted, prior to the cleaning and plating processes [4]. Furthermore, a baking heat treatment is conducted to diffuse hydrogen away from the material, with parameters adjusted to the specific strength level of the substrate and size of the sample [4]. An example of such treatment for high strength steel is: 177 to 205 °C for 4 h or more [3,4]. 2. Failure analysis In the present case, a cylinder assembly of a 17-4 PH steel was refinished via electrodeposition of a hard chromium layer on the internal surface of the component. During inspection procedure, a large crack was observed, with the direction approximately parallel to the main axis of the cylinder, with a length between 60 and 65 mm, located at a region of the main body of the cylinder assembly free from welds. The crack was apparent at the internal surface as well. No cracks were observed around the welds or threads. Fig. 1 shows the cylinder assembly and the region where the defect is located. The main body of the assembly can be considered a tube with internal diameter of 25.40 mm and wall thickness of 1.55 mm, which is the section with the thinnest wall of the component. 3. Materials and methods Samples were cut for microstructural characterization, fractography, chemical analysis, hydrogen desorption tests and microhardness Vickers measurements. The cutting of samples was made on a low speed diamond saw with alcohol in order to minimize heating of the sample and loss of hydrogen due to diffusion. The samples for microstructural characterization and microhardness measurements were mounted in cold resin, ground with 100 to 2400-grit water cooled SiC paper, polished with diamond paste of 6, 3 and 1 μm and etched by swabbing with Vilella's solution for 80 s. Five microhardness measurements were made only at the substrate with a load of 500gf. The fractography sample was cleaned in an immersion of acetone by ultrasonic agitation. The chemical composition was analyzed by optical emission spectroscopy and the observations of microstructure, hard layer surface and fractography were made via scanning electron microscopy (SEM) in secondary (BSE) and backscattering (BSC) electrons modes and also via energy dispersive spectroscopy (EDS). Estimates of the phases present were performed with Thermodynamic calculatios using Thermocalc software (version 2018b) with the TCFE8 Steels/Fe-alloys database [8]. The amount of hydrogen dissolved in the material was determined by hydrogen desorption in a temperature programmed desorption (TPD) apparatus from 20 to 600 °C and heating rate of 10 °C/min. The approximate dimensions of the sample were 1.5 × 4 × 8 mm. The TPD technique is based on the heating of a sample at a constant rate up to a temperature that assures the complete desorption of the hydrogen dissolved and/or trapped in the sample. During the heating the sample is exposed to a reference gas (a mixture argon-1.5% hydrogen) with a known thermal conductivity. The hydrogen desorbed off the sample induces a change in the thermal conductivity of the gas, which is measured by a detector. To determine the amount of hydrogen trapped, the resulting curve is integrated and compared to the result from the curve for the reference gas, giving the total amount of hydrogen in the sample. 4. Results Based on the chemical analysis of the steel substrate, it was identified as a 17-4 PH stainless steel with a microstructure consisting of fine tempered martensite. Table 1 presents its chemical composition and Fig. 2 shows the microstructure of the substrate. A chemical mapping made by EDS confirmed the chromium layer only at the inner wall of the cylinder assembly and is presented in Fig. 3. The observation of the transversal section of the cylinder revealed that cracks were initiated at the interface between the substrate 260
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Table 1 chemical composition of the substrate (wt%). C
Si
Mn
P
S
Cr
Ni
Mo
Al
Cu
Co
Ti
Nb
V
W
B
Sn
As
Fe
0.0034
0.43
0.54
0.016
0.009
15.63
4.05
0.32
0.014
3.52
0.073
0.024
0.189
0.092
0.035
0.002
0.007
0.022
Bal.
Fig. 2. Microstructure of the substrate (17-4 PH steel), identified as tempered martensite.
Fig. 3. (a): SEM image of the steel substrate and; (b) chromium chemical mapping; (c) iron chemical mapping; 261
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 4. Transversal view of the cylinder assembly, with the main crack and other secondary cracks radiating from the interface between chromium layer and steel substrate end from the main crack.
and hard chromium layer as indicated in Fig. 4. It is interesting to note that close to the origin of the cracks there was an apparent failure of the chromium layer. Based on this observation, SEM images were taken from the surface of the hard chromium layer, revealing several points with a failure with a circular morphology, as indicated in Fig. 5. Such defects were verified in several sites along the hard chromium layer. The microhardness average value on the steel substrate was 419.1 ± 3.2 HV and was consistent with a microstructure of tempered martensite, precipitation hardened [9]. The conversion of this value to Rockwell C scale was based on ASTM E140-12b [10] standard, resulting in a value between 42 and 43 HRC. The fractography revealed considerate evidences of hydrogen embrittlement in the samples, with a predominant intergranular fracture along prior austenite grain boundaries distributed throughout the thickness of the cylinder assembly. Regions of quasicleavage aspect were also evidenced. Such features are characteristic of martensitic samples exposed to a hydrogen environment, as reported in [11,12]. Fig. 6 shows both fracture morphologies. Cracks along the hard chromium layer were observed and, as reported by Lin et al. [13], the cracking of the chromium layer occurs to thickness greater than 0.5 μm and it is related to the formation of chromium hydride, which tends to decompose during electroplating, causing a 15% reduction in volume and consequent tensile stress and cracking. Fig. 7 shows the cracking along the hard chromium layer deposited. The temperature programmed desorption test indicated that the hydrogen desorption main process occurred between 190 and 370 °C, with multiple peaks occurring at 261, 301 and 325 °C. These peaks can be related to the multiple hydrogen traps distributed throughout the microstructure as: dislocations, lath and grain boundaries, Cu-rich precipitates and carbides (NbC and M23C6). However, due to the low carbon concentration on the alloy, the mass fraction of carbides is expected to be low. This is corroborated by the estimates from thermodynamic calculations showing that the mass fraction of M23C6 and MC carbides would be about 0.067 and 0.034%, respectively, which would result in a not relevant amount of trapping sites. Komazaki et al. [14] studied a copper-added ultra-low carbon steel and related the trapping capacity to the iron/matrix/copper particles interface and/or the copper precipitates themselves. Li et al. [15] pointed out that the lath boundaries, prior austenite grain boundaries and the nano-sized copper precipitates act as trapping sites. The thermodynamic calculations indicated that the mass fraction of copper precipitates can reach up to 3.6%. Fig. 8 shows the variation of the mass fraction of precipitates with temperature. These traps present different binding energies, which would reflect on the temperature that hydrogen is desorbed from each microstructural feature, as well as the area under the curve.
Fig. 5. Failure of the hard chromium layer, presenting a circular morphology. 262
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 6. Fracture morphology observed in the main crack: (a) quasi-cleavage aspect; (b) intergranular.
Fig. 7. SEM image of cracks along the hard chromium layer.
Fig. 9 presents the desorption graph with the curve fitting and deconvoluted peaks. The amount of hydrogen trapped in the microstructure of the sample could be determined, because it is proportional to the area under the desorption curve and the weight of the sample. The value calculated of total trapped hydrogen was 3.4 ppm (parts per million). 5. Discussion As high strength steel 17-4 PH is very prone to hydrogen embrittlement, it is critical to minimize hydrogen pick up during processing. As described in the previous section, there are several sources of hydrogen during cleaning, pickling or plating processes and these shall be controlled accordingly. As reported by Louthan [16], the concentration of nascent hydrogen from catholic reactions is very high and hydrogen can be absorbed in the metal. 263
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 8. Variation of the mass fraction of the copper precipitates, MC and M23C6 carbides with temperature, obtained by thermodynamic calculation.
Fig. 9. Graph of temperature programmed desorption of a sample, with multiple peaks related to different microstructural features.
In the same work it is reported that the martensitic structure can be severely degraded by relative low concentrations of hydrogen with the degree of embrittlement increasing with the strength level of the steel. Wanhill et al. [12] informed that the high and ultrahigh strength martensitic steels, with hardness greater than 37 HRC, can be significantly embrittled by hydrogen with solute concentrations as low as 1 ppm. The embrittling effect is augmented if tensile residual stresses are developed in the material, as result from processes as grinding or machining [4,12]. In order to mitigate the embrittlement, if the hardness of the component are above a specified amount, stress relieving heat treatments are indicated before the plating stage and baking heat treatments are indicated after the plating, the latter to promote hydrogen embrittlement relief [1,4,17,18]. It is important to remark that the baking heat treatment shall not be realized a long time after the plating process, as an interaction of hydrogen with residual stresses can occur, promoting irreversible damage to the material. The steel component analyzed presented a hardness value between 42 and 43 HRC. In this case, both stress relieving and baking heat treatment are recommended, in order to avoid potential hydrogen embrittlement [1]. However, based on the measurements of trapped hydrogen in the sample (3.4 ppm), a considerate amount this element was left in the sample, which can indicate severe hydrogen pickup during the processing and that the baking heat treatment was ineffective or did not occur. An interesting feature to consider was the occurrence of the frequent plating failures, with circular shapes, as presented in Fig. 5. As described in [6,19], hydrogen bubbles on the surface of the substrate can lead to the growth of pores, as the plating form around the bubbles, as shown schematically in Fig. 10. This phenomenon, identified as “gas pit” [20], is an indication of intense hydrogen evolution during the plating process and trapping of gas bubbles on the surface, corroboration the hydrogen embrittlement hypothesis. 264
Engineering Failure Analysis 103 (2019) 259–265
L.S. Araujo, et al.
Fig. 10. Schematic representation of the bubble formation influence on the hard chromium layer deposition, forming a “gas pit”.
6. Conclusions Based on the observations made in this analysis the hypothesis of hydrogen embrittlement of the 17-4PH steel substrate could be confirmed based on the following aspects:
• The failure of the steel cylinder assembly associated with the hydrogen embrittlement of the steel, with the cracking originating on the interface between the substrate and plating; • The main source of hydrogen was associated to the hard chromium plating process; • The crack propagation was predominantly intergranular, along prior austenite grain boundaries. However, regions with a quasicleavage morphology were observed as well, both associated with the influence of hydrogen; • The stress relieving and/or baking heat treatment, crucial steps to prevent hydrogen embrittlement in the high strength steel • •
substrate (42–43 HRC), were not conducted properly or were ineffective, as a high amount of trapped hydrogen was measured in the sample (3.4 ppm) by means of a TPD test; The trapping of hydrogen in the microstructure was attributed to the lath boundaries, prior austenite grain boundaries and copper precipitates; H2 bubbles formed during the plating process prevented the formation of hard chromium layer and acted as additional sources of hydrogen to the steel.
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