PSI - Issue 79

Hiroshi Nishiguchi et al. / Procedia Structural Integrity 79 (2026) 517–523

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1. Introduction Hydrogen embrittlement, which reduces material strength, remains a critical concern in hydrogen-related applications. Extensive studies have been conducted to elucidate its mechanisms and effects on structural materials. Early works by Troiano (1960) and Beachem (1972) proposed fundamental models for hydrogen-assisted cracking, while Hirth (1980) and Oriani and Josephic (1974) investigated hydrogen influence on mechanical properties and trapping behaviour in steels. Later studies by Birnbaum and Sofronis (1994, 1995) introduced the concept of hydrogen-enhanced localized plasticity (HELP), providing further insights into the interaction between hydrogen and dislocations. These studies collectively form the scientific foundation for understanding and mitigating hydrogen embrittlement in modern hydrogen infrastructures. Although certain materials, such as aluminum alloy A6061-T6 and austenitic stainless steel SUS316L, exhibit high resistance to hydrogen embrittlement, commonly used and cost effective materials such as carbon and low-alloy steels are susceptible (NASA, 1997). Fatigue crack growth behaviour in hydrogen environments has been extensively investigated, revealing that crack propagation is significantly accelerated while maintaining crack sharpness (Matsuda et al., 2016; Murakami et al., 2008; Yamabe et al., 2017). However, it has also been reported that crack growth acceleration has an upper limit, typically about 30 times faster than that in the absence of hydrogen (Matsuda et al., 2016; Yamabe et al., 2017). To mitigate this issue, various methods for preventing hydrogen permeation have been investigated. Previous work has demonstrated that a heat-treatment coating with aluminum powder effectively prevents hydrogen ingress. (Nishiguchi et al., 2024). Different steels heat-treated with aluminum powder were exposed to hydrogen gas at 100 MPa for 24 h; testing confirmed a hydrogen cut ratio of 100%. Furthermore, tensile tests and surface observations with the replica method revealed that microcracks began to initiate at coating part when the applied stress exceeded the yield strength. Nevertheless, this coating process has a limitation in that the high-temperature treatment induces an annealing effect, resulting in a decrease in the mechanical strength of the substrate. The objective of this study is to clarify the hydrogen cut ratio when the strength of the base material is improved through subsequent heat treatments, such as quenching, applied to four types of coated materials. Furthermore, this study aims to elucidate the hydrogen ingress characteristics by observing the changes in the coating condition induced by the heat treatment. 2. Experimental method In this study, four types of steel—S45C, SCM435, FC200, and SUS304—were used for the experiments after coating application. Specimens were prepared by cutting 4-mm-thick chips from 14-mm-diameter round bars using a fine cutter. The specimens were subsequently polished with emery paper (#100–#600) before coating. The coating process enables the control of the aluminum surface concentration by adjusting the heat-treatment conditions. In this study, a uniform coating with an aluminum concentration of 50% was applied to a depth of 20 μm below the surface of the base material. Table 1 lists the additional heat-treatment conditions. The coated S45C, SCM435, and FC200 steels were subjected to either quenching or quenching followed by tempering. Although SUS304 steel is inherently resistant to hydrogen ingress and does not require coating, solution heat treatment was performed instead of these heat treatments. Solution heat treatment involves uniformly heating a metal to a high temperature, followed by rapid cooling. Austenitic stainless steels generally do not experience significant increases in tensile strength or hardness through quenching because they do not transform to martensite. Hydrogen permeation measurements were conducted after exposing the heat-treated coated specimens to hydrogen gas at 100 MPa and 85 °C for 24 h using a thermal desorption analyzer (TDA, gas chromatography based). Two conditions were compared: (A) coated specimens that were exposed to hydrogen and had the coating removed before measuring the hydrogen content, and (B) uncoated specimens from which the coating was removed prior to hydrogen exposure. The hydrogen cut ratio was calculated from these results. The conditions of the coatings after heat treatment were observed using an optical microscope. Vickers hardness measurements were performed on all specimens under a load of 1 kgf for 30 s. Additionally, the composition of the coated layer was analyzed using energy-dispersive X-ray spectroscopy (EDS).