Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔK_th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔK_th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔK_th in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔK_th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔK_th in the hydrogen gas environment.