Hydrogen embrittlement of CrMo and CrMoV pressure vessel steels

The variation of $\text{K}{}_{\mathrm{i}}$ with time to fracture and the threshold values of $\text{K}{}_{\mathrm{ISH}}$ in an hydrogen environment were measured for four CrMo and CrMoV pressure vessel steels in various conditions of heat treatment. The CrMoV grades displayed higher $\text{K}{}_{\mathrm{ISH}}$ thresholds, suggesting that they are less susceptible to hydrogen embrittlement than CrMo grades of comparable strength. The findings were analysed with regard to the concurrent presence of various alloying elements and their effects on the microsegregation of detrimental impurity elements at the austenitic grain boundaries, where these harmful elements can, in conjunction with hydrogen, cause intercrystalline embrittlement. Studies were also made of the kinetics of stable crack growth in these materials.