Abstract
The relationship between changes in hydrogen distribution under tensile stress and hydrogen embrittlement with intergranular (IG) fracture of a tempered martensitic steel was investigated using the frozen-in hydrogen distribution technique. The hydrogen distribution under stress-free conditions and under elastic stress at room temperature (R.T.) was frozen in liquid nitrogen at -196 °C, at which temperature hydrogen diffusion is suppressed, followed by tensile testing. Tensile testing at various rates with a frozen-in hydrogen distribution in the absence of stress led to neither a ductility loss due to hydrogen nor a dependence of the hydrogen embrittlement susceptibility (HES) on the tensile rate at -196 °C. In contrast, tensile testing with a frozen-in hydrogen distribution under an elastic stress of 1300 MPa preloaded at various rates led to an increase in the HES and promotion of IG fracture at -196 °C as the preloading rate at R.T. decreased. The dependence of the HES on the two rates confirmed that it is possible to conduct tensile testing while maintaining and freezing the hydrogen distribution at the start of the test at -196 °C. It was found the hydrogen-related IG fracture may be promoted by decohesion due to hydrogen accumulated on prior austenite grain boundaries by stress-induced diffusion under elastic stress.