Extraction of reversible hydrogen trapped on prior austenite grain boundaries and promoting intergranular fracture in the elastic region of tempered martensitic steel by utilizing frozen-in hydrogen distribution at -196 °C

Factors that promote hydrogen-related intergranular (IG) fracture in the elastic region of tempered martensitic steel have been identified utilizing frozen-in hydrogen distribution and tensile tests in liquid nitrogen at -196 °C. Tensile test results at room temperature (R.T.) after precharging with 12.1 mass ppm of hydrogen showed hydrogen embrittlement associated with IG fracture. Although tensile test results at -196 °C after precharging with the same amount of hydrogen did not show hydrogen embrittlement with IG fracture, those at -196 °C after preloading with elastic stress just before fracture strength at R.T. showed hydrogen embrittlement with IG fracture. As preloading stresses increased and preloading speeds decreased, subsequent hydrogen embrittlement susceptibility at -196 °C increased. In addition, although tensile testing at -196 °C after hydrogen precharging, preloading at R.T., and unloading at -196 °C resulted in IG fracture, testing conducted after unloading at R.T. did not. It is noted that reversible hydrogen that accumulates on prior austenite grain boundaries (PAGBs) in the presence of load and desorbs in the absence of load in the elastic region at R.T. is directly responsible for IG fracture. These findings revealed that hydrogen-related IG fracture in the elastic region of tempered martensitic steel was not caused by stress-free hydrogen distribution, but was promoted by reversibly accumulated hydrogen mainly due to stress-induced diffusion onto PAGBs during stress loading at R.T.