The effect of strain rate on hydrogen embrittlement susceptibility of high strength tempered martensitic steel was investigated by tensile tests under cathodic hydrogen charge. Fracture elongation was decreased with a decrease in strain rate and increase in diffusible hydrogen concentration. The mechanism of hydrogen embrittlement susceptibility was investigated based on vacancy-type lattice defects formation by positron lifetime spectroscopy. The clear correlation was not comfirmed between

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... in rate and the parameters of average positron lifetime, dislocation density and vacancy density. These parameters decreased with a decrease in fracture elongation, it means that these parameters did not reflect hydrogen embrittlement susceptibility. On the other hand, vacancy clustering was promoted with a decrease in strain rate. Therefore, it is assumed that strain rate dependence of hydrogen embrittlement susceptibility is determined by vacancy clustering process.