Evaluation of hydrogen embrittlement susceptibility of high strength steel by the Weibull stress

The parameters for levels of the evaluation of hydrogen embrittlement susceptibility of high strength steel, which are independent of stress concentration and applied stress, are examined using circumferentially notched round-bar specimens with different notch root radius. The hydrogen embrittlement tests are performed for quenched and tempered JIS SCM440 steel with the tensile strength of 1403 MPa. The applied stress is changed from 0.33 to 0.72 times the tensile strength of the notched specimen and the stress concentration factor (Kt) ranges from 2.1 to 6.9. The initiation of hydrogen-induced crack is detected by acoustic emission measurement. Specimens are unloaded immediately after detecting the first acoustic emission and the crack initiation points are observed with SEM. The results are summarized as follows. (1) The Weibull stress (sigma(w)) and diffusible hydrogen contents in fracture process zone (Hc* (ave.)) enable the hydrogen embrittlement susceptibility evaluation independently of stress concentration and applied stress levels of circumferentially notched round-bar specimens. (2) Initiation points of hydrogen embrittlement fracture are located in the region where the maximum principal stress exceeds 0.8 times its peak value. The significance of hydrogen embrittlement susceptibility evaluation using the sigma(x)-Hc* (ave.) relation is discussed in terms of the extent of fracture process zone ahead of the notch root.