Parameters for the evaluation of hydrogen embrittlement of high strength steel

The influences of the applied stress and the stress concentration factor (Kt) on hydrogen embrittlement were investigated in the commercial JIS SCM440 steel with the tensile strength of 1403 MPa. The applied stress was changed from 0.33 times to 0.72 times the tensile strength of the notched specimen, while the Kt was altered from 2.1 to 6.9. The hydrogen embrittlement property was evaluated with the resistance to the diffusible hydrogen in the steel. The diffusible hydrogen concentrations were analyzed by thermal desorption analysis. The results are as follows. (1) The hydrogen embrittlement occurs under the lower diffusible hydrogen concentrations with increasing Kt. However, the H_D-t (diffusible hydrogen concentration-time to failure) curves become almost the same when the Kt and the applied stress are high. (2) The criterion of the hydrogen embrittlement cannot be determined by the combination of the maximum local axial stress in the specimen and the maximum diffusible hydrogen concentration locally accumulated in the specimen. In the discussion part, it was proposed that it is very important to consider the stress distribution in the specimen for the evaluation of hydrogen embrittlement.