H d) and time to failure (t f) for the modified ausformed and tempered (MAQT) martensite and the conventional QT martensite with a tensile strength of 1580 MPa 5) .

In order to evaluate the hydrogen embrittlement susceptibility of high strength steel independently of stress concentration factor Kt and specimen size, Local Approach method modified to evaluate the hydrogen content distribution in the specimen was used. In the method it was considered that the size of micro cracks depends on the hydrogen content. The stepwise hydrogen embrittlement tests were carried out to obtain the hydrogen embrittlement susceptibility of the material which was 1400 MPa grade JIS SCM440 (0.40C-0.24Si-0.81Mn-1.03Cr-0.16Mo (mass%)). Stress was applied to the specimen, which is circumferentially notched round-bar type with the stress concentration factor of 4.9, after hydrogen pre-charging and homogenization treatments. The applied stress on the first step was set at 702 MPa and the stress of 14 MPa was increased on and after the second step. The holding time of stress was over 12 h on the first step and over 2 h on and after the second step. The diffusible hydrogen content was measured by thermal desorption analysis immediately after fracture. The distribution of stress and hydrogen content near notch root was calculated by FE-analysis. In the modified Local Approach method the new parameter Σ w,cr was introduced including the stress factor and hydrogen content factor. The parameter Σ w,cr obtained by the stepwise test obeyed the Weibull distribution as the Weibull stress in the conventional Local Approach. The hydrogen embrittlement susceptibility of any specimens with various stress concentration factors was able to be evaluated uniquely by the Local Approach considering the effect of hydrogen content distribution.

An evaluation method for hydrogen embrittlement (HE) susceptibility for ultra high strength steel was studied. The study showed that the evaluation results obtained by the method were independent of the stress concentration factor and the dimension of the specimen. A commercial 0.40C-0.24Si-0.81Mn-1.03Cr-0.16Mo (mass%) steel with a tensile strength of 1 400 MPa was used. The Local Approach, which was originally used for the evaluation of brittle fracture property, was applied to evaluate the HE susceptibility. The method was modified to consider the effect of hydrogen content to evaluate the HE susceptibility. The critical HE data, which were required in the modified Local Approach, were obtained by the stepwise test in which the stress increasing process and the stress holding process were alternatively repeated until the specimen fractured. The dimensions of the specimen used in the stepwise test consisted of a diameter of 10 mm and a stress concentration factor of 4.9. To evaluate the HE susceptibility for specimens with other dimensions, a critical hydrogen content for failure called Hc, which represents the maximum hydrogen content among the unfracture specimens on the HE test with constant loading, was used. It was found that the HE susceptibility could be evaluated by the modified Local Approach considering hydrogen content distribution in the specimens and that the probability of the HE fracture on the components could be designed by using the evaluation results.

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