Determination Method of Weibull Shape Parameter for Evaluation of the Hydrogen Embrittlement Susceptibility of High Strength Steel

Abstract

An experimental method to obtain the shape parameter m in the Weibull stress for evaluation of hydrogen embrittlement (HE) was examined in a commercial JIS SCM440 steel with the tensile strength of 1403 MPa. The stepwise HE tests were performed by using circumferentially notched round bar specimens with the stress concentration factor of 4.9 after hydrogen pre charging and homogenization treatments. The applied stress of the first step loading was set at 702 MPa and the stress increment from the second step was 14 MPa. The holding time was over 12 h for the first step and over 2 h for each step from the second step. It was conformed by acoustic emission measurement that the first cracking and final fracture occurred in the same 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. By this procedure, it was able to obtain the distribution of critical Weibull stress under the condition that the local diffusible hydrogen content in fracture process zone was constant. The shape parameter m was determined to be 46 for the present steel. In the discussion part, it was confirmed that the evaluation of the HE susceptibility based on the local approach is appropriate and it was pointed out that the hydrogen content distribution in the fracture process zone must be taken into account for evaluating the HE.

Evaluation of Delayed Fracture Property of High Strength Bolt Steels

“…41) They also obtained critical NTS for delayed fracture by increasing applied stress in stepwise manner to obtain shape parameter in Weibull stress for evaluation of delayed fracture. 42 43) and the present authors [44][45][46][47][48][49] carried out slow strain rate test (SSRT) of hydrogen-precharged CNRBSs, and obtained the relationship between NTS and hydrogen content. For convenience and simplification, Hagihara et al proposed Conventional Strain Rate Test (CSRT) with a strain rate which is 2 orders of magnitude higher than that of SSRT.…”

A New Evaluation Method of Hydrogen Embrittlement Fracture for High Strength Steel by Local Approach

“…The stress was maintained over a time after the applied stress was increased in that test. The hydrogen content distribution almost reached the equilibrium condition during the holding time, 17) so we were able to estimate the hydrogen content distribution in the specimen in a critical HE condition by using the calculated stress distribution and Eq. (1) described in Sec.…”

“…(1) Stepwise Test under Low Applied Stress Condition 17,18) Hydrogen charging, Cd coating, and a hydrogen content homogenizing treatment of the specimens were conducted by the same procedure as the constant loading test. The current density was 1 A/m 2 and the hydrogen charge was for 5 h. In the first step, a stress of 702 MPa was applied on the specimens for 12 h. The specimens that fractured in this step were not used for the evaluation.…”

“…In the second step, the stress was increased by 14 MPa and applied on the specimen for 2h. 17) If the specimens did not fracture during this time, the applied stress was increased by another 14 MPa for another 2 h. This step was repeated until the specimen fractured. The fracture specimens were used for the hydrogen content measurement with the Q-mass.…”

“…We have tried to evaluate the HE susceptibility by the method using the Local Approach concept 8,[16][17][18] and showed that the HE susceptibility of high strength steels could be evaluated by the modified Local Approach by considering the effect of the hydrogen content distribution in the specimen. 18) However, the HE susceptibility of various specimens could not be evaluated by using the proposed method in the region of high applied stress.…”