Evaluation of Hydrogen Embrittlement Susceptibility of High Strength Steel Based on Local Approach

Abstract

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.

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

“…(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.…”

“…The hydrogen content distribution in the specimens was calculated by Eq. (1) 7,18) ........ 1where Ds h(element) is the difference in the hydrostatic stress of each element and the hydrostatic stress of the element which has an almost uni-axial stress condition that is located 20 mm away from the notched region. DV is the partial molar volume of the hydrogen in bcc-Fe (2ϫ10 Ϫ6 m 3 /mol), R the gas constant, T the test temperature (ϭ300 K), and H the diffusible hydrogen contents of the specimens.…”

“…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. The reason was considered that the material constants in the method were obtained using the critical HE data in only the low stress range.…”

“…In this paper, we show the process of introducing the modified Local Approach method based on an HE mechanism which is different from the mechanisms considered in the previous paper. 18) Then we evaluate the HE susceptibility with the material constants in the modified Local Approach, g and m, obtained from the critical HE fracture data in the wide range from low to high applied stress. And we discuss the important factors for evaluating the HE susceptibility property by this method in some detail.…”

Evaluation of Delayed Fracture Characteristics of High-strength Bolt Steels by CSRT

“…Then, the methods of evaluating the delayed fracture characteristics irrespective of the specimen configuration were studied. [2][3][4]7,8) Wang et al [2][3][4] have demonstrated by the slow strain rate test (SSRT) that the relationship between the maximum principal stress and instantaneously accumulated hydrogen concentration at the fracture initiation point during the test is independent of the notch geometry.…”