Evaluation of delayed fracture property and fractography of notched specimens for high tensile strength steel

Abstract

The evaluation of delayed fracture is essential owing to the widespread application of high-strength steel to reduce automotive weight. Furthermore, it is necessary to clarify the influence of local stress concentration at the notch root as delayed fracture occurs because of the accumulation of diffusible hydrogen in the stress concentration area. In this study, we used the slow strain rate technique (SSRT), constant load test, and conventional strain rate technique (CSRT) of notched specimens under cathodic hydrogen charging to obtain the tensile strength, diffusible hydrogen content, and ratio of brittle fracture surface, which consists of quasi-cleavage fracture and intergranular fracture, under various testing conditions. The results show that the relationship between the tensile strength and brittle fracture ratio of notched specimens is in good agreement with that of smooth specimens under various testing conditions. However, some data of brittle fracture ratio obtained using smooth specimens need to be modified to comply with the primary crack area. Therefore, if the primary crack can be determined, the brittle fracture ratio will be an important index for delayed fracture or hydrogen embrittlement criterion. Additionally, the relationship between tensile strength and the accumulated diffusible hydrogen content, which is estimated by FEM of notched specimens, has a good agreement with that of smooth specimens obtained by either SSRT and CSRT. However, the fracture strength curve of CSRT is higher than that of SSRT. The result indicates that the interaction between hydrogen and dislocation in the microstructure is needed to consider the influence of strain rate of the testing methods.