Application of local approach to hydrogen embrittlement fracture evaluation of high strength steels

A new method for evaluating the hydrogen embrittlement (HE) susceptibility of ultra high strength steel was studied in order to propose a new method for assessing the delayed fracture property. The material used was 1400MPa tempered martensitic steel with the chemical composition 0.40C-0.24Si-0.81Mn-1.03Cr-0. 16Mo(mass%). The local approach originally used for evaluating the brittle fracture property was applied to HE susceptibility assessment after modifying the method to include the effect of hydrogen content. Critical HE data used in the modified local approach was obtained by a stepwise test in which alternating processes of stress increase and stress holding were repeated until the specimen fractured. The specimen used in the stepwise test was 10 mm in diameter and the stress concentration factor was 4.9. Assessment of HE susceptibility for specimens with other dimensions entailed the use of a critical hydrogen content for failure, Hc, representing the maximum hydrogen content among the unfractured specimens in the HE test with constant loading. Matters to be noted for obtaining the material parameters are discussed.