Hydrogen Embrittlement (HE) Susceptibility of High Strength Fastener Grade Steels

Hydrogen embrittlement (HE) susceptibilities of two different grades of tempered martensite steels 35CrMo4 and 41Cr4 were determined by using the incremental step load (ISL) technique. Environmental hydrogen embrittlement (EHE) susceptibilities of these steel grades were determined by in-situ charging of hydrogen at two different potentials -1V_SCE and -1.2V_SCE, during ISL testing. The ISL test results show that 35CrMo4 is more susceptible than 41Cr4. The fracture surface morphology is entirely intergranular showing the severity of embrittlement in 35CrMo4, while cleavage like features coexist with some ductility and intergranularity in 41Cr4. From scanning and transmission electron microscopy studies, the lower susceptibility of 41Cr4 can be attributed to its low inclusion content (MnS) and higher matrix-cementite interfaces and cementite content, influencing the trapping characteristics of the material.