The role of reversed austenite in hydrogen embrittlement fracture of S41500 martensitic stainless steel

Up to now, the precise role of reversed austenite (RA) in hydrogen embrittlement (HE) of steel is still not fully understood. This work presents new observations and interpretation of fracture surface features immediately beneath the fracture surface for S41500 martensitic stainless steels (MSS) with aim to reveal the role of RA in HE resistance. The MSS were tensile tested with slow strain rate under electrochemical hydrogen charging condition. Steel containing more RA was found to have less hydrogen embrittlement susceptibility. Focused ion beam (FIB) was used to prepare sample for TEM observation of fracture path after HE fracture. It clearly shows that RA near the fracture surface has transformed to the newly formed martensite (NFM) and cracking occurs along both the tempered martensite/NFM boundaries and the lath boundaries. Three dimension atom probe (3DAP) confirms that RA is the H trapping site. Thus the beneficial role of RA is that it can act as a stable hydrogen trapping site which can increase the HE resistance by reducing hydrogen content at lath and grain boundaries. But its beneficial effect should not be overestimated since cracking along tempered martensite/NFM boundaries can occur after martensitic transformation as a result of hydrogen redistribution.