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The role of δ-Fe in hydrogen embrittlement (HE) of 17-4 PH steel is studied in this work. Scanning Kelvin probe force microscopy result indicates that δ-Fe is a hydrogen trapping site. Accordingly, δ-Fe can reduce the hydrogen concentration of surrounding martensite and prior austenite grain boundaries (PAGBs) and imped the brittle fracture along lath boundaries and PAGBs, which can be beneficial to the HE resistance improvement. However, a cleavage fracture of δ-Fe can occur under the synergetic action of hydrogen-enhanced localized plasticity (HELP) and hydrogen enhancement of the strain-induced generation of vacancies (HESIV). These findings indicate a new path to improving HE resistance of high strength martensitic steels. • SKPFM result shows that hydrogen tends to segregate in delta ferrite. • Delta ferrite acting as a hydrogen trapping site decreases the tendency of brittle fracture. • Hydrogen segregation in delta ferrite leads to a strain localization and a subsequent transgranular cleavage fracture. [ABSTRACT FROM AUTHOR]

Abstract A nanotwinned 304 steel with a favorable combination of strength and ductility has been made in this work through the dynamic plastic deformation (DPD) treatment followed by a proper heat treatment. The DPD, DPD-annealed and coarse grain (CG) 304 samples were tensile tested with slow strain rate under electrochemical hydrogen charging condition. Compared with CG sample, DPD-annealed sample with a mixed structure of nanotwins (41% volume fraction), recrystallization grain (RXG, 32% volume fraction) and dislocation structures (27% volume fraction) possesses a high HE resistance. Hydrogen causes a 12.5% reduction in yield strength and 5.4% reduction in tensile strength of DPD-annealed sample because hydrogen enhances dislocations/twin boundaries (TBs) interactions and the subsequent dislocation mobility during the TB-mediated slip transfer. Hydrogen charging also causes a 41% reduction in strain induced martensite of DPD-annealed sample. Strain localization is supposed to be mitigated by the bundle form of TBs and the micro-sized RXGs, accounting for the high HE resistance of DPD-annealed alloy. Our findings suggest a new strategy to design high strength alloys with low HE susceptibility. Highlights • The nanotwinned austenitic steel shows a high hydrogen embrittlement resistance. • H-charging suppresses the martensite transformation of DPD-annealed alloy. • Slip localization at twin boundary (TB) is mitigated by the bundle form of TB. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]