On the improved hydrogen embrittlement resistance in stress relieved laser powder bed fused 316L stainless steel over the hot isostatic pressed equivalent

The onset and mechanisms of hydrogen embrittlement are investigated in stress relieved (SR) and hot isostatic pressed (HIP) laser powder bed fused (L-PBF) 316L austenitic stainless steel. An improved resistance of SR L-PBF 316L to hydrogen-assisted degradation is uncovered by complementary modelling and in-depth experimental characterisation, including transmission electron microscopy. Novel insights show that the dislocation cells, characteristic for the SR L-PBF microstructure, form a beneficial hydrogen trap and establish an effective barrier against hydrogen-induced crack propagation and hydrogen embrittlement. Furthermore, the chemical heterogeneity in the dislocation cells contributes to crack arrest. In contrast, the mobile dislocations in HIP L-PBF 316L make this homogeneous microstructure significantly more prone to hydrogen embrittlement due to the promoted hydrogen-induced martensite formation, driving brittle crack propagation. Therefore, a dual HEDE-HELP synergetic mechanism is proposed for the hydrogen-induced embrittlement of SR L-PBF 316L, whereas HIP L-PBF 316L is dominated by the HEDE embrittlement mechanism. These insights highlight the potential of tailored post-processing in L-PBF microstructures, offering promising strategies to reduce hydrogen-assisted degradation in austenitic stainless steels.