The effect of austenite phase transformation on hydrogen distribution and embrittlement mechanisms of heterogeneous martensite stainless steel manufactured by laser powder bed fusion

Abstract

The hydrogen distribution and hydrogen embrittlement (HE) mechanisms of laser powder bed fusion (LPBF) heterogeneous martensite stainless steel (MSS) is complicated. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature of deformation-driven phase transformation render systematic studies of HE mechanisms challenging. In this study, we systematically quantified hydrogen traps and analyzed the synergistic action of heterogeneous austenite on the HE mechanisms of MSS manufactured by LPBF. The microstructure of LPBF-processed MSS contained 17 % dislocation cell-decorated bulk austenite distributed in the molten pool boundaries and approximately 8 % thin austenite at the martensite lath interfaces. Thermal desorption analysis revealed that there is a dominant hydrogen trapping sites in the austenite or at the interface between the matrix and the austenite. Bulk austenite can be considered an obvious sink hydrogen trap, which is more stable and impedes crack propagation. In contrast, the thin austenite releases hydrogen at a higher rate, i.e. it acts as a shallower trap. The transformation from thin austenite to martensite near the crack accelerates hydrogen-induced cracking. The possible crack initiation sites were determined to be the martensite laths, or phase boundaries between the transformation martensite/matrix. The cracks and propagate as transgranular fractures along the low-angle grain boundary, which generates higher HE susceptibility, explained by hydrogen-enhanced decohesion. Therefore, the findings of this study further advance the mechanistic understanding of austenite on HE and can guide new MSS with higher HE resistance assisted by microstructure designs.