Role of retained austenite morphology and stability in hydrogen-assisted cracking and subsequent arrest behavior in nano-bainitic steels

This study investigates hydrogen embrittlement (HE) in two nano-bainitic steels (designated as NB-Si and NB-Al) with high retained austenite (RA) but different RA morphologies—thin inter-lath RA films versus blocky RA islands—and different apparent RA stability during deformation. While both steels exhibited severe ductility loss after electrochemical hydrogen charging, NB-Si showed higher diffusible hydrogen and more extensive crack propagation. In contrast, NB-Al displayed lower hydrogen uptake and crack-tip blunting. Microstructural characterization showed a higher local fraction of RA adjacent to damage in NB-Al, whereas RA was scarce around cracks in NB-Si. The results suggest that the local persistence (stability) and spatial occurrence of RA near damage—rather than the initial bulk RA fraction alone—are key factors controlling hydrogen-assisted cracking.