Grain Refinement Alters Deformation Mechanisms to Mitigate Hydrogen Embrittlement in an Interstitial Nitrogen‐Alloyed Austenitic Stainless Steel

This study investigates the effects of grain size on deformation behavior and hydrogen embrittlement (HE) in an interstitial nitrogen‐alloyed austenitic stainless steel (QN1906). Using slow strain rate testing, thermal desorption spectroscopy, and electron microscopy, grain refinement is revealed from 56 to 4 μm significantly enhances yield strength (from 369.8 to 523.6 MPa) via the Hall‐Petch effect, while reducing HE susceptibility (with elongation loss decreasing from 33.3% to 24.2%). Coarse‐grained sample exhibits pronounced twinning‐induced plasticity, whereas fine‐grained sample shows suppressed twinning, thereby shifting the deformation mechanism to dislocation slip. Hydrogen accelerates both dislocation activity and twinning, decreasing twin spacing, which is attributed to the hydrogen‐enhanced localized plasticity mechanism. Hydrogen‐induced cracks nucleate at twin intersections and propagate along grain boundaries or twin boundaries attributed to strain concentration and hydrogen segregation. Grain refinement mitigates HE by reducing hydrogen content (from 1.22 to 0.39 wppm), slowing hydrogen diffusion, and alleviating local strain concentration at twin intersections. These findings highlight the critical role of grain size in balancing strength and HE resistance through tailored deformation mechanisms and hydrogen transport control.