Effect of grain-size distribution on elevated temperature tensile behaviour of a Cr–Mn–N austenitic stainless steel

This study investigates how grain size and grain-size distribution affect high-temperature deformation stability in a commercial Cr–Mn–N austenitic stainless steel. Cold rolling and subsequent annealing produced four microstructures: ultrafine-grained (UFG), coarse-grained (CG), fine-grained unimodal (FG-Unimodal), and heterogeneous fine-grained bimodal (FG-Bimodal). Tensile tests at 600 °C showed that UFG had higher ductility and more stable flow than CG, with elongations of 34% and 18%, respectively. After deformation, UFG contained dense intragranular low-angle grain boundaries (LAGBs), dislocation-wall structures, and local recrystallization features, indicating dynamic recovery and polygonization that improved strain compatibility and delayed grain-boundary damage. In contrast, CG showed localized misorientation gradients and grain-boundary-dominated failure. For the fine-grained states, FG-Unimodal showed a higher elongation than FG-Bimodal (34.0% vs. 23.5%). This result indicates that a narrow grain-size distribution promotes more uniform deformation, whereas a bimodal grain-size distribution increases strain partitioning and accelerates strain localization and local softening.