Microstructural degradation and impact toughness of simulated heat-affected sub-zones in RA-rich medium-Mn steel

Medium-Mn steels exhibit excellent combinations of strength and ductility due to the presence of metastable retained austenite (RA). However, the influence of welding thermal cycles on the fracture behavior of individual heat-affected zone (HAZ) subzones remains insufficiently understood. In the present study, Gleeble-based simulations of intercritical (ICHAZ), fine-grained (FGHAZ), and coarse-grained (CGHAZ) heat-affected zones were performed on an intercritically annealed 5Mn steel with a ferritic–austenitic duplex microstructure containing approximately 30 % RA. Increasing a peak temperature progressively degraded the duplex microstructure through RA destabilization and fresh-martensite formation. The ICHAZ retained a partially duplex character, whereas the FGHAZ became predominantly martensitic, and the CGHAZ transformed into a fully martensitic microstructure. No softened HAZ region was identified; under the applied cooling conditions, diffusional transformations were suppressed and martensitic transformation predominated. Importantly, the hardness distribution was non-monotonic: the maximum hardness occurred in the fully austenitized region before pronounced prior-austenite grain coarsening, whereas further overheating toward the CGHAZ center reduced hardness despite complete martensitic transformation. Instrumented Charpy impact testing combined with fractographic analysis showed progressive deterioration of impact toughness and transition from fully ductile fracture in the base material to mixed ductile–brittle fracture in the simulated HAZ subzones. The results further showed that welding-induced microstructural degradation affected not only the total absorbed energy but also the relative contributions of crack initiation and propagation. The beneficial contribution of RA to impact toughness was strongly dependent on its stability: sufficiently stable RA was associated with enhanced plastic accommodation and crack-propagation resistance, whereas thermal destabilization and fresh-martensite formation were associated with reduced dynamic fracture resistance.