Hot Deformation Behaviors and Dynamic Softening Mechanisms of As-Cast XM-19 Super Austenitic Stainless Steel

The hot deformation behaviors and dynamic softening mechanisms of XM-19 super austenitic stainless steel (SASS) were investigated using the isothermal compression test in the temperature range from 1025 to 1250 °C and a compression rate of 0.01–10 s−1. A hot processing map with a strain of 0.9 was constructed, and the analysis results show that the optimal thermal deformation parameters are a temperature range of 1200–1250 °C and a strain rate range of 0.03–0.2 s−1. The thermal activation energy at 0.7 strain is calculated to be 614.3 kJ/mol by developing constitutive equations under various deformation parameters, which is essentially higher than the range of thermal deformation activation energy of typical austenitic stainless steels. At a high temperature of 1250 °C, the synergistic effect of adiabatic heating and increased dislocation density drives the recrystallization fraction to surge from 20% to 78% as the strain rate rises from 0.01 to 10 s−1, while at a fixed strain rate of 0.1 s−1, the increase in deformation temperature from 1025 °C to 1250 °C promotes dynamic recrystallization (DRX), leading to a parallel rise in recrystallization fraction to 25%. The nucleation mechanism of XM-19 SASS is primarily driven by discontinuous dynamic recrystallization (DDRX), with a supporting role of continuous dynamic recrystallization (CDRX). The contribution of CDRX decreases gradually with increasing deformation temperature.