Correlation of microstructure and dynamic softening mechanism of UNS S32101 duplex stainless steel during elevated temperature tensile testing

Abstract

The dynamic substructural development and softening mechanism of UNS S32101 duplex stainless steel were comprehensively investigated by employing hot-tensile tests at various strain rates of 0.1–10 s−1 at a fixed temperature of 1200 °C. Different flow behaviors were attributed to the microstructural evolution and restoration process under various hot-deformation conditions. The alternative restoration mechanisms of ferrite in the current alloy were closely associated with the evolution of the misorientation angle in the (sub)grains, depending on the applied strain rates. Therein, three distinct softening mechanisms were found in ferrite, i) subgrain coalescence (SC) at 0.1 s−1, ii) continuous dynamic recrystallization (CDRX) at 1 s−1 and iii) subgrain rotation-assisted discontinuous dynamic recrystallization (SR-assisted DDRX) at 10 s−1. During SR-assisted DDRX process, new DRX nuclei were preferentially formed at the high-angle grain boundaries/phase boundaries (HAGBs/PBs) through the growth of highly misoriented subgrains. In contrast to ferrite, the available dynamic softening behavior of austenite, unlike the classical DDRX mechanism characterized by strain-induced boundary migration (SIBM), is affected by a limited number of pre-existing HAGBs. At lower strain rates of 0.1 and 1 s−1, the nucleation process of DRX in austenite is analogous to the CDRX behavior, whereas the growth characteristics conform to DDRX, thus, it can be called dynamic recovery-assisted DDRX (DRV-assisted DDRX). At a high strain rate of 10 s−1, DRX nucleation mainly took place through the strain-induced twin boundaries (TBs) transformation into HAGBs, and then rapidly grew via SIBM, referred to as TB-assisted DDRX.