Dynamic restoration of the ferrite and austenite phases during hot compressive deformation of a lean duplex stainless steel

Abstract

The active restoration processes within the ferrite and austenite phases were investigated in a Ni-free duplex stainless steel with the 75/25 austenite/ferrite ratio performing compression tests over the wide range of temperature and strain rate (700–1000 °C and 0.001–0.1s−1, respectively). The substructural features under different Zener-Hollomon parameters (Z) were examined using the electron backscatter diffraction analysis. A significant portion of flow softening was attributed to the active softening mechanisms in the continuous network of ferrite phase. The ferrite was softened through continuous dynamic recrystallization (CDRX) at high Z values, but the dynamic recovery (DRV) surmounted CDRX at low Z parameters. Interestingly, the discontinuous dynamic recrystallization (DDRX) mechanism was testified by increasing temperature and strain rate, i.e. at medium Z values. DDRX occurred by the growth of high-angle subgrains near the austenite/ferrite interfaces. With decreasing Z values, an increase in subgrain and grain sizes was observed. Contrarily to ferrite, the austenite phase was softened mostly through the dynamic recovery mechanism. At higher Z values microband arrays were formed in austenite holding specific character resembling to band-like misorientation gradients. However, under the lower Z values, a small fraction of recrystallized grains was formed through the CDRX/DDRX mechanisms. This was justified considering the lower strain accommodation in the austenite phase during deformation of duplex stainless steel.