Reverted austenite formation in a cryorolled lean duplex stainless steel

Strain-induced martensite formation and α’-martensite to austenite reversion (α’ →γ) were investigated in a cryorolled UNS S32304 lean duplex steel. The material was rolled at 77 K up to 50% thickness reduction and annealed up to 800 o C for 1 h. The microstructural evolution of the steel, with focus on α’-martensite to austenite reversion was followed by several characterization techniques, complemented with thermodynamic calculations. For 50% reduction, practically all austenite has transformed into α’-martensite. From EBSD and magnetization results, the α’-martensite to austenite reversion occurs in the temperature range of 500-800 o C. At 800 o C, the austenite phase is recrystallized, and presents a bimodal grain size distribution, mainly attributed to the fragmentation of α’-martensite prior to reversion. EBSD and EDS results indicate that the α’ → γ mechanism at 600 °C is likely displacive, while between 700-800 °C a gradual transition from a displacive to a diffusional mechanism occurs. From magnetic measurements, a comparison between the cryorolled steel and the same material deformed at room temperature up to 80% reduction, following annealing, was made. Except at the beginning of the α’ → γ transformation (500-600 °C), cryorolling does not accelerate the reversion of α’ martensite to austenite during annealing, contrary to expectations. However, it has a strong effect on the morphology of reverted austenite. This highlights the possibility of using cryorolling to produce a wider variety of microstructures during thermomechanical processing.