Reverse transformation mechanism of martensite to austenite and amount of retained austenite after reverse transformation in Fe - 3Si - 13Cr - 7Ni (wt-%) martensitic stainless steel

The reverse transformation mechanism of martensite to austenite and the volume fraction of retained austenite have been studied in an Fe-3Si-13Cr-7Ni (wt-%) martensitic stainless steel by means of dilatometry, transmission electron microscopy and X-ray diffraction. Below a heating rate of 10 K s^-1, the reverse transformation of α′ to γ occurs by diffusion, whereas it occurs by a diffusionless shear mechanism above 10 K s^-1. After reversion treatment at low temperatures, filmlike retained austenite is observed along α′ lath boundaries, while reversion treatment at high temperatures produces granular retained austenite inside the α′ laths in addition to filmlike retained austenite. The volume fraction of retained austenite at room temperature increases with increasing reversion treatment temperature, exhibiting a maximum at ∼625°C, above which it decreases with increasing reversion temperature.