AISI 304 steel: anomalous evolution of martensitic phase following heat treatments at 400 °C

AISI 304 steel with a biphasic structure (α′+γ), induced by plastic deformation at room temperature, shows an anomalous evolution of martensite content when heated at 400 °C. High-temperature X-ray diffraction has been used to investigate the phenomenon. Isothermal measurements at 400 °C for times up to 2.0 × 104 s show that martensite progressively transforms to austenite but an increase of martensite content is observed after cooling to room temperature. The result confirms that the heat treatment causes composition inhomogeneities and a local increase of MS (martensite start temperature) in the zones depleted of alloying elements so that a certain amount of martensite can form during cooling to room temperature. The temperature range of martensitic transformation is 240–190 °C and was determined by dynamic modulus measurements.