Mechanisms of the Reverse Martensite-to-Austenite Transformation in a Metastable Austenitic Stainless Steel

The martensite-to-austenite reversion mechanisms under continuous heating and annealing of metastable austenitic stainless steel subjected to cold swaging were studied. The reversion-temperature-time diagram was constructed using high-resolution dilatometry. The diagram revealed a sequence of martensitic and diffusional reversion and recrystallization. Martensitic and diffusional reversion might be separated by using the heating rate of >10 °C/s. The reversion started via the martensitic mechanism, and the diffusional mechanism developed during subsequent heating. However, both mechanisms enhance simultaneously during continuous heating at slow heating rates (<10 °C/s). At higher temperatures, recrystallization occurred. Post-mortem microstructure analysis has allowed classifying the reverse annealing modes into low- (500–650 °C), medium- (~700 °C), and high-temperature (~800 °C) regimes. During low-temperature annealing, the development of the phase reversion, recovery, recrystallization, and carbide precipitation was characterized by both a high amount of new austenite grains and restriction of their growth that resulted in the formation of an ultrafine grain structure with an average grain size of 100–200 nm. Medium-temperature annealing was associated with the formation of almost a fully recrystallized austenitic structure, but the lamellar regions were still detected. Austenitic grain growth and dissolution of carbide particles were enhanced during high-temperature annealing.