Austenite reversion in low-carbon martensitic stainless steels – a CALPHAD-assisted review

Low-carbon martensitic stainless steels with 11.5–16 wt-% Cr and < 0.07 wt-% C are characterised by high corrosion resistance, strength, ductility and impact toughness, obtained by formation of fine-grained reverted austenite from lath martensite upon inter-critical annealing. The review treats the mechanisms governing the formation and stabilisation of reverted austenite and is assisted by the computation of phase equilibria. Literature data on Cr and Ni concentrations of the reverted austenite/martensite dual-phase microstructure are assessed with respect to predicted concentrations. Reasonable agreement was found for concentrations in martensite. Systematic excess of Cr in austenite of approx. 2 wt-% relative to calculations was suspected to originate from the growth of M23C6 with a coherent interface to austenite. Within large scatter, measured values of Ni in austenite were on average 2 wt-% below predictions. Complex equilibration of the microstructure and experimental error are discussed as possible origins of the discrepancies.