Martensitic stainless steel: evolution of austenite during low temperature annealing and design of press hardening alloys

In steel, combining a martensitic matrix with a second, ductile austenite phase is a common strategy to achieve highest strength and good ductility. However, in order to obtain superior mechanical properties, the austenite has to be tailored in morphology and stability, requiring fundamental knowledge of the microstructural mechanisms. In this study, austenite evolution in two stainless martensitic steels Fe-13.5Cr-0.44C and Fe-10.6Cr-0.40C (wt.%) is analyzed. In this context, the influence of chromium on the low temperature austenite reversion mechanism proposed by Yuan et al. [1] is investigated to improve its understanding and allow future application. The influence of chromium content and quench temperature on the evolution of austenite in the quenched state and after annealing at 400°C for 30min is investigated. Using atom-probe tomography, an analog behavior is observed in both alloys with carbon partitioning from martensite to austenite and M3C carbide formation during annealing. The annealing treatment is analyzed by high resolution dilatometry and a dilatometric model is assembled to simulate the opposing influences of the microstructural phenomena. In this investigation, it is shown that no macroscopic austenite reversion takes place during annealing at 400°C for 30min and that the measured increase of the austenite content is an artefact. It is concluded, that the amount of austenite is actually set by quenching below the chromium-dependent martensite start temperature and during annealing this retained austenite is stabilized by carbon partitioning. In this context, the reliability of measuring retained austenite in the present martensitic steel using EBSD as well as XRD is discussed.