Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion

The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0-5 wt% Mn, 0-2 wt% Si, constant 13.5 wt% Cr and 0.45 wt% C) and seven martensite tempering conditions (300-500 °C, 0-30 min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35 vol%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5 GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion.