Modelling of delta ferrite to austenite phase transformation kinetics in martensitic steels: Application to rapid cooling in additive manufacturing

In this paper, the high temperature transformation kinetics of delta ferrite to austenite (δ → γ) phase transformation is modeled by thermodynamic and diffusion calculations. It appears that, in martensitic steels, the δ → γ transformation is very fast (a few nanoseconds) as soon as the first austenite nuclei appears. Classically the austenitic phase will thus systematically be observed in the material during conventional elaboration processes. However, in powder metallurgy and additive manufacturing, it is possible to obtain sufficiently high quenching rates (up to 106 °C/s) so that the γ phase does not have time to appear. The calculations presented here allow to rationalize the understanding of the microstructures of powders and different additive manufacturing materials. They enable to understand why ferrite or martensite is sometimes obtained in the final microstructure. From the calculations made, an original CCT (Continuous Cooling Transformations) diagram starting from the δ phase is proposed. It allows to set up a strategy of grade design or process definition according to the final microstructure targeted.