Computer Modeling of the Ferrite‐To‐Austenite Transformation in a Super‐Duplex Stainless Steel and the Important Role of Trans‐Interface Diffusion

Duplex stainless steels have high strength and corrosion resistance. They are prone to embrittlement due to intermetallic phases, such as the sigma phase, which forms during inappropriate heat treatments. The ferrite‐to‐austenite phase transformation is important here, as ferrite becomes enriched in Cr and Mo and depleted in Ni and N, which facilitates the nucleation and growth of intermetallic phases. A new simulation model for multiphase, multicomponent systems is proposed, which solves volume diffusion with a moving boundary with high accuracy. A model for trans‐interface diffusion is presented and volume diffusion and trans‐interface diffusion are solved simultaneously for all atomic species. The interface migration is determined by minimization of the total Gibbs energy. Simulations of the austenite growth during cooling are compared to experimental data obtained from in situ high‐energy X‐ray diffraction. A promising agreement is obtained between computer simulation and experiment for the austenite growth during slow cooling of a hot‐rolled sample with a lamellar structure of austenite in a ferrite matrix. It is shown that omission of trans‐interface diffusion can initially more than halve the transformation speed and leads to numerical instability. There are good reasons that the presented model can be applied to diffusional transformations in other systems.