Simulating the ferrite-to-austenite transformation in stainless steel welds

Under some conditions, the transformation of as-solidified ferrite to austenite in stainless steels has characteristics of a massive transformation. Calculations were carried out to determine if experimental evidence from Koseki of such a transformation could be modeled by a diffusion-controlled transformation. Two different conditions at the transformation interface were considered: ortho-equilibrium and para-equilibrium. Simulations solved the multi-component diffusion problem for various thermal conditions, including isothermal and continuous cooling conditions. It was found that neither interface condition could explain the experimental results of Koseki. The ortho-equilibrium interface condition resulted in transformation rates that were too slow. In addition, continuous cooling conditions led to non-uniform compositions, contradicting the experimental evidence. The para-equilibrium condition resulted in sufficiently fast transformation rates but the temperature of the transformation was considerably higher than that found experimentally. It is suggested that a condition of limited partitioning of substitutional solute elements at the interface could better explain the experimental results, but implementation of this condition at the interface is not possible with currently available simulation capabilities.