Modeling of ferrite formation in a duplex stainless steel weld considering non-uniform starting microstructure

A one-dimensional numerical diffusion model has been developed to simulate the kinetics of the austenite (γ) to ferrite (δ) transformation in 2205 duplex stainless steel during welding. In this model, it is assumed that the transformation is driven by the diffusion of nitrogen under para-equilibrium conditions. Transformation kinetics from both uniform and non-uniform starting microstructures are investigated. The uniform starting structure is accounted for by using a pair of γ and δ grains of constant sizes, whereas non-uniform structures are simulated by considering four γ and δ grains of varying sizes. Interactions between neighboring grains, particularly hard and soft impingements, are taken into account by properly adjusting the boundary conditions. It is found that the transformation may take 30% more time for the non-uniform starting microstructure, where the ratio of thickest to thinnest grains is about 2, than for the uniform structure under typical weld heating conditions. Time–temperature-transformation and continuous-heating-transformation diagrams are constructed using the numerical diffusion model, providing a graphical means for predicting the kinetics of the γ → δ transformation. The computed results were confirmed by experiments using an in-situ X-ray diffraction technique, thus validating the model.