Prediction of the amount of retained δ-ferrite and microsegregation in an austenitic stainless steel

A newly developed numerical procedure for simulation of multicomponent and multilayered phase diffusion has been applied to a microscopic simulation technique that simulates the diffusional reactions within a one-dimensional volume element between dendrites, in order to predict the amount of retained F-ferrite and microsegregation during solidification and continued cooling of a 304 austenitic stainless steel. Satisfactory agreements were obtained between the simulation and experimental data for the amount of retained F-ferrite and distribution of the main alloying elements, Cr and Ni, into austenite and ferrite. It was found that generally acceptable results can be obtained by using thermodynamic, kinetic parameters reasonably assessed, and giving values with physical basis to cooling rate and dendrite arm spacing as input data for the simulation. However, at least four elements, Fe, Cr, Ni and C, that give effects either on the overall diffusion kinetics due to low diffusivity or on the phase equilibria under consideration should be included in the simulation to obtain a practically useful result. Among the input data sets, the dendrite arm spacing (size of volume element) influences the simulation results most critically, and should therefore be predicted accurately using more generalized simulations, for example, those combined with macroscopic simulations.