A 2D analysis of the competition between the equiaxed ferritic and the bainitic morphology based on a Gibbs Energy Balance approach

Abstract Upon austenite decomposition, either of two ferritic morphologies may result depending on the steel composition and the transformation temperature, i.e. equiaxed (i.e. quasi spherical) or plate-like ferrite. A 2-D model based on the Gibbs Energy Balance (GEB) model and the Zener–Hillert equation is presented to explain the conditions leading to either morphology. To this aim the kinetics of isotropic growth and plate-lengthening, plate-thickening are modeled with appropriate assumptions regarding the interface conditions respectively. The competition between the two morphologies is determined by the fastest minimization of the total Gibbs energy of the system during transformation. The model is first applied to a ternary alloy containing an austenite forming element (Fe-0.23C-1.86Mn) and the predicted morphology competition is in good agreement with experimental observations. In the refined model, the effect of nucleation on the growth of a plate is rationalized. For the plate-like bainitic transformation, a new concept of ‘single plate-thickening stasis’ is proposed, in addition to the commonly discussed ‘global stasis of the bainitic transformation’, i.e. the incomplete bainitic transformation. A general model of the global transformation kinetics bridging the single plate stasis and the global stasis is depicted. Finally, without changing any of the model assumptions or transition conditions the model is applied to a ternary alloy containing a ferrite forming element (Fe-0.375C-1.48Si) The predicted morphology competition is fully in line with the experimental observations for samples subjected to isothermal decomposition at different temperatures.