MODELING DIFFUSIONAL GROWTH DURING AUSTENITE DECOMPOSITION TO FERRITE IN POLYCRYSTALLINE FE-C ALLOYS

A grain-boundary-nucleated, diffusional growth model of austenite decomposition to proeutectoid ferrite is developed for polycrystalline iron-carbon alloys. The diffusion equation is solved under restricted diffusion conditions using the quasi-static method and employing local thermodynamic equilibrium at the disordered austenite: ferrite interface. Decomposition kinetics for a model polycrystalline material consisting of a log-normal distribution of spherical grains are calculated numerically. Effects of temperature, overall carbon concentration, volume change, austenite grain size and carbon buildup in the centers of the austenite grains are included in the treatment. A scaling factor is deduced that enables the effect of austenite grain size on transformation kinetics to be characterized provided kinetic information is available for one grain size. Experiments carried out on a laboratory steel verified the applicability of the scaling factor. Also, partial I-T and C-T diagrams can be computed from the model and sample calculations are presented for an iron + 0.036 wt% carbon steel.