Theory for reaustenitisation from ferrite/cementite mixtures in Fe-C-X steels

Phase transformation theory for the formation of austenite from substitutionally alloyed ferrite/cementite mixtures is discussed. The local equilibrium assumption is adopted. Diffusion of carbon in ferrite and cementite is ignored, together with the ternary diffusional interactions. The ferrite and cementite phases are assumed to be semi-infinite in extent, so that soft impingement effects are not accounted for. Subject to these assumptions, exact solutions are presented for one-dimensional (planar) growth. The solution method involves the determination of the appropriate tie-lines at the cementite/austenite and austenite/ferrite interfaces. Potential numerical difficulties which arise in determining the solution are discussed. It is found that for reasonable values of the diffusion coefficients involved, the governing equations can be simplified. The results are presented in the form of reverse time-temperature-transformation diagrams, and comparisons with recent results for reaustenitisation from binary Fe-C steels are made. The effects of silicon and manganese as ternary alloying additions are quantified.