Determination of reaustenitisation kinetics in a Fe-0.4C steel using dilatometry and neutron diffraction

The contraction associated with austenite formation in a high purity alloy of nominal composition Fe-0.4C (wt%) has been determined using a non-contact laser dilatometer. Both isothermal and continuous heating experiments have been carried out. Interpretation of the results requires accurate estimates of the lattice parameters of the phases at the reaction temperatures; these have been measured using neutron diffraction, with the lattice parameters of cementite being measured on a further steel of composition Fe-2.77C (wt%). Using a rule of mixtures, it is shown that the dilatometrical results cannot be rationalised unless the dissolution of cementite is accounted for. The experimental data have been compared with predictions from a theoretical model which assumes that the growth of austenite is diffusion-controlled and associated with the dissolution of cementite, in two distinct ways. First, experimental and theoretical time-temperature-transformation (TTT) diagrams are computed, the experimental curves being deduced from the dilatometrical data. Second, the contraction associated with reaustenitisation during continuous heating is modelled and compared with experiment. This has involved modifying the theoretical model to account for anisothermal transformation, and the details are presented. Despite the fact that the model assumes a very simplified geometry, it is shown that theory and experiment are in broad agreement, with the model able to explain the major characteristics of the transformation.