DETERMINATION OF REAUSTENITIZATION KINETICS IN A FE-0.4C STEEL USING DILATOMETRY AND NEUTRON-DIFFRACTION

The contraction associated with austenite formation in a high purity a lloy 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 requi res accurate estimates of the lattice parameters of the phases at the reaction temperatures; these have been measured using neutron diffract ion, with the lattice parameters of cementite being measured on a furt her 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 ass umes that the growth of austenite is diffusion-controlled and associat ed with the dissolution of cementite, in two distinct ways. First, exp erimental and theoretical time-temperature-transformation (TTT) diagra ms are computed, the experimental curves being deduced from the dilato metrical data. Second, the contraction associated with reaustenitisati on during continuous heating is modelled and compared with experiment. This has involved modifying the theoretical model to account for anis othermal transformation, and the details are presented. Despite the fa ct 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. (C) 1998 Else vier Science S.A. All rights reserved.