Formation of martensite - austenite constituents in the intercritically reheated HAZ of a HSLA steel

Abstract

The aim of the present study was to investigate the mechanisms of formation of Martensite - Austenite (M-A) constituents in an intercritically reheated Heat Affected Zone (HAZ) of a HSLA steel. Austenite preferentially forms during heating at 725°C at the prior austenite grain boundaries and between bainitic laths by dissolution of cementite. Nucleation is followed by a rapid growth of austenite. During cooling, the austenite fraction decreases very slightly during cooling between 750°C and 600°C, before decreasing more significantly at 600°C, when bainitic transformation begins. Thermodynamic calculations of transformed austenite fraction in equilibrium and para-equilibrium conditions show that during heating and cooling the material always tends to either equilibrium or para-equilibrium conditions. However thermodynamics cannot account for the kinetics experimentally observed, especially during cooling. Kinetics of dissolution of a cementite plate in a ferritic lath, of growth and of shrinking of the austenitic phase at 775°C and during cooling is modelled. The predictions of the model are in very good agreement with the experimental observations. In particular, the experimental evidence of two consecutive kinetic regimes during cooling could be interpreted in terms of a carbon diffusion-limited growth process.