Numerical Investigations of Phase Transformations Controlled by Interface Thermodynamic Conditions during Intercritical Annealing of Steels

Austenite formation was numerically investigated using Thermo-Calc/DICTRA in a deformed ferrite/pearlite microstructure to produce dual-phase steels. This work aims to better understand how the interface conditions (local equilibrium with negligible partitioning-LENP-or local equilibrium with partitioning-LEP) control the austenite growth kinetics during the intercritical annealing. Inspired by our experimental observations, two nucleation sites were considered. The austenite formed from pearlite islands showed a regime transition from LENP to LEP when the holding stage started. For the growth of austenite from isolated carbides, three stages were identified during the heating stage: first, slow growth under LEP; then, fast growth under LENP; and finally, after dissolution of the carbide, slow growth again. LENP and LEP interface conditions may coexist thanks to these regime transitions. In the case of competition, LEP conditions hinder austenite growth while it is promoted by LENP interface conditions. Such differences in growth kinetics explain, in part, the morphogenesis of dual-phase microstructures.

Automated summary

Austenite formation in deformed ferrite/pearlite microstructure for dual-phase steels was numerically investigated. The study focused on understanding the role of interface conditions (local equilibrium with negligible partitioning-LENP-or local equilibrium with partitioning-LEP) in controlling austenite growth kinetics during intercritical annealing. Two nucleation sites were considered based on experimental observations. The regime transition from LENP to LEP was observed for austenite formed from pearlite islands during the holding stage. Three stages were identified for austenite growth from isolated carbides during the heating stage: slow growth under LEP, fast growth under LENP, and slow growth again after carbide dissolution. These regime transitions allowed coexistence of LENP and LEP interface conditions. In competition, LEP conditions hindered austenite growth while LENP interface conditions promoted it, explaining the morphogenesis of dual-phase microstructures.