Autocatalytic Mechanism of Pearlite Transformation in Steel

A model of pearlite colony formation in carbon steels is developed, with a parametrization based on ab initio calculations. The model describes the processes of decomposition of austenite and formation of cementite through a metastable intermediate structure, with a crucial role of ferromagnetic order arising under the cooling. The autocatalytic mechanism of pearlite colony formation is analyzed and conditions for its implementation are established. We demonstrate that pearlite with lamellar structure is formed by the autocatalytic mechanism when thermodynamic equilibrium between the initial phase (austenite) and the products of its decomposition (cementite and ferrite) cannot be reached. The transformation diagram is suggested, and various scenarios of decomposition kinetics are investigated by phase-field simulations. By using a model expression for the free energy with first-principles parametrization, we find conditions of the formation of both lamellar and globular structures, in good agreement with experimental data.