Modeling Competitive Precipitations Among Iron Carbides During Low-Temperature Tempering of Martensitic Carbon Steel

Abstract

Upon tempering of martensitic carbon steels, transition carbides (TCs), as precipitated precursors that compete for available C atoms among multiple iron carbides, are usually regarded as either the hexagonal e-Fe2C or the orthorhombic η-Fe2C, both of which have tremendous influences on the microstructural evolution and the final properties of steels, but still cannot be clearly distinguished experimentally, so far. It is timely to grasp the corresponding atomic-scale nature to predict the real structure of the TCs. Based on maximal entropy production principle (MEPP) and atomic-scale computations, a Fokker-Planck type equation (FPE) following a modified multi-scale modeling framework, is solved for competitive precipitations among the potential iron carbides, including e-Fe2C, η-Fe2C and θ-Fe3C, during the low-temperature tempering of carbon steel. The obtained microstructural evolution path dominated by synergy of thermodynamics and kinetics indicates a precipitation sequence for TCs, i.e. e → η, which is further verified by the minimum energy path (MEP) in terms of the solid-state nudged elastic band (SSNEB) method. On this basis, the potential hexagonal-orthorhombic transformation (HOT) of TCs is discussed, by which the probable structure of TCs can be accurately identified in experiment. The current modeling framework is expected to be a worthy paradigm for reference to predict the structural evolution of TCs in engineering materials.