Kinetic Transitions and Alloying Element Partitioning During Austenite Growth from a Ferrite-Cementite Mixture: Role of Heating Rate

Abstract

Austenite formation from a ferrite-cementite mixture is a crucial step during the processing of advanced high strength steels (AHSS). The ferrite-cementite mixture is usually inhomogeneous in both structure and composition, which makes the mechanism of austenite formation very complex. In this contribution, austenite formation upon continuous heating from a designed spheroidized cementite structure in a model Fe-C-Mn alloy was investigated with an emphasis on the role of heating rate in kinetic transitions and element partitioning during austenite formation. Based on partition/non-partition local equilibrium (PLE/NPLE) assumption, austenite growth was found alternately contribute by PLE, NPLE and PLE controlled interfaces migration during slow-heating, while NPLE mode predominately controlled the austenitization by a synchronous dissolution of ferrite and cementite upon fast-heating. It was both experimentally and theoretically found that there is a long-distance diffusion of Mn within austenite of the slow-heated sample, while a sharp Mn gradient was retained within austenite of the fast-heated sample. Such a strong heterogeneous distribution of Mn within austenite cause a large difference in driving force for ferrite or martensite formation during subsequent cooling process, which could lead to various final microstructures. The current study indicates that fast-heating could lead to unique microstructures which could hardly be obtained via the conventional annealing process.