The Iron-Carbon System: Genesis and Morphology of the Eutectic Involving Hyper-Cementite Carbide

A straightforward technology for the thermal cyclic processing of the Fe-C melt has been developed to induce significant super-cooling before crystallization. Eutectic crystallization of pro-eutectic alloys under substantial super-cooling is demonstrated to be a complex process, comprising several partial crystallization processes and the synchronous dissolution of crystalline phases: austenite and two metastable carbides, Fe3C and Fe7C3. The kinetics of the eutectic transformation L→L+Fe7C3 in its microscopic and thermal (DSC) imaging has been studied. In general, crystallization proceeds according to the scheme L→L+Fe7C3+γ→L+Fe7C3+γ+Fe3C→ Fe7C3+γ+Fe3C. Consequently, plate-like eutectic (Fe7C3+γ) with an austenite matrix and ledeburite (Fe3C+γ) with a cementite matrix are formed. A schematic diagram of the metastable phase equilibria in the Fe-C system is provided. In the conducted experiments, phase transformations occur in two subsystems: Fe-Fe3C (at low supercooling) and Fe-Fe7C3 (a subsystem of metastable equilibria of the second, higher degree of metastability at large supercooling). This is confirmed by the replacement of the carbide phase and different equilibrate concentrations of austenite in metastable equilibrium with each of the carbides.