Evolving mechanism of eutectic carbide in as-cast AISI M2 high-speed steel at elevated temperature

The evolution in type, size and shape of carbides in as-cast American Iron and Steel Institute (AISI) M2 high-speed steel before and after annealing were investigated. The micromechanism which was responsible for those changes was also analyzed and discussed. At the initial stage of reheating, metastable M₂C-type carbide decomposed continuously. M₆C-type carbide nucleated at the interface of M₂C/γ firstly and grow from surface to center. Then MC-type carbide nucleated at both surface of M₆C/M₆C and inner of M₆C. With the increasing decomposition of the metastable M₂C-type carbide, the rod-shaped construction of eutectic carbide began necking, fracturing and spheroidizing gradually. Held enough time or reheated at higher temperature, particle-shaped product aggregated and grew up apparently, while secondary carbide precipitated in cell and grew up less significantly than the former. Based on the above microstructural observation, the thermodynamic mechanism for decomposition of M₂C carbide, for spheroidization of products, and for the growth of particles were analyzed. The rate equations of carbides evolution were derived, too. It shows that the evolving rate is controlled by diffusion coefficients of alloy atoms, morphology of eutectic carbides and heating temperature.