Formation Behavior of M2C and M6C Eutectic Carbides in M42 High-Speed Steel

M2C eutectic carbide favours the mechanical properties of high-speed steels, but is often largely replaced by coarse M6C eutectic carbide in as-cast M42 steel. To deeply understand the formation behavior of M2C and M6C carbides, M2C and M6C eutectic alloys were prepared according to the composition of M2C and M6C eutectic mixtures in M42 steel, and their solidification behavior was investigated. Only one type of eutectic carbide is formed in water-quenched M2C and M6C eutectic alloys, i.e., M2C and M6C, respectively. Both M2C and M6C carbides appear in the alloys cooled at 3°C/min. However, the M2C eutectic alloy was more significantly affected in terms of carbide type by the low cooling rate. According to thermodynamic calculation, M6C carbide in the M2C eutectic alloy is only slightly more stable in thermodynamics above 1210.1°C, below which M2C carbide becomes stable. For the M6C eutectic alloy, however, only M6C eutectic carbide is thermodynamically stable. Furthermore, thermodynamic results reveal that besides raising the content of C and V, reducing the content of Mo can also greatly promote the formation of M2C carbide in M42 steel, which updates the traditional opinion on the influence of Mo element. The results in this work provide the underlying insights needed to promote the formation of M2C carbide in M42 steel by fine-tuning the composition.