Formation mechanism of MC and M2C primary carbides in as-cast M50 bearing steel

Based on thermodynamic calculations and experimental characterizations, the characteristics, formation mechanism, and evolution process of two typical primary carbides in as-cast M50 bearing steel were systematically investigated. According to X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations, MC and M2C appear as the V-rich FCC structure and Mo-rich HCP structure, respectively. Thermodynamic calculations show that both carbides cannot precipitate from liquid directly under thermoequilibrium solidification. However, due to the solute-enriched phenomenon during solidification, primary carbides can still form. It is further found that, though both M2C and MC nucleate at these segregated zones, M2C precipitate earlier than MC. It is caused by the distinct segregation extents of Mo and V and undercooling of two primary carbides, which consequently also lead to the difference of growing evolution process and the final carbides morphology. Besides, the Kikuchi patterns obtained by electron backscattered diffraction (EBSD) and the TEM dark-field micrograph show that nucleation position and growth direction of MC and M2C carbides have a selective orientation at the austenite boundaries. Finally, two significant ideas of increasing nucleation by breaking dendrite and adjusting the proportion of Mo and V contents are proposed to control large-sized primary carbides in engineering.