Transformation behavior of primary MC and M2C carbides in Cr4Mo4V steel

Abstract The transformation of primary MC and M 2 C carbides, during high-temperature treatment, was investigated by quasi in-situ observation of Cr4Mo4V steel. It was found that the V-rich rod-like MC carbides have great thermal stability, and disintegrating and coarsening are their dominant transformation mechanisms. The M 2 C carbides mainly possess rod-like ( M 2 C-R ) and lamellar ( M 2 C-L ) morphologies. The M 2 C-R carbides are Mo-enriched, and their transformation follows the paths: the separate precipitation of MC and M 2 C-R  +  γ → MC +  M 6 C . The V- and Fe-rich MC carbides nucleates randomly in the interior of M 2 C-R before the nucleation of Mo- and Fe-rich M 6 C carbides, which is controlled by the diffusion of V. Soaking for longer durations can cause spheroidization and coarsening of the MC carbides, along with the dissolution of M 6 C carbides. The evolution of Fe- and Mo-rich M 2 C-L carbides consists of two stages: the partial dissolution of M 2 C-L into the matrix and the transformation of M 2 C-L → MC +  γ . These novel findings contribute towards the understanding of how primary carbides evolve during heat treatment and provide a potential method for carbides refinement in Cr4Mo4V steel.