Primary carbide transformation in a high performance micro-alloy at 1000 °C

Abstract The transformation from M 7 C 3 → M 23 C 6 in a high performance micro alloy (HP-MA) was studied using different techniques. Following the cooling during the centrifugal casting, the as-cast HP-MA alloy consists of an austenitic matrix with the primary carbide network consisting of a combination of M 7 C 3 (M being mainly chromium) and MC (M being mainly niobium). During the heat treatment at 1000 °C, the primary chromium carbides transform from M 7 C 3 to M 23 C 6 . The incompletely transformed primary carbide consists of an outer shell of M 23 C 6 carbide (coherent with the austenitic matrix) and a core of M 7 C 3 type carbide. The experimentally determined M23C6 shell thickness agrees reasonably well with the estimated diffusion distance of carbon in M 23 C 6 based on Zener's relation implying that the M 7 C 3 → M 23 C 6 transformation is mainly controlled by the diffusion of the carbon from M 7 C 3 to the matrix through the M 23 C 6 shell. This mechanism is discussed vis-a-vis the literature reported mechanism on the transformation.