Characteristics and transformation of primary carbides during austenitization in Cr4Mo4V bearing steel

The characteristics and transformation of M2C and MC, the most common primary carbides in Cr4Mo4V bearing steel, have been investigated via scanning electron microscopy, energy dispersive spectrometry and X-ray dif-fraction at high austenitization temperatures. It was first found that the transformation of rod-like M2C and spherical MC primary carbides during heating occurs at approximately 1180 and 1200 degrees C, respectively. In the transformation process of M2C, the periphery first transforms into an MC-type carbide, and then the internal untransformed M2C and newly-formed MC-type melt, and re-solidify in a M2C" eutectic carbide structure with a composition change and a finer lamellar spacing. In contrast, MC carbides melt directly with the re-solidified product also being constituted of finer M2C" eutectic carbides. The transformation process is controlled by the diffusion of alloying elements Mo and V, while the matrix provides Fe for the formation of M2C". In addition to elucidate the transformation mechanisms, this study also highlights the essential effect of the melting matrix on the modification of primary M2C and MC carbides in Cr4Mo4V steel.