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

The characteristics and transformation of M 2 C and MC, the most common primary carbides in Cr4Mo4V bearing steel, have been investigated via scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction at high austenitization temperatures. It was first found that the transformation of rod-like M 2 C and spherical MC primary carbides during heating occurs at approximately 1180 and 1200 °C, respectively. In the transformation process of M 2 C, the periphery first transforms into an MC-type carbide, and then the internal untransformed M 2 C and newly-formed MC-type melt, and re-solidify in a M 2 C" 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 M 2 C" 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 M 2 C". In addition to elucidate the transformation mechanisms, this study also highlights the essential effect of the melting matrix on the modification of primary M 2 C and MC carbides in Cr4Mo4V steel. • Transformation of primary M 2 C and MC carbides in Cr4Mo4V were investigated at high austenitization temperatures. • Primary M 2 C carbide is rod-like Mo − M 2 C, while M 2 C is spherical V − MC, and they both remain unchanged below 1180 ℃. • The transformation of Mo − M 2 C consists of a solid-solid phase transformation, melting, and re-solidification. • The transformation of V − MC consists of melting and re-solidification, and the product is finer M 2 C". [ABSTRACT FROM AUTHOR]