A New Approach for Refining Carbide Dimensions in M42 Super Hard High-speed Steel

Abstract

Obtaining small carbides is crucial but difficult for high-speed steels. A new approach for refining carbide dimensions in M42 super hard high-speed steel by increasing cooling rate and spheroidizing treatment was proposed. The morphologies and properties of eutectic carbides formed at different cooling rates were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and differential scanning calorimeter (DSC). The results show that eutectic carbides change from a lamellar shape into a curved-rod shape as cooling rate increases. Despite different morphologies, the two carbides are both of M₂C type with a hexagonal close-packed structure and display a single crystal orientation in one eutectic colony. The morphology of M₂C mainly depends on the growing process of eutectic carbides, which is strongly influenced by cooling rate. Compared with lamellar carbides, M₂C carbides with curved-rod shapes are less stable, and decompose into M₆C and MC at lower temperatures. They are more inclined to spheroidize during heating, which ultimately and distinguishably refines the carbide dimensions. As small carbides are much easier to dissolve into matrices during austenization, the process described herein improves the supersaturation of alloying elements in martensite, which leads to an increment of hardness in M42 steel.