Effect of solidification upon features of primary carbides in niobium-titanium high-speed steels

Multi-stage solidification process is responsible for final microstructure of high-speed steels (HSSs). The typical HSSs grades contain large amount of eutectic carbides, mostly of M₆C and M₂C types, inhomogeneously distributed in a metal matrix. Application of niobium and titanium in alloying these steels results in increase of primary MC carbides precipitation temperature. In the investigated Nb-Ti HSSs characteristic carbon chains surrounding large MC particles were observed. These carbides have been analysed using scanning and transmission electron microscopy as well as high-temperature calorimetry. The results indicate that in the Nb-Ti HSSs an eutectic constituent has a form of M₂₃C₆ and M ₂C carbide chains around large MC type primary carbides of niobium and titanium. The carbide chains have contributed to good embedding of the MC particles in the matrix and consequently better service properties of the tools.