Structure of high-speed steel with high solidification rates

1. The interaction of the particles in spraying to a significant degree determines the conditions of solidification and the final structure of high-speed steel powder. After colliding with larger drops the highly dispersed, rapidly solidifying particles play the role of foreign internal centers of solidification on the surface of which heterogeneous formation of nuclei of the crystallizing phase starts. 2. After cooling of the molten metal at rates of 104–4·105 deg/sec the primary phases in the structure of the high-speed steel are martensite, austenite, and types MC, M2C, and M6C carbides. An increase in the cooling rate within these limits leads to an increase in the share of austenite and a corresponding decrease in the share of martensite in the structure of the steel with a simultaneous reduction in the carbon and alloy element content in the martensite. 3. To a certain degree the structural inhomogeneity characteristic of cast high-speed steels is maintained after cooling at high rates. The branches of the dendrites consists of lamellar martensite and high-alloy austenite and the carbides are concentrated in the interdendritic space, forming a continuous network in the volume of the particles. 4. An increase in the cooling rate in solidification to 106 deg/sec and more causes sharp changes in the structure of the high-speed steel. The sharply expressed dendritic structure at low cooling speeds disappears, the processes of precipitation of primary carbides are partially or completely suppressed, and the primary phases in the structure become α- and γ-solid solutions.