Abstract
Cooling conditions of quasi-spherical particles in atomization process belong to important factors influencing nucleation and primary microstructure development in rapidly solidified particles. In the dependence on the cooling rate of molten particles determined by combined convection and radiative heat transfer coefficient from particle surfaces to the atomizing gas, morphologically different microstructures can be formed in the droplets with the same diameters. Five main types of microstructures were experimentally identified in the rapidly solidified powder particles of hypereutectic iron based alloy Ch3F12: microstructure with dispersed carbides, microstructure with star-like carbides, microstructure with primary carbides in the centers of eutectic colonies, microstructure with eutectic colonies without primary carbides and microstructure with eutectic spherulites. Based on the mathematical model of heat transfer by rapid cooling and solidification of undercooled melt droplets, the influence of initial atomizing gas velocity on the heat transfer coefficient, theoretical temperatures of homogeneous nucleation and time of quasi-isothermal periods of spherical particles from Ch3F12 alloy is analysed in this paper.

Key words
gas atomization process, hypereutectic alloy, rapid solidification, homogeneous nucleation, microstructure development, numerical simulation

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