Forced heat exchange and cooling rate of drops at producing metal powders by melt water atomizatio

Abstract

of the technological process for the production of metal products using the powder metallurgy method. Among the wide variety of methods for making powders, in terms of efficiency and productivity, the spraying of melts with high-pressure water is favorably distinguished, in which the geometric shape and microstructure of particles significantly depend on the cooling rate, which is associated with the peculiarities of heat transfer in the melt drop – water – gas phase system. However, experimental confirmation of one or another mode of heat transfer is not possible, which is associated with the small size of particles, the number of which in 1 cm 3 of the spray zone is about 10 7 . The influence of the thermal physics of the sputtering process on the solidification rate of metal droplets is analyzed. Based on the analysis of the known regularities and phenomena of heat and mass transfer, the heat transfer coef-ficients, the heat flux density for various heat transfer modes, as well as the contribution of the conductive, convective and radiation components of heat transfer are estimated. It is shown that the fraction of heat transfer by radiation is no more than 0.1 %. It has been theoretically proven that droplets are cooled in the bubble mode of water boiling. For tool steel, the particle cooling rate calculated on the basis of the heat transfer coefficient in the developed bubble mode is about 10 6 K/s. Such a cooling rate during bubble or pseudobubble boiling is provided when the resulting vapor film is "torn off" by the water flow, simulating "pseudobubble" cooling. The formation of a cellular structure of powders, which is observed in practice during spraying with water under pressure, confirms the correctness of the theoretical calculations. The results of the work can be used to substantiate the thermal regimes for obtaining powders with various structures, including amorphous ones.