Effect of Na2O on Properties, Structure, and Crystallization of CaO-Al2O3-Based Mold Fluxes

This article investigates the effect of Na2O on melting properties, viscosity, flux structure, crystallization behavior, and heat transfer of CaO-Al2O3-based mold fluxes with Na2O concentration varied from 3.5 to 12.1 mass pct. With the increase in Na2O content, both melting temperature and viscosity at a given temperature decrease. The Raman spectroscopy indicates that increasing the Na2O content decreases the degree of polymerization, in line with the decrease in viscosity. Flux crystallization is studied using single hot thermocouple technology (SHTT) and double hot thermocouple technology (DHTT). The results show that flux crystallization tendency is enhanced with the increase in Na2O content. This increase is more significant when the Na2O content changes from 9.1 to 12.1 mass pct. X-ray diffraction analysis of the fluxes heat treated at different temperatures reveals that the crystal precipitates vary with the flux composition and temperature. Heat transfer measurement using the IR emitter technique (IET) shows that the increase in Na2O concentration impedes the heat transfer, in line with the increased crystallinity of the fluxes.