Effect of substituting Na2O for SiO2 on the non-isothermal crystallization behavior of CaO-BaO-Al2O3 based mold fluxes for casting high Al steels

Abstract The non-isothermal crystallization behaviors of CaO-BaO-Al2O3-based mold fluxes in which SiO2 had been replaced by Na2O were investigated, using an infrared furnace combined with digital microscopy as well as X-ray diffraction. The initial crystallization temperatures, crystalline phases generated at different cooling rates, crystallization rates, Ozawa indices NO (as determined using Mo's equation) and effective crystallization activation energies (as determined using the Friedman equation) were evaluated. The initial crystallization temperature increased along with the Na2O content in the flux. In addition, the precipitation of CaF2 crystals was inhibited while the growth of CaAl2O4 crystals was enhanced at Na2O concentrations over 4 wt%. Increasing the cooling rate inhibited the precipitation of MgAl6O10, and of CaSiO3, MgAl2SiO6 and NaAlSi3O8, respectively, in fluxes containing 0, 4 and 8 wt% Na2O (NS1, NS2 and NS3 samples). The peak instantaneous and average crystallization rates in these specimens were increased as the Na2O level was raised, as a direct result of changes in their NO values. The NS1 flux consistently had the lowest NO value, while the NS2 had the largest value in the primary stage of crystallization but the central value in the secondary stage, and the NO values of the NS3 flux exhibited the opposite trend. The effective crystallization activation energies decreased with increasing Na2O levels, and this was evidently an important factor affecting the initial crystallization temperature.