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

The non-isothermal crystallization behaviors of CaO-BaO-Al 2 O 3 -based mold fluxes in which SiO 2 had been replaced by Na 2 O were investigated, using an infrared furnace combined with digital microscopy as well as X-ray diffraction.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 N O (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 Na 2 O content in the flux. In addition, the precipitation of CaF 2 crystals was inhibited while the growth of CaAl 2 O 4 crystals was enhanced at Na 2 O concentrations over 4 wt%. Increasing the cooling rate inhibited the precipitation of MgAl 6 O 10 , and of CaSiO 3 , MgAl 2 SiO 6 and NaAlSi 3 O 8 , respectively, in fluxes containing 0, 4 and 8 wt% Na 2 O (NS1, NS2 and NS3 samples). The peak instantaneous and average crystallization rates in these specimens were increased as the Na 2 O level was raised, as a direct result of changes in their N O values. The NS1 flux consistently had the lowest N O value, while the NS2 had the largest value in the primary stage of crystallization but the central value in the secondary stage, and the N O values of the NS3 flux exhibited the opposite trend. The effective crystallization activation energies decreased with increasing Na 2 O levels, and this was evidently an important factor affecting the initial crystallization temperature. [ABSTRACT FROM AUTHOR]