Crystallization Behaviors of Mold Fluxes Containing Li₂O Using Single Hot Thermocouple Technique

Abstract

Minor addition of Li₂O (<2 mass%) can significantly alter the melting temperature and viscosity of a mold flux. However, the crystallization tendency of mold fluxes containing Li₂O more than 2 mass% hasn't been understood. In this paper, single hot thermocouple technique, X-ray diffraction, and Back Scattered Electron (BSE) were employed to investigate the crystallization behavior concerned with CCT and TTT diagrams of mold fluxes containing high content of Li₂O.
The critical cooling rate and crystallization rate of samples were increased with the addition of Li₂O, whether there is Na₂O or not. But the incubation time was reduced obviously with Li₂O more than 6 mass% in mold fluxes. The critical cooling rate and incubation time of NL_1.5 sample (Na₂O=8 mass%, Li₂O=1.5 mass%) were close to those of L₇ sample (Na₂O=0 mass%, Li₂O=7 mass%). Moreover, there is no difference in the crystal phase of the two samples, and the crystal phases precipitated at high temperature and low temperature were respectively Ca₂(SiO₄) and Ca₄Si₂O₇F₂. Li-cuspidine was promoted to precipitate when the basicity of the sample was increased. The small, dense grains of cuspidine were observed in NL_1.5 sample, and the thickness of slag film was 0.4 mm more than that of L₇ sample. The heat flux of NL_1.5 sample measured was 10% less than that of L₇ sample, as expected. Therefore, high content of Li₂O can be substituted by the combination of Na₂O and minor of Li₂O, as is the composition in the commercial mold fluxes.