Crystallization Products and Structural Characterization of CaO-SiO₂-Based Mold Fluxes with Varying Al₂O₃/SiO₂ Ratios.

Abstract

During the casting of high aluminum steel, the dramatic increase in the Al₂O₃/SiO₂ ratio is inevitable, resulting in significant changes of the crystallization behavior, which would result in heat transfer and lubrication problems. Crystallization products and structure characterization of glassy CaO-SiO₂-based mold fluxes with different Al₂O₃/SiO₂ ratios were experimentally investigated using a differential scanning calorimetry technique and Raman spectroscopy. With increasing Al₂O₃/SiO₂ ratios, the following results were obtained. The crystallization temperature and the crystallization products are changed. With increasing Al₂O₃/SiO₂ ratios from 0.088 to 0.151, the crystallization temperature first increases greatly from 1152 °C to 1354 °C, and then moderately increases. The crystallization ability of the mold flux is strengthened. The species of the precipitated crystalline phase change from two kinds, i.e., Ca₄Si₂O₇F₂ and Ca₂SiO₄, to four kinds, i.e., Ca₄Si₂O₇F₂, Ca₂SiO₄, 2CaO·Al₂O₃·SiO₂ and CaAlOF₂, the crystallization ability of Ca₄Si₂O₇F₂ is gradually attenuated, but other species show the opposite trend. The results of Raman spectroscopy indicate that Al mainly acts as a network former by the information of [AlO₄]-tetrahedral structural units, which can connect with [SiO₄]-tetrahedral by the formation of new bridge oxygen of Al⁻O⁻Si linkage, but there is no formation of Al⁻O⁻Al linkage. The linkage of Al⁻O⁻Si increases and that of Si⁻O⁻Si decreases. The polymerization degree of the network and the average number of bridging oxygens decrease. Further, the relatively strong Si⁻O⁻Si linkage gradually decreases and the relatively weak Al⁻O⁻Si gradually increases. The change of the crystalline phase was interpreted from the phase diagram and structure.