Crystallization products and structural characterization of CaO-SiO 2 -based mold fluxes with varying Al 2 O 3 /SiO 2 ratios

Abstract

During the casting of high aluminum steel, the dramatic increase in the Al 2 O 3 /SiO 2 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 Al 2 O 3 /SiO 2 ratios were experimentally investigated using a differential scanning calorimetry technique and Raman spectroscopy. With increasing Al 2 O 3 /SiO 2 ratios, the following results were obtained. The crystallization temperature and the crystallization products are changed. With increasing Al 2 O 3 /SiO 2 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., Ca 4 Si 2 O 7 F 2 and Ca 2 SiO 4 , to four kinds, i.e., Ca4Si2O7F2, Ca 2 SiO 4 , 2CaO·Al 2 O 3 ·SiO 2 and Ca 12 Al 14 O 32 F 2 , the crystallization ability of Ca4Si2O7F2 is gradually attenuated, but other species show the opposite trend. The results of Raman spectroscopy indicate that Al3+ mainly acts as a network former by the information of [AlO 4 ]-tetrahedral structural units, which can connect with [SiO 4 ]-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.