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Published on: March 26, 2013
Effect of Cooling Rate on Phase and Crystal Morphology Transitions of CaO⁻SiO₂-Based Systems and CaO⁻Al₂O₃-Based
Mei Leng1,2, Feifei Lai3,4, Jiangling Li5,6
1College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. 201809021083@cqu.edu.cn.
Abstract:
The phase and crystal morphology transitions of two typical types of mold fluxes were investigated fundamentally using differential scanning calorimetry (DSC) and confocal scanning laser microscopy (CSLM) techniques. For the traditional CaO⁻SiO₂⁻CaF₂-based mold flux, different cooling rates can change the phases and the crystal morphologies. Faceted cuspidine and CaSiO₃ are co-precipitated when the cooling rate is less than 50 °C·min-1. The phases transform from Ca₄Si₂O₇F₂ and CaSiO₃ to Ca₄Si₂O₇F₂ at the cooling rate of 50 °C·min-1. Cuspidine shows four different morphologies: faceted shape, fine stripe, fine stripe dendrite, and flocculent dendrite. The crystalline phases of CaAl₂O₄ and Ca₃B₂O₆ are co-precipitated in the CaO⁻Al₂O₃-based mold flux. Neither the phases nor the crystal morphologies change in the low cooling rate range (5 °C·min-1 to 50 °C·min-1). With decreasing temperature, the morphology of CaAl₂O₄ firstly becomes dendritic, and then the dendritic quality gradually changes to a large-mesh blocky shape at the cooling rates of 100 °C·min-1, 200 °C·min-1, and 500 °C·min-1. Different cooling rates do not show an obvious impact on the morphology transition of CaAl₂O₄. The strong crystallization ability and large rate of crystallization affect the control of the heat transfer of the CaO⁻Al₂O₃-based mold flux during casting. The big morphology difference between primary crystals of the CaO⁻SiO₂⁻CaF2-based mold flux and the CaO⁻Al₂O₃-based mold flux is probably one of the biggest factors limiting lubrication between the CaO⁻Al₂O₃-based mold flux and high-Al steel during casting.
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