The properties of magnesia-carbon bricks will change greatly due to different firing conditions. In order to study the mechanism of the performance change of magnesia-carbon bricks, technicians of Japan Heizaki Sowing and Milling Company investigated the thermal shock resistance, matrix structure and composition distribution of magnesia-zirconia bricks based on low-graphite magnesia-carbon bricks with 8% graphite content by changing firing time.
Magnesium-carbon bricks were prepared with 8% flake graphite and 92 fused magnesia and 1% 99 aluminium powder. The bricks were reduced and fired at 1400 C for 10 h and 30 h respectively. Thermal shock resistance and microstructure of the specimens were investigated. The thermal shock resistance test is to immerse the test block in hot metal at 1600 C for 90 seconds, then water-cooled for 30 seconds, and cycle three times to compare the appearance and section of the test block with cracks. The microstructures of the matrix were observed by FE-SEM and EDS. The section pictures of the specimens after thermal shock resistance test were observed. It was found that the cracks were obvious and the thermal shock resistance of the specimens was poor after 30 h reduction firing. Microscopic photographs of Al, Mg, Si and Ca in the matrix of samples sintered for 30 hours show that the original micro-powder Mg spinel exists in addition to silicon and Ca. This indicates that the secondary bonding in the matrix is active after long firing, and the high impurity content promotes the sintering of low melt, which results in the increase of elastic modulus and the decrease of thermal shock resistance.