Abstract

Alumina-magnesia castable refractories, which mainly used for the steel ladle, show large permanent linear change in high temperature accompanied by the spinel formation reaction. This reaction makes the material porous, when heated without restraining, however the materials in actual use is restrained two dimensionally by the surrounding structure, which should cause different structure and characteristics of the material comparing to the laboratory evaluations. In this study, the authors investigated the influence of two-dimensional restraint on structural, physical and chemical characteristics of alumina-magnesia castable using the specimens heated under restraint for correct understanding of the properties in actual use. The authors also made investigations into the influence of the magnesia content on the characteristics of alumina-magnesia castable heated under restraint condition. To prepare two-dimensional restrained specimens, alumina-magnesia castable refractories were surrounded by other high strength castable and heated at 1000°C, 1300°C and 1500°C. Then various characteristic evaluations and the microstructure observations were carried out. The permanent expansions of restrained specimens were drastically suppressed to approximately zero, which can be used as evidence that the restraint during heating was satisfactorily carried out. The restraint during heating affected the material strength. The strength rose greatly especially after high temperature heating. The apparent porosity was decreased and the pore size became smaller. It is suggested that the increase of strength is caused by densified structure. Under unrestrained condition, the expansion after heating enlarged as the magnesia content increase. On the other hand, under restraint condition, any specimens did not substantially expand. Nevertheless, the apparent porosity of restrained specimens also rose by heating; this may indicate that the specimens expanded in the unrestrained axis direction.