Characterization of carbon-bonded alumina filters with active or reactive coatings in a steel casting simulator

Carbon-bonded alumina filter materials—with and without coatings of carbon-bonded MgO and carbon-free Al2O3—were tested in a steel casting simulator in contact with steel melts containing defined endogenous alumina inclusions. For the evaluation of the filtration efficiency of the different filter surfaces, prismatic samples were immersed in the steel melt under an argon atmosphere at 1650 °C. The carbon-bonded alumina filter material reacted in situ in the steel melt, forming gaseous alumina sub-oxides. These sub-oxides reacted with the dissolved oxygen of the steel and formed a thin crystalline α-alumina layer on the area in contact with the steel. The secondary alumina layer was formed on the carbon-bonded alumina surface as well as on the alumina-coated carbon-bonded alumina substrate. The newly formed layer reflected the roughness of the substrate, influencing the wettability and, therefore, the deposition behavior of the inclusions on the filter material surfaces tested. Sintered fine endogenous inclusions were identified in various shapes on this thin layer. In the case of carbon-bonded magnesia on a carbon-bonded alumina substrate, carbothermal reduction of the magnesia and subsequent reoxidation to secondary magnesia in the form of a thin layer on the contact area with the steel was observed. Based on the carbothermal reduction of alumina and magnesia, MgAl2O4 spinel whiskers were formed in situ in the hollow filter struts.