Reaction mechanism between the carbon bonded magnesia coatings deposited on carbon bonded alumina and a steel melt

The carbon bonded magnesia (MgOC) coatings deposited on the carbon bonded alumina (Al2O3C) substrate are intended to be used as reactive filters for filtration of steel melts containing high oxygen levels and inclusions. The aim of this study was to describe the chemical reactions running at 1600°C at the interface between the MgO–C coating and the liquid 42CrMo4 steel, at the interface between the MgOC coating and the Al2O3C substrate as well as at the interface between Al2O3 inclusions and the liquid steel. The MgOC/Al2O3C interface was prepared by slip casting. The steel/MgOC interface was formed between the quickly molten metal and the MgOC/Al2O3C stack, the steel/Al2O3 interface between the quickly molten metal and the Al2O3 particles added to the steel melt. In order to imitate the temperature profile during a casting process and concurrently to avoid the metal convection, the spark plasma sintering was used for the steel melting. The chemical reactions at the interfaces were concluded from the phase formation after different reaction times. The results of the electron microscopy, X-ray spectroscopy and electron backscatter diffraction indicate the reduction of MgO and Al2O3 by carbon contained in the binder, the formation of CO gas, the evaporation of metallic Mg and Al and the dissolution of the vaporised metals in the steel. The above experimental methods revealed formation of MgAl2O4 whiskers and dense MgAl2O4 layers at the MgOC/Al2O3C and at the steel/MgOC interfaces, respectively. The exogenous Al2O3 inclusions inserted into the steel melt were also converted to MgAl2O4, which proves that Mg from MgOC was dissolved within the steel melt.