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Carbon-bonded alumina filter materials—with and without coatings of carbon-bonded MgO and carbon-free Al 2 O 3 —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, MgAl 2 O 4 spinel whiskers were formed in situ in the hollow filter struts.

Spinel-based ceramics for molten steel filtration were prepared from a mixture of alumina, magnesia and carbon precursors. Samples were analyzed in the form of bars heat treated at different temperatures, or as reticulated foams, after immersion in a steel melt at 1640 °C. An effect of the composition of the filters on the amount of spinel formed after the heat treatments was observed, which favored the MgO-rich composition. The formation of spinel successfully decreased the disadvantageous shrinkage occurring because of high temperature sintering and pyrolysis phenomena. The presence of carbon affected the rate of spinel formation, and led to additional carbide-based phases after heating at 1600 °C. The immersion of reticulated foams, only pre-heated at 800 °C, into molten steel at 1640 °C led to the virtual transformation of the starting oxides into spinel, and the foams survived well the thermal shock. These materials are therefore well suited for the fabrication of advanced ceramic filters for molten metal filtration.

Slip casted, carbon bonded nozzles with and without alumina based active coatings have been simultaneously tested in a novel casting simulator according to their clogging behavior against endogenous as well as exogenous non-metallic inclusions. Computer tomography images as well as SEM micrographs support the investigation of the clogging areas. The nozzle with the alumina active coating leads to increased clogging phenomena based on endogenous and exogenous inclusions as well as steel clogged particles. Correlating the clogging to a type of improved “filtration efficiency”, a higher filtration of steel melts is expected with the application of alumina active coatings on carbon bonded filters.

Carbon-bonded alumina filters for steel melt filtration are increasingly used by steel foundries. Their potential regarding material characteristics and filtration efficiency has not been fully exploited yet. A new filter composition based on the synthetic coal tar pitch Carbores ® P, has been explored. Its compatibility with a water-based dispersing system allows additions of carbon black, graphite, and additives to reach a residual carbon content of up to 30 wt.%. For determining physical properties and variables of the respective compositions, 10 pores per inch (ppi) foam ceramic filters were manufactured via the Schwartzwalder process. The influence of different binder contents on the processing properties and mechanical characteristics of the system was evaluated. Based on these studies, an increase in cold crushing strength of approx. 30% has been achieved in comparison to current state of the art materials.