Functionalized Carbon‐Bonded Filters with an Open Porous Alumina Coating: Impact of Time on Interactions and Steel Cleanliness

Understanding interactions between filter and molten steel is essential to improve the purity of casted products by filtration. Characteristic, in situ formed layers on the surface of carbon‐bonded alumina filters result from these interactions. To comprehend their formation, this study illustrates the time dependency of the layer buildup. Therefore, reactions at the filter/steel interface under quasi static conditions are examined using spark plasma sintering (SPS) equipment. Immersion tests in a steel casting simulator, which provides close‐to‐reality conditions, complement these investigations. Microstructure and phase analyses reveal that interfacial reactions between filter and steel lead to a thin in situ formed layer on the filter surface. During a “reactive” stage, large polycrystalline alumina structures are formed. Thereby, material is transported both from the carbon‐bonded material underneath (i.e., gaseous reaction products) and from the molten steel (i.e., precipitating particles and endogenous inclusions) to the filter/steel interface. The formation of these alumina particles comes to an end as soon as the carbon supply, which triggers the dissolution and precipitation processes at the interface, is cut‐off. From that point on, endogenous inclusions are deposited on them (“active” stage). The filters were most efficient during the reactive stage, that is, as long as the interfacial reactions take place. Interfacial reactions of molten steel and filter material distinctly influence the filtration performance. We observe that large polycrystalline alumina structures are formed in situ. The required material is supplied as well from the carbon‐bonded material underneath (i.e., gaseous reaction products) as from the molten steel (i.e., precipitating particles and endogenous inclusions). During that stage, filtration is most efficient because endogenous inclusions from the steel are entrapped and strongly bound to the filter.