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Non-metallic inclusions can affect significantly the mechanical properties of metallic materials. Mainly for safety relevant components, it is important to reduce size and number of non-metallic inclusions. Recent research work is focused on the development of new reactive, active, and functionalized filters for the reduction of non-metallic inclusions. A detailed knowledge on the genesis of inclusions is essential to improve the entrapment capability. The paper is focused on the microstructure of inclusions formed in cast steel 42CrMo4 after the application of two filter variants. Both, inclusions on the filter surface as well as inclusions in the as-cast steel are investigated using scanning electron microscopy. Their chemical composition and their crystal structure are identified by combined EBSD and EDS measurements. The three-dimensional morphology of the inclusions is analyzed by deep etching technique as well. Finally, the clusters and agglomerates exhibit a quite complex microstructure. Thus, the majority of inclusion clusters consists of alumina, spinel, mullite, MnS, and TiOx. This agrees well to the scenario of the evolution of inclusions described in the literature.

The carbon bonded magnesia (MgO C) coatings deposited on the carbon bonded alumina (Al 2 O 3 C) 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 MgO C coating and the Al 2 O 3 C substrate as well as at the interface between Al 2 O 3 inclusions and the liquid steel. The MgO C/Al 2 O 3 C interface was prepared by slip casting. The steel/MgO C interface was formed between the quickly molten metal and the MgO C/Al 2 O 3 C stack, the steel/Al 2 O 3 interface between the quickly molten metal and the Al 2 O 3 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 Al 2 O 3 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 MgAl 2 O 4 whiskers and dense MgAl 2 O 4 layers at the MgO C/Al 2 O 3 C and at the steel/MgO C interfaces, respectively. The exogenous Al 2 O 3 inclusions inserted into the steel melt were also converted to MgAl 2 O 4 , which proves that Mg from MgO C was dissolved within the steel melt.

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.

Surface functionalized carbon bonded foam filters have been evaluated with the aid of an industrial steel melt according to their performance to filtrate non-metallic inclusions. Exogenous alumina based non-metallic inclusions have been introduced in the steel melt in order to distinguish the influence of the filter surface chemistry on the deposition of non-metallic inclusions. The used filters have been examined after casting according to the population and chemistry of the deposited inclusions. Based on the deposition results an alumina coating leads to the highest deposition of alumina inclusions.

In this paper, the effect of a mullite coating on a carbon-bonded alumina filter used in steel melt filtration on the mechanical properties of the cast steel G42CrMo4 (1.7231) at both quasi-static and cyclic loading is presented. The investigations cover quasi-static tensile tests at different temperatures. Additionally, cyclic tests in high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) regimes were performed. Fracture surfaces as well as cross-sections are examined by scanning electron microscopy and light microscopy to determine the non-metallic inclusions responsible for failure. In comparison to the uncoated filter, a mullite coating on the filter leads to a deterioration of deformation characteristics and fatigue lifetime of the cast steel. This effect is attributed to a higher amount of large non-metallic inclusions, which were not retained by the coated metal melt filter.

Carbon-bonded alumina filters for steel melt filtration are increasingly used by industries. Their potential regarding the material characteristics and the filtration efficiency appears, however, not used in full measure up to now. In the course of the present work, a new approach, due to the application of active oxide coatings, whose chemical phase equates to that of the nonmetallic oxidic inclusions in the steel melt, was created. The main emphasis consists of the generation of cold-applied, active alumina, spinel, and mullite coatings. Coatings have been developed due to the variation of sintering temperatures and a thermal pretreatment, which lead to increasing cold crushing strength. A basic understanding regarding the adhesion and the interface between the oxide coating and the carbon-bonded filter substrate has been created. Therefore, the essential factor is traceable back to thermal-related mechanisms.

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.

New carbon bonded Al2O3 filters with Magnesia–carbon (MgO–C) “reactive coatings” for high filtration efficiency of steel melts are studied with the aid of a special steel casting simulator in full controlled atmosphere. Due to the in situ reduction of MgO in the MgO–C coating and the formation of Mg vapor a reaction with the oxygen dissolved in the steel melt is triggered and leads to the formation of secondary MgO on the filter surface. With this approach not only primary and secondary inclusions can be effectively removed from the steel melt, but as well as less tertiary and quartenary inclusions can be expected due to less oxygen dissolved in the steel melt.

Ceramic foam filters (CFFs) have been used by steel foundries for several years as an effective physical refining solution after the ladle treatment. Recently, new approaches involving “active” and “reactive” coatings applied on the filters have been proposed, in order to improve their efficiency during operation. In this work, two different coating methods are presented. The first exploits the reaction of carbon-bonded alumina–magnesia filters when immersed in a steel melt at about 1650 °C. In this case, the coating is formed directly during operation: due to the carbothermal reduction of magnesia and dissolution of alumina, in situ spinel formation is observed. This also results in a volume expansion that counteracts the shrinkage of the filter material related to sintering and pyrolysis phenomena. Next, multi-walled carbon nanotubes (MWCNTs) are applied on carbon-bonded alumina filters by means of spray coating to increase the reactivity of the surface. The performance of the coated filters is assessed in comparison to uncoated ones, using a special steel casting simulator under controlled atmosphere. The microscope investigations after 10 and 30 s of immersion indicate a better performance of the MWCNTs-coated filters.