Using a special steel casting simulator, carbon-bonded alumina filters are immersed in a steel melt, which contains artificially-generated endogenous alumina particles. Uncoated and MWCNTs-coated ceramic filters are dipped and rotated for 10 and 300 s in the melt at 1 650 C. Before and after the immersion test, the same samples are analyzed by means of computer tomography, in order to investigate the kinetic of inclusions deposition on the filter surface and possibly to measure the thickness of the in situ formed layers as a function of the immersion time. In addition, samples of the solidified steel are taken after the tests and analyzed by light and electron microscopy. The population of detected inclusions is classified in terms of size and chemistry in order to compare the filtration efficiency of the carbon-bonded filters.

The steel 18CrNiMo7-6 (AISI 4317) is treated in three different crucibles based on carbon-bonded alumina. Non-metallic inclusions in the steel are characterized by means of optical microscopy and scanning electron microscopy. Strength, ductility, and dynamic fracture toughness of the steel are evaluated at different temperatures. Furthermore, fatigue lifetimes in the very high cycle regime are determined. The treatment in a carbon-bonded alumina (A-C) crucible resulted in a relatively high inclusion content involving a high number of duplex inclusions consisting of MnS and Si-Al-O. In contrast, less but large pure MnS inclusions are observed when the steel is treated in carbon-bonded alumina-zirconia-titania crucibleswith or without a coating of carbon nanotubes (AZT-C-n and AZT-C). The steel treated in the A-C crucible exhibits the highest strength and fatigue lifetime, but the lowest energy dissipation. The relatively low inclusion content in the AZT-C treated steel results in high energy dissipation during tensile deformation. However, large MnS inclusions in the AZT-C and AZT-C-n treated steels act as crack initiation sites and reduce the fatigue lifetime. The dynamic fracture toughness is not affected by the different melt treatments. This result is explained by cell-like structures within the material that are characterized by a lower strength and an increased concentration of MnS inclusions.

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.

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