Reduction of the non-metallic inclusions (NMI) content is crucial for obtaining high quality steel and other alloys. This is achievable by different refining methods, among which metal melt filtration by ceramic foam filters is a very promising one. Steel refining by ceramic foam filters can be divided into two principal mechanisms: active and reactive filtration. To study the influence of different reactive filter coatings on NMI population and mechanical properties of 42CrMo4 steel prismatic (125×20×20 mm3) ceramic foam filters based on carbon-bonded alumina were produced by the replica technique. Functional coatings were applied on the filters by cold spray coating and the filters were heat treated again under reducing atmosphere. Several types of filter coatings were investigated: reference filter with no coating; alumina (Al2O3); alumina-based slurry (AC5); carbon nano-tubes (CNT) with alumina nano-sheets (ANS); and calcium hexaluminate (CA6).
Filters were immersed in a 42CrMo4 steel melt within a specially designed steel casting simulator. The solidified steel was analyzed with respect to the size distribution and the chemical composition of the remaining non-metallic inclusions (NMI) by means of sections imaging, chemical extraction and fractography. Cyclic loading and quasi-static tests were performed in order to determine the fatigue limit, the strength, deformability and toughness of the steel after filter immersion. The main results can be summarized as follows:
1) The application of the CA6-coated filter led to the most significant changes in NMI population in comparison to the reference state by reducing the total number of NMIs to 70%.
2) The chemical analysis of NMIs revealed mostly pure alumina and other oxides (e.g. CaO, SiO2, and mullite (Al2O3-SiO2)). All tested filter coatings reduced the total amount of alumina and increased the total amount of other oxides.
3) Plate-like alumina inclusions were the most detrimental in terms of fatigue strength causing 100% of fatigue cracks. Treatment with the CA6-coated filter resulted in the lowest fatigue limit due to the largest NMIs and in the same time in higher deformability and energy dissipation due to the lowest total NMIs density.
The implementation of an active + reactive filter system for two-stage steel cleaning is believed to be an effective method for removal of both small and large NMIs from the steel.