Abstract

Non-metallic inclusions in cast metal parts often have a negative impact on properties such as fracture toughness, tensile strength and fatigue resistance. Foundries require high purity metal melts in order to limit the casting repairs and possibly avoid rejected castings. This can be achieved by applying a filtration process after the ladle treatment. In the case of steel, ceramic foam filters (CFFs), especially those based on zirconia and carbon-bonded alumina, have been successfully employed for years. However, their potential has not been fully exploited yet: the filtration efficiency of ceramic foam filters in ferrous melts is less than 75% for inclusions in the particle size range 1-100 μm. It was reported that over 90% of the large nonmetallic inclusions with a size of 100 µm are efficiently filtered by the novel filters. In contrast, numerical simulations also showed that the filtration efficiency of small inclusions with a size of 1 µm and 10 µm is less than 10% and 18%, respectively. In order to improve the filtration efficiency, promising approaches which involve “active” and “reactive” coatings applied on the filters have been proposed.

Recently, carbon-bonded alumina filters were coated with multi-walled carbon nanotubes and immersed in a steel melt containing endogenous alumina inclusions. Microscope investigations after the test suggested a better performance of these MWCNTs-coated filters over uncoated Al2O3-C samples for short immersion times (10/30 s). Starting from this formulation, other additions of TiO2 and alumina nanosheets (in the substrate material and in the coating, respectively) were explored. In the case of titania, the reducing atmosphere of the heat treatment was expected to generate free electrons and produce point (or even surface) defects that could alter the chemical properties of the host system. The coated samples were tested for different times in contact with a corrosive steel melt clean from inclusions at 1580°C. Investigation of the filters after the test was carried out by optical microscopy, scanning electron microscopy and electron backscatter diffraction. In addition, steel samples were thoroughly analyzed with a special ASPEX-SEM to evaluate the influence of each specific ceramic filter on the steel composition and purity. Finally, the results from the different filter formulations were compared.