Abstract

Ceramic filters play an essential role relating to the cleanliness of cast steel bodies; they smooth the fluid steel flow and interact with, and capture inclusions contained within the molten steel as it enters the mould. A coating on such a filter is able to increase their mechanical strength and enhance the mechanisms of inclusion capture.
In this study, commercially available Al2O3-C filters were coated by two techniques: with an alumina slip via cold spraying, and with alumina via flame spray technology. The cold sprayed alumina coating requires an additional thermal treatment at 1400 °C, whereas filters with a flame spray coating can be directly used after the coating process. The microstructures of the coatings have been investigated by light microscopy, SEM, and CT and the phase composition has been analyzed by x-ray diffraction. The effect of the filters, with and without coating, on the cleanliness of steel was evaluated in a metal casting simulator by dipping and rotating the filters into molten steel (Grade 1.6587) at 1580 °C for 20 seconds. The levels of inclusions contained in the steel before and after filter interaction have been counted, and the chemistry and size identified with PSEM. In addition, the surface of the filters has been analyzed in order to identify the amount and type of trapped inclusions.
After the additional thermal treatment, the cold spayed coating is composed of homogenous corundum (α-Al2O3) particles and exhibits a porosity of about 35 %. The alumina coating interacts with the Alumina-C filter substrate forming an in situ alumina-layer, which promotes the deposition of inclusions during casting. However, there is some interaction between the metal and the base filter structure, that can negatively impact on the casting quality. In contrast, flame spraying results in a coating composed of overlaid γ-Al2O3 droplets. The flame sprayed coating is comparatively dense with a porosity of 7 %, this prevents any interaction of the filter substrate with the molten steel. It has been found that the Al2O3-C filter with cold sprayed coating has the highest inclusion filtration efficiency by trapping very small inclusions with diameter < 1 µm. Without alumina coating the Al2O3-C filter overall shows a good filtration of inclusions in range of 1 – 5 µm. A thermal spray coating is recommended for filtration of cast steel products that require a particularly high level of cleanliness.