Abstract

To remove nonmetallic inclusions and ensure the requirements of high purity metal castings, ceramic filters have been used in metal casting applications such as foundries for several years. These inclusions are responsible for the strength, the elongation, the fracture toughness and the fatigue performance of the metal components. During loading, stress concentrations are generated in the proximity of inclusions. In order to meet the increasing demands for high purity metals such as high security steels, ceramic foam filters (CFF), especially those based on zirconia and carbon bonded alumina, have been successfully employed for years. Zirconia filters, however, have the disadvantage of exhibiting creep, which decreases the flow rate during casting because of the changing filter geometry. In contrast, carbon bonded systems exhibit negligible creeping due to the high amount of carbon.

In terms of this contribution calcium aluminate composition coatings containing carbon will be explored. Such kind of functional coatings on a carbon bonded filter substrate provide a number of advantages for capturing fine alumina and calcium aluminate inclusions. If calcium aluminate compositions containing carbon come in contact with steel melt following mechanisms are activated:

I) The calcium aluminates react with the carbon, form calcium aluminate suboxides or also calcium and aluminum which are deposited on a calcium aluminate decarburized or partially decarburized zone in contact with the metal melt and generate a thin very active calcium aluminate layer due to the reaction of these suboxides with the oxygen of the steel melt. This thin very active layer between the decarburized zone and the steel melt is contributing as an active collector for endogenous inclusions.

II) In addition the high vapor pressure of calcium with an associated intense bath stirring promotes collision and coalescence of the alumina fine inclusions in the melt. With the aid of calcium vapor and the resulting coalescence of the alumina inclusions through collision, their removal from the steel is enhanced compared to the small non-buoyant alumina inclusions which must first cluster on their own before they are able to be separated by the liquid steel.

III) Depending on the applied composition of calcium aluminates the softening point and or the melting point of the mixture can be adjusted in order to promote an additional capturing by increasing the roughness of the thin active layer which copies the surface of the carbon free calcium aluminate layer below. A higher roughness leads to higher wetting angle against the iron melt which promotes a higher agglomeration via collision of the fine inclusions.

IV) The endogenous inclusions are trapped mechanically better during their collision on the active surface and more contacting surface is available for sintering fixing of the inclusions on the surface of the thin active layer.