Metal melt filtration steadily gained popularity in the foundry industry aiming for castings with high purity regarding the content of non-metallic inclusions. In continuous casting of steel, however, the principle of filtration is still not applicable with satisfying cost-benefit ratio in most cases. The high quantity of steel processed during the casting sequences results in severe conditions for cellular filter ceramics including thermal shock, mechanical load at high temperature as well as erosion and corrosion effects. Furthermore, the service life of integrated filters is directly linked with the period of application of the tundish. The ongoing clogging of the filter due to the separation of non-metallic inclusions inevitably results in increasing resistance to the melt flow, decreasing output as well as filter failure and premature stopping of the casting process in the worst case.
Consequently, another approach is necessary in order to use the advantages of ceramic foam filters for continuous casting: For long casting sequences, the filter has to be exchangeable during the casting operation without disrupting the same. To evaluate this concept in industrial feasibility tests, suitable filter systems had to be developed. Cylindrical foam filters with 200 mm diameter and height were prepared by the replica technique using different coating methods including centrifugation, dip coating and spray coating. Due to its good thermal shock behavior and refractoriness, carbon-bonded alumina was chosen as base material. The filters were investigated regarding their mechanical and structural properties in order to evaluate the most suitable manufacturing procedure.
Triple-coated filters thereby showed the best features and were chosen for further investigations. In order to test the material in contact with melt, 100 kg of molten 42CrMo4 steel were cast on the filter at 1650 °C under argon atmosphere with the aid of a steel casting simulator. The filter survived the test without critical deformation or fracture and exhibited the initial formation of typical alumina clogging layers. Consequently, the developed filter systems are suitable for future industrial investigations. First feasibility tests in a continuous casting plant showed promising results.