Removal of Alumina Inclusions from Molten Steel by Ceramic Foam Filtration
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00537-9
Abstract The efficiency of removal of solid alumina inclusions by filtration and the distribution of inclusions captured through the thickness of the filter was investigated for an aluminum killed 316 stainless steel casting. A mold design was developed using modeling software to produce two castings that fill simultaneously, one with a filter and the other without a filter. The design was optimized to produce the filtered casting and unfiltered casting from a single ladle pour, while also matching the fill rates and avoiding turbulence and reoxidation during pouring. Samples from the filters and the castings were analyzed using an SEM with EDS and automated feature analysis to measure the efficiency of inclusion removal for a 10 ppi zirconia foam filter. Results showed that inclusion removal efficiency
depends strongly on the initial inclusion concentration and that the alumina inclusions are captured within the filter at the filter web-steel interface. This study also documented that inclusion floatation inside the mold cavity plays a role in reducing the inclusion concentration in the casting. The distribution of alumina inclusions captured through the filter thickness was quantified using elemental mapping and the inclusion distribution was found to decrease exponentially, following first-order capture kinetics.
Introduction
Several studies have shown that ceramic filters can effectively remove inclusions2–9 from the steel melt. The melt flowrate inside the mold cavity influences the inclusion removal efficiency. High flow rates4 or melt velocities10 through the filter lower the removal efficiency due to the decreased residence time of the steel inside the filter. Filter geometry also plays an important role in inclusion removal. An increase in filter thickness11 or change in aspect ratio12 can also increase the residence time of steel melt in the filter, which helps to increase the inclusion removal efficiency. Inclusion removal efficiency (g) is defined in Eqn. 2, where Ci and Co are the inclusion concentrations in the steel melt at the filter inlet and at the outlet, respectively.
In foundry steelmaking, ceramic filters are commonly used to remove nonmetallic inclusions. Nonmetallic inclusions in steel can reduce mechanical properties, impact machinability, produce surface defects and increase scrap rates.1 Aluminum is a strong deoxidizer and at sufficient levels of addition, generates solid alumina inclusions in the steel melt. The equilibrium reaction of the formation of alumina inclusions in steel melt during deoxidization is shown in Eqn. 1. 2½Al þ 3½O ¼ ðAl2 O3 Þsolid
Eqn: 1
Keywords: steel, nonmetallic inclusions, filtration, floatation, removal kinetics, mathematical modeling
g¼ Received: 26 June 2020 / Accepted: 08 October 2020
International Journal of Metalcasting
Ci Co Ci
100%
Eqn: 2
In a previous study,13 the kinetics of inclusion capture by filtration was reported to follow first-order kinet
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