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Metal Casting Technologies : March 2007
46 www.metals.rala.com.au BacktoBASICS M Filtration of Steel Castings any steel foundrymen would contend that inclusions are an unavoidable consequence in the production of steel castings, a "fact of life". Unfortunately, designers and buyers of steel castings are increasingly not prepared to accept this "fact" and quality standards demanded on steel castings are increasing dramatically. In recent times, much effort has been spent on improving the "as-cast" quality of steel castings in an effort to maintain existing markets and develop new ones. Much of this effort has revolved around producing "cleaner" steels through improved melting and refining techniques, higher quality refractories, improved mould and core binders, and improved casting methods including filtration of liquid steel. Of course this effort has not been without some sound economic basis, as it has been estimated that the removal and repair of inclusions in steel castings can account for 15 to 20% of total manufacturing costs. INCLUSIONS IN STEEL CASTINGS Inclusions are introduced into steels during every stage of processing, from melting, trimming additions, deoxidation, tapping and teeming, as well as being generated within the mould itself. Inclusions are generally classified as exogenous or indigenous. Exogenous inclusions arise from external sources and are typically particles of sand, refractory, moulding materials, metal and ladle slags, and agglomerates of any of these. Indigenous inclusions originate from chemical reactions between elements within the steel itself during melting and deoxidation and reoxidation, which occurs during tapping and teeming. Such inclusions are essentially silicates, oxides, nitrides and sulphides, or more often complexes of these. All inclusions will have an adverse effect on the quality of castings produced either cosmetically or structurally, and the end result is usually costly rework or even scrapping of castings. FILTRATION OF LIQUID STEEL In recent times, advancements in ceramic and application technology have combined to make the use of filters a viable means of reducing inclusions in steel castings. Ceramic filters, such as those shown in figure 1, have the capacity to remove not only the relatively large exogenous inclusions, but also a high percentage of indigenous inclusions which may be sub- micron in size. The mechanical attraction of these inclusions onto the filter surface and its substrate is assisted by the substantial increase in surface area of the ceramic foam. This increase in surface area will, in turn, decrease the velocity of the liquid steel stream resulting in a reduction in mould and core erosion, reduced reoxidation and improved surface finish. FILTRATION MECHANISMS It is generally accepted that there are three main mechanisms of filtration, namely: Particle Sieving Particle sieving the process by which large particles are trapped on the surface of the filter much like a sieve. Particle sieving removes inclusions such as sand and slag agglomerates which are larger than the pore size of the filter. Cake Filtration Cake filtration is where the larger particles which have built up on the entry face of the filter by the particle sieving process act to filter out smaller particles. Both indigenous and exogenous inclusions are removed by this mechanism. Deep Bed Filtration Deep bed filtration operates in two ways: 1. Mechanical entrapment due to the tortuous path the inclusions take through the filter body. 2. Chemical attraction between the filter body and slags and liquid oxides. FLOW MODIFICATION The previously described filtration mechanisms have been concerned with removal of inclusions before the filter. The use of ceramic foam filters combined with a properly designed J. F. Meredith Solutions - Casting Solutions Pty Ltd Fig. 1. Selection of foam ceramic filters