Initially, the chemical analysis of each buildup was obtained by using an X-ray spectrometer.
Cathodoluminescence and optical microscopy were used to help classify each buildup. When viewing a polished sample section, the microscope was used to identify grain size and orientation.
These results will be used to verify the findings in the cathodoluminescence study and observe any possible nucleation sites where the buildup might begin growing.
In this case the ceramic buildup was a composite of magnesia and alumina in the form of spinel.
In one case, for example, it was determined that buildup occurred as a result of residual MgO carried over from melting treated ductile returns, combining with silica [SiO.sub.2] from the molten metal and ferroalloy additions, and iron oxide (FeO) from the rusty steel returns to form the forsterite.
This study also demonstrated the following: * As the concentration of metallic oxides and sulfides within the molten metal increases, there is an increased propensity to form a ceramic buildup or clog in the inductor channels or throat openings.
Table : 1 Chemical Analysis of Sample Buildup [SiO.sub.2] [Al.sub.2][O.sub.3] [Fe.sub.2][O.sub.3] 31.0% 17.4% 3.5% MgO CaO MnO 45.2% 1.6% 0.29%
Residual oxides from remelting solid charges can be a source of buildup. For example, as steel is remelted, iron oxide ([Fe.sub.2][O.sub.3]) and alumina ([Al.sub.2],[O.sub.3]) are generated.
Spent foundry sand can also be a possible source for the silica ([SiO.sub.2]) content in the buildup. Mullite (3Al.sub.2][O.sub.3], [2SiO.sub.2]) is frequently deposited as a result of sand contamination.
Many different buildup compositions are generated from this source.
The size of the scrap or charge material will also contribute to the throat buildup. The smaller the scrap size, the more surface area that is available to oxidize.