• The magnesium gas in the treated iron returns reacted with a silica lining to form a magnesio-silicate when the molten bath contained 1.8% Si.
• The magnesium sulfide reacted with a silica lining to form a magnesio-
silicate when the bath contained 1.8% Si and the partial pressure of the sulfur gas at the melt interface was near zero, which existed just prior to a back charge.
• The magnesium oxide released in
the base iron reacted with the silica lining to form a magnesio-silicate.
• The aluminum solutioned in both the steel and treated iron reacted with the silica lining to form an alumino-silicate when the molten bath contained 1.8% Si and 0.016% Al.
place below the MgO-SiO2 03
perature of 2,894F (1,590C), a solid mixture of MgO, Al2
temperature of 2,809F (1,543C) and below the Al2
-SiO2 Fig. 2. Shown is the stirring pattern in a coreless induction furnace.
silica dissolved sufficient amounts of the oxides of magnesium and alumi- num to lower the melting point of the lining below the pour temperature, lining wear resulted and was confined to the bottom of the furnace (Fig. 3). The reduction of a silica lining by magnesium, aluminum and other reac- tive elements in the molten iron takes place because the molten bath is in a deoxidized state. Consequently, the main source of oxygen for the reac- tive elements is the silica lining. One possible approach to increasing the life of a silica lining is to oxygenate the molten bath upon charging the furnace by adding a compound like iron oxide or magnesium carbonate to provide oxygen to the reactive elements. MC
formed. Thus, if a silica lining were to be solely acted upon by either magnesium or aluminum, the hot face of the lining would not melt because the melt temperature of 2,400F was below the solidus temperature. However, when a silica lining was simultaneously acted upon by the ox- ides of magnesium and aluminum, the melting point of the ternary mixture was lowered to the ternary eutectic temperature of 2,489F (1,365C). The composition of the ternary eutectic mixture was 20.73% MgO, 17.48% Al2 and 61.79% SiO2, which equates to 3MgO, Al2O3
O3
solidus tem- and SiO2
O3 , and 6SiO2 . Thus, when About the Authors
William Duca is president of Duca Manufacturing Inc., Boardman, Ohio. Russ Seider performs technical sales for Pryor Giggey Co., Chilton, Wis. Jerry Beaird is melting superintendent for Rochester Metal Products, Rochester, Ind.
For More Information
Fig. 3. This elephant foot wear pattern in a coreless induction furnace is caused when silica dissolves sufficient amounts of magnesium and aluminum oxides to lower the melting point of the lining.
34
“Understanding Bottom Wear in Coreless Induction Furnaces,” W.J. Duca, J. Beaird, MODERN CASTING July 2003.
MODERN CASTING / August 2010
• The aluminum oxide suspended in the steel and treated iron reacted to form an alumino-silicate. Since the above reactions took solidus
is
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