position for each lining sample was plotted on a ternary phase diagram (Table 2, Fig. 1). The analysis indicated that when the temperature of the hot face lining reached 2,650F, it would melt and be swept away by the stirring action in the furnace.


Where Does the


Magnesium Come From? A typical back charge


for the 35-ton coreless induction furnace con- sisted of 55% ductile iron returns, which were previously treated with magnesium-ferro-silicon (MgFeSi) containing 6.25% magnesium (Ta- ble 3).


MgFeSi was added to


is above the top of the power coil (Fig. 2). As the charge sub-


Fig. 1. A ternary phase diagram shows the compositions for MgO, Al2 fall within the liquid range at 2,650F (1,454C).


the base iron to deoxidize and desulfurize the iron to enable the graphite to grow as a spheroid rather than as a flake. Since the oxides and sulfides were formed on a molecular scale, not all of them floated to the surface of the metal to become part of the slag layer. Consequently, some magnesium oxide and magne- sium sulfide remained suspended in the molten iron and were trapped in the treated iron as it solidified. MgFeSi is added to the ductile base


iron in an amount in excess of what is needed to lower the dissolved sulfur and oxygen levels to ensure nodularity. Since the solubility of magnesium in molten iron is low, the magnesium was present primarily as a gas. When the treated iron solidified, the magnesium gas condensed and solidified in the treated iron. The amount of magnesium in the treated iron sample was 0.039%.


What Is the Source of Aluminum? A typical back charge for the 35-ton


coreless induction furnace consisted of 45% steel. Aluminum is added to molten steel to deoxidize the metal. Consequently, when the steel solidi- fied, the excess aluminum solidified in the steel, while some of the aluminum oxide remained suspended. The steel charged into the furnace typically con- tained between 0.0199% and 0.0459% aluminum. If all of the aluminum in the steel were to remain in the base iron,


MODERN CASTING / August 2010


up to 0.008 to 0.21% aluminum could be present. Since the balance of the back charge consisted of 55% ductile iron returns, which were previously treated with 1.5% MgFeSi containing 0.78% alu- minum, the treatment process could introduce up to 0.0064% aluminum into the base iron. Post inoculation of ductile iron could add up to 0.01% aluminum to the treated iron returns. The actual amount of aluminum in the treated iron sample was 0.0094% to 0.0103%.


Charging the Furnace When treated ductile iron returns


and steel are charged into a coreless induction furnace, they submerge in the liquid iron and melt by conduc- tion. A coreless induction furnace is characterized by a non-symmetrical load, where the bottom of the melt is at or slightly above the bottom of the power coil, while the top of the melt


Table 3. MgFeSi Chemistry Element


Magnesium Calcium


Aluminum Rare earths Barium Silicon Iron


Percentage 6.25% 0.9%


0.78% 1.04% 1.47%


44.01% 45.55%


O3 , and SiO2


merges in the molten iron, energy is transferred from the molten metal to the charge, causing the temperature of the charge materials to increase and the temperature of the molten metal to decrease to about 2,400F (1,316C). Since the charge materials tend to float in the upper portion of the molten bath, the molten metal there cannot superheat until sufficient energy has been provided to satisfy the heat of fusion requirement to melt all of the charge. Because the temperature of the metal in the lower portion of the furnace is hotter than in the upper portion, en- ergy is transferred to the


upper portion of the melt, causing the bottom charge materials to melt first. The magnesium that had solidified


in the treated ductile iron returns melts, vaporizes and becomes suspended as magnesium gas in the molten base iron. The aluminum that had solutioned in the steel and treated ductile iron returns remained in solution in the molten base iron. The magnesium sulfide trapped in the returns became suspended in the molten base iron. The oxides trapped in the returns and steel also became suspended. The magnesium, aluminum, sulfides and oxides were released and circulated in the bottom stirring pattern.


Source of Lining Wear


When magnesium, aluminum and magnesium sulfide in the treated iron returns are released in the molten bath, they have an affinity for their lowest free energy state, which is the oxide of that element. Since the oxygen content of the base iron, steel and treated iron returns is low, one source of oxygen is the refractory lining. To determine whether the reactive elements and their sulfides can obtain oxygen from a silica lin- ing, the events were studied that took place immediately after a back charge was made while the molten iron was at 2,400F.


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