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Fig. 3. Higher silicon leads directly to higher nodule count, more ferrite and more early carbon precipitation. Keep the percent silicon steady at the minimum level, using just enough to avoid carbides.


Fig. 4. Reduced shrinkage is observed in the MgFeSi alloy treated with lanthanum (right) vs. mischmetal.


time. After holding for 30 minutes at tapping temperature, the subsequently magnesium-treated and inoculated iron will become slightly more shrinkage prone and strongly more carbide prone. Ladle and stream inoculation may not eliminate the carbides. Tis effect was observed in a metalcasting facility using medium frequency furnaces, with one furnace melting and the other furnace delivering iron for tundish treatments. Six tundish treatments were made over one hour to drain the furnace, before switching to the next furnace. As a consequence, the first three treatments were conducted as desired and the last three treatments produced carbides within the structure, despite the use of powerful, continuous stream inocula- tion with high potency inoculants. Te casting facility learned some of the lost nucleation effect could be restored by adding crystalline graphite to the iron while replacing carbon losses during holding, but that one hundred percent graphite electrode turnings were reported to be the best type of carbon replacement material to eliminate the carbide tendency. Te rate of loss in carbon and nucle-


ation effect from holding base iron is believed to be different for various hold- ing furnaces and temperatures. Holding magnesium-treated iron


can induce shrinkage. After 25 to 30 minutes of holding iron in an autopour without any freshly treated iron addi- tions, the state of nucleation changes and it becomes shrinkage prone. Tis change can be detected using thermal analysis. Te effect can be corrected by additions of proprietary sulfur and oxygen-coated inoculant.


38 | MODERN CASTING February 2013


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