Treated Ductile Iron Returns’ Contribution to Lining Wear
Silica lining wear in coreless induction furnaces melting ductile iron can be caused by magnesium, aluminum and other reactive elements in the molten metal.
William J. Duca, Duca Manufacturing Inc., Boardman, Ohio Russ Seider, Pryor Giggey Co., Chilton, Wisconsin Jerry Beaird, Rochester Metal Products, Rochester, Indiana
T
he historical explanation for silica lining wear in the lower portion of the lining in a core- less induction furnace when melting gray iron is due to
the reduction of silica by carbon in the molten iron. However, this does not adequately address the situation when melting ductile base iron. The silica lining’s wear profile, which
resembles an elephant foot, occurs in the lower part of the furnace. Typically, silica lining is made of a dry, vibrated silica refractory grain bonded with boron oxide. According to free energy data (free energy is the energy in a sys- tem that can be converted into work),
Table 1. Refractory Lining Chemistry Compound
SiO2 Al2
Ce2 O3 MgO O3
CaO FeO
MnO Na2
K2 O La2 O3 TiO2 Totals O
Sample 1 62.16% 19.59% 12.82% 1.16% 1.11% 0.17% 2.12% 0.71% 0.16% 0 0
100%
Table 2. Major Oxides and Melting Temperatures Oxide
SiO2 Al2
O3 MgO 32
Sample 1 65.73% 20.71% 12.82%
carbon will react with silica to form carbon monoxide when melting gray iron containing 3.4% carbon and 2.4% silicon at 2,650F (1,454C). Likewise, carbon will react with silica to form carbon monoxide when melting ductile base iron containing 3.8% carbon and 1.8% silicon at 2,650F. The silicon dissolves in the molten
iron, while the carbon monoxide leaves as a gas. The reduction of silica by carbon results in a loss of lining material. For example, the lining in a 35-ton, 7,000-kW line frequency coreless induction furnace has to be replaced after melting 8,000 tons of gray iron. However, when melting
Sample 2 57.78% 15.66% 24.24% 0.35% 0
0.29% 0.83% 0.85% 0 0 0
100%
Sample 3 63.93% 18.4%
11.29% 0.96% 2.28% 0
1.85% 0.78% 0.19% 0.2%
0.11% 100%
ductile base iron, the lining has to be replaced after melting 2,600 tons. Since the drop in lining throughput is not proportional to the difference in the free energies, the wear of a silica lining when melting ductile base iron must involve a more pronounced wear mechanism. Less than 10 years ago, a study
showed that a silica lining also was reduced by magnesium and magnesium sulfide from treated iron returns, but when oxygen is stripped from silica by magnesium or magnesium sulfide to form magnesium oxide, virtually no loss of lining material occurs. How the magnesium and magnesium sulfide contribute to the primary lining wear needed further explanation.
Lining Wear’s Cause A spent silica lining from a ductile
Sample 2 59.15% 16.03% 24.82%
Sample 3 68.29% 19.65% 11.29%
to total 100% for analysis, and the com- , Al2 O3 MODERN CASTING / August 2010
iron melting campaign revealed the surface of its lower portion in the el- ephant foot region had melted, whereas the upper portion had not. For a silica lining to melt at temperatures below the melting point of silica, its refrac- tories had to be compromised by the introduction of other oxides. A scan- ning electron microscope revealed the compositions of three hot facing lining samples from the wear zone in a 35- ton coreless induction furnace melting ductile base iron. The primary oxides found in the composition included silica, alumina and magnesia (Table 1). The amounts of SiO2
and MgO were refigured
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