Induction furnace principle
Used by many specialty steelmaking shops and foundries, induction furnaces are cylindrical, open-topped, tiltable refractory crucibles with a water-cooled induction coil installed on the outside, around the side wall. The coil is powered by alternating current, which induces eddy currents in the metallic charge that generate heat. The refractory wall of the crucible is usually thin enough to achieve good penetration of the electromagnetic field into the charge.
Induction furnaces are used mainly for remelting and alloying and have very limited refining capabilities; in other words, they are not used for carbon, phosphorus, or sulfur removal. The slag is cold and not very active, and often there is no slag at all. However, the electromagnetic field stirs the melt well, and this is beneficial for alloying. Most furnaces’ coils are powered by line frequency, but there are also furnaces powered by medium frequency, utilizing solid-state frequency converters. The electrical system always includes capacitor banks to compensate for the high inductance of the furnace coil. Efficiency of converting electric power into heat is about 75 percent, and power consumption is around 550 kilowatt-hours per ton of steel.
In commercial operation, a hot heel is often left in the furnace after tapping in order to decrease the thermal shock on the lining generated by the water-cooled coil. Smaller furnaces use prefabricated crucibles, but larger furnaces have a rammed—that is, compacted and dried—refractory mass as lining. Computer control is well utilized in this system, monitoring, for instance, the crucible lining thickness by the electrical performance of the furnace coil. The capacity of the furnace varies from a few kilograms to 50 tons.
Many induction furnaces are installed and operated in vacuum chambers. This is called vacuum induction melting, or VIM. When liquid steel is placed in a vacuum, removal of carbon, oxygen, and hydrogen takes place, generating a boil in the crucible. In many cases, the liquid steel is cast directly from the furnace into ingot molds that are placed inside the vacuum chamber.