In the majority of installations, the melt melted in the induction furnace is ferrous, having the melting point from about 11500 C in case of grey-iron to about 15400 C in case of low carbon steel. Alloying elements like manganese, chromium, nickel, molybdenum, titanium, tungsten, etc and their Ferro-alloys have high melting points. The normal operating temperature in an induction furnace is, therefore, from around 1350-14000 C in case of soft-gray iron to as high as 16500 C in case of low carbon steels, and even higher, up to 17000 C in certain specialty superalloys containing tungsten. The lining material has to be, therefore, some form of refractory which can withstand such temperature.
In addition to the refractoriness, the lining material should possess many other properties to qualify as a good lining material. These are listed below.
- It should be capable of holding the liquid metal at its normal working temperature, and should also be able to withstand occasional over-temperature conditions, without getting fused or losing its hot strength.
- It should be chemically compatible with the metal being melted and the slag being generated so that erosion due to reaction with metal and/or slag is minimum.
- It should be able to withstand the stresses developed due to its own expansion and contraction caused by thermal cycles of furnace operation.
- It should be able to withstand the mechanical forces arising during metal charging, de-slagging, and furnace tilting.
- It should have a low thermal capacity so that during thermal cycles of furnace operation, less heat is absorbed by the lining.
- It should have low thermal conductivity so as to reduce heat losses into the induction coil. (However, high thermal conductivity has an advantage in case of metal penetration through the lining, as the penetrating metal stream will solidify much before it can reach the coil.)
- It should be easily installable and sinterable into a uniform and high-density mass of adequate strength.
- After use, it should be easily knocked-out without damaging the coil.