For safe furnace operation and long lining life, it is essential that the maximum temperature is not unduly exceeded beyond the safe temperature. In fact by paying more attention to metal temperature control by raising it not more than what is necessary, one can considerably increase the lining life. At low metal temperature, there is a tendency to oxidize silicon form the metal and produce a highly siliceous slag that may build up on the furnace walls. Si + 02 = Si02 With the increase in metal temperature, silicon oxidation becomes less and at some temperature, the melt remains under equilibrium. At still higher temperatures, the carbon in the melt reacts with silica lining to reduce it is silicon and liberates gaseous carbon monoxide.
Si01 +2C = Si + 2 CO
This carbon monoxide can be seen bubbling near furnace walls when metal is held at high temperature. The slag attack on lining also increases with increases in temperature.
Thus at higher temperatures, the lining is under server attack both form metal as well as from slag. In addition, the following practices also help to increase the lining life.
- Continuous use of the furnace round the clock gives less thermal fluctuations and results in a long life. Intermittent use results in repeated and steeper thermal shocks and causes rapid failure or lining.
- If a single type of alloy or grade is being repeatedly melted, the lining life is more. e.g. a furnace melting only unalloyed grey-iron has longer lining life as compared to a furnace melting various grades of grey irons, alloy-irons, ni-hard, etc.
- Care in charging the furnace to avoid/minimize the impact by charging heavy pieces.
- Removal of slag before charging liquid metal from other furnaces, ladies to the induction furnace to ensure that only clean metal is in contact with the lining.
- Avoid any possibility of bridging of a charge in the furnace. It can cause excessive wear of bottom lining in case of the superheating of the melt below the bridge.
- Do not use heavily rusted scrap. Its use will increase the volume of slag produced. Slag removal will be difficult and lining life will also be somewhat reduced.
- While using turnings/borings of steel scrap, do ensure that these are dry and are free from oil or moisture. Otherwise, there is a risk of splashing or explosion, with at times, heavy impact loading on the lining.
- During melting and melt-treatment, if any, slag formed should be removed at regular intervals. It will minimize slag/lining reactions or lining erosion and help in improving lining life.