Induction furnace is an electric heating device that converts electrical energy into thermal energy by using the principle of electromagnetic induction to induce current inside metal materials in an alternating magnetic field, thereby heating the materials until they melt. Induction furnaces are divided into two types according to their structure: coreless induction furnaces and cored induction furnaces. Coreless induction furnaces are widely used because of their fast heating speed, intermittent operation, less element burnout during smelting, and easy use and maintenance. In recent years, high power density, rapid melting, and large-scale development have become the current development trend of coreless induction furnaces.
1 Erosion
Erosion is manifested as thinning of the refractory bricks furnace lining and enlargement of the crucible diameter. The causes of erosion are mechanical erosion and chemical erosion.
1.1 Mechanical erosion
Mechanical erosion is caused by the following reasons: (1) Collision and friction of the charge. (2) Electromagnetic stirring. In the area of the furnace lining in contact with the molten metal, the molten metal continuously flushes the furnace lining under the action of electromagnetic stirring.
1.2 Chemical erosion
(1) Molten metal reaction
Magnesium furnace linings have good tolerance to Fe, Mn, and Ni, but poor tolerance to C and Si. At high temperatures, the activity of [C] and [Si] in the molten metal increases, and an interfacial reaction occurs with MgO on the surface of the furnace lining, causing damage to the furnace lining. When using a siliceous furnace lining for melting, SiO2 reacts with [C] in the molten metal to form the following reaction: SiO2(s)+2[C]→[Si]+2CO(g). The equilibrium temperature of this reaction for ductile iron melting is 1467°C, 1540°C for gray iron, and 1580°C for forgeable cast iron. When the melting temperature exceeds the equilibrium temperature, SiO2 in the furnace lining is reduced by [O], and the furnace lining becomes thinner.
(2) Slag erosion
Due to the presence of rust in the charge, unclean return charge, and oxidation of the molten metal at high temperatures, a large amount of slag containing FeO is produced. During the smelting process, the viscosity of the molten metal is significantly reduced due to electromagnetic stirring, and it penetrates into the pores of the lining when it contacts the surface of the refractory bricks furnace lining. At high temperatures, FeO in the slag reacts with SiO2, Al2O3 or MgO in the furnace lining to produce low-melting-point substances (such as FeO·SiO2, whose melting point is only 1170°C). Under the stirring of high-temperature molten metal, it is very easy to melt and quickly leave the reaction zone and enter the liquid. The greater the difference between the alkalinity of the lining material and the alkalinity of the slag, the easier it is for the lining to be corroded.
2 Leakage
The leakage phenomenon is manifested as metal penetrating into the furnace lining. The causes of leakage are: (1) The furnace density is too low; (2) The metal has melted before the sintering layer is formed; (3) Chemical reaction.
3 Slagging
The slagging phenomenon manifests as the furnace wall becoming thicker and rougher. The cause of slagging is the adhesion of metal oxides, recycled materials containing oxidants and sand to the furnace wall. The effects of slagging on the furnace lining are as follows: (1) Slagging is the infusible oxides, sulfides or phosphides floating on the molten metal that adhere to the lining parts where the molten metal flow rate is low and heat loss occurs; (2) It reduces the furnace capacity; (3) It reduces the melting efficiency of the furnace; (4) It makes the metal hidden in the slag easy to overheat; (5) It reacts with the refractory bricks materials of the lining; (6) It is easy to cause mechanical damage to the lining during slag removal; (7) It changes the thermodynamic state of the lining at the slag-hanging part. Quartz sand lining slagging occurs when the recycled materials contain more quartz sand (waste sand adhered to the surface of cast iron). These sands will sink to the bottom of the furnace and adhere to the furnace wall to thicken the lining. In severe cases, the bottom of the furnace will form a V shape and the furnace volume will decrease. The quartz sand furnace lining is an acidic furnace lining. Acidic slag will adhere to the acidic furnace lining, especially when the slag line is fixed and the tapping temperature is low, slagging will occur seriously at the slag line, affecting the charging and furnace capacity.
