The lining of an induction furnace

The lining of an induction furnace is typically made of refractory materials of various particle sizes bonded together (common refractory materials mainly fall into four categories: magnesia, quartz, high-alumina, and composite materials). High-alumina refractory materials are characterized by direct bonding, resulting in high erosion resistance, high mechanical strength, and good thermal shock resistance. Damage to high-alumina materials mainly manifests as thermal erosion caused by flowing molten steel and chemical erosion caused by slag components penetrating into the material.

During the smelting process, the molten metal penetrates the refractory matrix through capillary channels, eroding the lining. Components penetrating into the refractory matrix include: CaO, SiO2, and FeO from the slag; Fe, Si, Al, Mn, and C from the molten steel; and even metal vapors and CO gas. These penetrating components deposit in the capillary channels of the refractory material, causing a discontinuity in the physicochemical properties of the refractory working surface compared to the original refractory matrix. Under rapid temperature changes, this leads to cracking, spalling, and structural loosening; strictly speaking, this damage process is far more severe than the dissolution damage process.

The metallic materials added to the furnace introduce various oxides, and the composition of slag varies depending on the material and the furnace batch. Most of the oxides, carbides, sulfides, and various forms of composite compounds present in the slag will chemically react with the furnace lining, generating new compounds with different melting points. Some low-melting-point oxides generated during the reaction, such as fir olivine (FeOSiO2) and manganese olivine (MnOSiO2), generally have melting points around 1200℃. Low-melting-point slag has excellent fluidity and may act as a flux, causing severe chemical erosion of the furnace lining, thereby reducing its service life.

The high-melting-point slags generated during the reaction, such as mullite (3Al₂O₃·2SiO₂) and forsterite (2MgO·SiO₂), as well as some high-melting-point metallic elements with melting points exceeding 1800℃, exhibit complex intermingling and mutual solubility between the high-melting-point and low-melting-point slags suspended in the molten metal. These slags readily adhere to the furnace wall and accumulate, causing severe slag adhesion, affecting the electric furnace's power, melting rate, and capacity, and ultimately impacting the furnace lining's lifespan.

As furnace capacity increases, the proportion of heat lost from the molten steel surface decreases, resulting in higher slag temperatures and better slag fluidity compared to smaller-capacity furnaces. This exacerbates erosion of the furnace lining. Large induction furnaces often employ a mixed steel-slag tapping method, requiring slag with excellent fluidity to withstand the tapping conditions. Consequently, severe erosion occurs at the slag line, another reason for the reduced lining lifespan. For these reasons, the lining lifespan of large induction furnaces is shorter than that of medium and small induction furnaces. To improve lining lifespan, the lining thickness should be appropriately increased. However, as the furnace lining thickness increases, the resistance increases, reactive power loss increases, and electrical efficiency decreases. Therefore, the furnace lining thickness is limited to a certain range. Thus, a reasonable wall thickness must be selected to ensure both high electrical efficiency and the service life of the furnace lining.