During the operation of medium-frequency furnaces, the refractory material used for the furnace lining is only 70-110mm thick. The inner side is in contact with the high-temperature molten metal, while the outer side is in close contact with the water-cooled coils. This results in a significant temperature difference between the inner and outer sides of the refractory material, operating under relatively thin cross-sections and in highly corrosive environments during many smelting operations. The main process conditions affecting furnace lining damage include: smelting temperature, degassing time, primary degassing volume, slag chemical composition, and the type of steel (iron) produced. The main factors influencing furnace lining damage are: slag chemical erosion, refractory material structural spalling, and thermal erosion.
Composition of the furnace lining of an intermediate frequency furnace
The lining of an intermediate frequency furnace is typically made of refractory materials of various particle sizes bonded together (common refractory materials mainly fall into four categories: magnesia, quartz, alumina, and composite materials). Its characteristics include direct bonding, resulting in high erosion resistance, high mechanical strength, and good thermal shock resistance.
Mechanism of damage to the lining of an intermediate frequency furnace
Taking magnesia refractories as an example, the damage mechanism of magnesia furnace lining materials is explained as follows: The main manifestations of magnesia material damage are thermal erosion caused by flowing molten steel and chemical erosion caused by the penetration of slag components into the material. During the smelting process, the molten steel solution penetrates into the interior of the refractory matrix through the capillary channels, eroding the furnace 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 vapor 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, cracks, spalling, and structural loosening will occur. Strictly speaking, this damage process is much more severe than the dissolution damage process.
The metal 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 complex 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 and thus 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, have melting points that can reach over 1800℃. There is also a relatively complex interpenetration and mutual dissolution between the high-melting-point slags and low-melting-point slags suspended in the molten metal. These slags are very easy to adhere to the furnace wall and accumulate, causing serious slag adhesion, affecting the power, melting rate and capacity of the electric furnace, and even affecting the furnace lining life.
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 intensifies 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, erosion is severe at the slag line, another reason for reduced lining lifespan. Due to these factors, 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 lining wall thickness increases, resistance increases, reactive power loss rises, and electrical efficiency decreases. Therefore, lining wall thickness is limited to a certain range. Thus, a reasonable wall thickness must be selected to ensure both high electrical efficiency and a sufficient lining lifespan.
Solution Design
The aforementioned erosion, under cyclical temperature fluctuations, leads to so-called structural spalling. During production, slag penetrates into the pores of the refractory matrix, forming a large, thickened refractory layer. The physical and chemical properties of the refractory material impregnated by the slag change. Due to the difference in thermal expansion coefficients between the impregnated layer and the remaining slag layer, significant stress occurs at the interface between the two layers when the temperature changes, leading to cracks parallel to the working surface and ultimately causing the furnace lining to spall. The slag penetrating into the refractory matrix dissolves the refractory particles, weakening the bond between them, thus reducing the material’s refractoriness and high-temperature resistance. Therefore, the refractory material in the slag-infiltrated layer deteriorates more rapidly under the erosion of flowing molten steel.
The basicity of the slag should be compatible with the furnace lining material. Magnesia furnace lining materials can be corroded by high-CaO and SiO2 slags. The amount of CaF in the slag should be controlled; excessive CaF will corrode the basic furnace lining, causing premature melting of the slag line zone. When the levels of fluoride ions and metallic manganese ions in the slag are high, or when the molten pool temperature reaches above 1700℃, the viscosity of the solution will decrease sharply, accelerating the damage rate of the furnace lining and significantly reducing its lifespan. The service life of the furnace lining is longer during slag-free smelting under vacuum than during non-vacuum smelting.
The high iron oxide content in the furnace lining disrupts its microstructure, reduces its refractoriness, and lowers the viscosity of the CaO-Al₂O₃-SiO₂ slag, allowing it to penetrate deeper into the material. However, the presence of iron oxide in the original lining promotes rapid sintering and reduces porosity and permeability. In particular, the presence of iron oxide in the molding material significantly contributes to rapid sintering and prevents sand inclusions and erosion.
Increasing the magnesium oxide content and slag viscosity helps reduce slag erosion of the furnace lining and improves slag collection. When slag basicity is low, erosion of the magnesia lining is more severe, reducing its lifespan; conversely, higher slag basicity results in less erosion and a longer lining lifespan. Increasing slag basicity and MgO content, while decreasing FeO content, helps reduce slag erosion of refractory materials. Therefore, high-magnesium oxide materials should be selected when using slag-forming agents. A well-designed slag structure accelerates slag formation, shortens smelting time, and reduces the iron oxide content in the slag.
The appropriate slag should be selected based on the material of the furnace lining. Basic slag is suitable for magnesia linings, but can be corroded by high-CaO and SiO2 slags. Excessive CaF2 will also corrode basic linings, causing premature melting of the slag line zone. Acidic slag is suitable for quartz linings, while magnesia-alumina linings are only suitable for weakly basic or neutral slags. Alumina linings exhibit typical amphoteric properties at high temperatures under different pH levels, allowing them to adapt to slags of varying pH, but they are slightly less adaptable than acidic and basic linings. Therefore, some manufacturers use high-purity magnesia sand and add a certain amount of spinel to modify the matrix properties of pure magnesia lining materials. However, experiments show that even high-purity corundum materials have significantly lower erosion resistance than sintered magnesia sand with lower purity. Acidic slag is suitable for quartz linings, while magnesia-alumina linings are only suitable for weakly basic or neutral slags. Alumina furnace linings exhibit typical amphoteric properties at high temperatures and different pH levels, allowing them to adapt to slags of varying pH, although they are slightly less adaptable compared to acidic and basic furnace linings.
In summary, considering the main damage mechanisms of magnesia furnace linings, through continuous summarization and exploration, it is possible to improve the material’s resistance to slag penetration by limiting open porosity and permeability, and to improve the high-temperature erosion resistance and spalling resistance of the lining matrix by increasing high-temperature flexural strength and harsh softening temperature.
The performance of the furnace lining depends on a variety of factors, such as the particle size distribution of the material, the physicochemical properties of the material, and the sintering temperature of the furnace lining.
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