A medium-frequency furnace is a device that uses induction currents—generated by a medium-frequency alternating magnetic field—to heat and melt metal; refractory materials are required to protect the equipment during operation. The following is an overview of medium-frequency furnaces and the refractory materials used with them:
Performance Requirements for Refractory Materials in Medium-Frequency Furnaces
- High Refractoriness: During metal smelting, the temperature of molten steel can reach 1,700°C and molten iron can exceed 1,500°C; therefore, refractory materials must possess sufficient refractoriness to ensure they do not soften or melt at these high temperatures.
- Good Chemical Stability: Slag erosion is a common cause of refractory material degradation. Materials should be selected to have a chemical composition compatible with the slag to minimize chemical reactions, reduce the intensity of interfacial reactions, and enhance resistance to slag erosion.
- Excellent Thermal Shock Resistance: Operations such as charging, tapping, or process adjustments involve drastic changes in temperature and pressure. Consequently, refractory materials require good resistance to rapid cooling and heat to ensure the furnace lining does not crack, spall, or collapse due to thermal stress.
- Sufficient High-Temperature Strength: During smelting, furnace walls are subjected to the impact of metal charges, the hydrostatic pressure of the melt, electromagnetic stirring forces, and stresses caused by temperature gradients. Thus, refractory materials must possess adequate high-temperature strength to maintain structural stability and resist cracking or erosion under these various forces.
- Low Thermal and Electrical Conductivity: To minimize heat loss and improve thermal efficiency, refractory materials should have low thermal conductivity. Additionally, to prevent induction current losses and safety hazards, electrical conductivity must be low, and the inclusion of metal or conductive materials within the lining during the ramming process is strictly prohibited.
Types and Characteristics of Refractory Materials
Acidic Refractory Materials
- Main Components: Typically composed primarily of quartz sand, mixed with high-temperature sintering agents and mineralizers.
- Advantages: Low cost, widespread availability, good electrical insulation, low construction requirements, few defects during application, and stable production.
- Disadvantages: Relatively low refractoriness (generally suitable only for small medium-frequency furnaces); undergoes secondary phase changes during heating, resulting in poor dimensional stability and suboptimal chemical stability; prone to erosion through reactions with slag.
- Applications: Primarily used in foundries for melting gray iron, ductile iron, and carbon steel; also used for smelting titanium alloys and high-temperature non-ferrous metals.
Neutral Refractory Materials
- Main Components: Dry ramming mixes primarily composed of corundum sand and powder, mixed with magnesium-aluminum spinel powder and sintering agents.
- Advantages: Corundum features a high melting point and high hardness; consequently, these materials offer excellent thermal shock resistance, volume stability, and high-temperature strength, as well as good chemical stability and resistance to erosion by various types of slag.
- Disadvantages: Issues with phase changes and a high coefficient of thermal expansion, though these can be mitigated by adding spinel micropowder.
- Applications: Suitable for smelting various alloy steels, carbon steels, and stainless steels; particularly well-suited for high-temperature cast steel applications or linings for large furnaces.
Basic Refractory Materials
- Main Components: Dry ramming mixes made from fused or high-purity magnesia sand and powder, mixed with magnesium-aluminum spinel powder and sintering agents.
- Advantages: High refractoriness (approaching 2800°C), excellent high-temperature performance, strong erosion resistance, long service life, and relatively low cost.
- Disadvantages: High coefficient of thermal expansion; prone to cracking.
- Applications: Primarily used for smelting various high-alloy steels, carbon steels, high-manganese steels, tool steels, and stainless steels.
Spinel Furnace Lining Materials
- Main Composition: Formed using aluminum oxide and magnesium oxide; utilizes high-temperature fired or electro-fused pre-synthesized spinel, processed into various particle size specifications as required.
- Advantages: Combines the benefits of white fused alumina and magnesia linings; offers excellent erosion resistance, thermal shock stability, and dimensional stability. It overcomes certain drawbacks associated with traditional basic and neutral lining materials and represents the future direction for linings in large-scale medium-frequency furnaces and high-temperature furnaces.
- Disadvantages: Manufacturing processes are relatively complex, and costs are higher.
- Applications: Widely used in high-temperature, highly corrosive melting environments, such as large-scale medium-frequency furnaces and the smelting of high-temperature alloys.
Methods for Selecting Refractory Materials for Medium-Frequency Furnaces
- Based on the type of metal being melted: Different metals and their alloys possess distinct melting points, chemical properties, and melting requirements. For instance, acidic refractories are suitable for melting gray iron, ductile iron, and carbon steel, whereas neutral or basic refractories are preferable for alloy steel and stainless steel.
- Considering operating conditions: Factors such as operating temperature, heating and cooling rates, furnace atmosphere, and melting duration must be taken into account. Refractories with high thermal shock resistance should be selected for furnaces operating at high temperatures or involving rapid heating and cooling cycles; conversely, materials with high chemical stability are required if the furnace atmosphere is strongly oxidizing or reducing.
- Factoring in production scale and costs: For large-scale medium-frequency furnaces or enterprises with high production volumes, priority should be given to refractories offering long service life and stable performance; although the initial investment may be higher, the overall cost is reduced in the long run. For small enterprises or smaller-scale operations, cost-effective refractories that ensure product quality are a suitable choice.
Use and Maintenance of Refractory Materials for Medium-Frequency Furnaces
- Proper Furnace Lining Construction: Construct the lining in strict accordance with the supplier’s instructions—including material mixing, ramming, and vibration—to ensure the lining’s density and uniformity; carefully control environmental conditions such as temperature and humidity during construction to avoid compromising the refractory material’s performance.
- Appropriate Furnace Drying/Sintering Schedule: Newly installed or repaired linings require a drying and sintering process before use to remove moisture and allow the refractory material to sinter and harden. This process should follow the principles of gradual heating and staged holding periods to prevent lining cracks caused by rapid temperature increases.
- Routine Inspection and Maintenance: Regularly inspect the lining during furnace operation to monitor for erosion, cracks, or spalling; promptly repair localized damage to prevent issues from escalating; keep the furnace interior clean to avoid erosion of the lining by slag or molten metal.
- Proper Shutdown and Cooling: Follow established procedures for cooling during shutdown to avoid cracking the lining due to rapid cooling; perform necessary inspections and maintenance only after the lining has fully cooled.
