Characteristics and Applications of Silicon Carbide Refractory Wear-Resistant Castables
The most outstanding property of silicon carbide castables is their wear resistance, followed by their erosion resistance, offering a dual advantage in both. The manufacturing process still uses traditional refractory castables as the framework, introducing a certain proportion of silicon carbide particles to form a composite structure of "refractory aggregate + silicon carbide." Silicon carbide itself has high hardness and high grain boundary strength, allowing it to preferentially bear stress under erosion and friction conditions, thus mitigating matrix wear. Consequently, the overall wear resistance coefficient decreases, and the service life can typically be extended by one to two times. Simultaneously, silicon carbide has a large wetting angle with molten metal and slag, making it difficult to be wetted. Acidic and alkaline media also have difficulty corroding its surface, allowing the furnace lining to remain intact in multiphase corrosive environments involving solid, liquid, and gaseous states.
Mechanism of Improved Thermal Shock Resistance
Silicon carbide has high thermal conductivity and low thermal expansion. Its addition reduces the internal temperature gradient of the castable, buffering thermal stress caused by rapid heating and cooling. When silicon carbide is combined with silicon nitride, an interwoven fibrous silicon nitride network forms in the matrix, further absorbing thermal shock energy and improving thermal shock resistance by approximately two times. Furthermore, its high thermal conductivity rapidly homogenizes the furnace lining surface temperature, reducing localized hot spots. This inhibits slag crusting or nodulation on the surface, maintaining furnace stability and reducing cleaning intensity.
Oxidation Problems Arising from High Silicon Carbide Content
As the proportion of silicon carbide increases, the thermal conductivity of the castable also increases. While this is beneficial for heat dissipation, it is accompanied by a significant tendency to oxidize. When the temperature exceeds 1000 degrees Celsius, the silicon carbide surface easily reacts with oxygen to form silicon dioxide. This, along with volume changes, leads to microcracks, thus weakening the strength. To suppress oxidation, manufacturers commonly introduce metallic silicon powder. Metallic silicon preferentially combines with oxygen at high temperatures to form a dense glassy phase that covers the surface of silicon carbide particles, blocking oxygen diffusion channels and thus slowing down the oxidation rate.
The Influence of Different Bonding Phases on Oxidation Resistance
The microstructure of the bonding phase determines the difference in oxidation resistance. In silicon nitride-bonded silicon carbide castables, the matrix is interwoven with fibers, resulting in high permeability, but oxygen can still diffuse along the micropores, offering limited protection to the silicon carbide particles. In contrast, in silicate-bonded or oxynitride-bonded products, the silicon carbide particles are encapsulated by a continuous matrix, resulting in a tortuous oxygen diffusion path and slow oxide layer thickening. Therefore, they exhibit superior oxidation resistance. When designing formulations, a balance between thermal shock resistance and oxidation resistance is often achieved by adjusting the type and proportion of the bonding phase.
High-Temperature Strength and Chemical Stability
Silicon carbide maintains high hardness and high flexural strength at high temperatures and does not soften significantly with increasing temperature. Its chemical stability is outstanding; except for strong alkalis, it is inert to most acid and alkali solutions. In non-ferrous and ferrous metal smelting environments, it exhibits excellent resistance to molten iron, molten steel, molten copper, and high-calcium and high-iron slag. Thanks to the synergistic effect of low expansion and high thermal conductivity, the furnace lining can maintain structural integrity during long-term operation with frequent start-ups and shutdowns and sudden temperature changes, reducing maintenance frequency and improving operating efficiency.
Application Principles of Silicon Carbide Castables
When selecting materials for practical applications, careful consideration should be given to the furnace type, operating temperature, corrosiveness of the medium, and thermal shock frequency. High-silicon carbide wear-resistant grades should be prioritized in heavily eroded areas. In areas with strong oxidizing atmospheres, metallurgical silicon micropowder modification or silicon oxynitride bonding systems are recommended. For areas with extremely high thermal shock requirements but low oxidation, the silicon nitride bonding ratio can be appropriately increased. During construction, the amount of water added and the vibration time must be strictly controlled to ensure uniform dispersion of silicon carbide particles and avoid local enrichment or sedimentation, so as to fully utilize its combined advantages of wear resistance, corrosion resistance, and thermal shock stability.
Optimal Operating Environment for Silicon Carbide Castables
Above 1500℃ and in a reducing or weakly reducing atmosphere are prerequisites for the performance release of silicon carbide castables. Within this range, the SiC particles inside the material remain stable, and the surface layer undergoes slight graphitization, spontaneously forming a dense barrier. The resistance to erosion and corrosion increases with increasing temperature.
SiC should form a protective SiO₂ film in high-temperature oxidizing environments, but the high porosity of the castable makes the film prone to cracking due to mechanical impact and thermal stress. Oxygen diffuses inward along the cracks and pores, causing SiC to pulverize layer by layer, the matrix strength to continuously decrease, and eventually peeling off. Even at low oxygen partial pressures, oxygen can still slowly penetrate into the capillaries, leading to an overall quality decline; therefore, high-oxygen environments should be considered prohibited areas.
Advantages of Composite Formulations
To delay accidental oxidation, metallic silicon is often introduced into the formulation. It preferentially combines with free oxygen, locally reducing the oxygen partial pressure and providing sacrificial protection for SiC. Additionally, impregnation with sodium salt sol or silica sol allows nano-sized particles to seal open pores, forming a continuous mullite-glass phase after medium-temperature curing, providing both oxidation and penetration resistance. Pure calcium aluminate cement with an appropriate amount of alumina micropowder is used as the binder. Ceramic bonding can be achieved at around 1450℃, reducing the catalytic oxidation of SiC by free CaO.
Applications and Nomenclature of Silicon Carbide Castables
1. Circulating Fluidized Bed Boilers: Located in the dense phase zone of the furnace, in the return feeder and slag cooler, temperatures reach 900–1000℃, but particle erosion velocity is high. Using "high wear-resistant castables" with SiC ≥ 55% increases service life from 12 months for ordinary low-cement castables to 18–24 months.
2. Blast Furnace Main Iron Tray: Molten iron temperature reaches 1500–1550℃, and intermittent tapping causes strong thermal shock. SiC–Al₂O₃–C composite castables possess dual functions of slag and iron erosion resistance and thermal shock resistance, with a throughput of 120,000–150,000 tons.
3. Cement Rotary Kiln Feeding Strip: Kiln lining frequently adheres and detaches. SiC-based "anti-scabbing castables" utilize high thermal conductivity (λ ≥ 15 W·m⁻¹·K⁻¹) to quickly balance the lining temperature difference, reducing the frequency of scabbing by 40% and increasing kiln operating rate by more than 3%.
Precautions for Using Silicon Carbide Castables
1. Strictly prohibit low-temperature oxidation environments: At temperatures below 1200℃ and in an oxidizing atmosphere, the oxidation rate of SiC is low, but continuous weight loss occurs, resulting in lower matrix strength than ordinary high-alumina castables, leading to a "performance inversion."
2. Heating regime: The initial baking should follow a "three-step" curve (150℃×8 h, 350℃×12 h, 600℃×8 h) to fully expel the sol and organic binder, preventing internal carbonization and cracking.
3. Repair principles: When local erosion exceeds 1/3 of the lining thickness, use a composite method of "excavation and patching + overall casting" with castables of the same material. The interface between the old and new surfaces should be moistened with SiC fine powder slurry to ensure thermal expansion matching.
In summary, the value of silicon carbide castables lies in the three harsh operating conditions of "high temperature, reduction, and strong erosion." With proper selection of atmosphere and temperature range, and by suppressing oxidation through methods such as metal silicon buffering, sol-gel sealing, and gradient ceramic bonding, its service life can be increased by more than half compared to ordinary low-cement castables. Low Cement Silicon Carbide Castable. Conversely, if placed in a low-temperature or high-oxygen environment, it will fail prematurely due to continuous oxidation. Users should define the furnace operating conditions before selecting materials, and then match the appropriate SiC content and composite process to transform this "high-temperature bonus" into tangible economic benefits.
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