In the selection of refractory materials for powder metallurgy electric furnace linings, there are many types of refractory materials, mainly heavy refractory bricks, including refractory clay bricks, high alumina bricks, silica bricks, fused alumina bricks, silicon carbide bricks, carbonaceous bricks, etc.
Clay bricks
Clay bricks are composed of refractory clay or kaolin (mainly composed of Al₂O₃, SiO₂, and H₂O, and containing 6%–7% metal oxides such as K₂O, Na₂O, CaO, MgO, TiO₂, and Fe₂O₃). Clay bricks are products made by mixing soft clay (binder) with calcined clay, molding the mixture, and then sintering it at 1300–1400℃. Clay bricks contain 30%–40% Al₂O₃, with the remainder being SiO₂.
Clay bricks are weakly alkaline refractory materials, resistant to acidic slag corrosion, but with poor resistance to alkaline slag corrosion. They exhibit excellent resistance to rapid heating and cooling; the softening temperature under load is 125–300℃, the refractoriness is 1610–1750℃, and the maximum service temperature is 1300–1400℃. Clay bricks can be used to construct furnace walls, furnace roofs, furnace bottoms, combustion chambers, etc. However, it has a corrosive effect on Fe-Cr-Al resistance wires and is not suitable as a support brick for them. Moreover, it is easily damaged by CO and H2 in a controlled atmosphere.
High alumina bricks
High-alumina bricks refer to refractory materials with an Al2O3 content of over 48%. They are made from high-alumina bauxite (Al2O3·SiO2) and fired at around 1500℃. High-alumina bricks have a refractoriness of 1750~1790℃, a softening temperature under load of 1420~1500℃, and good resistance to chemical attack, but poor resistance to rapid heating and cooling. High-alumina bricks generally exhibit significant re-firing shrinkage, which is particularly important for those containing 60%~75% Al2O3. When using high-alumina muffle furnace tubes fired at insufficient temperatures or when using these high-alumina bricks to construct furnace roofs, the large re-firing shrinkage can easily cause the roof to sink or collapse after a period of high-temperature use.
Corundum bricks
Corundum bricks are high-grade refractory materials with an Al₂O₃ content greater than 98%. Depending on the raw materials used, corundum bricks are further divided into fused alumina bricks and alumina corundum bricks.
Corundum can be natural or synthetic. Synthetic corundum is generally formed by high-temperature calcination of industrial alumina (mainly γ-Al₂O₃), or it can be made from non-crystalline alumina raw materials through high-temperature electric furnace melting; this is often called fused alumina.
Fused alumina is a refractory product made primarily from fused alumina sand, with the addition of an appropriate amount of aluminum phosphate solution as a binder. Its forming methods include ramming (or machine pressing) molding and plaster casting. Its firing temperature is 1700~1800℃, refractoriness is 1950℃, load softening start temperature is 1770℃, resistance to rapid heating and cooling is 50 cycles, density is 3.1~3.4 g/cm³, and service temperature is 1800~1850℃.
Due to its excellent resistance to rapid heating and cooling, high hardness, and good wear resistance, corundum products can be used as furnace tubes in high-temperature molybdenum wire furnaces. It is worth noting that corundum products require very high firing temperatures. If the firing temperature is too low, re-firing shrinkage can easily occur during use, reducing wear resistance and worsening resistance to rapid heating and cooling. This is particularly important for furnace tubes in horizontal continuous molybdenum wire furnaces.
Alumina corundum bricks are made by firing industrial pure alumina powder with appropriate amounts of oxides (titanium oxide, chromium oxide, etc.) at temperatures above 1600℃, or by firing pure alumina products with an alumina content of over 98%. These products have an operating temperature of 1750-1850℃ and are generally used in high-temperature kilns, electric furnace linings, and furnace tubes.
Silica bricks
Silica bricks are silica-based refractory materials containing more than 93% SiO2. They are made by crushing quartzite and adding lime slurry or other binders.
Silica bricks are acidic refractory materials, exhibiting strong resistance to acidic slag but weak resistance to alkaline slag; this should be considered when using them. Silica bricks have a refractoriness of 1710~1730℃, and their softening temperature under load is almost equal to their refractoriness, generally above 1620~1640℃, which is their greatest advantage. Their disadvantage is poor resistance to rapid heating and cooling; they are sometimes used to construct high-temperature sections of heating furnaces, but are not suitable for intermittent furnaces.
Magnesia bricks and magnesium-aluminum bricks
Magnesium bricks are made from calcined magnesite (MgCO₃) and contain more than 80% magnesium oxide. Magnesia bricks are alkaline refractory materials with good resistance to alkali erosion and fair resistance to iron oxide erosion. They have a refractoriness of up to 2000°C and a load-softening temperature of 1550–1600°C. They undergo slight volumetric shrinkage during prolonged use at high temperatures, have poor resistance to rapid cooling and heating, and can withstand only 3–5 water quenches. Magnesia bricks are commonly used to line the hearths of heating furnaces. It should be noted that magnesium bricks are alkaline refractories and react to varying degrees with clay bricks, high-alumina bricks, and silica bricks at high temperatures.
Magnesia-alumina bricks are designed to improve the thermal stability of magnesia bricks; they are produced by adding 5%–10% Al₂O₃ to the mix to form magnesia-alumina spinel (MgO·Al₂O₃)-bonded magnesia bricks. With a refractoriness of 2135°C, they exhibit better thermal shock stability and resistance to rapid cooling and heating than standard magnesia bricks, and their load-softening onset temperature is also higher, making them refractory materials with excellent performance.
Silicon Carbide Refractory Products
Silicon carbide refractory products are made by molding silicon carbide as raw material, adding a binder (or not adding a binder), and then firing. The products are divided into three categories:
(1) products with clay as a binder;
(2) products with other mineral binders (ferrosilicon, quartz, etc.);
(3) products without binder (recrystallization).
Silicon carbide refractories possess high mechanical strength, excellent abrasion resistance, resistance to rapid heating and cooling, and superior thermal conductivity (5-10 times greater than conventional refractories). They also exhibit a high softening point under load, with deformation initiation around 1600℃. Their disadvantages include high cost, susceptibility to oxidation at operating temperatures above 1300℃, and susceptibility to corrosion by alkaline slag. The denser the silicon carbide material, the better its oxidation resistance.
Silicon carbide refractory products are commonly used as linings for electric furnaces, muffle walls in muffle furnaces, refractory heating plates, and heating elements.
Graphite refractory products
Graphite refractory products use graphite as raw material and soft clay as binder. They are available in superior, first, second and third grades, with a refractoriness of 3000℃, a service temperature of 2000℃, and a load softening start temperature of 1800~1900℃.
Alumina powder
Alumina powder has a melting point of 2015℃. It is very stable in both oxidizing and strongly reducing atmospheres, and can be used in any type of atmosphere below 1900℃.
Pure alumina has several crystalline structures at low temperatures, such as hexagonal (denoted by α), spinel (a mixture of oxides with the chemical formula RO·R₂O₃, denoted by β), and cubic (denoted by γ). However, all these crystalline forms become high-temperature stable α-Al₂O₃ (corundum) when heated to temperatures above 1600℃; this transformation is irreversible.
Alumina powder can be used as both a refractory material and a good thermal insulation material at temperatures below 1900℃. This is why furnace builders often fill the furnace tubes around which heating elements are wrapped with 50-100mm thick layers of Al₂O₃ powder. It is worth noting that if the furnace atmosphere is carbonaceous, carbon particles are more likely to deposit in the alumina powder, which is detrimental to Ni-Cr resistance wires and can even damage them.
Alumina products are commonly used as refractory linings for sintering furnaces; they are also used as furnace tubes or flame shields for electric furnaces wound with molybdenum or tungsten wires, operating in hydrogen or decomposed ammonia atmospheres; they can also be made into small boats, observation port sleeves, crucibles, etc. Alumina crucibles are used to melt various metals and oxides, exhibiting extremely high chemical stability.
