Refractory Bricks

What are the commonly used refractory materials for heat treatment furnaces? Where are these refractory materials used?

Mar 12,2024

In metallurgical factories, the main objects of heat treatment are metal ingots, plates, tubes, wires, profiles, rails, wheels, etc. In machinery manufacturing plants, heat treatment targets various parts, cutting tools, measuring tools, forgings, castings, etc.

Due to the wide variety of heat treatment processes and the wide variety of workpiece shapes and sizes, various heat treatment furnace types and structures are produced. Therefore, there are many classification methods for heat treatment furnaces.

The classification methods can be as follows:

According to temperature, there are high-temperature heat treatment furnaces with a furnace temperature greater than 1000°C; medium-temperature heat treatment furnaces with a furnace temperature of 650~1000°C; low-temperature heat treatment furnaces with a furnace temperature less than 650°C.

According to the heat source, those using fuel as the heat source include coal-fired furnaces, oil-fired heat treatment furnaces and gas-fired heat treatment furnaces. Electrical heat sources include resistance furnaces, induction heating devices, etc.

According to the operation mode, there are periodic operation heat treatment furnaces and continuous operation heat treatment furnaces.

According to the heat treatment process, there are quenching furnace, tempering furnace, annealing furnace, normalizing furnace and carburizing furnace.

According to the heating process, there are direct heating heat treatment furnaces and indirect heating heat treatment furnaces - such as muffle furnaces, radiant tube furnaces, etc.

According to the heating medium in the furnace, some use gas as the heating medium. Such as air, flue gas, controlled atmosphere, vacuum, etc.; some use liquid as the heating medium, such as molten salt, molten lead furnace; and others use solid as the heating medium, such as flowing particle furnace.

According to the mechanization method, there are trolley furnaces, roller hearth furnaces, chain furnaces, rotary hearth furnaces, vibrating hearth furnaces, bell furnaces, etc.

Generally speaking, heat treatment processes include quenching, asking, annealing, normalizing, carburizing, oxygenating, etc. Heating of workpieces includes three basic processes: heating, heat preservation, and cooling. The high heating temperature is determined by the treatment process and the material of the workpiece. Generally speaking, from the perspective of heating temperature, heat treatment furnaces are divided into: high-temperature heat treatment furnaces, with furnace temperatures between 1000 and 300°C; medium-temperature heat treatment furnaces, with furnace temperatures between 650-1000°C. ℃, low temperature heat treatment furnace, the furnace temperature is less than 650 ℃.

When steel is heated in air or furnace gas, at high temperatures, the iron and carbon in the steel will react chemically with the atmosphere, resulting in oxidation-reduction, decarburization-carburization and other reactions. For general steel or parts, oxidative decarburization will increase metal consumption. For steel or parts with strict surface requirements, oxidative decarburization will cause serious surface defects. A controlled atmosphere is used in the heat treatment furnace that requires high surface quality of some workpieces. Controlled atmospheres are divided into two categories: protective atmosphere and chemically active atmosphere. The protective atmosphere protects the surface composition of the workpiece from changing. Generally, the surface still maintains the metallic luster after heating, so it is also called bright heat treatment; while the chemically active atmosphere can protect the workpiece during the heating process. Perform chemical heat treatment such as carburizing or heliumizing on the surface of the workpiece.

Commonly used refractory materials in heat treatment furnaces include refractory clay bricks, high alumina bricks, anti-carburization bricks, refractory concrete, silicon carbide products, various refractory fibers and refractory coatings. Among them, refractory clay bricks are most commonly used in heat treatment furnaces.

(1) Refractory clay bricks.

The raw materials of refractory clay bricks are mainly refractory clay and kaolin. The chemical composition of refractory clay bricks is Al2O3 (mass fraction 30%~40%), SiO2 (mass fraction 50%~65%), and the rest are various metal oxides. Ordinary clay bricks are weakly acidic, with a load softening point of 1350°C. They have a small expansion coefficient, thermal conductivity and specific heat capacity, and have good thermal vibration stability. The maximum operating temperature is 1300~1400°C.

Ordinary clay bricks are most commonly used in heat treatment furnaces, and are generally used to build furnace tops, furnace bottoms, furnace walls, combustion chambers, etc. In the heat treatment furnace, a chemical reaction occurs between the iron-chromium-aluminum alloy and the clay bricks. The clay bricks will cause the iron-chromium-aluminum alloy to be corroded and damaged. Clay bricks are susceptible to corrosion and destruction by CO and H2 in a controlled atmosphere.

(2) High alumina bricks.

The mass fraction of Al2O3 in high alumina bricks is greater than 48%, and the rest is mainly SiO2 with very few impurities. Ordinary high alumina bricks are divided into three grades according to the mass fraction of Al2O3, that is, the mass fraction of Al2O3 is greater than 48%, the mass fraction of Al2O3 is greater than 55%, and the mass fraction of Al2O3 is greater than 65%. High alumina bricks have the advantages of high refractoriness, good high-temperature structural strength and good chemical stability, but are expensive. They are mostly used in high-temperature heat treatment furnaces and resistance wire or resistance strip bricks, thermocouple conduits, muffle furnace cores, etc. .

(3) Anti-carburization bricks.

In a reducing atmosphere containing CO and H2, Fe2O3 in refractory clay bricks or refractory high alumina bricks will react with the atmosphere as follows:

Fe2O3+3CO=2Fe+3CO2 (1)

Fe2O3+3H2=2Fe+3H2O (2)

Produce metallic iron. Iron is the catalyst for the following reaction, which accelerates the decomposition of CO, precipitates carbon black, and deposits inside the brick.

2CO=CO2+C (3)

The formation of iron and its subsequent formation of Fe3C makes the brickwork loose. At the same time, the deposition of carbon black in the brickwork causes the volume of the bricks to expand. This will lead to premature failure of ordinary refractory clay bricks or refractory high alumina brick furnace linings. Therefore, the mass fraction of Fe2O3 in anti-carburization bricks is required to be less than 1%.

(4) Silicon carbide refractory products.

The chemical composition of silicon carbide refractory products is mainly SiC. It has high refractoriness and high-temperature structural strength, good wear resistance, thermal shock stability, thermal conductivity and electrical conductivity. Depending on its manufacturing process, it can be used as an electric heating element of a high-temperature furnace, a furnace floor of a high-temperature furnace, etc. Silicon carbide products are easily oxidized above 1300°C and easily corroded by alkaline slag.

(5) Lightweight refractory bricks.

Refractory bricks can be divided into heavy refractory bricks, light refractory bricks and ultra-light refractory bricks according to their different volume densities. The main raw materials of lightweight clay bricks are the same as clay bricks, but wood chips, anthracite or coke powder are added to them. These substances are combustible and are burned during the preparation and sintering process of refractory bricks, thus increasing the porosity of refractory bricks. , the volume density becomes smaller and the thermal insulation is enhanced. Chemical foaming agents can also be added to the ingredients to reduce the volume density of the bricks.

When using lightweight or ultra-lightweight bricks as furnace lining, the heat storage loss of the furnace lining can be reduced, energy can be saved, and the time for the empty furnace to heat up can be shortened. However, lightweight bricks and ultra-lightweight bricks also have many shortcomings. Due to their small volume density, their compressive strength is reduced and their load softening point is also reduced; because their porosity increases, their residual shrinkage (or expansion) increases, and their resistance to Chemical attack ability is reduced.

(6) Fire-resistant fiber.

Refractory fiber, also known as ceramic fiber, is a new type of refractory material that has both fire resistance and heat preservation functions. Depending on the raw materials, there are aluminum silicate, quartz, alumina and graphite refractory fibers. Refractory fiber has the advantages of light weight, small specific heat capacity, high temperature resistance, good thermal shock stability, and good chemical stability. It can be filled in bulk or made into finished products such as felt, board, rope, brick, etc., and can also be used in furnace walls. The transition layer between the inner side or the fire-resistant layer and the insulation layer.

(7) Unshaped refractory materials.

Unshaped refractory materials can be made into prefabricated blocks of various shapes to facilitate mechanized construction, or they can be cast as a whole on the heating furnace to strengthen the integrity of the furnace body and facilitate the improvement of the furnace structure. For example, the salt bath crucible of the salt bath furnace can be manufactured, and the prefabricated parts of the furnace roof and furnace lining can also be manufactured. Unshaped refractory materials can be divided into castable materials, plastic materials, spray materials, ramming materials, smear materials, projection materials, etc. according to different production methods. Commonly used unshaped refractory materials include refractory castables, refractory plastics, refractory ramming materials, refractory coatings and refractory cement.

Refractory concrete is an unshaped refractory material commonly used in heat treatment furnaces. It can be divided into silicate refractory concrete, aluminate refractory concrete, phosphate refractory concrete and water glass refractory concrete. Refractory concrete is composed of three parts: cement, aggregate and admixture, and sometimes an accelerator is added. Its advantages are that it can be poured into an integral furnace lining, which facilitates the manufacture of complex components, is fast in repairing and laying furnaces, has a long furnace life, and is low in cost; its disadvantage is that its refractory resistance is lower than that of refractory bricks.

(8) Refractory mud.

Refractory slurry is a refractory material that is used to fill the brick joints to bond and fix each other when using shaped bricks to build a furnace body. At the same time, it gives the brickwork a certain strength and airtightness. Refractory mud is composed of clinker and binder. Its refractory degree is determined by the refractory degree of the raw materials and the ratio of its ingredients, which is generally slightly lower than the refractory degree of the refractory bricks laid. In order to make the refractory slurry and bricks have the same process performance, the composition and performance of the selected refractory slurry should be close to the corresponding properties of the refractory bricks, that is, various refractory bricks should use their specified refractory slurry. For example, when laying clay bricks, clay clinker powder with a mass fraction of 50% to 70% and bonded clay (raw clay) with a mass fraction of 30% to 50% are generally used as refractory slurry. The raw clay plays a bonding role. It has a strengthening effect, but the amount should not be used too much, because if the amount of raw clay is too much, the volume will shrink greatly during sintering, which will easily cause the joints to crack.

Sometimes in order to improve the air tightness of brickwork, plastic mud mixed with water glass can also be used. In addition, phosphate refractory mud has high bonding strength, good high-temperature volume stability and chemical stability at high temperatures. Therefore, when building the bath tank of a high-temperature salt bath furnace, phosphate refractory mud can be used as a caulking material. , you can also use high-aluminum clinker powder to make refractory mud

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