Commonly used refractory materials in heat treatment furnaces include refractory clay bricks, high alumina bricks, anti-carburization bricks, refractory concrete, silicon carbide products, various aluminum silicate refractory fibers and refractory coatings. Among them, refractory fireclay bricks are the most used in heat treatment furnaces.
01 Refractory Clay Bricks
The raw materials of refractory clay firebricks are mainly refractory clay and kaolin. The chemical composition of refractory fireclay bricks is Al2O3 (mass fraction is 30%~40%), SiO2 (mass fraction is 50%~65%), and the rest are various metal oxides. Ordinary clay firebricks are weakly acidic, with a load softening point of 1350℃, a small expansion coefficient, thermal conductivity and specific heat capacity, good thermal vibration stability, and a maximum use temperature of 1300~1400℃. Ordinary clay bricks are most 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 will occur between the iron-chromium-aluminum alloy and the clay fire bricks, and the bricks will corrode and damage the iron-chromium-aluminum alloy. Clay refractory bricks are easily damaged by the erosion of CO and H2 in a controlled atmosphere.
02 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 aluminum bricks are divided into three grades according to the mass fraction of Al2O3, namely, 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 they are expensive and are mostly used in high-temperature heat treatment furnaces and resistance wire or resistance belt shelf bricks, thermocouple tubes, muffle furnace cores, etc.
03 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) Metallic iron is generated. Iron is also a catalyst for the following reaction, which accelerates the decomposition of CO, precipitates carbon black, and deposits inside the brick body. 2CO=CO2+C (3) The formation of iron and the subsequent formation of Fe3C by iron make the brick body loose. At the same time, the deposition of carbon black in the brick body causes the volume of the brick to expand. This will cause the furnace lining of ordinary refractory clay bricks or refractory high-alumina bricks to be damaged prematurely. Therefore, the mass fraction of Fe2O3 in anti-carburization bricks is required to be less than 1%.
04 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 the manufacturing process, it can be used as electric heating elements of high-temperature furnaces, furnace bottom plates of high-temperature furnaces, etc. Silicon carbide products are easily oxidized at temperatures above 1300°C and easily corroded by alkaline slag.
05 Lightweight insulation bricks
Anthracite or coke powder, these substances are combustibles, which are burned during the preparation and sintering process of refractory bricks, thereby increasing the porosity of refractory firebricks, reducing the bulk density, and enhancing thermal insulation. Chemical foaming agents can also be added to the ingredients to reduce the bulk density of the bricks. When lightweight or ultra-lightweight bricks are used as furnace linings, the heat storage loss of the furnace lining can be reduced, energy can be saved, and the time for empty furnace heating can be shortened. However, lightweight bricks and ultra-lightweight bricks also have many shortcomings. Due to their low bulk density, their compressive strength is reduced, and the load softening point is also reduced; because their porosity increases, their residual shrinkage (or expansion) increases, and their resistance to chemical erosion decreases.
06 Aluminum silicate refractory fiber
Aluminum silicate refractory fiber, also known as ceramic fiber, is a new type of refractory material with both refractory and heat preservation effects. According to different 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, good chemical stability, etc. It can be filled in bulk, or made into finished products such as felt, board, rope, brick, etc. It can also be used on the inner side of the furnace wall or the transition layer between the refractory layer and the insulation layer.
07 Amorphous refractory material
Amorphous refractories materials can be made into prefabricated blocks of various shapes for easy 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 tank crucible of the salt bath furnace can also be made into prefabricated parts of the furnace roof and furnace lining. Amorphous refractories materials can be divided into castables, plastic materials, spray coatings, ramming materials, coating materials, projection materials, etc. according to different production methods. Commonly used unshaped refractory include refractory castables, refractory plastics, refractory ramming materials, refractory coatings and refractory cement. Refractory concrete is an unshaped refractories commonly used in heat treatment furnaces, which 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: binder, aggregate and admixture, and sometimes a coagulant is added. Its advantages are that it can be cast into an integral furnace lining, which is convenient for manufacturing complex components, fast furnace repair and furnace building, long furnace life and low cost; the disadvantage is that the refractoriness is lower than that of refractory bricks
08 Refractory mud
Refractory mud is a refractory material used to fill the brick joints when using formed bricks to build the furnace body so that they are bonded and fixed to each other, and at the same time make the brickwork have a certain strength and airtightness. Refractory mud is composed of clinker and binder. Its refractoriness is determined by the refractoriness of the raw materials and their proportions, which is generally slightly lower than the refractoriness of the refractory firebricks. In order to make the refractory mud and the brick body have the same process performance, the composition and performance of the selected refractory mud should be close to the corresponding performance of the refractories bricks, that is, various bricks should use the specified refractory mud. For example, when laying clay bricks, clay clinker powder with a mass fraction of 50%~70% and combined clay (raw clay) with a mass fraction of 30%-50% are generally used as refractory mud, in which raw clay plays a bonding and reinforcement role, but the amount cannot be too much, because if the amount of raw clay is too much, the volume shrinks greatly during sintering, which easily causes cracking of the masonry joints. Sometimes, in order to improve the air tightness of the brick body, 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, so when laying the bath of a high-temperature salt bath furnace, phosphate refractory mud can be used as a caulking material, or high-aluminum clinker powder can be used as refractory mud.
