Common refractory materials for heat treatment furnaces include refractory clay bricks, high-alumina bricks, anti-carburization bricks, refractory concrete, silicon carbide products, various refractory fibers, and refractory coatings.
1. Lightweight Refractory Bricks
Refractory bricks can be divided into heavy refractory bricks, lightweight refractory bricks, and ultra-lightweight refractory bricks based on their bulk density. Lightweight clay bricks are made from the same raw materials as clay bricks, but with the addition of sawdust, anthracite, or coke dust. These materials are combustible and are burned during the sintering process, increasing the porosity of the refractory bricks, reducing their bulk density, and enhancing their thermal insulation properties. Chemical foaming agents can also be added to the mix to reduce the bulk density of the bricks.
Using lightweight or ultra-lightweight bricks for furnace linings can reduce heat loss in the lining, saving energy, and shortening the time it takes to heat up an empty furnace. However, lightweight and ultra-lightweight bricks also have several drawbacks. Their low bulk density reduces their compressive strength and softening point under load. Their increased porosity increases residual shrinkage (or expansion), reducing their resistance to chemical attack.
2. Refractory Fiber
Refractory fiber, also known as ceramic fiber, is a new type of refractory material that combines refractory and thermal insulation properties. Depending on the raw material, it can be made from aluminum silicate, quartz, alumina, or graphite. Refractory fiber offers advantages such as light weight, low specific heat capacity, high temperature resistance, excellent thermal shock resistance, and chemical stability. It can be used as bulk fill or in finished products such as felt, boards, ropes, and bricks. It can also be used on the inside of furnace walls or as a transition layer between the refractory and insulation layers.
3. Monolithic Refractory Material
Monolithic refractory materials can be prefabricated in various shapes for easy mechanized construction. They can also be cast integrally in the heating furnace to strengthen the furnace’s integrity and facilitate improvements to the furnace’s structure. For example, salt bath crucibles for salt bath furnaces can be manufactured, as can prefabricated components for furnace roofs and linings. Monolithic refractories can be categorized by their production methods as castables, plastics, sprayables, ramming materials, coatings, and projectiles. Commonly used monolithic refractories include castables, plastics, ramming materials, refractory coatings, and refractory cements.
Refractory concrete is a commonly used monolithic refractory material in heat treatment furnaces and can be categorized as silicate, aluminate, phosphate, and water glass. Refractory concrete consists of three components: binder, aggregate, and admixtures, sometimes with the addition of a setting accelerator. Its advantages include the ability to be cast into a monolithic lining, facilitating the fabrication of complex components, expediting furnace repair and construction, extending furnace life, and reducing costs. Its disadvantages are lower refractoriness than refractory bricks.
4. Refractory Mortar
Refractory mortar is a refractory material used to fill the gaps between bricks when building a furnace body with formed bricks, ensuring they adhere and securely bond together. It also provides the brickwork with a certain degree of strength and airtightness. Refractory mortar is composed of clinker and a binder. Its refractoriness is determined by the refractoriness of the raw materials and their proportions, and is generally slightly lower than that of the refractory bricks being laid. To ensure that the refractory mortar and the bricks have the same processing properties, the composition and properties of the selected refractory mortar should closely match the corresponding properties of the refractory bricks being laid. In other words, each type of refractory brick should use the refractory mortar specified for that brick. For example, when laying clay bricks, the refractory mortar generally consists of 50% to 70% clay clinker powder by weight and 30% to 50% binding clay (raw clay) by weight. The raw clay provides bonding and reinforcement, but the amount should be limited. Excessive raw clay shrinks significantly during sintering, potentially causing cracking in the mortar joints. Sometimes, plastic slurry mixed with water glass can be used to improve the airtightness of the brickwork. In addition, phosphate refractory slurry 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 slurry can be used as a caulking material. High-alumina clinker powder can also be used as refractory slurry.