Types of Refractory Materials Used in Different Parts of the Heating Furnace
A heating furnace is a device (industrial furnace) used in the metallurgical industry to heat materials or workpieces (usually metals) to rolling or forging temperatures. It can be divided into continuous heating furnaces and chamber heating furnaces. Heating furnaces are widely used in many industries, including petroleum, chemical, metallurgy, machinery, heat treatment, surface treatment, building materials, electronics, materials, light industry, daily chemicals, and pharmaceuticals.
Structural Components of a Heating Furnace
Based on furnace temperature distribution, the furnace chamber is divided into a preheating section, a heating section, and a soaking section along its length.
The preheating section, located at the feed end where the furnace temperature is lower, utilizes the heat from the furnace gas to improve the furnace's thermal efficiency.
The heating section is the main heating section, where the furnace gas temperature is higher to facilitate rapid heating.
The soaking section is located at the discharge end, where the furnace gas temperature is very close to the metal material temperature, ensuring a uniform cross-sectional temperature of the discharged billet.
Refractory Materials for Heating Furnace Body
(1) Brick-built Furnace Body. The furnace lining is constructed of insulating bricks and refractory bricks. The furnace insulation layer is constructed of clay or high-alumina insulating bricks, perlite bricks, diatomaceous earth bricks, and refractory fiber felt, with a thickness of 113–300 mm. The working layer of the furnace wall is constructed of clay refractory bricks, with a thickness of 230–400 mm. Openings can be made using brick arches, irregularly shaped bricks, or long strips of clay refractory bricks.
(2) Precast Block Furnace Body. Precast blocks are made of refractory castables such as aluminate cement, phosphate low-grade cement, and water glass. If clay-bonded refractory castables are used, anchoring bricks are required. The furnace roof precast blocks are available in arched and long strip shapes; if reinforced with steel bars, they must be placed in the non-working layer.
(3) Refractory Plastic Rammed Furnace Body. The working layer of the furnace lining, made of refractory castable and anchored, is constructed by ramming. Refractory castable blanks are filled in the gaps between anchor bricks or hanging bricks and compacted using a pneumatic hammer or tamping machine. The refractory lining, including the furnace bottom, is generally constructed continuously in layers and sections, with the surface roughened, vent holes punched, and expansion joints cut. The advantages of refractory castable furnace linings are strong integrity, good sintering properties, and high high-temperature strength.
(4) Casting the furnace body with refractory castable. The working layer of the furnace lining is cast on-site using refractory castable. The structure of the furnace walls and top is the same as that of the refractory castable furnace body. After the anchor bricks or hanging bricks are installed, the refractory castable mixture is laid from one side and then compacted using a vibrator. Construction should be continuous and timely curing should be carried out.
Refractory Materials for Combustion Chambers
Used in coal-fired heating furnaces, these are classified into various types, including reciprocating grate and cyclone combustion types, and generally consist of a bottom, walls, and roof. The lining of a typical combustion chamber is usually constructed with clay bricks or high-alumina bricks. Due to high operating temperatures and large temperature fluctuations, slag erosion, and mechanical damage during slag removal, its lifespan is approximately one year. The inner furnace, serving as the passage between the combustion chamber and the furnace lining, operates at around 1600℃. When constructed with sillimanite or corundum bricks, its lifespan is only about 0.5 years.
Refractory Materials for Furnace Doors and Tapping Channels
Used to seal openings in the furnace walls, these are divided into side-opening and lift-type doors. Furnace doors are generally lined with materials such as clay bricks, refractory castables or refractory plastics, high-alumina insulating bricks, or refractory fiber felt. Side-opening furnace doors can be used for one furnace cycle, while lift-type furnace doors, due to temperature fluctuations and mechanical impacts, have a lifespan of approximately one year.
The tapping trough is used in heating furnaces where steel billets tap from the side. Except for water-cooled cast iron tapping troughs, they are generally constructed with high-alumina bricks or magnesia bricks, with a service life of 3-6 months. When using sintered or electrofused mullite bricks, the strength and wear resistance are high, but the thermal shock resistance is poor, making them prone to cracking, and their service life is about 1 year. Using heat-resistant steel fiber-reinforced corundum refractory castable to cast an integral tapping trough on-site has a service life of over 2 years.
Ceramic Sliders Made of Refractory Materials
Small pusher-type heating furnaces use ceramic sliders composed of brown corundum-silicon carbide slide bricks. Two or four rows of refractory bricks are used to build the base walls along the length of the furnace. High-alumina silicon carbide seat bricks are then laid on the base walls, and the slide bricks are installed to form the ceramic slide. The steel billet moves on the slide, achieving heating from both the top and bottom, with advantages such as low energy consumption and no black marks on the steel billet.
Refractory Insulation for Bottom Water-Cooled Pipes
In large and medium-sized pusher-type or walking beam furnaces, the bottom water-cooled pipes consist of thick-walled water-cooled pipes distributed longitudinally and laterally, providing support. The purpose of insulation is to reduce the heat carried away by the cooling medium, reduce black marks on the steel billets, and improve heating quality. Insulation methods include inlaying irregularly shaped clay bricks or suspending horseshoe-shaped bricks, welding precast blocks made of refractory plastics or refractory castables, and on-site wrapping with refractory fiber felt.
Refractory Materials for Flues and Chimneys
Flue linings are generally constructed with clay bricks, but can also be constructed using precast refractory castable blocks suspended or poured on-site. Sometimes, refractory spray coatings are also used. Red brick or concrete chimneys are constructed with refractory bricks in high-temperature areas. Metal chimneys have metal anchoring nails welded to the inner wall, sometimes with metal mesh added, and are lined with lightweight refractory castables or refractory spray coatings. This method is convenient to construct, has strong integrity, and a long service life. Refractory materials are used in all parts of the heating furnace.
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