The heat treatment furnace is a general term for industrial furnaces used for various metal heat treatments. It includes types such as the hood furnace, roller hearth furnace, chain furnace, drag furnace, and wire rod quenching furnace. These furnaces are used for various metal heat treatments and generally have lower temperatures than heating furnaces. Heat treatment furnaces can adopt various furnace types but require strict control over the furnace temperature and atmosphere. Most heat treatment furnaces use gas fuel for heating, with some employing electric heating for precise temperature control. Modern heat treatment processes, especially chemical heat treatment, are increasingly complex, with a variety of furnace designs available. For continuous heat treatment processes, a suitable continuous heat treatment furnace is required; for cyclic heat treatment processes, a forced circulation control atmosphere and the mechanization and automation of furnace operation are necessary. Metallurgical plants commonly use the following types of heat treatment furnaces: Hood Furnace: A periodic heat treatment furnace where the heated material is inside a hood. It is mainly used for the annealing treatment of thin plate stacks or steel coils. The hood furnace typically consists of an outer hood, an inner hood, and a hearth. The heated material is placed inside the inner hood (muffle hood) and heated between the inner and outer hoods. A controlled atmosphere is usually introduced into the inner hood. After the heating, holding, and slow cooling processes are completed, the outer hood is lifted, and the stack or coil cools inside the inner hood; once cooled to a specified temperature, the inner hood is removed, and the sheets or coils cool in the air before unloading. Figure 1 shows a hood furnace for treating thin plate stacks, with burners installed on the hearth. There are two types of hood furnaces for treating cold-rolled steel coils: single stack and multi-stack. The multi-stack type can accommodate 2 to 8 stacks on a rectangular hearth, with each stack covered by a separate circular inner hood. A circulation fan is installed below the inner hood to enhance control atmosphere and promote heat transfer between the coils. Roller Hearth Furnace: Utilizes furnace internal rollers to transport heat treatment materials. Rollers are installed at regular intervals along the entire length of the furnace, with the material running on the rollers. Burners can be arranged above and below the furnace chamber to provide heat (Figure 2). Rollers come in two types: ring rollers (with disc-shaped roller rings) and flat rollers. The former is only used for heating sheets, while the latter can be used for sheets, shapes, tubes, and bars. The outer roller sleeve material is typically heat-resistant steel, with some also using silicon carbide. High-temperature furnaces (1000-1150°C) use water-cooled shafts with insulating inner linings and heat-resistant steel rollers, or fully water-cooled roller shafts. To prevent roller bending, rollers in operation at high temperatures must continuously rotate; when the furnace is idle or not discharging, they must also oscillate or rotate at a low speed of 0.5 to 1.5 revolutions per minute. Roller hearth furnaces, due to the material being heated on both sides, offer faster and more even heating and are widely used in heat treatment processes such as normalizing, annealing, quenching, and tempering.
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