Types of Industrial Annealing Furnaces

Annealing furnaces are a general term for industrial electric furnaces used to perform various surface treatments on metal workpieces. Their operating temperatures are generally lower than those of heat treatment furnaces. Cart-type furnaces can utilise multiple types of heating elements, but require stricter control over temperature regulation and furnace atmosphere. Most annealing furnaces employ fossil fuels for heating. To achieve precise temperature control, some furnaces are electrically heated. Modern heat treatment processes (particularly chemical heat treatment) have grown increasingly complex, with numerous configurations for cart-type furnaces. Continuous heat treatment processes necessitate suitable continuous furnaces. For cyclic heat treatment operations, forced circulation with controlled atmosphere is required, and furnace operation should be automated and mechanised. The following furnaces are commonly employed in metallurgical plants.

Bell-type Furnace

A type of heat treatment furnace wherein the objects to be heated are periodically heated within a bell-shaped chamber. It is primarily used for annealing sheet metal stacks or coiled steel. Bell-type furnaces typically comprise a casing, an inner bell, and a furnace base. The objects to be heated are placed within the inner bell (muffle bell), with heat supplied between the inner bell and the rear cover plate. Typically, manufacturers introduce controlled atmosphere into the inner cover. Following heating, holding, and slow cooling, the rear cover is lifted to cool the plate stack or coil within the inner cover. Once cooled to discharge temperature, the inner cover is removed, and the plate or coil is air-cooled before discharge. The burner is mounted on the furnace platform. Two types of hood furnaces exist for treating cold-rolled steel coils: single-stack and multi-stack. In multi-stack configurations, 2 to 8 stacks of coils can be placed on a rectangular furnace platform, each stack secured by a separate circular inner hood. Circulating fans are installed beneath the inner hoods to enhance air control and promote convective heat transfer between coils.

Roller-hearth furnaces

Heat-treated materials are conveyed through the furnace chamber via rollers, which are spaced at regular intervals along the entire furnace length. Materials traverse the rollers while burners heat both above and within the furnace chamber. Two roller types exist: ring rollers (featuring disc-shaped roller rings) and flat rollers.

The former are exclusively for heating plates, while the latter accommodate plates, profiles, pipes, and bars. The material for the outer casing of the rollers is typically heat-resistant steel, with some applications employing carbon-carbon composites. Box furnaces (1000–1150°C) utilise water-cooled shafts and heat-resistant steel furnace rollers with insulating linings, or fully water-cooled rollers. To effectively prevent roller bending, rollers operating at high temperatures must be kept in continuous rotation. When the furnace is empty or not discharging, they must also oscillate or rotate at a low speed of 0.5 to 1.5 revolutions per minute. Due to heating from both sides of the material, roller hearth furnaces provide faster and more uniform heating. They are widely used in heat treatment processes such as quenching, tempering, annealing, and normalising.