An annealing furnace heats materials to high temperatures to change their properties, commonly used in steelmaking to relieve internal stresses and produce a more uniform structure. The furnace must heat the material to a specific temperature, and the process can take 15 minutes to several hours. Natural gas is commonly used as a heating fuel, and modern furnaces use computer systems for control and monitoring. Furnaces come in various sizes and capacities, and recent innovations have improved efficiency and control.
An annealing furnace is equipment used to heat a material to very high temperatures, to change its hardness and strength properties. Annealing is commonly associated with steelmaking, to relieve internal stresses that can lead to failure in service and to produce a more uniform or homogeneous internal structure. This process is also commonly used with various metals, glass, and other materials to make them less brittle and more workable.
For metals, an annealing furnace must be able to heat the material to a specific temperature, known as the recrystallization temperature. This produces changes in the atomic structure and causes the metal to soften. While annealing temperatures vary by material and application, a typical temperature for annealing steel is approximately 700°F (1.292°C). An annealing furnace is designed for very uniform heating followed by a gradual cooling process. To avoid oxidation of the material, the heating chamber is airtight, with a slightly positive pressure. The atmosphere within the chamber typically consists of a combination of nitrogen and hydrogen.
The two main types of annealing furnaces are batch and continuous. In a batch furnace, single or multiple stacks of coils are usually placed on a fixed base within the heating chamber. A continuous annealing furnace will heat a material as it moves along a conveyor or on a rotating hearth. A reel of material is unwound, treated as it moves through the oven, and then rewound. The annealing process can take anywhere from 15 minutes to several hours, depending on the type of material, treatment application and coil size.
Natural gas is commonly used as a heating fuel in an annealing furnace. A series of burners are arranged around the inner surface of the heating chamber. A fan is often used to circulate and convey the heat around the material being annealed. Most modern annealing furnaces use a computer system to control the natural gas supply, monitor and analyze conditions within the heating chamber, and operate motors and other mechanical equipment used to transport the annealing material. The heating chamber is designed to maintain a high degree of temperature accuracy and uniformity when heating the material.
An annealing furnace can be purchased in a wide variety of sizes and in a range of capacities. Units are manufactured for laboratory and small scale applications up to very large furnaces such as those typically used in steel foundries. Annealing furnaces have become more efficient in recent years thanks to innovations, such as improved chamber design and seals to reduce heat loss and emissions; better temperature control and treatment uniformity; combined natural gas and electric heating for greater efficiency; and improved process control and monitoring capabilities.
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