Annealing is a heat treating process that is commonly used in a number of industrial and commercial settings to heat and slowly cool a number of materials in order to alter their physical properties for improved strength and ductility. This particular type of heat treatment is exceedingly common. Though most often used to relieve the internal stresses of glasses and metals, ceramic annealing is also available.
Several occasions occur during which manufacturers will employ annealing with any of these materials. Stock forms of raw materials are often annealed for the improved flexibility needed for future machining. Semi-finished parts likewise undergo process annealing, some several times over in between manufacturing processes such as rolling, drawing, forging, spinning, extruding, heading and welding, all of which cause internal stress and reduce the workability of materials. Annealing alleviates this stress and is therefore used on finished products before they are to be introduced to the market or fitted to machinery or equipment. Industries including automotive, food processing, aerospace, tool and die, plumbing and many others utilize this treatment for pipes, tubes, cutlery, engine components and paneling. Several advancements in annealing technology allow for not only improved processing, but also significant gains in efficiency. Improved chamber or annealing furnace designs allow for better seals which reduce escaping heat and emissions, provide better temperature control which results in more uniform heat treatments, combine natural gas and electric heater to cut costs and improve monitoring capabilities such as computerized systems and furnace programming.
The first step in any annealing process is to heat the material, be it copper, steel, glass or ceramic. Materials are loaded into batch furnaces or placed on circulating conveyors in continuously run operations. The temperature is raised to the re-crystallization temperature of that material and ‘soaked' until the piece is uniformly heated. The thickness of a part or form therefore has a significant impact on the heat treating process. At this temperature, the atomic structure changes as do the physical properties of the metal. The stresses inherent in the materials relax as crystal defects or dislocations are removed. The refined grains then redistribute and begin to reform in finer strain free lattices that nucleate and replace deformations caused by previous stresses. Once the materials reach the equilibrium state with uniform composition, they may be slowly cooled to ensure a fine grain. Heat treating metals through the annealing process is fairly simple. Simplicity, however, does not negate the necessity for extreme precision throughout the process. Annealing furnaces must be able to maintain a high degree of temperature accuracy and heating uniformity to ensure that the material is heated evenly throughout. The thickness, heat capacity, thermal conductivity and thermal expansion coefficient of a material play a large role in the ease of annealing. The details of annealing, such as timing and temperature, are dependent upon the precise composition of the alloy or other material to be processed. While re-crystallization temperatures vary widely, a range from 500 degrees Fahrenheit to 1400 degrees Fahrenheit generally satisfies the needs of glass and metallurgical industries.