Annealing involves heating steel to a high temperature (above 750ºC) followed by very slow cooling in order to make the metal as soft as possible.
This, very time-consuming process is also known as full annealing since there are many types of intermediate or quicker annealing processes which make the material soft enough for a particular purpose but not as soft as possible. Annealing is also applied to many other non-ferrous metals and alloys.
Softening processes are used to improve hot and cold working characteristics, to increase machinability, to reduce internal stress due to working, welding etc, and also to condition components for subsequent hardening treatments. Occasionally they are used to impart particular final properties, as with low carbon transformer core material, which is annealed to optimise its magnetic characteristics.
The control of furnace atmosphere is vitally important since the prolonged treatment times required for many annealing processes would produce significant surface deterioration due to scaling if oxygen ingress were to occur. Atmospheres used for the annealing of steel include inert gases such as nitrogen and argon, cracked ammonia, exothermic gas mixtures and vacuum.
The use of continuous furnaces greatly improves the cost effectiveness when large volumes of small to medium sized components are to be annealed. The speed of throughput is variable and is the mechanism used to control time at annealing temperature. The uniformity of loading of the furnace belt or trays is another critical factor and sufficient even spacing of components and weight across the belt is vital.
When batch furnaces are used it is often a requirement, particularly with large components, that contact thermocouples are used, strategically placed over the surfaces of the component to provide a permanent trace record of the thermal history of the annealing process.
See also full annealing, process annealing, recrystallisation annealing, sub-critical annealing.