Annealing is one of the basics of heat treatments. It is used in a number of ways to improve ductility, relieve stress, assist in refining grain size, and more. Annealing is done in both batch and conveyor ovens. While a relatively simple process, annealing can be broken down into three main types.
Heating to a temperature of at least 50°F (28°C) above Ac3 (the temperature at which the transformation of ferrite to austenite is completed). The more open structure of the austenite is then able to absorb carbon from the iron-carbides in carbon steel. The material is held at this temperature for a specified time to allow the material to transform into Austenite or Austenite-Cementite. Following this, it is cooled at a controlled rate of about 36°F/hr (20°C/hr) in an oven to about 122°F (50 °C). It can then be cooled in room temperature air with natural convection. Knowing the isothermal transformation diagram and the continuous cooling diagram are essential in properly performing the annealing process. These diagrams predict the microstructure after transformation and the time required.
Ideal when subsequent machining and/or hardening is required. The spheroidized condition is the equilibrium state of steal in its softest condition. This microstructure has good cold-forming characteristics. The larger the spheroids, the more distance between them and the easier it is for steel to be cold formed.
Process annealing or normalizing, is used to counteract the hardening effects of cold-working. After the piece is worked, it may become too hard and brittle for further processing or to put into use. Process annealing involves heating the metal to Ac1 (the high temperature austenitic state) until the stresses have been removed. Then the material is slowly cooled to avoid adding new stresses. The result is a structure with a low level of residual stresses and good mechanical properties.