This is a kind of heat treatment after which a metal or alloy acquires a structure close to the equilibrium one. A material is exposed to an elevated temperature for an extended time period and then slowly cooled. Normally, annealing is carried out to:
- To improve machinability,
- To relative internal stresses,
- To increase softness, ductility and toughness,
- To produce a specific micro-structure,
- To remove gases,
- To refine grain size due to phase recrystallisation,
- To modify electrical and magnetic properties.
The approximate temperatures to which the steel-sample should be heated will depends upon its carbon content. The recommended temperatures are shown in the following table:
There are a variety of annealing heat treatments are possible. These are characterized by the changes that are induced, which many times are microstructural and are responsible for the alteration of the mechanical properties.
An annealing process consists of three stages:
(i) heating to the desired temperature,
(ii) holding or 'soaking' at that temperature, and
(iii) cooling, usually to room temperature.
In these annealing procedures, time is an important parameter. There exist temperature gradients between the outside and interior portions of the piece during heating and cooling. The magnitudes of temperature gradients depend on the size and geometry of the piece. Soaking time may be given at the rate of 3-4 minutes for every one mm thickness of the cross-section of material. If the rate of temperature, change is too great, temperature gradients and internal stresses may be induced that may lead to warping or even cracking. Moreover, the actual annealing time must be long enough to allow for any necessary transformation reactions. An annealing temperature is also an important consideration. Since diffusional processes are normally involved and therefore annealing may be accelerated by increasing temperature. The various types of annealing operations are:
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