Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

There are three stages in the annealing process

1.Recovery phase :

which results in softening of the metal through removal of crystal defects (the primary type of which is the linear defect called a dislocation) and the internal stresses which they cause. Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains.

2.Re-crystallization:

where new strain-free grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once re-crystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties

3.Grain growth:

Once the process of recrystallization has occurred and the structure is made of completely new crystal grains, the grains will continue to grow. The driving force here is the lowering of the energy of the material because there will be less high energy grain boundaries per volume. The grain boundaries will migrate and the larger grains will grow at the expense of the smaller ones (Much like soap bubbles.)

Full annealing is the process of slowly raising the temperature about 50°C above the Austenitic temperature line A3 or line ACM in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50°Cinto the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon).

It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite-Cementite as the case may be. It is then slowly cooled at the rate of about 20°C/hr in a furnace to about 50°Cinto the Ferrite-Cementite range. At this point, it can be cooled in room temperature air with natural convection.

The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.