Annealing

          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:

1. To improve machinability,
2. To relative internal stresses,
3. To increase softness, ductility and toughness, 
4. To produce a specific micro-structure, 
5. To remove gases,
6. To refine grain size due to phase recrystallisation,
7. 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: 

1. Full annealing,
2. Isothermal annealing,
3. Process (or) sub critical annealing,
4. Spheroidise annealing,
5. Diffusion annealing, and 
6. Recrystallisation annealing.

Spheroidisation annealing

          It is a type of annealing which causes practically all carbides in the steel to agglomerate in the form of small globules or spheroids. There may be wide range of hardness with such a structure for any grade of steel since the size of the globules has a direct relation to hardness. Spheroidising may be accomplished by heating to a temperature just below the lower critical and holding for sufficient period of time. A more desirable and commonly used method for spheroidising is to heat to temperature just above the critical and cool very slowly (6°C per hour) through the critical range or to heat to a temperature within the critical range but not above the upper critical and cool slowly.

          This treatment is used for practically all steels containing over 0.6 percent carbon that are to be machined or cold formed.  

Diffusion annealing

          This process, also known as homogenizing annealing, is employed to remove any structural non-uniformity, like dendrites, columnar grains and chemical inhomogeneities are generally observed in the case of ingots, heavy plain carbon steels castings. These defects promote brittleness, and reduce ductility and toughness of steel.

Process:

• Steel is sufficiently above the upper critical temperature (say 1000-2000°C), and held at this temperature for prolonged time, usually 10-20 hours, followed by slow cooling.
• Segregated zones are eliminated, and a chemically homogeneous steel is obtained by this treatment as a result of diffusion.
• Heating to such a high temperature results in considerable coarsening of austenitic grains & heavy scale formation. The coarse austenite thus obtained further transforms to coarse pearlite on cooling, which is not a desirable structure as mechanical properties are impaired.
• The main aim of homogenising annealing is to make the composition uniform, i.e., to remove chemical heterogeneity.
• The impact energy and ductility of the steel increases as the homogenizing temperature increases and the hardness yield strength and tensile strength decrease with an increase in the homogenizing temperature.
• Homogenizing annealing has a few shortcomings as well. It results in:
• Grain coarsening of austenite, there by impairing the properties.
• Thick scales on the surface of steels.
• It is an expensive process.

Recrystallisation Annealing

• Practically all steels, which have been heavily cold worked, are subjected to this treatment.
• The process consists of heating steel above the recrystallisation temperature, holding at this temperature and cooling thereafter. It results in decrease in hardness or strength and increase in ductility. The process is used both as an intermediate operation and as a final treatment. The treatment is very important and is frequently employed in industries manufacturing steel wires, sheets and strips.
• As little scaling and decarburisation occurs in recrystallisation annealing, it is preferred over full annealing.
• High carbon steels and alloy steels require higher recrystallisation temperatures.