Types of Annealing Used In Manufacturing

The various types of Annealing process are :

1. Full Annealing :

Full annealing consists of :

a. Heating the steel slightly above the critical point.

b. Holding it at this temperature for a considerable period.

c. Slowly Cooling.

This process is known as full annealing because it wipes out all traces of previous structure by complete phase recrystallisation.

2. Process Annealing :

When a steel is cold - worked the hardness and elastic properties particularly considerably increase, while the ductility remarkably suffers and the steel becomes unsuitable for further plastic deformation. The ductility of a steel may then be restored by so-called recrystallisation or process annealing.

Furthermore, the effect of process annealing is to relieve internal stresses resulting from any previous heat treatment.

3. Spheroidize Annealing :

The machinability of high carbon tool steels is at its best condition when the structure is composed of grained ( or globular ) pearlite. All alloy steels, including those of the carbide class, as well as ball bearing steels, should have a structure of globular pearlite in the deliverable state.

The process of producing a structure of globular pearlite is known as spheroidising or spheroidise annealing.

4. Homogenizing :

Homogenizing or diffusion annealing is applied to steel ingots (both carbon and alloy steel) and heavy complex casting for eliminating chemical in homogeneity within the separate crystals of diffusion

Homogenizing naturally causes a very rapid growth (for heating to a very high temperature) of austenitic grains. After homogenizing, therefore, the steel ingots necessarily undergo an ordinary phases (full) annealing for the fine-grain structure to be formed.

5. Isothermal Annealing :

Isothermal annealing is carried out as for ordinary annealing to form austenite. It is then cooled comparatively rapidly in air or by a blast in a furnace to a temperature of 50*C to 100*C below point i.e , 600*C to 700*C. The steel is held isothermally at this temperature (at constant temperature) during a certain period of time to provide for complete decomposition to pearlite and hence the name Isothermal annealing. This is followed by comparatively rapid cooling.

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