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Annealing process

Annealing (Softening)  is a heat treatment process in which steel is heated beyond the critical temperature (to 𝛾 -iron ) (or above recrystallize temperature), maintaining a suitable temperature for a suitable amount of time and then allow slow cooling (furnace cooling).

Fig.- full annealed steel knife

Purpose of annealing : 

  1. To relive the internal stresses
  2. To increase ductility ( hence toughness also increase)
  3. To reduce hardness
  4. Refinement of grain structure
  5. Improve plasticity, hence facilitate machining and cold deforming processes.
In annealing, slow cooling result in formation of spherical carbide and coarse perlite, which are very soft product.


Type of annealing :

  1. Full annealing
  2. Process annealing
  3. Spherodise annealing
  4. Diffusion annealing ( Homogenizing or Uniform annealing)

 1. FULL ANNEALING

        In full annealing, steel is heated beyond the critical temperature ( heated 50°- 70° C above upper critical temperature for hypo-eutectoid steel, heated above lower critical temperature for in case of hyper-eutectiod steel) and after definite period of time, it is slow cooled or furnace cooled.

For, hypo-eutectoid steel,     γ iron → perlite + ɑ iron 

For, hyper-eutectoid steel,    γ iron → perlite + FeვC

For,  eutectoid steel,             γ iron → perlite 

                                                                                  (Perlite = ɑ iron + FeვC mixture )

        Due to full annealing, Hardness increase and softness decrease.

2. PROCESS ANNEALING

        Process annealing basically empoyed for process product (eg. cold work product) to make them soft, so that they can be further used in process (ex. further processing of thin sheet, wire, etc.). Process annealing used for low carbon steel.

3. SPHERODISE ANNEALING

        Spherodise annealing is used for high carbon steel which are difficult to be machined. Speciment is heated just below re-crystallization temperature , hold for some time and then furnace cooled.
        By this process, carbide in steel is transformed in to a globular form ( or sphere form). Due to spherodise annealing,softness increase hence machinability increase.

4. DIFFUSOIN ANNEALING ( HOMONIZING OR UNIFORM ANNEALING)

        Heat the ingots, castings or forging billets to a temperature that is slightly lower than the solid phase line for long time, then slow cool down to eliminate the unevenness (or non-homogeneity) of chemical composition.

eg. In semiconductor, silicon are annealed so that boron, phosphorus etc can diffuse into substitution portion in crystal lattice.

        After diffusion annealing, complete / full annealing and normalizing are needed to refine grain structure.

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