Annealing

Annealing

Annealing is the heating of steel to austenizing temperature and then cooling slowly in the furnace. The purpose of annealing is :

1. To reduce hardness
2. To improve machinability
3. To increase or to restore ductility
4. To relieve internal stresses
5. To reduce or eliminate structural inhomogeneity
6. To refine grain size
7. To prepare steel for subsequent heat treatment.

Slow cooling results in the formation of spheroidal carbide and (coarse) lamellar pearlite. These products are very soft. The cooling rate during annealing varies from depending upon the alloying element in the steel and lower rate of cooling is used for alloy steels as compared to plain C-steels. Annealing results in the formation of ferrite, spheroidal element and coarse pearlite. All these phases and micro – constituents are relatively soft and therefore this is known as softening treatment and produces relatively lower hardness.

There are 4 types of annealing process:

1. Full Annealing : Primary objective of this process is to reduce hardness and increase ductility.

The process involves :

• Heating the steel to about 50 to 75°C above the upper critical temperature for hypoeutectoid steels and above the lower critical temperature for hyper eutectoid steel.
• Holding at this temperature for a sufficient time depending upon the thickness of object the holding time is 3 – 4 min/mm of thickness of the largest sections.
• Followed by slow cooling in the furnace. The rate of cooling varies from 30°C – 200°C per hour depending upon the composition and stability of austenite.

Following stages are there in annealing process:

• Recovery : During recovery, some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at the elevated temperature. There is some reduction in the number of dislocations, and dislocation configurations are produced having low strain energies. In addition, physical properties such as electrical and thermal conductivities recover to their pre-cold-orked states.
• Recrystallization : Even after recovery is complete, the grains are still in a relatively high strain energy state. Recrystallization is the formation of a new set of strain-free and equiaxed grains (i.e., having approximately equal dimensions in all directions) that have low dislocation densities and are characteristic of the pre-cold-worked condition. The driving force to produce this new grain structure is the difference in internal energy between the strained and unstrained material. The new grains form as very small nuclei and grow until they completely consume the parent material.
• Grain Growth : After recrystallization is complete, the strain-free grains will continue to grow if the metal specimen is left at the elevated temperature this phenomenon is called grain growth. Grain growth does not need to be preceded by recovery and recrystallization, it may occur in all polycrystalline materials-metals and ceramics alike.
  Grain growth occurs by the migration of grain boundaries. Obviously, not all grains can enlarge, but large ones grow at the expense of small ones that shrink. Thus, the average grain size increases with time, and at any particular instant there exists a range of grain sizes. Boundary motion is just the short-range diffusion of atoms from one side of the boundary to the other.

2. Process Annealing : It is usually carried out to remove the effects of cold working and to soften it to make it suitable for further plastic deformation as in the case of sheet and mill industries. It is the recrystallization of cold worked steel by heating below the lower critical temperature. The exact temperature depends upon the extent of cold working grain size, composition and holding time.

3. Spheroidize Annealing : This process is applied to medium and high carbon steels which are difficult to machine. These steels are heat treated (annealed) to develop spheroidite structure of Fe3C embedded  in a matrix of α-phase of iron. These steels are heated below lower critical temperature (A1) at about 600°C, soaked at this temperature for about 18–24 hours and then slowly cooled.

4. Diffusion Annealing : Diffusion annealing or homogenizing is applied to allow steel ingots and heavy complex casting for eliminating the chemical in homogeneously is applied to alloy within the separate crystals by diffusion. Homogenizing is carried out at temperature 1000 – 1200°C.

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