Annealing, Types of Annealing and Advantages of Annealing.

Annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable.
In annealing process, a material is heated above its recrystallization temperature,maintaining a suitable temperature and then cooling.

In cases of copper, steel, silver and brass, this process is performed by heating the material for a while and then slowly letting it cool to room temperature in still air.

Copper, silver and brass can be cooled slowly in air, or quickly by quenching of water unlike ferrous metal like steel which must be cooled slowly to anneal.

By annealing, the metal is softened and prepared for further work such as shaping, stamping or forming.

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Types of annealing:

1 Complete Annealing:
In this process, steel is heated to 30 to 50 degrees Celcius above the critical temperature of steel and this temperature is maintained for specified period of time, heat preservation for a period of time after slow cooling .
The cooling rate may be about 10 degree C per hour.
After that, material is allowed to cool down slowly inside the furnace without any forced cooling.
Complete annealing is used in worked sheets, forging and casting made from medium and high carbon steels.
Low carbon steel has low hardness and is not applicable to machining.

After complete annealing process, Fe3C2 precipitated in mesh along the grain boundary, the strength, hardness, plasticity and toughness of steel are significantly reduced.

Purpose of Complete Annealing:
Complete annealing is done to get all the changes in the properties of the metals like
1 Producing equibrilium microstructure.
2 Increase in ductility
3 Reduction in hardness, strength, brittleness and
4 Removal of internal stresses.
The microstructure after annealing contains coarse ferrite and pearlite.

2. Process Annealing:
This process is mainly suited for low carbon steel. In this process material is heated up to a temperature just below the lower critical temperature of steel or above its recrystallisation temoerature and then is allowed to cool slowly for sometime.
Cold worked steel normally have increased hardness and decrease ductility making it difficult to work. Process annealing improves this characteristics by making it more ductile and decreasing its hardness. This is mainly carried out on cold rolled steel like wire drawn steel, etc.

Purpose of Process Annealing:

1 Process annealing is done for recrystallization of metal.

2 During process annealing, new equiaxed, strain-free grains nucleate at high-stress regions in the cold-worked microstructure, and hence hardness and strength decrease whereas ductility increases.

3 The main aim of the process annealing is to restore ductility of cold worked metal.

3 Stress relief annealing:
In stress relief annealing, the metal is heated to a lower temperature about 650 degree and is kept at this temperature for some time in the furnace to remove the internal stress of metal followed by slow cooling.

Large castings or welded structures tend to possess internal stresses mainly caused during their manufacturing and uneven cooling.
No phase transformation takes place during stress relief annealing.

Purpose of stress Relief annealing:

The main aim of the stress relief annealing is to remove the internal stresses produced in the metal due to
i) Plastic deformation.
ii) Non-uniform cooling
iii) Phase transformation

4 Spheroidizing Annealing:
Spheroidizing Annealing process is for high carbon and alloy steel in order to improve their machinability.
In spheroidizing annealing, the steel is heated to a temperature below A1 temperature, kept at the temperature for sometime followed by slow cooling. The holding time varies from 15-25 hours.
It is mainly used for eutectoid steel and hypereutectic steel such as carbon tool steel, alloy tool steel, bearing steel etc.

This process improves the internal structure of the steel. This can be done by two methods:

i) The material is heated just below the lower critical temperature about 700 degrees and the temperature is maintained for hours and then allowed to cool down.

ii) Heating and cooling the material alternatively between the temperature just above and below the lower critical temperature.

Pupose of Spheroidizing Annnealing:

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a) The main aim of spheroidizing annealing is to improve the machinability of steel.
b) This process reduces harness, uniform structure and prepare the material for quenching.
c) In this process cementite is converted into spherical form.

5 Isothermal Annealing:
In isothermal annealing process, the steel is heated above the upper critical temperature. When the steel is heated above upper critical temperature, it converts rapidly into austenite structure.
After that, the steel is cooled to a temperature below the lower critical temerature 600 to 700 degree Celcius.The cooling is done by force cooling methods.
This temperature is maintained for a specific time period to produce a homogenous structure in the material.
Isothermal Annealing process is mainly applied to low carbon and alloy steel to improve their machinability.

6 Diffusion Annealing:
This process is known as diffusion annealing as in this process the iron and carbide diffuses with each other. For diffusion higher temperature is required, so the steel is heated above the upper critical temperature. The temperature is nearly about 1000 to 1200 degrees Celcius.
The heat preservation time in this process is nearly 10 to 15 hours.

After diffusion annealing, complete annealing and normalizing are done to refine the tissue.

This process is applied to high-quality steel and segregation of serious alloy steel casting and ingots.

Purpose of Diffusion Annealing:
The main purpose of diffusion annealing is to eliminate dendritic segregation and regional segregation.in the solidification process and to homogenize the composition and organization.

7 Incomplete Annealing:
In this incomplete annealing process, the steel is heated to about upper critical temperature. The heat treatment process is obtained by slow cooling after thermal insulation.

Purpose of Incomplete Annealing:
This process is mainly used to obtain spherical pearlite tissues for the hypereutectic steel to eliminate the internal stress, reduce the hardness and improve the machinability.

8 Normalization:
Normalization is a type of annealing process which is carried to enhance the hardness of steel. In this process, the steel is heated above its upper critical temperature.
Due to the heating process involved in the normalization, there is the formation of the Austenite structure.

Purpose of normalization:
Normalization is mainly used to increase the toughness and the ductility of material. Apart from that it also increases machinability of the steel and make it more stable.

Advantages of Annealing Process:

1 It increases the toughness of the material.
2 It increases the ductility of the metal.
3 It increases the machinability of metal.
4 It helps to enhance the magnetic property of metals.
5 It also helps in reducing the brittleness of metals.

One thought on “Annealing, Types of Annealing and Advantages of Annealing.

  • April 21, 2020 at 1:34 am
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    I had no idea that annealing was a heat treatment in order to alter metals. My brother-in-law is really into making swords and other models out of like styrofoam. I’ll talk to him about doing it out of metal and using annealing to do so.

    Reply

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