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 the 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 the air, or quickly by quenching 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.
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 a specified period of time, heat preservation for a period of time after slow cooling.
The cooling rate may be about 10 degrees C per hour.
After that, the 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 the complete annealing process, Fe3C2 precipitated in mesh along the grain boundary, and 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, the material is heated up to a temperature just below the lower critical temperature of steel or above its recrystallization temperature and then is allowed to cool slowly for some time.
Cold-worked steel normally has increased hardness and decreased ductility making it difficult to work. Process annealing improves this characteristic 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 the 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 of annealing is to restore the ductility of the cold-worked metal.
3 Stress relief annealing:
In stress relief annealing, the metal is heated to a lower temperature of about 650 degrees and is kept at this temperature for some time in the furnace to remove the internal stress of the 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 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:
The 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 that temperature for some time followed by slow cooling. The holding time varies from 15-25 hours.
It is mainly used for eutectoid steel and hyper eutectic 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 of 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.
Purpose of Spheroidizing Annealing:
a) The main aim of spheroidizing annealing is to improve the machinability of steel.
b) This process reduces harness, and uniform structure and prepares the material for quenching.
c) In this process cementite is converted into a spherical form.
5 Isothermal Annealing:
In the isothermal annealing process, the steel is heated above the upper critical temperature. When the steel is heated above the upper critical temperature, it converts rapidly into an austenite structure.
After that, the steel is cooled to a temperature below the lower critical temperature of 600 to 700 degrees 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.
The 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 diffuse 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 the 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 hypereutectoid steel to eliminate the internal stress, reduce the hardness and improve the machinability.
Normalization is a type of annealing process that is carried out 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 ductility of the material. Apart from that it also increases the machinability of the steel and makes it more stable.
Advantages of the 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.