Annealing is one of the heat treatment processes, which is done to steels for obtaining some properties. Annealing plays a predominant role in deciding the strength toughness and various other physical factors of the steel produced. In general annealing is of different types. Each type plays a significant role in forming the steel with the required properties. Some of the annealing processes are
1) Full annealing
2) Bright annealing
3) Box annealing
4) Isothermal annealing
5) Spherodize annealing
6) Sub critical annealing
a. Stress relief annealing
b. Re crystallization annealing
c. Process annealing
In general full annealing is one of the most commonly used annealing process. The purpose behind employing this heat treatment process is
1) To relieve internal stresses
2) To reduce hardness and increase ductility
3) For refining of grain size
4) To make isotropic in nature in mechanical aspects
5) For making the material having homogeneous chemical composition
6) For making the material suitable for high machining processes
7) To make steel suitable for undergoing other heat treatment processes like hardening, normalizing etc.
Full annealing is done by heating the steels to A3 temperature for hypo eutectoid steels and A1¬ for hyper eutectoid steels and then allowed to stay there for a time period and then subjected to slow cooling.
In some cases surface brightness of the component is highly desired in such cases this process is used. In this process the heating process is done in the presence of inert media so as to prevent oxidation of the surface metal. In general the materials used to provide inert environment around the body are either argon, nitrogen. In addition to these any reducing media acts as a protective shield around the object. In this process even the color of the surface is retained.
This annealing process can be called by various names such as black annealing, pot annealing, close annealing. In this process keeping the steel to be annealed in a closed medium carries the annealing process. The surroundings of the steel material are covered with cast iron chips, sand, and charcoal. The final annealing process is same as that of full annealing but the only difference is the medium used for doing this process. The background of this process is to prevent oxidation of the steel metal.
This process is other wise called as cycle annealing. In this process the material is heated to just above the temperature of A3¬ and then faster cooling rates are adopted than that of a normal annealing processes till the temperature reaches just below A1¬ temperature. The steel material is kept at that temperature for obtaining uniformity and then cooled to normal room temperature.
Isothermal annealing seems to be more advantageous over the conventional annealing processes some of the advantages of this process are
1) When slower cooling required materials are to be annealed then this process seems to be advantageous than the normal process. This process is even effective when the reduction of hardness is required.
2) Due to the generation of intermediate equalization of temperature homogeneity is more comparatively
3) The parts, which are annealed by this process, have high machinability and improved surface finish of the machined part can be obtained.
In general this process is used for low and medium carbon steels. This process is even used for some of the alloy steels for obtaining improved machinability. This improvement in the machinability is due to the formation of spheroidized structure.
Any methods through which spheroids are formed are called as spheroidized annealing. If an annealed product contains globules of cementite in the matrix of ferrite in the microstructure then it is termed as a spheroid. In general this microstructure is formed by various ways some of them are
1) Hardening and high temperature tempering
2) Holding the product at the temperature just below the A1¬ temperature
3) Thermal cycling around A1
This process is done over cold worked steels. In this process the cold worked steel is heated to a temperature just above the lower critical temperature. Due to the heating of the steel to lower critical temperature this process is named as sub critical temperature. In general this process is done to
1) To relieve internal stresses developed due to cold working process
2) To refine the grain structure
3) To reduce hardness and to improve ductility of the material
Sub critical annealing is done by three processes for obtaining various varieties of properties on cold worked steels.
1) Stress relief annealing
2) Re crystallization annealing
3) Process annealing (intermediate annealing)
Stress relief annealing
In this process the cold worked steel is heated to a temperature around 5250C i.e. just below the recrystallization temperature. So due to this heating there arises no change in the microstructure of the material. The body is kept at that temperature for around two to three hours and then subjected to air-cooling. As there is no change in the microstructure this heating process has no adverse affect on the hardness and strength of the material. This annealing process reduces the risk of deformation of the product during machining.
In this annealing process the cold worked steel material is heated to a temperature above A1 i.e. around 625 to 6500C. During this annealing process the cementite present in the microstructure starts converting in to spheroids and hence high machining ability is obtained. Due to heating of temperature up to A¬1¬ the grain structure changes and not only this internal stresses developed in the cold working process is removed.