Heat Treatment: Annealing

Annealing is made to increase machining abilities and to soften the metal. Generally the purpose of annealing is as follows.

To refine the grain size.

To increase mechanical properties such as strength and ductility.

During casting, to remove those holes that are trapped in metal.

To make changes to the magnetic and electrical properties.

Annealing are processed in two types

Full Annealing

To soften the metal, Full annealing to refine the grain structure and relieve the strain.

In this process hypoeutectoid steel heated over 32 to 50 degree above than the upper critical temperature and for hypereutectoid steel heated over the 32 to 50 degree above than the lower critical temperature.

Hold this temperature until internal structure is not change, this maturation period is 1 mm per thickness square area 3 to 4 minutes.

And in the furnace it slowly cools down.

Process Annealing

Process annealing is used to remove internal stress due to previous setup.

In this process, the steel is heated to a lower temperature than the lower critical temperature.  Keeping it cool for a while at a temperature, it is cooled gradually.

This process increases in machinability.

It happens a recrystallization in steel.

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Normalizing of Steel

The finished product of casting is processed to get good properties by heat treatment. Certain properties or desirable condition is obtained by heating or cooling of the metal by without any changing of chemical composition in solid state. 

The metal consists of micro structure of grain particals. It is a mechanical and metallurgical process in which, By the changing of grain size or micro structure we can obtain some challenging properties of metal.

Following properties to be obtained by the heat treatment process.

• To Soften metal.
• To increase the hardness and strength of metal.
• To change the magnetic properties.
• To Increase the resistance of wear.
• To change grain size.
• To change the electric properties.
• To Improve machinability and quality of casting.

Considering its different properties in way of engineering, its 1st topic is normalizing 

Normalizing

Normalizing process is used to soften the metal. By the normalizing we can achieve highest machinability and improve tensile strength. By this improvement in electric and magnetic properties of metal.

The cold working process like hammering, knockout, bending causes strain on metal, its remove by normalizing.

It used to refine grain structure and uniformity of grain size and composition. It makes the material brittle and unreliable. This process less expensive than annealing.

For Hypo eutectoid steels the heating temperature is 45 to 50 degree above to its upper critical temperature and for Hyper eutectoid steels, the temp. is 45 to 50  above to its lower critical temp. The metal is kept for a while at this temperature and then cooled down by air.

The upper and lower critical temp of steed depends upon the total percentage of carbon consists in steel.

For best quality and machinability it performed after forging and casting.

In Hypo-eutectoid steel less than 0.8% of C in its composition. It is composed by pearlite and α-ferrite and Hyper-eutectoid steel has between 0.8% and 2% of C, composed by pearlite and cementite. It is very important to understand Iron-Carbon phase diagram to understand the Hypo-eutectoid and Hyper-eutectoid.

This process provides a uniform structure consisting of ferrite and pearlite for hypoeutectoid

steels, and for hypereutectoid steels pearlite and cementite.

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