Mechanical Engineering : Heat Treatment of steel

Heat treatment of metal is a term used to describe an operation or a series of operations which involves heating and cooling of a metal or an alloy when it is in the solid state for obtaining some desirable properties (without changing the chemical composition ).

The main objective of Heat Treatment of Steel

1. Enhancing the hardness of the steel.

2. Stress reliving from the metal after hot rolling or cold working.

3. To improve the machinability.

4. To make the metal soft.

5. The structure of the metal can be modified to improve the electrical and magnetic propeeties.

6. To change the grain size of the metal.

7. After heat treatment metals are more resistant to heat, corrosion and wear.

Various Heat Treatment processes

a. Normalizing:

In Normalizing the soaking temperature is 30-50 degree centigrade within the range of austenite field.
After soaking the alloy is cooled. The cooling rate and temperature helps to get smaller grains.
Normalizing improves toughness and strength.

b. Annealing:

Process involes heating the steel above the critical temperature, maintaining the temperature and and then cooling
Increases ductility, softens the metal, and improve cold working properties.

c. Spheroidising:

It is done at about 727 degree centigrade temperature. This process is limited to the steels having a carbon percentage of 0.5%.
High temperature recrystallizes the ferrite. The Iron Carbide present in the pearlite takes the shape of ball or sphere. This is called 'ball up'
Because of these changes hardness and strength reduces.

d. Hardening and Tempering:

Hardening and tempering is done using very high temperatures. The temperature depends on the amount of carbon present in the steel .
Tempering means heating the steel again but this time the temperature is not that high.

e. Surface Hardening or Case Hardening :

The main reason for hardening steel is to retard wear on bearing and rubbing surfaces. But harder steel is brittle and that's why its not fatigue and shock resistant. So the hardening process is done on the surface only. It is very much cost effective.