Heat Treatment | Definition and Types of Heat Treatment Processes

What is heat treatment?

Heat treatment process is the controlled heating or cooling of plain carbon steel or alloy steel. Heat treatment of steel is done to change its molecular structure, in order to obtain certain mechanical properties. These mechanical properties could be hardness, strength or ductility. Various types of heat treatment processes are done to obtain various types of mechanical properties.

Note: For heat treatment there should be some quantity of carbon in the iron.

There are basically four main types of heat treatment processes.

Types of heat treatment processes

1. Annealing

In annealing steel is heated to a temperature which is slightly above the critical temperature, followed by slow cooling.

Annealing is used to reduce hardness, improves grain structure and increases ductility.

2. Normalizing

Normalizing is similar to annealing, but in normalizing the heated steel is cooled in air. Normalizing is used to remove the effect of all the previous heat treatment processes.

It relieves internal stresses in the material and refines the grain structure. It also improves machinability of the material.

3. Quenching

In quenching steel is heated to the critical temperature and then cooled rapidly in water or air.

Quenching is used to increase hardness and wear resistance. In this process steel becomes brittle and its ductility is reduced. To remove some of the brittleness of the quenched material tempering can be performed.

4. Tempering

Tempering is reheating of quenched component to a temperature which is below transformation range, followed by cooling at a desired rate.

Tempering is used to restore ductility and reduce the brittleness due to quenching.

Selection of a suitable heat treatment process always depends on the required properties of the steel.

Defects of heat treatment processes

• Decarburization

Loss of carbon from the metal surface due to prolonged heating is known as decarburization.

• Over heating

Heating metal for a long period of time at high temperature causes loss of ductility and impact strength of material.

• Oxidation

Oxidation of metal surface is also a defect which normally arise due to heat treatment processes. It can be prevented by using an inert atmosphere.

• Soft spots

Localized decarburization and bubble formation in metal results in soft spots.

• Quenching cracks

Cracks arising in metal surfaces due to higher cooling rates is known as quenching cracks.

• Warping

Non-uniform heating or cooling of the metal causes warping.

Benefits of various heat treatment processes

• It can reduce stresses in the metal so that it becomes easy to machine or weld.
• It can change mechanical properties of the metal which makes it suitable for further machining processes.
• It can increase strength of metals.
• It can make metals more ductile.
• It can make metals more flexible.
• It can increase wear resistance (hardness) of metals.
• It can increase or decrease brittleness of metals.
• It can improve electrical properties of the metals.
• It can improve magnetic properties of the metals.

Also read:

What is hot working?

What are fiber reinforced plastics?

What are ceramics?

What is a coupling?

Featured image attribution: By Laurens Sanders – Industrial Gears Watteeuw, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=41262166