Types of Heat Treatment in Steel and Surface Treatment

Here, we have covered the four basic types of heat treatment in steel that are Annealing, Normalizing, Quenching and Tempering. Purpose of heat treatment of steel are also explained here.

Heat Treatment in Steel

Steel’s heat treatment is a normal thermal process which modifies only the structure of steel. Generally, heat treatment of steel uses phase transformation during heating and cooling that changes the microstructure and properties of steel.

In case of thermo-mechanical treatment (TMT) process of steel, the shape and structure of the steel components are modified. Whereas, during the thermo-chemical process of steels, the surface chemistry and structure of the steel are modified.

Heat Treatment Process for Steel

The general heat treatment process for steel is as follows:

• Selection of steel type
• Machining
• Inspection
• Washing
• Quenching
• Washing, if oil quenching
• Tempering
• Post processing (as required)
• Inspection

Types of Heat Treatment Process for Steel

The four basic types of heat treatment in steel are:

1. Annealing,
2. Normalizing,
3. Hardening,
4. Tempering.

1. Annealing

Annealing is a type of heat treatment process for steel that is used to improve the structure of the steel and to soften it. Mostly, it is used for high carbon steel and alloy steel and applied to the parts, those have been either cold worked, hot worked or cast.

In annealing process steel is heated above its upper recrystallization temperature first. The temperature is then maintained and reduced to the lower recrystallization temperature and maintained once again. After that, it is gradually cooled to the room temperature. This process ensures that the steel has reached a uniform temperature and microstructure before proceeding to the next cooling stage.

Purpose of Annealing Process

• To obtain softness.
• To increase ductility.
• To reduce or eliminate structural inhomogeneity.
• To improve machinability.
• To relieve internal stresses.
• To refine the grain size.

2. Normalizing

In the normalizing process the steel is heated above its critical temperature. The steel is then left at room temperature to be cooled slowly by air.

This process is used to make the steel structure fine, uniformize the carbides and remove residual stresses, making the steel easier to cut. Strength and hardness of steel also improve.

Purpose of Normalizing Process

• To increase the toughness of a steel.
• To relieve internal stresses.
• To refine the grain size.

3. Hardening

In quenching process, the steel is heated to above its critical temperature and held at that temperature for specific time. Then the steel part is removed and cooled in water, brine or oil quickly. This process is used for increasing the hardness and strength of the steel without changing its shape. But after this process the steel becomes more fragile. This process is also known as hardening process.

Purpose of Quenching Process

• To improve hardness.
• To improve strength

4. Tempering

Tempering process is done after hardening / quenching process because after this process steel becomes fragile which may crack.

In tempering process, the quenched steel is reheated and cooled to its lower critical temperature. This process adds ductility, toughness and high resistant to the steel. However, this process slightly reduces the hardness and strength of quenched steel.

Purpose of Tempering Process

• To improve the toughness of steel.
• To improve the ductility of steel.

Quenching and tempering under the right heat treatment conditions for the specific type of steel is essential to ensure the quality required for the application.

Purpose of Heat Treatment of Steel

The purposes of heat treatment of steel include the following:

• To change electrical and magnetic properties.
• To soften the steel for machining easily.
• To change the microstructure.
• To increase resistance to heat and corrosion.
• To refine the grain structure.
• To remove internal stresses.
• To reduce the brittleness.
• To increase surface hardness.
• To make the steel ductile.
• To vary strength and hardness.

Surface Treatment of Steel

Surface treatment of steel is the process by which the hardness of only the surface layer is increased instead of achieving uniform hardness throughout the steel. This process is used in manufacturing of products such as mechanical parts, machining tools and functional parts.

The process is widely used for machinery parts such as bearings, gears and cylinder shafts.

Types of Surface Treatment of Steel

Here are some types of the steel surface treatment of steel:

• Uniform attack corrosion
• Color Painting
• Powder Coating
• Galvanization
• Black Oxide Finish
• Pickling and Passivation
• Phosphating Treatment
• Case Hardening
• Nitriding

FAQs on Types of Heat Treatment in Steel