What is the Heat Affected Zone in Welding?

The heat affected zone (HAZ) is the portion of the base metal, adjacent to a weld, the structure or properties of which have been altered by the heat of welding. In steel and other metals, it is essential that the microstructure and mechanical properties in the heat-affected zone be controlled to ensure that the joint will have the required strength and toughness.

In some cases, the heat-affected zone may be only a very small fraction of the total joint volume, while in others it can be a major contributor to the overall properties of the weldment.

What are the effects of welding on the heat-affected zone?

There are 3 key factors that need to be controlled in order to minimize the negative effects of welding on the heat-affected zone:

Time at temperature – The longer the time that the metal is exposed to high temperatures, the greater the chance of microstructural changes occurring. Therefore, it is important to minimize the time that the metal spends in the heat-affected zone.
Temperature – The higher the temperature, the greater the chance of microstructural changes occurring. Therefore, it is important to minimize the temperature of the heat-affected zone.
Cooling rate – The faster the cooling rate, the greater the chance of microstructural changes occurring. Therefore, it is important to maximize the cooling rate of the heat-affected zone.

How heat affected zone avoided in welding?

The best way to avoid the negative effects of welding on the heat-affected zone is to control the time at temperature, temperature, and cooling rate. This can be done by:

preheating,
post-weld heat treatment,
and the use of filler metals with low hydrogen content.

By controlling the heat-affected zone, it is possible to minimize the negative effects of welding and ensure that the joint has the required strength and toughness.

How far is the heat-affected zone?

The heat-affected zone extends from the weld metal to a distance of about 1.5 mm (0.06 in) on either side of the weld. In some cases, the heat-affected zone may be only a very small fraction of the total joint volume, while in others it can be a major contributor to the overall properties of the weldment.

What is the fusion zone and heat-affected zone in welding?

The fusion zone is the area where the base metal and filler metal have been completely melted and fused together. The heat-affected zone is the area where the base metal has been heated by the welding process but has not been melted.

Good to know:

Submerged arc welding

It is an arc welding process in which the weld is shielded by granular fusible material. This dressing also called slag, covers and protects the weld pool from atmospheric contamination.

The main advantage of submerged arc welding is its high deposition rate, which results in increased productivity.