What is Heat Affected Zone in Welding? (7 things to know!)

What is Heat Affected Zone (HAZ)?
Definition: Heat Affected Zone is defined as a portion of a base metal lying next to the fusion line of the weld which has not melted but whose microstructure has been altered by the heat of welding.

In simple words, heat affected zone is the zone of the base metal which is affected by the extreme heat.

The heating and cooling cycles involved in the welding process affects the weld region very much.

And because of the extreme heat near the weld region, the microstructures and the properties of the metal at that region changes.

Also the non uniform stress field is generated due to local expansion and contraction of the metal, which leads to residual stresses and thermal distortion.

Heat Affected Zones in Welding

Heat Affected zones of welding are mentioned below.

  • Coarse grained supercritical heat affected zone
  • Fine grained supercritical heat affected zone
  • Intercritical heat affected zone
  • Subcritical heat affected zone

Let me explain these heat affected zones using a diagram.

heat affected zone diagram

Heat Affected Zone diagram is shown above.

The leftmost vertical line in the diagram is the weld region where the fusion of metal occurs.

The region which is towards the right side of the vertical line is the heat affected zone.

#1) Coarse grained supercritical heat affected zone

In this HAZ, full phase transformation took place and it is followed by grain coarsening.

This zone lies in contact with the fusion line (or weld bead) and it gets heated a lot.

This is probably the most dangerous area of the heat affected zone.

#2) Fine grained supercritical heat affected zone

In this weld zone, the full phase transformation takes place and it is normally a fine grained region with moderate hardness.

#3) Intercritical heat affected zone

Intercritical HAZ is a region of the welding that is partially recrystallized.

#4) Subcritical heat affected zone

In subcritical HAZ, tampering takes place and this zone acquires a mixed structure.

Heat Affected Zone color and temperature

The chart mentioned below shows the temperature of heat affected zone and its color.

heat affected zone color and temperature

Whatever the welding process is, the heat affected zone undergoes a thermal cycle in which rapid heating occurs and it is followed by cooling at variable rate.

This thermal cycle (of rapid heating and cooling) is responsible for the color and final metallurgical structure that is formed in the heat affected zone of welded parts.

What causes cracking in heat affected zone? And how to control it?

Cracking in the heat affected zone occurs due to rapid cooling of the metal.

Rapid cooling results in a harder microstructure that results in cracking.

In order to control the cracking, the region which is to be welded is preheated.

In this way, as the cooling rate decreases, the hardness also decreases and that prevents cracking in HAZ.

Heat Affected Zone microstructure depends on…?

The heat affected zone microstructure depends on the composition of steel as well as thermal cycles of the welding.

If the carbon content is higher in the metal, then the chances of cracking may occur.


External links:
Heat affected zone: Image by Ytrottier, CC BY-SA 3.0, via Wikimedia Commons

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