illustrated diagram of the heat affected zone

What is the heat affected zone (HAZ?)

The heat affected zone is an area of the base material that hasn’t melted during the welding process, but has undergone changes in material properties as a result of its exposure to high temperatures.

Where does the heat affected zone occur?

Heat affected zones generally appear adjacent to the weld that is formed. The size of the area is influenced by the heat input of the welding method, duration of exposure to heat, and the thermal diffusivity of the base material. The longer it takes for the material to cool down after welding is performed, the larger the heat affected zone will be. Laser welding, for example, uses a very low heat input, which means the components being welded will cool faster.

How can you evaluate a heat affected zone?

Often, heat tint is visible and can indicate the temperature the material reached. Light yellow is the coolest (550° F) and dark blue is the hottest (1,100° F).

However, these colors are visible because of oxidation. Factors like surface condition, contamination and oxygen availability can affect the appearance of a heat affected zone. Additionally, the amount of chromium present in the material being welded will reduce the intensity of heat tint due to it’s resistance to oxidation.

While the heat tint is the most visible indicator that a material has undergone change, other chemical and metallurgical changes can occur within the HAZ. They include:

  • Surface Nitriding – increased hardness, and decreased weldability.
  • Corrosion – caused by precipitation of chromium carbides near grain boundaries. Without chromium content of 10.5%, steel loses it’s “stainless” quality.
  • Hydrogen embrittlement – hydrogen gas diffuses into the molecular structure of the material. This creates a pressure that will ultimately cause cracking if the hydrogen is not removed.
  • Phase changes – Austenitic steel can become Martenistic after exposure to extreme heat, becoming harder and more brittle, or becoming generally weaker.

Are there ways to reverse the molecular changes?

While we can never eliminate the creation of a heat affected zone entirely, we can deploy a pre-weld or post-weld heat treatment. Heating a material evenly using a heat treatment can help restore some of the properties that may have been lost while the weld was performed.

If you work with Joining Technologies, our engineers and technicians will develop the best strategy for your specific project to ensure the best result possible.

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