Which factors determine structural changes in the heat-affected zone?

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Welder in a blue uniform and protective helmet using shielded metal arc welding to join metal pieces, with bright sparks and smoke visible at the welding point. — welding Shielded metal arc welding.

In the context of welding, the heat-affected zone (HAZ) is the region of base metal adjacent to the weld that, while not melted, experiences alterations in its microstructure or mechanical properties due to the thermal cycle of the welding process.1

The structural changes within this zone are determined by three primary factors:

Peak Temperature: The maximum temperature reached at any specific point within the HAZ during the welding cycle.1
Time of Exposure: The duration for which the material is subjected to these elevated temperatures.1
Cooling Rates: The speed at which the material cools after the heat source has passed, which significantly influences the final metallurgical state of the zone.1

These factors interact with the specific characteristics of the base metal, which generally fall into categories such as materials that remain unaffected by heat, those that undergo structural hardening, or those hardened through precipitation processes. Proper management of these variables is essential, as the thermal expansion and contraction of the HAZ contribute to the development of residual stress within the welded joint.1