How can welding heat change the microstructure of metal next to a weld?

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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 fusion welding, the area immediately surrounding the weld metal is known as the heat-affected zone (HAZ). Although this region does not reach the melting point, it undergoes significant changes in its microstructure and mechanical properties due to the thermal cycle of the welding process.1

The specific structural alterations in the HAZ are determined by three primary factors: the peak temperature reached at a given point, the duration of exposure to that heat, and the subsequent cooling rate. Depending on the base metal's composition, these thermal cycles can lead to several outcomes:1

Structural Hardening: Certain materials undergo phase transformations that result in increased hardness.1
Precipitation Hardening: Some alloys are hardened through precipitation processes triggered by the heat.1
Residual Stress: The expansion and contraction of the HAZ, restrained by the surrounding cooler metal, induce internal residual stresses. If not managed through proper heat treatment or fabrication procedures, these stresses can lead to distortion or bowing of the final weldment.1

Materials are generally categorized by their sensitivity to these thermal effects, ranging from those that remain largely unaffected to those that experience significant hardening or property degradation.1