WeldHeatAffectedZone

Weld heat-affected zone (HAZ) is the portion of base metal that experiences temperatures high enough during welding to alter its microstructure and properties without melting. It lies adjacent to the weld metal and extends into the surrounding base material; the extent depends on heat input, material properties, and welding process. The HAZ undergoes a range of thermal cycles that can produce coarse-grained, fine-grained, intercritical, or subcritical subregions.

In carbon steels, the coarse-grained HAZ near the fusion line is often the toughest and most brittle, while the fine-grained HAZ may retain more toughness; in stainless steels, high temperatures can cause sensitization and carbide precipitation in certain subzones; in aluminum alloys, thermal exposure can dissolve or overage strengthening precipitates and reduce strength.

The properties of the HAZ are usually different from the base metal and, in many cases, from the weld metal, with hardness, ductility, and toughness affected. The HAZ is a common site for cracking if residual stresses are high, microstructural flaws exist, or hydrogen-assisted cracking susceptibilities are present. Characterization methods include hardness testing across the weld, metallographic examination of microstructure, and impact or fracture toughness testing. Mitigation strategies include controlling heat input, preheating where appropriate, interpass temperature management, selecting compatible filler materials, applying post-weld heat treatment, and optimizing joint design and welding technique to minimize detrimental transformations.