What is the Heat Affected Zone (HAZ) in welding?

The Heat Affected Zone (HAZ) is the area of base metal adjacent to the weld that experiences thermal cycles but does not melt. The HAZ undergoes microstructural changes due to heating and cooling, which can affect mechanical properties - potentially causing hardening, softening, or reduced corrosion resistance depending on the material. Controlling HAZ size and properties is critical for weld quality, achieved through proper heat input and welding parameters.