Heat-affected zone (HAZ)

The heat-affected zone is the band of parent metal adjacent to a weld that is not melted but is altered by the weld thermal cycle, ranging from a coarse-grained region next to the fusion line through to a tempered zone farther out. In higher-tensile and quenched-and-tempered steels the coarse-grained HAZ can lose toughness and harden, raising the risk of hydrogen cracking, which is why low-hydrogen consumables, controlled preheat, and limited heat input are specified. Weldability tests such as CTOD assess HAZ fracture toughness for critical structure.