What is the heat affected zone (HAZ) in welding?

The heat affected zone (HAZ) in welding refers specifically to the area of the base metal that is adjacent to the weld and undergoes microstructural changes due to the heat of the welding process. When metal is heated during welding, its physical and chemical properties in this region can change significantly, even though the material does not completely melt. The HAZ is critical to understanding how welding affects the strength and integrity of the overall welded assembly. It's important to evaluate the conditions within the HAZ as these can impact the performance and durability of the welded joint in service, making this definition essential in the context of welding processes. Understanding the HAZ enables inspectors to assess potential issues such as hardness, brittleness, or other mechanical property changes that can arise from the thermal cycle of welding.