What is the role of the heat-affected zone in welding?

The heat-affected zone (HAZ) in welding is primarily recognized for its role in altering the microstructure of the base material without melting it. When heat is applied during the welding process, the areas adjacent to the weld pool experience temperature changes that can lead to metallurgical changes, such as grain growth, phase transformation, or hardening, depending on the material and the cooling rate. These alterations can significantly affect the mechanical properties of the welded joint, such as strength and ductility.

While there are indeed zones where the metal may be melted (which would represent the weld pool) and areas where there is non-fusion (which describe what happens further away from the heat source), these characteristics do not define the central role of the HAZ. Instead, the focus is on how the heat affects the properties of the surrounding material. Therefore, the most accurate description of the HAZ's role is that it is the zone where the microstructure is altered due to the thermal effects of welding.