What does the heat-affected zone refer to in welding?

The heat-affected zone (HAZ) refers specifically to the region of the base metal which, while it does not reach the melting temperature, experiences changes in its physical or metallurgical properties due to the heat generated during the welding process. This area can undergo alterations such as changes in microstructure, hardness, and ductility, which can influence the overall strength and performance of the welded joint.

In contrast, the other options describe different aspects of welding. Some pertain to materials directly involved in creating the weld joint itself, like added metal or melted regions, but the HAZ is distinct in that it specifically addresses the effects of heat on the surrounding base metal rather than indicating materials that are involved in melting or mixing. Understanding the HAZ is critical as it can impact the integrity and longevity of welded structures, making it a vital consideration in welding practices.