What is the heat-affected zone (HAZ) and why is its microstructure important to weld performance?

• A. The weld metal region where the composition changes due to welding.
• B. The area of base metal whose properties were altered by heat; microstructure changes govern hardness, strength, and brittleness in the HAZ.
• C. A separate heat source region outside the weld.
• D. A non-existent zone in modern welding.

Answer: (B)

The heat-affected zone is the portion of the base metal near the weld where the heat from welding has changed the microstructure without melting the metal. That area is between the untouched base metal and the melted weld metal, and its properties shift because the heating and subsequent cooling alter grain size, phases, and residual stresses.

Those microstructural changes are why the HAZ matters for weld performance. Different microstructures can be harder and more brittle or softer and tougher, which directly affects hardness, strength, and the risk of cracking under service conditions. If the heating or cooling drives grain growth or unwanted phase transformations, the HAZ can become a weak link even though the base metal wasn’t melted or the weld metal was, because its response to loads and thermal cycles is governed by that altered microstructure.

This description is why the HAZ is defined as the heat-altered region of base metal, not the weld metal itself or an external heat source, and why its microstructure is critical to overall weld performance.