What is the heat-affected zone (HAZ)?

• A. The region of base metal whose microstructure and properties have been altered by the heat of welding but not melted.
• B. The region of base metal that melts during welding.
• C. The weld bead.
• D. The shielding gas boundary.

Answer: (A)

Understanding what the heat-affected zone is starts with recognizing that welding heat can change a metal’s structure without melting it. The heat-affected zone is the portion of the base metal that has been heated by the welding process enough to alter its microstructure and mechanical properties, but not hot enough to melt. This zone sits around the weld bead, between the fully melted weld metal (the fusion zone) and the untouched base metal. The changes come from the thermal cycle and cooling rate, which can cause grain growth or other transformations that change hardness or brittleness, depending on the material and weld conditions. The region that actually melts is the weld metal, not the HAZ. The weld bead is the deposited filler metal, and the shielding gas boundary is just the atmosphere around the arc and doesn’t define a metallurgical region.