What is the heat-affected zone (HAZ) and why is its size controlled?

• A. The melted weld metal; its size is controlled to ensure full fusion
• B. The base metal region altered by welding heat; its extent is controlled to maintain mechanical properties and reduce cracking risk
• C. The region cooled rapidly after welding; its size is controlled to prevent warping
• D. The entire welded structure; its size is controlled to meet weight requirements

Answer: (B)

The heat-affected zone is the portion of the base metal that is heated by welding but does not melt. In this region the metal’s microstructure and properties are altered by the heat, which can change hardness, strength, ductility, and residual stresses.

Its size is controlled because how much of the base metal is heated (the heat input and the duration of heating) directly affects those properties. A larger HAZ means more of the base metal experiences changes that can reduce toughness or encourage cracking and distortion. By managing heat input, travel speed, interpass temperature, preheating, post-weld heat treatment, and cooling rates, the HAZ is kept within bounds to preserve mechanical properties and minimize cracking risks.

The other descriptions aren’t correct: the melted weld metal is the weld metal itself, not the heat-affected zone; the HAZ is defined by heating of the base metal, not merely cooling; and it isn’t the entire welded structure.