What is undercut in a weld?

• A. A shallow cavity at the weld toe that increases thickness.
• B. A slag inclusion within the weld metal.
• C. Porosity pockets in the weld metal.
• D. A groove along the base metal adjacent to the weld root that reduces section thickness.

Answer: (D)

Undercut is a groove melted into the base metal next to the weld toe or root that is not filled by weld metal, leaving a notch that reduces the joint’s cross‑section thickness. This thinning creates a stress concentration and weakens the weld, especially under load or fatigue. It’s not a feature of the weld metal itself like slag inclusion or porosity; those are different defects. The description of a groove in the base metal adjacent to the weld root that reduces section thickness matches undercut, making it the correct choice. Causes include too high travel speed or excessive heat input, or insufficient filler metal, and remedies involve adjusting welding parameters and technique to ensure the joint is fully filled and the toe is smooth.