An undercut in welding is best described as...

• A. A weld defect caused by gas porosity
• B. A slag inclusion in the weld
• C. A groove in the base metal at the toe of the weld
• D. A raised bead on top of the weld

Answer: (C)

Undercut is a groove melted into the base metal at the weld toe, left unfilled by the weld bead. This removes material from the joint edge, creating a notch that weakens the weld by reducing the base metal thickness and concentrating stress, especially under bending or fatigue. It often results from too much heat input or too fast travel speed, improper torch angle, or insufficient filler deposition.

It's not a porosity issue (gas pockets in weld metal), not slag inclusion (slag trapped in the weld), and not an elevated, raised bead on top.