What is an undercut, and what conditions typically cause it?

• A. A groove melted into the base metal at the toe of the weld, reducing strength.
• B. A slag inclusion.
• C. A pinhole in weld.
• D. A lack of fusion.

Answer: (A)

Undercut is a groove melted into the base metal right at the weld toe, where the weld bead meets the base metal. It creates a notch along the edge instead of a smooth transition, which reduces the effective cross‑section carrying the load and can concentrate stress, weakening the joint.

This defect shows up when heat input outpaces the metal’s ability to fill the edges with filler metal. Think of too much heat from high current or a long arc length, or techniques that don’t deposit enough filler or fill the edge quickly enough. A weld may burn away the edge faster than the bead can build up, especially if travel speed is not balanced with deposition, or the bead contour and travel angle aren’t right. Reducing heat input and improving filler deposition and bead shape helps prevent undercut.