Undercut — What It Is and Why It Matters
Undercut is a groove melted into the base metal along the toe of a weld that is not filled by weld metal. It creates a notch that reduces the base metal's effective cross-section and acts as a sharp stress concentrator — making it one of the most significant weld defects for fatigue-loaded structures.
Undercut is caused by excessive heat input, excessive travel speed, improper electrode angle, or too long an arc length. It is most common along the top toe of horizontal fillet welds (where gravity pulls the puddle away) and on the edges of overhead welds.
Prevention involves reducing heat input (lower amperage or voltage), slowing travel speed to allow the weld pool to fill the melted groove, maintaining proper work angle, and pausing briefly at the toes during weave beads. Mild undercut (under 1/32" deep) is often acceptable under structural codes, but deeper undercut must be repaired by rewelding.
Frequently Asked Questions
How much undercut is acceptable?
AWS D1.1 permits undercut up to 1/32" deep for most structural applications. For cyclically loaded (fatigue) connections, the limit is more restrictive. ASME codes for pressure vessels have their own acceptance criteria. Always check the specific code governing your work.
How do I prevent undercut?
Reduce amperage or voltage, slow your travel speed, maintain proper torch angle (directing heat into the joint rather than the base metal edges), and pause briefly at the toes of weave beads to allow filler metal to fill the melted groove.