Undercut in Welding: Causes and Prevention
Undercut is a groove melted into the base metal beside a weld that has not been filled with weld metal. It is different from overlap, where metal rolls onto the surface without proper fusion. Clean the weld before deciding which condition you are seeing.
Inspect the edge, not only the bead surface
Look along both toes under good light after removing slag or residue. A shadow can exaggerate a groove, so change the viewing angle and inspect the actual profile. Do not grind the edge smooth before documenting what is there.
| Possible contributor | What to compare on a practice coupon |
|---|---|
| Excess heat for the joint | A controlled reduction within the product/procedure limits |
| Travel too fast | Whether the pool has time to fill the edge |
| Long arc or unsuitable voltage | Arc stability and where heat is directed |
| Incorrect working angle | Whether both edges receive controlled heat and filler |
| Oversized movement | A simpler run with a visible pool |
These conditions can interact. Repeating the same joint while changing one variable gives a more useful result than changing electrode, current and travel together.
Prevent it during practice
Prepare a consistent joint and select consumables suited to the position. Position the work so you can see both toes. Clean between passes and inspect each one rather than waiting until the joint is full. Keep a short note of what happened at the start, middle and finish; a defect only near the end may relate to fatigue or position.
Decide whether repair needs qualified review
Acceptable undercut depends on the applicable standard, loading and joint details. There is no universal “small groove is fine” allowance for every weld. A load-bearing joint with a groove needs evaluation against its requirements. Do not simply deposit another pass over an unidentified defect.
Is undercut always caused by excessive amperage?
No. Heat, arc characteristics, angle, travel and filler placement all matter. Diagnose the complete setup rather than assuming one control caused it.
Can a smooth bead still have undercut?
Yes. The top may look regular while a groove remains at the edge. Appearance also cannot establish internal fusion. For critical work, follow the specified inspection and repair process.
Continue learning
Sources and limits
This is an educational reference. Follow the exact equipment and consumable instructions; safety-critical welding needs appropriate qualifications, procedures and inspection.
