Troubleshooting guide › Weld defects › Undercut
Undercut is a groove melted into the base metal along the edge of the weld that the filler metal did not fill back. It thins the plate right where stress is highest, so inspectors reject it quickly. It comes from too much heat or the wrong torch movement, and it is fixable by technique.
Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.
A sharp notch or channel running along one or both toes of the weld, between the bead and the plate. You can catch a fingernail in it. On a fillet weld it is most often on the vertical plate.
Illustration, not a photo. Orange marks the defect.
The arc digs into the plate faster than the filler can fill the groove.
The pool does not have time to flow back to the edges.
Pointing the arc at the plate edge instead of the joint washes metal away from that edge.
A wide arc spreads heat beyond where the filler lands.
In a weave, pausing too briefly at the edges leaves them unfilled.
A big electrode in the vertical or overhead position runs too hot to control.
| Process | Typical reason on this process | What to do |
|---|---|---|
| MIG / MAG | Voltage too high for the wire speed, travel too fast, torch angled too much sideways on a fillet. | Drop voltage one step, slow down, aim the wire at the root with the torch 40-45 degrees between the plates. |
| ARC / MMA | Current too high for the electrode size, long arc, weaving too wide with no pause at the edges. | Set current to the electrode range, short arc, pause at each side of the weave. |
| TIG | Travel too fast with too little filler, arc too long, torch angle wrong on the vertical plate. | Add filler more often, shorten the arc to 2-3 mm, split the arc evenly between both plates. |
| Submerged arc | Travel too fast, current or voltage too high, wire not aligned with the joint. | Reduce travel speed or current, lower voltage, correct wire position. |
| Welding carriage / automation | Torch position drifted, speed set for a smaller leg than required, weave width too wide with no dwell. | Reset the torch position on the X-Y slide, match speed to the leg size, add 0.1-0.4 s dwell at each side. |
ARC (MMA), E6013 3.15 mm electrode on 6 mm mild steel, flat fillet. A starting point only; confirm on a test piece of the same plate.
| Current | about 90-120 A |
| Arc length | equal to the core wire diameter |
| Electrode angle | 45 degrees to both plates, 10-20 degrees drag |
| Travel | steady; pool wets both plates |
Typical values for mild steel. Your wire, plate condition and machine will move them. Our technical team can set them with your operator on a video call.
Run a fingernail or a fillet gauge along the toe. A groove you can feel is undercut. Measure its depth with a welding gauge: the limit depends on the code, the plate thickness and the loading (structural codes typically allow fractions of a millimetre on statically loaded joints and much less, or none, on fatigue-loaded ones). Use the limit from the applicable code or drawing, and repair by the approved procedure, not by a blanket rule.
Yes. Clean the toe with a wire brush and run a small stringer bead along the groove at reduced current. Do not fill it with a wide weave; that adds heat and more undercut.
The arc is biased towards the vertical plate and melts it faster than it can be filled. Point the wire more at the bottom plate, keep the torch at about 45 degrees, and slow down slightly.
The same depth of undercut removes a bigger share of a thin plate, so the strength loss is larger. Keep current at the low end of the range on thin plate.
A machine with a wide, stepless current range makes fine adjustment possible; on a tapped transformer set you may have to compromise on travel speed instead.
Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.
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