Undercut: causes, limits and repair
A groove melted into the parent metal along the toe of the weld and left unfilled. Easy to see, easy to measure, and more serious than it looks.
Why it matters
Two reasons. It removes section — a 1 mm groove in a 6 mm plate is a sixth of the thickness gone. And it puts a sharp notch exactly at the weld toe, which is where bending stress peaks and where fatigue cracks start anyway. Under static load undercut is a nuisance; under fatigue load it is the crack initiation site.
What causes it
Current too high. The commonest cause by far. The arc digs into the parent metal faster than the filler can fill the groove behind it.
Arc too long. A long arc spreads and washes the toes without depositing there.
Travel speed too high. The pool does not have time to flow out to the edges before it freezes.
Wrong torch or electrode angle. Especially on a fillet in the horizontal position, where too much angle towards the vertical plate melts its edge and lets gravity pull the metal down and away.
Weaving badly. Pausing in the middle and sweeping quickly at the edges is exactly backwards. The pause belongs at the edges of the weave, where it lets the pool fill the toe.
Fixing the technique
Drop the current 10 % and see what changes; it is the fastest diagnostic. Shorten the arc. Slow down. On a horizontal fillet, aim the arc more at the bottom plate and let the pool wash up to the vertical one, rather than pointing into the corner.
Undercut on the last run of a multi-run weld is often a cap-run problem alone: the fill runs were fine and the cap was made too hot and too fast because the joint was nearly finished.
Repair
Grind the groove out to clean metal with a smooth transition — no sharp step at the ends of the ground area. Then a stringer run at reduced current, and blend the toe afterwards. Grinding alone, without adding metal, is only acceptable if the remaining thickness still satisfies the drawing, and that is a decision for the inspector rather than the welder.
Related shape defects with the same root causes are collected in the defect catalogue, group 500.
Frequently asked questions
How much undercut is allowed?
It depends on the quality level and the thickness. Level B allows 0.05·t up to 0.5 mm, level C 0.1·t up to 0.5 mm, level D 0.2·t up to 1 mm — and there are conditions on smoothness. The acceptance calculator does the arithmetic for your thickness.
Can undercut be filled in with a small run?
Sometimes, and it must be agreed first: on many contracts repair welding needs its own approved procedure. Where it is allowed, the groove is ground clean and a small stringer run is laid with reduced current — never a quick dab with the same settings that caused it.
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