Discontinuity
Undercut
A groove melted into the base metal at the weld toe and left unfilled by weld metal, reducing the section of the parent material and leaving a sharp notch exactly where stress concentrates.
What it looks like
- A continuous or intermittent channel running along the toe of the weld
- Often deeper and more obvious on the upper toe of a horizontal fillet weld
- Feels like a step when a fingernail or gauge is drawn across the toe from plate to bead
- On film, a darker irregular band along the edge of the weld image where metal is missing
Causes
- Excessive current melting more base metal than the deposition rate can refill
- Travel speed too high, so the puddle does not have time to wash back into the melted edge
- Incorrect electrode angle directing too much arc force at the plate rather than into the joint
- Excessive weaving without a pause at the edges of the weave to fill the toe
- Arc length too long, widening the arc cone and spreading heat onto the base metal
Prevention
- Reduce current or slow travel until the puddle can fill the melted edge completely
- Pause briefly at each side of a weave so weld metal wets the toe before returning
- Hold a short arc and a work angle that keeps arc force in the joint rather than on the plate
- Use a slightly smaller electrode in positions where puddle control is difficult
How it is found
- VT
- Undercut is surface-breaking by definition, so direct examination with a fillet or undercut gauge measures its depth reliably.
- MT
- Confirms whether a sharp undercut notch has begun to crack, which is the real hazard rather than the groove itself.
- RT
- Shows as a shadowed band along the weld edge, useful for confirming undercut on the inaccessible root side of pipe.
How inspectors think about it
- Undercut is assessed on depth and on how much of the weld length it runs, because a shallow intermittent groove behaves very differently from a continuous deep one.
- The notch effect matters most on joints loaded in fatigue, where a sharp toe groove is a ready-made crack starter even when the depth looks trivial.
- Inspectors also weigh where the undercut sits: a groove in a thin member removes a much larger fraction of the section than the same depth in heavy plate.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
OverlapWeld metal that has rolled over the toe and sits on unmelted base metal, forming a lip with a sharp crevice underneath that carries no load and hides an unfused interface.UnderfillA groove weld whose face or root sits below the surface of the adjoining base metal, so the joint has less cross-section than the surrounding material it is supposed to match.Toe crackA crack starting at the junction of the weld face and the base metal, where the abrupt change in section concentrates stress and the heat-affected zone is at its hardest and least forgiving.Excessive reinforcementA weld face built up far higher than the joint needs, producing an abrupt change of section at the toes that concentrates stress despite the extra metal looking like extra strength.
Can you name it on sight?
The gauge trainer's identification mode shows you a weld and four names. Reading about porosity and picking it out of a line-up are not the same skill.