Undercut is a groove melted into the parent plate along the weld toe and left unfilled. It’s easy to spot, easy to measure, and easy to underestimate: a half-millimetre notch at the toe sits at the exact location where geometric stress concentration peaks — which makes undercut one of the most consequential surface defects for fatigue-loaded structures.
It’s also one of the few imperfections where ISO 5817 gives clean, memorable numeric limits — making it the textbook example of how quality levels B, C and D actually differ.
What is undercut?
ISO 6520-1 distinguishes:
| Type | Code | Where |
|---|---|---|
| Continuous undercut | 5011 | Uninterrupted groove along the toe |
| Intermittent undercut | 5012 | Short, separated grooves |
| Root undercut (shrinkage groove) | 5013 | Groove at the root of a butt weld |
The mechanism: the arc melts a channel into the parent metal at the pool edge, and surface tension plus travel speed prevent the pool from flowing back to fill it. The result is a notch with weld metal on one side and plate on the other — the worst possible orientation, transverse to the principal stress in most joints.
ISO 5817 acceptance limits
Undercut depth h scales with material thickness t:
| Type | Code | Level B | Level C | Level D |
|---|---|---|---|---|
| Continuous undercut | 5011 | h ≤ 0.05t, max 0.5 mm | h ≤ 0.1t, max 1 mm | h ≤ 0.2t, max 2 mm |
| Intermittent undercut | 5011 | h ≤ 0.1t, max 1 mm | h ≤ 0.2t, max 2 mm | h ≤ 0.2t, max 2 mm |
| Root undercut | 5013 | Not permitted | h ≤ 0.05t, max 0.5 mm | h ≤ 0.1t, max 1 mm |
Smooth transitions are generally required — a sharp-bottomed groove is sentenced more severely than a smooth one of the same depth. Values are indicative references: verify the current edition of ISO 5817 and any overriding spec. Fatigue codes can be stricter than Level B for critical details.
On a 10 mm plate that means: 0.5 mm tolerated at Level B, 1 mm at C, 2 mm at D — the same weld passes or fails depending on the level your project specifies. How levels are assigned: ISO 5817 quality levels explained · Level B vs C vs D side by side.
What causes undercut
Electrical and travel parameters
- Current too high — the arc digs more parent metal than the pool can refill
- Travel speed too fast — pool solidifies before backfilling the melted edge
- Arc length / voltage too high — wider, shallower arc melting the toes
Technique
- Wrong work or travel angle — the arc heats one toe asymmetrically; classic in horizontal (PB/2F) fillets where gravity drains the pool off the vertical leg
- Excessive weaving — pool edges freeze at the extremes of the weave
- Wrong electrode diameter for the joint and position
Process-specific
- In MAG spray transfer at high current, undercut risk grows with travel speed — exactly the corner of the parameter envelope production wants to run in for throughput. This makes undercut a drift defect: a cell that produced acceptable toes at qualification slides into undercut as contact tips wear, stick-out shifts, or speed is nudged up. Continuous parameter and thermal monitoring catches that drift before the seam fails VT.
Detection and measurement
Undercut is a visual-testing item (ISO 17637): raking light and a depth gauge (or profile gauge / laser profilometry for automated lines). Two practical rules:
- Measure depth at the deepest point, not an average — sentencing uses the maximum.
- Record position and length: intermittent vs continuous changes the applicable limit.
Full VT procedure context: ISO 17637 visual weld inspection. For automated geometry checks, see weld bead geometry measurement.
Prevention
- Respect WPS current and speed — undercut is the canonical symptom of running hot and fast.
- Fix the angle: keep the electrode/torch bisecting fillet joints; slight drag angle; aim the arc so both toes wet evenly.
- Control arc length — shorter arc, narrower dig.
- Limit weave width or switch to stringers in position work.
- Watch the toes, not the bead: a bright, shiny toe channel behind the pool is undercut forming in real time — operators (and vision systems) can see it before the weld is finished.
Undercut is one of the defects in our complete reference: welding defects guide with ISO 5817 acceptance criteria.
Frequently Asked Questions
What is undercut in welding?
Undercut (ISO 6520-1 code 5011/5012) is a groove melted into the parent metal along the weld toe that is left unfilled by weld metal. It reduces the load-bearing cross-section locally and creates a sharp notch exactly where weld geometry already concentrates stress, making it one of the most fatigue-relevant surface imperfections.
How much undercut is acceptable under ISO 5817?
For continuous undercut, ISO 5817 limits depth h to 0.05t (max 0.5 mm) at Level B, 0.1t (max 1 mm) at Level C, and 0.2t (max 2 mm) at Level D, where t is material thickness. Intermittent undercut gets slightly more allowance, root undercut less. Smooth transitions are generally required. Always verify the current edition of the standard and your governing code.
What causes undercut in welding?
The classic causes are welding current too high for the position, travel speed too fast for the weld pool to backfill the melted groove, wrong torch or electrode angle, excessive arc length or voltage, and excessive weaving. In fillet welds in the horizontal position, gravity pulling the pool away from the vertical leg is a frequent contributor.
Can undercut be repaired?
Yes. Light undercut within acceptance limits can be left or blended by grinding to a smooth transition. Undercut beyond limits is excavated if needed and filled with an additional stringer pass under a qualified procedure, then re-inspected visually — and the repair recorded against the joint ID for traceability.
