A groove melted into the base metal along the weld toe — the notch that steals cross-section and hands fatigue a starting point. It is almost always a parameter or technique problem, which makes it one of the most fixable defects on the list.
Undercut is a groove melted into the base metal at the weld toe — a notch where the weld metal does not reach the parent metal surface. The arc removes more base metal than the deposit replaces, leaving a depression along one or both toes of the weld.
Undercut matters twice over. It reduces the load-bearing cross-section of the weld, and its sharp root acts as a stress riser — the classic starting point for fatigue cracks in cyclically loaded structures. It also telegraphs a technique problem: the parameters, angle, or travel speed are working against the pool.
Toe undercut, groove at the toe, arc gouging, excessive toe sharpness.
Undercut reads like a machining pass: clean-edged, sharp-bottomed, and parallel to the toe — not a ragged hole.
Every undercut is a balance failure: the arc melts base metal faster than the filler fills it. The six boxes below are the levers that break that balance.
A sound weld toe needs one thing: the volume of filler delivered must match the volume of base metal melted. Undercut is what happens when the arc out-melts the deposit. The arc energy at the leading edge melts the base metal; the deposit fills it only if the pool stays fluid long enough to wash into the toe. Break that balance in any of four ways — heat, speed, direction, or weave — and the toe stays open.
High current density melts a wide, deep trench at the toe — spray MIG and hot SAW are the champions. The filler cannot fill what the arc removes faster than it arrives.
Travel speed outruns the pool: the molten toe solidifies before filler reaches it. A fast, steady hand with a hot arc undercuts the full bead length.
A steep work angle, long arc, or excessive drag pushes the arc force into one toe and digs it out. That is why undercut so often lives on one side only.
A weave that dwells at the edges re-melts the toes while the center gets a thin deposit. Dwell belongs in the center, never at the edges.
The result is a notch with two costs. First, the load-bearing section of the weld is reduced by the depth of the groove — in a fillet weld, that is a direct cut into the effective throat. Second, the sharp root concentrates stress: in a fatigue-loaded structure, toe cracks initiate at the undercut root far earlier than they would at a smooth toe.
Root-side undercut is the same disease in a different place: a root pass run hot on a backing melts the root face wider than the weld fills it — a groove on the inside of the joint, found only on RT or from the back side.
Undercut is a rhythm problem: steady speed, steady angle, steady stick-out. Lock the rhythm and the groove disappears.
Set amperage and voltage to the qualified range for the position — not the bench setting. Match wire size to the amperage and electrode to the material. Clean and grind the toes so the arc does not wander on mill scale.
Hold a 10–15° drag angle and a steady travel speed — the pool should wash into both toes ahead of you. Watch the toes: the moment a groove starts, the parameter or the angle is off. In fillets, point the arc at the root, not at the vertical toe.
Inspect each pass for undercut before it gets buried. On multipass welds, position stringers to overlap the previous toe. On the next pass, weld the side with the groove first — a cool toe and a hot arc dig.
The classic mistake: slowing down to "give the toes time to fill." A slower, hotter pool digs deeper — longer arc time at the toe melts more base metal. Correct the cause — current, angle, or speed — not the symptom.
When undercut is already in the weld, the fix depends on depth — and on the code. Measure before you decide.
| Step | Action | Why |
|---|---|---|
| 1 | Measure the depth, length, and location of the undercut with a gauge. | The code verdict is made on the numbers, not the look. |
| 2 | Compare against the governing document's limits. | AWS D1.1 and ISO 5817 allow shallow undercut in some categories — depth decides. |
| 3 | Within limits — grind smooth, blending the notch into the toe. | A feathered toe removes the stress riser without adding metal. |
| 4 | Over limits — clean the groove and deposit a low-heat stringer pass along the toe. | The repair must fill the notch without creating overlap or new undercut. |
| 5 | Grind the repair flush and smooth; re-inspect with VT, and MT where the code asks. | The repaired toe must look like a toe, not a patch. |
Grinding out deep undercut can drop the section below minimum thickness — check the leg or wall size before you chase it. And a repair pass that sits proud of the plate is just overlap in a new costume. See overlap for the warning.
Each process undercuts with its own signature. Read yours against the list.
The MIG story is voltage and angle. Voltage too high widens the arc and digs the toes; a steep push or drag angle directs it into one toe; spray transfer on thin material with fast travel leaves a classic groove. Fix in order: voltage, then angle, then speed.
Amperage too high for the filler feed rate — the pool grows faster than the rod fills it. A long arc fans heat onto the toes. On fillets, torch angle into the vertical member; on aluminum, the cleaning action can melt the toe before the rod arrives.
Current too high, arc too long, rod angle too steep — the three classic stick undercuts. On 7018, a wide weave that dwells at the edges digs both toes. Position work needs a current cut; the angle stays 10–15° drag with a short arc.
Same family as MIG, same causes: voltage too high and gun angle too steep. Self-shielded wire with long stick-out raises the voltage and digs. High-deposition settings in position work undercut before the pool can wash in.
Voltage too high on thick plate digs the toes — the arc eats the sidewalls while the flux blanket hides the groove until it is too late. Balance voltage, flux depth, and travel speed; check the toes between passes.
High current density on thin sheet with fast travel outruns the pool and cuts a groove at the edges. A worn orifice amplifies it. Drop the current or slow the travel — the pool must wash into the toes before it freezes.
Five numbers control undercut: current, voltage, speed, angle, and weave. Check them in that order.
| Parameter | Check | Typical Fix |
|---|---|---|
| Current | Too high for the wire size and position | Use the qualified range; cut current for out-of-position work |
| Voltage | MIG/FCAW too high — the arc widens and digs | Reduce in 1–2 V steps; watch the toes between passes |
| Travel speed | Too fast — the pool freezes before it fills the toe | Steady speed; the pool must wash into both toes |
| Work angle | Steep angle digs one toe | 10–15° drag (MIG/FCAW); 10–15° with stick and TIG; point at the joint root |
| Weave width & dwell | Too wide, or dwell at the edges | Max 2–3× wire or electrode diameter; dwell in the center |
| Arc length | Too long (stick/TIG) fans heat at the toes | Short, tight arc; steady hand |
| CTWD / stick-out | Too long raises the voltage and digs | 10–15× wire diameter (MIG); per WPS for FCAW |
Use the Heat Input Calculator to see how current, voltage, and speed work as one number, the Welding Calculator to set parameters per joint, and the Amperage Guide to dial current for the position.
Undercut is rarely a hardware failure — but the hardware feeds the balance.
Power sources: a machine that drifts from its dialed output changes heat input mid-bead. Torches: a clean, correctly sized nozzle keeps the arc where it belongs; a worn one lets it wander. Consumables: wire and electrode sized to the current keep current density in the arc, not in the toe.
Undercut is found with the eye and measured with a gauge — the fastest inspection in the book, and one every welder can do.
Acceptance is a numbers game: AWS D1.1 typically limits undercut to about 1/32 in. (0.8 mm) on statically loaded welds and tightens that (roughly 1/64 in.) where fatigue governs; ISO 5817 and API 1104 set their own limits by quality level. Depth, continuity, and the weld's place in the structure all play. Read the governing document before judging a weld acceptable — there is no universal pass/fail rule.
The questions welders actually ask about undercut — answered straight.
No — it is code-dependent. AWS D1.1 typically allows undercut up to about 1/32 in. (0.8 mm) deep on statically loaded welds, and tightens that (roughly 1/64 in.) where fatigue loads apply. ISO 5817 and API 1104 have their own limits by quality level. Measure with a gauge, read the governing document, then judge.
Yes, within limits. Shallow undercut can be filled with a stringer pass along the toe — run at conservative parameters so the repair does not create overlap or new undercut, then grind smooth. Deep undercut should be excavated and re-welded, or accepted after grinding only if the remaining section still meets thickness requirements. Check the code either way.
Check voltage first — too high is the most common cause; the wide arc digs the toes. Then check gun angle (keep 10–15° drag, not steeper), then travel speed. A worn contact tip or oversized stick-out raises the voltage and digs too. Fix the cause, not the bead.
It can. A weave wider than about 2–3× the wire or electrode diameter, or one that dwells at the edges, re-melts the toes while the center gets a thin deposit. Weave tight, dwell in the center, and watch both toes.
They are mirror images at the same toe. Undercut is a groove melted into the base metal that the weld does not fill — base metal removed. Overlap is weld metal rolled over the base without fusing — metal added but not bonded. Both are toe defects, both are stress risers, and both are fixed by the same angle, speed, and heat balance.