What Causes Undercut in Welding? Causes and Fixes

What causes undercut in welding is usually too much heat at the weld toe: amperage or voltage set too high, travel speed too fast, or the gun or rod held at the wrong angle. Turn the heat down, slow your travel, and hold a 10 to 15 degree travel angle, and most undercut goes away.

Undercut is a groove melted into the base metal along the edge of the bead that the filler metal never fills. It looks small, but it thins the joint right where stress is highest.

The fixes are the same for MIG, flux core and stick welding. Start with the most common cause and work down the list.

Undercut Quick-Fix Table

Symptom	Likely cause	Fix	DIY or pro
Groove on both toes, wide flat bead	Amperage too high	Lower amps or wire feed speed	DIY
Wide arc, groove on both toes, MIG	Voltage too high or arc too long	Lower voltage, shorten the arc	DIY
Narrow, ropey bead with grooves at the edges	Travel speed too fast	Slow down so filler reaches the toes	DIY
Groove on one toe only	Wrong work angle	Point the arc more at the undercut side	DIY
Grooves at the edges of a weave	No pause at the toes	Dwell briefly at each side	DIY
Undercut on a code or structural weld	Heat, speed or angle	Inspect, then repair to the WPS	Pro (CWI)

What Is Undercut in Welding?

Undercut is a groove or depression that runs along the toe of the weld. WeldGuru explains that it forms when the arc melts base metal and the filler doesn’t flow back to fill that space.

You can usually see it with the naked eye or feel it with a fingernail. On fillet welds it shows up on the top plate first, because gravity pulls the puddle down and away from the upper toe.

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What Causes Undercut in Welding: 7 Causes, Most Common First

1. Amperage Set Too High

Too much current melts more base metal than the filler can replace. Weld.com names the top cause plainly: “your amperage is too high.” In stick welding, that means the electrode runs too hot.

Fix it by dropping the amperage in small steps until the bead wets into both toes without a groove. On a MIG machine, wire feed speed sets the amperage, so lower that.

2. Voltage Too High or Arc Too Long

Higher voltage puts more heat into the weld zone and widens the arc. Xiris notes that excess voltage or arc length “broadens the arc cone and erodes the sidewalls.”

For MIG and flux core, lower the voltage a step at a time. For stick, hold a tighter arc. A long arc spreads heat unevenly and digs into the edges.

3. Travel Speed Too Fast

When you move too fast, the arc melts the toes but the puddle freezes in the center before filler can flow out to the edges. The bead looks narrow and tall with a groove on each side.

Slow down until the puddle fully wets both toes. WeldGuru puts the rule plainly: never weld in a hurry.

4. Wrong Work or Travel Angle

A bad gun or electrode angle heats one edge more than the other. One toe overheats and undercuts, and the other side freezes early. Undercut on only one side of a fillet weld is the classic sign.

Clause5 and Xiris both point to a 10 to 15 degree drag or travel angle. On a T-joint, aim the arc a little more toward the thicker or lower plate so heat doesn’t wash out the top toe.

5. No Pause at the Edges of a Weave

If you weave across the joint without stopping at each side, the arc melts the toe but never stays long enough to fill it. WeldGuru advises a short pause at each edge of the weave.

Move quickly across the center and dwell briefly at the toes. On wide joints, switch to stringer beads if the weave keeps undercutting.

6. Wrong Electrode or Wire Size

An oversize electrode needs more heat and dumps a large puddle that can wash the edges. An undersize electrode can’t supply enough filler to reach the toes. Both leave grooves.

Match the rod or wire diameter to the plate thickness and joint, then set amps inside the maker’s range on the box or data sheet.

7. Dirty Metal or Poor Joint Prep

Mill scale, rust and oil block fusion at the toes, so the filler doesn’t wet out. Xiris also lists sharp bevels and narrow joints as a cause, because the arc erodes the sidewalls.

Clean the weld zone to bright metal and open up tight joints so the arc can reach the root.

Why Undercut Weakens a Weld

Undercut thins the base metal right at the weld toe. Xiris calls it a stress riser and warns it can still grow fatigue cracks under cyclic loading, even when it falls inside code limits.

The groove also traps water and dirt. Over time that leads to rust and cracks at the toe, which is the worst place for them.

How Much Undercut Is Acceptable Under AWS D1.1?

Under AWS D1.1, the common limit is 1/32 in deep, with tighter or looser limits by loading and thickness. The Fabricator cites the code limit as 1/32 in, with an exception of up to 1/16 in for an accumulated length of up to 2 in in any 12 in of weld.

Clause5’s summary of D1.1:2025 Table 8.1 lists these visual limits:

Connection	Condition	Max undercut depth
Statically loaded	Material under 1 in thick	1/32 in (with accumulated-length exceptions)
Statically loaded	Material 1 in and over	1/16 in
Cyclically loaded	Primary tension members	0.01 in
Cyclically loaded	All other cases	1/32 in

Table numbers and wording change between D1.1 editions, so check the edition named in your contract or WPS. Pipeline work follows API 1104 instead, which limits undercut next to the cap to 1/32 in or 12.5% of wall thickness, whichever is smaller.

How to Repair Undercut

1. Clean the groove: brush or grind off slag and oxide so the new bead can fuse.
2. Fill shallow undercut: run a small stringer bead along the toe at slightly lower heat, as WeldGuru describes.
3. Remove deep undercut first: on code work, Clause5 notes D1.1 repairs remove the defect by grinding, air carbon arc gouging or chipping to sound metal.
4. Reweld to the WPS: use the approved procedure and filler.
5. Reinspect: check the repair to the same acceptance criteria.

Grinding out undercut only works when the joint is thick enough to spare the metal. On thin sheet, fill it instead.

When to Call a Pro

• The weld is on a structural, pressure, lifting or vehicle-frame part.
• The job falls under AWS D1.1, API 1104 or another code and needs inspection.
• Undercut appears on a cyclically loaded part, like a trailer hitch or a machine frame.
• You can’t tell how deep the groove is. A Certified Welding Inspector (CWI) can measure it with an undercut gauge.

How to Prevent It

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• Run a test bead: weld a scrap piece of the same thickness and check the toes before the real joint.
• Set heat in the middle of the range: start mid-range on the electrode box or machine door chart, then adjust.
• Hold 10 to 15 degrees: keep a steady drag or push angle.
• Keep a short, steady arc: a stable arc puts heat where you aim it.
• Slow down at the toes: pause at each edge of a weave.
• Clean the joint: remove rust, scale, oil and paint first.

Frequently Asked Questions

What Is the Main Cause of Undercut in Welding?

Too much heat is the main cause, usually from amperage set too high. High voltage, a long arc, fast travel and a poor gun or rod angle add to it. Lower the heat a step, slow down, and keep a 10 to 15 degree travel angle to stop it.

Is Undercut a Weld Defect?

Undercut is a discontinuity that becomes a defect when it exceeds the code limit. Under AWS D1.1, the common limit is 1/32 in deep, with tighter limits on cyclically loaded primary tension members. Shallow undercut within limits can still act as a stress riser on parts that see fatigue.

Can You Weld Over Undercut?

Yes, for shallow undercut. Clean the groove, then run a small stringer bead along the toe at slightly lower heat so it fuses into the base metal. On code work, deep undercut is usually removed to sound metal first, then rewelded to the approved procedure and reinspected.

What Is the Difference Between Undercut and Underfill?

Undercut is a groove melted into the base metal at the weld toe. Underfill means the weld face sits lower than the surface of the base metal because there isn’t enough filler in the joint. Undercut is a toe problem, and underfill is a joint-filling problem.

Why Do I Get Undercut on the Top of a Fillet Weld?

Gravity pulls the puddle down, so the top toe gets heat but little filler. Point the arc slightly lower toward the bottom plate, cut the heat a little, and pause at the top toe. Fast travel and a long arc make top-toe undercut worse.

Final Words

Undercut almost always traces back to heat, speed or angle. Lower the amps or voltage, slow down, hold a 10 to 15 degree angle, and pause at the toes.

On hobby work, a small stringer bead fixes most shallow undercut. On structural or code work, measure it against the right D1.1 edition and let a qualified inspector sign off.