Undercut is a groove melted into the base metal along the weld toe that doesn’t get filled by weld metal. It reduces the cross-section of the base metal at the exact point where stress concentrates, making it both a code violation and a structural weak point. The fix is straightforward: reduce heat, correct your electrode angle, and slow down.
Undercut is one of the easiest defects to see during visual inspection, and it’s one of the most common reasons for weld rejection on structural work. Every welder produces undercut occasionally. The difference between a good welder and a struggling one is recognizing it immediately and knowing how to adjust.
What Undercut Looks Like
Undercut appears as a narrow groove or channel along one or both toes of a weld bead. Run your finger along the weld toe. If you feel a depression where the weld meets the base metal, that’s undercut.
On fillet welds, undercut typically shows on one toe more than the other, depending on electrode angle. On groove welds, it appears at the cap toe where the weld face meets the base metal surface.
Toe Undercut vs. Root Undercut
Toe undercut occurs at the face of the weld where it meets the base metal. This is the most common type and the one visible during surface inspection.
Root undercut occurs on the back side of a groove weld at the root. It forms when the root pass melts too deeply into the base metal at the root edges. Root undercut is harder to detect because it’s on the inside surface (on pipe) or the back side (on plate).
Causes of Undercut
| Cause | Mechanism | Fix |
|---|---|---|
| Excessive amperage | Too much heat melts the base metal at the toe faster than filler metal fills it | Reduce amperage by 10-15% |
| Wrong electrode angle | Arc directed away from the toe concentrates heat on base metal without filling | Direct electrode into the joint, not away from it |
| Travel speed too fast | Weld puddle doesn't have time to fill the melted toe area | Slow down, especially at the toes during weaving |
| Arc length too long | Wide, spread-out arc melts a wider area than the puddle can fill | Shorten arc to one electrode diameter (SMAW) or reduce stickout (GMAW) |
| Wrong electrode size | Electrode too large for the joint requires excessive heat | Use smaller electrode, reduce heat accordingly |
| Excessive weave width | Wide weave spends too much time at center, not enough at toes | Narrow the weave, pause at each toe |
| Gravity (horizontal/overhead) | Puddle sags away from upper or outer toe | Angle electrode upward, reduce heat, faster travel |
Undercut by Position
Flat (1G/1F)
Undercut in flat position usually means too much heat or too fast travel. It’s the least likely position for undercut because gravity holds the puddle in place. If you’re getting undercut in flat, your settings are too aggressive.
Horizontal (2G/2F)
The most common position for undercut. Gravity pulls the puddle to the lower toe, leaving the upper toe unfilled. Fix: angle the electrode 5-10 degrees upward, pause at the upper toe during weaving, and reduce amperage slightly.
Vertical (3G/3F)
Vertical up welding can produce undercut if you don’t pause at the toes during the weave pattern. The puddle rolls over the toes without fusing. Fix: hold at each toe for 1-2 seconds, let the puddle wet out to the base metal before moving to the other side.
Overhead (4G/4F)
Overhead undercut happens when the arc melts the base metal but the puddle drips before it fills the groove. Fix: reduce heat significantly (15-20% below flat settings), tight arc, stringer beads instead of weave.
Measuring Undercut
Gauges
Bridge cam gauge: The most common tool. The pointed probe slides into the undercut groove, and a scale reads the depth directly. Measure at the deepest point.
Undercut gauge: A specialized gauge with graduated steps. Place each step into the undercut until one matches the depth.
Fillet weld gauge: Some multi-purpose gauges include an undercut measurement feature.
Where to Measure
- Check both toes of the weld for the entire length
- Measure at the deepest point of each section of undercut
- Record the location and depth for the inspection report
AWS D1.1 Acceptance Limits
| Loading Condition | Maximum Undercut Depth | Additional Notes |
|---|---|---|
| Statically loaded (non-tubular) | 1/32" (0.031") | In any direction |
| Cyclically loaded, transverse to stress | 0.01" (approx. 1/100") | Very tight limit due to fatigue |
| Cyclically loaded, parallel to stress | 1/32" (0.031") | Same as static |
| Tubular connections | Per Table 9.15 | Depends on loading and joint type |
The cyclically loaded limit of 0.01 inch is extremely tight. On fatigue-critical structures (bridges, crane runways, dynamically loaded frames), even minor undercut at the weld toe becomes a crack initiation point.
Why Undercut Matters Structurally
Undercut does two things that weaken a joint:
Reduces the base metal cross-section. The groove removes material right at the point where the weld meets the base metal. This is already a stress concentration point due to the change in section geometry.
Creates a notch effect. The sharp transition from the weld toe into the undercut groove acts as a stress riser. Under cyclic loading, this stress concentration initiates fatigue cracks that propagate into the base metal.
Even shallow undercut (0.01 to 0.02 inch) can reduce fatigue life by 50% or more compared to a smooth weld toe. This is why cyclically loaded codes have such tight limits.
Repair Methods
Stringer Bead Fill
The most common repair. Run a small stringer bead along the undercut using:
- Low amperage (10-15% below normal)
- Small electrode (one size smaller than the production weld)
- Arc directed into the undercut groove, not onto the weld face
- Consistent travel speed to produce a flat, even fill
The repair bead should fill the undercut to flush or slightly above the base metal surface without excessive buildup on the weld face.
TIG Repair
TIG is ideal for undercut repair because of the precise heat control:
- Use the minimum amperage needed to establish a puddle
- Feed a small filler rod (1/16 inch) into the groove
- Walk the puddle along the undercut, filling as you go
- Produces the most controlled repair with minimal distortion
Grinding (Cosmetic Only)
If the undercut is very shallow and the weld has sufficient reinforcement, grinding the weld toe to a smooth transition can be acceptable. This doesn’t add material but removes the stress concentration. Only valid if:
- The remaining throat/leg size still meets the minimum
- The code or specification allows ground repairs
- The undercut depth is marginal (just barely over the limit)
Prevention Techniques
Pause at the toes. During any weave pattern, hold at each toe for 1-2 seconds. This lets the puddle fill the toe area before you move back to the center.
Match electrode angle to joint. Point the electrode toward the area where metal needs to go. On a fillet weld, split the angle between the two plates. On a groove weld, direct the arc into the bevel face.
Control arc length. A long arc spreads heat over a wider area than the puddle can fill. Keep it tight: one electrode diameter for stick, 3/8 to 1/2 inch stickout for MIG.
Reduce heat for position. Every position change upward from flat needs a heat reduction. If you’re getting undercut in horizontal, you’re probably too hot.
Use stringer beads in overhead. Wide weave beads in overhead almost always produce undercut at the leading toe. Stringer beads give better control.
Undercut by Process
MIG Undercut
MIG undercut is most often caused by excessive voltage (which widens the arc and melts more base metal) or too-fast travel speed. Reducing voltage by 1-2 volts and slowing down usually eliminates MIG undercut. On thinner material, switching from spray transfer to short circuit gives better control.
Stick Undercut
Stick undercut results from long arc length (which spreads the heat zone), excessive amperage, or wrong electrode angle. E6010 electrodes are more prone to undercut than E7018 because of the more aggressive arc. Keeping the arc tight and pausing at the toes during weaving corrects most stick undercut.
TIG Undercut
TIG rarely produces undercut because the process has such precise heat control. When it does occur, it’s usually from excessive amperage or travel speed. The fix is straightforward: reduce amps or add more filler at the toes to fill the molten groove.
Flux-Core Undercut
FCAW undercut follows the same patterns as MIG. The additional factor is the slag coverage. Slag can mask undercut during welding, making it invisible until you chip and inspect. Always inspect both toes after slag removal.
Common Mistakes
Trying to fix undercut by running hotter. More heat makes undercut worse. Always reduce, not increase.
Ignoring one toe while watching the other. Both toes need attention. If the electrode angle favors one side, undercut forms on the other.
Not measuring. Undercut that “looks fine” might measure 1/32 inch or more. Use a gauge. Visual estimation is unreliable on shallow grooves.
Repairing undercut with the same settings that caused it. If your production weld produced undercut, don’t use the same settings for the repair. Drop the heat, use a smaller electrode, and slow down.
For other weld defects, see the porosity guide, lack of fusion, and weld spatter guide. For inspection techniques that detect undercut, visit the visual weld inspection guide. Return to weld defects or the welding techniques pillar.