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Undercut is a groove melted into the base metal beside the weld toe but not filled with weld metal. It leaves a sharp, reduced section along one or both edges of the bead. In a stick weld, it usually comes from too much heat, excessive travel speed, an overly long arc, or poor electrode angle.
A small amount may be acceptable under a particular welding code, but visible undercut is a stress concentrator. It can reduce the effective thickness of the joint, trap slag, and give cracks a convenient starting point. On structural, pressure-containing, lifting, or safety-critical work, judge it against the applicable code rather than relying on appearance alone.
What Undercut Looks Like
Inspect the weld after it has cooled and all slag has been removed. Undercut appears as a narrow channel or groove running along the edge of the bead. The weld may look wide enough, but the parent metal immediately beside it has been gouged away.
Run a clean fingertip carefully across the joint, or use a small pick without forcing it into the weld. A sound transition should be reasonably smooth. With undercut, your finger or pick catches in a line beside the toe. A bright inspection light held at a low angle makes shallow grooves easier to see.
Do not confuse undercut with weld spatter, a slag line, or a concave fillet. Spatter sits on the surface and can usually be chipped or wire-brushed away. Slag may form a dark line that disappears after cleaning. A concave fillet has a curved face, but its edges are not necessarily gouged. Undercut is missing base metal.
How to Confirm Undercut
Start with visual inspection. Clean the joint with a chipping hammer and wire brush, then inspect both toes of the weld from several angles. If the groove continues along the bead and remains after cleaning, it is likely undercut.
For a measurable check, use a weld profile gauge or a small bridge cam gauge. Measure the deepest point of the groove, not the distance across it. Typical project limits vary, but many specifications restrict undercut to roughly 1/32 inch (0.8 mm) or less, while some allow more on particular joints. Always use the drawing, welding procedure specification, or governing code for the actual limit.
| What you see | Likely condition | Quick check |
|---|---|---|
| Groove beside the bead toe | Undercut | Remains after chipping and brushing; catches a gauge or pick |
| Dark, glassy line on top of the weld | Slag inclusion or trapped slag | May lift with careful chipping; inspect for a cavity underneath |
| Rounded metal droplets around the joint | Spatter | Sits above the surface and can be removed without exposing a groove |
| Low, smoothly curved weld face | Concave profile | Measure the fillet legs and throat; the toes are not necessarily gouged |
Why Stick Welds Develop Undercut
Excessive current is one of the most common causes. Too much amperage makes the arc dig aggressively into the edges while the puddle fails to fill the melted areas. The correct setting depends on electrode type, diameter, position, joint design, and machine polarity. As a starting point, a 1/8-inch E6013 may run around 90 to 130 amps, while a 1/8-inch E7018 commonly falls in a similar broad range. Use the electrode package and welding procedure as the primary guide.
Traveling too fast leaves a narrow bead and does not give the molten metal time to flow into the toes. This often produces a bead that is tall in the center but underfilled at the edges. Slowing slightly can help, but moving too slowly can create excessive buildup, slag entrapment, and overheating.
A long arc spreads the arc and increases spatter. It can wash the edges of the joint without depositing enough metal there. Keep the arc length close to the electrode core diameter; with a 1/8-inch rod, an arc around 1/8 inch is a useful starting point. Shorter is generally better than dramatically longer, provided the rod does not stick.
Incorrect electrode angle or weaving can direct force and heat toward one toe. A wide weave, rapid side-to-side motion, or pausing too long at the edges can melt the base metal unevenly. In many fillet welds, a work angle near 45 degrees and a travel angle of about 5 to 15 degrees are reasonable starting points, but joint access and position matter.
Contamination, poor fit-up, wind, and an unstable power source can make control harder. They are less often the sole cause of undercut, but they can produce a rough puddle that hides the problem.
How to Prevent Undercut
Clean mill scale, rust, paint, oil, and moisture from both sides of the joint. Set the machine within the rod manufacturer’s range, then make a short test weld on similar material. Watch the puddle, not just the arc. The molten metal should wet smoothly into both toes without digging a trench.
Use a short, steady arc and moderate travel speed. If the bead is narrow with visibly recessed edges, slow down slightly or reduce amperage. If the puddle is sluggish and the bead piles up, increase speed a little or adjust current within the permitted range.
For a fillet weld, a small triangular weave can help fill the toes, but a weave is not a cure for incorrect settings. Keep the weave narrow and pause only briefly at each edge. On thin steel, a straight stringer bead is often easier to control and produces less distortion.
Rod choice also matters. A forgiving general-purpose E6013 electrode is often a good choice for clean mild steel and beginners. For stronger, code-controlled work, low-hydrogen E7018 may be required, but it demands better storage and handling. A cheap rod is fine for brackets, repairs, and noncritical practice when its properties match the job; it is not a substitute for a specified electrode on structural work.
How to Repair Undercut
Do not simply lay another bead over a dirty groove. First remove slag, spatter, and any sharp unfused edge. For shallow undercut, a compatible filler pass may correct the profile if the welding procedure permits it. Use lower heat and enough deposition to blend into the surrounding metal without creating another groove.
Deep, continuous, cracked, or inaccessible undercut may require grinding back to sound metal and rewelding. Avoid grinding the base metal into a deeper notch. After repair, clean and reinspect the entire toe. On critical work, use the required nondestructive test, such as dye penetrant or magnetic-particle inspection.
Useful Inspection and Welding Gear
A basic weld profile gauge, bright work light, chipping hammer, and stiff wire brush are enough for most shop inspections. A gauge is inexpensive and more reliable than judging a groove by eye alone.
Protect your eyes and face while chipping and grinding. A properly fitted auto-darkening welding helmet with a grind mode is convenient, though a basic fixed-shade helmet can be perfectly adequate if you are switching it correctly. Wear safety glasses under the helmet, hearing protection while grinding, gloves, and flame-resistant clothing. Never inspect a hot weld by touch; let it cool, then clean it thoroughly before deciding whether the groove is a defect.