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Undercut is a groove melted into the base metal alongside the weld bead and left unfilled. On structural steel, it reduces the effective cross-section of the joint and creates a sharp notch where cracks can start. It is usually caused by too much heat at the toes of the weld, but technique and joint preparation matter just as much.
Before welding, check the drawing, welding procedure specification (WPS), and governing code. Acceptable undercut limits vary by project, weld type, and inspection category. A weld can look sound and still fail visual inspection if the groove is deeper or longer than the specified limit.
Why Undercut Happens
The most common cause is excessive amperage for the electrode and position. High current melts the edges of the joint faster than the electrode can deposit filler metal. A long arc makes this worse by spreading heat and increasing arc force at the toes.
Fast travel can also leave the edges melted but underfilled. Conversely, moving too slowly with an oversized electrode may create a wide, fluid puddle that runs away from the toes. Excessive weaving is another common problem: the electrode pauses at the sides, erodes the base metal, and then crosses the center without depositing enough metal at the edges.
Other causes include a dirty or rusty surface, incorrect electrode angle, poor fit-up, and welding over a sharp edge. Moisture in low-hydrogen electrodes does not directly cause most undercut, but it can create hydrogen-related cracking and makes a structural repair more serious.
Set Up the Joint Before Striking an Arc
Remove mill scale, paint, oil, rust, and moisture from at least 1 inch on both sides of the joint, or farther if the project procedure requires it. Use a grinder or needle scaler, then wipe away dust and abrasive residue. Do not weld through primer unless the coating is specifically approved for welding.
Check the root opening, bevel angle, and alignment against the WPS. Excessive root opening encourages the welder to increase amperage and travel speed, which can produce an undercut or burn-through. A tight, uneven, or misaligned joint also makes it difficult to hold a consistent arc length.
Use the correct rod diameter for the material thickness and welding position. A 1/8-inch electrode is easier to control on many fillet welds than a 5/32-inch rod, while a larger electrode can improve deposition rate on thick plate when the joint and procedure permit it. Store low-hydrogen rods in a proper holding oven when required by the manufacturer or project specification.
Control Current, Arc Length, and Travel Speed
Start at the electrode manufacturer’s recommended amperage and adjust in small steps. As a rough starting point, a 1/8-inch E7018 often runs around 90–130 amps, depending on position, machine, and joint. A 3/32-inch E7018 commonly runs around 70–110 amps. These are starting ranges, not substitutes for the WPS.
| Symptom | Likely cause | Correction |
|---|---|---|
| Groove along one or both toes | Amperage too high, arc too long, or travel too fast | Reduce current slightly, shorten the arc, and slow enough to fill the toes |
| Wide, flat, fluid puddle | Too much heat or excessive electrode manipulation | Lower amperage and use a smaller, tighter weave |
| Undercut on the upper toe of a fillet weld | Electrode angle or gravity is pulling the puddle away | Adjust work angle, shorten the arc, and pause briefly at the upper toe |
| Ropey bead with poor sidewall fusion | Traveling too fast or using too little current | Slow down slightly or increase current within the approved range |
Keep a short arc, approximately the diameter of the bare electrode core. With a 1/8-inch rod, that means roughly 1/8 inch or less. A long arc increases spatter, reduces control, and makes the bead more likely to wash away from the edges.
Move steadily rather than chasing the puddle. For a fillet weld, use a work angle close to 45 degrees between the two plates and a travel angle of roughly 5 to 15 degrees. Watch the molten puddle, not just the arc. The toes should wet into the base metal without forming a dark groove behind the puddle.
Use the Right Bead Technique
Stringer beads are usually the safest choice for structural work. They produce a narrower heat-affected zone and make it easier to control the toes. If a weave is allowed, keep it narrow—often no more than about two to three electrode diameters—and pause only long enough at each sidewall to establish fusion.
For a large fillet or groove weld, use multiple smaller passes instead of one oversized pass. Clean all slag completely between passes with a chipping hammer and a grinder and flap discs where needed. Do not grind away a defect and bury it under the next pass. If undercut remains, fill it with a controlled repair pass only when the procedure permits it.
Maintain interpass temperature within the specified range. Excessive heat can make the puddle overly fluid and increase distortion. Let the joint cool as required rather than using water or compressed air, which can create hard, crack-sensitive areas in some steels.
Inspect Before Moving On
Stop after the first pass and inspect both toes under good lighting. A fillet weld gauge and inspection light help identify shallow grooves that are easy to miss while the steel is hot. Measure the deepest point, not the average appearance, and compare it with the drawing or applicable code.
For a minor defect, grind to sound metal with a smooth transition, then weld the repair using the approved process. Feather the ends of the repair so it does not create another abrupt notch. If the undercut is deep, continuous, cracked, or located at a highly stressed connection, stop and ask the responsible welding inspector or engineer before repairing it.
Good visibility also prevents technique errors. Use a properly shaded auto-darkening welding helmet with a clear lens, and keep the lens clean. Seeing the puddle clearly is cheaper than correcting a rejected structural weld.