Undercut is a groove or channel melted into the base metal along the edge of a weld that doesn’t get filled back in with filler metal. It looks like a narrow trench running parallel to the weld bead, right where the weld meets the parent material. This missing material weakens the joint and, if deep enough, can lead to cracking under stress. It’s one of the most common weld defects across all fusion welding processes.
How Undercut Forms
During welding, the arc melts both the filler metal and a thin layer of the base metal along the edges of the joint. Normally, molten filler flows into those melted areas and solidifies to create a smooth, continuous profile. Undercut happens when the base metal melts away but the molten filler doesn’t flow in to replace it. You’re left with an incomplete groove at the weld toe, the point where the weld bead transitions into the base metal.
Whether the melt fills in properly depends on the temperature, the viscosity of the molten metal, and its surface tension. If conditions push the molten pool away from the edges or the metal cools before it can flow into the groove, you get undercut. The surface condition of the base metal also plays a role. Mill scale, rust, or contamination can interfere with how the melt wets and fills the joint edges.
What Causes Undercut
Undercut almost always traces back to one of a few welding parameters being off, or to the welder’s technique. The most common causes are:
- Travel speed too high: Moving the torch or electrode too fast means the filler metal doesn’t have time to deposit into the melted edges. This is especially common with new wire-feed welders who travel or weave too quickly.
- Excessive voltage or amperage: Too much heat melts more base metal than the filler can replace. High welding current is a frequent culprit in stick welding.
- Excessive arc length: Holding the electrode too far from the work spreads the arc, reducing penetration control and increasing the area of base metal that melts without being filled.
- Improper torch angle: If the electrode or torch isn’t centered on the joint, more heat goes into one side than the other. This often produces undercut on just one side of the weld.
- Overheated base material: When the base metal gets too hot from multiple passes or slow cooling, each subsequent pass melts more material than intended.
Stick welding is particularly prone to undercut because the welder controls so many variables by hand: arc length, travel speed, electrode angle, and weave pattern all happen simultaneously. MIG and flux-core welding can produce undercut too, especially when voltage is set too high or the welder moves too fast. TIG welding tends to give more control over heat input, but incorrect torch angles or excessive amperage will still cause the same problem.
Why Undercut Matters Structurally
The groove left by undercut acts as a stress concentrator. When the welded joint is loaded, forces don’t distribute evenly across the cross-section. Instead, stress piles up at the sharp notch of the undercut, sometimes several times higher than the average stress in the surrounding metal. This is the same principle that makes a small nick in a piece of glass cause it to crack along that line.
Fatigue cracks in welded structures most commonly start from local imperfections like undercut. Under repeated loading (vibration, thermal cycling, wind on a structure), that stress concentration drives a crack that grows with each cycle. The severity depends on the depth and shape of the undercut, the geometry of the weld bead, the type of material, and the stress range the joint sees in service. A shallow undercut on a lightly loaded joint may be harmless. The same depth on a bridge girder or pressure vessel could be the starting point of a failure.
How Much Undercut Is Acceptable
Most structural welding codes set specific depth limits for undercut. Under the American Welding Society’s D1.1 structural steel code, the rules work on a sliding scale. If undercut runs continuously along the full length of a weld, it can be no deeper than 1/32 of an inch (about 0.8 mm). For shorter, isolated spots of undercut on certain connections (stiffener plates, continuity plates, groove welds), the limit increases to 1/16 of an inch (about 1.6 mm), but only if the undercut doesn’t exceed 2 inches in any 12-inch length of weld.
So if you have a 36-inch weld with three separated spots of undercut, each in its own 12-inch segment, those spots can be up to 1/16 inch deep as long as none exceeds 2 inches in length. But if undercut runs the full 36 inches, the stricter 1/32-inch limit applies to the whole thing. Other codes for pipelines, pressure vessels, and aerospace work have their own limits, often tighter than D1.1.
Measuring and Inspecting for Undercut
Visual inspection catches most undercut. An experienced inspector can spot the shadow of a groove running along the weld toe, especially under good lighting at a low angle. But to determine whether undercut meets code, you need an actual depth measurement.
The standard tool for this is a bridge cam gauge, a small handheld instrument that sits across the weld with both feet resting on the parent material. A sliding pointer drops into the undercut groove and reads the depth directly. The key to accurate readings is making sure both feet of the gauge contact flat base metal, not the weld bead itself. Bridge cam gauges also measure fillet weld leg length, reinforcement height, and weld preparation angles, making them one of the most versatile tools in a welding inspector’s kit.
Preventing Undercut
Most undercut prevention comes down to controlling heat input and technique. Hold your electrode or torch at a consistent 90-degree work angle to the joint, with a 10 to 15 degree travel angle. For stick welding, that means dragging the electrode; for MIG, a slight push angle works better. This keeps heat distributed evenly across both sides of the joint rather than concentrated on one.
Slow down. Traveling too fast is the single most common mistake, especially for welders new to wire-feed processes. A smooth, steady travel speed gives the filler metal time to flow into the melted edges. If you’re weaving, keep the pattern tight and controlled. Wide, fast weaves pull heat across more base metal than the filler can cover.
Reduce voltage or amperage if you’re seeing undercut consistently. Sometimes the fix is as simple as using a smaller electrode. In stick welding, switching to a rod one size down reduces the heat footprint and makes it easier to keep the molten pool manageable.
How Undercut Gets Repaired
Because undercut is a loss of material, grinding alone doesn’t fix it. Grinding can smooth out the sharp notch, which reduces the stress concentration, but the base metal is still thinner than it should be. Under the D1.1 code, the approved repair method is to clean the undercut area and deposit additional weld metal to fill the groove back to the original surface profile.
In practice, this means lightly grinding the undercut to remove any contamination or oxide, then running a small fill pass over the affected area. The goal is to replace the missing material precisely, not to build up excess reinforcement. Overwelding the repair creates its own problems, including new stress risers at the toes of the repair pass. If the undercut is severe enough that significant base metal was lost, the damaged section may need to be gouged out entirely and re-welded according to a qualified procedure.