What Causes Undercut In Welding – How To Fix And Prevent It
Undercut is a welding defect where a groove is melted into the base metal at the toe of the weld and left unfilled by filler metal. It is primarily caused by excessive heat (amperage), improper torch angles, or moving the weld puddle too quickly across the joint.
To fix it, you must reduce your amperage, adjust your work angle to direct more heat toward the thickest metal, and slow down your travel speed to allow the puddle to fill the edges properly.
Finding a groove or a “ditch” along the edge of your weld bead can be incredibly frustrating, especially when the rest of the bead looks solid. We have all been there, pulling back the hood only to see that the base metal has been eaten away without being replaced by filler.
Understanding what causes undercut in welding is the first step toward achieving those professional, stack-of-digits beads that are both beautiful and structurally sound. This common defect is more than just a cosmetic issue; it creates a weak point that can lead to structural failure under stress.
In this guide, I will walk you through the technical and mechanical reasons behind this flaw. We will cover everything from machine settings to hand techniques so you can stop grinding out mistakes and start laying down perfect welds every time you step into the garage.
Understanding what causes undercut in welding
Before we dive into the fixes, we need to define exactly what we are looking at. Undercut is essentially a “missing” piece of the base metal at the toe of the weld. The toe is the point where the face of the weld meets the base metal.
When the arc is too intense or the puddle is not managed correctly, the base metal melts away, but the filler metal fails to flow into that empty space. This leaves a sharp, notch-like depression. In the world of structural integrity, this is known as a stress riser, which is a fancy way of saying “the place where the metal will likely crack.”
When diagnosing what causes undercut in welding, we usually look at three main categories: machine parameters, operator technique, and material preparation. If any of these are out of balance, the physics of the molten puddle will work against you rather than for you.
The Role of Excessive Amperage and Heat
The most frequent culprit behind undercut is simply having your machine set too hot. High amperage creates a very aggressive, fluid arc that digs deep into the base metal. While penetration is usually a good thing, too much heat causes the metal to liquidize and move away from the arc’s center faster than the filler can replace it.
If you are using a Stick welder (SMAW) or a TIG welder (GTAW), you have direct control over this heat. When the amperage is cranked up too high for the thickness of the material, the arc force literally “blows” the molten metal out of the groove. This is particularly common in horizontal fillet welds where gravity also plays a role.
To prevent this, always refer to a welding procedure specification (WPS) or a standard settings chart for your specific rod or wire diameter. If you see the edges of your joint melting away like wax before you even move the torch, it is time to reach for the dial and turn the heat down.
Travel Speed: Why Moving Too Fast Ruins the Bead
Your travel speed—the rate at which you move the torch along the joint—is the second most common factor in what causes undercut in welding. If you move too fast, the filler metal does not have enough time to “wet out” and flow into the edges of the path carved by the arc.
Think of it like pouring thick syrup on a plate. If you move the bottle quickly, you get a thin, broken line. If you move slowly, the syrup spreads out and fills the space. In welding, the molten puddle needs a fraction of a second to reach the edges of the heat zone.
When you rush the weld, the center of the bead might look okay, but the edges will remain hollowed out. This is a classic mistake for beginners who are nervous about burning through the metal. Instead of moving faster to avoid burn-through, it is better to lower the heat and maintain a consistent, steady pace that allows the puddle to bridge the gap.
Mastering Torch and Electrode Angles
The geometry of how you hold your torch or stinger significantly dictates where the heat goes. If your work angle is tilted too far toward one side of the joint, the arc will chew into that side while leaving the other side cold. This is a very common cause of undercut on the top plate of a horizontal lap joint.
In addition to the work angle, the travel angle (whether you are pushing or pulling the weld) matters. For example:
- Too much of a “drag” angle can push the metal toward the back of the puddle, leaving the front edges starved.
- An incorrect “push” angle in MIG welding can cause the arc to wander and gouge the side walls.
- Long-arcing, or holding the electrode too far from the work, spreads the heat out and makes the arc unstable, which almost always results in undercut.
Maintaining a tight, consistent arc length is critical. In Stick welding, you want the tip of the rod to be just a hair above the metal. If you pull it back, the voltage jumps, the heat spreads, and the undercut begins to form immediately along the toes of the bead.
Process-Specific Causes: MIG, TIG, and Stick
While the general principles remain the same, each welding process has its own unique quirks regarding what causes undercut in welding. Identifying these nuances helps you troubleshoot your specific setup more effectively.
Stick Welding (SMAW)
In Stick welding, the type of electrode you use plays a massive role. Rods like 7018 are prone to undercut if the arc length is too long. Because 7018 is a low-hydrogen, fast-freeze rod, the puddle doesn’t always flow as easily as a 6013. If you are weaving the rod, failing to “pause” at the sides of the weave is a guaranteed way to leave a groove behind.
MIG Welding (GMAW)
For MIG, the shielding gas and wire feed speed are the big variables. If your wire speed is too low relative to your voltage, the arc becomes long and erratic, “digging” into the metal without providing enough filler. Additionally, using a gas mix with too much CO2 can create a more violent arc that increases the risk of undercut compared to a smoother Argon/CO2 blend.
TIG Welding (GTAW)
TIG welding is all about the “dip” of the filler rod. If you are not adding enough filler metal to the puddle, or if you are adding it too late, the heat of the tungsten will melt the base metal away. This is often seen in outside corner joints where the heat has nowhere to go, causing the edge to “sink” or undercut if the filler isn’t introduced aggressively enough.
Material Selection and Surface Preparation
You might be surprised to learn that what you do before you even strike an arc can influence what causes undercut in welding. Contaminants like mill scale, heavy rust, or oil can interfere with the way the molten metal flows.
When the puddle encounters impurities, the surface tension changes. Instead of the metal flowing smoothly to the edges of the joint, it may pull inward or “bead up,” leaving the edges exposed and melted out. This is why professional welders spend so much time with angle grinders and wire brushes.
Furthermore, the thickness of your material matters. If you are welding a thin sheet to a thick plate, the thin sheet will reach its melting point much faster. If you don’t aim your arc toward the thicker “heat sink” plate, you will almost certainly undercut or melt through the thinner piece.
How to Identify and Measure Undercut
Identifying undercut is usually done through visual inspection. You are looking for a dark line or shadow running along the edge of the weld bead. If you can hook your fingernail into the groove, it is likely significant enough to require a fix.
In professional or industrial settings, we use a bridge cam gauge or a V-WAC gauge to measure the depth. Most welding codes, such as AWS D1.1, allow for a small amount of undercut (typically up to 1/32 of an inch), but for a DIYer or hobbyist, the goal should always be zero.
If you spot undercut, don’t just weld over it. You often need to clean the area with a grinder to remove any slag or oxidation trapped in the groove, then lay a small “stringer” bead specifically into that depression to fill it back up to the level of the base metal.
Practical Tips to Prevent Undercut Today
If you are struggling with this defect, try these three practical adjustments during your next practice session. They solve the majority of issues for garage welders.
- The “Pause” Technique: If you are weaving your weld (moving side to side), always pause for a “one-count” at the edges. This allows the filler metal to catch up to the heat and fill the toe.
- Watch the Puddle, Not the Arc: Stop looking at the bright light and start looking at the back edge of the molten puddle. You should see the metal “tying in” smoothly to the base plate. If you see a hole forming, slow down.
- Adjust Your Work Angle: If the undercut is only on the top piece of a joint, tilt your torch 5-10 degrees more toward that top piece. Use the arc force to “push” the metal into the corner.
Using a welding helmet with a clear, high-quality lens (like a True Color lens) also makes a world of difference. If you can’t see the toe of the weld clearly, you can’t possibly know if you are leaving an undercut behind until the slag is chipped away.
Frequently Asked Questions About what causes undercut in welding
Can undercut be fixed without grinding the whole weld off?
Yes, in most cases, you can fix undercut by cleaning the groove with a wire brush or a thin grinding wheel and then running a small repair bead over the affected area. You don’t usually need to remove the entire weld unless the undercut is exceptionally deep or the weld is structurally compromised.
What causes undercut in welding when using 7018 rods?
The most common cause with 7018 is long-arcing. If you lift the rod too far away from the puddle, the voltage increases and the arc becomes much hotter and wider, which melts the base metal edges without depositing enough filler. Keep that rod tight to the joint.
Is undercut always a reason to fail a weld inspection?
Not always, but it is a red flag. Most codes allow for very minor undercut (under 1/32″), but any undercut that is sharp or deep is usually a “reject” because it acts as a starting point for cracks, especially in parts subject to vibration or heavy loads.
Does the type of shielding gas affect undercut?
Absolutely. For MIG welding, using 100% CO2 provides deep penetration but can be “harsh,” leading to more undercut. Switching to a 75% Argon / 25% CO2 mix provides a much smoother arc and better puddle control, which significantly reduces the risk of defects.
Conclusion: Mastering the Puddle
At the end of the day, preventing undercut comes down to puddle control. Whether it is turning down the heat, slowing your hands, or fixing your torch angle, the goal is to ensure that every bit of metal you melt away is replaced by the filler rod or wire in your hand.
When you start paying attention to what causes undercut in welding, you stop being a “trigger puller” and start being a craftsman. It takes practice and a bit of patience to get the settings dialed in, but the result—a strong, clean, and professional weld—is well worth the effort.
Grab some scrap metal, turn your settings down a notch, and focus on that “pause” at the toes. Before you know it, those frustrating grooves will be a thing of the past, and your projects will be stronger than ever. Keep the sparks flying, stay safe, and I’ll see you at the workbench!
