How to identify an undercut in a weld for SMAW

Spotting Undercut in SMAW Welds: What It Is, How It Shows Up, and How to Handle It

If you’re working with Shielded Metal Arc Welding (SMAW) and paying attention to every bead like a detective, you’ll quickly learn that some defects are quiet performers. Undercut is one of those sneaky flaws that can slip by if you’re not looking closely. It isn’t dramatic like a crater or a crater crack—yet it can quietly undermine the strength of a joint. In the SMAW HT A School world, understanding undercut means you can stop it before it becomes a problem in real-life projects.

What is undercut, exactly?

Put simply, an undercut is a groove or crater at the edge or toe of the weld bead. It happens when the base metal near the edge of the weld melts away faster than the weld metal fills in. The result is a depression or notch right where the weld meets the base metal. Think of it as a tiny bite taken out of the workpiece at the edge, where the metal should be smoothly fused. This isn’t about color changes or splatter alone; it’s about material loss at the weld’s edge, which can weaken the joint.

The correct answer to “how can you identify an undercut in a weld?” is straightforward: it appears as a groove or crater at the edge of the weld bead. If you’re ever describing what you see to a coworker or a mentor, that’s the phrase to use. It’s precise and easy to verify on inspection.

How to spot it—the telltale signs you should trust

Undercut isn’t always obvious at first glance. Here are practical cues you can rely on during a typical SMAW inspection:

• The edge of the weld bead has a notch or V-shaped indentation right at the toe. This is the classic visual clue.

• The groove runs along the edge where the weld meets the base metal, not in the center of the bead.

• Sometimes the surface over the groove looks smoother or thinner than the surrounding weld metal, almost like a faint scar.

• In more severe cases, the groove can extend under the weld bead, creating a deeper notch that’s easier to feel with a file or pry bar.

• Discoloration nearby can show up, but it’s not a reliable indicator of undercut on its own. Don’t rely on color alone; you want the geometry at the toe.

A quick sensory check helps, too. Run your finger along the toe of the bead (gently, with PPE on). If you feel a noticeable depression or sharp edge along the weld’s edge, you’re likely dealing with undercut. It’s not a disaster right away, but it’s a defect you’ll want to address.

Why undercut matters—why you should care

You might wonder, “So what if there’s a groove at the edge?” Here’s the short version: undercut concentrates stress. That notch becomes a potential crack initiation site when the joint is loaded or vibrated. In everyday terms, think of bending a plastic straw with a small notch near the bend—the area is weaker than the rest. A welded joint with an undercut behaves similarly under load.

Beyond strength, undercut can affect fatigue life. If a structure endures cycles of stress—think bridges, frames, or pressure vessels—the notch can grow and propagate cracks over time. Visually, it may not be the worst-looking defect, but its mechanical impact can be meaningful. From a workmanship perspective, undercut is a sign that heat input, travel speed, or technique needs adjustment.

What causes undercut, and how to prevent it

Understanding the why helps you prevent it in future welds. Several factors commonly contribute to undercut in SMAW:

• Too high heat input for the joint: If the amperage is too high or the arc length is long, the weld may dig into the edges more than it builds up. The base metal at the toe gets melted away faster than the filler metal can fill it.

• Travel speed too fast: If you move the electrode quickly, the molten pool doesn’t have time to fill the edge as it should, leaving a groove as the bead toes off.

• Weaving technique too aggressive: Wide weaving or erratic motion can drag the pool away from the edge, creating a groove at the toe.

• Poor joint fit-up or dirty edges: If the edges aren’t properly prepared or they’re contaminated, the weld can form less fusion at the toe and leave a notch behind.

• Wrong electrode size or polarity for the joint: An electrode that’s too small or used with an incompatible polarity can affect wetting and edge filling.

• Arc length too long: A longer arc tends to heat and melt the toe more than the interior of the bead, pulling metal away from the edge.

Let me explain with a quick analogy: imagine painting a fence with a brush. If your brush is too big for the edge and you “overpaint” the edge, the paint near the boundary can wear away the underlying wood a little, leaving a thin line of wood showing beneath. In welding, the “edge” is the boundary between melted metal and the solid base metal; if you mismanage heat and motion, you end up cutting into that edge and leaving a groove.

Practical fixes and how to implement them

If you spot undercut, you’ve got a few reliable levers to pull. Here are practical steps you can take to fix the current weld and prevent recurrence:

• Check heat input: Reduce amperage or adjust your polarity to suit the electrode and joint. If you’re using too much heat, dial it back a notch and test with a small, controlled weld.

• Control arc length: Keep the arc short. A short arc reduces the excessive heating of the toe and helps the bead wet the edge more evenly.

• Moderate the travel speed: Slow down enough so that the molten pool can fill the edge as you move. If you were rushing, ease off a bit.

• Refine the technique: If you’re weaving, try stringer beads or a narrower weave near the toe to improve edge filling. The goal is to promote uniform wetting along the edge, not to scorch the metal.

• Prepare the joint properly: Clean and square up the edges. Remove any rust, oil, paint, or scale. A tight fit-up keeps the edges from opening up and losing base metal at the toe.

• Choose the right electrode and electrode angle: A slightly different angle (often more of a push or slight 15-20 degree tilt) can help the molten metal flow toward the toe rather than away from it.

• Inspect after each pass: If you’re multi-pass welding, check the toe after each pass. If you see any groove reappearing, adjust your next pass accordingly—don’t let small deviations accumulate into a larger undercut.

A quick checklist you can run in the shop

• Is the edge visibly grooved at the toe? If yes, that’s undercut.

• Am I using the correct amperage for the electrode and joint?

• Is my arc length short enough and my travel speed controlled?

• Are the edges clean, square, and well-fit?

• Am I applying a technique that encourages good edge wetting (stringer beads or light, controlled weaving near the toe)?

• Do I need a re-weld or a small rebuilding pass to fill the groove and restore the edge?

In many cases, a light re-weld over the groove with the right settings and technique will fill in the notch and produce a stronger joint. If the groove is deep, a multi-pass approach might be necessary to rebuild the edge and then finish with a proper cap bead.

Seeing the bigger picture—how to think about weld quality

Here’s a practical mindset you can carry into every weld: rough beauty isn’t the goal; reliable performance is. An undercut may look minor, but it’s a signal that the edge isn’t being fused as well as it should be. By paying attention to heat, speed, and edge preparation, you protect the joint’s integrity and its ability to hold under real-world conditions.

If you’re teaching someone else or collaborating on a project, explain it this way: the weld bead should appear as a continuous, evenly filled line along the joint toe. When you see a groove or crater there, you’re looking at undercut. The fix isn’t just cosmetic; it’s about restoring the edge to a place where it can support the designed load.

A few friendly tangents you might find useful

• The role of joint design matters. Some joints are more forgiving than others. A butt joint with a small root opening behaves differently from a fillet in a corner. Understanding the geometry helps you predict where undercut might occur and plan your technique accordingly.

• Tooling and workflow matter. The grinder is your ally once the weld has cooled and you’re preparing the surface for the next pass or for a final finish. Quick, careful grinding to remove the notch (without removing too much material) can save you from a failed joint later.

• Training and habit-building pay off. Practicing with a consistent setup—same electrode, same amperage, same travel speed—helps your eyes learn what a good edge looks like. Your future self will thank you for the muscle memory.

In the end, the ability to identify and address undercut is a mark of a thoughtful welder. It shows you’re not just making flames and beads; you’re shaping a joint that can stand up to real-world use. The next time you’re inspecting a completed weld, take a moment to scan the toe. If you see that groove or crater at the edge, you’ve found the telltale sign. And with a few careful tweaks, you’ll turn that notch into a solid, well-welded connection.

Final nugget to take away

Undercut is a groove or crater at the weld toe. It happens when the edges are melted away faster than the weld metal fills them. It weakens the joint by concentrating stress and can lead to cracks under load. Recognize it by its edge-notch appearance, adjust your heat and technique, clean and prepare the joint properly, and you’ll keep your SMAW beads sturdy and dependable.

If you’re curious about how this plays out in different metals and thicknesses, or you want quick, real-life examples of how small changes in amperage or travel speed shift the weld toe, I’m happy to chat. Welding is as much about feel as it is about numbers, and getting a feel for the edge is a vital part of mastering SMAW.