Why undercut happens in SMAW welding and how to prevent it

Outline (brief skeleton)

• Hook: Undercut is that sneaky groove at the edge of a weld, and it shows up when heat is not just doing its job.

• Define undercut in SMAW and why it matters for strength and appearance.

• The main culprit: current setting too high.

• How excessive heat input erodes the base metal edges and prevents proper fill.

• What the weld looks like when you’ve overcooked it.

• Quick look at other factors (electrode size, angle, base metal cleanliness) and why they’re not the primary cause but still matter.

• How to spot the problem and fix it on the fly.

• Practical tips to dial in the heat and keep the weld solid.

• A simple, rider-friendly checklist for your next weld.

• Final takeaway: understanding heat control saves more than just looks.

Undercut in SMAW: what it is and why it matters

Picture this: you’ve laid down a neat bead, you flip your helmet up, and there’s a little groove along the edge of the weld where the base metal looks a bit sunken. That’s undercut. In shielded metal arc welding (SMAW), undercut is a groove that erodes the edge of the base metal instead of getting filled with filler metal. It weakens the joint where the metal should be strong, so even if your bead looks smooth elsewhere, that edge groove can become a big trouble spot.

Now, what actually causes this unwelcome groove? The most common answer in a school setting is that the current setting is too high. Think of current as the volume knob for heat. When you push it too high, you push heat into the joint faster than the molten metal can fill, and the base metal edges start to melt away or erode. The result is a neat-looking bead with a notch or groove right at the weld toe. No filler metal has bridged that groove completely, so the strength of the joint can suffer.

Why is high current the main suspect?

High current means more heat per unit length of weld. In SMAW, you’re melting a lot of metal, and the edges of the weld pool want to flow away from the edge as you paste in filler metal. If the heat is excessive, the base metal edges can burn back faster than the filler metal can fill in. The rest of the bead might look fine, but at the toe—where the bead meets the base metal—you end up with that undercut notch.

This isn’t just a cosmetic issue. Undercut can concentrate stress, especially in load-bearing joints. A good bead with proper fusion and penetration should fill the joint evenly and leave a smooth transition from the weld bead to the base metal. When undercut appears, it’s a sign you may be overcooking the joint with heat input.

Other factors worth knowing (but not the main culprit here)

• Larger electrode size: An electrode that’s thicker can carry more current, which sometimes means more heat. But it doesn’t automatically cause undercut. If you’re using a bigger electrode, you’ll want to adjust your current to match the size and the metal thickness. Misalignment here can lead to other problems, but undercut is not inherently caused by electrode size alone.

• Electrode angle: The angle you hold the electrode can influence the shape of the weld bead and how heat is distributed. If the angle is off, you might get irregular beads or poor fusion at the edges, but again, the direct cause of undercut is typically excessive heat rather than angle alone.

• Base metal cleanliness: A dirty surface can lead to porosity or weak fusion, but undercut is more about how the heat interacts with the edges than about surface cleanliness by itself. Cleanliness matters for overall quality, just not as the primary trigger for undercut.

Let me explain with a simple mental model

Imagine baking a cake. If you bake it at too high a temperature, the edges burn before the middle sets. The center might look okay, but the edges are compromised. In welding, the bead is the “middle” and the base metal edges are the “edges” of the cake. When heat input is too high, those edges melt away or erode rather than being filled in by the filler metal. That is undercut: a sung into edge where the weld didn’t fill as intended.

How to diagnose and fix the issue

• Look for the telltale groove at the weld toe. If you see a clean notch along the base metal edge, that’s a strong sign heat input was too aggressive.

• Check your current settings against the electrode type and metal thickness. If you’re using a common SMAW electrode like E7018 for a moderate thickness joint, ensure your amperage is within the recommended range for that electrode and metal.

• Observe the bead profile. If the bead looks wide, flat, or you’re not getting good fusion at the toe, the heat might be too intense for the joint.

• Do a quick test weld on scrap metal. Start with a lower current, run a short bead, and inspect the toe. If the toe looks smoother and the bead appears well-fused with no groove, you’re on the right track.

Practical tips to dial in the heat and keep the weld solid

• Start with the manufacturer’s recommended current range for the electrode and metal thickness. If you’re unsure, a safe approach is to start on the lower end of the range and creep up as needed after a quick test bead.

• Use the right electrode size for the joint. If the joint is thin, don’t grab a heavy electrode just because you can. You want enough metal to fill the joint without dumping extra heat.

• Control heat input with travel speed. Moving a bit quicker reduces the time heat is in contact with any one spot, which helps prevent the edges from burning back. Think of it as keeping a steady pace rather than racing or stalling.

• Maintain a consistent angle and travel direction. A slight forward tilt helps ensure the bead fills toward the toe rather than eroding the edge. If your angle is too steep or too shallow, you’ll see other weld defects, which complicate things.

• Ensure the base metal is clean enough to fuse well, but don’t obsess over perfection. Rust, oil, or paint can be cleaned, but some mill scale is common on structural steel. The key is a clean, rust-free, oil-free surface.

• Let the filler metal do its job. In SMAW, the slag can affect heat transfer and how the bead solidifies. Don’t fight the slag; let it form and then chip between passes so you can observe the toe clearly.

• Get comfortable with a quick “bead test.” Before you tackle a full joint, practice on scrap with the same electrode and metal. Fine-tune current and travel speed until the toe shows a clean edge without a groove.

A practical, friendly checklist to keep on hand

• Are you using the correct electrode for the metal thickness?

• Is the current within the electrode manufacturer’s suggested range?

• Is your travel speed steady, not too fast and not too slow?

• Is the electrode held at a slight forward angle toward the direction of travel?

• Is the base metal clean and free of debris, oil, or heavy rust?

• Do you see a smooth toe without a groove when you inspect the bead?

• Have you done a quick test weld to verify the settings before committing to the joint?

A few closing thoughts

Undercut is one of those welding quirks that reminds you heat control is the real boss behind a solid weld. When you hear a line like “current setting too high,” you’re really hearing: you’ve given the weld too much heat too fast for the edges to keep up. The result is a groove where there should be metal. By dialing in current to match the electrode and metal, you give the filler metal a chance to bridge the joint cleanly and leave a strong, unified seam.

If you’re staring at a rejected toe in the middle of a project, don’t panic. Recheck the heat, adjust the current, and run a short test bead. You’ll likely see improvement quickly. The more you work with SMAW, the more you’ll sense the right balance of heat, speed, and angle. And when you nail it, that edge will blend in, the bead will look solid, and you’ll feel the confidence rise with your next weld.

Final takeaway

In SMAW, the most common path to undercut is excessive heat from too-high current. It’s a straightforward thing to manage once you know to tune the heat to the job. Keep an eye on your current, your electrode size, your travel speed, and your toe cleanliness, and you’ll keep undercut at bay while you build reliable, strong welds that stand up to real-world use.