What Is Undercut In Welding And How Do You Prevent It?

Undercut in welding is a groove that forms along the weld toe, where the weld metal meets the base metal. It happens when the welding arc melts away the base metal faster than the weld puddle can fill it back in. This leaves a shallow channel or depression that runs along the edge of the weld, and it weakens the joint because it reduces the thickness of the base metal at that point.

Preventing undercut comes down to controlling your technique and settings before you strike the arc. Proper travel speed, correct amperage, the right electrode angle, and good joint preparation all play a direct role in whether undercut forms. In many cases, undercut is a sign that the welder is moving too fast, using too much heat, or holding the torch at an angle that pushes molten metal away from the joint edge.

Undercut is one of the most common weld defects found during visual inspection. It is also one of the easiest to fix if you know what causes it. This article explains what undercut looks like, why it happens, and the practical steps you can take to keep it out of your welds.

What Does Undercut Look Like?

Undercut appears as a continuous or intermittent groove along the toe of the weld. The toe is the line where the weld face meets the flat surface of the base metal. Instead of a smooth transition from weld to base metal, you see a small trench or channel cut into the base metal.

In some cases, the groove is barely visible and requires a weld gauge to detect. In others, it is obvious to the naked eye, especially after the slag is removed. The groove runs parallel to the weld length and sits right at the edge of the weld bead.

Undercut can occur on one side of the weld or both sides. It can also appear on the root side of a weld, which is called root undercut. Root undercut forms at the bottom of a joint and is often harder to see without special inspection tools.

What Causes Undercut In Welding?

Undercut forms when the welding process removes base metal faster than the molten weld pool can replace it. The arc melts the base metal, and the molten puddle flows into the joint. If the arc is too hot or directed at the wrong angle, it digs a trench into the base metal that the weld metal does not fill.

Several specific factors contribute to this problem:

• Excessive current or amperage. Too much heat melts a wider and deeper area of base metal, making it harder for the puddle to fill the cavity.
• Travel speed too fast. Moving the torch quickly leaves less time for the molten puddle to flow into the melted groove and fill it.
• Incorrect electrode angle. If the electrode is tilted too steeply, the arc force pushes molten metal away from the joint edge rather than into it.
• Long arc length. A long arc spreads the heat over a wider area and concentrates melting at the edges rather than the center of the joint.
• Poor joint fit-up. Gaps or misalignment between the pieces being welded can cause the puddle to flow away from the edges.

Each welding process has its own common causes. In stick welding, a long arc and high current are frequent culprits. In MIG welding, travel speed and voltage settings matter most. In TIG welding, torch angle and filler metal addition technique play the biggest role.

Why Is Undercut Dangerous?

Undercut is dangerous because it reduces the load-bearing cross-section of the welded joint. The groove cuts into the base metal, which means the metal at that point is thinner than it was designed to be. When a load is applied, stress concentrates at the bottom of the groove, and that is where cracking often begins.

This is especially critical in structural applications such as bridges, pressure vessels, and heavy equipment. Fatigue loading, vibration, and cyclic stress can cause cracks to start at the undercut and propagate through the weld or base metal over time. In extreme cases, the joint can fail completely.

Many welding codes and standards have specific acceptance criteria for undercut depth. In general, undercut deeper than 1/32 inch (0.8 mm) is rejected in most structural applications, though exact limits vary by code and joint location. Some codes allow shallower undercut in non-critical areas, while others require complete removal of any visible undercut in high-stress zones.

How Do You Prevent Undercut In Welding?

Prevention starts with setting up the weld correctly before you begin. The most important variables are heat input, travel speed, and electrode manipulation. Get those right and undercut becomes rare.

Lower the amperage. If you are seeing undercut, the first thing to try is reducing the current by 10 to 15 percent. Less heat means less base metal melting, which means the puddle can fill the joint without digging a trench.

Slow down the travel speed. Moving the torch more slowly gives the molten puddle time to flow into the edges and fill any cavity the arc has created. The goal is to let the puddle wet out to the edges of the joint rather than pulling away from them.

Shorten the arc length. A tight arc concentrates the heat in the joint center and reduces melting at the edges. This is one of the simplest fixes, particularly in stick welding and TIG welding.

Adjust the electrode angle. In most positions, a drag angle of 5 to 15 degrees from vertical is recommended for stick welding. The exact angle depends on the process and joint configuration, but the principle is the same: direct the arc force toward the weld pool, not away from it.

Use proper joint preparation. Clean the base metal of rust, oil, paint, and moisture before welding. Contaminants can cause the puddle to behave unpredictably and can contribute to poor wetting at the weld toes.

Add filler metal at the right moment. In TIG welding, undercut often happens when the welder does not add enough filler metal as the puddle travels. Timing the filler addition so that it fills the leading edge of the puddle prevents the groove from forming.

Can Undercut Be Repaired?

Undercut can be repaired, but the approach depends on the severity and the application. Minor undercut within code limits can often be left as-is, though some inspectors require it to be ground smooth to reduce stress concentration.

For undercut that exceeds code limits, the standard repair is to grind out the defect and reweld the area. The groove is removed with a grinder, the joint is cleaned, and a new weld pass is laid over the area. This must be done carefully to avoid creating a new defect in the repair pass.

Grinding alone is not a repair. Removing the undercut by grinding makes the joint thinner, which does not restore the original strength. The area must be filled with additional weld metal to bring the joint back to its designed cross-section.

How Do Different Welding Processes Affect Undercut?

Each welding process has its own tendency toward undercut, and the prevention methods differ slightly.

In stick welding (SMAW), undercut is most often caused by excessive current, long arc length, or a steep electrode angle. Using a shorter arc and reducing amperage usually solves the problem. Weaving the electrode slightly at the edges can also help fill the toes.

In MIG welding (GMAW), undercut frequently comes from high voltage, fast travel speed, or an incorrect gun angle. Reducing voltage and slowing down are the first adjustments to make. Keeping the gun at a push angle of 10 to 15 degrees helps the puddle wet out properly.

In TIG welding (GTAW), undercut typically happens when the welder runs too hot or fails to add enough filler metal. Lowering the amperage and adding filler at the leading edge of the puddle are the most effective corrections.

In flux-cored welding (FCAW), undercut often results from excessive voltage or a travel speed that is too fast for the wire feed rate. Adjusting the voltage down and matching travel speed to the wire feed rate usually resolves it.