Consequences of Welding Undercutting: What You Need to Know

welder setting up plasma cutter

Welding Buddy Experts

June 22, 2022
A man welding forming undercuts

What You Need to Know

You're in the middle of a welding project when you notice that your welds are starting to look a little uneven. Upon closer inspection, you realize that you've been inadvertently causing welding undercuts.

Welders know that the undercut is a defect that should be avoided, but it's often hard to do so. Undercuts are common issues that can occur when welding. However, it can lead to some serious consequences if not remedied. This blog post will discuss the types of undercuts, the consequences of welding undercuts, and how to prevent them. We hope this information helps you avoid any damage to your project!

A Closer Look at Welding Undercutting

Welding undercut happens when the molten welding pool metal does not completely fill the welding joint, resulting in a groove parallel to or near the weld metal. As a result, you can incur undercuts on your entire weld or portions of your weld. It is often caused by welding too quickly or using too much heat, resulting in the welding pool unable to penetrate the entire metal plate thickness.

Types of Undercut

There are two different undercuts types, and they show up in unique ways.

  • External 

Also known as a crown undercut, this type of undercut happens at the toe of your weld. External undercuts can occur when you join two metal pieces together at an angle (often at 90 degrees). This process is also known as a fillet weld.

  • Internal

Also known as root undercut, you will find this welding defect at the root of the weld or the bottom-most part of your weld. Welders will often find this type of undercut in butt welds, a process where you weld metal ends without overlapping them. 

A man measuring undercuts

How to Use a Welding Undercut Gauge

If you see pits on your weld, does it mean it's a weld defect and you'll have to start from scratch? According to AWS, undercuts are acceptable if these conditions are met:

  • The undercut is not more than 1/32 inches deep in its entire length.
  • Undercuts greater than 1/32 inches deep are not greater than 1/16 inches deep for every undercut length of 2 inches.

Thus, a weld gauge is a must-tool that every welder should have in their arsenal, so you can accurately measure undercuts or pits. 

To check how big or small an area of an undercut is:

  • Place your welding gauge on the base metal
  • Put the tip of the welding gauge pointer into the undercut 
  • Read the undercut measurement from the scale of the welding gauge
  • Use the retaining screw to hold readings for future references

Effects of Welding Undercutting

You can imagine your weld with undercuts like the worn-out roads with holes. If cars pass or it rains, the holes trap liquid and dirt, creating bigger holes. It's the same idea for your parent metal. Undercuts cause metal fatigue strength and speed up corrosion in your weld. 

Another effect is if the undercut is more than 6mm deep, it can compromise the joint's structural integrity and lead to cracking under loading and pressure. 

Like any welding defect, undercuts shouldn't be overlooked as they will cause large accidents if the undercut weld is used for infrastructures such as bridges or buildings. If you suspect that your welding equipment may be causing a welding undercut, it is important to have it inspected by a qualified welding engineer. 

Prevention Tips for Undercutting

Welding undercut can be prevented by following proper welding procedures and using quality welding equipment.

Be Comfortable

It's incredibly important to have a level welding surface to maintain the correct direction of motion, drag angle, and weld bead consistency. This will help you to produce a quality weld every time.

Make Sure You See Your Weld Metal

Welder's helmets that don't have auto-darkening features are difficult to use because the welder has to keep taking the helmet off to see what they're doing, resulting in welding defects such as undercuts or burns.

Get the Right Arc Length

Controlling your arc length is extremely important. Welding too close or too far away from the weld puddle will result in welding defects. An indicator that your arc length is incorrect is welding spatter. When the welding current is too high, it causes molten metal to spatter. Another indicator of an incorrect welding arc length is welding resistance. When welding, you should maintain a consistent welding speed. 

Lower Your Current

If instead of portions, you see long lines of undercuts, we suggest that you reduce your current. This will decrease overheating and help you get proper fusion—it also improves your welds’ appearance. Using a lower welding current can create a cleaner weld that is less likely to have imperfections. 

Consider Which Electrode You Are Using

While deeply penetrating electrodes are necessary for many applications, the welder needs to be mindful of their technique to avoid welding undercuts. If the arc length is too far or the weave width is too wide, welding undercuts will almost certainly be present in the weld. 

Some electrodes—with stick welding, for instance—are naturally more prone to undercutting than others. The welder needs to be aware of this and use the correct electrode for the application. 

Allow the Base Metal to Cool

You must be aware and patient when heating the base metal if you're welding. It will warp or distort from excess heat. You need to weld in small segments and then let the metal cool down for 5–10 minutes between welding. The waiting period will vary depending on the material and welding tool you use. If you're not sure how long to wait, err on the side of caution and give it more time to cool down. Otherwise, you risk ruining your work with a welding undercut.

Pause on the Weld Puddle

Welding takes a lot of patience and a steady hand, especially if you want to avoid welding undercuts. Undercuts can happen when the welding bead does not travel in the leading 1/3 of the weld pool, resulting in the heat from the arc being directed towards the molten puddle instead of the weld itself. 

Welding gauge with a weld metal


Welding undercuts can be minor or severe, and unfortunately, they can also lead to bigger welding problems like weak welds. Fortunately, you can prevent undercutting and other welding defects by paying attention to your welding equipment, technique, and speed. With a little practice, you'll be welding like a pro! 

Therefore, it's important to know the different types of undercuts, use a welding undercut gauge, and know what to look for when inspecting your welds. Check out our other blogs for more informative reads on welding.

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