What Is Undercut in Welding?

Welding is not as simple as it sounds. The first time you tried welding something, it probably ended up deformed. It could be because of a weak point in the weld, making it not as durable as you wanted. Well, let us tell you now – the deformity was actually an undercut weld.

What is undercut in welding? Undercut (or undercutting) in welding is the term given to a groove that develops at the base or root of the metal. A groove, in simpler terms, is a long and narrow cut in hard material. It is a widespread problem, and even the most experienced welders sometimes face this issue.

Undercutting can actually cause a lot of problems with your weld, leading to a loss in productivity and money. In this article I’ll go a little deeper on what causes undercut and how you can avoid it.

What are the main causes of undercutting?

If you are frequently faced with an undercut, then there is something wrong with your welding technique. You could also be overheating the metal. A few common causes of undercutting are:

Maintaining too long of an arc

The appropriate arc length will vary with the electrodes. The metal portion of the core is a starting point. Make sure your arc does not exceed the diameter of the metal portion of the core.

For example, you should hold a 1/8-inch 6010 electrode at about 1/8 inch off the base material.

Improper selection of a gas field

Specifically for MAG (metal active gas) welding processes, the right selection of the gas is pertinent. A wrong choice is the primary cause of undercuts, so watch out for that.

To avoid this from happening, first, assess the material type and thickness. Then you will identify the correct gas composition accordingly.

Here’s a bit of advice for you: For carbon steel welding, a combination of carbon dioxide with inert gases works exceptionally well.


Trying to weld too quickly by using high temperatures is calling for trouble. By increasing the temperature, you get the main metal to melt faster.

Pay attention to the welding temperature. Watch out for thinner metals and reduce the current and arc speed accordingly. By investing a little bit of time and attention, you can avoid an undercut.

Improper electrode angle

If you want to master the craft of welding, then knowing your electrode angle and size is essential. Here are a few tips:

  1. Direct the heat towards edges and areas with thicker composition. Keep heat away from the free edges
  2. You should avoid using electrodes that are larger than is necessary
  3. Choose the appropriate thickness of the plate. If the melted metal becomes larger than the plate, it causes an undercut

How to fix undercutting

Even though undercutting is common, it is best to try and avoid it altogether. It is costly to fix and results in a reduction in speed and productivity. So, preventive measures are the way to go.

Undercuts 0.5 mm deep or lower do not need to be fixed. In low-carbon steel plates, you can even let go of a 10 mm thick cut. They are easily fixable, though.

For an external undercut, you can place a stringer bead along the entire length of the cut. In some cases, you may even need to use a grinder to blend the pieces together.

Internal undercuts and how to fix them

Internal undercuts typically occur at the butt joint weld near the base metal. They can also be found right next to the roof of the weld.

Another type of internal undercut is one found within a weld. It is a groove left near the sidewall of the roof of the weld.

Preventive measures for both internal and external uppercuts are the same. You can fix an internal undercut by weld beads. Place a weld bead within the weld area. These beads will form inclusions in the weld fixing the flaw.

You can also use the weaving technique to fix the flaw.

Weaving technique

Weaving is basically used to regulate heat in a weld puddle. To use this technique, all you have to do is pause at each side of the puddle. When using the weaving technique, make sure to hold the duct in the sidewall of the groove. Also thoroughly clean the work surface before and during welding.

Stringer vs weaving

Stringer bead and weaving are two techniques that you can use to fix an undercut. Different welders have different opinions on which is better. To help you understand the pros and cons of both, here’s a video.

Here are a few tips from Lincoln Electric’s Karl Hoes and Miller Electric’s John Leisner to help you ace your welding techniques:

Prep the surface

Always prepare the surface before starting the weld. Stick welding does provide a lot of leeway to dirt and rust. Do not use that as an excuse for laziness.

Make sure to clean the surface with a wire brush or grinder before welding. This removes dirt, rust, and grime from the area to be welded. A grinder also removes cracks on the surface. This makes for a uniform weld.

Work angle

Stick and wire welding require different angles. For wire welding, Hoes and Liesner recommend holding the gun at a 10 to 15-degree angle. Angle the gun in the same direction as which you are pushing the weld.

For stick welding, you have to drag the weld. Hold the gun at a 20 to 30-degree lead angle in the direction of the drag.

Arc spacing

For wire welding, the recommended distance is 3/8 to ½ of an inch.

For stick welding, the arc distance should not exceed the diameter of the electrode’s core. Typically this equals up to a distance of 1/8 inch between the rod tip and workpiece.


Travel speed too slow will get you shallow penetration. This will result in too much metal deposit. Travel speed too high produces a shallow weld. This creates a narrow and highly crowned bead.

Ideally, you can set the travel at 40 inches per minute for most joints. For wire welding, you should also watch out for the ridge. It should be approximately 3/8 of an inch behind the wire electrode, according to Hoes.

Gas selection

100% pure carbon dioxide is a tried and tested shielding gas that has proven its credibility. However, in certain situations, it is better to use a combination of gases.

  • For welding at high amperage levels: 75% argon and 25% Co2. This combination produces nice looking welds. Co2 alone causes a lot of splatters.
  • For metals with high mill scales or rust: 85% argon and 15% Co2. This combination is also ideal for heavier steel plates.
  • For spray transfer welding: 90% argon and 10% Co2.
  • Aluminum: 100% argon or an argon helium mixture
  • Stainless Steel: 90% argon, 7.5% helium and 2.5% Co2

Other common welding flaws to watch out for

Burn through

If you end up using excessive heat for thin metal plates, you can burn through the metal. It is a very common problem faced by welders. You need to be particularly careful with plates ¼ inches or thinner.

If a burn through does happen, you can quickly rectify the problem. Lower the voltage or the wire feed speed. Increasing travel speed also helps solve the problem, especially with aluminum.

Aluminum is prone to heat build-ups. When dealing with the metal, use a weaving technique. This minimizes heat input and reduces penetration risk.

Incomplete joint penetration

Insufficient heat input, improper gas mixture and wire diameter all lead to improper joint penetration. It occurs as a result of a shallow fusion between the base and weld metal at joining.

Using high wire feed speed alone or with high voltage can solve the problem. You can also use a lower travel speed. This allows larger amounts of weld metals to penetrate the base.

Incomplete fusion

Incomplete fusion occurs as a result of incorrect gun angle, insufficient heat or impurities. It is when the base metal and the weld metal or weld bead do not mix. It is also called a cold lap or lack of fusion.

The easiest way to avoid this is to clean your surface properly. Then determine the appropriate angle required. You can use the guide mentioned above.

Make sure to watch out for the arc. Increase travel speed as necessary to stop the arc from getting too far ahead of the weld puddle.

Lastly, ensure that there is enough heat to weld both the metals together completely.

Almost all welders, even the most experienced ones, face these welding flaws. Do not get disheartened. Eventually, you will become experienced enough to be able to avoid these issues with minimal effort.

With practice, you will know which angle to use, which selection of gas to use, and even what speed to maintain. For now, we say practice makes a man perfect. And before you start complaining, ask yourself. Did you take all preventive measures to avoid undercutting?