What Is Undercutting In Welding?

December 12, 2018

What Is Undercutting In Welding?


Undercutting is a common problem for welders, so I thought I would write an article for those who are new to welding. Welding defects can be in all sizes, shapes, and degrees. Welding defects are usually application-oriented or material-oriented, but there are other ways that defects such as undercutting can occur.  

So what is undercutting in welding? Undercutting is a groove that develops in the base metal near the toe or root of the weld. Undercutting happens when the weld metal fails to fill in that grooved area. This results in a feeble weld that is likely to have some cracks along the toes of the weld.

Undercutting is common and costly. It leads to a decrease in productivity and a reduction in speed. This is a problem that even the most skilled welder can create accidentally. There are several methods for welding operators to get rid of welding defects and return to work quickly.

In this article, I will go through the causes of undercutting, how undercutting can be prevented, and how you can fix some common undercutting problems.

What Are The Main Causes Of Undercutting?

Undercutting is one of the main issues that diminish welding quality. Preventing problems such as undercutting and its derivatives means saving time and money in welding operations.

An undercut is usually produced from overheating and poor welding technique.

  • Undercutting can be caused by maintaining to long of an arc length.
  • Another common reason for undercutting is the improper selection of a gas shield.
  • Also, an undercut can happen if a welder tries to weld quickly with a high welding current.
  • Rust can also cause undercutting so proper prep beforehand can help eliminate the problem.
  • Improper electrode angle.

The undercutting of steel has always been a serious concern in the steel industry, and in recent years a lot of effort has been made to solve the problem of undercutting.

An undercut typically happens when the main material and the weld metal do not fuse properly during a welding procedure. In other words, it happens when you fail to fill the corrugated area with the weld metal.

When this welding defect occurs, a weak source is prone to cracking.

So what can you do about undercutting?

How to Prevent Undercutting

1. Accurate Heat Input

One of the most important reasons for the undercutting during welding is the use of high heat during an operation near the free edges. This causes the melting of the main metal or the previous weld metal. To avoid this, reduce the welding current, which may require a reduction in arc speed. Also, pay attention to the welding temperature when approaching free and thinner edges.

2. Working at a Medium Speed

Fast welding is another cause of undercuts. When welding too quickly, some of the base metal becomes molten metal and it cannot be returned to its original shape due to rapid solidification. This creates a depression along the edges. The most ideal welding speed is medium. Very slow welding does not yield satisfactory results. Once again, being too fast can cause problems such as undercutting.

3. Correct Electrode Angle and Size

Knowing the angles of welding plays a vital role in an error-free welding operation. If one of the welding workers is adding more heat to the free edges at the wrong angle, then an undercut is likely to be produced. Remember to direct the heat to the edges and areas with thicker components compared to the free edges. You should also try to avoid using an electrode that is larger than necessary. An undercut can be produced if the size of the molten metal becomes too large for the thickness of the plate.

4. Correct Weaving Technique

Weaving is used to regulating heat in a weld puddle. Make sure to pause on each side of the weld to prevent undercutting on edges. In other words, when a welder performs a welding operation with a fast weaving technique, the problem of undercutting may occur. If using a weaving technique, hold the duct in the sidewall of the groove. A dirty work surface is another common cause of fusion deficiency. Appropriate and thorough cleaning of the surface before and during welding is a great way to find success in welding.

5. Gas Protection Selection

Choosing the wrong gas protection during welding is also one of the main causes of undercuts. The correct gas composition is congruent with the material type, source derivatives, and thickness. For example, the use of carbon dioxide with inert gases provides quality results during the carbon steel welding. If necessary, increase the shielding gas flow in the regulator to reduce the chances of creating undercuts.

6. Correct Welding Position

Sometimes welding in a horizontal and vertical position can cause welding cuts. An incorrect welding position can lead to an undercut due to insufficient metal filling along the connecting edges. Try to weld in a flat position to avoid this.

7. Use a Multi-Run Welding Technique

This is the best option to reduce the amount of lower segments, or undercuts, during welding. A multi-operation technique covers all of the above-mentioned problems. It helps to generate good resources with improved mechanical properties.

8. Use the Right Travel Speed

Using a travel speed that is too fast can cause an undercut. To prevent this issue, keep track of how much weave is used, carefully observe the speed of the weld, and try not to hold the electrode near the vertical plate when attempting to make a horizontal fillet weld.

9. Make Sure Your Arc Voltage is Not Too High or Too Long

An undercut can occur when the arc voltage is arc is too long or too high. Once again, too much heat in one area leads to undercuts.

How To Fix Undercutting Problems

An undercut can be corrected by placing a stringer bead for the entire length of the undercut. If necessary, you can blend it with a grinder to achieve desired weld profile.

Typically, undercuts do not need to be fixed if they are not any larger than 0. 5 mm deep. In a low-carbon steel plate, they can be left uncorrected if they are 10 mm thick, but not more than 1 mm deep.

What is Internal Undercutting?

This article is mainly about external uppercutting, which is the main type of uppercutting.

Internal undercutting, or root undercut, typically occurs in a butt joint weld near the base metal. They occur right next to the root of a weld.

In pipe welding, internal uppercuts can form near the inside of the pipe next to a narrow edge.

Another type of internal undercutting can be found within a weld. It is perceived as a groove left near the sidewall at the top of a root weld.

Internal uppercuts are fixed with weld beads that are positioned in the weld area. These beads can form inclusions in the weld.

The prevention methods for internal welding are similar to regular uppercut prevention methods. Pay extra attention to the roots to prevent internal uppercuts.


The undercut is created when there are no gaps in the connection between metals during welding. Undercutting is mainly due to the excessive spread of heat that creates gaps and depressions at the weld. Poor technique is also one of the main causes of undercuts.

This phenomenon weakens the source and may cause cracking later. There are many different reasons for undercutting. Some undercuts can be left alone, but some may require a repair.

Do you have any techniques you would like to share? Let us know in the comments below. Don’t forget to check out all of our Metal Fabrication Projects, and Fabrication Equipment plans, if you are looking to expand your metal fabrication shop capabilities you will want to check it out.

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