What is an Undercut in Welding?

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When welding, it is important to have a good understanding of undercutting. Undercutting is a process that helps welds join together more effectively, and it’s something you should know about if you want to improve your welding skills. In this blog post, we will explain what undercutting is and how you can do it yourself. Armed with this information, you can create better welds and achieve better results in your welding sessions.

Is Undercut in Welding a Defect?

An undercut is a type of failure in welds where the molten metal does not completely cover the desired weldment. It is a term used in welding when the bottom of the weld is not smooth. It can occur when the filler metal flows too quickly or when heat is not applied to the weld joint. In contrast, overcut occurs when too much filler material is used, resulting in a weld extending beyond the metal’s underlying portion.

Undercutting is considered a common problem in welding. It can create a weak spot in the weld that can eventually fail. Undercutting can also cause excessive heat and sparks, leading to weld failure. It occurs due to improper welding techniques, lack of quality control, or defects in the metal being welded.

The undercut is avoided at all costs because it can cause major problems with the integrity of the weld. Therefore, if an undercut is detected during the welding process, it should be corrected immediately to avoid potential problems.

Different Types of Undercut

There are many undercut types, but they all have one thing in common: they create a recess in the weldment that exposes more metal. The most common types of undercut are shoulder and root undercut.

Shoulder Undercut

Shoulder or external undercut in welding is the most common type of undercut. It usually occurs when the welds are too close to each other, causing a sharp transition between the area of the weld and the surrounding metal. This phenomenon can cause corrosion and other problems.

External undercut is accomplished by lowering the weldment below the surface of the workpiece. The resulting recess is created by pushing down on either side of the weld while welding. The disadvantage of this type of undercut is that it creates a weak spot in the weld that may require additional reinforcement.

Root Undercut

A root or internal undercut in welding is less common but more problematic because it creates gaps within the welds that allow moisture and other contaminants into the joint. An internal undercut is formed by making an incision into the workpiece itself; this technique allows for a deeper recess, which results in a stronger weld.

However, root undercut can be difficult to achieve because it requires accurate placement and control of the welding torch.

magnifying glass on welded steel

What Is the Main Cause of Undercut in Welding?

Many welders are familiar with the term “undercut” and know that it is a problem that can often occur when welding. However, the actual causes of the undercut are still being determined. This section will emphasize the main known causes of undercut welds and how to avoid them.

When experienced welders produce high-quality joints, they must consider the potential for the undercut. As we discussed above, an undercut is a condition in which the upper portion undercuts the lower portion of a weld bead. It can lead to instability and failure in the weld. Many factors can contribute to undercutting. Here are some of them.

One of the major causes is excessive heat generation, which can happen when welding with improper equipment or techniques. It can create a greater arc length, which results in more heat being applied to the metal and creates more chances for undercutting. Poor welding techniques can also lead to undercutting, as well as poor quality welds. When these factors are combined, they can cause significant problems for the integrity of the weldment.

High current and high travel speed also cause undercutting and produce a lot of heat in the welding arc. Eventually, this heat can cause the metal to expand, pushing against the weld bead and creating an undercut. The higher the current and speed, the more likely this will happen.

Another factor that contributes to undercut is high arc voltage, i.e. how much power is being applied to the welding arc. Higher voltages produce more heat, which can also lead to the expansion of the metal and undercutting of the weld bead.

Unfortunately, there is a lot of misinformation out there regarding electrode size and undercut welding. Many welders believe that the smaller the electrode size, the better. Unfortunately, that’s not always true! Using a too-small electrode can cause an undercut in the weld because the molten metal cannot flow freely over the joint.

This problem is especially common with MIG welding, where a small wire must make delicate connections. If the wire is too small, it can easily break under the pressure of the arc and leave an undercut in the weld. Over time, this tiny hole will become bigger and bigger until it causes major problems.

Ultimately, welders need to consider all these factors when welding so they don’t end up with an unstable joint that could eventually fail. For example, an undercut could weaken a weld.

Effective Ways to Fix Undercut in the Welding

There are a few different ways to fix an undercut, but the most effective approach is usually a combination of several.

The first step is to determine the extent of the undercut. Then, you can try to fix it with a welder’s torch if it’s just a small amount. It involves heating the weld and then using the flame to smooth out any bumps or ridges in the metal. Always be careful not to overheat the weld, or you’ll risk creating more problems.

If the undercut is more extensive, you may need to remove part of the weld layer using a wire brush or other abrasive tool. It will help expose the underlying metal so you can weld it back together properly. Remember to use caution; if you’re unsure how strong your weld layer is, don’t try to fix it using these methods!

In some cases, fixing an undercut isn’t possible, either because it’s too deep or because there’s too much damage. In these cases, you may need to replace the welder altogether.

How to Prevent Undercutting

There is a lot of debate about what constitutes an undercut in welding. Generally, undercutting is a condition where the weld metal is exposed below the surrounding material. It can lead to problems with the weld’s heat and integrity. There are several ways to prevent undercutting in welding:

  1. Use a consistent weld seam width. It will help keep the molten metals from flowing too closely together, and it will also ensure that there are no gaps beneath the welds.
  2. Position the pieces to be welded so that they are not skewed or tilted towards each other. If they are tilted, the molten metals will flow down into any gaps beneath the welds, creating the potential for undercutting.
  3. Always use enough flux when welding. Flux helps to create a smooth, solid surface on which to form the weld joint and prevents any metal from coming in contact with itself. Undercuts can occur if there is too little flux or if it is applied incorrectly.
  4. Make sure that your arc is properly controlled and lined up with your target area before you start welding. A poor arc can cause heat distortion and can lead to undercutting.
  5. Raise your welding gun’s temperature! Higher temperatures will help increase molten metal flow and prevent small droplets from freezing, which would cause smaller bubbles in the weld puddle and create more porosity.
  6. Use a thicker filler material to provide more support for the weldment so that it doesn’t deform as easily under stress.
  7. Increase your weld speed to prevent undercutting. Speeds above 60% should be used when working with thinner metals or during colder weather since these conditions increase the chance of pieces breaking off due to cold cracking.


To avoid undercutting, welders must be familiar with the main causes of this particular issue and make appropriate adjustments in their welding process. Always try to ensure that your welding process is safe and efficient; following proper safety procedures can minimize the likelihood of encountering undercut welding problems.

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