Undercut in welding: the beginner’s mistake

Welding is a complex process that requires a high degree of skill and precision. It involves joining two or more pieces of metal together using heat and pressure. However, even the most experienced welders can make mistakes, and one of the most common errors is undercutting.

What is an undercut in welding 1

What do we know about it?

Undercutting occurs when a groove is formed in the molten metal, which can significantly reduce the durability of the weld.

An undercut in welding is a serious problem because it weakens the joint and makes it more prone to failure.

It can also lead to cracks and other defects that can compromise the integrity of the weld.

This is why it is important to understand what welding undercut is and how to prevent it. In this article, we will explore the causes of undercutting and provide practical tips for preventing it.

Whether you are a novice or an experienced welder, understanding how to avoid undercutting will help you produce stronger, more durable welds that will stand the test of time.

Undercut in welding is the most frequent seam defect

An undercut is a groove on or in the seam that reduces the thickness of a narrow part of the joint. It can be caused by various factors such as improper welding parameters, poor surface preparation, or incorrect electrode manipulation.

An undercut in welding can also capture water and dirt, leading to corrosion. There are two types of undercut: internal and external.

What actions may lead to an undercut in the welding process?

Let’s take a closer look.

Internal undercut (root undercut)

Internal undercut occurs inside the layer of molten metal and represents cavities. It’s impossible to notice it visually since it’s hidden inside or under the beam. Ultrasound or radiography is the most informative way to notice an internal undercut.

External undercut

An external undercut forms on the outer side of a weld. It’s the most common type of undercut weld and can be easily detected via a visual control. As a rule, this type is formed when a weld gun comes too close to the surface.

To avoid undercuts, surfaces must be meticulously prepared, including making grooves as even as possible, wiping, cleaning, degreasing, and sandpapering them smoothly.

External undercut

What should I do?

The general recommendation for a regular butt joint is to move the electrode in a consistent, controlled manner to avoid internal undercuts.

Starting position

Begin by positioning the electrode at a 90-degree angle to the joint, and then move it forward at a steady pace, maintaining a consistent arc length and travel speed.


Avoid excessive weaving or side-to-side movement, as this can cause uneven heating and cooling of the metal, leading to undercuts.

Instead, focus on maintaining a straight, uniform path along the length of the joint, and pause briefly at the end of each pass to allow the metal to cool before continuing.

Finally, inspect the weld regularly for signs of undercutting, such as visible grooves or depressions along the edges of the joint, and make any necessary adjustments to your technique as needed.

Power characteristics

Proper voltage and no excessive heat are key to reducing the probability of defects. The wire electrode should be led with slow, accurate movements, filling a weld bead evenly and delaying the electrode on the most insecure spots.

What is an undercut in welding 6

Close to standards

It is important to use standard electrodes, shielding gas, and other materials for optimal conjunctions. Regular inspection and testing are also crucial to identify any undercut in welding early on and prevent joint failure.

By taking these precautions and following best practices and standards’ recommendations for welding processes, welders can ensure strong and reliable joints that meet industry standards.

Inappropriate welding parameters

Improper parameters of welding are the main cause of undercutting. High current, high travel speed, and high arc voltage are the primary culprits.

Shielding gas

The choice of proper shielding gas or even a combination of gases is also critical in determining the weld puddle (the weld pool) and whether undercutting occurs.

For instance, a carbon steel welding defect is less likely when some standard inert gas is combined with carbon dioxide gas.

Filler material

Improper filler metal, electrode angle, and wrong electrode size can all lead to undercut formation. Incompatible shielding gas can also disrupt the weld puddle wetting properties, heat transfer coefficients, and density, contributing to undercutting.

The weaving technique and other welding practices

The weaving technique and excessive arc length can also cause undercutting issues. Angles correct and convenient for stick welding don’t work for wire welding. If the wire welding requires a 10-15 degree angle, the stick welding needs a twice or thrice bigger angle.

Joint’s tidiness

Lack of edge preparation, such as dirt or grit on welding surfaces and improper surface shape, can cause undercutting, disrupting the weld pool wetting.

What is an undercut in welding 4

An undercut in welding may be appropriate

The amount of an undercut in welding that is acceptable in welding depends on the application and the welding code or standard that is being followed.

Generally, most welding codes specify a maximum depth and width of an undercut in welding that is allowed.

For example, the American Welding Society (AWS) D1.1 structural welding code specifies that the maximum depth of undercut should not exceed 0.0625 inches (1.6 mm) or 10% of the thickness of the weld metals, whichever is less.

It is important to note that the acceptance of an undercut also depends on its location. For example, an undercut on the surface of a weld is generally less acceptable than an undercut located in the root of the joint.

Internal or external?

The surface undercut can weaken the joint and increase the risk of failure, while the undercut in the root is less likely to affect the integrity of the weld.

Furthermore, the acceptance of an undercut can also depend on the type of material being welded its operating conditions, and what filler rod is used.

For example, an external undercut in a high-stress area of pipe welding is generally less acceptable than an undercut in a non-critical area.

Types of welding prone to undercut

Some types of welding, such as fillet weld, are more prone to undercuts than others.

In addition, some materials, such as stainless steel, are more sensitive to undercut than others and may require more stringent acceptance criteria.

In conclusion, the acceptable amount of undercut in welding depends on various factors such as the welding code or standard being followed, the location of the undercut, and the type of material being welded.

How to remove the undercut in welding?

It is essential to follow the appropriate welding procedures and techniques to minimize undercut and ensure the joint’s integrity.

Regular inspections and testing can help ensure that the welding meets the required acceptance criteria.

If a weld has undercuts, it is essential to remove them to ensure the integrity of the joint.

Here are some common methods for removing undercuts:

  1. Grinding is a common method for removing undercuts. This process involves using a grinding wheel or disc to remove the excess filler metal from the weld. Care should be taken not to remove too much material, which can weaken the weld.
  2. Chipping is another common method for removing undercuts. This process involves using a chipping hammer or a welder’s chipping tool to remove the excess material from the weld. Care should be taken not to damage the weld metal or the weld while chipping.
  3. The filling is a process that involves adding more material to the undercut area to build up the weld. This process is often used when the deep undercut cannot be easily removed by grinding or chipping.
  4. However, it is essential to ensure that the additional material is compatible with the original weld and that the correct welding procedures are followed.
  5. Welding over the undercut – this method involves re-welding over the undercut area to fill the groove. Care should be taken to ensure that the correct welding procedures are followed and that the new weld is compatible with the original weld.

It is important to note that the removal of undercuts should be done carefully and only by trained professionals. Careless removal can cause further damage to the weld metal, compromising the structural integrity of the component.

Regular inspection and testing of the weld after the removal of undercuts can help ensure that the weld meets the required acceptance criteria.

External undercut 1


This topic may seem complicated to you, but if you study the material in detail, you will understand everything. Undercuts are not easy to avoid. Even with lots of time and practice, your experience does not guarantee you will never face this issue.

Generally speaking, a welder should keep an eye on a weld puddle (the weld pool), trying to evenly wet all the surfaces of the base material and spread weld beads.

The closer he sticks to the standards (welding temperature, filler material, arc length, shielding gas) the less likely will an undercut weld be.


What causes undercut in welding?

An undercut in welding is one of the most common welding defects that occur when the weld bead creates a groove or depression in the base metal near the joint. This can weaken the weld and increase the risk of failure.

Undercut is caused by several factors, including improper welding technique, incorrect welding parameters such as high welding speed or high current, or an improper welding setup such as an improper electrode angle.

What is undercut in MIG welding?

In MIG welding, undercut refers to a groove or depression in the base metal near the weld joint. It is caused by the welding arc burning into the base metal, creating a groove or undercut.

It is important to follow the correct welding parameters, including the proper wire feed speed, voltage, and amperage, to prevent undercut in MIG welding.

What is acceptable undercut in welds?

The acceptable undercut welds vary depending on the welding code and application. In general, the depth should not exceed 0.5 mm or 0.02 inches.

What is undercut and how do you prevent it?

Undercut is a groove that weakens a weld joint. To prevent it, meticulously prepare surfaces, use proper voltage, and lead the electrode with slow, accurate movements. Use standard materials.

Christian Griffin

Since childhood, I have been fond of metalwork. I believe that this material, despite its strength, could be more malleable, if you know how to work with it correctly. I recently opened my shop where I sell tools for metalwork.

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