What is Undercut in Welding, Causes and How to Avoid Them

Marcus Colson Last updated on September 4, 2023
Reading Time: 4 Minute

Welding is not as easy as it looks. If it is not done properly, it can cause some defects. One of the defects that often occur is undercut.

So, what is undercut in welding?

I am very excited to share my knowledge and experience about welding defects especially undercut.

So, through this article, I’ll explain undercut welding meaning, what causes it, and tips to prevent it.

What is Undercut in Welding?

First, let me define undercutting in welding.

Undercutting is a groove or depression that forms at the base of the weld joint. It develops along the edge of the base metal.

Then, how can it occur?

Undercutting happens when the weld metal melts the base metal, and the molten metal flows away, leaving a gap.

This groove or depression weakens the joint and can cause structural failure.

It also makes the weld look unsightly and reduces its aesthetic value.

Unfortunately, this defect not only looks bad. But it also affects the strength of the metal.

I’ll give you a simple illustration to understand this defect.

For example, I want to weld two metal plates so that they can form 8 mm of thickness. So, after welding these plates, the final thickness should be 8 mm.

However, when an undercut occurs on the weldment area, it will reduce the thickness and strength of the welded plates.

As a result, the undercut area may not be as strong as other welded areas. Therefore, it can be broken easily.

That is why undercut is bad in welding.

Read Also : Excess Reinforcement in Welding: Causes and How to Avoid

What is the Cause of Undercut in Welding?

After knowing the definition of undercut, let’s discuss what causes undercut in welding. Several factors can contribute to undercutting.

Mostly, undercut occurs because of three mistakes.

First, the welder may select the wrong welding parameters.

Then, it also can occur because of electrode and shielding.

Last, an undercut may appear because of improper welding techniques.

Let me explain these causes one by one. Understanding the causes will make it easier to prevent undercut formation.

Here are the main problems that cause a welding defect.

1. Incorrect Welding Parameters

Before starting the work, the welders must select the right parameters for a satisfying result. If these parameters are inappropriate, the defect may appear.

Here are several aspects that cause undercuts.

  • High Current

The first parameter that may affect the formation of undercut is high current.

The higher the current is, the more energy is distributed to the weld. So, the base of the metal may melt too much.

When the metal melts more than it is supposed to, it will form a gap.

Then, the excessed amount of melted metal will fill in this gap. As a result, the crater will appear at the edges.

  • Fast Moves

When the heat source moves too fast, the metal may not be able to spread evenly before it becomes solid.

Besides, it also can cause the freezing of metal in the cavity. That is why an undercut appears along the edges of the welded metal.

  • High Voltage

Another factor that can cause defects in your welding is high voltage.

When the voltage surges, the weld zone will be hotter. Therefore, the base metal will melt more than usual.

Besides, too much heat will create a bigger cavity.

2. Electrode and Shielding

Electrode and shielding also affect the process of undercuts formation. These are several aspects that will cause defects.

  • Wrong Selection of Material

To prevent undercuts, I always select the filler material with the same thermal properties as the base metal.

If their thermal properties are different, the heat distribution will be uneven. Then, it can cause advanced solidification and undercut formation.

  • Improper Electrode Angle

When the electrode angle is too steep, it can cause the molten metal to flow away from the base metal.

Then, it will create a groove.

  • Incorrect Electrode Size

If the electrode size is too small or too large for the thickness of the metal, it can result in an undercut.

3. Improper Welding Technique

The aspects that I have explained are something that you can adjust before starting the welding project.

But to ensure the welding is successful, you must ensure the technique is correct.

These are common problems that impact the final result.

  • Not Preparing The Surface

The welding surface must be clean and free of dirt.

So, the base and filler can blend well. That is why I always check whether the welding surface is properly shaped.

  • Too long Arc

I’ll tell you my experience when I use a longer arc. The longer the arc is, the more uneven the heat distribution.

So, some areas may melt properly. Besides, others solidify faster because they do not receive sufficient heat.

This will lead to undercutting.

Tips to Prevent Undercuts

After explaining some factors that lead to an undercut in welding, I now want to give you some tips on preventing this defect.

  • Proper Welding Technique

Use the proper welding technique to ensure the weld metal melts into the base metal without creating a gap.

Maintain the correct electrode angle, travel speed, and arc length to ensure a proper weld.

  • Use The Correct Welding Current and Electrode Diameter

Select the correct welding current and electrode for the type and thickness of the base metal.

This will ensure that the weld metal melts the base metal just enough to form a strong bond without creating a gap or depression.

Read Also : Burn Through Welding Causes and Tips to Avoid Them

  • Proper Joint Fit-Up

Use proper clamping and alignment tools to ensure the joint is tight and even. This will prevent excessive gaps or depressions, which can lead to undercutting.

  • Proper Cleaning and Preparation

Clean and prepare the base metal properly before welding. Remove any rust, paint, or contaminants that can cause undercutting.

Use a wire brush, grinder, or chemical cleaner to remove contaminants.

  • Post-weld Inspection

Inspect the weld joint after welding to ensure that there is no undercutting.

Use a magnifying glass or visual inspection tools to check for any grooves or depressions along the edge of the base metal.

To prevent any defects, you need to do these tips.

Also, consider the welding parameters and practice a lot to ensure the best result.

Now, you know what is undercut in welding.

As this welding defect can weaken the weld joint and reduce its aesthetic value, you have to prevent it by avoiding the causes and implementing the tips above.

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Marcus Colson
Marcus Colson

Welding is more than a hobby for me - it's a passion. The art of fusing metal together to create something new and functional never gets old. From intricate sculptures to sturdy structures, I love the endless possibilities that welding offers.

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