What is Undercut in Welding? A Complete Discussion

While welding with different tools and techniques, it’s pretty common to come across various kinds of faults and problems. Amongst them, undercut in welding is a common fault faced by many welders out there.

But hey, every problem must have a solution, right? Not to worry, we’re about to go deep in this writeup and discuss all there is to know about undercut in welding. So, if you’re really into this, I’d suggest you go through the article step by step.

Let’s begin!

Defining Undercut in Welding

First thing you’ll need to know is, an undercut in welding is a defect that occurs during the process of welding. It’s basically the formation of a depression in the metal weld. It’s generally caused by under-filling or the lack of fusion in the process of welding.

The tricky part is, that this defect might be visible or invisible to inspection.

Undercuts are seen when the flux does not melt entirely in front of the puddle while one side of a fillet weld is formed. You’ll probably see this a lot with stainless steel. Since it takes more heat to melt than regular steel, it creates problems in proper fusion happening between both sides.

And there you go! An unwanted undercut.

Undercut Types

Undercut defects come in various types. Among them, the most common ones are probably the internal and the external one. Let’s discuss the variants in detail.

Internal Undercut

Internal Undercut

You’ll notice an internal undercut when the weld metal isn’t deep enough within the joint to cover the backside completely. On the backside, it creates a concave surface and can be caused for several reasons. These include poor joint fit-ups, improper efficiency in technique or insufficient wire diameter.

However, poor techniques may end up creating the appearance of the weld not penetrating at all. Therefore, the fact that it’s an internal undercut might get a bit confusing. So, don’t flinch in pulling it out and giving it a proper check.

External Undercut

External Undercut

Well, as the name suggests, an external undercut is just the opposite of an internal one. It happens because of too much deposit of weld metal on the weld joint’s face side. This in turn, causes a convex surface.

Too large of a wire diameter for application can be a primary cause. Or, in another case, allowing too much heat to build up in the puddle can be the cause of a “Blow through.”

If filler metal of a larger diameter than the base material or the sheet metal is used, it causes a blow through or an external undercut. So be careful; it often occurs when not enough attention is paid.

The Causes of Undercut in Welding

Causes of undercut in welding

Now that we’ve talked about the definition and the types of undercut issues. Let’s elaborate a bit on what actually causes it in the first place.

  • Use of Wrong Sized Filler Rods

When you’re making a tack weld, choosing too large of a filler rod can be the cause of an undercut. It’s unwise to continue without knowing the proper size of the filler rods. A good way to decide is to see if there is a 1/8″ gap between the two pieces of metal.

Then, in that case, use 030″ diameter filler rods, and for larger gaps, it’ll need .045″ diameter filler rods.

  • Improper Gas Shielding

Too much shielding gas or flux in the puddle is a BIG no when it comes to welding. It’s one of the prime reasons for an unwanted undercut. Filler rods have a certain amount or percentage of hydrogen in them. This reacts with the air’s oxygen to create water vapor.

It actually helps to reduce the oxidation of the metal. But excessive amounts can tamper with the fluidity of the weld pool. The weld pool can turn into a hot, sticky mess. We don’t want that now, do we?

  • Fast Travel Speeds

Keep a consistent arc length and keep a slow travel speed if you want to get things done right. Otherwise, the metal can begin to melt faster than the rod itself. This results in poor fusion and the formation of an undercut in the weld bead.

  • Poor Fit-up

Here we are, with yet another common cause for undercuts. Often, it’s found that there’s a gap between the pieces of metal that are to be welded together. Lack of proper measurement and fit up of the two pieces are the main culprits here.

When this occurs, you’ll be able to see a large gap between both sides, or one side will seem larger than the other. Voila! There’s your undercut.

  • Shield Gas Contamination

Contamination of the shielding gas is yet another commonly seen reason of undercut. Contaminated or dirty shielding gas inhibits or causes problems in the flow of the weld puddle. This results in the arc to burn through on top, leaving a gap between the two pieces of metal you intended to join together.

  • High Voltage

High voltage makes the arc heat up the weld puddle quicker than it can fill. A gap between the wire electrode and the weld puddle is formed, and thus, we see an undercut. Using lower amperage settings are advised when high voltages are used.

  • Inappropriate Filler Metal

When the welding arc melts the base metal before intended, an undercut occurs. This creates a noticeable gap between the filler metal and the melted plate. This is pretty often caused by a filler metal that’s a tad bit too large for the joint that’s too be welded.

It can also occur due to shooting an overly hot arc or traveling at speeds that are excessively high.

  • Gas Flow Inconsistencies

Undercut can also happen if the shielding gas is too low in amount. You might want to use a gas lens to smooth out the flow of the gas. Otherwise, it can cause too much turbulence around the arc due to the gas being blown away by air currents.

  • Welding Too Fast

Welding faster than needed can also cause undercut. If you do it too fast, the arc can melt both sides that can cool down into two separate pools before the filler rods melt in. The result will be a weak and inadequate weld, which we’d obviously want to avoid.

  • Too Much Arc Force

An excessive amount of arc force can cause an undercut as well. This forces the puddle further from the filer material, which leads to a gap between the two metal pieces.

  • Unclean or Rusty Metal

Arcs burn through rust real fast. Plus, they can leave a hole that’ll leave no penetration for the filler rod. This causes it to pop up. Unclean metal parts are hard to deal with as well. These also are the reasons for undercut in welding.

Repairing Undercut in Welding

To fix an undercut in welding, the first thing you should focus on is, finding the exact area where it’s located. However, the difficulty of the repair depends on the circumstances at hand entirely.

For example, a filler wire might just be enough to fill it up and make it strong again.

If it’s similarly something rather small like a groove on aluminum, then there is a high possibility of two points where the arc burnt through instead of numerous spots.

The issue at hand can be fixed by firstly rewelding those areas. Then applying some filler wires over the top will help out (Make sure you don’t use too much). But to ensure that it doesn’t happen again, you might use additional wires to compensate for the movement.


Now, let’s answer a few questions that are concerned with the undercut topic itself.

Does undercut waken a weld?

Yes, it does. Undercut in welds is known as primarily a defect. It almost cuts down the weld’s strength by half and disrupts the overall balance.

Is it possible to prevent undercuts?

Yes, it is indeed possible. You have to be well versed in the causes and make sure those don’t happen during the welding procedure. Precaution before welding is advised as well.

Can undercut on steel be fixed?

Yes, it can be fixed by rewelding the area properly.

A Few Parting Words

Like all defects, undercut too can be prevented and repaired. But the first thing you have to acquire is knowledge revolving undercut in general. I believe I’ve given you just that in this entire article. I really hope you enjoyed the read. I bet you found it useful as well.

So, let’s not stretch things any further and bid you farewell.

Hi! My name is Josh Mathews and I’m a professional welder residing in the state of Texas. As I’m a writer by heart, I believe it would be wonderful to share the knowledge I’ve gained over the years working in the welding industry, which worked as my initial motivation. I want others who read through our blogs to get inspired and enter the welding scene one day.

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