What Causes Undercut in Welding & How Do You Prevent It?

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The complex welding universe includes many problems, one of which is undercut.

In fact, this welding defect is pretty common and occurs with frequently used materials and joints. 

So, those just starting out must have basic knowledge of this unwelcome flaw to help them tackle it. 

This article covers what welding undercut is, its causes, solutions, and removal techniques.

What is Undercut?

undercut in a weld
Undercut on the top of the weld

An undercut is an accidental groove running along the weld bead. It is a long trough in the base metal at the edge of the weldment. I say trough because it can literally trap liquid inside of it. 

This points us towards the adverse effects of undercuts. In short, undercuts weaken the joint in several ways. 

Most apparent, an undercut causes an absence of material in areas where it is needed. This is unacceptable. 

For example, suppose you are welding two 6mm plates (0.236″ or 15/64″) with a butt joint. Ideally, you would want the weldment and its surrounding area to be at least 6mm thick. 

But if there is an undercut, the thickness will be less than 6mm at the weld toe or root. This compromises the joint’s structural integrity at these points. In most cases, these low-thickness areas are the first to crack under loading. 

Undercut also decreases a joint’s strength by trapping water and dirt. The captured water accelerates corrosion in this already weakened area, which causes premature mechanical failure of the part. 

Types of Undercut

An undercut is classified as either internal or external. 

External Undercut

weld defect under cut

External undercut appears at the toe of the weld. In a fillet weld, it is on the front of the joint, which is visible during welding. 

External undercuts are more common since the front surface is closer to the weld gun. 

Internal Undercut

root undercut

Internal undercut forms on the bottom side of a butt joint. Technically speaking, at the root of the weldment. It is also known as root undercut. 

10 Quick Tips to Prevent Undercut in Welding 

undercut in weld bead

Here are a few tips and tricks you can follow to prevent undercut in welding:

  1. Use the Right Current: If you see long, consistent undercuts, try lowering the current. This reduces overheating to get proper fusion. 
  2. Use the Correct Voltage: Find the voltage knob’s sweet spot that produces no undercuts, a smooth finish, and an even bead profile.
  3. Never Weld in a Hurry: Move the stick gradually to provide ample time for mixing and avoid early solidifying. 
  4. Use Compatible Fillers: Consult a chart or an experienced welder before choosing which filler material to use. 
  5. Keep a Good Work Angle: Maintain an angle that deposits filler evenly in the weld pool. You can judge this by observing each side of the bead and noticing any differences. 
  6. Choose the Correct Electrode Size: A reasonable electrode fills up your groove just right and delivers energy evenly. 
  7. Use Case-Appropriate Shielding: Make an informed choice based on the intended weld and ensure that the gas mix is appropriate for the job. 
  8. Weave With a Steady Hand: When weaving, slightly delay your stay at the edges to ensure proper fusion at these high-risk points. 
  9. Maintain a Stable Arc: Your arc length is incorrect if there is excessive spatter or resistance to your movement. Adjust it using these indicators and maintain it throughout the weld. 
  10. Prepare the Edge Well: Maintain the desired fillet grooves, smoothen them, and clean them like a professional. 

Main Causes of Undercut

welding undercut

From the discussion above, it should be clear that undercutting is not something to be ignored. It brings severe consequences if you create this defect in your welds

So, let’s explore the various causes of undercutting and how they play a role in its formation. 

Wrong Selection of Welding Parameters

In most cases, inappropriate welding parameters are the main culprit behind undercuts.

High Current

Current determines how much energy goes into the weld. The base metal melts more than required if it is too high, forming a deeper weld pool.

The extra amount of liquefied metal flows down into the pool, away from the boundaries. This leaves a crater at the edges. 

High Travel Speed

High electrode travel speed and undercutting have a very interesting relationship. If the travel speed is too fast, the electrode moves away from the weld pool prematurely.

This means that there is not enough time for the molten metal to spread out before solidifying. 

Moving the electrode (heat source) away too fast allows the metal to “freeze” in the center of the weld pool/cavity. This creates undercuts along the edges of your weld. 

High Arc Voltage

Voltage, like current, is an indicator of the total heat input into the weld. As the voltage goes up, more heat enters the weld zone, and more base metal melts. 

Excess heat creates a larger than required cavity which is not always completely filled. This translates to undercuts appearing on the sides of the weld due to a lack of enough filler material. 

Electrode & Shielding

Undercutting is highly dependent upon the choice of electrode and shielding gas. These factors play a major role in determining how the weld pool acts and whether undercutting occurs.  Some of the important considerations are detailed below.

Improper Electrode Material

Filler material and base metal should have similar thermal properties. If there is a significant difference between both, you risk uneven heat distribution in the weld zone. 

Not only does this lead to premature solidification, but it also disturbs the metals’ wetting characteristics. Both are leading causes of undercut formation. 

Electrode Angle 

Your work angle is crucial in determining whether the weld will be defect-free or not. This holds true for undercutting as well. 

An improper electrode angle causes uneven heat input into the weld area. One edge may overheat and melt into the weld pool, creating an undercut, while the other side might solidify early and cause the same flaw. 

Wrong Electrode Size

Choosing the correct electrode size is not easy. If it is oversized, excessive filler enters the pool and accumulates in the mid-section of the groove due to higher surface tension. Owing to the larger stick diameter, the heat input is also more scattered. Both reasons contribute to undercutting. 

A small electrode also creates issues. The filler metal is insufficient to fill the joint, creating an undercut at the edge of the bead. 

Incompatible Shielding Gas

Many things can go wrong if you use the wrong combination of shielding gases or do not regulate the pressure per the weld requirements. 

The gas may not adequately shield the weld pool from the environment. If air reacts with molten metal, it disrupts its wetting properties, heat transfer coefficients, density, etc. Each of these property changes is linked to undercutting. 

Welding Technique 

Up till now, we discussed the things you decide before welding. Welders can fix these with a bit of help and some experience. 

Next up are the items you control while welding and are purely dependent on how much you practice. 

Weaving Technique

weave bead motion
Weave bead motion: Move across the middle & hold on the sides

Weaving is used for a variety of weld jobs. A welder’s inexperience can cause undercuts at the points of the weave pattern that touch the groove fillet’s edges. 

This often happens because the user passes the electrode quickly by the joint’s edges. This motion creates localized high travel speeds that allow premature cooling of the molten pool. 

Excessive Arc Length

Holding a long arc length creates undercutting issues. 

This happens because a long arc does not provide evenly distributed heat energy to the weld zone. 

This means some areas receive more heat and melt properly, and other areas solidify early because of low heat.

This imbalance disturbs the weld pool dynamics and causes undercutting. 

Lack of Edge Preparation

Dirt or grit on the welding surfaces tends to block heat transfer. This causes undercutting due to a lack of fusion between the base metal and filler. 

If the surface is not ground to the proper shape before welding, its ridges and craters can also cause undercutting through bad heat penetration, bad liquid flow, and over/under-sizing. 

What is the Acceptable Amount of Undercut?

If you observe undercuts during your quality checks, you need to decide whether it is acceptable. 

There are two ways to tackle this situation. 

If you are not adhering to strict quality assurance rules, you can decide based upon visual checks, joint type and thickness, and strength of the material. Usually, welders consider anything below 0.5 mm (.02 in.) to be safe. 

The second, more formal way is to consult the standard guidelines set by the American Welding Society (“AWS”). 

According to AWS D1.1 Code, any undercut below 1/32” is harmless, even if it runs the entire weld length. If it goes above this, you need to keep an eye open during the inspection. The upper limit is 1/16”. Even a minute section of undercut deeper than 1/16” renders the part rejected. 

Within this range of 1/32” – 1/16”, the rules get a bit tricky. The quality inspector needs to find the total length of undercuts falling into this range. This includes long, single undercuts and smaller ones in bits and pieces. In a 12” section, the total length of these must not exceed 2” for the part to pass. 

Removing Undercuts for a Bad Weld

Once it is determined that an undercut is unacceptable, there are a few ways to fit it. Like other cavity-based defects, like porosity or surface craters, undercuts can be filled with new weld beads. 

Since undercuts are often narrow and long, stringing or weaving are popular methods to fill them up. The welder can decide between these methods by evaluating the width of the undercut; wider sections require weaving. 

Another option is to grind the flaw off with an angle grinder thoroughly. This technique is only viable if the joint is thick enough to withstand the decrease in strength. For thinner weldments, professionals sometimes add a filler layer after grinding to restore the original thickness.

Quick FAQs

What is the main cause of undercut in welding?

Undercut can occur for several reasons: Excessive heat from high current and voltage settings. If the travel speed is too fast, the electrode moves away from the weld pool prematurely. If edges are not prepared properly dirt or grit on the welding surfaces tends to block heat transfer which can cause undercutting.

Does undercut weaken a weld?

Undercut can weaken the joint’s structural integrity. In most cases, these low-thickness areas are the first to crack under loading. Undercut can also decrease a joint’s strength by trapping water and dirt. Captured water accelerates corrosion in this already weakened area.

How do you correct undercut in welding?

Since undercuts are often narrow and long, using stringer or weaving beads are popular methods to fill them up. Another option is to grind the flaw off with an angle grinder thoroughly. This technique is only viable if the joint is thick enough to withstand the decrease in strength.


It has been a long read, but this is a complex topic. Undercuts are not easy to avoid. Even with lots of time and practice, your experience does not guarantee you will never face this issue. 

So, you need to know how to deal with it like a professional when it does happen. Plus, practice all you can to keep undercutting to a minimum.

We hope that you find this article informative and learn something from it. Please do not hesitate to reach out to us if you have questions or comments. Weld Guru is your go-to source for all your welding information.

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About Jeff Grill

Jeff Grill hails from Long Island, a 118 mile stretch of land that starts just off the coast of Manhattan and stretches deep into the Atlantic ocean. He has always been interested in welding from an early age and has the cuts and bruises to prove it as he set out to work with a variety of metals.

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