What is an Undercut in Welding and How Do You Prevent it?

There are many problems involved in the complex welding universe, one of which is the undercut.

In fact, this welding defect is very common and often occurs with the materials and joints used.

Therefore, those who are just starting out should have basic knowledge to help them deal with this unwanted blemish.

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

What is undercut?

Undercut at top of weld

An undercut is a recessive 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 it.

This points us to the adverse effects of undercuts. In short, undercuts weaken the joint in a number of ways.

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

For example, let’s say you’re welding two 6mm plates (0.236″ or 15/64″) together with a butt joint. Ideally, you want the weldment and the area around it to be at least 6 mm thick.

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

The undercut also reduces the strength of the joint by trapping water and dirt. The captured water accelerates corrosion in this already vulnerable area, causing premature mechanical failure of the part.

type of undercut

An undercut is classified as either internal or external.

External undercut

The outer undercut is visible at the toe of the weld. In a fillet weld, this is the front of the joint, visible during welding.

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

Interior undercut

The internal undercut is formed on the underside of a butt joint. Technically speaking, at the core of the weldment. It is also known as root undercut.

10 Quick Tips for Preventing Undercuts in Welding

Here are some tips and tricks you can follow to prevent undercuts in welding:

  1. Use the correct current: If you see long, frequent undercuts, try reducing the current. This minimizes overheating to achieve proper fusion.
  2. Use the correct voltage: Find the sweet spot of a voltage knob that produces no undercuts, a smooth finish, and a uniform bead profile.
  3. Never weld in haste: Stir the stick slowly to provide sufficient time for mixing and avoid rapid solidification.
  4. Use compatible fillers: Consult a chart or an experienced welder before choosing which filler material to use.
  5. keep a good working angle: Maintain an angle that deposits the filler evenly into the weld pool. You can judge this by looking at each side of the bead and noticing any differences.
  6. Choose the Right Electrode Size: A proper electrode fills your groove properly and delivers energy evenly.
  7. Use case-appropriate shielding: Make an informed choice based on the desired weld and ensure that the gas mixture is suitable for the job.
  8. Knit with a steady hand: When weaving, slightly delay your stay on the edges to ensure proper fusion at these high-risk points.
  9. Maintain a steady arc: If there is excessive splash or resistance in your motion then your arc length is wrong. Adjust it using these indicators and maintain it throughout the weld.
  10. Prepare edge well: Maintain the desired fillet grooves, smooth them, and clean them like a professional.

Main reasons for undercut

It should be clear from the above discussion that undercutting cannot be ignored. If you create this defect in your weld it can have serious consequences.

So, let us find out 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 necessary if it is too high, forming a deep weld pool.

The excess amount of liquefied metal flows down into the pool away from the boundaries. This causes pitting 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 prematurely moves away from the weld pool.

This means that there is not enough time for the molten metal to expand before it solidifies.

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

High arc voltage

Voltage, like current, is an indicator of the total heat input to the weld. As the voltage increases, more heat enters the weld area, and more base metal is melted.

The excess heat creates a larger cavity than necessary which is not always completely filled. This translates to undercuts appearing at the edges of the weld due to lack of sufficient filler material.

Electrodes and shielding

Undercutting is highly dependent on the choice of electrode and shielding gas. These factors play a major role in determining how the weld pool performs and whether undercutting occurs. Some important points are given below.

Improper electrode material

The filler material and the base metal must have similar thermal properties. If there is a significant difference between the two, you risk uneven heat distribution in the weld zone.

This not only leads to premature solidification, but also disturbs the wetting characteristics of the metals. Both are the major causes of undercut formation.

Electrode angle

Your work angle is critical in determining whether or not the weld will be defect free. This is also true for undercutting.

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

Wrong electrode size

Choosing the right size of electrode is not easy. If it is too large, excessive filler enters the pool and is deposited in the central part of the groove due to the high surface tension. Due to the larger stick diameter, the heat input is also more dispersed. Both causes contribute to undercutting.

A short electrode also creates problems. 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 don’t control the pressure according to weld requirements.

The gas may not adequately protect the weld pool from the environment. If air reacts with the molten metal, it impairs its wetting properties, heat transfer coefficient, density, etc. Each of these quality changes is associated with undercutting.

Welding techniques

So far, we discussed the things you need to decide before welding. Welders can fix these with a little help and some experience.

Next are the items you control when welding and purely depend on how much you practice.

Knitting techniques

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

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

Excessive arc length

Having a long arc length creates undercutting problems.

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

This means that some areas get more heat and melt properly, and other areas freeze quickly due to less heat.

This imbalance disturbs the mobility of the weld pool and causes undercutting.

Lack of advance preparation

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

If the surface is not machined to proper size prior to welding, its ridges and craters can also cause poor heat penetration, poor fluid flow and undercutting through over/under size.

What is the acceptable amount of undercut?

If you see a cut during your quality check, you need to decide whether it is acceptable.

There are two ways to deal with this situation.

If you are not following strict quality assurance regulations, you can make a decision based on visual inspection, joint type and thickness, and material strength. Generally, welders consider anything below 0.5 mm (.02 in) to be safe.

The second, more formal method is to consult the standard guidelines set forth 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 must keep an eye open during inspection. 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 trickier. The quality inspector needs to ascertain the total length of undercuts falling in this category. This includes long, single undercuts and shorter pieces and pieces. In a 12″ section, the total length of these should not exceed 2″ for the part to pass through.

Removal of undercuts for bad welds

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

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

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


What is the main reason for undercut in welding?

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

Does undercut weaken a weld?

The undercut may undermine the structural integrity of the joint. In most cases, these low thickness areas are the first to crack under loading. Undercuts can also reduce the strength of joints by trapping water and dirt in them. The accumulation of water in this already weak area accelerates corrosion.

How do you fix undercuts in welding?

Since undercuts are often narrow and long, using stringers or weaving beads are popular methods for filling them in. Another option is to thoroughly grind the defect with an angle grinder. This technique is only viable if the joint is thick enough to withstand the reduction in strength.


It’s a long read, but it’s a complicated topic. Avoiding the undercut isn’t easy. Even after lots of time and practice, your experience does not guarantee that you will never encounter this problem again.

So, you need to know how to deal with it like a pro when it happens. Also, practice as much as possible to keep undercutting to a minimum.

We hope you found this article informative and learned something from it. Please don’t hesitate to contact us if you have any questions or comments. Toolsworking is your favorite source for all your welding information.



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