The welding operations are not as simple as it seems. If you tried welding for the first time, it most likely resulted in deformation. This may be due to a weak spot in the weld, which results in the weld being less durable than expected. As of now, we can tell you that the deformity is caused by undercutting in welding. Here we also explain what is undercut welding. The term undercut in welding refers to a groove that develops at the base or root of the metal.

The definition of a groove can be simplified to describe a long and narrow cut made in hard material. Welders of all levels are sometimes faced with this widespread problem. The practice of undercutting can cause several problems with your weld, resulting in a loss of productivity and money. In this article, we will provide a little more insight into what causes undercutting and how to avoid it. Also, check our other article that differentiates between good welding and bad welding.

What is Undercut Welding?

Undercutting occurs when under-filling or insufficient fusion of weld metal causes a depression on the face of the metal. Depending on the degree of depression, it may be visible or invisible to the naked eye. This occurs when the flux does not melt completely before the puddle while welding a fillet.

It mostly occurs in stainless steel because it takes more heat to melt this type of steel than regular steel, preventing proper fusion between both sides, resulting in an undercut. The result is what we regard as an undercut in welding. Also read this guide on what is overlap in welding.

Undercut Welding Types

Now you are well aware of what is undercut welding. So, let’s look at the types of undercut welding. Undercuts can be classified into different types. Internal and external undercuts are the most common types.

Internal Undercut

Weld metal that is not deep enough inside the joint to cover the joint’s backside is an internal undercut. As a result, the backside of the weld is concave, and the cause of this can be attributed to several factors, including unsuitable wire diameters for the welding application, a poor fit-up of the joints, or incorrect welding techniques. 

Welds performed with poor technique will usually appear to be not penetrating. Moreover, it can be difficult to tell if you have an internal undercut simply by looking at your weld bead, so don’t hesitate to remove it for inspection.

External Undercut

There is a difference between an internal undercut and an external undercut. An excess weld metal can cause a convex surface due to excess weld metal being deposited on the face side of the weld joint.

You may experience this problem if you use too large a wire diameter or allow too much heat to accumulate in your puddle and cause “blow-through.” It is common for filler metal with a larger diameter than the base material (sheet metal thickness) to blow through and cause an external undercut if you do not pay close attention.

Main Causes of Undercutting in Welding

If you constantly encounter an undercut during welding, there is something wrong with your technique. It is now clear to you what is undercut welding all about, so let us now look at what causes it.  It is also possible that the metal is overheating. There are several common reasons for undercutting, including:

Excessive Arc Maintenance

It is important to note that the appropriate arc length will depend on the electrodes. The metal portion of the core provides a starting point for the design. Ensure that the diameter of the metal portion of the core does not exceed the diameter of the arc. Using an electrode of 1/8-inch 6010, for example, you should hold it about 1/8 inch off the base material.

Selecting a Gas Field Incorrectly

In particular, when using MIG (active metal gas) welding processes, it is important to select the appropriate gas. The most common cause of undercuts is a wrong choice, so be careful not to make the wrong choice. The first step in preventing this is to assess the type and thickness of the material. Based on this information, you can determine the appropriate gas composition.

Rapid Travel Speed

If you travel too quickly with your arc, the metal will melt faster than the rod can be melted into it. Consequently, the weld bead becomes undercut due to poor fusion lines. You should slow down your travel speed and maintain a more consistent arc length to resolve this problem.

High-Voltage Use

As a result of the arc heating the weld puddle more quickly than it can be filled, high voltage welds produce an undercut. The result is the creation of an undercut between the weld puddle and the wire electrode. You should use lower amperage settings when welding at higher voltages.

Gases Contaminated with Shielding

Contaminated shielding gas is one of the most common causes of an undercut in welding. If you use a dirty or contaminated shielding gas, the flow of your weld puddle is inhibited, resulting in an arc burning through the top, which results in a gap between your two pieces that need to be welded together.

Inconsistent Flow of Shielding Gas

A lack of shielding gas is another cause of an undercut during welding. You may encounter this problem if you do not use a gas lens to smooth the flow of gas, which directs the shielding gas over your weld puddle and prevents it from being blown away by the air currents or excessive turbulence around the arc. Another possible reason is that your regulator is set too high, and you are wasting gas by not having a smooth flow out of the timeline.

Use of Wrong Filler Metal

It is common for an undercut to occur when the welding arc melts the base metal too rapidly, creating a space between the melted plate and the filler material. Often, this occurs when a filler metal is too large for the joint to be welded, an overheated arc is used, or excessive speed is maintained during the welding process.

Overspeeding Welding Process

If you weld too fast, it can cause an undercut in welding. In the case of filling large gaps between metal pieces, the arc will melt both sides, which will cool down into two separate pools before the rod melts into them. This results in an insufficient weld that is weak and does not penetrate the material.

How to Prevent Undercut in Welding

You are well aware of what is undercut in welding, now take a look at preventions. Similar to any welding defect, the first and most important step in preventing undercuts is determining what causes them. Identifying the cause of any welding defect is sufficient to eliminate it.

Moreover, you should also be familiar with preventive methods to reduce undercuts in your welds and know the basics. As it is now clear to you what is undercut welding and what are its causes. Let’s take a look at what can prevent undercut welding from occurring in the first place. The most effective preventive measures are listed below.

Welding in Passes

If you want to overcome undercutting in welding, you must do welding in passes. There are many advantages of using this technique. The primary benefit is that it will reduce the chances of undercut in welding. The fusion properties of the metal are stabilized through this process, resulting in a reduced rate of undercutting. 

Specify the Length of the Arc

It is possible to cause scattering and undercutting when welding at distances greater than the diameter of the rod from the molten metal. The electrode should not be more than an eighth inch away from the base when welding an item eight inches in diameter.

Using the Proper Weaving Technique

In addition to mastering the weaving technique, another way to prevent undercutting is to ensure your thread is tightly woven. Regular pauses between each side of the weld will minimize the amount of molten metal that splashes back into the welding area. When the electrode weaves at a high rate of speed, undercuts can also occur due to the high speed.

Use Shielding Gases in an Effective Manner

Imagine a situation in which molten metal comes into contact with air. When this occurs, it is essential to protect the metal from oxygen and other atmospheric gases so that the subsequent reaction does not result in the undercutting of the metal. A combination of gases should be used in the shield, according to the type and thickness of the metal being shielded.

Choosing the correct shielding gas is a tricky task for inexperienced welders. If you are one of those people who fall into that category, you can watch the video to learn more about choosing the right gas for your needs. It is vital for you to know because it helps you to understand what is undercut welding.

Ensure Correct Position and Angle

Numerous risks are associated with welding, but the angle at which you are welding is extremely important. So, you can never undermine the importance of position and angle while welding. If the angle or position is not right while welding, it can cause undercutting in welding. 

Improve the Efficiency of Travel

Occasionally, welding processes can fail as a result of low travel speeds. Low travel speeds cause undercuts, burn-throughs, incomplete penetrations, inter spattering, cracking, and other defects. The travel speed of the weld metal must be adequate to fill the joint area.

Preparation of Metal Surfaces

Metal surfaces that will be welded must be cleaned before welding to prevent flaws such as undercuts. Due to deposits such as rust, paint, moisture, and oil, certain surfaces interfere with the welding process. As soon as you reach the debris, stop, so that trapped bubbles are allowed to escape before the metal solidifies.

Stable Work Environment

Welders should use sturdy clamps when fastening structures for welding, minimizing the number of undercuts made during the welding process. To ensure that the motion path during welding is correct, you must have a solid working bench.

What are Undercut Welding Measurements?

It is vital to know the measurement of undercut welding because it helps you to get a clear idea of what are undercuts in welding. According to the rule, Undercuts shall not exceed 1/32 inches. Any 12-inch weld with an accumulated length of up to 2 inches is exempt from that rule, which requires an undercut of 1/16 of an inch. To sum up, remember that you cannot exceed 1/32 of an inch in depth with the undercut. Under industry standards and regulations, anything below that is considered acceptable.

As such, it should be your goal to avoid undercuts whenever possible. It is possible to learn the proper techniques and hone your skills. In most cases, undercut welds result from welders who don’t know how to adjust their technique to improve their welds. A beginner may need time and practice to improve, and that’s fine.

Undercuts should be measured with a bridge cam gauge. Undercut gauges can be inserted into gouges to tell how deep the undercut is and whether it is permissible.

How to Repair Undercuts in Welding

The decision of whether to repair an undercut can be a difficult one in the welding industry. Before deciding, it is essential to weigh the costs against the benefits. It is possible to repair an undercut, regardless of choice, and it can be an easy process depending on the severity of the undercut.

Weaving and stringer beads are two of the most commonly used techniques for filling the undercut groove. Additionally, it is possible to grind down the weld, but this could reduce the overall strength of the joint. A mechanically nicked or gouged surface adjacent to the weld metal is not permitted. Grinding, chipping, and gouging are also methods of removing metal.

As-rolled steel may be used. Removing any unwanted portions of the weld is essential to prevent the base metal from being substantially removed from the weld. Welding can only be performed on surfaces that have been thoroughly cleaned. Whenever there is a deficiency in size, weld metal must be deposited to compensate. A certain amount of undercut is acceptable in a weld, regardless of why the undercut cannot be repaired.

How Much Undercut is Acceptable?

For a professional welder, it is vital to know what is the acceptable limit of the undercut. A section should not have more than 0.5 mm of undercut if it complies with the recommended thickness for a section, and the undercut should not interfere with the minimum thickness required for the section.

It is unacceptable to have undercuttings deeper than 1/32 inch in depth. It is imperative for welders to know this range because it helps them to get a clear idea of whether it is undercut or not. A welder should also be familiar with what is arc blow in welding.


Your ultimate goal should be to produce a sturdy, high-quality weld with little to no undercut. If a weld is undercut, the integrity of the weld will be reduced, and it will also look less professional. To correct it, additional work must be performed. You are mistaken, if you believe that rushing through a project and undercutting are normal in welding. You are not increasing your productivity by doing undercut in welding operations. 

Take your time, pay attention to what you are doing, and practice your skills as much as possible. You will need to put forth a significant amount of effort to improve your welding skills. Although it is true that any welder can’t guarantee that he will never face an undercut in welding. But if you follow the above guidelines, there will be minimum chances of undercutting. Hope this article will help you to understand undercutting in welding.

Pin It on Pinterest