Introduction to Undercut in Welding

The integrity of weld joints is vital in every application and industry. One common problem welders encounter that can compromise a weld’s quality is welding undercut. This defect, characterized by a groove or depression along the weld's edge, can weaken connections and promote corrosion. In this article, we delve into the essential techniques and strategies to prevent undercut in welding for seasoned welding professionals and beginners alike. 

Jump to a section: Causes of Undercutting | Type of Undercut | How to Prevent Undercutting | Electrode & Shielding | Prevention Checks for Undercutting | FAQ

What is Undercut in Welding?

Welding undercut is a weld discontinuity that occurs when the base metal has not been adequately filled back in from the applicable welding process and leaves a groove or a depression in the edge of the completed weld.

Undercutting can significantly impact the quality of welded joints, with its repercussions varying depending on the specific application. This undesirable groove in the weld bead can undermine the strength of the final weld, posing a substantial threat to the structural integrity of the joint. In applications like process piping, an undercut can also create a place for contaminants to build up, potentially accelerating corrosion rates beyond anticipated levels.

See the photo below for a visual example of undercut in a tubing to plate connection. Both filet welds and flare bevel groove weld joints were utilized in this assembly.

undercut tubing to plate connection

Dangers of Welding Undercuts

The presence of an undercut can pose significant risks and hazards to the quality and longevity of welded joints. It has the potential to compromise a weld’s structural integrity, reduce load-bearing capacity, and create premature corrosion. 

The biggest danger in undercutting a weld is that it reduces the load-bearing capacity of the joint. Undercut can create areas of stress concentration, making the joint more susceptible to cracks and fractures that could ultimately lead to total structural failure.

Causes of Undercutting in Welding

Undercut can be caused by a range of factors, individually or in combination. Excessive heat during the welding process can lead to undercut, as it causes excessive melting and erosion of the base metal. Likewise, an overly rapid travel speed can limit the time available for proper filler metal deposition, potentially resulting in undercut. Using an inadequate welding electrode or filler metal size in relation to the joint's dimensions can also contribute to this flaw. Lastly, improper joint preparation, such as excessive gaps or root openings, can create conditions conducive to undercut formation. Recognizing and addressing these causative factors is crucial for achieving high-quality, defect-free welds.

Issues with Welding Procedure

A properly written welding procedure should address all of the essential variables involved in the welding process; things like electrode size, material thickness of the base metal to be welded, amp and volt requirements, weld position and progression. If these parameters are not properly addressed and adhered to, undercut can result in the finished product.

Incorrect Welding Parameters

When amperage or voltage is too high for the materials being welded on, this means that more base metal will be melted than can be filled in. This is one of the most common contributing factors in undercut defects.

Inadequate Skills of a Welder

Beginner or novice-level welders may have a hard time developing techniques to eliminate excessive undercut in a welded joint. It can take time and patience to develop the right touch and “feel” necessary to produce a sound weld free from undercut. The properly performed weld is made by a combination of many different variables; travel speed, amps and volts, rod angle, arc length, and overall technique. For example, in weaving a vertical weld with 7018 electrode in the uphill progression, mistakes such as not holding long enough on the sides will invariably result in undercut. Gaining proficiency in welding techniques and recognizing the subtleties involved is essential for consistently producing high-quality, undercut-free welds.

Types of Undercut

Undercut can occur in both internal and external forms, with open root joint applications being particularly susceptible to this flaw. As with any potential welding defect, it is important to learn proper weld quality testing techniques to ensure the structural integrity of a weld. 

External Undercut

External welding undercut occurs on the outer surface of a welded joint. It is characterized by a groove or depression along the edge or "toe" of the completed weld, and it is typically visible upon visual inspection. This defect arises when the welding process does not adequately fill the joint, resulting in a concave or undercut area that can weaken the weld's load-bearing capacity and structural integrity. 

Note the undercut in the weld in the photo below especially at the termination point in the weld.

external undercut

Internal Undercut

Internal welding undercut occurs within the weld itself, beneath the surface, and is not visible through standard visual inspection. Unlike external undercut, which is observable on the outer surface of the weld, internal undercut may be hidden within the joint. It is characterized by a groove or depression that extends into the base material or the internal portions of the weld. As it may not be visible in a completed weld, it may be necessary to use a radiographic examination to determine if there is an internal undercut present.

The photo below shows an example of an internal undercut in the root pass of a butt-welded piece of pipe.

internal undercut

How to Prevent Undercutting

There are a number of methods and techniques that can prevent getting undercut in the completed weld joint. Choosing the right welding process for the joint being welded and understanding and adhering to the parameters in that welding process will make all the difference in achieving a sound weld, free of undercut.

Choosing the Right Welding Technique

Applying the right welding technique to prevent undercut depends on a number of factors, including the material being welded, the application, thickness, joint configuration, along with other considerations. Many commonly employed welding processes possess inherent characteristics and techniques that can be leveraged to mitigate or prevent undercut effectively.

TIG Welding

TIG welding is an excellent process that can be utilized in many applications to eliminate undercut. TIG offers a greater level of operator control than many other processes, which can eliminate the possibility of undercutting. By using the right size filler metal the welder can determine that the correct amount of filler metal is being deposited into the weld joint making it possible even to fill large gaps in a single weld pass with no undercut. 

Stick Welding

SMAW or “stick” can be utilized to make welds with the proper profile by choosing the right size electrode for the material thickness and position of the weld. In stick welding, techniques, like proper travel and work angle are vital to ensure a proper weld profile free of undercut. Arc length also plays a key role in achieving an undercut-free weld. Too short or too long an arc length will greatly affect the mode of arc transfer and the amount of weld metal being deposited into the weld.

MIG Welding

MIG welding can be used effectively to eliminate undercut by establishing proper welding parameters and monitoring them throughout the welding process. Regulating wire speed will help ensure that sufficient filler metal is being deposited in the weld joint. When properly performed in combination with all other factors, this will result in a weld free from undercut. 

Given that MIG welding is “semi-automatic,” the welder will have limited ability to change welding parameters once the arc has been established. Ensuring a precise fit-up of the weld joint with minimal deviations in factors like root opening, bevel angle, and others is crucial.

Adjusting Welding Parameters

Achieving a sound weld with the proper profile will be determined by a skilled operator adhering to the parameters of the welding process. Before the welding begins, factors such as material type and thickness, weld joint configuration, welding positions, welding process, and electrodes and filler metals to be used must be evaluated and considered. 

Right Amperage Proper Voltage

The current used in welding is determined by the proper setting of either amperage in SMAW and TIG welding or voltage in MIG or FCAW welding. Too many amps or volts will result in too much “heat” in the weld puddle, which will mean more base metal is brought into the molten state and not enough filler metal is then deposited which can only lead to undercut. 

Appropriate Travel Speed

The correct travel speed is critical in achieving a proper weld profile free from undercut. If the travel speed is too fast there won't be sufficient weld metal deposition in the joint, resulting in the potential formation of undercut. Striking the right balance between travel speed and weld deposition rate is essential for a successful weld.

Skilled Handling of the Torch, Electrode Holder, “Stinger,” or MIG Gun

One of the greatest contributing factors that lead to undercut is an improper work angle or travel angle. This angle, which may vary depending on the welding process and the welding position, influences how much molten weld metal is deposited in the joint. An incorrect angle can lead to a poor weld profile, often resulting in undercut. 

The angle of the electrode plays a crucial role in determining how the molten weld metal is deposited into the joint. The specific welding process and the welding position also influence this factor. An incorrect electrode angle can lead to an inadequate weld profile that fails to fully fill the joint, frequently resulting in undercut.

Using Proper Equipment and Tools

Of course, having the right tools and equipment to prepare a weld joint will help to achieve a sound weld. If the joint requires a bevel to be made at a specific angle, then you must make sure that you have a torch, grinder or specific tool capable of making the bevel correctly. This may even include tools for measuring and verifying the angle. A poorly prepared weld joint with an improper angle may be difficult to fill in adequately which can often lead to indications of undercut.

Electrode & Shielding

Within the welding process itself there are some factors that can contribute to the formation of undercut in the finished weld.

Improper Electrode Material

It’s critical to ensure that the electrodes you chose are compatible with the base metal being welded. Each type of electrode has a specific coating composition that determines its performance. If an electrode is used that is not metallurgically compatible with the material being welded, it may not allow for proper penetration and fusion of the metals which could then lead to undercut.

Electrode Angle

A poor work angle can gouge out too much base metal and make it difficult to fill in the weld joint properly, resulting in undercut. In most cases, the travel angle should be a slight “drag” angle, approximately 10 to 15 degrees, to allow the weld metal to fill back in the weld joint. (It’s important to note this is a general guideline and may vary depending on a number of different factors such as weld position, material thickness, joint configuration and welding process.

Wrong Electrode Size

Trying to fill a large weld joint with an electrode that is too small can lead to undercut by not allowing the welder to deposit enough weld metal into the joint. Even attempting to correct this by slowing down the travel speed can result in staying too long in the molten weld puddle which can and often does lead to undercut. Make sure you are using the correct size welding electrode and filler metal for the material thickness of the parts being welded.

Incompatible Shielding Gas

Certain welding processes and materials to be welded may have very specific shielding gas requirements. For example some welding processes on certain types of material may use argon, helium, nitrogen while some others may use carbon dioxide or a mixture of shielding gasses. Using a shielding gas that is not compatible with the welding process and the material being joined can adversely affect the stability of the arc and result in inadequate protection which could, in turn, lead to undercut.

Prevention Checks for Undercutting in Automated Welding

In automatic welding, making adjustments to the welding parameters after starting the process can be difficult, often resulting in having to stop and restart the operation altogether. This is a time-consuming and complicated process, so it is important to check all the determining variables prior to the start of welding operations. 

Setting and Checking Machine Parameters

Great care should be taken when configuring welding parameters to ensure that the amps, volts, wire speed and travel speed, among others, have been checked and are in compliance with the welding procedure.

Proper Maintenance and Calibration of Welding Machines

Before welding begins, it is important to check the equipment to ensure it is in proper working condition. The wire feeder and the rollers that feed the wire should be checked for proper tension. The liners in MIG guns should be changed when necessary to allow the filler metal to be fed into the weld puddle at a consistent rate. 

Verify the flow of shielding gas and make sure the regulator is correctly adjusted. Additionally, inspect the gun or torch for build up of debris that might impede the gas flow. Check the hoses for signs of damage that could affect the flow of shielding gas.

Also, consider checking the lead and ground cables to make sure they are not worn and frayed. Ensure that the connections are sound and snug so there will not be a loss of current at the weld puddle that will adversely impact the welding process.


 Let’s address some of the more frequently asked questions related to welding undercut.

What is the main cause in welding undercut?

Undercut is most commonly caused by improper welding technique and not adhering to welding parameters. Undercut is almost inevitable when a welder is welding too hot, too fast, and with an incorrectly sized electrode.

Why is undercut harmful to a weld?

Undercut can weaken the strength of a structural joint by reducing the effective thickness of the pieces being joined. Undercut can create a point of stress which could lead to cracking and ultimately lead to total failure. 

In pipe welds, internal undercut can lead to a place where contaminants can build up and rapidly increase the rate of corrosion and eventually lead to containment failure.

Can damage from an undercut be fixed?

External undercut can almost always be fixed by simply adding another pass where the undercut occurred. It may be necessary to clean the undercut and maybe grind a bit to ensure enough space to deposit another pass free from fusion defects. However excessive grinding, which can remove too much of the weld or base material, may exacerbate the undercut. Therefore, it's crucial to limit grinding to the minimum required.

Internal undercut may involve grinding all the way through to the root pass and performing the weld again. In extreme cases, cutting the joint apart starting the welding process over may be the only solution.

Thanks for reading.

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