What is a welded undercut?

What is a welded undercut?

What is a welded undercut?

The integrity of welded joints is critical to every application and industry. A common problem welders encounter that can affect weld quality is weld undercut. This defect is characterized by grooves or depressions at the edge of the weld, which weakens the connection and promotes corrosion. In this article, we take an in-depth look at basic techniques and strategies for preventing undercuts in welds, for experienced welding professionals and beginners alike.

Undercut on welds: what it is and how to avoid it

What is an undercut in welding?

A weld undercut is a welding discontinuity that leaves a groove or depression in the edge of the finished weld when the base metal is not adequately filled from the applicable welding process.

Undercuts can significantly affect the quality of welded joints, and their effects vary depending on the specific application. Such poor grooves in the weld bead can destroy the strength of the final weld, posing a significant threat to the structural integrity of the joint. In applications such as process piping, undercuts may also provide a site for contaminants to accumulate, which may accelerate corrosion rates beyond desired levels.

Dangers of welding undercuts

The existence of undercut can cause significant risks and hazards to the quality and life of the welded joint. It has the potential to compromise the structural integrity of the weld, reduce load-bearing capacity and cause premature corrosion.

The biggest danger of undercutting welds is reducing the load-bearing capacity of the joint. Undercuts create areas of stress concentration that make joints more susceptible to cracks and fractures, which can ultimately lead to overall structural failure.

Causes of undercutting during welding

Undercutting can be caused by a variety of factors, individually or in combination. Excessive heat during welding can cause undercutting because it causes excessive melting and corrosion of the base metal. Likewise, too fast a travel speed limits the time for proper fill metal deposition, possibly resulting in undercutting. Using an inappropriate welding rod or filler metal size relative to the joint size can also cause this defect. Finally, improper joint preparation, such as excessive gaps or large root openings, may create conditions conducive to undercut formation. Recognizing and addressing these causative factors is critical to achieving high-quality, defect-free welds.

Welding process issues

A properly written welding procedure should address all the basic variables involved in the welding process; such as electrode size, material thickness of the base metal to be welded, current and voltage requirements, welding location and progress, etc. Failure to properly handle and adhere to these parameters can result in undercuts in the finished product.

Welding parameters are incorrect

When the amperage or voltage is too high for the material being welded, it means more base metal will melt than can be filled. This is one of the most common causes of undercut defects.

Inadequate welder skills

Beginner or novice-level welders may have difficulty developing techniques to eliminate excessive undercutting in welded joints. It takes time and patience to develop the right touch and “feel” to produce a good weld without undercuts. Properly performed welding is a combination of many different variables; travel speed, amps and volts, rod angle, arc length, and overall technique. For example, when weaving vertical welds with 7018 electrodes on an uphill pass, errors such as holding on to the side long enough will always result in undercuts. Being proficient in welding techniques and recognizing the subtleties is critical to consistently producing high-quality, undercut-free welds.

Undercut type

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

external undercut

External weld undercut occurs on the outer surface of the welded joint. It is characterized by a groove or depression along the edge or “toe” of the completed weld and is usually visible by visual inspection. This defect occurs when the welding process does not adequately fill the joint, resulting in recessed or undercut areas that weaken the load-bearing capacity and structural integrity of the weld.

internal undercut

Internal weld undercuts occur within the weld itself, beneath the surface, and are not visible through standard visual inspection. Unlike external undercuts, which are observable on the outer surface of the weld, internal undercuts may be hidden within the joint. It is characterized by grooves or depressions that extend into the base metal or into the weld. Because it may not be visible in the finished weld, radiographic inspection may be necessary to determine the presence of internal undercuts.

How to prevent undercutting

There are various methods and techniques to prevent undercutting in completed welded joints. Choosing the correct welding process for a welded joint and understanding and adhering to the parameters in that welding process are crucial to achieving a good weld without undercuts.

Choosing the right welding technique

Using the correct welding technique to prevent undercut depends on a variety of factors, including the weld material, application, thickness, joint configuration, and other considerations. Many commonly used welding processes have inherent properties and techniques that can be exploited to effectively mitigate or prevent undercutting.

argon arc welding

TIG welding is an excellent process for eliminating undercuts in many applications. TIG offers a higher level of operator control than many other processes, which can eliminate the possibility of undercutting. By using the correct size of filler metal, the welder can be sure that the correct amount of filler metal is being deposited into the weld joint, allowing even large gaps to be filled in a single weld pass without creating an undercut.

Stick welding

Welds with the correct profile can be made with SMAW or “stick welding” by selecting the appropriate size electrode based on material thickness and weld location. In rod welding, techniques such as proper stroke and working angle are crucial to ensure a correct weld profile, without undercuts. Arc length also plays a key role in achieving undercut-free welding. An arc length that is too short or too long can greatly affect the way the arc transfers and the amount of weld metal that is deposited into the weld.

Gas metal arc welding

Undercutting can be eliminated effectively using MIG welding by establishing appropriate welding parameters and monitoring them throughout the welding process. Adjusting the wire speed will help ensure that adequate filler metal is deposited in the weld. When performed correctly in conjunction with all other factors, this will result in a weld with no undercuts.

Given that MIG welding is “semi-automatic,” the welder’s ability to change welding parameters is limited once the arc is established. It is critical to ensure precise fit-up of welded joints with minimal deviations in factors such as root openings, bevel angles, etc.

Adjust welding parameters

A skilled operator follows the parameters of the welding process to determine whether a good weld with the proper profile is achieved. Before welding begins, factors such as material type and thickness, weld configuration, welding location, welding process, and the electrode and filler metal to be used must be evaluated and considered.

Correct amperage Correct voltage

The current used in welding is determined by the correct setting of the amperage in SMAW and TIG welding or the voltage in MIG or FCAW welding. Too much current or volts will cause too much “heat” to be generated in the weld pool, which means more of the base metal goes into the molten state and then not enough filler metal is deposited, which only results in undercutting.

appropriate driving speed

The correct travel speed is essential to obtain the correct welding profile without undercuts. If the travel speed is too fast, there will not be enough weld metal deposited in the joint, and undercuts may form. Striking the right balance between travel speed and weld deposition rate is critical to successful welding.

Proficient in operating a cutting torch, electrode holder, “Stinger” or MIG welding gun

One of the biggest contributors to undercutting is improper working angle or travel angle. This angle may vary depending on the welding process and welding position, affecting the amount of molten weld metal deposited in the weld. Incorrect angles can result in poor weld profiles, often resulting in undercuts.

The angle of the electrode plays a critical role in determining how molten weld metal is deposited into the joint. The specific welding process and welding location will also affect this factor. Incorrect electrode angles can result in inadequate weld contours that completely fill the joint, often resulting in undercutting.

Use appropriate equipment and tools

Of course, having the right tools and equipment to prepare the joint for welding will help achieve a good weld. If a joint requires a bevel made at a specific angle, you must make sure you have a torch, grinder, or specific tool that can make the bevel correctly. This might even include tools for measuring and verifying angles. Welds that are improperly prepared and angled may be difficult to adequately fill, which often results in signs of undercutting.

Electrodes and shielding

Within the welding process itself, there are a few factors that can cause undercuts to form in the finished weld.

Improper electrode material

It is crucial to ensure that the electrode you choose is compatible with the base metal you are welding. Each type of electrode has a specific coating composition that determines its performance. If the electrode used is metallurgically incompatible with the material being welded, proper penetration and melting of the metal may not be achieved, which may result in undercutting.

Electrode angle

A poor working angle can dig out too much of the base metal and make it difficult to properly fill the weld, resulting in undercuts. In most cases, the angle of travel should be a slight “drag” angle, about 10 to 15 degrees, to allow the weld metal to refill the weld. (It is important to note that this is a general guideline and may vary based on many different factors, such as welding location, material thickness, joint configuration, and welding process.

Wrong electrode size

Attempting to fill a large weld joint with a welding rod that is too small may result in the welder being unable to deposit enough weld metal into the joint resulting in undercutting. Even attempts to correct this by slowing down the travel speed can result in staying in the molten pool for too long, which can, and often does, result in undercutting. Make sure the electrode and filler metal sizes you use are appropriate for the material thickness of the parts you are welding.

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 materials may use argon, helium, nitrogen, while other welding processes may use carbon dioxide or a mixture of shielding gases. The use of a shielding gas that is incompatible with the welding process and the materials being joined can adversely affect the stability of the arc and lead to insufficient protection, which can lead to undercutting.

Preventive inspection of undercut in automatic welding

In automated welding, adjusting the welding parameters after starting the process can be difficult, often resulting in the operation having to be completely stopped and restarted. This is a time-consuming and complex process, so it is important to check all decision variables before starting the welding operation.

Set and check machine parameters

Extra care should be taken when configuring welding parameters to ensure that current, voltage, wire feed speed, travel speed, etc. are checked and comply with the welding procedure.

Proper maintenance and calibration of welding machines

Before welding begins, it is important to inspect the equipment to ensure it is in proper working order. The wire feeder and wire feed roller should be checked for proper tension. The MIG gun liner should be replaced when necessary to allow filler metal to be fed into the weld puddle at a consistent rate.

Verify shielding gas flow and ensure regulator adjustment is correct. Also, check the gun or torch for debris buildup that could impede airflow. Check the hose for signs of damage that may affect the flow of shielding gas.

Also, consider inspecting the leads and ground cables to make sure they are not frayed and frayed. Make sure the connections are strong and tight to avoid current losses in the weld pool, which can adversely affect the welding process.


Let’s address some of the more common issues related to welding undercuts.

What is the main reason for welding undercut?

The most common causes of undercuts are improper welding technique and non-compliance with welding parameters. Undercutting is almost inevitable when a welder welds too hot, too fast, and with the wrong electrode size.

Why is undercut harmful to welds?

Undercuts can weaken structural joints by reducing the effective thickness of the parts being joined. Undercuts can create stress points that can lead to cracking and ultimately complete failure.

In pipeline welds, internal undercuts can cause contaminants to accumulate and rapidly increase corrosion rates, ultimately leading to containment failure.

Can damage caused by undercuts be repaired?

External undercuts can almost always be fixed by simply adding another path where the undercut occurred. The undercut may need to be cleaned up and may need to be sanded a bit to ensure there is enough room to deposit another weld bead without fusion defects. However, excessive grinding removes too much of the weld or base material, potentially exacerbating undercutting. Therefore, it is crucial to limit grinding to the minimum required.

Internal undercuts may require grinding all the way to the root pass and welding again. In extreme cases, cutting the joint open and starting the welding process again may be the only solution.

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