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Welding Undercut 101: What It Is, Causes, and Remedies

Undercutting is a common welding defects in welding that weakens the strength and stability of welded structures.

So, how does undercutting occur in welding? There are actually many reasons for it. It could be due to improperly adjusted welding current or speed, incorrectly chosen electrode angle or size, or even inappropriate use of shielding gas. These issues can all disrupt the welding process and lead to undercutting.

How should we deal with undercutting in welding? In this article, we will provide detailed explanations for both experienced welding professionals and beginners on the causes, hazards, and countermeasures of undercutting in welding.

What is An Undercut in Welding?

Welding undercut is the undesirable groove or depression formed along the edges of a welded joint, the base metal has melted but has not been fully filled by the filler metal. It usually appears as a notch or indentation on one or both sides of the weld bead. The depth and width of the undercut can vary, but it typically extends into the base metal adjacent to the weld.

Excessive undercut can weaken the strength of welded joints and generate stress concentration at the undercut, potentially leading to structural damage. Especially for the welding of low alloy high-strength steel, the edge structure of the undercut is hardened, which is more likely to cause cracks.

Please refer to the diagram below for a visual example of welding bottom cutting.

undercut welding images
undercut welding images

Types of Welding Undercut

According to the position of the undercut on the top and bottom of the weld seam, it can be divided into external undercut (on the side with the larger groove opening) and internal undercut (on the bottom side of the groove).

External Undercut

External undercut typically occurs on the larger opening side of a bevel, namely the outer surface of the weld seam. Its main characteristic is the presence of noticeable grooves or depressions at the edge or toe of the weld seam. The formation of external undercut is often related to factors such as the flow of the weld pool, welding speed, welding angle, and welding current during the welding process.

If the weld pool fails to adequately fill the bevel during welding, or if the melted metal does not shrink properly during cooling, grooves or depressions may form at the edge of the weld seam, known as external undercut.

External undercut not only weakens the structural strength of the weld seam but also may become an area of stress concentration, increasing the risk of fracture of the weld joint during service.

Internal Undercut

Internal undercut occurs on the bottom side of the bevel, within the weld seam, at the junction of the weld metal and the base metal. The formation of internal undercut is typically due to uneven flow and solidification of the weld pool at the bottom of the bevel during the welding process.

When welding thick plates or employing multi-pass welding processes, internal undercut may occur due to restricted flow of the weld pool at the bottom of the bevel or improper control of welding parameters. Since internal undercut is hidden within the weld seam and difficult to detect directly through visual inspection, non-destructive testing methods such as radiographic testing and ultrasonic testing are required for identification and evaluation.

The presence of internal undercut similarly negatively impacts the performance of the weld seam, reducing the load-bearing capacity and corrosion resistance of the weld joint.

The following figure shows examples of electric images for external undercut and internal undercut.

In addition to external and internal undercuts, the types of welding undercuts can also be divided into the following types, as shown in the example diagram:

Types of welding undercut

Impact of Welding Undercut on Component Performance

Under Static Loads

  • In the plastic state of the component: Undercutting reduces the cross-sectional area of the joint and increases local stresses. If the undercutting is parallel to the applied stress and in a plastic state, it won’t affect the joint’s performance significantly.
  • In the brittle state of the component: Any undercut increases the risk of brittle fracture, with very low allowable undercut values for high-strength materials or thick-walled weldments.

Under Dynamic Loads

  • Sharp weld toe grooves penetrate into sections and cause propagation of microcracks; circular grooves show no microcracks on the surface.
  • The relationship between undercut depth and fatigue strength: Deeper undercuts lead to greater reductions in fatigue strength.


  • Undercuts or weld toe grooves in corrosive environments accelerate local corrosion due to the accumulation of corrosion products.
    Undercuts in wet environments or containing metallurgical residues pose greater hazards.
  • These factors highlight the importance of minimizing undercutting to maintain the structural integrity and performance of welded components, especially under varying loading conditions and in corrosive environments.

Root Causes of Undercutting in Welding

Undercutting can stem from multiple factors. Undercut arises from excess heat, rapid travel speed, unsuitable electrode/filler size, or improper joint prep. Some common causes include:

Insufficient Filler Metal

Several factors can contribute to insufficient filler metal deposition. Using a small electrode size limits the amount of filler metal available for the weld. Improper electrode extension, where the electrode sticks out too far from the welding torch, can also reduce the efficiency of filler metal transfer. Additionally, steep electrode angles can direct the filler metal away from the edges of the weld bead, leading to undercut.

The influencing factors of undercutting (arc pressure)

Excessive Current

Excessive welding current can cause the edges of the joint to melt and flow into the weld pool, leaving impressions resembling drainage channels along the length of the weld. Additionally, excessive current can lead to undercutting, root overheating, burn-through, and other issues on both sides of the weld seam, especially in flat, vertical, and horizontal welding positions, where weld beads may form at the root; in overhead welding positions, the root may exhibit concavity.

Welding Slow Speed

Welding at a slow speed can allow the edges of the joint to melt excessively before the weld pool moves on. This excessive melting of the edges and subsequent draining into the weld pool can create undercut defects.

Improper Welding Technique

In welding training and teaching, due to the emphasis on meeting production quotas, many workers tend to favor using higher currents. However, using excessively high current can accelerate the melting of the base metal, increase the size of the weld pool, and prevent the welding rod’s melted metal from adequately filling the groove formed by the melting of the base metal edges. As a result, the width of the weld bead becomes sufficient, leading to undercutting.

Incorrect Filler Metal

Using an incorrect filler metal for the welding application can create greater temperature gradients between the center of the weld and the edges. This uneven heating can lead to the edges melting excessively and draining into the weld pool, causing undercut.

undercutting in welding
undercutting in welding

Insufficient Welder Skills

Novice welders may struggle with eliminating undercut in joints. Inconsistent arc movement can lead to uneven heating and melting of the base metal. Improper weaving, where the electrode is moved in a specific pattern to distribute the filler metal, can result in inadequate coverage along the edges of the weld bead. Excessive arc length, which is the distance between the tip of the electrode and the base metal, can reduce the heat transfer efficiency and affect the penetration of the filler metal. Slow travel speed, which is the rate at which the welding torch moves along the joint, can also allow the arc to dwell in one area too long, melting excessive base metal and potentially causing undercut.

Heat Input

Excessive heat input, especially when using high welding currents and fast welding speeds, can reduce the material in the fusion line area, leading to local undercutting of the weld toe. When welding T-joints, using a larger heat input in single-pass welding is more likely to cause undercutting. This is because the weld pool size is larger, and the weld metal sags before solidifying into a triangular shape, leaving a gap on the upper surface, resulting in undercutting.

More Other Reasons

  1. Incorrect shielding gas: Improper selection or flow rate of shielding gas can affect the weld pool’s protection, leading to incomplete fusion and undercut along the edges of the weld.
  2. Welding material specification: Using larger-sized welding materials can lead to melting excess portions of the base metal during welding, resulting in undercutting.
  3. Dampened Electrode: A dampened electrode can cause instability in the arc, affecting the transfer of heat and filler metal to the joint. This instability can contribute to the development of undercut defects.

Hazards of Welding Undercuts

Welding undercuts pose severe risks to joint quality and durability. They weaken structural integrity, lower load-bearing capacity, and promote premature corrosion. The primary hazard is the reduction in load-bearing ability, which leads to stress concentration, cracks, and potentially, complete structural failure. Addressing undercuts is crucial for ensuring weld safety and reliability.

Hazards of undercutting in welding

Addressing Undercut Issues

Removing undercut from defective weld seams is an important step aimed at rectifying errors in the welding process and enhancing welding quality. Below are some suggestions and steps to help eliminate undercut defects in weld seams:

  1. Filler Metal: Apply additional filler metal to the grooves or areas affected by undercut. Ensure that the filler metal matches the original weld composition and is applied with precision to blend seamlessly with the surrounding weld.
  2. Thorough Cleaning: Prior to repair, thoroughly clean the affected area to remove slag, debris, and other contaminants that could compromise the quality of the repair. Use appropriate tools such as a wire brush or grinder to ensure a clean and smooth surface for welding.

How to Prevent Welding Undercutting

undercutting diagram
undercutting diagram

To prevent or minimize undercut during welding, you can take several measures and employ proper techniques. Here are some strategies to help stop undercut when welding:

Correct Selection of Welding Parameters

Ensure that you are using the correct welding parameters such as current, voltage, travel speed, and electrode size/type according to the welding procedure specifications and the material being welded. Adjusting these parameters appropriately can help control heat input and ensure proper fusion without causing undercut.

According to relevant welding specifications, select welding process parameters, cultivate good quality awareness in daily work, participate in various training and standardize welding requirements seriously. Process examples:

Type of welding rodGradationWelding rod diameter/mmWelding current/AArc voltage/VWelding speed cm/min
E4303First layer3.290-14021-3010-30
Middle layer3.2100-16024-3410-30
Outer layer4.0130-22021-3710-35
Cover surface4.0130-22021-3710-35

Choosing the Right Welding Technique

Use proper welding technique, including maintaining the correct travel speed, maintaining a consistent arc length, and using the correct welding angle. Avoid excessively wide weaving patterns and ensure smooth and steady movement along the joint to achieve even fusion and minimize the risk of undercut.

TIG Welding

TIG welding technology is renowned for its high level of control and precision. Welders can precisely manage welding current, welding speed, and filler metal usage, ensuring clear weld bead profiles without undercutting. Additionally, it should be noted that after welding is completed, it is necessary to distinguish between bad welding and good welding to ensure the quality and reliability of the welded joint.

MIG Welding

MIG welding technology is characterized by high efficiency and stability, making it suitable for automation and semi-automation welding. Properly setting welding parameters, including wire feed speed, welding current, and voltage, ensures sufficient filler metal in the weld joint, effectively preventing undercutting. Control of wire feed speed is a key factor in ensuring adequate filler metal deposition in the weld joint.

Stick Welding

Shielded metal arc welding (SMAW), commonly known as stick welding, is a widely used welding method known for its adaptability to materials of varying thicknesses. By selecting the appropriate electrode size, adjusting travel and work angles, and controlling arc length, undercutting can be effectively prevented, thus enhancing the quality and reliability of the weld joint. Moreover, maintaining a proper arc length and stable welding speed is also crucial in preventing undercutting.

Laser Welding

Laser welding is a high-precision, high-energy welding method suitable for welding thin sheet materials. Due to its fast welding speed and small heat-affected zone, it can effectively reduce the occurrence of undercutting. However, laser welding requires high demands on equipment and operational techniques, necessitating specialized training and experience to ensure welding quality.

Prepare The Joint Properly

Ensure that the joint is properly prepared with the correct bevel angle, fit-up, and cleanliness. Proper joint preparation provides better access for the weld metal and helps ensure adequate penetration and fusion along the joint, reducing the likelihood of undercut.

Maintain The Correct Welding Angle

The angle between the welding gun or electrode and the workpiece should be maintained within an appropriate range, generally between 60 ° and 80 °. For manual welding operations, the vertical line between the welding head and the workpiece should be strictly controlled at around 90 degrees, and adjusted appropriately according to the different welding positions.

Selecting the Correct Shielding Gas

Different welding processes and materials require the use of different types of shielding gases. For instance, Argon (Ar) is commonly used as a shielding gas for welding materials such as stainless steel, aluminum alloys, and titanium alloys due to its inert properties. Carbon dioxide (CO₂), on the other hand, is often used for welding steel due to its relatively lower cost.

Using shielding gases that are incompatible with the welding process and the materials being joined can result in decreased arc stability, inadequate weld protection, and increased risks of undercutting and other welding defects.

Electrode Angle

During the welding process, even slight variations in electrode angle can significantly affect the depth, width, and shape of the weld bead. Improper electrode angle may result in insufficient penetration of weld metal into the weld joint, leading to undercutting. In most cases, the travel angle should slightly present a “dragging” position, approximately 10 to 15 degrees, allowing the welding metal to fill back into the weld joint.

Correcting Excessive Undercut Depth in Weld Seam

For minor undercut areas, a grinding wheel can be used to smooth out the transition. When selecting a grinding wheel, the coarseness should be determined based on the size and requirements of the weld bead, typically ranging from 100 to 240 grit. During grinding, it is important to control the speed and pressure to avoid overgrinding or damaging the weld bead.

Prevention Checks for Undercutting in Welding

Prior to welding, Conducting a thorough pre-weld assessment ensures smoother operations and reduced downtime.

Setting and Verifying Machine Parameters

  • Thoroughly inspect and set the welding parameters, including amps, volts, wire speed, and travel speed, according to the welding procedure specification.
  • Ensure all settings are accurate and comply with industry standards and the material being welded.

Proper Maintenance and Calibration of Welding Machines

  • Conduct pre-weld inspections to verify the equipment’s condition.
  • Check the wire feeder and rollers for proper tension to ensure consistent wire feeding.
  • Replace worn MIG gun liners to maintain a stable filler metal flow.
  • Verify the flow of shielding gas and adjust the regulator accordingly.
  • Inspect the gun or torch for debris accumulation that could obstruct gas flow.
  • Check gas hoses for damage or leaks that might compromise the protective atmosphere.

Inspection of Lead and Ground Cables

  • Inspect lead and ground cables for wear, fraying, or damage that could lead to current loss at the weld puddle.
  • Ensure all connections are tight and secure to maintain optimal current flow.

Joint Preparation

  • Verify that the joint preparation, including gap size, root opening, and bevel angles, is within specified limits to reduce the risk of undercutting.

By conducting these comprehensive pre-weld checks, you can significantly reduce the likelihood of encountering undercutting issues during welding operations.


Welding undercut can compromise the integrity of welded joints, leading to potential structural failures. By understanding its causes and implementing appropriate remedies, welders can produce high-quality welds with minimal undercut, ensuring the reliability and longevity of welded components.

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What is the depth of undercut in welding?

The depth of undercut is measured from the original surface of the base metal to the lowest point of the groove or depression along the toe of the weld. This depth can vary depending on factors such as welding parameters, material properties, and welding technique, but typically ranges from a fraction of a millimeter to several millimeters deep.

Why is undercut bad in welding?

When undercut occurs, it creates a groove or depression along the base metal adjacent to the weld, leaving insufficient filler material to bond effectively. This lack of material can lead to stress concentration points, which can result in premature failure under load or stress.

Can undercutting damage be repaired?

Damage caused by undercutting can be repaired. When the undercut depth is shallow, not exceeding 10% of the base material thickness, it can be smoothed using a grinder or abrasive wheel. If the undercut is severe or occurs in critical areas, repair may require welding. Weld repair involves re-welding at the undercut to fill the groove and restore the integrity of the weld joint.

Tagged: Sheet Metal Fabrication Guide

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