What is Undercut in Welding? [Incl. Pictures!]

Undercut is a welding defect that occurs when the base metal is melted or heated too much or too quickly by the welding process. It creates a gap or crater-like divot in the base metal next to the weld bead.

Undercutting can be a serious problem in welding.

It weakens the structural integrity of the welded joint, making it more susceptible to cracks and failures.

Depending on its size, it can be a discontinuity or a defect. It is a condition in which the base metal has been melted during the welding process, but has been insufficiently filled by the filler metal, presenting, as a result, a crater.

We’re going to cover what undercut is in welding in more detail, what causes it, and how to prevent it.

Definition of Undercut in Welding

Undercut is a type of weld defect that occurs when the weld reduces the cross-sectional thickness of the base metal.

This type of defect reduces the strength of the weld and workpieces.

Undercutting is a surface discontinuity that occurs in the base metal directly adjacent to the weld bead.

Depending on its size, it can be a discontinuity or a defect.

It is a condition in which the base metal has been melted during the welding process, but has been insufficiently filled by the filler metal, presenting, as a result, a crater.

An undercut is a surface and subsurface defect resulting from grooves within the parent metal area.

Like all other defects, instead of improving the joint strength, it introduces weak points that often propagate structural failure.

Undercut is a type of weld defect that can occur due to a variety of reasons such as excessive current, incorrect electrode angle, incorrect travel speed, and incorrect welding technique.

The most common reason for undercut is excessive current, causing the edges of the joint to melt and drain into the weld, leaving a drain-like impression along the length of the weld.

Causes of Undercut

Undercut is a welding defect that occurs when the metal at the base of the weld joint is melted away, resulting in a groove or depression.

This can weaken the joint and make it more susceptible to failure. There are several causes of undercut, including:

High Speeds of Electrode Movement

The speed and direction of electrode movement play a significant role in the quality of the weld.

If the electrode is moved too quickly, it can cause undercutting.

This happens because the arc is not able to melt the metal properly, resulting in a groove or depression.

Incorrect Welding Parameters

Incorrect welding parameters, such as the wrong current, voltage, or travel speed, can cause undercutting.

If the current or voltage is too high, it can cause the metal to melt too quickly, resulting in a groove or depression.

If the travel speed is too slow, it can cause the metal to overheat, resulting in a groove or depression.

Poor Welding Techniques

Poor welding techniques, such as improper angle, electrode placement, or manipulation, can cause undercutting.

If the angle of the electrode is incorrect, it can cause the arc to melt the metal unevenly, resulting in a groove or depression.

If the electrode is not placed correctly, it can cause the arc to miss the metal, resulting in a groove or depression.

Incompatible Weld-Parent Material

Incompatible weld-parent material can cause undercutting.

If the weld-parent material is not compatible with the filler material, it can cause the metal to melt unevenly, resulting in a groove or depression.

This can weaken the joint and make it more susceptible to failure.

How Much Undercut is Acceptable?

The acceptability of undercut in welding depends on various factors, including the welding code or standard being followed, the type of weld, the materials involved, and the specific application of the welded component.

General guidelines for acceptable levels of undercut are as follows:

1. For Structural Welding: Codes like the American Welding Society’s D1.1 (AWS D1.1) often specify the maximum allowable undercut. Typically, for structural steel welding, undercuts should be less than 1/32 inch (0.8 mm) deep. However, for thicker materials, a slightly larger undercut may be permissible.
2. For Critical Applications: In critical applications like pressure vessels, aerospace, or pipelines, the standards for undercut can be much stricter. Often, any visible undercut is unacceptable due to the high risk of failure under pressure or stress.
3. For General Fabrication: In less critical applications, a small amount of undercut might be acceptable if it does not compromise the integrity and strength of the weld. This should be determined based on the specific requirements of the project and the judgement of a qualified welding inspector.
4. Standards and Specifications: It’s important to refer to the specific standards or specifications applicable to your welding project. Different standards might have varying thresholds for what is considered an acceptable undercut.
5. Inspection and Testing: In many cases, the acceptability of undercut is determined through inspection and testing processes. Methods like visual inspection, ultrasonic testing, or other non-destructive testing techniques can be used to evaluate the severity of undercut.

Always refer to the welding code or standard relevant to your work for specific guidance.

Effects of Undercut on Weld Quality

Undercutting in welding can have significant effects on the quality of the weld.

Here are some of the effects that you should be aware of:

Impact on Structural Integrity

Undercutting can weaken the weld, making it more prone to failure. It can cause stress concentrations in the weld, which can lead to cracking and ultimately, failure.

When the undercut is severe, it can significantly reduce the cross-sectional area of the weld, which can compromise its strength.

Visual and Aesthetic Concerns

Undercutting can also have visual and aesthetic concerns.

It can create a rough surface finish, which can make the weld look unattractive.

It can lead to porosity, which can cause the weld to appear porous and weak.

To avoid these issues, it is essential to ensure that the welding parameters are correct, and the welding technique is appropriate.

Proper selection of the electrode and shielding gas, as well as maintaining the correct angle and speed, can help prevent undercutting.

Prevention and Remediation Techniques

Welding Technique Adjustments

One of the most important factors to consider when preventing undercut in welding is proper welding technique.

• Use the correct welding parameters, such as the right welding current and voltage, based on the type and thickness of the material being welded.
• Maintain a consistent travel speed and angle throughout the welding process.
• Avoid excessive weaving or oscillation of the welding torch, which can cause uneven heat distribution and lead to undercut.
• Use the correct welding technique for the type of joint being welded, such as fillet, butt, or lap joint.

Material and Equipment Considerations

In addition to proper welding technique, there are several material and equipment considerations that can help prevent undercut:

• Use the correct electrode and shielding gas for the type of material being welded.
• Ensure that the base material is clean and free from any contaminants or debris that can cause defects in the weld.
• Use the correct size and type of welding equipment for the job, such as the appropriate welding torch, wire feeder, and power source.
• Inspect the welding equipment regularly for any signs of wear or damage that could affect the quality of the weld.

Inspection and Detection of Undercut

To ensure the quality of your welds, it is important to perform regular inspections to detect any undercut.

Undercutting can be detected visually or through non-destructive testing methods.

Here are some methods you can use to inspect and detect undercut:

Visual Inspection

Visual inspection is the most common method used to detect undercut.

Undercutting appears as a groove or depression along the toe of the weld.

It is important to inspect the entire weld length to ensure no undercut is present.

You can use a magnifying glass or microscope to inspect the weld more closely.

Non-Destructive Testing

Non-destructive testing (NDT) methods can also be used to detect undercut.

These methods include ultrasonic testing, radiographic testing, and magnetic particle testing.

NDT methods can detect undercut that is not visible to the naked eye.

However, they require specialized equipment and trained personnel to perform the tests.

Measurement

Undercut can also be measured to determine its severity.

The depth and width of the undercut can be measured using a depth gauge or a microscope.

The severity of the undercut can be determined by comparing the measured values to the allowable limits specified in the welding code.

Prevention

Preventing undercut is always better than detecting and correcting it after the fact.

To prevent undercut, you should ensure that the welding parameters are set correctly, the joint design is appropriate, and the base metal is clean and free from contaminants.

You should also ensure that the filler metal is added in the correct amount and at the correct angle.

Frequently Asked Questions

How can you prevent undercuts during the welding process?

To prevent undercuts during the welding process, it is essential to follow proper welding procedures. This includes maintaining the correct welding angle, ensuring the right amount of heat is applied, and using the appropriate filler metal.

What are the main causes of undercuts in welds?

Undercuts in welds can be caused by a variety of factors, including improper welding techniques, incorrect welding angles, and insufficient heat.

Can you share images that illustrate what an undercut looks like in welding?

Yes, an undercut in welding appears as a groove or depression along the weld’s edge. The base metal has been melted during the welding process but has been insufficiently filled by the filler metal, presenting a crater.

What are some effective remedies to fix undercuts in welding?

One effective remedy to fix undercuts in welding is to grind down the undercut area to remove the defect. Another option is to use a different type of filler metal that is better suited for the specific welding job.

What is the acceptable amount of undercut in a weld according to industry standards?

According to industry standards, the acceptable amount of undercut in a weld is typically no more than 1/32 of an inch.

What techniques can be used to remove undercuts in MIG welding?

To remove undercuts in MIG welding, use a grinder to remove the affected area.