What Causes Undercut in Welding and Possible Remedies

By Joe Stephens •  Updated: 09/10/20 •  9 min read

Undercuts are one of the most common welding defects that every welder has to deal with from time to time. Undercutting leaves the welded area weak and prone to cracking. Fortunately, with proper welding techniques, undercuts can be minimized significantly. This is why it is essential to learn what causes undercut in welding so that effective prevention methods can be applied.

What is Undercut in Welding?

An undercut is a groove-shaped defect that typically shows up along the edges of the base metal in a weld. This defect is usually only common with fillet and butt joints.

Example of Welding Undercut

The main concern with undercuts is how much they can reduce the thickness of the parent metal. This thickness reduction then leads to a loss of quality and strength of the weld and welded structure.

The first thing that’s important to understand when it comes to undercuts is that it can show up in two different ways. These include the Internal or Root Undercut and the External or Crown Undercut.

Example of Root Undercut
Internal Undercut or Root Undercut

The Internal Undercut or Root Undercut forms near the root of the weld on the parent metal and will appear as a depression on the sidewall. In Radiographic Testing, a Root Undercut will show up near the center of the welded area as a dark line.

Example of Crown Undercut

External Undercut or Crown Undercut

An External Undercut or Crown Undercut forms near the toe or crown of the weld area on the parent metal. In Radiographic Testing, the Crown Undercut will show up on the edge of the welded area as a dark line.

Causes Of Undercut In Welding

Now that you have an understanding of what an undercut is, it’s time to start learning about what can cause it.

1. The Current Is Too High

A very high current makes the edges of the parent metal melt due to the high amounts of heat. The molten metal caused by this excessive heat then drains into the welded area, increasing the probability of an undercut forming.

2. Excessive Amounts Of Voltage

A very high voltage can cause the electrode to melt into large beads that move from side to side, resulting in a low fusion of the metal with the filler material. The molten electrode mixes with the molten metal leading to the formation of undercuts.

3. Poor Weaving Skills

Poor skills in weaving during welding could lead to increased welding speeds; therefore, cause undercutting. Very high weaving speeds cause uneven distribution of heat, leading to inconsistent melting of the metal, resulting in undercuts.

4. Use Of The Incorrect Filler Material

Temperature differences are created around the weld due to incorrect filler materials, leading to inconsistent melting. The irregular generation of molten metal leads to the formation of an undercut.

5. Insufficient Supply Of Filler Material

A good weld has adequate amounts of filler material deposited in the weld area. However, having insufficient amounts of filler material can cause undercutting.

6. An Improper Angle When Placing The Electrode

When the electrode is incorrectly positioned, too much or too little molten metal is generated in the welded section, leading to undercuts. Holding the rods at a wrong angle leads to uneven distribution of heat, with the edges getting more, thus increasing the probability of undercuts forming.

7. Welding Surfaces That Have Debris

Welding surfaces with deposits like paint, dust, rust, and moisture are more prone to undercuts forming. The particles interfere with the metallic properties making a proper fusion of the molten metal difficult.

8. Poor Selection Of Shielding Gas

Contaminated or improperly mixed shielding gas leads to interference with the metal properties, leading to undercuts.

9. Having An Improper Welding Position

Undercuts form due to the practice of holding the electrode vertically as the welding process takes place on a horizontal surface. Additionally, working on unstable benches will interfere with the quality of the welding. Therefore, undercuts result from the insufficient and unequal distribution of the filler material on the metal surface during welding.

10. Incorrect Electrode Size

Large electrodes melt an excessive amount of metal, leading to undercuts. Conversely, electrodes that are too small generate an insufficient amount of molten metal, leading to welding defects. A welding rod of the correct size should also carry high amounts of current without interfering with the molten metal supply or deform the metal.

11. Excessive Arc Length

The arc is the gap from the tip of the welding rod and the metal to be welded. If the arc’s size is larger than the electrode’s diameter, the molten metal covers a broader area than initially intended, leading to increased undercuts.

How To Prevent Undercut In Welding

As with any welding defect like porosity or welding cracks, the first and most important way to prevent undercuts is by studying what causes them. Just having this knowledge can significantly reduce any welding defect.

Alongside being knowledgeable, there are numerous preventive methods you can apply to further reduce undercuts in your welds. The following methods are the most useful ones you should be practicing.

1. Using The Multi-Pass Welding Process

With multi-pass welding, after the first weld is placed and cooled another weld is placed on top of the first. This process helps in stabilizing the fusion properties of the metal, thus reducing undercut occurrence. If you need help mastering the multi-pass welding process, you can find an amazing guide here.

2. Regulate The Length Of The Arc

Welding with a rod that is too far from the molten metal leads to spattering and can cause the formation of undercuts. Generally, the arc length should not be more than the diameter of the rod’s metallic part. For example, when welding an eight-inch diameter, the electrode should be about an eighth of an inch from the base.

3. Proper Weaving Technique

Mastering the weaving technique is another way of preventing undercutting. Timely pauses on each side of the weld allow for beads’ build-up, thus minimizing undercuts from forming. More undercuts form when the electrode weaves at high speeds due to the splashing of the molten metal. Generally, the rod should move within the intended path without deviating too far around.

4. Appropriate Use Of Shielding Gas

Using the proper shielding gas protects the molten metal from being exposed to oxygen and other atmospheric gases. If the molten metal gets into contact with air, the resulting reaction could lead to the formation of undercuts. The mixture of gases in the shield should be appropriate for the metal type and thickness.

Determining which shielding gas to use can be difficult for welders, especially if they’re inexperienced. If you’re one of those people having trouble choosing the correct gas, we’ve included a great video below that should help you out.

5. Correct Position And Angle

As mentioned previously, holding the gun, rod, or wire at incorrect angles while you are welding can significantly increase the chance of undercutting. This is why it’s highly important to maintain the proper angle throughout the welding process. Here are a few tips that might help:

Wire Welding – As you are pushing in the direction of the weld, the gun should be at a 10-15 degree angle.

Stick Welding – You need to keep an angle of 20-30 degrees as you are dragging.

6. Optimize Travel Speeds

While welding, having a too slow or fast travel speed can be a reason for undercuts to form. The unsuitable travel speeds can cause burn through, incomplete penetration, increased weld splatter, cracks, and much more. You need to have a travel speed that permits the weld metal to completely fill the joint.

7. Prepare Metal Surfaces

Surfaces that have deposits like rust, paint, moisture, and oil create welding defects. The debris on the metal surface interferes with fusion. Therefore, it is advisable to clean the metal to be welded to prevent the formation of welding flaws like undercuts. If the debris is not removable, slow travel speeds to allow trapped bubbles to blow out before the molten metal solidifies.

8. Stable Work Environment

A stable and balanced working bench is essential to getting the path of motion correctly during welding. A good welder should use sturdy clamps to fix the structures being welded, minimizing undercuts’ chances.

How To Repair Undercut In Welding

The question of whether or not to repair an undercut is an important one in the welding industry. The cost considerations versus the benefits should be analyzed to arrive at a decision. Either way, fixing an undercut is possible and can be easy depending on the severity.

Using the weaving technique and stringer beads technique are the two most popular methods. You are basically looking to making another weld pass to fill the groove of the undercut. There is also the option of grinding the weld down, but this can reduce the strength of the overall structure.

According to AWS standards in regards to repairing undercuts, they state:

“The removal of weld metal or portions of the base metal may be done by machining, grinding, chipping, or gouging. It shall be done in such a manner that the adjacent weld metal or base metal is not nicked or gouged. Oxygen gouging shall only be permitted for use on as-rolled steels. Unacceptable portions of the weld shall be removed without substantial removal of the base metal. The surfaces shall be cleaned thoroughly before welding. Weld metal shall be deposited to compensate for any deficiency in size.”

For whatever reason, if you’re unable to fix the undercut, you might not need to worry too much. This is because there is actually an acceptable amount of undercut allowed in a weld.

What Is The Acceptable Amount Of Undercut Allowed?

An undercut should not go beyond 0.5 mm, and it should not interfere with the minimum requirements for the thickness of a section. Undercutting that goes beyond the 1/32 inch is therefore not recommended. Therefore, if a three-inch structure has a 1/16 inch deep undercut over two inches, it is acceptable.

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Joe Stephens

Joey has over 20 years of experience working in the welding industry and now works with providing readers with intensive reviews. Joey has also self-published an e-book and has written countless articles regarding welding information and safety.

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