What is Undercut in Welding?- Causes, Effects, and Prevention

Welding is an important part of manufacturing, construction, and various industrial processes in establishing strong, long-lasting connections of metal parts.

But, as with any fabrication process, welding has its own complications, one of which, the welding undercut, can be common.

In this discussion, we will define what a welding undercut is, examine the contributors to it, and discuss practical measures to combat and remove this defect.

What Is Undercut?

In the process of welding, undercut is defined as a groove or erosion along the edge of the weld bead when the base metal melts away, but not enough filler material is added to fill the area, resulting in a pronounced undercut.

An undercut defect is detrimental because it reduces the strength of the weld, making the joint more probable to degradation, corrosion, cracking, or even structural failure when subjected to repeating stresses.

Undercut is defined as the notch created when arc force has removed metal from the edge of a joint, while that area was not sufficiently filled with weld metal. This typically exists at the toe of a weld bead and is often caused by excessive current.

In practice, undercut is a very commonly encountered challenge in Gas Metal Arc (GMA) welding, especially where there isn’t enough oxygen present to help stabilize the arc! Several other factors take place, including electrode angle, excessive weaving of the bead width, or motions.

There are several mitigation strategies to ensure that you do not undercut; often, these work well. You can weld in the flat position, utilize multiple passes instead of one pass, change shielding gas, and pay special attention to your hand movements to ensure that you are filling the base metal along the edge of the weld bead.

Causes of Undercut:

Undercut is caused by one or more of the following:

1. Excessive welding current.
2. Arc voltage is too high.
3. Excessive travel speed which does not allow enough filler metal to be added.
4. Erratic feeding of the electrode wire.
5. Excessive weaving speed.
6. Incorrect electrode angles, especially on vertical and horizontal welds.

Preventing Undercut:

Undercut can be prevented by:

1. Using an appropriate welding current based on the size of electrode and welding position;
2. Decreasing the welding voltage;
3. Utilizing a proper electrode angle;
4. Using a travel speed that is slow enough so that the weld metal can totally fill all of the melted out area of the base metal;
5. Using the correct filler metal;
6. Cleaning the nozzle, the inside of the contact tube, or removing the jammed electrode wire.

The presence of an undercut in the sidewalls of welding groove does not pose a significant risk to the strength of the weld, if it is completely removed before placing the next bead over that area.

Usually, the well-practiced welder will find the most reliable results from a rounded chipping tool or an appropriate grinding wheel to remove the undercut.

In the case of a relatively shallow undercut, it is sometimes feasible to assess whether the next weld pass will heat enough to fully fuse the undercut with the next weld pass.

This is often an advantage for the welder because they can finish the next weld pass indicating the time spent on the undercut would be extra time spent without the intent to apply a weld pass following the preparation phase.

Occasionally an undercut may warrant the repairing of the weld region. This may simply require grinding and blending the area or depositing additional weld metal. When using grinding to remove the undercut, a pencil grinder is preferred.

Additionally, the welder should make sure the grinding marks cross the weld rather than run parallel. The goal is to achieve a surface finish of better than 250 micro-inches.

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