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

Welding plays a crucial role in manufacturing, construction, and a wide range of industrial applications by creating strong, lasting bonds between metal components. However, like any fabrication process, welding comes with its own set of challenges—one common issue being undercut. In the following discussion, we’ll take a closer look at what welding undercut is, explore the factors that contribute to its occurrence, and consider practical approaches for addressing and eliminating this defect.

What Is Undercut?

In welding, the term “undercut” describes a groove or indentation that appears along the edge of a weld bead. This occurs when the base metal melts but the filler material does not sufficiently fill the area, leaving a noticeable depression. Such a flaw can compromise the strength of the weld, making the joint more susceptible to cracking, corrosion, or even structural failure—especially when the weld is subjected to repeated stress.

Undercut refers to the groove formed when the arc force removes metal from the edge of a joint, but that area is not adequately filled with weld metal. This issue typically appears along the toe of the weld bead, and it is often the result of using excessive current.

In practice, undercut is a fairly common challenge in Gas Metal Arc (GMA) welding, particularly when there isn’t enough oxygen present to help stabilize the arc. Other factors, like holding the electrode at the wrong angle or weaving the bead too widely, can also lead to undercut.

If you want to avoid undercutting, there are a few strategies that tend to work well. Welding in the flat position and using multiple passes rather than just one can help. It’s also worth considering a change in shielding gas, and paying close attention to your hand technique so that you fill in the base metal properly 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. Use proper welding current for electrode size and welding position.
2. Reducing the welding voltage.
3. Use a proper electrode angle.
4. Using a travel speed slow enough so that the weld metal can completely fill all of the melted-out areas of the base metal.
5. Select the right filler metal.
6. Cleaning the nozzle, inside of the contact tube, or removing the jammed electrode wire.

If an undercut appears along the sidewalls of a welding groove, it does not necessarily threaten the weld’s structural integrity—as long as it is completely removed before the next bead is applied in that area. For this task, most experienced welders find that a rounded chipping tool or an appropriate grinding wheel offers the most reliable results for clearing out the undercut.

When the undercut is fairly shallow, it is often possible to judge whether the following pass will achieve enough fusion to fully eliminate the flaw. In many cases, this means the welder can simply address the undercut during the next weld pass, rather than stopping for a separate removal process.

Some situations do require more deliberate repair. These may involve grinding and blending the affected area, or even depositing additional weld metal. When grinding is chosen, it’s best practice to use a pencil grinder and to make sure the grinding marks cross the weld rather than run parallel to it. The goal is to achieve a surface finish of 250 micro-inches or better.

FAQs