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What Is Meant by Undercutting a Welded Joint?

Lillian Teague
Table of Contents

Welding flaws and imperfections occur in all types of welds. Known as defects or unacceptable weld profiles, imperfections negatively impact the strength and durability of a weld. Undercutting is a weld defect resulting in a reduction in the thickness of the material being welded.

The specific welding code determines the amount of undercut allowed.

Being able to correctly identify this defect enables the welder to take steps to avoid undercutting.

Identifying Undercutting

Undercutting appears as a groove in the welding material located directly along the edges of the weld. Seen at the fusion point, undercutting reduces the thickness of the metal being welded together. X-ray films show the undercut as a thin, dark line that is slightly irregular along the weld.

Fillet Undercutting

Fillet welding uses two pieces of metal plate to make overlapping joints, corner joints or t-joints. Undercutting appears along the edge, or toe of the weld. Esab, a welding machine and equipment manufacturer, reports that undercutting is most commonly seen in lap fillet welds but may also be seen in other types of fillet welding.

Internal Undercutting

Internal undercutting, also known as root undercut, occurs in a butt joint weld along the base metal next to the root of the weld. In pipe welding, this is at the inside of the pipe along the narrow edge. Another form of internal undercutting is found within the weld. It is seen as a groove left along the side wall at the top of the root weld. When the subsequent weld bead is placed in the weld area it can form inclusions in the weld. The undercutting, if not correctly adjusted for, may show up in X-rays as internal undercutting.

External Undercutting

External undercutting, or a crown undercut, is evident from the outside of a butt joint weld. It appears as an erosion or groove running along the edge of the weld. Bernard, a welding equipment manufacturer, warns against external undercutting as it weakens the weld and makes it susceptible to cracking along the weld edges. (ref 5)


Undercutting results from a variety of conditions, including fast travel speed, high current adjustment, long arc gap and improper welding technique. When traveling too fast, the surface tension of the weld draws the melted base metal into the weld, resulting in undercutting. Esab recommends decreasing arc travel speed in order to reduce the surface tension, especially if previous undercutting is continuous In addition to slower travel speeds, reducing the welding current and voltage may reduce undercutting. Bernard recommends using a weaving technique to go back and forth to each side of the weld. Additional recommendations for preventing weld undercutting include adjusting weld angle, adjusting electrode positioning and reducing arc length.