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D1.1:2025 Acceptance Criteria for Undercut

D1.1:2025 Table 8.1 Item (7) defines separate undercut limits for statically loaded and cyclically loaded nontubular connections. The criteria are applied during visual inspection per Clause 8.9. Undercut is measured as depth from the base metal surface at the weld toe — it is a groove melted into the base metal that has not been filled by weld metal.

Statically Loaded Nontubular Connections

The static criteria split by material thickness at the 1 in [25 mm] boundary, with additional exceptions for accumulated length on thinner material.

The exceptions in (a) and (b) only apply to material less than 1 in thick. For thicker material, the flat 1/16 in limit applies with no accumulated-length allowance. In practice, the 1/16 in allowance on thicker material reflects the reduced sensitivity of heavier sections to stress concentrations at the weld toe.

Cyclically Loaded Nontubular Connections

Cyclic loading imposes stricter limits because undercut creates a stress concentration that accelerates fatigue crack initiation. The criteria distinguish between primary tension members and all other members.

The 0.01 in limit on primary tension members is the strictest undercut tolerance in D1.1. At that depth, undercut is barely visible to the naked eye — most inspectors use a fillet weld gauge or a depth micrometer to measure undercut this shallow. For welds on secondary members or members not carrying computed tensile stress, the 1/32 in limit matches the static thin-material baseline.

What Causes Weld Undercut

Undercut forms when the welding arc melts a groove into the base metal at the weld toe that is not filled by deposited filler metal. The defect appears as a narrow channel running along one or both sides of the weld bead. Three parameter errors account for the majority of undercut seen in structural fabrication.

Travel speed too fast

When the electrode moves too quickly, the arc melts base metal at the weld toe but advances before filler metal can flow into the groove. This is the most common cause of undercut in production welding, particularly with FCAW and GMAW where wire feed speed is constant. Slowing travel speed allows the weld pool to wet the toe and fill the groove before solidification.

Amperage too high

Excessive current widens the arc cone and increases the volume of base metal melted at each point along the joint. The weld pool becomes wider than the filler metal can bridge, leaving unfilled grooves at the toes. This is especially problematic on fillet welds where the horizontal leg melts into the vertical member. Reducing amperage or switching to a smaller electrode diameter narrows the melt zone.

Incorrect electrode angle

Holding the electrode at too steep an angle — or favoring one side of the joint — concentrates arc force unevenly. One toe gets excessive heat and melts a groove while the other toe may show adequate fusion. On T-joints, angling the electrode too far toward the vertical member undercuts the horizontal plate. Maintaining a consistent 45-degree work angle on fillet welds distributes heat evenly across both legs.

Improper weave technique

On weave beads, pausing too briefly at the weld toes does not allow filler metal to fill the edges of the bead. The center of the weave gets adequate fill while the toes are left undercut. Pausing slightly longer at each toe — particularly on vertical-up welds with SMAW — allows the pool to wet and fill the base metal groove before moving across the bead width.

When and How to Repair Undercut

If undercut exceeds the applicable Table 8.1 limit, the weld does not pass visual inspection per Clause 8.9 and must be repaired. D1.1:2025 Clause 7.25.1.2 addresses the repair procedure for undercutting specifically.

For undercut that is only slightly deeper than the acceptance limit, grinding alone may bring the weld toe profile into compliance. Grinding blends the transition between weld metal and base metal, effectively reducing the measured depth of the undercut groove. This approach works when the undercut is shallow enough that removing a small amount of material still leaves the base metal above its minimum required thickness.

For deeper undercut, the surfaces shall be prepared per Clause 7.29 (weld cleaning) and additional weld metal deposited to fill the groove. The repair weld must use a qualified WPS and meet the same acceptance criteria as the original weld. In practice, a single stringer bead along the undercut toe using a small-diameter electrode (such as 3/32 in E7018 for SMAW) at reduced amperage fills the groove without introducing new undercut on the opposite side.

The removal of weld metal or base metal during repair may be done by machining, grinding, chipping, or gouging per Clause 7.25. Oxygen gouging is permitted only on as-rolled steels. The repaired or replaced weld shall be retested by the same method originally used, and the same acceptance criteria apply.

Undercut is one of eight discontinuity categories evaluated during visual inspection. For the complete list of D1.1 Table 8.1 acceptance criteria — including cracks, incomplete fusion, and porosity — see the weld defects overview. When a repair weld is required, it must be performed using a qualified WPS — see Procedure Qualification Records (PQR) for D1.1’s two qualification paths.