What Is Undercut? Weld Defect

Undercut is a weld defect that appears as a groove or channel melted into the base metal along the toe or root of a weld that is not filled by weld metal. It reduces the effective wall thickness of the parent material and creates a sharp stress concentration that can initiate fatigue cracks, especially in cyclic service. Undercut is one of the most common defects found during visual inspection of piping and structural welds.

When Undercut Occurs

Undercut forms when the welding arc melts the base metal at the weld toe but insufficient filler metal flows into the resulting groove. It is most common on the vertical or overhead portions of circumferential pipe welds and along the upper toe of horizontal fillet welds.

Causes and Remedies

Cause	Remedy
Excessive current	Reduce amperage to prevent over-melting of the base metal
Excessive travel speed	Slow down to allow filler metal to fill the melted groove
Improper electrode angle	Aim the arc into the joint, not at the sidewall
Incorrect weave technique	Pause briefly at each toe during oscillation to fill the edges
Wrong electrode size	Use a smaller electrode for the cap pass
Long arc length	Maintain a short, controlled arc
Incorrect shielding gas	Adjust gas mixture for better wetting (e.g., add CO2 to Ar for GMAW)

Acceptance Criteria

Code/Standard	Maximum Allowable Undercut
ASME B31.3 (normal service)	0.8 mm (1/32 in) depth
ASME B31.3 (severe cyclic)	Not permitted
AWS D1.1 (non-tubular, static)	1 mm (1/32 in) for members under primary stress
AWS D1.1 (cyclically loaded)	0.25 mm (0.01 in)
ASME Section VIII	Per UW-35: 0.8 mm (1/32 in)
API 1104 (pipeline)	0.8 mm depth, max 50 mm length in 300 mm
EN ISO 5817 Level B	0.5 mm maximum

Common Mistake

Do not attempt to repair undercut by simply adding a cosmetic weld pass over the defect. The repair pass must fully fuse into the base metal groove. If the undercut has already created a stress-cracking initiation site, grinding to sound metal before re-welding is mandatory.

Detection and Measurement

Undercut is primarily detected by visual inspection and measured with a depth gauge, weld pit gauge, or profile comparator. For critical service, surface NDT methods such as magnetic particle testing (MT) or liquid penetrant testing (PT) are used to confirm there are no cracks at the undercut root.

Undercut depth is measured perpendicular to the base metal surface at the deepest point. Continuous undercut is more severe than intermittent undercut of the same depth, as the uninterrupted groove creates a longer stress raiser.

In carbon steel piping, undercut deeper than the code allowance must be repaired by grinding (if within minimum wall tolerance) or by weld buildup followed by re-inspection.

Read the full guide to fillet welds