6.5 Undercutting, Crater Cracks & Arc Burns

6.5 Undercutting, Crater Cracks & Arc Burns

Surface profile discontinuities concentrate stress and shorten fatigue life. API 1104:2021 splits their acceptance between the radiographic rules of 9.3 and the visual rules of 9.7.

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1. Undercutting — Two Evaluation Routes

Undercutting is a groove melted into the parent material adjacent to the weld toe (external, EU, at the cover pass) or root (internal, IU), left unfilled by weld metal.

Radiographic Evaluation (9.3.11)

EU and IU in any combination are defects when their aggregate length:

• exceeds 2 in. (50 mm) in any continuous 12 in. (300 mm) of weld; or
• exceeds one-sixth of the weld length.

Visual / Mechanical Evaluation (9.7, Table 8)

When depth is determined visually or mechanically, Table 8's three tiers apply — and when both mechanical and radiographic measurements exist, the mechanical measurement governs:

Undercutting is excluded from the accumulation-of-imperfections summation (9.3.12) precisely because it has its own limits here.

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2. Crater (Star) Cracks (9.3.10)

Crater cracks form at bead stopping points from solidification shrinkage — the classic star pattern. API 1104's crack rule has exactly one exception:

• Any crack, any size, any location → defect, EXCEPT
• a crater (star) crack not exceeding 5/32 in. (4 mm) in length.

The same 5/32 in. (4 mm) allowance appears in the PT acceptance standards (9.5.2): linear indications evaluated as crater/star cracks beyond 5/32 in. are defects, and any other crack indication is a defect outright.

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3. Arc Burns

An arc burn is localized melting of the pipe surface from a stray strike or bad ground, quenched by the surrounding cold steel into hard, brittle untempered martensite with possible micro-cracks. Handling:

1. Remove by grinding to sound metal with a smooth contour, then verify with MT or PT. Minimum design wall thickness must be maintained — otherwise the joint is cut out.
2. Unrepaired arc burns may be accepted only through the Annex A engineering route: Table A.6 lists acceptable limits for unrepaired arc burns (based on the premise that total arc-burn depth including its HAZ is less than half the burn width).
3. Cracked arc burns — cracks visible to the eye or on conventional radiographs — are NOT covered by the Table A.6 allowance at all.

4. Which Measurement Governs, and Where Each Method Applies

Section 9.7.2 settles a classic dispute: when both mechanical (depth gauge) and radiographic measurements of undercut are available, the mechanical measurement governs. The radiographic 9.3.11 length-only rule exists because film cannot measure depth; the moment a gauge touches the pipe, Table 8's depth tiers take over.

5. Companion Rules for Surface Evaluation

• MT (9.4): rounded indications are evaluated to the scattered-porosity criteria (9.3.9.2); base-material imperfections found by MT are reported to the company for disposition.
• PT (9.5): indications of 1/16 in. (2 mm) or less are nonrelevant; linear indications are those longer than three times their width. Relevant linear indications evaluated as IF are defects beyond 1 in. (25 mm) total length per 12 in. or 8% of weld length; anything evaluated as a crack (other than a ≤ 5/32 in. crater/star crack) is a defect.
• Base material imperfections (9.3.13 / 9.4.3 / 9.5.3): always reported to the company; disposition as the company directs.

6. Formation and Prevention Notes

External undercut follows high current, excessive travel speed, long arc length, or bad electrode angle at the cap; internal (root) undercut follows excessive root-pass penetration or a misdirected arc at the root. Prevention is parameter discipline within the qualified WPS envelope — voltage, amperage, travel speed, and weave limits — plus correct electrode angle at the toes. EU is accessible to the depth gauge; IU normally reaches evaluation through radiography (9.3.11) or, when accessible, mechanical measurement, and 9.7.2's "mechanical governs" rule resolves any conflict between the two.