6.3 Slag Inclusions (ESI & ISI) & Hollow Bead (HB) Porosity

6.3 Slag Inclusions (ESI & ISI) & Hollow Bead (HB) Porosity

Slag inclusions and hollow-bead voids are the everyday volumetric discontinuities of SMAW pipeline welding. API 1104:2021 evaluates slag in Section 9.3.8 and hollow bead in Section 9.3.9.3. When measuring a radiographic slag indication, its maximum dimension counts as its length.

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1. Slag Inclusions (9.3.8) — Large-Diameter Pipe (OD ≥ 2.375 in.)

Elongated Slag Inclusions (ESI) — continuous or broken slag lines ("wagon tracks") at the fusion zone — are defects when:

• an individual ESI indication exceeds 2 in. (50 mm) in length;
• the aggregate length of ESI exceeds 2 in. (50 mm) in any continuous 12 in. (300 mm) of weld; or
• the width of an ESI indication exceeds 1/16 in. (1.6 mm).

Wagon-track rule: parallel ESI lines separated by approximately the width of the root bead are considered a single indication — unless the width of either line exceeds 1/32 in. (0.8 mm), in which case they are evaluated as separate indications.

Isolated Slag Inclusions (ISI) — irregularly shaped slag pockets anywhere in the weld — are defects when:

• the aggregate length of ISI exceeds 1/2 in. (13 mm) in any continuous 12 in. (300 mm) of weld;
• the width of an ISI indication exceeds 1/8 in. (3 mm); or
• more than four ISI indications at the maximum 1/8 in. (3 mm) width appear in any continuous 12 in. (300 mm) of weld.

Finally, the aggregate length of ESI and ISI indications together may not exceed 8% of the weld length.

2. Slag Inclusions — Small-Diameter Pipe (OD < 2.375 in.)

• ESI: individual length may not exceed three times the thinner specified wall thickness; width limit stays 1/16 in. (1.6 mm).
• ISI: aggregate length may not exceed two times the thinner wall AND width may not exceed one-half the thinner wall.
• The 8% aggregate rule still applies.

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3. Hollow Bead Porosity — HB (9.3.9.3)

Hollow bead is elongated linear porosity occurring along the root-bead centerline — trapped hydrogen/CO bubbles from fast vertical-down cellulosic root passes. On a radiograph it shows as a smooth, dark, tubular shadow centered in the root bead.

HB is a defect when:

• an individual HB indication exceeds 1 in. (25 mm) — note the 22nd edition relaxed this from the 21st edition's 1/2 in.;
• the aggregate length of HB exceeds 2 in. (50 mm) in any continuous 12 in. (300 mm) of weld; or
• the aggregate length of all HB exceeds 8% of the weld length.

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4. Comparison Matrix

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5. Worked Evaluations

Scenario A — ESI on 0.375 in. wall, 24 in. OD pipe

One ESI line, 1.25 in. long × 0.040 in. wide. Individual limit is 2.0 in. (there is NO t/2 reduction for radiographic ESI — that reduction belongs to nick-break slag). Width 0.040 ≤ 1/16 in. ACCEPTED, provided the 12 in. aggregate stays ≤ 2.0 in.

Scenario B — ISI count rule

Five ISI indications, each at the maximum 1/8 in. width, in one 12 in. segment, aggregate 0.55 in. Width: at the 1/8 in. limit (OK individually); aggregate: 0.55 > 0.5 in. and more than four at maximum width — REJECTED on two grounds.

Scenario C — HB on a short weld

Weld length 7.5 in. (2.375 in. OD nipple). One HB indication, 0.9 in. Individual: 0.9 ≤ 1.0 in. OK. Aggregate: 0.9 > 8% × 7.5 = 0.60 in. → REJECTED by the 8% rule.

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6. Causes, Prevention, and Radiographic Recognition

What the Definitions Anchor

Section 3 defines a slag inclusion as a nonmetallic solid entrapped in the weld metal or between the weld metal and the parent material (3.1.54), and porosity as gas trapped by solidifying weld metal before the gas has a chance to rise to the surface of the molten puddle and escape (3.1.35). Hollow bead is the elongated linear porosity that occurs in the root bead (3.1.18). Per 9.3.8.1, ESIs are usually found at the fusion zone, while ISIs are irregularly shaped and may be located anywhere in the weld.

How They Form — and How Crews Prevent Them

• ESI / wagon tracks: slag trapped in the toes alongside an underfilled or crowned root bead when the following pass fails to remelt it and float it out. Prevention: correct bead profile, proper electrode angle and current, and thorough interpass chipping, grinding, or wire-brushing with the power or hand tools the WPS designates (5.3.2.12).
• ISI: slag pockets left between passes by incomplete cleaning, excessive weaving that undercuts the bead toes, or slag frozen into the deep valleys of a rough ripple pattern. Prevention: clean every pass to bright metal before depositing the next.
• HB: gas — chiefly hydrogen and carbon monoxide generated by cellulosic electrode coatings — entrained along the solidifying centerline of a fast vertical-down root pass. Excessive travel speed, a long arc, moisture, or a contaminated joint face aggravate it. Prevention: qualified travel speed and arc length, dry electrodes and joint faces, and the root-bead sizing the WPS specifies.

Telling Them Apart on Film

ESI images as linear dark lines hugging the fusion-zone edges — often the paired wagon tracks of the single-indication rule. ISI images as irregular, isolated dark spots anywhere in the weld cross-section. HB images as a smooth-walled, tubular dark line centered in the root bead — distinct from the sharp, straight-edged dark line of IP and from the broad, diffuse density change of internal concavity.