6.2 Incomplete Fusion (IF & IFD), Internal Concavity (IC) & Burn-Through (BT)

6.2 Incomplete Fusion (IF & IFD), Internal Concavity (IC) & Burn-Through (BT)

Fusion failures and root-pass cavities attack the structural cross-section of the weld. API 1104:2021 handles them in Sections 9.3.4 through 9.3.7.

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1. Incomplete Fusion — IF (9.3.4) vs. IFD (9.3.5)

IF is a lack of bond between weld metal and base metal that is open to the surface — at the root of the bead or the top of the joint. IFD (cold lap) is a lack of bond between adjacent beads or between weld metal and base metal that is NOT open to the surface — typical of GMAW short-circuit transfer when weld metal runs over unmelted base metal.

The surface-connected distinction drives the limits: surface-open IF is more injurious and gets the tighter limits.

Radiographically, IF shows as a thin dark line parallel to the weld edge with one perfectly straight side (the original bevel face); IFD shows as a faint, wavy dark shadow following a pass boundary.

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2. Internal Concavity — IC (9.3.6)

Internal concavity ("suck-back") is a root bead that is fused to and completely penetrates both bevels, but whose center sits below the pipe's inside surface. The 22nd-edition acceptance rule is density-based: IC of any length is acceptable provided the density of its radiographic image does not exceed that of the thinnest adjacent parent material. Any area denser than that reference (meaning the remaining section is thinner than the thinnest parent wall) is a defect. Section 9.8 adds the visual rule: IC detected visually is a defect when the depth of concavity exceeds the cap height at the same location.

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3. Burn-Through — BT (9.3.7)

A BT is a portion of the root bead where excessive penetration blew the weld puddle into the pipe, leaving a cavity or icicle. Causes: excessive heat input, excessive root gap, or grinding the root bead too thin before the hot pass.

Pipe OD ≥ 2.375 in. (60.3 mm)

A BT is a defect when its image density exceeds that of the thinnest adjacent parent material AND any of these hold:

• maximum dimension exceeds 1/4 in. (6 mm); or
• maximum dimension exceeds the thinner of the specified wall thicknesses joined; or
• the sum of maximum dimensions of separate BTs exceeds 1/2 in. (13 mm) in any continuous 12 in. (300 mm) of weld or the total weld length, whichever is less.

Pipe OD < 2.375 in.

Same two dimensional limits plus a count limit: more than one BT of any size is a defect.

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4. Worked Evaluation

A 12 in. segment of a 16 in. OD × 0.375 in. wall weld shows:

1. IFD, 1.2 in. long → ≤ 2.0 in. individual and aggregate: PASSES.
2. BT #1, 3/16 in. (0.1875 in.), density darker than parent → ≤ 1/4 in. and ≤ wall: PASSES individually.
3. BT #2, 1/8 in. (0.125 in.), same density → passes individually; BT sum = 0.1875 + 0.125 = 0.3125 in. ≤ 1/2 in.: PASSES.

Disposition: ACCEPTED. (On pipe under 2.375 in. OD, the same two BTs would reject the weld — more than one BT of any size is a defect there.)

5. Formation Detail the Exam Loves

• IF causes: bevel contamination (mill scale, rust, oil), wrong electrode angle starving one sidewall, insufficient heat input.
• IFD causes: the GMAW short-circuit transfer mode is the classic cold-lap producer — low heat input lets the droplet wet onto unmelted base metal. This is why the welder-qualification rules restrict GMAW-S production passes to those proven in the test (6.2.2), and why procedure work for GMAW-S demands careful Section 9 examination.
• IC causes: root-pass gravity sag or excessive internal reinforcement consumption on overhead segments — fused on both sides, but the center drops below the pipe's inside surface. The 22nd edition judges it by radiographic density (9.3.6) or, visually, by comparing concavity depth to cap height (9.8).
• BT causes: hot pass too hot over a thin root, excessive root gap, or over-grinding the stringer bead. On small-diameter pipe (< 2.375 in. OD), the standard adds the strict count rule because one BT already consumes a large fraction of the short circumference.