Radiographic Interpretation Dispute: Slag Versus Lack of Fusion in a Tie-In
A 12-inch, grade X42 tie-in weld is being inspected by radiography. The radiograph shows a linear indication about 18 mm long in the fill region, oriented roughly parallel to the weld axis. One interpreter calls it slag inclusion and recommends acceptance because it is short; another calls it lack of fusion and recommends rejection. The weld was made with a cellulosic downhill procedure, and the welder reports that the joint was clean and the root gap was consistent. The project specification invokes API 1104 acceptance criteria. The inspector has the original radiograph, the welder's travel-speed and amperage records, and a ultrasonic unit available. The line is scheduled to be commissioned in 48 hours.
1. How should the inspector resolve the disagreement between the two interpreters, and what does the orientation and length of the indication tell you?
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The inspector should not resolve the dispute by choosing the more convenient interpretation or by the commissioning schedule. The indication must be characterized before disposition.
What the features suggest: a linear indication oriented roughly parallel to the weld axis in the fill region is consistent with either slag or lack of fusion, but the two have different radiographic signatures. Slag tends to be irregular, often with tapered ends and variable density along its length; lack of fusion tends to be straighter, with sharper edges and more uniform density. An 18 mm length is significant either way - under API 1104 criteria, elongated slag may be acceptable within length limits depending on the acceptance table, while lack of fusion is generally rejectable regardless of length.
Resolution steps:
- Have the radiograph re-interpreted by a second qualified interpreter, ideally with the original film and proper viewing conditions, and document both opinions.
- Use ultrasonic testing to characterize the indication: UT can distinguish planar from volumetric defects and measure through-thickness extent, which radiography cannot do reliably.
- Review the welder's records for travel speed and amperage to see whether conditions favored lack of fusion (fast travel, low heat input) or slag (poor cleaning between passes).
- If the indication is confirmed planar (lack of fusion), reject and repair. If confirmed volumetric and within acceptance limits, accept with documentation.
The inspector should also resist schedule pressure: a 48-hour commissioning deadline is not a basis for accepting an unresolved indication.
2. If the indication is confirmed as lack of fusion, what repair approach is appropriate and what should be verified before the line is commissioned?
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If confirmed as lack of fusion, the weld is rejectable and must be repaired.
Repair approach:
- Locate and size the indication precisely with UT before excavation, so the repair extent is planned rather than guessed.
- Grind out the indication from the accessible side, confirming complete removal with MT or PT and checking remaining wall thickness against the minimum.
- Re-weld with a qualified repair procedure. The repair procedure should address the constraint and heat-sink differences of a repair compared with production welding.
- Use a qualified welder for the repair, with qualification covering the repair configuration.
- Apply preheat per the procedure and monitor interpass temperature.
Before commissioning, verify:
- Post-repair NDT - radiography plus UT where geometry permits - confirms the defect is removed and no new defects were introduced.
- The repair is documented in the as-built record with location, extent, procedure, welder, and NDT results.
- If the repair was partial-thickness, confirm the remaining wall thickness meets the minimum and the repair procedure was qualified for that configuration.
- Any hardness or mechanical testing required by the specification is complete.
The judgement is that schedule pressure must not substitute for defect characterization and qualified repair. A confirmed lack-of-fusion indication is a planar defect that can grow under cyclic service, so accepting it to meet a deadline would be a poor integrity decision.