Radiograph With Clustered Porosity, Undercut, and a Possible Root Lack of Fusion
A 25 mm thick carbon steel butt weld, single-V, FCAW-G, is radiographed after completion. The film shows a 6 mm long cluster of rounded pores located 80 mm from the start of the weld, plus 1.2 mm of undercut on the cap along 150 mm. The applicable acceptance criteria state that rounded indications classified as porosity are unacceptable if a single indication exceeds 3 mm or a cluster exceeds 4 mm in any 25 mm length; undercut is unacceptable if depth exceeds 0.8 mm or 10% of wall thickness, whichever is smaller. The weld has already had one repair at a different location, and the client specification permits a maximum of two repairs per joint. The welder says the porosity came from damp wire and proposes to cover it with a cap pass; production wants the joint shipped today. The film also shows a faint linear indication near the root that the interpreter calls possible lack of fusion.
1. Apply the acceptance criteria and classify each discontinuity. Decide whether the joint is acceptable as-is.
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Porosity: the cluster is 6 mm, which exceeds the 4 mm cluster limit, so it is rejectable. Undercut: 1.2 mm exceeds both the 0.8 mm limit and 2.5 mm, which is 10% of 25 mm; the smaller limit is 0.8 mm, so it is also rejectable. The linear root indication is not yet classified. Possible lack of fusion cannot be accepted on a hunch; it needs a better radiographic technique or UT, and if confirmed as lack of fusion it is a planar defect and rejectable. The joint is therefore not acceptable as-is, regardless of production pressure.
2. Determine the disposition and explain why a cover pass is not acceptable.
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A cover pass is not acceptable because it hides the defect and leaves the stress concentration at the root and cap. Grind the porosity and undercut to sound metal with a smooth taper; for the root indication, gouge to the root and verify by PT or MT before rewelding. Use the qualified repair procedure and any required preheat. Check the two-repair limit: one repair is already used, so this is the last permitted repair; if the root indication requires a separate excavation, combine it into one repair. Re-radiograph the finished repair and document the disposition.
3. Identify likely root causes and preventive actions for the porosity and undercut.
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FCAW-G porosity commonly comes from loss of gas shield, such as draft, spatter-blocked nozzle, hose leak, or too-long stick-out, or from damp flux-cored wire or a dirty start. The damp-wire claim is plausible only if the spool was stored in a humid shop or left open; verify storage and discard suspect wire. Undercut is more likely from travel or angle at the cap, or excessive current or voltage, than from the wire. Prevention: control wind, set preflow, clean the nozzle, store wire properly, use a parameter card, and inspect the first piece. Document the repair and the corrective action so the same combination is not repeated on the next joint.