Porosity on a Pressure Pipe Root Pass
You are TIG welding the root pass on 50 mm NB schedule 40 carbon steel pipe in the 6G position for a low-pressure air line. The procedure requires a consumable insert and argon purge. After the first three joints, radiographic inspection reports scattered porosity and one incomplete fusion defect at the 5 o'clock position. The gas flow meter reads 12 L/min, the purge is running, and the pipe ends were cleaned with a grinding disc that was previously used on galvanised steel. The ambient temperature is 4 degrees C and the pipe has been stored outside overnight.
1. Rank the likely causes of the porosity and incomplete fusion, and justify your ranking.
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The strongest candidate is contamination. A grinding disc previously used on galvanised steel can transfer zinc and other material into the joint face, and zinc in the weld pool produces porosity and fume. That fits scattered porosity across the joint. Second is moisture: pipe stored outside at 4 degrees C will have condensation on the bore and joint faces, and moisture is a classic hydrogen source for porosity in a TIG root.
Incomplete fusion at 5 o'clock is more likely a technique and access issue. In 6G, 5 o'clock is a difficult position where the arc can be held too far from the sidewall, travel can be rushed, or the torch angle can fail to direct heat into the root. Low ambient temperature also increases heat loss from the pipe, so the welder may be running too cold or moving too fast to compensate.
Gas flow at 12 L/min is plausible but should be checked for drafts and for turbulence drawing air in; a purge that is not verified at the joint is another possible contributor. Ranking: contamination first because it explains the scattered pattern and the disc history is specific evidence; moisture second; technique and heat input for the fusion defect; gas coverage as a check rather than the primary cause.
2. State the corrective actions and the evidence you would collect before the next joint is welded.
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Actions: remove the contaminated surface by grinding back to clean metal with a disc dedicated to carbon steel, and clean the joint faces and bore with a suitable solvent and lint-free cloth; dry the pipe and bring it to a temperature above the dew point before welding, or apply preheat within the procedure; verify purge quality at the joint, not just at the regulator; check gas flow and shield the joint from drafts; and review technique at 5 o'clock with the welder, including torch angle, arc length and travel speed.
Evidence: the revised cleaning method and disc identification, pipe temperature and preheat record, purge oxygen or flow verification, gas flow setting, and the welder's confirmation of technique. Then re-test the next joint by radiography before releasing the rest of the run. If the defect repeats, stop and review the procedure rather than accepting repeated repair.