Porosity on a Gas-Shielded Stainless Job
A shop is welding 6 mm Type 304 stainless plate in the flat position using GTAW with argon and 2.4 mm ER308L filler. The first articles passed, but a batch of ten parts now shows scattered porosity and a gray-black surface oxide along the bead. The welder recently switched from a water-cooled torch to an air-cooled torch, and the gas flow was raised from 9 to 20 L/min to 'get more coverage.' The argon cylinder was exchanged the same week. Fitting is good, the joint is clean, and the machine settings are unchanged at 110 A. The inspector wants the parts scrapped; production wants them reworked.
1. Diagnose the most likely causes of the porosity and oxide, and explain the reasoning behind each.
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Several clues point to shielding problems rather than base-metal contamination. Raising flow from 9 to 20 L/min on an air-cooled torch can create turbulence at the nozzle, entraining room air into the arc and producing porosity; excessive flow is a common cause of exactly this symptom. The switch to an air-cooled torch may also mean a smaller gas lens or a different cup, changing coverage. A newly exchanged cylinder raises the possibility of a poorly purged line, a leaking fitting, or the wrong gas, so verify the label and purge the hose. Gray-black oxide on stainless suggests inadequate shielding or overheating, consistent with air entrainment. Check for drafts, a clogged or spattered nozzle, and a damaged O-ring. Confirm filler and base metal are clean and dry. The diagnosis is supported by the fact that settings and fit-up did not change while gas delivery did.
2. Recommend a sequence of corrective actions and state what evidence would confirm the fix before the batch is released.
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Start by restoring known-good gas delivery: set flow back to roughly 9-12 L/min using a flowmeter at the cup, inspect the torch for leaks and a damaged cup, and purge the line after the cylinder change. Weld a test coupon and inspect it visually and with dye penetrant or a bend test. If porosity persists, check for drafts and add a wind screen, then verify gas purity. Only after a coupon passes should the batch be reworked. For the existing parts, grind out porosity to sound metal, re-weld, and re-inspect; do not simply cover it. Record the root cause and the corrected parameters so the first-article approval reflects the actual torch and gas setup.