Root Pass Porosity After Shielding Gas Change
A contractor is welding 12.7 mm thick ASTM A516 Gr.70 plate with the GMAW spray-transfer process using ER70S-6 filler at 1.2 mm diameter. The WPS specifies 75% Ar/25% CO2 at 15 L/min, 26 V, 240 A, and a 3 mm root gap. Midway through the job the supplier delivers 85% Ar/15% CO2 cylinders in error, and the crew uses them without checking labels. The first 6 m of completed butt weld passes visual inspection, but radiography of the next 4 m reveals scattered porosity concentrated in the root and a slightly narrower, more finger-like penetration profile. The welder insists nothing else changed. Ambient temperature is 18 C, humidity 55%, and the plate is clean and dry.
1. Explain why changing the shielding gas from 75/25 to 85/15 produced root porosity and altered the penetration profile.
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The shielding gas composition controls both arc physics and metallurgical protection. Reducing CO2 from 25% to 15% lowers the reactive oxygen potential of the gas, which changes the surface tension and flow of the weld pool. The higher argon content promotes a more constricted, finger-like penetration pattern in spray transfer, matching the observed change. More importantly, the lower CO2 level reduces the oxygen available to react with residual surface films and reduces the gas's ability to suppress nitrogen and hydrogen pickup. In the root region the shielding is least effective because of turbulence and the gap geometry, so any reduction in reactive gas makes the root more vulnerable to porosity. Additionally, the crew likely did not adjust voltage or current: the 85/15 gas at 26 V and 240 A may run with a slightly different arc length and transfer stability, creating spatter and uneven shielding. The correction is to verify gas composition against the WPS, restore 75/25, and re-qualify if the intended gas ever changes. The porosity is a process variable violation, not a welder skill issue.
2. What immediate actions and longer-term controls should the contractor take?
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Immediate: stop production, quarantine the 85/15 cylinders, and identify all welds made with them. Radiograph or UT the affected 4 m plus any suspect adjacent lengths, and cut out and repair the porous root. Re-verify gas flow with a flowmeter at the nozzle and check for leaks. Longer term: implement a receiving inspection that checks gas cylinder labels and certificates against the WPS, color-code cylinders, and store only approved mixes at the point of use. Add a pre-job checklist item requiring the welder to confirm gas composition and record the cylinder batch. Review the WPS to confirm which gas it qualifies; if the contractor wants to use 85/15 in future, a new PQR is required because shielding gas is an essential variable. Train the crew that unlabeled or mislabeled gas is a stop-work condition.