Porosity Discovered After Visual Pass on a GMAW Butt Weld
A GMAW (short-circuit transfer) butt weld joins two 10 mm carbon steel plates in the flat position. The WPS specifies ER70S-6 wire, 75% Ar/25% CO2 shielding gas at 14 L/min, and a single-V groove with a 2 mm root gap. Visual inspection after welding shows a uniform, slightly convex cap with good tie-in and no visible surface porosity or undercut. However, the inspector notices a faint bluish discoloration along the cap edges and the spatter pattern is heavier than typical for this shop. The gas flow meter was checked at the start of the shift and read 14 L/min. The welder reports the wire spool was changed mid-shift. The code requires visual inspection of 100% of the weld and permits surface porosity up to 1 mm individually.
1. Does this weld pass visual inspection? Explain what the visual evidence does and does not tell you.
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On the visual criteria alone, the weld passes: the cap is uniform, tie-in is good, and there is no visible surface porosity exceeding 1 mm or any undercut. Visual inspection is limited to surface and near-surface conditions accessible to the eye (with or without magnification).
However, the visual evidence is a screen, not a clearance. Several observations warrant follow-up even though they do not by themselves fail the weld:
- Bluish discoloration along the cap edges suggests heat-tint variation, which in carbon steel is generally benign but can indicate inconsistent heat input or shielding at the edges.
- Heavier-than-normal spatter can signal arc instability, wire feed issues, or shielding gas disruption - all of which can produce subsurface porosity that visual inspection cannot detect.
- Wire spool change mid-shift raises the possibility of a wrong wire, contaminated wire, or a feed problem.
The correct position is: the weld is visually acceptable, but the process indicators justify requesting volumetric examination (RT or UT) per the code or the inspection plan if the code requires it for this joint, and verifying the wire certificate and gas flow at the torch (not just the meter). Visual inspection cannot certify internal soundness.
2. The shop supervisor says "the gas meter read 14 L/min, so shielding was fine." Evaluate this reasoning.
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The reasoning is incomplete and potentially wrong. A meter reading at the regulator or flowmeter does not confirm flow at the torch nozzle. Common failure points between the meter and the arc include:
- Leaks or kinks in the gas hose, especially after a spool change when the torch may have been moved.
- A clogged or damaged diffuser/contact tip area, reducing effective coverage even with correct meter flow.
- Drafts or cross-ventilation in the shop blowing the shielding envelope away, which a flowmeter cannot reveal.
- Flow set at the start of the shift may not reflect conditions later; regulators drift and hoses can be disturbed.
For short-circuit GMAW on 10 mm plate, inadequate shielding readily produces subsurface porosity that a visual pass will not catch. The bluish edge tint and heavy spatter are consistent with marginal shielding or arc instability.
My response: acknowledge the meter reading but explain it is necessary, not sufficient. Recommend a torch-flow check (flow meter at the nozzle), inspection of the diffuser and liner, verification of the wire certificate against ER70S-6, and volumetric examination of the suspect weld. Document the process verification in the inspection record.