Shielding Gas Substitution on a Prequalified FCAW-G Procedure
A fabricator is welding A992 plate from 1/2 in. to 1 in. thick using prequalified WPS No. W-12: FCAW-G, E71T-1, 0.045 in. electrode, 75% Ar/25% CO2 at 35–45 cfh, producing 5/16 in. fillets and 1/4 in. partial-joint-penetration groove welds. A supplier shortage prompts the shop to switch the same WPS to 100% CO2, arguing that 'the electrode, the wire feed, the amperage and the travel speed are all unchanged, and CO2 is a common, cheaper gas.' There is no PQR in the shop's file - the procedure has always been used as prequalified. The welders' performance qualification test records show FCAW-G with the original 75/25 gas. The special inspector notices the cylinder labels and the flowmeter settings during a production shift and later finds roughly two weeks of welds made since the change.
1. Is the shop's change permitted under a prequalified WPS? Explain the governing principle.
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No. A prequalified WPS is a procedure that the code accepts without qualification testing only because it stays inside the prequalified limits. For FCAW-G, the shielding gas composition is one of those limits, and the prequalified tables give specific ranges tied to the electrode classification and gas mixture.
Changing from a 75% Ar/25% CO2 blend to 100% CO2 is not a within-limits adjustment: it changes arc characteristics, penetration profile, oxygen potential, and hydrogen level, and it moves the procedure outside the prequalified envelope. Once outside, the procedure is not prequalified, and the code's default requirement is qualification by testing that produces a PQR supporting a new WPS. The fabricator's argument that the electrode and current are unchanged addresses only some variables; a single uncontrolled variable is enough to invalidate prequalification. Nothing in the contract gives the fabricator authority to make an essential variable change without procedure qualification and acceptance by the EOR.
2. What happens to the welds already made with the wrong gas, and what is the inspector's immediate action?
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Those welds were made to a WPS that did not apply, so they are nonconforming work, regardless of how they look. Immediate actions:
- Stop the operation and notify the contractor, the EOR and the AHJ in writing; issue a nonconformance report.
- Bound the problem: use weld maps, date codes, shift logs and gas cylinder delivery records to identify every weld made after the gas change. Assume the two weeks identified is the exposure window unless records prove otherwise.
- Do not self-approve remediation. The EOR decides the disposition. Typical options: removal and rewelding; evaluation by NDT beyond the routine schedule plus possible mechanical testing; or acceptance by engineering evaluation where the EOR can demonstrate adequate capacity.
- Require the shop to revert to 75/25 on W-12, or qualify a new WPS before any further welding with 100% CO2, and hold production welding in the meantime.
- Confirm no welds were made after the change on quenched-and-tempered or high-restraint joints without the EOR's specific review.
An inspector does not back-fit an approval to keep the schedule; the record must show the deviation was raised and dispositioned.
3. If the shop chooses to qualify a 100% CO2 procedure, what else must be checked before welding resumes?
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Qualification of the WPS is necessary but not sufficient. Before production resumes, verify:
- PQR test results are per the required tests for the joint type and thickness, and the ranges bracket actual production (thickness, position, electrode diameter, current, preheat/interpass, gas flow).
- The new WPS is complete and issued - gas, flow rate, current, voltage, travel speed, preheat/interpass, cleaning, and joint details - and is available at the point of use.
- Welder performance qualification: compare the welders' existing qualification records against the essential variables of the new WPS. If the changed condition falls outside their qualification envelope, requalify; do not assume that a welder qualified on the old gas is automatically qualified on a procedure whose essential variables have changed.
- Consumable control: confirm the E71T-1 is stored and used per the manufacturer's recommendations, that any reclassified electrode has the correct classification, and that the gas mixer/flowmeter calibration is current.
- Production monitoring: re-establish the shift inspection routine and the documentation trail so a second silent change does not occur.