Gas Supply Shortfall and the Temptation to Swap Electrodes
A shop is mid-way through 1,200 ft of 1/4 in fillet welds on A36 angle frames. The only qualified WPS/PQR combination is FCAW-G: 0.045 in E70T-1, 75Ar/25CO2 at 35 cfh, DCEP, 200-240 A, 24-27 V, 50 °F minimum preheat, 300 °F maximum interpass, qualified on 1/4 in plate in the 2F and 3F positions. On Monday the gas supplier cuts deliveries and there is enough shielding gas for about two more days. The superintendent proposes switching to self-shielded 0.045 in E71T-8 on the same WPS because it is still FCAW and the classification looks close, then to 0.062 in E71T-8 to make up lost time. The customer's inspector arrives Thursday. No other WPS exists.
1. Can production continue under the existing WPS with E71T-8? Justify your answer.
Read the worked answer
No. The existing WPS is supported only by a PQR welded with a gas-shielded electrode.
- Moving from gas-shielded to self-shielded FCAW changes the electrode classification and the way the arc is protected; the mechanical test results and soundness on the PQR belong to E70T-1, not E71T-8.
- Self-shielded wires normally run on a different polarity and need a different stickout and slag practice. Those are conditions the original PQR never proved.
- Moving from 0.045 in to 0.062 in changes current density, deposition and heat input for the same nominal current range. The WPS ranges were never qualified at that size.
- Electrode classification and diameter changes sit in the essential variable tables of the structural codes, so the WPS is unsupported and any weld made under it is a nonconformance the inspector will write up on Thursday.
This is exactly the change that 'looks the same' on the shop floor and is not the same on paper.
2. Give your recommended course of action with supporting numbers. Facts: four welders, 35 cfh, about 6 h arc time per welder per day; an alternate supplier can deliver 15 cylinders on Wednesday at $180 each instead of $130; a self-shielded PQR with tensile, bend and lab turnaround takes 4-6 working days and $2,000-3,500.
Read the worked answer
Cost of running out of gas
- Idle crew: 4 welders x 8 h x 2 days x $58/h = $3,712, before schedule impact and the inspector's visit.
Cost of premium gas
- 15 cylinders x $50 premium = $750, and the profile stays inside the qualified WPS.
Cost and time of requalification
- $2,000-3,500 and 4-6 working days, plus a new WPS and a welder-qualification review. That is longer than the gas gap, so it does not solve this problem.
Recommendation
- Buy the premium gas, keep the qualified WPS and the qualified welders, and tell the inspector what happened so there is no surprise.
- Start the self-shielded PQR as a planned future capability with a proper schedule, not as a rescue on Monday.
3. If self-shielded FCAW is adopted later, what must be in place before any production weld is made under it?
Read the worked answer
Before production:
- A new PQR welded with the actual wire size, polarity and position range, with mechanical testing that covers the thickness range and any toughness requirement.
- A WPS written from that PQR, with current, voltage, polarity, stickout, travel speed, preheat and interpass.
- A welder qualification review against the code's essential variable table for performance qualification. Do not assume the existing FCAW qualification carries across; technique and polarity differ, and the conservative and defensible route is to requalify.
- Customer or engineer approval where the specification names the process, plus a revised inspection plan that accounts for the different slag and penetration characteristics.
- A safety review: self-shielded wires generally generate more fume, so ventilation and manganese exposure need to be assessed before the first production arc, not after.