FCAW Porosity and a Proposed Wire Switch
A fabricator is running 8 mm flux-cored fillets on 12 mm ASTM A36 with 1.2 mm E71T-1 wire, CO2 shielding gas at 16 L/min, DCEP, 25 V and 210 A. Halfway through the shift the welder reports elongated wormhole porosity running down the centre of the bead, an occasional pop, and one birdsnest at the drive rolls. The bay door has been open all afternoon for ventilation and the floor near the machine is damp from a leaking hose. Maintenance has already replaced the contact tip. The welder wants to switch to a self-shielded E71T-11 wire 'so the wind doesn't matter' and keep running on the same machine settings. The QC lead wants the beam finished today and suggests skipping post-weld inspection because the defects look small.
1. Give a systematic diagnostic sequence for the wormhole porosity and the birdsnest, and state the likely correction for each.
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Wormholes (elongated porosity along the bead centre):
- Check the actual gas flow at the nozzle with a flowmeter, not at the regulator. If it reads 16 L/min with the door open, much of it never reaches the arc. CO2 shielding is disrupted by a light breeze, so close the door or use screens and re-check.
- Inspect the nozzle for spatter bridging and the contact tip for recess or wear; a partially blocked nozzle gives exactly this kind of intermittent porosity.
- Check the wire: damp floor, an open spool, or rust on the wire surface introduces hydrogen. Store wire sealed and dry.
- Check contact-tip-to-work distance. An excessively long CTWD drops voltage and loses gas coverage.
Birdsnest: the wire stopped feeding at the tip while the drive rolls kept pushing. Check drive roll tension and alignment, liner condition and length, tip size versus wire, and for burnback or a kink in the gun lead.
The combination points first to lost gas coverage plus a feed problem, both of which are equipment and environment issues rather than a wire defect.
2. Evaluate the welder's proposal to switch to E71T-11 without changing machine settings.
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E71T-11 is a legitimate answer to a drafty shop, but it is not a drop-in substitution.
- Polarity: E71T-11 is designed to run electrode negative (DCEN). Leaving the machine on DCEP will give poor penetration, excess spatter and an unstable arc.
- Settings: voltage, wire feed speed and CTWD all differ from E71T-1; the wire runs on recommended parameters from the manufacturer's data sheet, not from the gas-shielded procedure.
- No gas: the gas solenoid and flowmeter are irrelevant, and the nozzle can be removed or kept as a guide.
- Properties: self-shielded wires typically have lower toughness than gas-shielded E71T-1 and may not meet a Charpy requirement in the specification. Check the job's mechanical requirements before substituting.
- Qualification: changing the consumable and process requires an approved WPS and, in most codes, re-qualification and welder familiarization.
So the answer is yes in principle, no in practice without engineering approval, a qualified procedure and correct polarity.
3. How would you respond to the suggestion to skip post-weld inspection, and what would you require before the beam ships?
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Skipping inspection is not an option. The acceptance criteria belong to the contract and the applicable code, not to the production schedule, and 'small' is not a disposition. Approving weld metal with known porosity also makes the fabricator responsible for any failure later.
Required before shipping:
- Stop and correct the process as diagnosed, then run a test fillet on scrap and bend or break it to confirm porosity is gone before restarting production.
- Identify the suspect length: all weld made since the problem started, plus the material welded with the birdsnest and any stop/start areas.
- Inspect 100% of that length visually, then with the NDT method the specification calls for. Repair by excavating to sound metal, verifying removal with MT or PT, and rewelding to a qualified WPS.
- Re-inspect the repairs to the same criteria and record everything.
- Fix the environment: close or screen the door, repair the leaking hose, store wire dry.
The schedule pressure is real, but a rejected beam at the customer's site costs far more than a day's delay.