Clustered porosity after a submerged-arc flux change on a 25 mm shell seam
A fabricator is welding the longitudinal seam of a 25 mm S355J2+N shell, double-V, multi-pass submerged-arc welding on a column-and-boom tractor. The qualified WPS uses 4.0 mm wire, DC+, 620 A, 28 V, 340 mm/min, and flux to ISO 14174, classification SA FB 1 55 AC H5. Halfway through the order the shop runs out of flux and buys flux of the same classification from a second manufacturer, keeping the same WPS. The next three seams show clustered porosity on radiographs, concentrated in the first 80 mm of the seam, at stop/starts and in the mid-wall; the operator also reports arc wander near the earth connection. The production manager proposes to keep welding, saying that "the flux class has not changed, so the WPS is still valid", and the vessel is due for delivery in six days.
1. Diagnose the likely causes of the porosity and the arc wander, and rank the checks you would make before any further welding.
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Start from the correlation: three seams, all after the flux change, with repeatable defect positions. That points to the flux and its handling first, not to the operator.
Flux. Basic agglomerated (FB) fluxes are hygroscopic. A different manufacturer means different packaging, particle size, coating and, critically, different as-delivered moisture and drying recommendation. Porosity concentrated at start/stop indicates poor flux coverage before the arc stabilises and flux that is not dry and fluid; porosity in the mid-wall indicates moisture or contamination entering the arc. Checks: batch certificate and the H5 hydrogen designation; date of manufacture; whether the bag was opened and stored in a humid shop; whether the WPS or manufacturer requires baking (commonly 300–350 °C for basic agglomerated flux) and holding at 100–150 °C; whether recovered flux was mixed in; condition of the wire (rust, lubricant, new lot) and contact tip.
Arc wander. Check the work return: position, condition, and whether it is on the far side of the joint, producing an asymmetric field. Also check stick-out, worn contact tip, flux coverage ahead of the arc, and a new wire lot with different cast/helix. Moving the earth and using run-on/run-off plates are immediate trials.
Ranking: flux batch, storage and drying → work return/earth position → start-stop technique and run-on plates → wire, tip and parameter verification with calibrated meters.
2. Is the production manager correct that the WPS remains valid after the flux change? Justify your answer with reference to qualification essential variables and quality-system requirements, and state what evidence you would require before authorising production.
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The statement is half right, and the half that is wrong is the dangerous half.
Under ISO 15614-1 the flux is a consumable essential variable: a change of flux designation or classification requires a new procedure qualification. A change of manufacturer while keeping the same ISO 14174 classification is normally not listed as an essential variable, so a full requalification is not automatically triggered by the brand change alone.
But the label is not the evidence. The H5 suffix is a hydrogen claim that must be substantiated by the new product's batch certificate. The WPS's mechanical properties and porosity behaviour were demonstrated with the other manufacturer's flux, and the defects now observed show that the process has in fact changed. ISO 3834-2 requires consumables to be verified and WPSs to be supported by valid evidence.
Minimum before restart: batch certificate including hydrogen designation; storage and drying records; a documented trial weld in the same grade and thickness using the dried new flux; radiography of that trial; and written confirmation from the responsible welding coordinator that the change is not essential under the applicable standard and contract. If the contract or customer specification treats a change of flux brand as requiring requalification, then requalify to ISO 15614-1. Record the decision and its basis.
3. State the disposition you would give the three welded seams and the conditions you would attach to restarting. Include one health-and-safety point arising from the flux change.
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The three seams are nonconforming until proven otherwise. The radiographs must be evaluated against the acceptance level in the contract (for example ISO 10675-1 with ISO 5817), the indications sized and located, and the seams rejected for repair.
Repair: gouge or grind out the indications to sound metal, confirm removal by PT/MT, re-weld with a qualified repair WPS using the same flux controls, and re-radiograph. Repeated excavation in the same area should be agreed with the responsible engineer.
Conditions for restart:
- flux dried per the manufacturer's instruction, taken from a sealed bag, held in a holding oven;
- run-on/run-off plates on every seam;
- verified work return position and condition;
- first production seam 100% radiographed before the rest of the order continues;
- trial weld accepted and records filed.
Health and safety: basic flux dust contains fluorides, and the flux oven and hot flux are burn hazards. Use local exhaust ventilation at the arc, dust extraction or careful bag handling when charging the oven, gloves and eye protection, and keep eating and drinking out of the welding area.