WPS / PQR essential variables, heat-input window, and qualification requirements per ASME IX with NORSOK M-630
A super duplex weld looks like an austenitic weld at the bench, behaves like a ferritic weld during cooling, and fails like a sensitised stainless if the procedure drifts from the qualified envelope. Heat input controls cooling rate, cooling rate controls the ferrite-austenite balance, and the balance controls strength, toughness, and pitting resistance. The welding procedure for Ferralium 255 (UNS S32550) is qualified per ASME IX with the NORSOK M-630 supplementary requirements: heat input 0.5 to 2.5 kJ/mm, interpass ≤ 150 °C, no preheat above ambient, no PWHT. Permitted processes: GTAW (TIG), GMAW (MIG), SMAW (stick), FCAW (flux-cored), SAW (submerged-arc). Matching filler is 25.10.4.L family (AWS A5.9 ER2594, A5.4 E2594-15/-16, A5.22 E2594-T1).
| Variable | Qualified envelope | Why it matters |
|---|---|---|
| Heat input | 0.5 to 2.5 kJ/mm | Controls cooling rate through 1000 to 600 °C window |
| Interpass temperature | ≤ 150 °C | High interpass slows cooling, risks sigma + Cr2N |
| Preheat | Ambient only (no preheat) | Preheat slows cooling rate, same risk as high interpass |
| Filler-metal classification | AWS A5.9 ER2594 (or matching A5.4 / A5.22) | Over-alloyed Ni for austenite formation in fast-cooled weld |
| Shielding gas (GTAW / GMAW) | Ar + 1 to 5% N2 + optional 30% He | N2 in shield maintains weld-metal N for pitting resistance |
| Backing gas (root) | Ar + 2 to 5% N2 | Prevents N loss from root, prevents oxide on root underside |
| PWHT / stress-relief | NOT PERMITTED | Any soak in 600 to 1000 °C precipitates sigma |
| Joint geometry | Per qualified PQR | Wider gap = more passes = more re-heat cycles on root |
| Test | Standard | Acceptance |
|---|---|---|
| Tensile transverse | ASTM E8 / A370 | Failure in parent metal at ≥ parent UTS |
| Bend (face + root) | ASME IX paragraph QW-462.3 | No defect > 3 mm in any direction |
| Charpy V-notch (weld + HAZ) | ASTM E23 at -46 °C | ≥ 45 J weld centreline, ≥ 35 J HAZ |
| Hardness traverse | ASTM E384 (Vickers HV10) | ≤ 310 HV10 (NACE MR0175 ≤ 32 HRC equivalent) |
| Ferrite content (weld) | ASTM E562 point count | 35 to 65% |
| Microstructure | ASTM A923 method A | No sigma, chi, Cr2N at 500× |
| Pitting test | ASTM G48 method A at 35 °C | Mass loss ≤ 4 mg/cm² in 24 hours, no pits |
| NDE | RT or UT 100% on root + cap | Per AWS D10.18 acceptance |
