Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex 2507 Welding Procedure
Welding Super Duplex 2507 (UNS S32750, EN 1.4410) demands tight control of heat input, interpass temperature, and shielding-gas chemistry to preserve the balanced ferrite-austenite microstructure and to avoid sigma-phase embrittlement in the heat-affected zone (HAZ). The qualified welding procedure specification (WPS) is normally written to ASME Section IX with additional requirements from NORSOK M-601. Recommended ranges for the core variables are heat input 0.5 to 2.5 kJ per millimetre, interpass temperature below 150 degrees Celsius, argon plus 2 to 5 percent nitrogen shielding for the GTAW root, and post-weld ferrite content 35 to 65 percent by Feritscope. Common processes are GTAW (TIG) for root and thin sections, GMAW (MIG) for fill on heavier wall, SAW for thick plate, and SMAW for repair and field welding.
Filler metal selection is covered separately on the welding-filler page (ER2594 overmatching, ER2553 matching, both nitrogen-enhanced). Sigma-phase metallurgy is on the sigma-phase page. Ferrite acceptance criteria and measurement methods are on the ferrite content page.
Welding Processes Used on Super Duplex 2507
| Process | Typical Application | Comments |
|---|---|---|
| GTAW (TIG, 141 / 142) | Root pass on pipe and pressure vessels; thin-wall fabrication | Cleanest weld; argon plus 2 to 5 percent nitrogen shielding mandatory; manual or orbital |
| GMAW (MIG, 131 / 135) | Fill and cap on pipe wall above 6 mm; structural fabrication | Solid wire ER2594 or ER2553; shielding Ar plus 2 percent CO2 plus 1 to 2 percent N2; pulsed mode preferred |
| FCAW (136 / 138) | Vertical and overhead fill in field welding | E2594T1 or E2553T1 nitrogen-enhanced flux-cored wire; metal-cored variants available for low-spatter |
| SAW (121) | Heavy plate fabrication, longitudinal seams in pipe mill | Solid wire plus basic flux; high heat input (3 to 5 kJ per mm) requires careful heat-input control or balanced two-wire setup |
| SMAW (111) | Repair welding, short field joints, overhead positions | E2594-15 or -16 basic-coated electrode; nitrogen pickup from coating decomposition; back-bake before use |
Heat Input Control
Heat input is the master variable in super duplex 2507 welding. Too low, and the weld metal cools fast enough to lock in ferrite excess (above 65 percent) and risk Cr2N nitride precipitation. Too high, and the cooling rate falls below the sigma-phase nose, allowing sigma to nucleate in the HAZ. The qualified band is normally 0.5 to 2.5 kJ per millimetre, with the actual production set-point determined during WPS qualification.
Heat input is calculated as:
Heat input (kJ per mm) = (Volts x Amps x 60) / (Travel speed in mm per minute x 1000)
| Heat Input, kJ per mm | Microstructural Outcome | Disposition |
|---|---|---|
| Below 0.4 | Ferrite excess; Cr2N nitride risk in HAZ; possible cracking in heavy restraint | Outside qualified range; not acceptable |
| 0.5 to 2.5 (qualified band) | Balanced ferrite-austenite; minimal sigma risk if interpass controlled | Acceptable per typical WPS |
| Above 2.5 | Sigma-phase risk in HAZ; toughness loss; pitting-resistance loss | Outside qualified range; not acceptable |
Interpass Temperature
Interpass temperature is the temperature of the previously deposited weld metal immediately before the next pass is started. For super duplex 2507, the standard limit is 150 degrees Celsius maximum. Higher interpass slows the cooling rate of subsequent passes, prolonging dwell in the sigma-phase nucleation band and degrading microstructure. Interpass is measured by surface-contact thermocouple or calibrated temperature crayons (Tempil sticks) on the deposited weld bead, not on the surrounding cold parent metal.
In high-production environments, interpass control is enforced by mandatory cooling pauses between passes, by water-mist cooling on the back of the weld (where service permits), or by alternating between two adjacent joints to allow each to cool while the other is being welded.
Shielding Gas Selection
| Process | Recommended Shielding | Backing Gas (root pass) |
|---|---|---|
| GTAW root | Argon plus 2 to 5 percent nitrogen | Argon plus 2 to 5 percent nitrogen (purge) |
| GTAW fill | Argon plus 2 percent nitrogen (or pure argon) | (Not applicable) |
| GMAW fill | Argon plus 2 percent CO2 plus 1 to 2 percent N2 | (Not applicable) |
| FCAW fill | Argon plus 20 to 25 percent CO2 (per electrode datasheet) | (Not applicable) |
| SAW | Basic flux (per electrode datasheet) | (Not applicable) |
| SMAW | Basic-coated electrode (no shielding gas) | (Not applicable) |
Nitrogen addition (2 to 5 percent in argon) compensates for nitrogen loss during the high-temperature welding cycle, preserves the austenite-stabilising effect of nitrogen, and lowers the weld-metal ferrite content into the acceptable 35 to 65 percent range. Without nitrogen addition, the GTAW root in particular tends ferrite-rich and is at risk of Cr2N nitride precipitation.
Joint Preparation and Fit-Up
- Bevel: V-bevel 60 to 70 degrees included angle for pipe and plate up to 25 mm wall. Compound bevel (J or U) for heavier wall to reduce filler-metal volume and heat input.
- Root face: 1.5 to 2.0 mm for GTAW root; 0 to 1.0 mm for fully open root.
- Root gap: 2.0 to 3.0 mm for open-root GTAW; 0 mm with consumable insert.
- Cleanliness: Joint surfaces and 25 mm beyond on each side mechanically cleaned to bare metal with stainless wire brush or grinding wheel reserved for stainless. No carbon-steel tooling or brushes; iron contamination is a stress-corrosion-cracking initiator in service.
- Tacking: Tack welds made with the same filler and procedure as the production pass. Tacks consumed by the root pass; not left in place.
- Backing gas purge: Confirm by oxygen sensor (oxygen below 0.5 percent in the purge volume) before striking the root pass. Inadequate purge produces oxidised root with degraded corrosion resistance.
Post-Weld Verification
| Test | Acceptance | Reference |
|---|---|---|
| Visual examination | No undercut, no overlap, no spatter, no surface cracks | ASME IX, AWS D1.6 |
| Liquid penetrant (PT) | No relevant indications | ASME V Article 6 |
| Radiography (RT) or ultrasonic (UT) | Per ASME VIII Div 1 or project spec | ASME V Article 2 / 4 |
| Ferrite content (Feritscope) | 35 to 65 percent in weld metal | NORSOK M-630, AWS A4.2M |
| Charpy V-notch at minus 46 deg C (qualification) | 45 J avg, 35 J min on weld metal and HAZ | NORSOK M-601, ASTM E23 |
| Sigma screening (ASTM A923 Method A) | No unaffected (sigma-bearing) microstructure | ASTM A923 |
| Pitting resistance (ASTM G48 Method A) | Below 4.0 g per square metre weight loss at 35 deg C, 24 hr | NORSOK M-601, ASTM G48 |
Related Super Duplex 2507 References
- UNS S32750 reference page
- Welding filler (ER2594, ER2553)
- Sigma-phase embrittlement
- Ferrite content (NORSOK M-630)
- Heat treatment
- Super duplex pipe and fasteners
- NORSOK M-630
Super Duplex 2507 Welding FAQ
What heat input range applies to super duplex 2507 welding?
0.5 to 2.5 kJ per millimetre is the standard qualified band. Below 0.4 kJ per mm produces ferrite-excess weld metal at risk of Cr2N nitride; above 2.5 kJ per mm slows the cooling rate enough that sigma phase nucleates in the HAZ. The actual set-point within the qualified band is fixed during WPS qualification and reproduced in production.
What is the maximum interpass temperature?
150 degrees Celsius. Higher interpass slows the cooling rate of each subsequent pass, prolonging dwell in the sigma-phase nucleation band (600 to 1000 deg C) and degrading microstructure. Interpass is measured by surface-contact thermocouple or temperature crayons on the deposited weld bead, not on the surrounding cold parent metal.
Why is nitrogen added to the GTAW shielding gas?
Nitrogen is the strongest austenite stabiliser in the super duplex chemistry. The high-temperature welding cycle drives nitrogen out of the weld metal toward the gas phase. Adding 2 to 5 percent nitrogen to the argon shielding gas replaces the lost nitrogen, preserves austenite formation, and lowers weld-metal ferrite content into the 35 to 65 percent acceptable range. Without nitrogen, the GTAW root tends ferrite-rich and is at risk of Cr2N nitride precipitation.
Is post-weld heat treatment required?
No. PWHT in the 600 to 1000 deg C band would re-introduce sigma phase. Where stress mitigation is required, it is achieved by controlled welding (low heat input, low restraint, controlled interpass) rather than by PWHT. If a full re-solution anneal is performed after welding, it must be at the full 1040 to 1100 deg C soak temperature followed by water quench, which restores microstructure but is rarely practical for completed fabrications.
Which welding standard is most commonly invoked for super duplex 2507?
ASME Section IX is the base WPS qualification standard. NORSOK M-601 is the dominant supplementary specification on Norwegian Continental Shelf and offshore projects worldwide; it adds the 0.5 to 2.5 kJ per mm heat-input band, the 150 deg C interpass limit, the Charpy V-notch requirement at minus 46 deg C, the ferrite-acceptance criterion, and the ASTM G48 pitting test on the qualified procedure.
Can carbon-steel wire brushes be used for joint preparation?
No. Carbon-steel tooling and brushes leave iron contamination on the stainless surface. Iron contamination is a stress-corrosion-cracking and rust-blooming initiator in chloride-bearing service. Joint surfaces are cleaned with stainless wire brushes reserved for stainless work, or with grinding wheels marked for stainless use only.
What is the minimum purge oxygen level for the GTAW root pass?
Below 0.5 percent oxygen in the backing-gas volume immediately before the root pass is struck, verified by inline oxygen sensor. Inadequate purge produces an oxidised root pass with chromium-depleted oxide that degrades corrosion resistance. The purge volume is normally argon or argon plus 2 to 5 percent nitrogen, sized to allow at least three volume changes before welding starts.
Satisfaction Survey
