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 Heat Treatment (Solution Anneal Only)
Super Duplex 2507 (UNS S32750, EN 1.4410) is supplied in the solution-annealed and water-quenched condition only. The full heat-treatment cycle is solution anneal at 1040 to 1100 degrees Celsius followed by rapid water quench. There is no precipitation hardening, no aging, and no temper. The water quench must drive the material rapidly through the 600 to 1000 degrees Celsius band to suppress sigma-phase formation, which would compromise both Charpy toughness (cut by 80 percent or more) and pitting resistance. Slow cooling (air cool, oil quench) is not acceptable. Heavy sections require special quench-tank design and intermediate forging temperatures to keep the centreline above the critical cooling rate.
The cycle is governed by ASTM A479, ASTM A182 F53, ASTM A1082, EN 10088-3 (1.4410), and NORSOK M-630. Manufacturer qualification per NORSOK M-650 is required for North Sea projects, and the qualification heat-treatment record (QTR) is reviewed against the chemistry, section thickness, and quench medium used in production.
Solution-Anneal Cycle
A correctly executed solution-anneal cycle dissolves intermetallic phases, restores the 50 to 50 austenite-ferrite balance, and locks the microstructure by quenching faster than the sigma-phase nose on the time-temperature-transformation curve.
| Step | Parameter | Acceptance Window |
|---|---|---|
| 1. Heat to soak temperature | Ramp rate | Limited only by furnace; no thermal-shock concern |
| 2. Soak temperature | 1040 to 1100 deg C (ASTM A479) / 1025 to 1125 deg C (ASTM A1082) | 1075 deg C is a common production set-point |
| 3. Soak time | 30 minutes per 25 mm (1 inch) of cross-section, 60 minutes minimum | Long enough to dissolve sigma and chi phases that may have formed in prior thermal cycles |
| 4. Quench medium | Water (agitated, ambient temperature) | Air cool and oil quench are NOT acceptable |
| 5. Quench rate target | Cool from soak temperature to below 300 deg C as fast as possible | Critical band is 1000 down to 600 deg C; transit must be measured in seconds, not minutes |
| 6. Post-quench condition | Solution-annealed, no further heat treatment | Mill test certificate reports condition as "solution annealed and water quenched" |
No Precipitation Hardening, No Aging, No Temper
A common engineering misconception is that super duplex 2507 has an aged or hardened temper similar to precipitation-hardening grades like 17-4 PH (UNS S17400). It does not. Strength in super duplex 2507 comes from the duplex microstructure itself, not from precipitates. Any aging or tempering operation between 600 and 1000 degrees Celsius would introduce sigma phase, destroying both toughness and corrosion resistance.
Mill test certificates report the heat-treatment condition as "solution annealed and water quenched" or equivalent. There is no H1100, H1150, H950, or similar designation in the super duplex 2507 specifications. Where a project drawing calls for an aged temper on super duplex 2507, the drawing is in error and should be referred back to the design authority before fabrication.
Sigma-Phase Risk in the 600 to 1000 Degree Celsius Band
Sigma is a hard, brittle iron-chromium-molybdenum intermetallic that nucleates rapidly at ferrite grain boundaries when super duplex is held in the 600 to 1000 degree Celsius temperature range. Even a few minutes in this band can be enough to initiate sigma formation and cause measurable loss of Charpy toughness. The water quench at the end of the solution-anneal cycle exists for one purpose: to transit this band fast enough that sigma cannot nucleate. The detailed nucleation kinetics, TTT diagram, and welding implications are covered on the sigma-phase embrittlement page.
| Temperature, deg C | Risk | Avoidance Strategy |
|---|---|---|
| Above 1100 | Grain coarsening, ferrite excess | Cap soak temperature at 1100 deg C unless specified otherwise |
| 1040 to 1100 | Solution-anneal soak window (intended operating point) | Hold for soak time, then quench |
| 1000 to 600 | Sigma-phase nucleation band (avoid at all costs) | Quench through this range as fast as possible |
| Below 600 | Sigma kinetics negligible | Material is safe once below 300 deg C |
Heavy-Section Considerations
The principal challenge in super duplex 2507 heat treatment is achieving an adequate quench rate at the centreline of heavy sections. Conduction-limited cooling means the centre of a 200 mm bar cools far slower than the surface, and at some critical thickness the centreline cannot be quenched fast enough to escape the sigma-phase nose. Practical limits depend on quench-tank design (volume, agitation, water temperature), but published mill experience indicates:
- Up to 100 mm cross-section: standard agitated water quench from a single-zone furnace is normally adequate.
- 100 to 200 mm cross-section: requires a dedicated high-flow quench tank with strong agitation; spray quench may be added at the surface.
- Above 200 mm cross-section: NORSOK M-650 manufacturer qualification is essential; mid-thickness Charpy and ferrite testing on a sacrificial test block is normally required by the project specification.
For forgings and heavy plate, the heat-treatment record (QTR) under NORSOK M-650 documents the soak time, quench medium, water temperature, agitation, and time-from-furnace-to-quench, all of which determine whether centreline microstructure meets the ferrite, hardness, and Charpy targets.
Post-Fabrication Heat Treatment
Cold work (bending, forming, threading by rolling) above approximately 10 percent strain may warrant a re-solution anneal to restore the microstructure. The decision is normally taken by the design authority based on the cold-work magnitude and the corrosion environment. Hot work above 1000 degrees Celsius (forging, hot upsetting) is followed by full re-solution anneal and water quench as standard.
Stress-relief heat treatment between 600 and 1000 degrees Celsius is NOT applied to super duplex 2507. The same temperature range that would relieve residual stress also nucleates sigma phase, and the pitting-resistance loss outweighs any benefit from stress relief. Where residual-stress mitigation is required, it is achieved by controlled welding (low heat input, low restraint) rather than by post-weld heat treatment.
Heat-Treatment Quality Controls
A controlled super duplex 2507 heat-treatment cycle is verified by the following measurements on every lot:
| Test | Acceptance | Reference |
|---|---|---|
| Tensile and yield strength | 800 / 550 MPa minimum | ASTM A370 |
| Hardness | 33 HRC max (general), 28 HRC max (NACE) | ASTM E18, NACE MR0175 |
| Charpy V-notch at minus 46 deg C | 45 J avg, 35 J min individual | NORSOK M-630, ASTM E23 |
| Ferrite content | 35 to 55 percent (base material) | NORSOK M-630, ASTM E562 |
| Pitting resistance, ASTM G48 Method A | No pits at 35 deg C, 24 hour exposure | NORSOK M-630, ASTM G48 |
Related Super Duplex 2507 References
- UNS S32750 reference page
- Sigma-phase embrittlement
- Ferrite content (NORSOK M-630)
- Mechanical properties
- Welding procedure
- ASTM A182 F53 forgings
- NORSOK M-630
Super Duplex 2507 Heat Treatment FAQ
What is the heat treatment of super duplex 2507?
Solution anneal at 1040 to 1100 degrees Celsius for 30 minutes per 25 mm of cross-section (60 minutes minimum), followed by rapid water quench. There is no precipitation hardening, no aging, no temper. The cycle restores the 50 to 50 austenite-ferrite balance and locks the microstructure by quenching faster than the sigma-phase nose on the TTT curve.
Why is rapid water quench required?
The 600 to 1000 degree Celsius temperature band is the sigma-phase nucleation window. Sigma is a brittle iron-chromium-molybdenum intermetallic that cuts Charpy toughness by 80 percent or more and reduces pitting resistance. Slow cooling (air cool, oil quench) allows sigma to form. Water quench, ideally with strong agitation, transits the critical band fast enough to suppress sigma nucleation.
Can super duplex 2507 be precipitation hardened?
No. Strength in super duplex 2507 comes from the duplex microstructure itself, not from precipitates. There is no aged temper analogous to H1100 or H950 used on precipitation-hardening grades like 17-4 PH. Aging in the 500 to 700 degree Celsius range introduces sigma phase and 475 deg C embrittlement; both are damage mechanisms, not strengthening mechanisms.
Is air cooling acceptable for thin sections?
No. Even thin sections are quenched in water for super duplex 2507. The mill specifications (ASTM A479, A1082, A182, EN 10088-3) require water quench, and acceptance criteria for ferrite content, Charpy toughness, and ASTM G48 pitting test cannot be reliably met from air-cooled material. Air-cooled super duplex 2507 will normally fail one or more of these tests.
Is post-weld heat treatment required for super duplex 2507?
No. Post-weld heat treatment in the 600 to 1000 degree Celsius band would re-introduce sigma phase. Where the project requires stress mitigation, it is achieved by controlled welding (low heat input, low restraint, controlled interpass temperature) rather than by PWHT. If a full re-solution anneal is performed after welding, it must be at the full 1040 to 1100 degree Celsius soak temperature followed by water quench.
What is the maximum section thickness that can be solution annealed?
There is no absolute upper limit, but practical capability depends on quench-tank design. Sections up to 100 mm are routinely processed in standard agitated water quench tanks. Sections from 100 to 200 mm need dedicated high-flow tanks with strong agitation. Above 200 mm, NORSOK M-650 manufacturer qualification with sacrificial test-block measurement is normally required to confirm centreline microstructure meets the ferrite, hardness, and Charpy targets.
Can sigma-phase damage be reversed by re-solution annealing?
Yes, in principle. Re-solution annealing at 1040 to 1100 degrees Celsius for an extended soak (90 minutes per 25 mm of cross-section) followed by rapid water quench will dissolve sigma phase and restore the duplex microstructure. The decision to re-treat versus scrap depends on the magnitude of the damage and project economics; for forgings or heavy plate, re-treatment is usually preferred. The repaired material must pass all original acceptance tests (ferrite, Charpy, ASTM G48) before it can be released.
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