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Sigma Phase Embrittlement in Super Duplex 2507
Sigma phase is a hard, brittle iron-chromium-molybdenum intermetallic that nucleates rapidly at ferrite grain boundaries when super duplex 2507 (UNS S32750, EN 1.4410) is held in the 600 to 1000 degree Celsius temperature band. It is the single largest microstructural risk in the alloy. Even a few minutes in this band can cause measurable damage: Charpy V-notch toughness drops by 80 percent or more, pitting resistance falls as chromium and molybdenum are robbed from the surrounding matrix, and hardness rises at the precipitate sites. Avoidance is achieved by rapid water quench at the end of the solution-anneal cycle and by controlled welding heat input (0.5 to 2.5 kJ per mm) with interpass temperature held below 150 degrees Celsius.
This page covers the metallurgy of sigma-phase nucleation, the time-temperature-transformation (TTT) curve, the practical thermal exposures that put super duplex at risk, the measurable consequences for mechanical and corrosion performance, and the avoidance and detection strategies built into the relevant standards (NORSOK M-630, ASTM A923, ASTM A1084).
Sigma-Phase Metallurgy
Sigma phase is a tetragonal intermetallic with approximate composition Fe-30Cr-4Mo, varying with the alloy chemistry. It nucleates preferentially at ferrite grain boundaries and at ferrite-austenite interfaces, where the matrix is already chromium-rich and atomic mobility is highest. The thermodynamic stability range for sigma in super duplex 2507 spans 600 to 1000 degrees Celsius; nucleation kinetics are fastest at approximately 850 to 900 degrees Celsius (the nose of the TTT curve).
| Phase | Composition (approximate) | Formation Mechanism | Effect |
|---|---|---|---|
| Sigma | Fe-30Cr-4Mo (tetragonal intermetallic) | Nucleation at ferrite grain boundaries between 600 and 1000 deg C | Brittle; cuts toughness 80 percent or more; lowers PREN locally |
| Chi | Fe-25Cr-15Mo (cubic intermetallic) | Companion to sigma; precipitates earlier and at slightly lower temperatures | Same direction of damage as sigma but typically at smaller volume fractions |
| Cr2N nitride | Chromium-rich nitride | Precipitates in HAZ and rapidly cooled weld metal at 700 to 900 deg C | Lowers pitting resistance locally; toughness less affected than by sigma |
| 475 deg C embrittlement (alpha-prime) | Cr-rich BCC decomposition product of ferrite | Forms in ferrite phase between 280 and 525 deg C over hundreds to thousands of hours | Slow embrittlement risk for long-term elevated-temperature service |
Time-Temperature-Transformation Behaviour
The sigma-phase TTT curve for super duplex 2507 has a classic C shape with the nose at approximately 850 to 900 degrees Celsius. Published nose times for measurable sigma formation (1 percent volume fraction) on UNS S32750 fall in the 1 to 5 minute range. This is far faster than for standard duplex 2205, where the nose time is in the 10 to 30 minute range, because the higher chromium and molybdenum in super duplex shift the equilibrium and accelerate nucleation.
| Temperature, deg C | Approximate Time to 1 Percent Sigma | Practical Implication |
|---|---|---|
| 1000 | 10 to 30 minutes | Avoid prolonged dwell at the upper edge of the band |
| 950 | 3 to 10 minutes | Sigma forms during slow cooling from solution anneal |
| 900 (nose) | 1 to 3 minutes | Fastest nucleation; the temperature most aggressively avoided |
| 850 (nose) | 1 to 5 minutes | Same risk as 900 deg C; broad nose region |
| 800 | 3 to 10 minutes | Welding HAZ exposure here is a common defect source |
| 700 | 10 to 60 minutes | Slower kinetics but still relevant for long welding cycles |
| 600 | 1 to 10 hours | Lower bound of the sigma stability range |
Mechanical and Corrosion Consequences
A super duplex 2507 component contaminated with sigma phase exhibits all of the following symptoms simultaneously, in increasing severity with sigma volume fraction:
| Property | Sigma-Free | 1 Percent Sigma | 5 Percent Sigma |
|---|---|---|---|
| Charpy V-notch at minus 46 deg C | 60 to 100 J | 30 to 50 J | Below 20 J (often single digits) |
| Yield strength | 550 to 720 MPa | Marginal increase | Up to 800 MPa (embrittled) |
| Hardness | 25 to 28 HRC | 28 to 32 HRC | Above 32 HRC, often above 35 HRC |
| CPT (ASTM G48 Method E) | 50 to 70 deg C | 40 to 50 deg C | Below 35 deg C |
| ASTM G48 Method A weight loss | Below 4.0 g per square metre | Above 4.0 g per square metre | Severe pitting |
Charpy is the most sensitive indicator. A Charpy result that drops below 45 J at minus 46 degrees Celsius on a heat that previously qualified is the strongest single signal that the heat-treatment quench was inadequate or that the welding cycle introduced sigma-phase contamination.
Practical Thermal Exposures That Cause Sigma
In normal fabrication, super duplex 2507 enters the sigma risk window during three operations: solution-anneal cooling, welding, and any out-of-spec post-fabrication heat treatment.
- Slow cooling from solution anneal: air cooling, oil quench, or even slow water quench in heavy sections allows the centreline to dwell in the 600 to 1000 deg C band. This is the single most common root cause of sigma contamination on bar and forgings above 100 mm cross-section.
- Excessive welding heat input: heat input above 2.5 kJ per mm slows the cooling rate of the weld metal and the heat-affected zone enough that sigma nucleates. The HAZ is more vulnerable than the weld metal because filler dilution does not benefit it.
- High interpass temperature: interpass above 150 deg C raises the starting temperature for each subsequent weld pass, prolonging dwell in the sigma window across the multi-pass cycle.
- Inadequate shielding-gas nitrogen: argon without nitrogen addition for GTAW root passes produces nitrogen-depleted weld metal and HAZ, which favours ferrite excess and accelerates sigma kinetics.
- Out-of-spec stress-relief heat treatment: a stress-relief cycle anywhere in the 600 to 1000 deg C band is a sigma-generation cycle. This is why super duplex 2507 components are not stress-relieved; residual stress is managed by controlled welding instead.
Avoidance and Detection
| Strategy | Where Applied | Reference |
|---|---|---|
| Rapid water quench from solution anneal | Mill heat treatment | Heat treatment page |
| Heat input 0.5 to 2.5 kJ per mm | Welding | Welding page |
| Interpass below 150 deg C | Welding | NORSOK M-601, ASME IX |
| Ar + 2 to 5 percent N2 shielding for GTAW root | Welding | NORSOK M-601 |
| Charpy V-notch at minus 46 deg C, 45 J min | Lot acceptance | NORSOK M-630 |
| ASTM A923 Method A (etch test) | Sigma screening, 100 percent of welded fabrications | ASTM A923 |
| ASTM A923 Method B (Charpy) | Quantitative sigma confirmation | ASTM A923 |
| ASTM A923 Method C (ferric chloride immersion) | Combined sigma and pitting screening | ASTM A923 |
| Image-analysis sigma volume fraction | WPS qualification, dispute resolution | ASTM E562, ASTM A1084 |
Related Super Duplex 2507 References
- UNS S32750 reference page
- Heat treatment
- Ferrite content (NORSOK M-630)
- Welding procedure
- Welding filler (ER2594, ER2553)
- Mechanical properties
- NORSOK M-630
Sigma Phase Embrittlement FAQ
What is sigma phase in super duplex 2507?
Sigma is a hard, brittle iron-chromium-molybdenum intermetallic with approximate composition Fe-30Cr-4Mo. It nucleates at ferrite grain boundaries between 600 and 1000 degrees Celsius. Even small volume fractions cut Charpy toughness sharply (80 percent or more at 5 percent sigma) and reduce pitting resistance because chromium and molybdenum are robbed from the surrounding matrix to feed the precipitate.
Why is super duplex more sigma-prone than standard duplex 2205?
Higher chromium and molybdenum content in super duplex 2507 (25Cr-3.5Mo nominal versus 22Cr-3Mo nominal in 2205) shifts the equilibrium toward sigma and accelerates the nucleation kinetics. Published TTT data show the nose time for 1 percent sigma in super duplex 2507 is 1 to 3 minutes; in 2205 it is 10 to 30 minutes. The faster kinetics make heavy-section heat treatment and welding noticeably more demanding for super duplex.
At what temperature is the sigma-phase nose?
Approximately 850 to 900 degrees Celsius. This is the temperature where nucleation kinetics are fastest. The full sigma stability range is 600 to 1000 degrees Celsius, so any thermal exposure in this band carries some risk; the mid-band is the most aggressive.
How is sigma phase detected?
The screening test on welded fabrications is ASTM A923 Method A: the cross-section is polished, etched in 40 percent NaOH, and examined under optical microscopy at 400x to 500x. Any unaffected (sigma-suspect) microstructure is dispositioned to Method B (Charpy at minus 40 deg C) or Method C (ferric chloride immersion) for confirmation. Quantitative sigma volume fraction can be measured by image analysis per ASTM E562 with the same NaOH etch.
How much sigma phase is acceptable?
Per ASTM A923 Method A, a polished and etched section is rejected if "unaffected" microstructure (sigma-bearing) is observed. The standard does not assign a numeric volume-fraction limit because measurable Charpy and corrosion-resistance loss begins at well under 1 percent sigma volume fraction. Practical industry practice is to treat any visible sigma as a non-conformance pending Method B or Method C confirmation.
Can sigma phase be reversed?
Yes, by 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. The high-temperature soak dissolves sigma back into the ferrite matrix; the rapid quench re-locks the duplex microstructure without giving sigma time to re-form. The repaired component must pass all original acceptance tests (Charpy, ferrite, ASTM G48, ASTM A923) before release.
What is the difference between sigma phase and 475 deg C embrittlement?
Sigma phase is a separate intermetallic precipitate that forms between 600 and 1000 deg C. 475 deg C embrittlement is a spinodal decomposition of the ferrite phase into chromium-rich (alpha-prime) and chromium-depleted regions, occurring between 280 and 525 deg C over hundreds to thousands of hours. Sigma is a fast (minutes) damage mechanism relevant to fabrication; 475 deg C embrittlement is a slow (hours to years) damage mechanism relevant to long-term elevated-temperature service.
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