Precipitation kinetics, detection methods, and process controls for sigma-phase avoidance in super duplex
Sigma phase (FeCrMo intermetallic, ordered tetragonal) is the dominant failure mode for super duplex stainless steel exposed to the 600 to 1000 °C temperature window. The precipitation kinetics for Ferralium 255 are fast: at the nose of the time-temperature-transformation (TTT) curve around 850 °C, sigma can form in under one minute. Even small fractions (above approximately 0.5% by area) destroy Charpy V-notch toughness, taking the room-temperature value from ~150 J to under 30 J and the -46 °C value from ~80 J to under 10 J. Sigma also depletes the surrounding ferrite of chromium and molybdenum, which destroys the local pitting resistance and triggers selective corrosion of the depleted zone in chloride or sour service. Avoidance is mandatory and the standard quality-control test is ASTM A923 method A (sodium hydroxide etch), method B (Charpy impact acceptance), or method C (ferric-chloride corrosion).
| Process step | Sigma risk | Mitigation |
|---|---|---|
| Solution-anneal slow quench | HIGH if cooling rate < 1 °C/s through 1000 to 600 °C | Water quench within 90 seconds of furnace exit |
| Hot-forming finish-temperature | HIGH if part finishes below 950 °C | Re-heat to 1080 °C and quench, do not let part stop in window |
| Welding heat-affected zone | HIGH if heat input > 2.5 kJ/mm or interpass temperature > 150 °C | Heat input 0.5 to 2.5 kJ/mm, interpass ≤ 150 °C, no PWHT |
| Multipass weld root re-heat | HIGH on root pass during fill-pass thermal cycle | Limit fill-pass heat input, use stringer beads not weave |
| Service exposure 300 to 1000 °C | HIGH on long-time hold above 300 °C | NORSOK caps service at 250 °C; ASME caps design at 316 °C |
| Stress-relief or PWHT cycle | HIGH any soak in 600 to 1000 °C | PWHT not permitted on super duplex pressure parts |
ASTM A923 provides three acceptance methods; the customer specification picks one or more. Method A (sodium-hydroxide etch) reveals sigma as a dark structure visible at 500× under bright-field metallography; the acceptance limit is "no continuous sigma network and no sigma at the ferrite-austenite grain boundary in the qualifying region." Method B (Charpy impact) requires the Charpy V-notch energy to be at or above the qualifying value (typically 45 J at -46 °C transverse). Method C (ferric-chloride corrosion per ASTM G48 method A) requires the 24-hour mass-loss to be below 4 mg/cm² at the qualifying temperature (35 °C for Ferralium 255). NORSOK M-630 mandates method A plus method C; ASME B16.34 typically requires method B.
