Solution-annealing schedule, quench requirements, and forbidden temperature windows for super duplex stainless steel
Super duplex stainless steels are supplied in one heat-treat condition only: solution-annealed and water-quenched. Unlike precipitation-hardening grades (Inconel 718, A286) or martensitic grades (410, 17-4PH), Ferralium 255 (UNS S32550) does not undergo aging, tempering, or strain-hardening. The required microstructure (35 to 55% ferrite balanced with austenite, no sigma / chi / Cr2N precipitation) is achieved by holding the part at 1080 °C ± 20 long enough for full austenitisation, then cooling fast enough through the 600 to 1000 °C window to suppress the brittle inter-metallic phases. Cooling rate is the critical variable: water-quench is mandatory for sections above 25 mm; air-cool is acceptable only for thin sections (≤ 10 mm) where natural cooling stays above the critical rate of approximately 1 °C/s through the precipitation window.
| Parameter | Specification |
|---|---|
| Hold temperature | 1080 °C ± 20 (per ASTM A479) |
| Hold time | 30 minutes per inch (1.2 min/mm) of cross-section, minimum 30 minutes |
| Quench medium | Water (mandatory above 25 mm cross-section) |
| Quench delay | ≤ 90 seconds from furnace exit to immersion |
| Cooling rate through 1000 to 600 °C | ≥ 1 °C/s |
| Furnace atmosphere | Neutral or slightly oxidising; no carburising / nitriding |
| Surface condition after quench | Pickle to remove scale, passivate per ASTM A967 |
| Temperature window | Phase formed | Consequence |
|---|---|---|
| 600 to 1000 °C | Sigma phase (FeCrMo intermetallic) | Severe loss of toughness and corrosion resistance; embrittlement |
| 700 to 900 °C | Chi phase (Fe36Cr12Mo10) | Companion to sigma; same effect on toughness |
| 700 to 950 °C | Cr2N nitride (chromium-depleted zone) | Loss of pitting resistance, sensitisation to chloride |
| 300 to 500 °C (475 °C embrittlement) | Spinodal decomposition of ferrite (alpha-prime) | Hardness rise, ductility loss on long exposure |
No stress-relief, post-weld heat treatment, or low-temperature soak in the 300 to 1000 °C range is permitted. Where the customer specifies a stress-relief on the carbon-steel section of a clad joint, the super duplex layer must be removed from the heat-treat cycle (clamp insulated, kept below 300 °C) to avoid embrittlement.
Cold work above 5 percent strain (cold-drawing, cold-forming, severe machining) raises hardness above the NACE MR0175 32 HRC limit and re-distributes the ferrite-austenite balance. Any cold-work above 5 percent triggers re-solution-annealing per the schedule above. For roll-threaded fasteners and cold-drawn bar feedstock the supplier verifies hardness traverse on the as-supplied product to confirm the 32 HRC cap is not exceeded.
