Duplex Stainless Steel: Phase Balance and Heat Treatment
The defining characteristic of duplex stainless steel is its mixed austenite-ferrite microstructure — approximately 50% each. When this balance is correct, you get twice the strength of 316L plus SCC immunity. When it's wrong, you get embrittlement, loss of corrosion resistance, or both. Understanding phase balance is essential for anyone specifying, fabricating, or inspecting duplex pipe.
The Ideal Microstructure: 40-60% Ferrite
The optimal ferrite content for duplex 2205 is 40-60%, measured by point count per ASTM E562 or magnetic methods per ASTM A923. Below 35% ferrite: the alloy behaves like austenitic stainless — you lose the strength advantage and SCC immunity. Above 65% ferrite: 475°C embrittlement risk increases, and hot workability decreases. The sweet spot at ~50% provides the best combination of mechanical properties, corrosion resistance, and fabricability.
Sigma Phase (σ) — The Enemy
Sigma is a hard, brittle Fe-Cr-Mo intermetallic that forms in the temperature range of 600-950°C. Even 1-2% sigma can reduce impact toughness by 90% and dramatically reduce pitting resistance. Sigma formation is a function of time AND temperature — 30 minutes at 850°C can produce measurable sigma in 2205. The solution annealing temperature (1040-1100°C) specifically dissolves sigma and chi phases.
475°C Embrittlement
Ferrite undergoes spinodal decomposition into Fe-rich (α) and Cr-rich (α') phases when held at 300-550°C. This causes severe hardening and embrittlement — ductility can drop to essentially zero. Named for the temperature at which the effect is most rapid. This is why duplex grades are NOT recommended for continuous service above 300°C, despite their high-temperature strength.
Heat Treatment: Solution Annealing Is Mandatory
| Grade | Solution Anneal Temp | Quench Method | Target Ferrite |
|---|---|---|---|
| 2205 (S32205) | 1040-1100°C | Water quench or rapid air cool | 40-55% |
| 2507 (S32750) | 1050-1125°C | Water quench mandatory | 40-55% |
| S32760 (Zeron 100) | 1080-1120°C | Water quench mandatory | 35-55% |
ASTM A923: Verifying Freedom from Detrimental Phases
ASTM A923 provides three test methods for detecting detrimental intermetallic phases in duplex stainless steel: Method A: Sodium hydroxide etch test — a quick screening method. The etched surface is examined at 400-500× magnification. Sigma phase appears as a distinct phase outlined by the etch. Method B: Charpy impact test — the most widely specified. Impact energy must meet minimum values at the specified test temperature. Low impact energy = sigma phase present. Method C: Ferric chloride corrosion test — most sensitive for pitting resistance degradation. Weight loss after 24-hour exposure to 6% FeCl₃ at the specified temperature (typically 25-50°C). For NACE MR0175 sour service, Method B (Charpy) is typically mandatory for duplex grades. TOKO TECH performs Method B and C on every duplex heat lot.