Duplex stainless steels combine the properties of austenitic and ferritic grades in a two-phase microstructure containing approximately equal proportions of austenite and ferrite. This dual structure provides high strength and excellent corrosion resistance, particularly against stress corrosion cracking.
Microstructure and Properties
The balanced austenite-ferrite microstructure (ideally 50/50, acceptable range 35–65% ferrite per ASTM E562) gives duplex steels yield strengths approximately double those of standard austenitic grades while maintaining good ductility and toughness.
The ferrite phase provides resistance to chloride stress corrosion cracking (SCC), while the austenite phase ensures good toughness and ductility. The high chromium content (22–25%) combined with molybdenum and nitrogen gives excellent pitting and crevice corrosion resistance.
Common Grades
**Lean duplex (e.g., 2304 / 1.4362)**: Lower alloy content, PRE ≈ 25. Cost-effective alternative to 316L with superior strength. Applications: storage tanks, structural components, water treatment.
**Standard duplex (2205 / 1.4462)**: The workhorse grade, ~70% of all duplex used. PRE ≈ 35. Applications: pressure vessels, heat exchangers, chemical processing, offshore structures.
**Super duplex (2507 / 1.4410)**: High alloy content, PRE ≈ 42. For severe chloride environments. Applications: seawater systems, subsea equipment, aggressive chemical processing.
Welding Duplex Stainless Steels
Welding duplex steels requires careful control of heat input and cooling rate to maintain the correct phase balance in the weld and HAZ. Excessive heat input promotes sigma phase formation; insufficient heat input results in excessive ferrite.
**Heat input**: Maintain within 0.5–2.5 kJ/mm for standard duplex. Maximum interpass temperature 150°C for 2205, 100°C for super duplex. Never use carbon steel wire brushes — only stainless steel or nylon.
**Filler metals**: Use over-alloyed consumables (e.g., 2209 for 2205 base metal) to compensate for nitrogen loss and ensure adequate austenite reformation. Autogenous welding (without filler) is generally not recommended as it produces excessive ferrite.
**Shielding gas**: Argon + 2% nitrogen for GTAW (TIG) to compensate for nitrogen loss. Pure argon or Ar/CO₂ mixtures should not be used for root shielding of duplex steels.
Post-weld testing should include ferrite measurement (using Fisher Feritscope or metallographic methods) and corrosion testing (ASTM G48 Method A at appropriate temperature) to verify that the phase balance and corrosion resistance have been maintained.