Duplex stainless steels (mixed austenite-ferrite microstructure, 21 to 26 percent Cr, 4 to 7 percent Ni, 2.5 to 4 percent Mo) provide approximately twice the yield strength of 316L and significantly better resistance to chloride pitting and stress corrosion cracking, making them the preferred pump material for offshore oil and gas, desalination, and seawater cooling applications where 316L is inadequate.
History
The first commercial duplex stainless steel, 3RE60, was developed by Sandvik AB in Sandviken, Sweden in 1930 to combat corrosion in the sulfite pulp paper industry. The modern second-generation duplex grades emerged after advances in argon-oxygen decarburization (AOD) refining in the 1970s allowed precise nitrogen control in the alloy. SAF 2205 (UNS S32205) was introduced by Sandvik and Avesta in 1978 and became the defining standard duplex grade for offshore and chemical pump applications. Super-duplex grade SAF 2507 (UNS S32750) was commercialized in 1985 for the most aggressive seawater service conditions.
Key Properties
- Density: 7.8 g/cm3
- Yield strength: 450 to 550 MPa — more than twice the strength of 316L stainless
- Tensile strength: 620 to 800 MPa
- PREN: 35 to 43 (standard 2205) and 40 to 45 (super-duplex 2507)
- Excellent fatigue resistance — suitable for reciprocating pump shafts
- Ferromagnetic — responds to magnets unlike austenitic 316L
Corrosion and Chemical Resistance
- Resists chloride pitting up to approximately 10,000 ppm Cl- at ambient temperature (grade 2205)
- Excellent resistance to crevice corrosion in seawater below 30 degrees C for 2205 and below 60 degrees C for 2507
- Not susceptible to stress corrosion cracking in typical marine environments
- Resistant to phosphoric acid, acetic acid, and urea synthesis process environments
- Not suitable for hydrochloric or sulfuric acid above moderate concentrations
- Avoid sustained temperatures above 300 degrees C — sigma phase embrittlement risk
Grades and Standards
- SAF 2205 / UNS S32205 / EN 1.4462 — standard duplex pump grade for general applications
- SAF 2507 / UNS S32750 / EN 1.4410 — super-duplex for seawater and aggressive chemical service
- ASTM A890 Grade 4A (CD4MCu) — cast duplex pump casing alloy
- ASTM A995 Grade 5A — cast super-duplex (CE8MN) for pump casings in severe service
- NORSOK M-630 standard governs duplex pump use on offshore installations
Applications
- Offshore seawater injection and produced water handling pump casings
- Desalination SWRO high-pressure pump components
- Chlorination system and bleach solution metering pump wetted parts
- Phosphoric acid and fertilizer process pump casings and impellers
- Pulp and paper digester circulation pump components
- Subsea umbilical and hydraulic supply pump casings
Advantages
- Twice the yield strength of 316L — enables lighter and thinner pump wall sections
- Far superior chloride corrosion resistance compared to standard austenitic grades
- Good weldability compared to ferritic stainless steel grades
- Lower nickel content than austenitic grades reduces alloy cost volatility
- Excellent erosion-corrosion resistance in high-velocity seawater service
Limitations
- More difficult to cast and machine than austenitic stainless grades
- Sigma phase embrittlement if incorrectly heat treated or exposed above 300 degrees C
- Higher material and fabrication cost than standard austenitic grades
- Not suitable for cryogenic service due to ductile-brittle transition at low temperature
- Limited availability in heavy section castings compared to standard 316L
Selection Criteria
- Specify duplex 2205 for pump components in seawater below 30 degrees C or chlorides up to 10,000 ppm
- Specify super-duplex 2507 for higher temperature seawater or concentrated chloride above 10,000 ppm
- Require full solution annealing and quenching certification for all cast and fabricated components
- Consult NORSOK M-630 for offshore pump specification requirements
- Verify that welding procedure includes controlled heat input to maintain correct ferrite-austenite phase balance
Top 10 Manufacturers
- 1. Sandvik AB (Sweden) — ~22% of duplex pump component market; inventor and leading supplier of SAF 2205 and 2507
- 2. Outokumpu Oyj (Finland) — ~18%; Supra 2304 and 2507 wrought strip and bar for pump shafts
- 3. Flowserve Corporation (USA) — ~10%; CD4MCu and CE8MN cast duplex pump casing components
- 4. ATI Inc. (USA) — ~8%; wrought duplex and super-duplex plate and bar for fabricated pump bodies
- 5. Sulzer Ltd. (Switzerland) — ~7%; duplex impellers for seawater and chemical process pumps
- 6. Fengyang Stainless Steel (China) — ~6%; ASTM A995 cast duplex pump body castings
- 7. Weir Group PLC (UK) — ~5%; super-duplex casings for offshore and desalination pump applications
- 8. Voestalpine AG (Austria) — ~5%; duplex round bar for precision-machined pump shafts
- 9. Plymouth Tube Co. (USA) — ~4%; seamless duplex tubing for pump column and discharge pipe
- 10. Aperam S.A. (Luxembourg) — ~4%; duplex stainless coil and sheet for pump fabricators worldwide