Duplex Stainless Steel (DSS) is a type of stainless steel with a microstructure consisting of approximately 50% ferrite and 50% austenite. It combines the excellent toughness and weldability of austenitic stainless steel with the high strength and resistance to chloride-induced stress corrosion cracking of ferritic stainless steel. Duplex stainless steels are generally classified into three grades according to their PRE value (Pitting Resistance Equivalent) or alloy content: lean duplex, standard duplex, and super duplex stainless steel. The following is a detailed description of the material grades:
I. Lean Duplex Stainless Steel
This grade has relatively low chromium and nickel contents and lower cost. Its strength is higher than that of 304 and 316L, and it is mainly used to replace 304 and 316L for weight reduction or improved stress corrosion resistance.
- UNS S32101 / EN 1.4162 (LDX 2101)Features: High nitrogen content, good corrosion resistance and formability.Applications: Replacement for 304, used in building facades, decorative tubes, and chemical equipment with mild corrosion.
- UNS S32304 / EN 1.4362 (UR 45N)Features: Corrosion resistance between that of 304 and 316.Applications: Mainly used in pulp and paper industry and heat exchangers.
II. Standard Duplex Stainless Steel
This is the most widely used grade of duplex stainless steel, with grade 2205 as the representative. Its corrosion resistance is superior to 316L, and its strength is twice that of conventional austenitic stainless steel.
- UNS S31803 / S32205 / EN 1.4462 (2205)Composition: Approximately 22% Cr, 5% Ni, 3% Mo, 0.15% N.Status: The “main force” of duplex stainless steels.Features: Excellent resistance to chloride-induced pitting corrosion, crevice corrosion and stress corrosion cracking.Applications: Oil and gas pipelines, chemical storage tanks, seawater desalination equipment, flue gas desulfurization units.
- UNS S32550 / EN 1.4507 (Ferralium 255)Composition: Copper and tungsten added on the basis of 2205, with higher strength and corrosion resistance.Applications: Severe chemical environments and flue gas desulfurization systems.
III. Super Duplex Stainless Steel
This grade contains very high levels of chromium, molybdenum and nitrogen, with a PRE value generally above 40. Its corrosion resistance even exceeds that of 316L and 904L, and it is specially designed for extremely harsh corrosive environments.
- UNS S32750 / EN 1.4410 (2507)Composition: Approximately 25% Cr, 7% Ni, 4% Mo, 0.27% N.Status: The most widely used super duplex stainless steel at present.Features: Extremely high resistance to pitting and crevice corrosion, and very high strength.Applications: Deepwater oil and gas exploitation, high-concentration chloride environments, pulp bleaching equipment.
- UNS S32707 / EN 1.4501 (SAF 2707 HD)Features: High alloy content and extremely high nitrogen content, classified as “high-nitrogen super duplex stainless steel”.Applications: Extreme corrosive environments such as highly acidic oil wells.
IV. Common ASTM Standard Grades for Pipe Fittings and Flanges
In industrial procurement, duplex stainless steel pipe fittings and flanges usually conform to the following standard designations:
- Plate / Strip: ASTM A240 (e.g., 2205, 2507)
- Tubing / Pipe: ASTM A789 / A790 (e.g., S32205, S32750)
- Pipe Fittings (elbows, tees): ASTM A815 (e.g., WP2205, WP2507)
- Flanges / Forgings: ASTM A182 (e.g., F51, F53, F55)
- F51 corresponds to 2205 (S32205)
- F53 corresponds to 2507 (S32750)
- F55 corresponds to S32707 (super duplex)
V. Key Properties and Precautions of Duplex Stainless Steel
- High strength: Yield strength is typically above 450–550 MPa, twice that of 304/316L, allowing thinner wall design to reduce cost and weight.
- Corrosion resistance: Especially resistance to chloride-induced stress corrosion cracking (SCC) is much stronger than 304 and 316L.
- Weldability: Weldable, but the heat-affected zone (HAZ) is prone to embrittlement or unbalanced phase ratio (excessive ferrite). Solution annealing is often required after welding (especially for thick-walled parts), and selection of welding materials is critical (high-nickel filler metals are usually used to balance phase ratio).
- Low-temperature toughness: Standard and super duplex grades maintain good toughness at low temperatures, while lean duplex grades experience reduced toughness at extremely low temperatures.
Summary Recommendations
- For cost reduction or weight replacement of 304/316, choose 2101.
- For general chemical or offshore engineering, choose 2205 (F51).
- For extreme corrosion or deepwater applications, choose 2507 (F53).
