2205 vs 2507 Duplex Stainless Steel: Composition, Properties and Selection

Duplex Stainless Steels at a Glance

Duplex stainless steels combine a two-phase microstructure of roughly equal parts ferrite and austenite. The dual structure delivers yield strength approximately double that of 304 or 316 austenitics, good toughness, and outstanding resistance to chloride stress corrosion cracking, which ordinary austenitic grades cannot offer. The two workhorse grades of the family are 2205, the standard duplex grade, and 2507, the super duplex grade with higher chromium, molybdenum and nitrogen.

Chemical Composition (ASTM A240)

Composition ranges per ASTM A240 for UNS S32205 (2205) and UNS S32750 (2507), in weight percent:

Element	2205 (S32205)	2507 (S32750)
Carbon (C)	0.030 max	0.030 max
Manganese (Mn)	2.00 max	1.20 max
Silicon (Si)	1.00 max	0.80 max
Phosphorus (P)	0.030 max	0.035 max
Sulfur (S)	0.020 max	0.020 max
Chromium (Cr)	22.0-23.0	24.0-26.0
Nickel (Ni)	4.5-6.5	6.0-8.0
Molybdenum (Mo)	3.0-3.5	3.0-5.0
Nitrogen (N)	0.14-0.20	0.24-0.32

Mechanical Properties (ASTM A240)

Property	2205 (S32205)	2507 (S32750)
Yield Strength (0.2% offset)	450 MPa min	550 MPa min
Tensile Strength	655 MPa min	795 MPa min
Elongation	25% min	15% min
Hardness	293 HBW max	310 HBW max

The higher nitrogen and molybdenum of 2507 also raise its proof strength, so thinner sections can often be substituted for 2205 or austenitic designs.

Corrosion Resistance and PREN

The pitting resistance equivalent number, PREN = Cr + 3.3 Mo + 16 N, is the standard screening index for chloride pitting resistance. 2205 reaches a PREN of about 34, while 2507 reaches about 42. In seawater service, 2205 is generally accepted for continuous immersion up to roughly 50 degrees C, whereas 2507 extends the limit to about 60 degrees C and additionally resists sour service with hydrogen sulfide, which is why super duplex is specified for offshore production, subsea equipment and flue gas desulfurization units handling aggressive chloride plus acid streams. Both grades resist chloride stress corrosion cracking far better than 304/316, and both should be solution annealed at 1020-1100 degrees C after hot forming to restore the ferrite-austenite balance.

Standards and Product Forms

Both grades are covered as plate and sheet by ASTM A240, as welded and seamless pipe by ASTM A789/A790, and as bar and forgings by ASTM A276/A182. In the European system 2205 corresponds to EN 1.4462 (X2CrNiMoN22-5-3) and 2507 to EN 1.4410 (X2CrNiMoN25-7-4) per EN 10088-2 and EN 10088-3. The Japanese standard JIS G4304/G4305 includes the 2205-type grade as SUS329J3L; super duplex has no direct JIS designation and is normally specified to ASTM or EN. Common product forms are plate, sheet, strip, seamless and welded pipe, tube, bar, fittings and forgings.

Selection Guidance

Choose 2205 for seawater-cooled heat exchangers, chemical tankers, desalination plants, pulp digesters and general chemical processing where chloride and stress corrosion resistance are needed at moderate cost; 2205 is typically 30-50% cheaper than 2507 per ton.

Choose 2507 for offshore topside and subsea equipment, high-chloride and sour gas service, FGD absorbers, and process streams where 2205 would pit.

For strongly reducing acids or hot seawater above the duplex limits, verify performance by testing, because no duplex grade matches the corrosion resistance of high-molybdenum nickel-base alloys in the most aggressive environments.

Frequently Asked Questions

What does duplex mean? Duplex stainless steel has a two-phase microstructure of about 50% ferrite and 50% austenite, combining the high strength of ferritic steels with the toughness and corrosion resistance of austenitic steels.

What is the main difference between 2205 and 2507? 2507 contains more chromium, molybdenum and nitrogen than 2205, raising its PREN from about 34 to about 42 and extending seawater service from roughly 50 degrees C to about 60 degrees C; it also has higher yield strength (550 MPa vs 450 MPa) and higher cost.

What is PREN? PREN is the pitting resistance equivalent number, PREN = Cr + 3.3 Mo + 16 N, a screening index for resistance to pitting in chloride media. Higher values indicate better resistance.

Can duplex stainless steels be welded? Yes. Welding is performed with matching duplex fillers, normally 22Cr-9Ni-3Mo type for 2205 and 25Cr-9Ni-4Mo type for 2507, with controlled heat input and interpass temperature. No post-weld heat treatment is required.

How much more expensive is 2507 than 2205? 2507 typically costs 30-50% more than 2205 because of higher alloy content and more demanding processing, so the grade should be selected only where the environment requires its higher PREN and strength.

Which duplex grade should I choose for seawater? For continuous seawater immersion below about 50 degrees C, 2205 is usually sufficient; above that temperature, or where crevice conditions are severe, 2507 or a higher-alloy super duplex grade is recommended.