Duplex 2205

What is Duplex 2205?

Duplex 2205 is a stainless steel with approximately equal proportions of ferrite and austenite. Its composition includes 22.0 to 23.0 percent chromium, 4.5 to 6.5 percent nickel, 3.0 to 3.5 percent molybdenum and 0.14 to 0.20 percent nitrogen. Carbon is limited to 0.03 percent. The nickel content is insufficient to produce a fully austenitic structure. Nitrogen restores the austenite fraction to about half and increases its strength.

The ferrite phase provides high yield strength and near-immunity to chloride stress corrosion cracking. The austenite phase provides toughness, ductility and weldability. The combined structure is stronger than either phase alone and has greater chloride resistance than standard austenitic grades.

Duplex 2205 has substantially higher yield strength than AISI 316. ASTM A276 specifies a minimum 0.2 percent yield strength of 450 MPa for UNS S32205 and 205 MPa for 316. In a yield-governed design, this difference can permit approximately half the wall thickness for the same duty. Duplex 2205 also has a pitting resistance equivalent number of 34.1 to 37.8, compared with 22.6 to 29.5 for AISI 316.

Laxcon Steels lists Duplex 2205 in its grades reference as a duplex grade. The same steel is written 2205, S32205, S31803, F60, F51, 1.4462 and SUS 329J3L.

Standard duplex, UNS S32205, ASTM A240 and ASTM A182 F60. EN 1.4462 is the same steel.

What is the chemical composition of Duplex 2205?

Composition limits in weight percent are carbon 0.03 maximum, manganese 2.0 maximum, sulphur 0.02 maximum, phosphorus 0.03 maximum, silicon 1.0 maximum, chromium 22.0 to 23.0, nickel 4.5 to 6.5, molybdenum 3.0 to 3.5 and nitrogen 0.14 to 0.2.

What is Duplex 2205 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of Duplex 2205?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties

ASTM A276 specifies UNS S32205 bar in Condition A, whether hot-finished or cold-finished, with a minimum tensile strength of 655 MPa and a minimum 0.2 percent yield strength of 450 MPa. Minimum elongation is 25 percent in 50 mm. Maximum hardness is 290 HB. ASTM A240 specifies the same tensile strength, yield strength and elongation for plate. It limits hardness to 31 HRC or 293 HB.

The density is about 7,800 kg per cubic metre, and the elastic modulus is about 200 GPa. The mean coefficient of thermal expansion over 0 to 100 degrees Celsius is approximately 13.7 micrometres per metre per degree Celsius. The corresponding value for AISI 304 is 17.2. This lower expansion reduces welding distortion and residual stress relative to an austenitic fabrication of the same geometry.

UNS S31803 and UNS S32205

The original 2205 composition was registered as UNS S31803. It permits 21.0 to 23.0 percent chromium, 2.5 to 3.5 percent molybdenum and 0.08 to 0.20 percent nitrogen. Mills later restricted the lower ends of these ranges. The restricted composition was endorsed as UNS S32205 in 1996. It raises the chromium minimum to 22.0 percent, the molybdenum minimum to 3.0 percent and the nitrogen minimum to 0.14 percent.

ASTM A276 specifies a minimum tensile strength of 620 MPa for S31803 and 655 MPa for S32205. Both grades have a minimum yield strength of 450 MPa. S32205 provides a higher minimum level of corrosion resistance because its permitted composition is less lean. Much current S31803 production also complies with S32205, but the designations are not identical specifications. Atlas Steels recommends stating S31803 or S32205 when ordering grade 2205. ASTM A240 defines 2205 as S32205. In forgings, the same steel is designated ASTM A182 grade F60.

Corrosion behaviour

The pitting resistance equivalent number ranges from 34.1 at the specified composition minima to 37.8 at the maxima. It is calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. Molybdenum contributes approximately one-third of the result, and nitrogen contributes a further tenth. The nitrogen minimum therefore has a significant effect on pitting resistance.

Atlas Steels gives a general critical pitting temperature of at least 35 degrees Celsius for 2205. It also states that the grade resists chloride stress corrosion cracking at temperatures up to about 150 degrees Celsius. Austenitic 304 and 316 become susceptible to chloride stress corrosion cracking above approximately 60 degrees Celsius. The difference provides about ninety degrees of additional operating range. General corrosion resistance is superior to that of 316 in most environments. Seawater resistance is also higher, although 2205 is not intended as a full seawater immersion alloy.

Welding, machining and forming

Solution treatment consists of heating to 1,020 to 1,100 degrees Celsius and then cooling rapidly. Heat treatment cannot harden the grade, but cold work causes work hardening. Standard welding processes use over-alloyed 2209 consumables. These consumables produce the required phase balance and avoid the excessive ferrite associated with an autogenous weld. Nitrogen in the shielding gas helps maintain adequate austenite. Welding requires low heat input. Neither preheating nor post-weld heat treatment is used, unlike normal martensitic practice.

Machining uses cutting speeds approximately 20 percent below those used for grade 304. The high yield strength increases the force required to shear the chip. Forming requires higher-capacity equipment and allowance for substantial springback. Ductility is lower than in an austenitic grade, although it is not low in absolute terms. Severe operations such as cold heading are generally unsuitable. More than moderate cold work may require an intermediate anneal.

Applications and designations

Applications include chemical processing, transport and storage. Other uses include oil and gas exploration, processing and support systems; marine and other high-chloride environments; pulp and paper digesters, liquor tanks and paper machine components; desalination and brine handling; and structural components where high strength is required. The European designation is material number 1.4462, with the name X2CrNiMoN22-5-3. The Japanese designation is SUS 329J3L. Purchaser designations include 2205, SAF 2205, S32205, F60, SS2205 and duplex2205.

In which product forms does Laxcon list Duplex 2205?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where Duplex 2205 is not the right choice

Duplex 2205 is unsuitable for service outside the approximate range of minus 50 to plus 300 degrees Celsius. Above 300 degrees Celsius, spinodal decomposition separates the ferrite phase into chromium-rich and iron-rich regions. This process is known as 475 degree embrittlement, and Atlas Steels states that it can occur after short exposure times. The steel becomes harder and stronger but loses toughness. Recovery requires full solution annealing, which is rarely practicable for installed equipment. Below about minus 50 degrees Celsius, the ferrite phase undergoes a ductile-to-brittle transition. Austenitic AISI 316L is suitable at both temperature extremes.

Duplex 2205 is also unsuitable for autogenous welding, deep drawing, cold heading and fabrication without controlled procedures. Welding without filler produces excessive ferrite and reduces both toughness and corrosion resistance. Prolonged exposure between approximately 600 and 1,000 degrees Celsius precipitates sigma phase and causes the same losses. Super duplex 2507 has a PREN of 37.7 to 47.6 for chloride conditions beyond the capability of 2205, including warm seawater and desalination brine. Where formability is more important than strength, 316L is easier to cold form and is more widely stocked.

Other duplex grades