Duplex & Super Duplex Seamless Pipe & Tube
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Duplex stainless – two phases in one material
Duplex steel has a two-phase microstructure of roughly equal parts austenite and ferrite. This combination delivers properties neither phase has on its own: yield strength about twice that of austenitic grades such as 304 or 316L, and much better resistance to chloride stress corrosion cracking. High chromium, molybdenum and nitrogen also provide strong pitting resistance. Duplex tube therefore often allows a thinner wall at the same pressure, which partly offsets the higher per-pound cost of duplex steel.
Duplex 1.4462 and super duplex 1.4410
1.4462 (X2CrNiMoN22-5-3, commonly 2205) is the most widely used duplex grade – tube descriptions list it as “duplex 1.4462” or “duplex S31803”. S31803 and the newer S32205 are two UNS variants of this grade that differ in minimum chromium, molybdenum and nitrogen content. 1.4410 (X2CrNiMoN25-7-4, 2507, UNS S32750) is a super duplex with higher levels of these elements, intended for seawater and the most aggressive chloride solutions. Tube labeled “duplex 08X22H6T” is made from steel to the GOST standard – a titanium-stabilized duplex without molybdenum, so its pitting resistance is lower than that of 1.4462. The category also includes tube in UNS S31254 (1.4547, known as 6Mo), a superaustenitic steel whose pitting resistance in chlorides is comparable to super duplex. The grade of each heat is confirmed by an EN 10204 3.1 certificate.
Chemical composition and PREN
| Grade | C | Cr | Ni | Mo | N | PREN |
|---|---|---|---|---|---|---|
| 1.4462 / 2205 | ≤ 0.030 | 21.0–23.0 | 4.5–6.5 | 2.5–3.5 | 0.10–0.22 | approx. 35 |
| 1.4410 / 2507 | ≤ 0.030 | 24.0–26.0 | 6.0–8.0 | 3.0–4.5 | 0.24–0.35 | > 40 |
Composition per EN 10088-1 (wt. %). Equivalents: 1.4462 = X2CrNiMoN22-5-3 = UNS S31803 / S32205 · 1.4410 = X2CrNiMoN25-7-4 = UNS S32750. PREN = Cr + 3.3 × Mo + 16 × N; typical values shown.
Duplex pipe and tube sizes and standards
Duplex and super duplex tube is available in outside diameters from 6.35 mm (1/4 in.) to 168.3 mm (DN 150), with walls from 0.89 to 13 mm. The range also includes inch-size tubing for hydraulics, instrumentation and heat exchanger bundles, plus several dozen hollow bars. Seamless duplex tube is produced to ASTM A789 (tubing for heat exchangers and general service), ASTM A790 (pipe) and EN 10216-5. Any tube can be cut to length.
Where is duplex pipe and tube used?
Typical applications include offshore platforms and subsea systems, shipbuilding, seawater desalination, seawater and brackish water cooling systems, the chemical and pulp and paper industries, chemical transfer lines and high-pressure hydraulics. Super duplex 1.4410 is selected where 2205 is at its limit – in warm seawater or in firewater systems on platforms. We cover these applications in more detail in our article Duplex 2205 (1.4462) in marine and offshore service.
Duplex or austenitic acid-resistant steel?
Compared with 316L and 316Ti, duplex is stronger and more resistant to pitting and, above all, to stress corrosion cracking, which is a common cause of austenitic tube failures in warm chloride solutions. It does have limitations: long-term service above about 250–300°C risks embrittlement of the ferrite, welding requires heat input control, and cold bending takes more force. Where chloride levels are low, 316L seamless tube or 316Ti / 1.4571 tube is enough, and in chloride-free environments, 304 stainless tube. For high-temperature service, 321 / 321H tube or heat-resistant tube is used instead of duplex.
See also: we also stock duplex and super duplex round bar. Tube in other grades can be found in the parent category seamless tube and hollow bar.