Duplex Stainless Steels: Structure, Grades and Classifications
Send an enquiryDuplex stainless steel is a family of stainless steels whose microstructure contains two phases, ferrite and austenite, in roughly equal proportion. The term describes a structure rather than a single chemistry: about fifteen commercial grades share it, and those grades differ from one another more than grade 304 differs from grade 316. Austenitic stainless steels are single phase, as are ferritic stainless steels; duplex steels combine both, with each phase contributing distinct properties. Ferrite provides strength and resistance to chloride stress corrosion cracking, while austenite provides toughness and ductility. A correctly balanced duplex steel yields at roughly twice the figure of 304 and resists chloride stress corrosion cracking far better than 316, although on pitting resistance the leaner grades overlap 316 rather than exceed it. An incorrectly balanced one, a condition welding heat can readily produce, retains neither advantage.
The duplex grades and their PREN bands
The Laxcon grade register carries 547 grades, of which 15 are duplex. The table below gives the pitting resistance equivalent number band each grade permits, calculated from its own composition window as PREN = Cr + 3.3Mo + 16N rather than from a nominal analysis. The method is set out in the PREN reference.
| Grade | Tier | Cr % | Ni % | Mo % | N % | PREN band |
|---|---|---|---|---|---|---|
| S 32101 | Lean | 21.0 to 22.0 | 1.35 to 1.70 | 0.10 to 0.80 | 0.20 to 0.25 | 24.5 to 28.6 |
| S 32202 | Lean | 21.5 to 24.0 | 1.00 to 2.80 | up to 0.45 | 0.18 to 0.26 | 24.4 to 29.6 |
| S 32003 | Lean | 19.5 to 22.5 | 3.00 to 4.00 | 1.50 to 2.00 | 0.14 to 0.20 | 26.7 to 32.3 |
| S 32304 | Lean | 21.5 to 24.5 | 3.00 to 5.50 | 0.05 to 0.60 | 0.05 to 0.20 | 22.5 to 29.7 |
| DIN 1.4362 | Lean | 22.0 to 24.0 | 3.50 to 5.50 | 0.10 to 0.60 | 0.05 to 0.20 | 23.1 to 29.2 |
| AISI 329 | Older standard | 23.0 to 28.0 | 2.00 to 5.00 | 1.00 to 2.00 | none specified | 26.3 to 34.6 |
| F - 50 | Standard | 24.0 to 26.0 | 5.50 to 6.50 | 1.20 to 2.00 | 0.14 to 0.20 | 30.2 to 35.8 |
| F - 51 | Standard | 21.0 to 23.0 | 4.50 to 6.50 | 2.50 to 3.50 | 0.08 to 0.20 | 30.5 to 37.8 |
| DIN 1.4462 | Standard | 21.0 to 23.0 | 4.50 to 6.50 | 2.50 to 3.50 | 0.10 to 0.22 | 30.9 to 38.1 |
| 2205 / S32205 / F60 | Standard | 22.0 to 23.0 | 4.50 to 6.50 | 3.00 to 3.50 | 0.14 to 0.20 | 34.1 to 37.8 |
| F - 61 | Super | 24.0 to 27.0 | 4.50 to 6.50 | 2.90 to 3.90 | 0.10 to 0.25 | 35.2 to 43.9 |
| S32760 / F55 | Super | 24.0 to 26.0 | 6.00 to 8.00 | 3.00 to 4.00 | 0.20 to 0.30 | 37.1 to 44.0 |
| F - 59 | Super | 24.0 to 26.0 | 5.50 to 8.00 | 3.00 to 5.00 | 0.20 to 0.35 | 37.1 to 48.1 |
| 2507 / S32750 / F53 | Super | 24.0 to 26.0 | 6.00 to 8.00 | 3.00 to 5.00 | 0.24 to 0.32 | 37.7 to 47.6 |
| S 32906 | Super | 28.0 to 30.0 | 5.80 to 7.50 | 1.50 to 2.60 | 0.30 to 0.40 | 37.8 to 45.0 |
The three tiers: lean, standard and super
The tiers correspond to three different compositional responses to the cost of nickel, not to grades of quality.
Lean duplex
Lean duplex grades replace most of the nickel with manganese and nitrogen. Grade S 32101 carries 1.35 to 1.70 percent nickel, against 8.0 to 10.5 percent in 304; manganese at 4.0 to 6.0 percent and nitrogen at 0.20 to 0.25 percent stabilise the austenite instead. The resulting PREN band of 24.5 to 28.6 straddles that of 316 at 22.6 to 29.5, at a fraction of the nickel content and roughly twice the strength. Where cost is driven by nickel content and the duty is structural rather than chemical, lean duplex grades are displacing 304 in tanks, bridges and reinforcement.
Standard duplex
Grade 2205 is the most widely used duplex grade. Its PREN band is 3.6 points wide, the narrowest of any duplex grade in the register and narrower than that of 316 at 6.9 points. Chromium is held to a single percentage point and nitrogen carries a true minimum of 0.14 percent rather than being merely tolerated. A narrow composition window reduces heat-to-heat variation: material supplied against the grade lies close to the material ordered.
Super duplex
By trade convention, a duplex grade counts as super duplex at PREN 40 and above. The convention is not guaranteed by a grade name. Calculated across its own specification limits, 2507 has a PREN band of 37.7 to 47.6, the widest of the commonly traded grades at 9.9 points. The lower bound follows from chromium at 24.0, molybdenum at 3.0 and nitrogen at 0.24 percent, all legitimate specification minima, and it sits below 40. A heat at that bound is compliant S32750 but is not super duplex on the convention. Where a specification requires PREN 40 minimum, the requirement belongs on the purchase order as a certified minimum on the heat analysis, not as a grade name. NORSOK M-650 qualification, which Laxcon holds through TUV India and which is listed on the quality page, exists because grade names alone are insufficient at this end of the range.
Mechanical properties compared with austenitic grades
| Grade | Yield, 0.2 percent proof, minimum | Tensile, minimum | Elongation, minimum | Hardness, BHN maximum |
|---|---|---|---|---|
| DIN 1.4462 | 450 MPa | 650 MPa | 25 percent | 270 |
| UNS S32205 | 450 MPa | 655 MPa | 25 percent | 290 |
| UNS S31803 | 448 MPa | 620 MPa | 25 percent | 290 |
| DIN 1.4460 | 450 MPa | 620 MPa | 20 percent | 260 |
| DIN 1.4362 | 400 MPa | 600 MPa | 25 percent | 260 |
These minima are published on the duplex steel page. The proof stress is roughly double that of an annealed austenitic grade, which converts directly into thinner sections, lighter vessels and lower tonnage. The elongation minimum of 25 percent shows that the strength is achieved while the steel remains ductile, which distinguishes duplex from a hardened martensitic grade.
Fabrication constraints
Duplex is not a direct substitute for austenitic stainless steel. Service failures of duplex trace to a common cause: the phase balance was destroyed by heat and was not checked afterwards. The principal constraints are:
- Heat input must be controlled. Too little heat causes the weld metal to solidify too ferritic, losing toughness and corrosion resistance. Too much promotes intermetallic phases. Carbon-steel welding practice errs in both directions.
- Interpass temperature has a ceiling. Multipass welding on heavy sections is where sigma phase appears. Sigma phase embrittles the steel and depletes chromium and molybdenum from the surrounding matrix.
- Prolonged service between roughly 300 and 1,000 degrees Celsius is excluded. Duplex has an upper temperature limit that austenitic grades do not, for the same phase-stability reason. It is a corrosion and strength material, not a high-temperature one.
- Duplex is magnetic. Ferrite is ferromagnetic, so a magnet adheres to duplex. A magnet is therefore not a valid test for stainless steel.
- Machining is more difficult. Higher strength and work hardening require lower speeds, more rigid setups and result in shorter tool life than 304.
Specifying duplex material
- Name the UNS number, not the trade name. 2205, S32205 and F60 designate the same steel; S31803 is a related but different composition window with a lower nitrogen minimum and a lower published proof stress. Ordering 2205 and receiving S31803 is a common substitution.
- State the PREN minimum and the formula. The nitrogen coefficient is 16 in the general form and often 30 for duplex specifically, a difference of four points on a grade carrying 0.30 percent nitrogen. Without the formula, the two parties to an order are not calculating the same number.
- Require the certified heat analysis. Every heat Laxcon casts is sampled from the ladle during pouring and analysed on optical emission spectrometers, and the analysis appears on the certificate for that heat. The heat analysis is the only document that records where inside the composition window the material landed.
- Specify the delivery condition. Duplex is supplied solution annealed and quenched. The quench fixes the phase balance; Laxcon's published quench delay is under one minute.
- State the testing required. Ferrite content, impact testing at temperature, and corrosion testing to ASTM G48 are the three tests that matter for duplex, and none is automatic.
Frequently asked questions
What does duplex stainless steel mean?
Duplex means the microstructure contains two phases, ferrite and austenite, in roughly equal proportion, rather than the single phase of a conventional stainless steel. The name describes the structure and not the chemistry, which is why some fifteen distinct grades all count as duplex.
Is duplex better than 316?
Duplex exceeds 316 in strength and in resistance to chloride stress corrosion cracking, usually by a wide margin, but the leaner grades overlap 316 on pitting resistance rather than exceed it, and no duplex grade exceeds 316 in fabrication simplicity, high-temperature service or formability. A 2205 section carries roughly twice the proof stress of 316 and a PREN band of 34.1 to 37.8 against 22.6 to 29.5, and it demands welding discipline that 316 does not.
What is the difference between 2205 and 2507?
2507 is a super duplex grade: more chromium at 24.0 to 26.0 percent against 22.0 to 23.0, more molybdenum at up to 5.0 percent against 3.5, and more nitrogen at 0.24 to 0.32 percent against 0.14 to 0.20. The additional alloying provides chloride resistance at higher temperature at considerably higher cost. Its PREN band is also much wider than that of 2205, so the certified analysis matters more, not less.
Is duplex stainless steel magnetic?
Duplex stainless steel is magnetic, because the ferrite phase is ferromagnetic, so a magnet will adhere to it. This is expected behaviour and is not a sign of contamination or of an incorrect grade being supplied.
Can duplex be used at high temperature?
Duplex is not suitable for prolonged service at high temperature. Above roughly 300 degrees Celsius the phase balance begins to degrade, and the intermetallic phases that form embrittle the steel. Where the duty is hot rather than corrosive, an austenitic grade such as 321, 309 S or 310 S is the appropriate family.
Which duplex grades do you supply, and in what form?
The grades documented on the duplex steel page cover S31803, S32205, S32750, 1.4462, 1.4460, 1.4410 and 1.4362, supplied by Laxcon as bar across its rolling and cold finishing range, and as seamless tube in the published phase one range.
Sources
Composition windows for all fifteen duplex grades are the published specifications in the Laxcon grade reference. PREN bands are calculated from those windows as Cr + 3.3Mo + 16N, taking the specification minimum and maximum for each element and treating an unspecified minimum as zero; the full method is in the PREN reference. Yield, tensile, elongation and hardness minima, and the chemistry tables by standard, are published on the duplex steel product page. Quench delay and heat treatment furnace data are on the technology page. NORSOK M-650 qualification and laboratory accreditation are on the quality page.
