What Is Duplex Stainless Steel?
Duplex stainless steel is a mix of roughly half austenite and half ferrite in its microstructure, which combines the strength of ferritic grades with the toughness and weldability of austenitic ones. The standard grade is 2205, also written UNS S32205 or S31803. It is part of the stainless steel family but behaves differently enough from 304 and 316L that buyers must select it deliberately, not as a drop-in replacement.
The practical payoff is about double the yield strength of 316L and far better resistance to chloride pitting and stress corrosion cracking. The trade-offs are tighter welding control, a 475 °C embrittlement limit, and higher cost.
2205 vs 316L vs 304: The Comparison That Drives the Choice
| Property | 304 | 316L | 2205 (S32205) |
|---|---|---|---|
| Structure | Austenitic | Austenitic | Duplex (austenite + ferrite) |
| Yield strength (MPa, annealed) | ~210 | ~220 | ~450 to 550 |
| Pitting resistance (PREN) | ~18 to 20 | ~23 to 26 | ~34 to 38 |
| Chloride / seawater pitting | Poor | Good | Very good |
| Stress corrosion cracking (SCC) | Poor above 60 °C | Fair | Excellent |
| Weldability | Easy | Easy | Tighter control needed |
| Formability | Excellent | Excellent | Lower than austenitic |
| Relative cost | 1.0 | ~1.2 | ~1.4 to 1.6 |
PREN (pitting resistance equivalent number) rises with chromium, molybdenum, and nitrogen content. 2205 owes its edge largely to nitrogen. To understand where 316L already wins, see 304 vs 316L and the 316L vs 304 marine comparison.
Where 2205 Pays Off
- Chloride and seawater service: heat exchangers, piping, and tanks handling brine, seawater, or chloride-bearing process fluids where 316L pits.
- Stress corrosion cracking resistance: hot chloride environments above ~60 °C where austenitic grades crack.
- High strength, lower weight: pressure vessels, storage tanks, and structural members where 2205’s roughly double yield lets you cut thickness and weight.
- Oil and gas, desalination, pulp and paper: the classic duplex applications.
Where 316L (or 304) Is Still the Better Buy
Duplex is not a universal upgrade. Stick with 316L or 304 when:
- Welding and forming dominate: complex forming, deep drawing, or tight-radius bending is easier on austenitic grades. For food and pharma, the established choice is covered in our food processing grade guide.
- Surface and hygiene matter most: austenitic grades take a more consistent polished finish; compare 2B, BA, and No.4 finishes.
- Cost-sensitive, mild environment: for indoor or decorative duty, 201 vs 304 or 430 for appliance panels may be enough.
The Grade Names Buyers Must Get Right
- S32205 vs S31803: both are called 2205. S32205 is the tighter, modern specification with a narrower chemistry range and higher guaranteed corrosion resistance. S31803 is the older, broader range. Specify S32205 for critical service; S31803 is acceptable for less demanding duty.
- Lean duplex (2101, 2304): lower alloy, lower cost, PREN near 316L. Use when you want duplex strength without full 2205 corrosion resistance.
- Super duplex (2507, S32750): higher PREN (~42+) for severe chloride and sour service. Higher cost and harder to weld.
- To avoid confusion across standards, compare ASTM, DIN, and JIS steel standards before substituting.
Welding and Fabrication Notes
Welding is where buyers get duplex wrong. The aim is to keep a balanced 50/50 phase structure in the weld and heat-affected zone.
- Filler: use an over-alloyed duplex filler (2209 for 2205) so the as-welded ferrite stays in range.
- Heat input: keep within a controlled band. Too low and ferrite is excessive (embrittlement); too high and intermetallic phases form.
- Interpass temperature: limit to about 150 °C for standard 2205; lower for super duplex. Use stringer beads, not wide weaves.
- Post-weld: standard 2205 usually does not need solution annealing if welded correctly; super duplex often does.
- For the austenitic comparison, see our notes on welding 304 vs 316L.
The 475 °C and Sigma Embrittlement Trap
Duplex embrittles when held in the 300 to 950 °C range, with a fast drop around 475 °C (alpha-prime) and sigma phase formation at higher temperatures. For service above 300 °C, duplex is the wrong choice; switch to a stable austenitic grade. This limit is the most common reason a duplex selection is quietly reversed at design review.
Thickness, Surface, and Inspection
2205 is available in sheet, plate, and pipe. Verify plate thickness tolerance for plate, and check the surface finish against the application; for corrosion-critical service, remove any scale from the cut edge. Run a pre-shipment inspection and confirm chemistry (Cr, Ni, Mo, N), PREN, and mechanical properties on the MTC. Hardness checks use the methods in our HB, HRC, HV guide. For a corrosion-by-environment view, see galvanized vs stainless.
Cost, Lead Time, and Next Step
2205 costs more than 316L, but its roughly double yield can offset that by allowing thinner sections, and its corrosion resistance can remove the need for a higher alloy or coating. Lean duplex (2101/2304) narrows the cost gap. Lead time for 2205 plate and pipe is longer than for 304/316L, and super duplex longer still.
If you are sourcing S32205/S31803, lean duplex 2101/2304, or super duplex 2507, use our quote spec template (grade, form, thickness, finish, quantity, incoterm) and request a quote. We will confirm the UNS number, PREN, MTC, and lead time against your specification.