When austenitic stainless (304/316L) can’t take the pressure — literally — engineers reach for duplex stainless steel. Duplex grades deliver roughly double the yield strength of 304/316L while matching or exceeding their corrosion resistance, particularly against chloride pitting and stress corrosion cracking. The trade-off: they’re harder to weld, more expensive, and require specific heat treatment that can be done wrong.
This guide covers what duplex stainless actually is, the main grades (2205, 2304, 2507), how they compare to 316L, when duplex is the right choice, and the manufacturing rules that prevent welding failures.
Quick summary — Duplex stainless steels have a 50/50 mix of austenite and ferrite, giving them 2× the strength of 316L and superior chloride resistance. 2205 is the workhorse (most common). 2304 is the “lean duplex” — cheaper, slightly lower performance. 2507 is the “super duplex” — premium performance for severe service. Choose duplex over 316L for: oil & gas, marine, desalination, chemical processing, where strength + chloride resistance both matter.
What Duplex Actually Is
Conventional stainless steels are either:
- Austenitic (304, 316L): face-centered cubic, low strength, very weldable
- Ferritic (430): body-centered cubic, magnetic, moderate properties
- Martensitic (410, 17-4PH): hardenable through heat treatment
Duplex is a 50/50 mix of austenite and ferrite, achieved through chemistry and controlled processing. The result is a microstructure that combines properties from both phases:
- High strength (from ferrite)
- Toughness and corrosion resistance (from austenite)
- Better chloride resistance than either alone
Chemistry vs austenitic
| Element | 316L | 2205 Duplex | Effect |
|---|---|---|---|
| Chromium | 16-18% | 22-23% | More Cr → better corrosion |
| Nickel | 10-14% | 4.5-6.5% | Less Ni → lower cost |
| Molybdenum | 2-3% | 3-3.5% | Chloride pit resistance |
| Nitrogen | trace | 0.14-0.20% | Austenite stabilizer, strength |
The reduced nickel is a major cost advantage when nickel prices spike — typically 30-40% less nickel than 316L in 2205.
The Main Duplex Grades
2205 (UNS S31803/S32205) — The Standard Duplex
The workhorse. ~80% of all duplex applications.
| Property | Value |
|---|---|
| Yield strength | 450 MPa (65 ksi) |
| Tensile | 620 MPa (90 ksi) |
| Elongation | 25% |
| PREN | 35-38 |
| Max service temp | 250°C (sustained) |
| Density | 7.8 g/cm³ |
2304 (UNS S32304) — Lean Duplex
Lower alloy content, lower cost. Good for moderate corrosion environments where 316L isn’t strong enough but full 2205 isn’t needed.
| Property | Value |
|---|---|
| Yield strength | 400 MPa (58 ksi) |
| Tensile | 600 MPa (87 ksi) |
| Elongation | 25% |
| PREN | 24-26 |
| Cost | ~20% less than 2205 |
2507 (UNS S32750) — Super Duplex
Premium grade for severe service.
| Property | Value |
|---|---|
| Yield strength | 550 MPa (80 ksi) |
| Tensile | 800 MPa (116 ksi) |
| Elongation | 25% |
| PREN | 40-43 |
| Cost | ~50-100% more than 2205 |
Hyper duplex (2707, Zeron 100)
PREN > 45. Used for the most demanding offshore, chlorinated, and acid environments. Specialty material.
PREN — The Single Most Important Number
PREN = Pitting Resistance Equivalent Number
PREN = %Cr + 3.3 × %Mo + 16 × %NPREN predicts resistance to chloride pitting. A grade is generally considered:
- PREN < 25: Standard environments
- PREN 25-32: Mild chloride exposure
- PREN 32-40: Marine, chloride-rich (2205 sits here)
- PREN > 40: Severe chloride (super duplex 2507)
PREN comparison
| Grade | PREN |
|---|---|
| 304 | 18-19 |
| 316L | 24-25 |
| 2304 (lean duplex) | 24-26 |
| 2205 (duplex) | 35-38 |
| 2507 (super duplex) | 40-43 |
| 6Mo austenitic (254 SMO) | 42-45 |
The key insight: 2205 has PREN ~36, vs 316L’s 24-25 — substantially better chloride resistance.
Duplex vs 316L: Direct Comparison
| Property | 316L | 2205 Duplex | Winner |
|---|---|---|---|
| Yield strength | 170 MPa | 450 MPa | 2205 (2.6×) |
| Tensile | 485 MPa | 620 MPa | 2205 |
| PREN | 24-25 | 35-38 | 2205 |
| SCC resistance | Moderate | Excellent | 2205 |
| Weldability | Excellent | Good (with care) | 316L |
| Formability | Excellent | Good | 316L |
| Toughness | Excellent | Good (drops below -50°C) | 316L (cryogenic) |
| High temp (>250°C) | OK to 800°C | Limited to 250°C | 316L (high temp) |
| Cost | Baseline | +15-25% | 316L (cost) |
| Cost (high Ni) | Volatile | More stable | 2205 (Ni stability) |
The structural advantage: For a tank, pressure vessel, or structural component, using 2205 lets you reduce wall thickness by ~30-40% vs 316L while maintaining safety margin. This often offsets the material cost premium, especially on large structures.
For deeper austenitic stainless comparison, see our 304 vs 316 guide.
Where Duplex Wins
Oil & gas
| Application | Why duplex |
|---|---|
| Offshore platforms | Marine + structural strength |
| Subsea manifolds | Chloride + pressure |
| Sour service piping | H2S + chloride |
| FPSO process equipment | Chloride + cyclic loading |
Chemical processing
| Application | Why duplex |
|---|---|
| Pulp & paper bleaching | Chloride + chemical attack |
| Chemical tank cars | Strength + chemical resistance |
| Heat exchangers | Chloride + pressure |
Desalination
| Application | Why duplex |
|---|---|
| RO plant high-pressure piping | Brine + pressure |
| Brine heaters | Concentrated chloride |
| Pump components | Chloride + erosion |
Marine & coastal
| Application | Why duplex |
|---|---|
| Bridge structural | Strength + coastal exposure |
| Marine hardware | Salt spray + strength |
| Coastal architecture | Long-term aesthetic + structural |
Renewable energy
| Application | Why duplex |
|---|---|
| Offshore wind structural | Marine + cyclic loading |
| Tidal energy components | Continuous immersion + strength |
Manufacturing Challenges: Why Duplex Is Harder
Welding — the critical issue
Duplex relies on a balanced 50/50 austenite/ferrite microstructure. Welding can disrupt this balance if heat input is wrong:
Too high heat input: ferrite phase grows excessively → loss of toughness and corrosion resistance Too low heat input: not enough austenite formation → high ferrite, brittleness
Welding rules:
- Heat input controlled: typically 0.5-2.5 kJ/mm (vs 316L’s wider range)
- Filler material: matching duplex filler (E2209 for 2205)
- Interpass temperature: max 150°C (vs 316L’s 200°C)
- Argon shielding: high-purity, never contaminated
- No preheat generally (vs preheat needed for martensitic stainless)
- Post-weld inspection: ferrite count via Fischer Feritscope (target 35-65%)
For more on welding, see our TIG vs MIG vs robotic guide.
Forming
Duplex requires 40-60% more force to bend than 316L. Design considerations:
- Larger inside radii (R ≥ 2.5T)
- Springback is significant
- Limited cold formability — avoid stress-relief temperatures
For bending math, see our K-factor explained.
Machining
Duplex is harder to machine than 316L:
- Higher cutting forces
- More work-hardening
- Better machining performance with rigid tooling
- Sharp cutting edges, generous coolant flow
Heat treatment risks
Duplex must be solution-annealed at ~1050°C, then quenched rapidly. The hazards:
- Slow cooling through 600-950°C: forms intermetallic phases (sigma, chi) → brittleness and corrosion
- Holding at 475°C: causes “475°C embrittlement” (alpha-prime precipitation)
- Field stress relief: cannot use standard procedures designed for 316L
This is why field repairs on duplex require qualified procedures and welder certifications.
Cost Analysis
Material cost comparison (per kg, illustrative):
| Grade | Relative Cost | Notes |
|---|---|---|
| 304 | 1.0× (baseline) | Common, stable supply |
| 316L | 1.4× | Higher Ni + Mo content |
| 2304 lean duplex | 1.3× | Less Ni than 316L |
| 2205 duplex | 1.6× | Premium duplex |
| 2507 super duplex | 2.5× | Severe service only |
The real cost calculation: For structural applications, the strength advantage of 2205 lets you use thinner sections. A 30% wall thickness reduction often makes 2205 the lower total cost vs 316L.
Specification Best Practices
A complete duplex spec includes:
- Grade: 2304, 2205, 2507 with UNS number
- Standard: ASTM A240 (plate/sheet), A790 (piping), A276 (bars), AMS 5895
- Heat treatment: Solution annealed condition, cooling rate
- PREN minimum: Specify required PREN value
- Welding procedure: Heat input limits, filler material
- Ferrite count: Range (typically 35-65%)
- Inspection: Hardness, PMI (positive material identification), ferrite count
- Documentation: Mill test certificate (EN 10204 3.1), heat treatment records
When NOT to Use Duplex
| Situation | Better Alternative |
|---|---|
| High-temperature service > 250°C | 316L or 310 (sigma phase risk in duplex) |
| Cryogenic service < -50°C | 304L / 316L (duplex toughness drops) |
| Hydrogen service | Specialty grades (hydrogen embrittlement risk) |
| Frequent field welding | 316L (easier to weld in field conditions) |
| Cosmetic / architectural | 316L (better surface finish, easier to polish) |
How DRAmetal Handles Duplex Stainless
Our duplex capabilities:
- Materials: 2304, 2205, 2507 with full traceability and PMI verification
- Welding: Qualified procedures (ASME IX) with ferrite count verification per AWS A4.2
- Forming: Press brake with springback compensation for duplex
- Heat treatment: In-house solution annealing for small parts; qualified partners for large work
- NDT: PMI, ferrite count (Fischer Feritscope), hardness, dimensional
- Industries: Oil & gas, marine, chemical processing, desalination, renewables
For our stainless steel material capabilities, see the material page. For duplex stainless fabrication, submit your specifications. The initial response target is within 24 working hours; grade, welding and inspection recommendations require engineering review.
Bottom Line
Duplex stainless steel is the right choice when you need both high strength and chloride resistance in a single material. 2205 is the default duplex grade for most applications. 2304 saves money when corrosion requirements are moderate. 2507 handles severe service where 2205 doesn’t have enough margin.
The main fabrication risks include inappropriate weld heat input, unsuitable thermal exposure and a service environment that has not been assessed for the specified grade. Compare the sheet-metal welding service and custom stainless-steel enclosure fabrication against buyer-released material, weld, cleaning and inspection requirements; corrosion performance and service life must be validated for the actual application rather than promised from an alloy name.
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