Basic Concepts and History
1. What is Duplex Stainless Steel?
Duplex stainless steel (DSS) is a type of stainless steel whose microstructure consists of approximately 50% austenite (γ) and 50% ferrite (α). The phrase “50% austenite (γ) and 50% ferrite (α)” refers to the microstructural phase composition of duplex stainless steel.
Austenite (γ):
- Face-centered cubic (FCC) crystal structure.
- Features: good toughness, high ductility, and strong corrosion resistance.
- Commonly found in austenitic stainless steels such as 304 and 316.
Ferrite (α):
- Body-centered cubic (BCC) crystal structure.
- Features: high strength, good thermal conductivity, and excellent resistance to chloride stress corrosion cracking.
- Typical of ferritic stainless steels such as 430.
In duplex stainless steels:
- Under the microscope, roughly half of the microstructure is austenite and the other half is ferrite.
- The ratio is not an exact 50:50 but typically falls within the 40–60% range, depending on heat treatment and alloy composition.
If the structure were entirely austenite → good toughness but insufficient strength, prone to stress corrosion cracking.
If the structure were entirely ferrite → high strength but poor toughness, prone to embrittlement.
By combining both phases in ~50:50 proportion → the weaknesses of each are compensated, giving duplex steels both high strength and good toughness, along with excellent corrosion resistance.
It is like a team where austenite represents the “flexible humanities student” and ferrite represents the “strong science student.” Either group alone would have shortcomings, but together they complement one another, forming a well-rounded and balanced team.
2. Brief History of Duplex Stainless Steel
- 1930s: First development in Europe of 25% Cr duplex stainless steels.
- 1970s: The growth of offshore oil and gas industries demanded better chloride pitting and SCC resistance; grade 2205 (UNS S32205) became the industrial benchmark.
- 1980s–1990s: Super duplex grades (Cr > 25%, Mo > 3.5%, N > 0.2%) such as 2507 emerged.
- Today: Duplex steels are widely applied in chemical processing, desalination, oil & gas, and pulp & paper industries.
3. Principle of Duplex Stainless Steel
Through alloy balance design, both ferrite (α) and austenite (γ) phases are stabilized at solution-annealed condition:
- Cr, Mo → ferrite stabilizers.
- Ni, N, Cu → austenite stabilizers.
- Controlled heat treatment and cooling rate → prevent excessive single-phase dominance.
Result: microstructure exhibits “island-shaped austenite” embedded in a ferritic matrix.
4. How Duplex Structure is Achieved
- Composition design: ensure the austenite equivalent (Ni eq.) ≈ ferrite equivalent (Cr eq.).
- Processing: solution annealing at 1020–1100 °C followed by rapid water quenching to preserve the α+γ phases.
Manufacturing and Composition
1. How is Duplex Stainless Steel Manufactured?
- Melting: Electric arc furnace (EAF) followed by AOD or VOD refining; strict control of C, N, S, and P.
- Hot/Cold Working: Rolled at 1000–1250 °C; avoid σ-phase precipitation.
- Recuit de mise en solution: 1020–1100 °C with rapid quenching to achieve the desired duplex structure.
2. Typical Compositions of Duplex Stainless Steels
| Grade | Cr (%) | Ni (%) | Mo (%) | N (%) | Caractéristiques |
| 2205 (UNS S32205) | 21–23 | 4,5–6,5 | 2,5–3,5 | 0.14–0.20 | Most common grade; ~2× yield strength of 316L; superior pitting resistance |
| 2507 (UNS S32750) | 24–26 | 6–8 | 3–5 | 0.24–0.32 | Super duplex; excellent seawater resistance |
| 2304 (UNS S32304) | 22–24 | 3–5 | 0.2–0.6 | 0,05–0,10 | Lean duplex; cost-effective with moderate corrosion resistance |
3. Carbon Content in Duplex Stainless Steels
Typically C ≤ 0.03%: Low carbon avoids carbide precipitation at grain boundaries, improving intergranular corrosion resistance.
4. Equivalent Grades in Different Standards
| Standard | Common Grade | UNS | EN/DIN | GB (Chine) |
| ASTM/UNS (USA) | 2205 | S32205 | 1.4462 | 022Cr22Ni5Mo3N |
| ASTM/UNS (USA) | 2507 | S32750 | 1.441 | 022Cr25Ni7Mo4N |
| ASTM/UNS (USA) | 2304 | S32304 | 1.4362 | 022Cr23Ni4MnN |
Properties and Characteristics
1. Advantages of Duplex Stainless Steel
- High strength (yield strength ≈ twice that of 304/316).
- Superior resistance to piqûres et corrosion caverneuse compared with 316.
- Excellent resistance to chloride fissuration par corrosion sous contrainte.
- Lower thermal expansion than austenitic steels, better thermal fatigue resistance.
2. Key Characteristics
- High strength combined with good toughness.
- Resistance to stress corrosion cracking.
- Good fatigue resistance.
- Moderate weldability.
3. Corrosion Resistance
- 2205: better resistance in chloride-containing environments than 316L.
- 2507: maintains performance in high chloride environments (seawater, brine, salt spray).
4. Pitting Resistance by Grade (PREN)
- PREN = %Cr + 3.3×%Mo + 16×%N
- Standard duplex (PREN 25–30): e.g., 2205
- Super duplex (PREN 35–40+): e.g., 2507
- Lean duplex (PREN 22–25): e.g., 2304
5. Mechanical Properties (2205 example)
- Yield strength ≥ 450 MPa
- Tensile strength ≥ 620 MPa
- Elongation ≥ 25%
- Good impact toughness even at -40 °C
6. Thermal Properties
- Thermal conductivity: higher than austenitic steels, close to ferritic steels → faster heat transfer.
- Thermal expansion coefficient: lower than 304/316 → better dimensional stability.
7. Magnetism
- Due to ferrite phase, duplex steels exhibit partial magnetism.
- Magnetic intensity varies with α/γ phase balance.
8. Machinability Rating
- More difficult to machine than 304/316.
- Requires carbide tooling and reduced cutting speeds.
Heat Treatment and Failures
1. 475 °C Embrittlement
- Long exposure at 300–500 °C leads to α′ decomposition in ferrite → severe toughness loss.
- Avoid prolonged service in this range.
2. Heat Treatment
- Solution annealing: 1020–1100 °C → rapid quenching.
- Post-weld treatment: sometimes required to prevent σ-phase or χ-phase precipitation.
Applications and Uses
1. Best Applications of Duplex Stainless Steel
- Desalination plants
- Offshore structures and subsea pipelines
- Chemical process equipment (heat exchangers, pressure vessels)
- Pulp and paper bleaching equipment
- Food industry and environmental equipment
2. Common Product Forms
- Pipes (seamless and welded)
- Plates (hot-rolled, cold-rolled)
- Bars, forgings, brides
- Welding consumables (wires, electrodes)
Advantages, Disadvantages, and Barriers
1. Advantages of Using Duplex Steels
- Yield strength twice that of common austenitic steels.
- Better SCC resistance in chloride environments.
- Allows thickness reduction → cost savings.
2. Disadvantages
- More difficult to machine and weld than 304/316.
- Embrittlement risk at 300–500 °C.
- Higher material cost compared with common grades.
3. Barriers to Use
- Requires expertise in fabrication and welding.
- Limited availability compared with 304/316.
- Need for specialized welding consumables.
Comparison with Other Stainless Steels
1. Duplex vs 304
- Strength: 2205 ≈ 2× yield strength of 304.
- Corrosion: 2205 far superior in pitting/SCC.
- Magnetism: 304 essentially non-magnetic; duplex steels are magnetic.
2. Duplex vs 316
- Strength: 2205 > 316.
- Corrosion: 2205 much better pitting/crevice resistance than 316L.
- Cost: 2205 is higher per ton, but total project cost may be lower due to thinner walls.
3. Duplex vs 201
- Composition: 201 has low Ni, high Mn, poor corrosion resistance.
- Properties: 201 stronger than 304 but much weaker than duplex steels.
- Application: 201 is mainly decorative; duplex steels are used in harsh corrosive and load-bearing environments.
Duplex stainless steels achieve the best balance of strength and corrosion resistance through their unique α+γ microstructure. They are increasingly critical in industries requiring seawater resistance, pressure handling, and chemical durability. Despite challenges in fabrication, their performance advantages ensure growing adoption worldwide.
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