Super Duplex Stainless Steel 32750 (also known as UNS S32750, SAF 2507, F53, Alloy 32750) is a next-generation duplex alloy designed for extreme corrosion environments. With balanced austenitic–ferritic microstructure and high contents of Cr (~25%), Mo (~3–5%), Ni (~6–8%), and N (~0.24–0.32%), SS 32750 offers a PREN > 41—surpassing many duplex and austenitic grades.
2. Alloy Identity & Standards
3. Chemical Composition & Microstructure
SS 32750 typically comprises:
- Cr: 24–26%
- Mo: 3–5%
- Ni: 6–8%
- N: 0.24–0.32%
- Also contains C ≤0.03%, Si ≤0.8%, Mn ≤1.2%, Cu ≤0.5%, P/S ≤0.035/0.02
Its microstructure is balanced (ferrite: austenite ~50/50), offering twice the strength of most austenitic steels and enhanced pitting protection.
4. Mechanical & Physical Properties
4.1 Mechanical Strength
- Tensile Strength: 800–1000 MPa (116–145 ksi)
- Yield Strength: ≥ 550 MPa (80 ksi)
- Elongation: ≥ 15–25%
- Hardness: ≤ 310 HB
- Impact: ≥ 80 J @ room temp
High strength enables thinner components—ideal for pressure vessel and piping.
4.2 Physical Characteristics
- Density: ~7.8 g/cm³
- Melting Point: ~1350 °C
- Elastic Modulus: ~200 GPa
- Thermal Expansion (20–300 °C): ~11.1 µm/m·K
- Thermal Conductivity: ~14 W/m·K
Its low thermal expansion and high conductivity reduce distortion when welded or stressed.
5. Corrosion Resistance
5.1 Localized and SCC Resistance
- Pitting/crevice corrosion: Outstanding (PREN >41; critical pitting >50 °C)
- Stress corrosion cracking: Superior to both austenitic and standard duplex grades in chlorides
5.2 General & Environmental Performance
- Highly resistant in acidic, alkaline, and halide environments.
- Approved for sour gas (H₂S) service due to NACE ratings.
5.3 Operating Temperature Range
- Ideal: −50 °C to +300 °C; may suffer toughness loss above this due to intermetallic phases .
6. Fabrication & Welding Guidelines
6.1 Machining
“Super duplex… it’s hell. …need a decent rigid machine… loves to move… avoid if you can.”
Use slow speeds (40–60 SFM), carbide tools, rigid setups, heavy coolant.
6.2 Welding
Acceptable methods: TIG, MIG, SMAW, FCAW. Avoid oxyacetylene to prevent carbon pickup. Maintain interpass < 150 °C. Use matching super duplex filler.
6.3 Heat Treatment
- Solution anneal: ~1050–1100 °C with rapid cooling
- Avoid: Slow cooling through ~600 °C to prevent sigma-phase formation.
7. Industrial Applications
Ideal for:
- Oil & Gas: Subsea piping, risers, wellhead equipment
- Chemical & Petrochemical: Vessels, scrubbers, evaporators
- Marine: Ship shafts, desalination, offshore platforms
- Pulp & Paper: Bleach plants, digesters
- Power & FGD: Flue gas systems, cooling circuits
- Food & Pharma: Chlorides and hygienic piping
8. Scrap Super Duplex: Recycling & Low-Cost Alternatives
8.1 Scrap Economics
- Scrap is magnetic with high Cr/Mo/N — valuable if properly sorted.
- Must avoid contamination with lower grades; buyers use XRF or magnetomics .
8.2 Low-Cost Duplex Alternatives
- 2205 duplex (PREN ~34): sufficient for many applications at lower cost
- Lean ferritics/austenitics unsuitable for high-end chloride service.
9. Fabrication Case Studies & Machining Insights
- Machinist Reddit threads note its stringy chips, tool wear, dimensional shifting—emphasizing the need for rigid setups and gentle chip control
- Sigma‑phase embrittlement risk: avoid high forging/holding time; rapid post-heat treatment cooling is essential.
10. Future Trends & Sustainability
- Demand rising in desalination, offshore, and green hydrogen sectors.
- Pressure on scrap purity and alloy circularity increases with Ni/Cr supply chain focus.
- Emergence of hybrid alloys with slightly reduced alloy content for lower cost while maintaining >PREN40.
11. Conclusion
SS 32750 / Super Duplex 32750 provides an unmatched combination of strength and corrosion resistance, ideal for critical environments. While its price and machining difficulty are downsides, its enhanced performance and recyclability make it a top choice for high-stakes engineering. Responsible scrap handling and informed alloy selection (e.g., 2205 vs 32750) can optimize performance and cost—essential for today’s sustainable, competitive steel economy.
