Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex Stainless Steel Composition (Cr-Ni-Mo-N)
The defining super duplex stainless steel composition is built around four elements: chromium (Cr), nickel (Ni), molybdenum (Mo) and nitrogen (N). The nominal balance is roughly 25Cr-7Ni-3.5Mo-0.3N. This combination delivers the balanced 50 percent austenite, 50 percent ferrite microstructure, plus a Pitting Resistance Equivalent Number (PREN) of 40 or higher that defines the super duplex class per NORSOK M-630.
The most common super duplex grade is 2507 (UNS S32750 / EN 1.4410), with no intentional copper or tungsten. Other grades in the family (Zeron 100, Ferralium 255, DP3W) carry intentional additions of copper, tungsten or both for service in specific media. The element-by-element breakdown below covers what each element does and the standard chemistry windows.
Canonical 2507 Composition (UNS S32750 per ASTM A479)
| Element | ASTM A479 Min % | ASTM A479 Max % | Typical Mill Heat |
|---|---|---|---|
| Carbon (C) | (no min) | 0.030 | 0.020 |
| Manganese (Mn) | (no min) | 1.20 | 0.80 |
| Phosphorus (P) | (no min) | 0.035 | 0.025 |
| Sulfur (S) | (no min) | 0.020 | 0.001 |
| Silicon (Si) | (no min) | 0.80 | 0.40 |
| Chromium (Cr) | 24.0 | 26.0 | 25.0 |
| Nickel (Ni) | 6.0 | 8.0 | 7.0 |
| Molybdenum (Mo) | 3.0 | 5.0 | 3.8 |
| Nitrogen (N) | 0.24 | 0.32 | 0.28 |
| Copper (Cu) | (no min) | 0.50 | 0.20 (residual, not intentional) |
| Tungsten (W) | (no intentional addition for S32750) | ||
| PREN | 41 to 43 (Cr + 3.3 times Mo + 16 times N) | ||
Role of Each Element
Chromium (Cr). Drives ferrite formation and chloride pitting resistance. The 24 to 26 percent window in 2507 places the alloy at the high end of the duplex family. Chromium is the dominant term in PREN.
Nickel (Ni). Drives austenite formation. The 6 to 8 percent window balances chromium and molybdenum to maintain the 50/50 phase split. Higher nickel pushes austenite up; lower nickel pushes ferrite up.
Molybdenum (Mo). Strong contributor to chloride pitting and crevice corrosion resistance, weighted 3.3x in the PREN formula. The 3 to 5 percent window in 2507 is the main differentiator from standard duplex 2205 (which sits at 3 to 3.5 percent).
Nitrogen (N). Strong austenite stabiliser, second in PREN weighting at 16x. The 0.24 to 0.32 percent window in 2507 lifts both yield strength and pitting resistance. Nitrogen also reduces the alloy's sensitivity to sigma-phase precipitation during welding.
Carbon (C). Held to 0.030 percent maximum in 2507. Higher carbon promotes M23C6 carbide precipitation at grain boundaries during welding, which depletes adjacent matrix in chromium and triggers intergranular corrosion (sensitisation).
Manganese (Mn). Held to 1.20 percent maximum. Mild austenite stabiliser; also helps nitrogen solubility in the melt.
Silicon (Si). Held to 0.80 percent maximum. Mild ferrite stabiliser; higher silicon promotes sigma-phase precipitation.
Phosphorus (P) and Sulfur (S). Tramp elements held low (0.035 and 0.020 percent maximum) to preserve toughness, weldability and corrosion resistance.
Copper (Cu). Listed as 0.50 percent maximum in 2507 (UNS S32750), a residual ceiling, not an intentional addition. Copper improves resistance in non-oxidising acids and is intentionally added in Ferralium 255 (UNS S32550, 1.5 to 2.5 percent) and Zeron 100 (UNS S32760, 0.5 to 1.0 percent).
Tungsten (W). No intentional addition in 2507. Tungsten lifts pitting resistance similarly to molybdenum and is intentionally added in Zeron 100 (0.5 to 1.0 percent) and DP3W (1.5 to 2.5 percent). Tungsten contribution to PREN is via the alternative PREW formula.
Copper in 2507 (Residual, Not Intentional)
A common point of confusion: ASTM A479 lists copper at 0.50 percent maximum for UNS S32750, but the original mill datasheets for SAF 2507 (Sandvik), UR 52N+ (Industeel) and Avesta 2507 (Outokumpu) all describe 2507 as containing no intentional copper. The 0.50 percent ceiling is a residual limit, not a target. Mill heats typically run at 0.10 to 0.25 percent copper, well below the cap. If a project specification expects intentional copper for resistance in dilute sulfuric acid, the correct grade is Ferralium 255 (UNS S32550), not 2507.
Composition Comparison Across Super Duplex Grades
| Element | 2507 (S32750) | Zeron 100 (S32760) | Ferralium 255 (S32550) | DP3W (S39274) |
|---|---|---|---|---|
| Chromium | 24.0 to 26.0 | 24.0 to 26.0 | 24.0 to 27.0 | 24.0 to 26.0 |
| Nickel | 6.0 to 8.0 | 6.0 to 8.0 | 4.5 to 6.5 | 6.0 to 8.0 |
| Molybdenum | 3.0 to 5.0 | 3.0 to 4.0 | 2.9 to 3.9 | 2.5 to 3.5 |
| Nitrogen | 0.24 to 0.32 | 0.20 to 0.30 | 0.10 to 0.25 | 0.24 to 0.32 |
| Copper | 0.50 max (residual) | 0.50 to 1.00 | 1.50 to 2.50 | 0.20 to 0.80 |
| Tungsten | (none) | 0.50 to 1.00 | (none) | 1.50 to 2.50 |
Phase Balance and Composition Control
Achieving the target 50/50 ferrite-austenite balance depends on tight control of the chromium and nickel equivalents. The Schaeffler-DeLong-WRC diagrams used for austenitic grades are extended for duplex steels using nitrogen-corrected chromium and nickel equivalents. In practice, mills target a ferrite content of 45 to 55 percent in the solution-annealed bar and plate. NORSOK M-630 requires base-metal ferrite content of 35 to 55 percent, and weld-metal ferrite content of 35 to 65 percent.
Standards Setting Composition
| Standard | Scope |
|---|---|
| ASTM A479 | Bar and shapes (UNS S32750 chemistry) |
| ASTM A182 F53 | Forgings, flanges and forged fittings (UNS S32750) |
| ASTM A789 / A790 | Tube and pipe (UNS S32750 and S32760) |
| EN 10088-3 | Werkstoff 1.4410 (X2CrNiMoN25-7-4) for S32750 |
| NORSOK M-630 | Material data sheet, Norwegian Continental Shelf |
Super Duplex Composition Frequently Asked Questions
What is the composition of super duplex stainless steel?
Super duplex stainless steel composition is built around 24 to 27 percent chromium, 4.5 to 8 percent nickel, 3 to 5 percent molybdenum and 0.10 to 0.32 percent nitrogen. The nominal 2507 (UNS S32750) chemistry is 25Cr-7Ni-3.5Mo-0.3N with no intentional copper or tungsten. Other super duplex grades carry intentional additions of copper, tungsten or both.
Does 2507 contain copper?
UNS S32750 (2507) lists copper at 0.50 percent maximum, which is a residual limit, not an intentional addition. Mill heats typically run at 0.10 to 0.25 percent copper. Original mill datasheets for SAF 2507, UR 52N+ and Avesta 2507 all describe 2507 as containing no intentional copper. For intentional copper in a super duplex grade, specify Ferralium 255 (UNS S32550) or Zeron 100 (UNS S32760).
Does 2507 contain tungsten?
No. UNS S32750 (2507) contains no intentional tungsten. Tungsten is intentionally added in Zeron 100 (UNS S32760, 0.5 to 1.0 percent) and DP3W (UNS S39274, 1.5 to 2.5 percent). The forging spec ASTM A182 F53 (for S32750) does not include tungsten in the chemistry window.
What is the role of nitrogen in super duplex?
Nitrogen is a strong austenite stabiliser and the second-largest term in the PREN formula (weighted 16x). The 0.24 to 0.32 percent window in 2507 lifts both yield strength and pitting resistance. Nitrogen also reduces the alloy's sensitivity to sigma-phase precipitation during welding, by slowing the kinetics of ferrite-to-sigma transformation.
Why is carbon held below 0.03 percent?
Carbon above 0.030 percent promotes M23C6 carbide precipitation at grain boundaries during welding heat cycles. This depletes the adjacent matrix in chromium and triggers intergranular corrosion (sensitisation) in chloride service. The 0.030 percent maximum in 2507 is the same limit applied to L-grade austenitic stainless steels for the same reason.
What is the typical PREN of 2507 from a mill heat?
Typical 2507 mill heats sit at PREN 41 to 43, calculated from the chemistry on the mill test certificate using Cr + 3.3 times Mo + 16 times N. A heat with 25 percent Cr, 3.8 percent Mo and 0.28 percent N yields PREN of approximately 42.0.
Are mill chemistries the same as ASTM minima?
No. ASTM A479 sets the minimum and maximum allowable composition window for UNS S32750. Mill heats typically run at the centre of the window (around 25Cr-7Ni-3.8Mo-0.28N) to deliver consistent PREN and phase balance. Mill test certificates show the actual heat chemistry, which is the basis for PREN calculation and for sour-service compliance under NACE MR0175.
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