What Is a Seamless Super Duplex Steel S32550 Tube?
Seamless Super Duplex Steel S32550 Tube — widely known as Ferralium 255 or Alloy 255 — is a 25% chromium, copper-bearing super duplex (ferritic/austenitic) stainless steel tube produced to ASTM A789 from a solid billet with no weld seam.
UNS S32550 combines 24.0–27.0% chromium, 2.9–3.9% molybdenum and a substantial 1.50–2.50% copper addition. The copper markedly improves corrosion behavior in sulfuric acid and seawater, giving the grade a PREN typically around 38 (calculated from heat analysis) together with 550 MPa minimum yield strength. It is a proven, economical super duplex choice for chemical, marine and pulp & paper service.
Key Features and Advantages
Seamless UNS S32550 tube combines the corrosion performance of nickel alloys with the strength and economy of a duplex structure, which is why it is specified for the most aggressive chloride service:
Copper for Acid and Seawater Service: the 1.50–2.50% Cu addition improves resistance in sulfuric acid and seawater environments beyond standard 25Cr duplex.
High Yield Strength: 550 MPa minimum yield — about double austenitic 316L — allows thinner-wall, lighter designs.
Chloride Stress Corrosion Cracking Resistance: inherent to the balanced duplex microstructure in hot brines.
Good Pitting and Crevice Resistance: PREN typically ≈38 from the 25Cr-3Mo-N chemistry.
Cost-Effective Super Duplex: lower alloy content than 25Cr W-grades while retaining super-duplex-level performance in many media.
Product Specifications and Data
To provide a comprehensive overview of this high-performance material, here are the detailed technical specifications and performance data for UNS S32550 seamless tube.
Equivalent Grade
| UNS | Common grade name | EN material number | EN steel name | JIS grade | GB/T grade |
|---|---|---|---|---|---|
| S32550 | 255 / Ferralium 255 | 1.4507 | X2CrNiMoCuN25-6-3 | Not confirmed | 03Cr25Ni6Mo3Cu2N (S25554, approx.) |
Grade correspondence does not make ASTM, ASME, EN, JIS and GB delivery requirements interchangeable. Confirm chemistry, heat treatment, mechanical properties, dimensions, testing and certification against the purchase specification. Approximate or unconfirmed correspondences retain their qualification. See the grade designation index.
Chemical Composition
The 25Cr-5Ni-3Mo-2Cu-N chemistry is what gives UNS S32550 tube its super-duplex performance level.
| Grade | C | Cr | Ni | Mo | Mn | P | S | Si | Cu | N | Others |
|---|---|---|---|---|---|---|---|---|---|---|---|
| S32550 | ≤0.04 | 24.0–27.0 | 4.5–6.5 | 2.9–3.9 | ≤1.50 | ≤0.040 | ≤0.030 | ≤1.00 | 1.50–2.50 | 0.10–0.25 | – |
Chemical requirements per ASTM A789/A789M-24, Table 1. Single values are maximum content unless a range is indicated. PREN (Cr + 3.3Mo + 16N) is calculated from the heat analysis; typical heats run about 38.
Typical Mechanical Properties
UNS S32550 delivers roughly double the yield strength of austenitic 304/316 tube while remaining ductile and tough.
| Grade | Yield Strength, min, ksi (MPa) | Tensile Strength, min, ksi (MPa) | Elongation in 2 in. or 50 mm, min, % | Hardness, max, Brinell / Vickers | Hardness, max, Rockwell |
|---|---|---|---|---|---|
| S32550 | ≥80 (550) | ≥110 (760) | ≥15 | ≤297 HBW / 295 HV | ≤31 HRC |
Tensile and hardness requirements per ASTM A789/A789M-24, Table 4, in the solution-annealed condition (1040 °C (1900 °F) minimum). Mechanical property requirements do not apply to tubes smaller than 1/8 in. (3.2 mm) OD or with walls thinner than 0.015 in. (0.4 mm).
Typical Physical Properties
These physical properties highlight the material’s ability to perform reliably across a range of environments.
| Density | Melting Point | Thermal Expansion | Thermal Conductivity | Modulus of Elasticity | Electrical Resistivity |
|---|---|---|---|---|---|
| 7.8 g/cm³ | 1415 – 1460 °C | 12.4 μm/m.°C (20-100°C) | 13.5 W/mK (20°C) | 190 GPa | 0.85 x 10-6 Ω·m (20°C) |
Size Tolerance Chart (mm)
Permissible variations for seamless super duplex tube in accordance with ASTM A789/A789M-24, Table 5:
| Outside Diameter, mm | Permissible Variation in OD, mm | Average Wall | Minimum Wall |
|---|---|---|---|
| Up to 12.7, excl | ±0.13 | ±15% | +30% / −0% |
| Over 12.7 to 38.1, excl | ±0.13 | ±10% | +20% / −0% |
| Over 38.1 to 88.9, excl | ±0.25 | ±10% | +20% / −0% |
| Over 88.9 to 139.7, excl | ±0.38 | ±10% | +20% / −0% |
| Over 139.7 to 203.2, incl | ±0.76 | ±10% | +20% / −0% |
Notes (ASTM A789/A789M-24, Table 5): When the ordered wall is 19 mm or over, or the ID is 60% or less of the OD, average wall may vary ±12.5% and minimum wall +25% / −0%. For thin-walled tubes, ovality is limited per 12.2: max and min diameter at any cross section shall deviate no more than twice the OD variation, with the mean diameter still within tolerance.
Length Tolerance
- For tube ordered to a specific cut length: +3 mm / −0 mm (OD 38.1 mm and under) or +5 mm / −0 mm (over 38.1 mm), for cut lengths up to 7.3 m (24 ft).
- For lengths over 7.3 m, the over-tolerance increases by 3 mm for each 10 ft (3 m) or fraction thereof over 24 ft, or 13 mm (1/2 in.), whichever is less.
- Tube is normally supplied in mill lengths or specific cut lengths as agreed in the order.
Disclaimer: Dimensional data above are summarized from ASTM A789/A789M-24, Table 5. The full and most current specification document must always be consulted for official design and procurement purposes.
Common Industrial Applications
The combination of high strength, outstanding chloride stress corrosion cracking resistance and pitting resistance makes UNS S32550 seamless tube a preferred choice for:
Chemical Processing
Sulfuric acid coolers, exchangers and lines handling mixed acids and brines.
Oil and Gas
Process tubing and seawater-cooled equipment on offshore platforms.
Marine and Seawater Service
Seawater lift, firewater and exchanger tubing exposed to splash-zone chlorides.
Pulp and Paper
Chloride-bearing liquor and bleaching circuits where 300-series grades fail by SCC.
Frequently Asked Questions
What sizes are available for S32550 (255) super duplex seamless tube?
The overall range is OD 6–257 mm; WT 0.5–35 mm. Requirements outside this range are reviewed against mill capability on request.
Which specification governs S32550 (255) super duplex seamless tube?
ASTM A789 / ASME SA789 is the governing specification for S32550 (255) super duplex seamless tube. The edition in force at order time is stated on the quotation and purchase order; chemistry, mechanical properties, heat treatment, dimensions and testing are reported against the ordered standard and material heat.
What inspection documents are provided?
Each order is documented with a mill test certificate stating heat chemistry and mechanical test results. EN 10204 3.1 certification, hydrostatic testing, eddy-current or ultrasonic inspection and PMI are performed when specified in the purchase order.
Ready to Request a Quotation?
Send the NPS and schedule (or actual OD and wall), quantity, length, end finish and the inspection items you need as acceptance criteria — you will get a written quotation stating the specification and edition, delivery condition, documents included and lead time.
