What Is a Seamless Super Duplex Steel S32950 Tube?
Seamless Super Duplex Steel S32950 Tube — known by the trade name 7-Mo PLUS — is a high-chromium (26.0–29.0%) duplex (ferritic/austenitic) stainless steel tube produced to ASTM A789 from a solid billet with no weld seam.
UNS S32950 pairs very high chromium with moderate molybdenum (1.00–2.50%) and nitrogen (0.15–0.35%), delivering a PREN typically around 37 together with 480 MPa minimum yield strength — still roughly double that of 316L. Its low nickel content makes it an economical duplex option for chloride-bearing process streams, and the grade is solution annealed at 990–1025 °C with air cool per ASTM A789 Table 2.
Key Features and Advantages
Seamless UNS S32950 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:
High-Chromium Duplex Chemistry
26.0–29.0% Cr gives strong general and localized corrosion resistance at moderate alloy cost.
Roughly Double the Yield Strength of 316L
480 MPa minimum yield supports wall-thickness reduction.
Chloride SCC Resistance
duplex structure resists chloride stress corrosion cracking in hot brines.
Economical Alloying
only 3.5–5.2% Ni — stable pricing versus austenitic grades.
Nitrogen-Strengthened
0.15–0.35% N stabilizes austenite and boosts pitting resistance.
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 S32950 seamless tube.
Equivalent Grade
| UNS | Common grade name | EN material number | EN steel name | JIS grade | GB/T grade |
|---|---|---|---|---|---|
| S32950 | SUPER-DUPLEX | Not confirmed | Not confirmed | Not confirmed | Not confirmed |
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 28Cr-4Ni-2Mo-N chemistry is what gives UNS S32950 tube its super-duplex performance level.
| Grade | C | Cr | Ni | Mo | Mn | P | S | Si | Cu | N | Others |
|---|---|---|---|---|---|---|---|---|---|---|---|
| S32950 | ≤0.030 | 26.0–29.0 | 3.5–5.2 | 1.00–2.50 | ≤2.00 | ≤0.035 | ≤0.010 | ≤0.60 | – | 0.15–0.35 | – |
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 37.
Typical Mechanical Properties
UNS S32950 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 |
|---|---|---|---|---|---|
| S32950 | ≥70 (480) | ≥100 (690) | ≥20 | ≤290 HBW / 290 HV | ≤30 HRC |
Tensile and hardness requirements per ASTM A789/A789M-24, Table 4, in the solution-annealed condition (990 – 1025 °C (1820 – 1880 °F), with air cool). 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 | 1420 – 1460 °C | 12.5 μm/m.°C (20-100°C) | 15 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 S32950 seamless tube a preferred choice for:
Oil and Gas
Process and utility tubing in chloride-containing production and injection systems.
Chemical Processing
Heat-exchanger tube for brines and organic-acid streams.
Cooling Water and Seawater
Exchanger tubing in brackish and seawater cooling circuits.
Power Generation
Condenser and auxiliary tubing where chloride SCC excludes austenitic grades.
Frequently Asked Questions
What sizes are available for S32950 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 S32950 duplex seamless tube?
ASTM A789 / ASME SA789 is the governing specification for S32950 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.
