What Is a Seamless Duplex Steel S31500 Tube?
Seamless Duplex Steel S31500 Tube is a lean duplex (ferritic/austenitic) stainless steel tube, commonly known by the trade name 3RE60 (a designation originated by Sandvik/Alleima and now widely used in the industry). It is produced to ASTM A789 from a solid billet with no weld seam, giving uniform structure and reliable pressure integrity.
With about 18.5% chromium, 5% nickel, 2.75% molybdenum and a deliberate silicon addition (1.40–2.00%), S31500 was developed for hot chloride environments. The dual-phase microstructure gives it roughly double the yield strength of austenitic 304/316 tube and a strong advantage against chloride stress corrosion cracking, while the silicon addition improves performance in hot, chloride-bearing waters — the reason the grade became a classic in the pulp and paper and process industries.
Key Features and Advantages
Seamless UNS S31500 tube offers a combination of properties that standard austenitic grades struggle to match, which is why duplex tubes are widely specified for aggressive service:
High Yield Strength: with a 440 MPa minimum yield strength, S31500 tube allows thinner walls and lighter structures than austenitic grades at the same pressure.
Chloride Stress Corrosion Cracking Resistance: the ferritic–austenitic structure gives markedly better chloride-SCC resistance than standard austenitic stainless steels in hot water and brine service. As with every duplex selection, the actual medium, temperature, chloride content and applied stress decide the operating window.
Silicon-Enhanced Hot Chloride Performance: the 1.40–2.00% Si addition is a deliberate metallurgical feature that improves resistance in hot chloride environments such as paper-machine and hot-water systems.
Good Weldability: S31500 can be welded with standard duplex practice and matching fillers without post-weld heat treatment in most applications.
Cost-Efficient Alloying: lower nickel and molybdenum content than austenitic grades gives a lower alloy cost per metre. Actual material cost and price stability depend on the quoted heat, size and market conditions, and are confirmed at quotation.
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 S31500 seamless tube.
Equivalent Grade
| UNS | Common grade name | EN material number | EN steel name | JIS grade | GB/T grade |
|---|---|---|---|---|---|
| S31500 | 3RE60 | 1.4424 | X2CrNiMoSi18-5-3 | 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 balanced 18Cr-5Ni-Mo-Si chemistry is what gives UNS S31500 tube its characteristic duplex performance.
| Grade | C | Cr | Ni | Mo | Mn | P | S | Si | N |
|---|---|---|---|---|---|---|---|---|---|
| S31500 | ≤0.030 | 18.0–19.0 | 4.3–5.2 | 2.50–3.00 | 1.20–2.00 | ≤0.030 | ≤0.030 | 1.40–2.00 | 0.05–0.10 |
Chemical requirements per ASTM A789/A789M-24, Table 1. Single values are maximum content unless a range is indicated. For S31500 the sulfur limit is ≤0.030 (not the ≤0.020 applied to S31803/S32205), and the grade specifies a deliberate silicon addition of 1.40–2.00 % for improved performance in hot chloride-bearing waters.
Mechanical Requirements
Minimum tensile and hardness requirements for UNS S31500 in the solution-annealed condition, per ASTM A789/A789M-24, Table 4. These are specification minima for the grade, not typical values from a given heat.
| 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 |
|---|---|---|---|---|---|
| S31500 | ≥64 (440) | ≥92 (630) | ≥30 | ≤290 HBW / 290 HV | ≤30 HRC |
Tensile and hardness requirements per ASTM A789/A789M-24, Table 4, in the solution-annealed condition (980 – 1040 °C (1800 – 1900 °F), rapid 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 |
|---|---|---|---|---|---|
| 7.7 g/cm3 | 1420 – 1460 °C | 12.5 μm/m.°C (20-100°C) | 15 W/mK (20°C) | 190 GPa | 0.80 x 10-6 Ω·m (20°C) |
Size Tolerance Chart (mm)
Permissible variations for seamless duplex tube in accordance with ASTM A789/A789M-24, Table 5:
| Outside Diameter, mm | Permissible Variation in OD, mm | Average Wall | Minimum Wall |
|---|---|---|---|
| Under 12.7 | ±0.13 | ±15% | +30% / −0% |
| 12.7 to under 38.1 | ±0.13 | ±10% | +20% / −0% |
| 38.1 to under 88.9 | ±0.25 | ±10% | +20% / −0% |
| 88.9 to under 139.7 | ±0.38 | ±10% | +20% / −0% |
| 139.7 up to and including 203.2 | ±0.76 | ±10% | +20% / −0% |
Notes (ASTM A789/A789M-24, Table 5): Each row applies to the whole band shown; the upper bound of a band is the start of the next band up to and including 203.2 mm OD. 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, chloride stress corrosion cracking resistance and good general corrosion resistance makes UNS S31500 seamless tube a preferred choice for:
Oil and Gas
Produced water, injection lines and process piping where chloride levels exclude standard austenitic grades.
Chemical Processing
Heat-exchanger tubing and transfer lines handling brines, organic acids and chloride-bearing media.
Desalination and Water Treatment
Tube for multi-effect and evaporator systems exposed to hot, concentrated brines.
Pulp and Paper
Classic 3RE60 service: digester and liquor piping where hot chlorides and SCC ruled out 300-series grades.
Frequently Asked Questions
What sizes are available for S31500 (3RE60) 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 S31500 (3RE60) duplex seamless tube?
ASTM A789 / ASME SA789 is the governing specification for S31500 (3RE60) 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?
For an S31500 seamless tube quotation, send the outside diameter, wall thickness and whether it is specified as average or minimum wall, cut length, quantity, applicable standard and edition, surface condition, and the inspection documents you need as acceptance criteria. Include the service conditions and any additional acceptance tests. You will get a written quotation stating the specification and edition, delivery condition, documents included and lead time.
