ASME SA789 UNS S31500 Duplex Stainless Steel Tube

ASME SA-789 / SA-789M is the Section II Part A Code Book product standard for seamless and welded ferritic/austenitic stainless steel tubing for general service. The 2023 BPVC edition reprints text identical with ASTM A789/A789M-18, yet a Code-stamped purchase should still name SA-789 on the order and the mill test certificate. Grade UNS S31500 is the lower-chromium, silicon-rich duplex in Table 1: chromium only 18.0–19.0%, nickel 4.3–5.2%, molybdenum 2.50–3.00%, silicon deliberately high at 1.40–2.00%, and nitrogen 0.05–0.10%, with carbon ≤0.030%.

SHH STEEL can produce and supply ASME SA789 UNS S31500 duplex stainless steel tube in seamless and welded (no filler metal) forms, in typical heat-exchanger OD × wall sizes, with EN 10204 3.1 MTCs that report ladle chemistry — including the distinctive silicon range — plus tensile, hardness, and the Table 2 heat-treatment record. We manufacture this grade under the SA-789 tables rather than offering an unnamed “18Cr duplex” substitute. Dual SA-789/A789 marking is available when the PO requests both names; this article remains ASME-first.

S31500 is an older duplex chemistry (often associated historically with 3RE60-type practice in trade literature) and sits below the 22Cr mainstream of SA789 S32205 and SA789 S31803 on chromium. Lean-duplex buyers comparing nickel cost sometimes look at SA789 S32304; higher-chromium 25Cr options start at grades such as SA789 S31200. Plate of related duplex chemistry is ordered under A240 / SA240. Hub: SA789/SA789M.

Heat analysis must meet Table 1. Product analysis applies only when Section 8 or the purchase order invokes it through A1016/A1016M, using A480/A480M product-analysis tolerances. The silicon range 1.40–2.00% is a defining feature of S31500 — far above the 1.00% maxima on most sister duplex rows — and the MTC must report the actual silicon value inside that band.

Reading the row: chromium at 18–19% with molybdenum 2.5–3.0% and modest nitrogen yields a lower PREN than 2205-class grades. Using PREN = %Cr + 3.3×%Mo + 16×%N on mid-range aims (Cr ≈18.5, Mo ≈2.75, N ≈0.075) gives PREN ≈ 18.5 + 9.1 + 1.2 ≈ 29. That estimate is from stated chemistry only; SA-789 publishes no PREN minimum for S31500. Silicon is not in the PREN formula but does affect oxidation and slag behaviour in welding — another reason not to swap S31500 for a low-silicon 22Cr grade by nickname.

Table 4 sets a slightly lower tensile floor for S31500 than for S31200 or S32205, with a higher elongation minimum. Lot sampling follows Section 9. Mechanical properties do not apply below 1/8 in. OD or 0.015 in. wall.

The 92/64 ksi pair with 30% elongation reflects this grade’s older duplex design envelope. Hardness ceiling matches several sister duplex rows at 290 HBW / 30 HRC. Section II Part D carries design allowables — not quoted here. Source: ASME BPVC.II.A-2023 SA-789 Table 4.

SA-789 allows seamless or welded manufacture; welded tube must be made without filler metal. The finished tube is heat-treated as a whole, and marking must state the route and the wall basis (average or minimum).

Seamless lots take flaring (10% min ID expansion). Welded lots take flange testing plus reverse flattening every 1500 ft [450 m]. Tension and hardness apply to both. SHH STEEL produces both routes for SA789 S31500 and records the route on stencil and MTC.

Order by OD × wall (average unless minimum is specified) and length. Table 5 tolerances apply by OD group; thin-wall ovality follows clause 12.2. Prefer tube / BWG language over pipe schedules as the primary size story.

Source: ASME BPVC.II.A-2023 SA-789 Table 5. State average versus minimum wall on every S31500 enquiry so the permissible variation band is unambiguous.

ASME SA-789 UNS S31500 governs this page. Trade nicknames and EN numbers that echo older 18Cr duplex practice are approximate only — confirm the product standard and Table 4 row before accepting a substitute.

Because silicon and chromium set S31500 apart, do not treat it as interchangeable with 2205 without a materials review.

Table 2 requires S31500 solution treatment at 1800–1900 °F [980–1040 °C] with rapid cooling in air or water — a lower temperature band than the 1870–2010 °F window used for S31803/S32205.

Seamless tubes may be quenched after hot forming when temperature stays at or above the minimum. Rapid cooling still matters for phase balance and to limit intermetallics. SHH STEEL delivers SA789 S31500 only in the Table 2 heat-treated condition with the record on the MTC.

SA-789 emphasizes general corrosion resistance and chloride stress-corrosion cracking performance versus austenitics. S31500’s estimated PREN near ≈29 from mid-range chemistry is modest by modern duplex standards — useful where the datasheet specifically names this UNS, less competitive where 22Cr PREN in the mid-30s is expected.

For higher chloride or seawater-leaning duty, step to SA789 S32205 or a super duplex rather than stretching S31500. Prolonged elevated-temperature exposure can embrittle duplex ferrite (including the 475 °C mechanism discussed in duplex literature); SA-789 itself warns about prolonged elevated-temperature embrittlement risk.

S31500 appears when an older specification or replacement BOM still calls this UNS for heat-exchanger and general corrosive tube duty. Match the grade to the written UNS and SA-789 tables rather than upgrading or downgrading by informal “duplex” language alone.

New designs that want mainstream 22Cr duplex usually specify SA789 S32205 instead.

Tube-to-tubesheet and attachment welds typically use matching or near-matching duplex filler under a qualified WPS. The mill welded product under SA-789 remains autogenous (no filler in the longitudinal seam). High silicon influences weld-pool fluidity and slag behaviour, so procedures qualified on low-silicon 2205 are not automatically valid for S31500.

Control heat input and interpass temperature, cool the HAZ promptly, and clean after welding. SHH STEEL can discuss filler families used with S31500; the fabricator owns the WPS/PQR.

Standard delivery is heat-treated and descaled/pickled, free of excessive mill scale per Section 13. Bright anneal or specified Ra finishes are agreement items.

Ra maxima on hygienic or fouling-sensitive exchangers are commercial overlays, not automatic with the SA-789 stamp. SHH STEEL quotes special finishes against the enquiry.

Testing includes Table 1 heat analysis, Table 4 tension and hardness, flare or flange tests by route, reverse flattening for welded tube, and hydro or NDE on each length. Hydrostatic calculation uses the duplex factor 64000 (441.2). Manufacturer chooses hydro versus NDE unless the PO specifies.

Marking must show seamless or welded and average or minimum wall. SHH STEEL 3.1 MTCs link heat, chemistry (including Si), mechanicals, heat treatment, and test method to each length.

Bundles leave SHH STEEL tagged with ASME SA789, UNS S31500, route, OD × wall, heat number, and PO reference, with end protection matched to the length mix.

Export packing ranges from wrapped bundles to wooden cases for bright or small-bore tube. Incoterms follow the order acknowledgment.

SHH STEEL produces and supplies ASME SA789 UNS S31500 as a manufactured Code Book tubing product — not a trader nickname for “any low-nickel duplex.” We report the full Table 1 row including silicon 1.40–2.00%, meet the 92/64 ksi tensile floors, and heat-treat inside the 1800–1900 °F band on every certified length.

If a new design would be better served by mainstream 2205, we say so and quote SA789 S32205. If the BOM truly locks S31500, we produce to that UNS. Dual SA-789/A789 marking is available on request.

SHH STEEL can produce and supply ASME SA789 UNS S31500 seamless and welded tube to the edition and finish on your enquiry. Capability and Stock / Available sizes follow.

Stock / Available sizes — Regular enquiry and production bands include OD about 12.7–50.8 mm with common BWG / metric walls; larger Table 5 OD groups by agreement. Cut lengths on request. No invented tonnage lists — send OD × wall, route, and quantity for a firm reply.

List ASME SA789 UNS S31500, seamless or welded, OD × wall, wall basis, length, and quantity on the enquiry.

ASME SA789 UNS S31500 is the high-silicon, lower-chromium duplex tubing grade retained for Code and replacement service when that UNS is written into the datasheet. SHH STEEL produces and supplies seamless and welded (no filler metal) SA789 S31500, heat-treated in the 1800–1900 °F band, tested to Section 9, and certified on EN 10204 3.1 MTCs. Confirm OD × wall, wall basis, route, and any dual marking on the purchase order. For technical follow-up, contact admin@steelspecs.com.