ASME SA789 UNS S31200 Duplex Stainless Steel Tube

ASME SA-789 / SA-789M is the Boiler and Pressure Vessel Code Book adoption in Section II Part A of seamless and welded ferritic/austenitic (duplex) stainless steel tubing for general service, identical with ASTM A789/A789M-18 in the 2023 BPVC text. Grade UNS S31200 under that Code Book product standard is a mid-alloy duplex with melting limits of chromium 24.0–26.0%, nickel 5.5–6.5%, molybdenum 1.20–2.00%, and nitrogen 0.14–0.20%, together with carbon ≤0.030%. When a Code-stamped vessel, exchanger, or piping datasheet calls for SA-789 S31200, the purchase order and mill test certificate should carry the ASME designation so Section II material identity stays clear.

SHH STEEL can produce and supply ASME SA789 UNS S31200 duplex stainless steel tube in both the seamless and the welded (no filler metal) routes, in typical heat-exchanger and general-service OD × wall sizes, with EN 10204 3.1 mill test certificates that report ladle chemistry, tensile and hardness results, and the heat-treatment record required by the ordered edition. We manufacture and stock-enquire this grade as a pressure-boundary tube product under the SA-789 chemistry, tensile, and heat-treatment tables — not as a catalogue nickname for “any 25Cr duplex.” Dual SA-789/A789 marking is available when a purchase order asks for both names on the stencil and MTC; for non-Code purchases the sister ASTM page is the twin reference, while this article stays ASME-first.

Commercially S31200 sits in the standard duplex band rather than lean duplex or super duplex. Buyers comparing chloride SCC resistance and strength against the more common 22Cr family will often look next at SA789 S32205 or the older SA789 S31803 designation; for higher PREN and offshore duty the step-up is usually SA789 S32750. Plate and sheet of related duplex chemistry are ordered under A240 / SA240 rather than under this tubing standard. The hub for the full grade set is SA789/SA789M.

Two chemistry layers matter for S31200 under ASME SA-789. The melting (heat or ladle) analysis in Table 1 is mandatory for every heat, while a product analysis on finished tube applies when Section 8 or the purchase order invokes it through the general-requirements path (Specification A1016/A1016M as referenced by the ordered SA-789 edition). Product-analysis tolerances typically flow from the A480/A480M annex table; they are not reprinted as a standalone chemistry table inside SA-789 itself. SHH STEEL reports the actual ladle values — and the product-analysis values when ordered — on the EN 10204 3.1 MTC for every heat.

Reading the row: chromium in the mid-20s with modest molybdenum and a controlled nitrogen band places S31200 in the standard duplex PREN window rather than the lean or super-duplex bands. Using the common formula PREN = %Cr + 3.3×%Mo + 16×%N on mid-range ladle aims (about Cr 25, Mo 1.6, N 0.17) gives roughly PREN ≈ 33, which is consistent with mid-alloy duplex practice; the standard itself does not publish a PREN minimum for this grade, so treat that figure as an engineering estimate from the stated chemistry, not a Code acceptance number. Ellipses in Table 1 mean the element need not be determined or reported.

ASME SA-789 Table 4 sets the tensile and hardness acceptance floor for UNS S31200 after the final heat treatment. Every lot sampled under the edition’s specimen rules must meet those numbers; hardness tests are part of lot acceptance, not an optional shop check. Mechanical property requirements do not apply to tubing smaller than 1/8 in. [3.2 mm] OD or with walls thinner than 0.015 in. [0.4 mm], per the Table 4 footnotes.

Those floors sit above typical 304/316 austenitic tube tensile rows for the same OD × wall, which is why duplex tube often allows thinner walls when design stress and corrosion both allow it. Design allowable stresses for Code vessels live in Section II Part D and are not quoted here; confirm the edition-specific allowable with your design engineer. Source for the numbers above: ASME BPVC.II.A-2023 SA-789/SA-789M Table 4 (identical with A789/A789M-18).

ASME SA-789 covers both seamless and welded tube. Section 5 requires that welded tube be made with no filler metal added — the longitudinal seam is autogenous (typically TIG or laser), then the finished tube is heat-treated as a whole. That is a different product story from filler-welded pipe under SA-312, and the stencil must state whether the tubing is seamless or welded.

Lot testing follows the manufacture route. Seamless tubes take a flaring test (minimum 10% inside-diameter expansion) on one end of one tube per lot. Welded tubes take a flange test on one end of one tube per lot, plus a reverse-flattening test on a specimen from each 1500 ft [450 m] of finished tubing. Tension and hardness tests apply to both routes on the lot basis in Section 9. SHH STEEL produces both seamless and welded SA789 S31200 under those rules and marks the route clearly on the tube and the MTC.

Dimensional supply for ASME SA789 S31200 is tube language — outside diameter × wall thickness (average wall unless the order specifies minimum wall) and, commercially, BWG / gauge — not ASME B36.19 pipe schedules as the primary size story. Ordering information in Section 3 asks for quantity, seamless or welded, grade, OD and wall, length, optional product analysis, hydro or NDE preference, and the specification designation. State on the purchase order whether wall is average or minimum, because Table 5 applies different permissible variation percentages to each.

Ovality rules in 12.2 apply to thin-walled tubes as defined in Table 5: the max and min diameters at any cross section may deviate from nominal by up to twice the OD tolerance, provided the mean diameter stays inside the given OD band. Cut-length tolerances also sit in Table 5. Source: ASME BPVC.II.A-2023 SA-789 Table 5. SHH STEEL quotes OD × wall (or BWG) and average-versus-minimum wall explicitly on every SA789 S31200 enquiry.

Equivalence language around S31200 tube is often looser than a Code or heat-exchanger purchase order can afford. ASME SA-789 UNS S31200 is the governing designation for this article; wrought names and EN numbers that point to similar 25Cr duplex families are approximate chemistry cousins, not automatic substitutes on an SA-789 mill test certificate. When a datasheet says “equivalent to S31200,” confirm which product standard, tensile/hardness table, and heat-treatment clause the mill will actually certify.

S31200 does not carry a widely used trade name the way 2205 (S32205) does, so purchase orders should spell the UNS number and the ASME SA789 product standard rather than relying on a shop nickname. Do not treat S31200 as interchangeable with S32205 or S31803 without checking nickel, molybdenum, nitrogen, and the Table 4 tensile floors for the ordered edition.

All tubes under ASME SA-789 shall be furnished in the heat-treated condition. For UNS S31200, Table 2 requires solution treatment at 1920–2010 °F [1050–1100 °C] followed by rapid cooling in water. That water-quench route is stricter than the air-or-water options listed for several sister duplex grades, and it matters for phase balance and corrosion performance after forming.

For seamless tubes, Section 6 allows an alternate to final furnace heat treatment: immediately following hot forming, while the tube temperature is not less than the specified minimum solution-treatment temperature, tubes may be individually quenched in water or rapidly cooled by other means. Skipping or under-cooling the solution treatment risks sigma-phase and other intermetallics that cut toughness and corrosion resistance, and it also raises the long-term risk of 475 °C embrittlement if the tube later sees prolonged service in that temperature band. SHH STEEL delivers SA789 S31200 only in the heat-treated condition required by Table 2, with the treatment record on the MTC.

Scope language in SA-789 emphasizes general corrosion resistance with particular attention to stress-corrosion cracking — the classic duplex selling point versus 300-series austenitics in chloride-bearing waters. S31200’s mid-20s chromium, modest molybdenum, and nitrogen band give a PREN around the low-to-mid 30s from the stated chemistry (estimate only; not a Code number), which is typically adequate for many process and cooling-water duties but below the PREN ≥40 class associated with super duplex.

In hotter, more aggressive chlorides, sour service, or seawater-leaning exchanger duty, buyers usually step up to SA789 S32750 or another super-duplex grade rather than stretching S31200. Prolonged exposure near 475 °C can embrittle duplex ferrite by spinodal decomposition; SA-789 itself notes that these steels are susceptible to embrittlement if used for prolonged periods at elevated temperatures. Keep design temperatures and post-weld heat practice inside the fabricator’s qualified window, and do not assume S31200 will match a 2205 or 2507 corrosion envelope without a materials review.

Because S31200 is a Code Book duplex tubing grade with a 100 ksi tensile floor and solid chloride-SCC resistance relative to 304/316, it shows up in heat exchangers, process coolers, condensers, and general corrosive tube duty where a mid-alloy duplex is specified by UNS number. The table below pairs typical application areas with a selection note so the grade is chosen for the fluid, temperature, and fabrication route — not only for the familiar “duplex” label on the BOM.

If the datasheet already names 2205 / S32205, stay on that grade rather than substituting S31200 by chemistry resemblance alone — nickel, molybdenum, nitrogen, and tensile floors differ. For lean-duplex cost-driven projects, compare SA789 S32304; for seawater and high-PREN duty, compare SA789 S32750.

Duplex S31200 tube welds with matching or near-matching duplex filler for tube-to-tubesheet joints, attachment welds, and fabrication joins when the fabricator’s WPS calls for filler; the SA-789 welded tube product itself, however, is made without filler metal in the longitudinal seam. Heat input control, interpass temperature limits, and rapid cooling after welding remain the fabricator’s responsibility under the applicable boiler or pressure-vessel Code — SA-789 governs the tubing product, not the field or shop weld procedure.

Ferrite–austenite balance in the weld metal and HAZ is sensitive to dilution and cooling rate. Overheating or slow cooling can promote sigma and other intermetallics that hurt toughness and pitting resistance; under-alloying the weld metal can leave too much ferrite. Qualify procedures on the actual heat and thickness, and clean thoroughly after welding. SHH STEEL can advise on typical filler families used with S31200 tube-to-tubesheet work, but the Code-stamped fabricator owns the WPS/PQR.

Standard mill delivery for ASME SA789 S31200 is heat-treated and pickled (or otherwise descaled), leaving a clean matte finish suitable for most heat-exchanger and general-service tube duty. Section 13 requires tubes to be free of excessive mill scale and suitable for inspection; a slight amount of oxidation is not considered scale. Any special finish — bright anneal under protective atmosphere, specified Ra on the bore or OD, or polish — is subject to agreement between manufacturer and purchaser and must be called out on the order.

When a fouling-sensitive or hygienic exchanger specification quotes an Ra maximum, treat that as a commercial finish requirement on top of SA-789 product, not as automatic with the grade stamp. SHH STEEL supplies the standard pickled heat-treated finish as default and quotes special finishes against the written enquiry.

Base ASME SA-789 testing for S31200 includes heat analysis to Table 1, tension and hardness tests to Table 4, flaring (seamless) or flange plus reverse-flattening (welded) tests on the lot basis in Section 9, and either hydrostatic testing or nondestructive electric testing of each length. The manufacturer may choose hydro versus NDE unless the purchase order fixes the method; both are acceptance routes inside the product standard. For the hydrostatic calculation, SA-789 substitutes 64000 (441.2) for the 32000 (220.6) factor used in A1016/A1016M — a duplex-specific detail buyers sometimes miss.

Marking under Section 14, in addition to A1016/A1016M requirements, must indicate whether the tubing is seamless or welded and whether the wall is average or minimum. SHH STEEL’s EN 10204 3.1 MTC ties heat number, chemistry, tensile/hardness, heat treatment, and test method to each length so receiving inspection can clear the shipment against the PO.

S31200 tube leaves SHH STEEL bundled, end-protected, and tagged for export or domestic delivery according to the length mix on the order. Bundle tags repeat the ASME SA789 grade, seamless or welded identity, OD × wall (or BWG), heat number, and PO reference so site receiving can check the shipment without opening every certificate packet first.

Export packing can be bare bundles, wrapped bundles, or wooden-cased lengths for bright-finished or small-bore material. We ship under the Incoterms and destination documents named on the order acknowledgment, with the MTC pack accompanying or preceding the cargo as agreed.

SHH STEEL produces and supplies ASME SA789 UNS S31200 as a Code Book tubing product with enforceable chemistry, tensile, hardness, heat-treatment, and marking rules — not as a trader nickname for “any 25Cr duplex tube.” We manufacture seamless and welded (no filler metal) routes under the SA-789 tables, report full ladle chemistry and tensile/hardness acceptance on the EN 10204 3.1 MTC, and stencil seamless-or-welded identity together with average-or-minimum wall so receiving inspection stays clean.

We keep the duplex conversation honest. If the service needs PREN in the 40+ band, we steer the enquiry toward SA789 S32750 or another super duplex rather than stretching S31200. If the BOM already names 2205, we quote SA789 S32205 on its own tensile and chemistry row. Dual SA-789/A789 marking is offered when requested so Code and non-Code jobs stay cleanly documented. That production-plus-advice posture is how SHH STEEL supports Shanghai export buyers and overseas fabricators on this grade.

SHH STEEL can produce and supply ASME SA789 UNS S31200 duplex stainless steel tube against the edition, manufacture route, and finish named on your enquiry. The capability table below summarizes the sizes, routes, and documents we regularly quote for this grade, followed by the Stock / Available sizes map buyers ask for when they need a quick sense of what to enquire against for heat-exchanger and general-service duty.

Stock / Available sizes — SHH STEEL regularly enquires and produces SA789 S31200 in heat-exchanger and general-service bands such as OD about 12.7–50.8 mm (½–2 in.) with walls in common BWG / metric thicknesses, and can extend into larger OD up to the Table 5 groups on agreement. Cut lengths, U-bends, and plain or beveled ends are quoted against the PO. We do not publish fictitious warehouse tonnages; send the OD × wall, length, seamless or welded preference, and quantity, and we will confirm production slot and any stockable near-sizes.

To enquire, list ASME SA789 UNS S31200, seamless or welded, OD × wall (average or minimum), length, quantity, test preferences (hydro or NDE), and any dual-marking or third-party need. We reply with mill route, lead time, and document set for that enquiry.

ASME SA789 UNS S31200 remains a solid mid-alloy duplex tubing choice when a Code Book purchase needs chromium in the mid-20s, a 100 ksi tensile floor, and better chloride SCC resistance than 304/316 without stepping into the super-duplex PREN band. SHH STEEL produces and supplies seamless and welded (no filler metal) SA789 S31200 tube, heat-treated per Table 2, tested per Section 9, and documented on EN 10204 3.1 MTCs for export and domestic Code work. Confirm OD × wall, average or minimum wall, manufacture route, and any dual SA-789/A789 marking on the purchase order so the stencil and certificate match the datasheet. For technical follow-up on this grade, contact admin@steelspecs.com.