ASTM A789 UNS S31200 Duplex Stainless Steel Tube

ASTM A789 / A789M is the product specification for seamless and welded ferritic/austenitic (duplex) stainless steel tubing intended for general corrosion service, with particular emphasis on resistance to chloride stress corrosion cracking. Grade UNS S31200 under that specification is a high-chromium duplex with heat analysis limits of carbon ≤0.030%, chromium 24.0–26.0%, nickel 5.5–6.5%, molybdenum 1.20–2.00%, and nitrogen 0.14–0.20% as listed in A789/A789M-24 Table 1. The balanced ferrite–austenite microstructure gives roughly double the yield strength of common 300-series austenitics and markedly better chloride SCC resistance than Type 304 or 316L in many aqueous duties.

SHH STEEL can produce and supply ASTM A789 UNS S31200 duplex stainless steel tube as both seamless and welded product (welded without filler metal), in the usual heat-exchanger and process OD × wall / BWG band, documented on EN 10204 3.1 mill test certificates that report ladle chemistry and the tensile plus hardness acceptance set. Dual A789 / SA789 marking is available only when the purchase order asks for both independent standards; ASME SA789 remains a separate Code Book adoption even when the grade text aligns for a referenced edition. Browse the grade family on the A789/A789M hub.

Commercially the same chemistry is still called 44LN, 1.4460, or S31200 duplex tube; none of those shop names replaces the UNS grade on the stencil or MTC. Sister grades under the same ASTM standard include the workhorse A789 S32205 (2205 family), lean options such as A789 S32304, and super duplex A789 S32750 when a higher PREN band is required. Plate buyers looking for matching sheet chemistry should reference A240 rather than this tube specification.

Heat analysis for UNS S31200 must meet the melting limits in ASTM A789/A789M-24 Table 1. The table below records those limits exactly as published in the public sample of the -24 edition; product analysis, when ordered, uses the A480/A480M annex tolerances, except that the product analysis tolerance does not apply to carbon when the specified maximum is 0.04% or less.

Reading the melt against a PREN estimate of Cr + 3.3Mo + 16N at mid-range chemistry (Cr 25, Mo 1.6, N 0.17) gives roughly PREN ≈ 33, which places S31200 in the standard duplex band rather than lean or super duplex. Nitrogen in the 0.14–0.20 window stabilizes austenite and contributes to pitting resistance; molybdenum at 1.20–2.00% is lower than 2205-class S32205 (3.0–3.5% Mo), so chloride pitting performance sits below that of S32205 even though chromium is higher. Always confirm the heat-certificate numbers against your process chloride and temperature window rather than relying on the PREN label alone.

ASTM A789 requires the material to meet the tensile and hardness properties prescribed in its tensile/hardness table (Table 4 in recent editions). The public A789/A789M-24 sample excerpt available to this writer did not include that table; the figures below are taken from older publicly circulated A789 Table 4 copies (cross-checked between two PDF reprints) and should be reconfirmed against the edition named on your purchase order before they are used for design acceptance.

Those minima already put S31200 well above the yield of annealed 304/316 tube, which is why duplex tube is often specified when wall thickness or weight must come down without sacrificing pressure capacity. Hardness is capped at 280 HBW for this grade — slightly lower than the 290 HBW ceiling used for many other duplex entries — so mills must keep cold work and quench practice under control. 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 footnote language in those older Table 4 copies.

Section 5 of ASTM A789 states that tubes shall be made by the seamless or welded process with no filler metal added. Seamless S31200 tube is typically produced by hot extrusion or pierce-and-draw followed by cold finishing to final OD and wall; welded tube is formed from strip or plate of matching chemistry and autogenously welded (TIG/plasma or equivalent) without filler, then usually cold-worked and re-annealed so the weld seam meets the same heat-treatment and mechanical floors as the parent metal.

Mandatory forming tests differ by route: seamless tube takes a flaring test (minimum 10% expansion of the inside diameter) on one end of one tube per lot, while welded tube takes 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. Both routes share tension and hardness sampling. SHH STEEL produces and supplies both seamless and welded A789 S31200 against the route named on the enquiry, with the weld seam and parent metal reported on the same EN 10204 3.1 certificate when welded product is ordered.

ASTM A789 covers average wall thickness as the default, or minimum wall when the order specifies it. Outside diameter and wall are ordered as tube sizes (metric or inch OD × wall / BWG), not as pipe schedules first — although schedule-equivalent walls can be quoted when a buyer converts from an NPS callout. Dimensional and general requirements fall under Specification A1016/A1016M unless A789 states otherwise.

When you convert a pipe schedule into tube wall, state clearly whether the acceptance is average or minimum wall — the two are not interchangeable on the mill ticket. SHH STEEL will confirm the OD × wall band, length tolerance, and finish before the heat is reserved so the MTC matches what the fabricator expects to install.

UNS S31200 is commonly cross-referenced to EN 1.4460 (X3CrNiMoN27-5-2) in European designations and appears in ASTM plate and pipe sister documents under the same UNS number. Exact chemistry windows and mechanical floors differ slightly between product forms, so an A789 tube heat should not be assumed interchangeable with an A790 pipe heat or an A240 plate heat without checking the applicable table.

Buyers who need Code-stamped tube for ASME Section VIII or similar service should call out SA789 explicitly; dual A789/SA789 stencil is only issued when both names are on the purchase order. For plate of matching chemistry see A240; for the broader duplex tube family stay on the A789/A789M hub.

All A789 tubes shall be furnished in the heat-treated condition per Table 2. For UNS S31200 the solution-annealing window in A789/A789M-24 Table 2 is 1920–2010 °F [1050–1100 °C] followed by rapid cooling in water (or by other means that achieve an equivalent quench). For seamless tubes, as an alternate to a final furnace anneal, tubes may be individually quenched immediately after hot forming while still at or above the specified minimum solution temperature.

That rapid quench is not optional detail: duplex alloys are prone to sigma and other intermetallic phases if they linger in the roughly 700–1000 °C window, and they are also susceptible to 475 °C (885 °F) embrittlement from spinodal decomposition in the ferrite when held for prolonged periods between about 400 and 500 °C. The A789 scope itself warns that these steels are susceptible to embrittlement if used for prolonged periods at elevated temperatures — so S31200 tube is a corrosion and strength grade for aqueous and moderate-temperature service, not a creep alloy for continuous high-temperature duty. SHH STEEL supplies S31200 in the solution-annealed and quenched delivery condition required by the standard; any subsequent hot forming by the fabricator must be followed by a proper re-anneal and quench if the phase balance is to be restored.

The primary reason buyers move from 304/316L to an A789 duplex such as S31200 is chloride stress corrosion cracking resistance, combined with higher strength that allows thinner walls. S31200’s high chromium (24–26%) and nitrogen addition support pitting and crevice resistance in many chloride-bearing waters, while the duplex microstructure itself is far less prone to classic austenitic CSCC under the temperature and chloride combinations that crack 300-series alloys.

Compared with A789 S32205, S31200 carries more chromium but less molybdenum, so its PREN sits in a similar mid-duplex band while the Mo-driven crevice performance is typically below 2205-class material. Against lean duplex grades such as A789 S32101 it offers higher Cr and Mo; against super duplex A789 S32750 it is a step down in PREN and is not a substitute where PREN ≥40 or seawater crevice duty is specified. Confirm corrosion selection with your process fluid, temperature, and any NACE / ISO 15156 or project chloride limits — PREN is a screening index, not a warranty.

UNS S31200 tube under A789 is used where general corrosion resistance and chloride SCC resistance matter more than extreme pitting PREN: chemical-process heat-exchanger and condenser tube, pulp-and-paper liquor lines, desalination pretreatment and brine-handling tube, and selected oil-and-gas utility and process circuits that do not demand super duplex. The high yield strength relative to 300-series austenitics also helps when pressure design drives wall thickness or when vibration and erosion-corrosion margins are tight.

It is not the first choice for untreated seawater crevice service or for continuous exposure in the 300–500 °C band where 475 °C embrittlement or sigma formation becomes a design concern — those duties push buyers toward higher-PREN super duplex or toward alloys qualified for the temperature. When your datasheet already names 2205 / S32205, ask whether S31200 is truly interchangeable before switching; the Mo difference is real in crevice chloride service. SHH STEEL produces and supplies A789 S31200 for the enquiry sizes and test scope your project lists.

Duplex S31200 welds readily with matching or overalloyed duplex fillers when a fabricated spool needs filler metal, but A789 welded tube itself is produced without filler metal — the autogenous weld must meet the same heat treatment and mechanical requirements as the parent tube. Heat input and interpass temperature have to stay inside a controlled window so the ferrite–austenite balance in the weld and HAZ does not drift into a brittle, sigma-prone, or overly ferritic structure.

Post-weld solution annealing and rapid quench restore phase balance after hot work or after heavy fabrication welds when the procedure requires it; many thin-wall autogenous tube welds are qualified without a separate PWHT when the mill anneal already covers the seam. Avoid prolonged soaks in the 600–1000 °C sigma nose and in the 400–500 °C embrittlement band during any stress-relief or coating-cure cycle. SHH STEEL can advise on typical WPS ranges for S31200 tube-to-tube and tube-to-tubesheet joints when the enquiry includes fabrication scope.

Standard delivery for A789 S31200 tube is solution-annealed and pickled (or otherwise descaled) to remove heat-tint and mill scale so the passive film can form uniformly. Bright-annealed or lightly polished OD/ID finishes are available by agreement when the heat-exchanger or hygienic duty needs a smoother bore or a cleaner OD for shell-side inspection.

Passivation after pickling or after fabrication welding is good practice in chloride service; any residual iron contamination from carbon-steel tooling should be removed before the tube enters corrosive duty. SHH STEEL states the finish route on the quotation and on the MTC so receiving inspection can match the PO line.

Each A789 tube is subjected to either a nondestructive electric test or a hydrostatic test per the standard and the ordering options in A1016/A1016M. Mechanical sampling covers tension (lot size rules in Notes), hardness on two tubes per lot, flaring for seamless or flange plus reverse flattening for welded, and full heat analysis on the MTC. Product analysis and any supplementary corrosion or ferrite tests are added when the purchase order asks for them.

SHH STEEL issues EN 10204 3.1 certificates as standard and arranges third-party inspection / 3.2 when the enquiry names an agency. Lot definition follows A789 Notes 1 and 2 for forming tests versus tension/hardness, including the continuous-furnace and direct-quench-after-hot-forming rules — so the certificate heat and lot identity stay traceable to the lengths you receive.

A789 S31200 tube is typically packed in hexagonal bundles or plywood cases with end protectors and waterproof wrapping for ocean freight, with bundle tags that repeat the heat number, size, and standard so receiving can match the MTC without opening every package. Cut-to-length exchanger tube may be boxed by length group; U-bends, when ordered, travel in purpose-built crates.

SHH STEEL ships FOB, CFR, CIF, or EXW under the Incoterm named on the order and can arrange destination marks, packing lists, and third-party tally when the project requires them. Export documentation includes the commercial invoice, packing list, and the EN 10204 3.1 (or 3.2) mill certificate set.

SHH STEEL is a Shanghai-based stainless steel manufacturer and exporter that produces and supplies duplex tube — including ASTM A789 UNS S31200 — for heat-exchanger, process, and general corrosion service. We are not a paper trader: mill rolling, heat treatment, testing, and documentation are coordinated so the lengths you receive match the grade, size, and test scope on the purchase order.

Buyers choose SHH STEEL for A789 S31200 when they need a clear seamless-or-welded option, EN 10204 3.1 traceability, and a supplier who understands duplex metallurgy — solution anneal windows, sigma and 475 °C risks, and the difference between this grade and 2205 or super duplex — rather than a generic stainless stockist. Enquiry response covers OD × wall, average versus minimum wall, finish, and whether dual A789/SA789 marking is required before the heat is reserved.

SHH STEEL can produce and supply ASTM A789 UNS S31200 duplex stainless steel tube against the edition, size, and finish route named on your enquiry. The capability table below summarizes what we regularly quote for this grade, followed by the Stock / Available sizes map buyers use when they need a quick sense of what to enquire against.

Stock / Available sizes

SHH STEEL quotes S31200 tube against a practical production-and-stock enquiry range rather than a frozen warehouse SKU list with published tonnages. The sizes below are the OD × wall combinations buyers most often request for duplex heat-exchanger and process tube; availability of any single line on a given week depends on mill drawing and inbound schedule, so treat the table as a supply map for enquiry — not a guarantee that every line sits on the floor.

When you send an enquiry, include the ASTM edition if known, seamless or welded, OD × wall (avg or min), length, finish, test extras, and destination port so we can confirm production lead time and whether dual Code marking is needed. Sister grades such as A789 S32205 and A789 S32750 can be quoted on the same enquiry if your project mixes duplex classes.

ASTM A789 UNS S31200 is a high-chromium standard duplex stainless steel tube grade for general corrosion service where chloride SCC resistance and elevated yield strength matter more than the highest PREN band. SHH STEEL produces and supplies seamless and welded S31200 tube to the A789 heat-treatment and test regime, with EN 10204 3.1 documentation and the size and finish options listed above. For availability, dual marking, or a comparison with S32205 or super duplex alternatives, contact admin@steelspecs.com.