One family, three rungs

Duplex stainless steels carry roughly half austenite and half ferrite in the delivered structure, and that balance is what the whole family trades on: strength near double the austenitic grades with better chloride resistance than the 18-8 grades they replace. The catalogue splits into three rungs. Lean duplex — LDX 2101 — substitutes nickel with manganese and nitrogen, the same trick the 201 family plays at the low end but engineered for structural strength rather than decoration. Standard duplex — 2205 and its pipe equivalents — is the volume grade. Super duplex — 2507 and S32760 — pushes chromium, molybdenum and nitrogen to the top of what the structure tolerates. What moves between the rungs is chemistry, price and a temperature ceiling, and knowing which rung the duty actually needs is where the money is.

The lean rung: LDX 2101

The LDX 2101 page describes the design intent directly: a low-nickel general purpose substitute for 304 and 304L. The chemistry runs 21.5 per cent chromium, 5 per cent manganese and 1.5 per cent nickel — the datasheet text quotes 0.45 per cent molybdenum while its own composition table lists 0.3 — plus 0.22 per cent nitrogen. Corrosion resistance is stated as superior to 304L and comparable to 316L, with greater chloride stress-corrosion cracking resistance than the conventional 300 series. The mechanical table is the argument for paying duplex prices on a lean grade: 65 ksi minimum yield rising to 69 ksi typical, 94 ksi minimum tensile, 30 per cent minimum elongation and a 290 HB ceiling against a 225 HB typical. The page draws the commercial conclusion itself — the higher strength enables thinner cross-sections, and thinner sections cost less to buy and to ship.

The standard rung: 2205

2205 — UNS S32205/S31803, EN 1.4462 — is the rung most orders land on. It carries 22 per cent chromium, 3 per cent molybdenum, 5 to 6 per cent nickel and 0.18 per cent nitrogen, and its page states pitting and crevice resistance superior to 316L or 317L in almost all corrosive media. Yield is about twice austenitic: ASTM A240 asks 65 ksi minimum against a 74 ksi typical, with 90 ksi minimum tensile, and the elevated-temperature table still shows 41 ksi yield at 572 °F. Plate runs 4.8 to 152.4 mm in twenty-five stock thicknesses, and the working window is -50 to +600 °F with restrictions noted for welded structures outside it. One number on the page settles most grade arguments: in wet-process phosphoric acid the corrosion rate is listed at 3.1 mils a year where 316L sits at 200. The 2205 pipe programme extends the same alloy into seamless, ERW, welded and fabricated forms under ASTM A790 / ASME SA790, with S31803 and S32205 as the two UNS designations buyers quote.

The super rung: 2507 and S32760

2507 lifts the alloy to 25 per cent chromium, 4 per cent molybdenum, 7 per cent nickel and 0.24 to 0.32 per cent nitrogen. A240 plate properties are 116 ksi minimum tensile, 80 ksi minimum yield and 15 per cent minimum elongation, and the page repeats the family’s commercial line: a light gauge of 2507 reaches the design strength of a thicker nickel alloy. It resists organic acids such as formic and acetic, inorganic acids carrying chlorides, and carbide-related intergranular attack, with weld wire listed as AWS ER2594, E2594-16 or E2553T-1. The ceiling matters more here than anywhere else in the family: use is limited below 600 °F (316 °C), where extended exposure costs toughness and corrosion resistance. S32760 — F55, 1.4501 — is published with PREN 40, 550 N/mm² minimum proof, 750 minimum tensile, 25 per cent elongation, 270 HB maximum and Charpy values of 80 J minimum at ambient and 45 J average at -46 °C. It is listed in NACE MR 01-75 for sour service and ships with 3.1 certification, 3.2 at a surcharge.

Where each rung pays

Map the duty to the rung and the choice usually makes itself. Lean duplex covers structural and general fabrications where 304L strength or nickel price is the constraint. Standard 2205 owns the middle: the page lists pressure vessels, tanks, piping and heat exchangers in chemical processing, oil and gas piping and tubing, effluent scrubbing, pulp and paper digesters and bleaching equipment, cargo tanks, food processing and biofuels. Super duplex is for warm, concentrated chloride: 2507’s application list names desalination equipment, flue gas scrubbing, offshore oil production and marine work, while S32760 adds explicit resistance to pitting and crevice corrosion in seawater at elevated temperature. If the duty is oxidising rather than chloride-driven, the austenitic ladder from 317 through 254 SMO to 904L remains the alternative — 317LMN travels as A240 sheet, tube and plate and coil for the mid-rung — and where reducing acids join the chlorides, 825 and C-276 sit above the whole stainless family. Titanium stays the last stop where chlorides defeat everything else.

Forms, supply and the commercial side

The duplex coil listing carries 2205 and 2507 in 0.2 to 60 mm thickness and 3.2 to 2,000 mm width, in finishes from No.1 and 2B through No.4, hairline and No.8 to BA, at a stated density of 7.93 g/cm³ and ±1 per cent tolerance. Coil weight runs 3 to 10 tonnes, MOQ is 1 tonne, supply 5,000 tonnes a month and lead time 7 to 15 working days after a 30 per cent deposit — commercially unremarkable, which is the point: duplex is a stock item, not a special melt. Pipe mirrors that: the A790 pages list finishes from annealed-and-pickled and bright-annealed to polished and mirror, inspection by TUV, SGS, BV, ABS and LR, and processing services covering welding, punching, cutting, bending and decoiling on the same enquiry. Where a job mixes families — duplex in the chloride zone, 304/304L elsewhere, square sections for handrail and support — the stainless pipe range and the large-diameter thick-wall programme let one supplier quote the whole material list, and welding and cutting of the transition pieces can ride the same purchase order.

What to put on the order

Seven lines settle a duplex enquiry. Quote the UNS number and the EN equivalent together — S31803 and S32205 are close enough to confuse a quotation and different enough to matter on a welded vessel. State the duplex rung you intend, not only “duplex”, because suppliers stock the ladder, not one grade. Give the media with chloride content, pH and velocity. Give design and operating temperature, which is what flags the 316 °C super-duplex ceiling and the -50 °F floor. Ask for solution-annealed condition and say whether a PREN or pitting-resistance requirement applies. Fix dimensions in the product’s own language — plate 4.8 to 152.4 mm, coil 0.2 to 60 mm, pipe to A790. And name the certificate class and any third-party inspection up front. An order carrying those lines prices cleanly across all three rungs; one that says only “duplex plate” invites the cheapest rung to be quoted, which is rarely the one the duty needed.