Specifications

Composition

S32760 also requires a minimum PREN of 40. Footnotes as given in the standards.

For less severe service, duplex 2205 round bars offer similar strength at lower cost.

Where super duplex bar is used

Seawater pump shafts, impeller shafts and propeller shafts must resist pitting, crevice corrosion under sleeves and seals, and corrosion fatigue, while carrying torque and bending loads. Super duplex bars combine high strength with a PREN of 40 or more, which makes them a common choice where 316 or 2205 shafts have suffered crevice corrosion. Subsea and splash-zone bolting, clamps and connectors are also machined from super duplex bar, as are valve stems, balls and seats, and small-bore fittings for super duplex piping. In cathodically protected subsea equipment, hydrogen embrittlement is a design consideration, so hardness and microstructure requirements from the project specification apply.

Manufacturing and testing

Bars are hot rolled or forged, solution annealed and quenched, then straightened and finished to size. The heat treatment of thick bars is critical, because slow cooling at the centre can form brittle intermetallic phases, so larger sizes are commonly tested at mid-radius or centre for ferrite, impact toughness and ASTM A923 compliance, and ultrasonically tested for internal soundness. Each bar is marked with grade, size and heat number. Bars can be supplied to NACE MR0175 / ISO 15156 on request.

Machining super duplex

Super duplex is harder to machine than 2205 and much harder than austenitic stainless steel: its strength and work-hardening call for rigid machines, sharp carbide or coated tools, conservative cutting speeds and generous coolant. Interrupted cuts and dwelling should be avoided to limit work-hardening. Ordering bright bars close to the finished size reduces machining time. Parts that are welded or hot worked after machining need to be solution annealed again.

Ordering

Please give the UNS grade and standard, shape and size, tolerance, black or bright, length, quantity, and the project's supplementary tests, NACE MR0175 / ISO 15156 and inspection requirements.

Super duplex compared with other shaft materials

For seawater shafts, designers usually compare super duplex with duplex 2205, 6% molybdenum austenitic steels, nickel alloys such as 625 and K-500, and titanium. Super duplex offers high strength with a PREN of 40 or more at a moderate cost, which suits large pump and propeller shafts. Alloy 625 is chosen where the temperature or chemistry goes beyond the duplex range, and Monel K-500 where very high strength in seawater is needed. Titanium is light and highly resistant but has a lower modulus, which affects shaft stiffness. The selection is normally made in the equipment specification, and we supply bar in whichever grade is specified, with the certification and tests required.

Applications and related products

Super duplex round bars are used for seawater pump, propeller and agitator shafts, valve stems and trim, subsea and offshore bolting, connectors and fittings, and desalination equipment parts. They are the starting material for super duplex fasteners and for machined pipe fittings and flanges. To complete a super duplex package we also supply super duplex pipes and tubes and super duplex sheets and plates. See all bar materials on the round bars, rods and wires page and grade guidance on the duplex and super duplex steel page.