ASTM A790 UNS S32750 Super Duplex Pipe for Desalination: Properties and Selection

What Is ASTM A790 UNS S32750 Super Duplex Pipe?

ASTM A790 is the specification for seamless and straight-seam welded ferritic-austenitic (duplex) stainless steel pipe for general corrosive service, covering sizes from NPS 1/8 up to NPS 30. UNS S32750, widely known as alloy 2507, is a super duplex grade with a nominal composition of 25% chromium, 7% nickel and 4% molybdenum plus nitrogen, giving a microstructure of approximately equal parts ferrite and austenite. The duplex structure produces a combination that neither austenitic nor ferritic grades can match: yield strength roughly double that of type 316L together with very high resistance to chloride pitting, crevice corrosion and stress corrosion cracking.

Chemical Composition and Microstructure (ASTM A790)

Element	UNS S32750 (wt%)
Carbon (C)	0.030 max
Silicon (Si)	0.80 max
Manganese (Mn)	1.20 max
Phosphorus (P)	0.035 max
Sulfur (S)	0.020 max
Chromium (Cr)	24.0 - 26.0
Nickel (Ni)	6.0 - 8.0
Molybdenum (Mo)	3.0 - 5.0
Nitrogen (N)	0.24 - 0.32
Copper (Cu)	0.50 max

Nitrogen is the key element for the super duplex performance: it stabilizes austenite, raises pitting resistance and increases strength. The pitting resistance equivalent number, calculated as PREN = Cr + 3.3 x Mo + 16 x N, is about 42 for S32750, far above the values of 2205 duplex (about 35) and 316L (about 25).

Mechanical Properties and Delivery Condition

Property	Requirement (ASTM A790)
Tensile strength, min	800 MPa (116 ksi)
Yield strength, min	550 MPa (80 ksi)
Elongation, min	15%
Hardness, max	32 HRC (310 HBW)

Pipe is supplied in the solution-annealed condition: heated to 1880-2060 deg F (1025-1125 deg C) and rapidly quenched in water. This treatment dissolves secondary phases and restores the balanced ferrite-austenite structure, which is essential for corrosion resistance. Seamless pipe is typically produced by hot or cold working, and welded pipe by forming strip or plate with a straight longitudinal weld; both forms must satisfy the hydrostatic test and the nondestructive examination requirements of the specification.

Why S32750 for Desalination Service

Desalination plants move large volumes of hot, oxygenated, chloride-rich seawater, a combination that aggressively attacks ordinary stainless steel through pitting and crevice corrosion. S32750 resists these mechanisms even at elevated temperatures, which is why it is used for seawater intake and brine lines, evaporator and heat exchanger tubing, high-pressure reverse osmosis piping and pump and valve components. Its high strength allows thinner walls than austenitic alternatives, reducing weight and material volume, and its resistance to chloride stress corrosion cracking eliminates the main failure mode of austenitic pipe in hot seawater service. In multi-effect distillation and reverse osmosis systems, S32750 piping has a proven record of long service life with minimal maintenance, with the final design life governed by wall thickness, flow velocity and the specific water chemistry.

Fabrication, Welding and Heat Treatment

Welding of S32750 requires controlled procedures: low heat input, interpass temperature limited to about 300 deg F (150 deg C), and filler metal of matching super duplex composition, typically a 25Cr-9Ni-4Mo-N type. The weld zone must be shielded with inert gas, including a nitrogen-containing backing gas on the root, to preserve the austenite-ferrite balance. After welding, the phase balance of the heat-affected zone must be verified, and full solution annealing and quenching are required if the service demands maximum corrosion performance. Because duplex steels are prone to sigma-phase embrittlement if cooled slowly, any hot forming or heat treatment must be followed by rapid water quenching.

Frequently Asked Questions

What is the difference between UNS S32750 and UNS S32760 (F55)?

Both are super duplex grades with similar strength. S32760 adds deliberate copper (0.5-1.0%) and tungsten (0.5-1.0%) and uses slightly less molybdenum (3-4%), which improves resistance in very aggressive chloride-rich and acidic environments. S32750 has marginally higher specified tensile strength; S32760 is often preferred for the most demanding marine and chemical duty.

Why is S32750 called a super duplex stainless steel?

The term super duplex refers to a pitting resistance equivalent number above 40. With a PREN of about 42, S32750 offers substantially better chloride pitting and crevice corrosion resistance than standard duplex grades such as 2205.

Can S32750 pipe be welded without post-weld heat treatment?

Welded joints made with matching filler and controlled heat input can be used in the as-welded condition in many services, but the weld and heat-affected zone must meet the required phase balance and corrosion performance, verified by testing. Solution annealing and quenching restore the best properties.

What is the maximum service temperature of S32750?

Super duplex grades are generally limited to about 575-600 deg F (300-315 deg C) continuous service because exposure above this range can promote embrittling phases. S32750 is not a heat-resistant grade; its value is strength plus corrosion resistance at ambient to moderate temperatures.

Is S32750 available as seamless pipe?

Yes. ASTM A790 covers both seamless and welded S32750 pipe, produced by hot or cold working in the case of seamless, and by strip or plate forming with a longitudinal weld in the case of welded pipe, both supplied solution annealed and quenched.

Which standards cover S32750 in forms other than pipe?

Plate, sheet and strip are covered by ASTM A240, pipe by A790 and A928 (welded, with filler), fittings by A815, and bars by A276 and A479. The corresponding European designation is EN 1.4410 (X2CrNiMoN25-7-4).