ASTM A790 UNS S32750 Super Duplex Stainless Steel Pipe: Composition, Properties and Fabrication

Scope of ASTM A790 and the S32750 Grade

ASTM A790/A790M is the material specification for seamless and welded ferritic-austenitic (duplex) stainless steel pipe intended for general corrosive service. UNS S32750, known commercially as 2507 super duplex and designated EN 1.4410, is the 25 percent chromium super duplex grade listed in that specification. The standard fixes chemical composition, tensile properties, hardness limits, heat treatment and test frequency; the purchase order adds size, schedule, quantity and any supplementary requirement.

Pipe is supplied in the solution-annealed condition. Each length receives either a hydrostatic test or a nondestructive electric test, and the documentation package normally includes a mill test certificate reporting heat number, chemistry and mechanical results.

Chemical Composition of UNS S32750

Element	Requirement, %
Carbon (C)	0.030 max
Manganese (Mn)	1.20 max
Phosphorus (P)	0.035 max
Sulfur (S)	0.020 max
Silicon (Si)	0.80 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

The high chromium and molybdenum content, combined with deliberate nitrogen addition, produces a roughly 50/50 ferrite-austenite microstructure in the annealed condition. Nitrogen is the element that stabilises the austenite phase during rapid cooling and simultaneously lifts pitting resistance, which is why the specification sets a minimum rather than a maximum for it.

Mechanical Properties and Heat Treatment

Property	Requirement
Tensile strength, min	800 MPa (116 ksi)
Yield strength, 0.2 % offset, min	550 MPa (80 ksi)
Elongation in 2 in., min	15 %
Hardness, max	32 HRC
Solution annealing temperature	1025 - 1125 °C (1880 - 2060 °F)
Cooling after annealing	Water quench or rapid air cool

The strength level is roughly twice that of 316L, which lets designers reduce wall thickness and still meet pressure requirements. Slow cooling through the 600 to 900 °C interval must be avoided: sigma and chi phases precipitate there, consuming chromium and molybdenum and degrading both impact toughness and pitting resistance. Cold forming beyond moderate deformation and any welding operation are therefore followed by a repeat of the full solution anneal and quench cycle.

Pitting Resistance and Chloride Service

Pitting resistance equivalent number is calculated as PREN = Cr + 3.3 Mo + 16 N. The nominal composition of the grade (25 % Cr, 4 % Mo, 0.27 % N) returns a value of approximately 43, compared with about 25 for 316L and about 35 for standard 2205 duplex. That margin is the reason S32750 is routinely selected for chloride-bearing process streams, offshore piping and seawater-cooled exchangers where 316L would be vulnerable to pitting and chloride stress corrosion cracking.

NACE MR0175 / ISO 15156 lists super duplex stainless steels for defined sour-service envelopes; the acceptable partial pressure of hydrogen sulfide depends on temperature, chloride level, pH and stress, so each application is qualified against the standard rather than judged from a single table. Two fabrication rules matter in practice: keep service conditions inside the envelope for which the grade was selected, and never allow a heat-tint oxide from welding to remain on the surface, because it is locally depleted in chromium and becomes the initiation site for pitting.

Dimensions, Testing and Delivery

Seamless pipe is commonly supplied from NPS 1/8 through NPS 24, with wall thicknesses following ASME B36.10M. Outside diameters from roughly 6 mm to 325 mm and wall thicknesses from 0.5 mm to 25 mm cover most light-wall and standard-wall enquiries, with individual lengths up to about 12 m. Welded pipe to the same specification serves larger diameters and heavier wall combinations. Testing includes the hydrostatic or electric test noted above, flattening or reverse-bend tests on welded product, and tensile tests at the frequency set by the specification; when internal soundness must be demonstrated, ultrasonic examination to ASTM E213 or eddy current examination to ASTM E309 is added to the order.

Fabrication and Welding Practice

Super duplex is weldable when heat input is controlled and interpass temperature is held low, typically below about 150 °C, so that the ferrite-austenite balance in the weld metal and heat-affected zone is retained. Filler metal classified under AWS A5.9 with matching or higher alloy content is used for the root and fill passes, and nitrogen-bearing shielding gas helps restore austenite in the root. Pickling and passivation after fabrication remove oxide and restore the passive film. Bending to moderate radii is practical; heavier cold deformation is followed by solution annealing.

Frequently Asked Questions

Q: What is the minimum yield strength of ASTM A790 UNS S32750 pipe?
A: 550 MPa (80 ksi) at 0.2 % offset, with a minimum tensile strength of 800 MPa (116 ksi) and 15 % minimum elongation in 2 in.

Q: How does S32750 differ from 2205 duplex?
A: S32750 contains about 25 % chromium, 4 % molybdenum and 0.27 % nitrogen against roughly 22 % Cr, 3 % Mo and 0.17 % N in 2205. PREN rises from about 35 to about 43, yield strength increases, and the higher alloy content improves performance in chlorides and mildly sour service at additional cost.

Q: Why does the specification cap hardness at 32 HRC?
A: The limit protects the annealed duplex structure. A reading above the cap indicates incomplete annealing or excessive cold work, and both conditions reduce resistance to chloride stress corrosion cracking even when tensile results still look acceptable.

Q: Can UNS S32750 pipe be welded in the field?
A: Yes, with qualified procedures. Controlled heat input, low interpass temperature, matching duplex filler metal and post-weld pickling are the controlling variables; the weld and heat-affected zone must be tested for ferrite content and corrosion resistance where the specification requires it.

Q: What is EN 1.4410?
A: It is the European designation for the same super duplex grade, written as X2CrNiMoN25-7-4 in EN 10088. Chemistry and mechanical requirements align closely with UNS S32750, so the two designations are treated as equivalent in most project specifications.

Q: Which test certificate is issued with the pipe?
A: Inspection certificate EN 10204 type 3.1 is standard, reporting heat analysis, product analysis, tensile results, hardness and the hydrostatic or electric test outcome for each lot.