ASTM A815 S32750 Duplex Stainless Steel Fittings: Elbows Explained

ASTM A815 and Grade S32750

ASTM A815 is the standard specification for wrought stainless steel pipe fittings - elbows, tees, reducers, caps and stub ends - intended for pressure piping service. Each fitting is marked with a class designator that identifies its chemical composition; S32750 fittings carry the class WPS32750. The composition of WPS32750 matches UNS S32750, the 25% chromium super duplex grade also specified as bar and forgings in ASTM A182 and as pipe in ASTM A790.

Super duplex S32750 is a two-phase material containing approximately equal proportions of austenite and ferrite. The ferrite phase provides high strength and exceptional resistance to chloride stress corrosion cracking, while the austenite phase contributes toughness and general corrosion resistance. This microstructure is the reason S32750 is specified where standard austenitic grades fail by pitting, crevice corrosion or stress corrosion cracking in seawater and chloride-bearing process streams.

Chemical Composition of WPS32750

The nitrogen content of 0.24–0.32% is critical: it stabilizes austenite, raises yield strength and contributes strongly to pitting resistance, giving the alloy a typical PREN of approximately 40 or higher, calculated as %Cr + 3.3 × %Mo + 16 × %N.

Heat Treatment and Microstructure

Fittings must be solution annealed at 1025–1125 °C and then quenched in water. The rapid cooling preserves the balanced ferrite-austenite structure and prevents the precipitation of brittle intermetallic phases such as sigma and chi, which form if the material is held in the 300–1000 °C range during cooling or subsequent processing. Correct heat treatment is verified by the mechanical test results and by metallographic examination where specified in the purchase order. Fittings supplied in the solution-annealed and quenched condition exhibit a hardness typically within 28–32 HRC.

Mechanical Properties

Dimensional Standards for Elbows

Elbows in S32750 are manufactured to the same dimensional standards as fittings in other materials. ASME B16.9 covers factory-made butt-welding fittings, including 45°, 90° and 180° elbows in long-radius (1.5 D) and short-radius (1.0 D) forms, with wall thickness matching the connecting pipe schedule such as 5S, 10S, 20S, 40S or 80S. ASME B16.28 covers short-radius elbows and returns, and MSS SP-43 covers lightweight corrosion-resistant fittings made from sheet or plate. Seamless and welded construction are both permitted, with welded fittings required to pass the hydrostatic or air test specified in A815.

Welding S32750 Elbows into the Piping System

S32750 is weldable but demands disciplined procedure control. Matching over-alloyed filler such as ER2594 (AWS A5.9) is recommended to keep the weld metal with a slight austenite excess. Heat input should be moderate and interpass temperature must be held below 150 °C to avoid excessive ferrite and intermetallic precipitation in the weld and heat-affected zone. Shielding gas should be argon-based with a small nitrogen addition to compensate for nitrogen loss during welding. Post-weld solution annealing is generally not required for a properly controlled weld, but any repair welding must follow the same strict controls.

Typical Applications

Offshore platforms and subsea manifolds handling seawater and produced fluids

Chemical and petrochemical process piping in chloride-bearing streams

Desalination plants and seawater cooling systems

Flue gas desulphurization (FGD) systems in power plants

Pulp and paper digesters and bleaching equipment

Frequently Asked Questions

What does WPS32750 mean on an A815 fitting?

WPS32750 is the ASTM A815 class designation. The letters indicate the material class for the fitting, and the number refers to UNS S32750, the 25% Cr super duplex grade. A fitting stamped WPS32750 is made from material meeting the S32750 composition and tested to the A815 requirements.

What is the difference between S32750 and S32760?

Both are super duplex grades. S32750 has 24.0–26.0% Cr, 6.0–8.0% Ni and 3.0–5.0% Mo with nitrogen 0.24–0.32%; S32760 adds tungsten and copper, with 24.0–26.0% Cr, 6.0–8.0% Ni and 3.0–4.0% Mo, and offers similar corrosion performance in most services. S32760 is more common in European-specified projects, while S32750 is the most widely stocked super duplex grade globally.

Why must the fitting be water quenched after solution annealing?

Water quenching after the 1025–1125 °C solution anneal is necessary to pass rapidly through the 300–1000 °C range where sigma, chi and other brittle intermetallic phases precipitate. Slow cooling destroys the toughness and corrosion resistance of the fitting.

Can A815 S32750 elbows be welded to S32750 pipe?

Yes. Butt-weld the elbow to pipe of the same grade using an over-alloyed filler such as ER2594, with controlled heat input and interpass temperature below 150 °C. The joint does not normally require post-weld heat treatment when the procedure is qualified and executed correctly.

What are the common angles and radii of S32750 elbows?

Standard elbows are 45°, 90° and 180°, in long-radius (1.5 times the nominal pipe size) and short-radius (1.0 D) forms per ASME B16.9 and B16.28. Custom angles can be cut from standard elbows where the project permits.

Is S32750 magnetic?

Partially. Because super duplex stainless steel contains roughly 50% ferrite, S32750 is magnetic, unlike fully austenitic grades. This is a quick field check to distinguish it from 316/316L, though not a substitute for PMI testing.