ASTM A789 Super Duplex Steel Pipe: UNS S32205, S31803, S32750 and S32760 for Heat Exchangers

What Is ASTM A789?

ASTM A789 is the specification for seamless and welded ferritic/austenitic (duplex) stainless steel tubing for general corrosive service. It covers both the standard duplex grades and the super duplex grades, and it is the specification most often called up for heater bundles, air cooler tubes, condenser tubes, tank coils and similar heat-transfer components in chemical, petrochemical, desalination and offshore service. Unlike the austenitic 300-series grades, duplex steels combine a ferrite-austenite two-phase microstructure, which gives them roughly twice the yield strength of 316L together with much better resistance to chloride stress corrosion cracking.

Covered Grades and Chemical Composition

Four grades are commonly specified: the duplex grades UNS S31803 and UNS S32205, and the super duplex grades UNS S32750 and UNS S32760. Composition limits (weight percent) are given below.

Element	S31803	S32205	S32750	S32760
Carbon (C)	≤ 0.030	≤ 0.030	≤ 0.030	≤ 0.030
Manganese (Mn)	≤ 2.00	≤ 2.00	≤ 1.20	≤ 1.00
Phosphorus (P)	≤ 0.030	≤ 0.030	≤ 0.035	≤ 0.030
Sulfur (S)	≤ 0.020	≤ 0.020	≤ 0.020	≤ 0.010
Silicon (Si)	≤ 1.00	≤ 1.00	≤ 0.80	≤ 1.00
Chromium (Cr)	21.0 - 23.0	22.0 - 23.0	24.0 - 26.0	24.0 - 26.0
Nickel (Ni)	4.5 - 6.5	4.5 - 6.5	6.0 - 8.0	6.0 - 8.0
Molybdenum (Mo)	2.5 - 3.5	3.0 - 3.5	3.0 - 5.0	3.0 - 4.0
Nitrogen (N)	0.08 - 0.20	0.14 - 0.20	0.24 - 0.32	0.20 - 0.30
Copper (Cu)	-	-	≤ 0.50	0.50 - 1.00
Tungsten (W)	-	-	-	0.50 - 1.00

Mechanical Properties per ASTM A789

Property	S31803 / S32205	S32750	S32760
Tensile strength, min (MPa)	620	800	750
Yield strength 0.2%, min (MPa)	450	550	550
Elongation, min (%)	25	15	25

These minimum values are roughly double the yield strength of annealed 316L (170 MPa minimum), which allows thinner tube walls for the same design pressure - an important economy in bundle and coil construction.

Corrosion Resistance and PREN

The pitting resistance of duplex grades is usually expressed by the pitting resistance equivalent number, PREN = Cr + 3.3 × Mo + 16 × N. Computed from nominal compositions, S31803 and S32205 fall in the range of about 34-36, while S32750 and S32760 reach values of about 40 or higher. By comparison, 316L computes to roughly 26. This is why the duplex and super duplex grades are specified for seawater-cooled heat exchangers, chloride-laden process streams and brine, where austenitic grades pit or suffer chloride stress corrosion cracking. The super duplex grades add copper and tungsten (S32760) to further improve resistance in acidic chloride media.

Fabrication and Service Considerations

Duplex tubing is normally supplied in the solution-annealed condition. Welding requires controlled heat input and, in most cases, an over-alloyed filler of the S32205 or S32750 type to maintain the austenite-ferrite balance in the weld metal; preheating is generally not required. Because duplex steels are prone to embrittlement phenomena in the 280-500 °C range, long-term service temperatures above about 300 °C are avoided and the tubes are designed within the temperature limits of the governing pressure code. After hot forming, solution annealing followed by rapid cooling is standard practice.

FAQ

What is the difference between S31803 and S32205?

S32205 has a tighter chromium range (22.0-23.0%), a higher minimum molybdenum (3.0-3.5%) and a higher nitrogen minimum (0.14-0.20%) than S31803, which improves pitting resistance and is the reason S32205 is now the most widely specified duplex grade.

What does PREN mean and why does it matter?

PREN (pitting resistance equivalent number) is a calculated index of pitting resistance: PREN = Cr + 3.3 × Mo + 16 × N. A higher value indicates better resistance to chloride pitting; super duplex grades reach about 40, versus roughly 26 for 316L.

Is A789 pipe seamless or welded?

ASTM A789 covers both seamless and welded duplex tubing for general corrosive service. For pipe products, ASTM A790 covers the same grade family.

What are super duplex tubes used for?

Typical duties include seawater heat exchangers and condensers, air coolers, heater bundles, tank heating coils, chemical process lines and other chloride-rich service where high strength and pitting resistance are both required.

Can duplex tubing be welded without post-weld heat treatment?

Yes, in most cases. Correct welding procedures with controlled heat input and matching or over-alloyed filler are required; post-weld solution annealing is normally reserved for severe service or after hot forming.

What is the difference between A789 and A790?

A789 covers seamless and welded duplex tubing for general corrosive service, while A790 covers seamless and welded duplex pipe. The grade chemistry and mechanical requirements are similar.