ASTM A789 Duplex Stainless Steel Tubing: Grades, Composition and Heat Treatment

What ASTM A789 Duplex Steel Tubing Covers

ASTM A789/A789M is the standard specification for seamless and welded ferritic/austenitic (duplex) stainless steel tubing for general corrosive service, with particular emphasis on resistance to chloride-induced stress corrosion cracking. Tubing is supplied in nominal wall thicknesses and is produced either by the seamless process or by the welded process with no filler metal added. ASTM A789/A789M is the tubing companion to the pipe specification ASTM A790/A790M, and duplex tube is commonly supplied together with fittings to ASTM A815 so that a complete corrosion-resistant system is delivered under one metallurgical philosophy.

The duplex structure is a balanced mixture of austenite and ferrite, usually close to an even split between the two phases. That balance produces the combination of properties that makes duplex tube attractive in aggressive duty: high yield strength, good toughness, and far better resistance to chloride stress corrosion cracking and pitting than conventional austenitic grades. Tube is normally delivered solution annealed and pickled, or bright annealed where internal cleanliness, low roughness and a controlled surface finish are critical for the process fluid.

Grade Coverage and Chemical Composition

The specification covers several duplex and super duplex grades. The most widely ordered grade is UNS S31803, frequently supplied as the closely related UNS S32205, whose chromium, molybdenum and nitrogen ranges are tighter and therefore more consistent from heat to heat. The table below gives the chemical composition of both grades in percent by weight.

Element, %	UNS S31803	UNS S32205
Carbon, max	0.030	0.030
Manganese, max	2.00	2.00
Phosphorus, max	0.030	0.030
Sulfur, max	0.020	0.020
Silicon, max	1.00	1.00
Nickel	4.5-6.5	4.5-6.5
Chromium	21.0-23.0	22.0-23.0
Molybdenum	2.5-3.5	3.0-3.5
Nitrogen	0.08-0.20	0.14-0.20

Nitrogen is the key addition for strength and for restoring corrosion resistance in the heat affected zone of a weld, so tube that will be welded in the field is usually ordered with nitrogen at the upper part of the range. Chromium and molybdenum jointly control pitting and crevice corrosion resistance; a pitting resistance equivalent number is a useful screening index when shortlisting candidate grades for a chloride-bearing duty.

Mechanical Properties of ASTM A789 Tubing

Mechanical requirements are specified for the finished tube in the annealed condition, and are verified on each lot by tensile testing in accordance with the specification.

Property	UNS S31803 / UNS S32205
Tensile strength, min	620 MPa
Yield strength, 0.2% offset, min	450 MPa
Elongation in 50 mm, min	25%
Hardness typically reported by mills	about 293 HB, equivalent to 31 HRC

Because the specified minimum yield strength is roughly double that of 304 or 316, a thinner wall often satisfies the same design pressure. That reduces weight and cost in offshore, skid-mounted and mobile equipment, and it also lowers the amount of welding required. The trade-off is a lower practical maximum service temperature than the austenitic grades, so temperature limits must be checked for every application.

Heat Treatment, Welding and Fabrication

Solution annealing: heat the tube uniformly to a minimum of 1040 degrees C, then cool rapidly enough to suppress sigma phase and chromium nitride precipitation.

Quenching: water quenching is preferred for heavier walls and for tube that will be cold formed afterwards; air cooling is only acceptable where the section is thin enough to cool quickly.

Pickling and passivation: oxide scale from annealing or welding is removed chemically so that the passive film can re-form uniformly on both surfaces.

Welding: duplex tube is welded without preheat and with controlled heat input, limited interpass temperature, and a filler selected to match or exceed the corrosion resistance of the base metal.

Service limits: prolonged exposure above about 300 degrees C is avoided because of 475 degrees C embrittlement, while temperatures in the 600 to 900 degrees C range promote sigma phase formation and loss of toughness.

Applications and Selection Notes

Oil and gas gathering lines, control and umbilical tubing, and wellhead equipment where chlorides and carbon dioxide are present.

Heat exchangers, condensers and cooling water systems in chemical and petrochemical plants.

Desalination and seawater handling, including marine engineering pipework and ballast systems.

Pulp and paper liquor circuits, food processing and pharmaceutical process lines where corrosion resistance and cleanability both matter.

When choosing between seamless and welded tube, the deciding factors are usually the pressure class, the availability of the required size, and the non-destructive examination regime required by the end user. Seamless tube is preferred for high-pressure and high-integrity duty, while welded tube offers shorter lead times and larger diameters.

Frequently Asked Questions

Q: What is the difference between ASTM A789 and ASTM A790?
ASTM A789/A789M covers tubing in nominal wall thicknesses, while ASTM A790/A790M covers pipe in standard pipe sizes. Both describe duplex stainless steel for corrosive and high-temperature service.

Q: Are UNS S31803 and UNS S32205 the same material?
They are closely related duplex grades. S32205 has tighter chromium, molybdenum and nitrogen ranges, which gives more consistent corrosion resistance, and it is the grade most often ordered for new projects.

Q: What heat treatment is required for ASTM A789 tubing?
The tube is solution annealed at a minimum of 1040 degrees C and then cooled rapidly enough to preserve the correct austenite and ferrite balance and to avoid harmful secondary phases.

Q: Can A789 duplex tube be welded in the field?
Yes. Duplex tube is welded with controlled heat input, a limited interpass temperature and no preheat, using a filler that matches the corrosion resistance of the base metal.

Q: Why is duplex preferred over 316L for seawater duty?
Duplex grades resist chloride stress corrosion cracking and pitting far better than 316L, so they remain sound in chlorinated seawater and hot brine where 316L can fail by cracking.

Q: What sizes are normally available?
Seamless tube is typically produced from about 6.0 mm to 630 mm outside diameter with wall thickness from 1 mm upward, and the welded route extends to larger diameters and thinner walls for large exchangers.