ASTM A789 Duplex Stainless Steel Pipe Guide
What ASTM A789 Covers
ASTM A789/A789M is the standard specification for seamless and welded ferritic-austenitic stainless steel tubing for general service. It is the duplex companion to A268 (ferritic) and A269 (austenitic) tubing, and it is widely used for heat exchanger and condenser tubes where chloride-bearing fluids would attack standard austenitic grades. Products under A789 are furnished in the solution-annealed condition to develop the balanced austenite-ferrite microstructure that gives duplex steels their strength and corrosion resistance.
Grades, Composition, and Mechanical Properties
| Grade | Nominal composition | Yield strength, min (MPa) | Tensile strength, min (MPa) | Elongation, min (%) |
|---|---|---|---|---|
| S31803 | 22Cr-5.5Ni-3Mo-0.14N | 450 | 620 | 25 |
| S32205 | 22Cr-5Ni-3.2Mo-0.17N | 450 | 655 | 25 |
| S32304 | 23Cr-4Ni-0.1N (lean duplex) | 400 | 600 | 25 |
| S32750 | 25Cr-7Ni-4Mo-0.27N | 550 | 795 | 15 |
| S32760 | 25Cr-7Ni-3.5Mo-0.25N-W-Cu | 550 | 750 | 25 |
Mechanical tests are performed per ASTM E8/E8M for tension. The phase balance, typically 40-60% ferrite, is controlled by chemistry and solution annealing temperature; the PREN of the super duplex grades exceeds 40, which is why they resist pitting and crevice corrosion in seawater.
A789 vs A790 vs EN 10216-5
| Standard | Scope | Notes for buyers |
|---|---|---|
| ASTM A789 | Seamless and welded duplex tubing for general service | Heat exchanger and condenser tubes; requires solution annealing |
| ASTM A790 | Seamless and welded duplex pipe for pressure and corrosion service | Piping sizes; covers lean, standard, and super duplex grades |
| EN 10216-5 | Seamless stainless tube for pressure purposes | European PED compliance; stricter low-temperature impact testing |
Applications and Ordering Details
Heat exchanger and condenser tubes in refineries, chemical plants, and offshore topsides.
Seawater cooling systems, desalination plants, and fire-fighting water lines.
Process piping and tubing in chloride-bearing, CO2/H2S-containing streams.
Ordering must state: grade and UNS number, seamless or welded, OD and wall thickness, length, condition (solution annealed), and testing requirements.
Testing Requirements
Per A789, tubes are subject to flattening, flange, and reverse-bend tests as applicable, plus hydrostatic testing or an alternative nondestructive test such as eddy current or ultrasonic. The purchaser may add intergranular corrosion testing, PMI (positive material identification) on every piece, and impact testing at low temperature, particularly for offshore service where the code demands it.
Common Misconceptions
'A789 tubes can skip annealing after cold working.' No. A789 requires solution annealing to restore the phase balance and corrosion resistance; cold-worked material must be annealed before delivery.
'S31803 and S32205 are identical.' S32205 has tighter composition limits (22-23.5% Cr, 3.0-3.5% Mo, 0.14-0.20% N) and higher minimum tensile strength; S32205 is the modern replacement for S31803 in most specifications.
'A790 is only for super duplex.' A790 covers the full duplex family, including lean (S32304), standard (S31803/S32205), and super duplex (S32750/S32760).
Frequently Asked Questions
What is ASTM A789?
It is the ASTM specification for seamless and welded ferritic-austenitic (duplex) stainless steel tubing for general corrosive service, especially heat exchanger and condenser tubes.
What is the difference between A789 and A790?
A789 covers tubing (smaller diameters, heat exchanger service); A790 covers pipe (larger diameters, pressure piping). Both cover duplex grades and require solution annealing.
Which grades does A789 cover?
Lean duplex S32304, standard duplex S31803 and S32205, and super duplex S32750 and S32760, with mechanical and corrosion requirements per grade.
What is the yield strength of A789 S32750 tubing?
Minimum 550 MPa yield and 795 MPa tensile, about double the yield strength of 316L, tested per ASTM E8/E8M.
Is A789 tubing seamless or welded?
Both are permitted. The order must state which route is required; welded tubing is made from duplex strip with full-penetration longitudinal welding, then solution annealed.
Why is duplex tubing annealed before delivery?
Solution annealing restores the balanced austenite-ferrite structure and dissolves harmful precipitates that can form during forming or welding, which is essential for corrosion resistance.
