ASTM A789 / A789M — Seamless and Welded Duplex Stainless Steel Tubing
ASTM A789/A789M (latest edition: A789M-23) specifies seamless and welded ferritic-austenitic (duplex) stainless steel tubing for general corrosion-resistant service. ASME SA-789 is adopted in BPVC Section II Part A. The companion pipe specification is ASTM A790. Duplex grades offer a unique combination of high strength (YS ≥ 450 MPa) and excellent resistance to chloride stress corrosion cracking (SCC), making them ideal for heat exchangers, seawater coolers, and desalination plant tubing.
1. Scope
A789 covers tubing in outside diameters from about 3.2 mm (⅛ in) upward, with walls from approximately 0.4 mm. Unlike A790 pipe (schedule-based, larger diameter), A789 tubing uses OD × wall-thickness designation and is typically used in heat exchangers, instrument lines, and condensers. The tubing may be seamless (drawn or extruded) or welded (EFW or ERW).
2. Grades Covered
| UNS No. |
Common Name |
Type |
PREN |
Typical Use |
| S32101 |
LDX 2101 (lean duplex) |
Lean Duplex |
~26 |
Cost-optimised; mild corrosion, structural |
| S32304 |
2304 (lean duplex) |
Lean Duplex |
~26 |
Moderate chloride, heat exchangers |
| S31803 |
2205 (original) |
Standard Duplex |
~32 |
General purpose duplex; seawater, brine |
| S32205 |
2205 (revised) |
Standard Duplex |
~35 |
Most common duplex; seawater, chemical, offshore |
| S32750 |
2507 (super duplex) |
Super Duplex |
~43 |
Highly aggressive chloride, deep sea, FGD |
| S32760 |
Zeron 100 |
Super Duplex |
~41 |
Offshore heat exchangers, seawater injection |
3. Chemical Composition
| UNS / Grade |
C max |
Cr |
Ni |
Mo |
N |
Others |
| S32304 |
0.030 |
21.5–24.5 |
3.0–5.5 |
0.05–0.60 |
0.05–0.20 |
— |
| S31803 |
0.030 |
21.0–23.0 |
4.5–6.5 |
2.5–3.5 |
0.08–0.20 |
— |
| S32205 |
0.030 |
22.0–23.0 |
4.5–6.5 |
3.0–3.5 |
0.14–0.20 |
— |
| S32750 |
0.030 |
24.0–26.0 |
6.0–8.0 |
3.0–5.0 |
0.24–0.32 |
Cu ≤0.50 |
| S32760 |
0.030 |
24.0–26.0 |
6.0–8.0 |
3.0–4.0 |
0.20–0.30 |
W 0.50–1.00, Cu 0.50–1.00 |
4. Mechanical Properties
| Grade |
UTS min (MPa) |
YS min (MPa) |
Elong. min (%) |
Hardness max (HRB) |
| S32304 |
600 |
400 |
25 |
32 HRC |
| S31803 / S32205 |
620 |
450 |
25 |
32 HRC |
| S32750 |
795 |
550 |
15 |
32 HRC |
| S32760 |
750 |
550 |
25 |
32 HRC |
5. Heat Treatment
All A789 duplex tubing must be furnished in the solution-annealed condition, water-quenched from 1020–1100 °C (2205) or 1050–1150 °C (2507/S32760) to achieve 40–60% ferrite balance. Refer to ASTM A923 for verification of freedom from detrimental intermetallic phases.
6. Testing Requirements
| Test |
Requirement |
| Tensile |
One per heat or heat-treatment lot |
| Hydrostatic |
Each tube unless eddy current (ET) is agreed as substitute |
| Flare (Welded tubes) |
Flare test or reverse flattening per A789 requirements |
| Eddy Current (ET) |
100% ET as alternative to hydrostatic when specified |
| Intermetallic Phase |
ASTM A923 Practice A — optional, specified in PO |
7. A789 vs A790 — Key Differences
| Aspect |
ASTM A789 (Tubing) |
ASTM A790 (Pipe) |
| Designation |
OD × wall thickness (e.g. 25.4 × 1.65 mm) |
NPS + Schedule (e.g. NPS 2, Sch 40S) |
| Size range |
Small OD, heat exchanger / instrument tubing |
NPS ½–12+ for pressure piping |
| Dimensional std |
OD tolerance per A789 |
ASME B36.19 (stainless schedules) |
| Primary use |
Heat exchangers, condensers, instrument lines |
Process piping, pressure vessels, headers |
| ASME equivalent |
SA-789 |
SA-790 |
8. Related Standards
| Standard |
Description |
| ASTM A790 / SA-790 |
Duplex SS seamless and welded pipe |
| ASTM A815 / SA-815 |
Wrought duplex SS butt-welding fittings |
| ASTM A182 F51/F53 |
Forged duplex SS flanges |
| ASTM A923 |
Detection of intermetallic phases in duplex SS |
| ASTM A213 / SA-213 |
Austenitic stainless steel boiler tubing (companion) |
