ASTM A790 applies to duplex stainless steel pipes, which are dimensioned by NPS and schedule, while ASTM A789 applies to duplex stainless steel tubes, which are dimensioned by outside diameter and wall thickness. Both standards cover the same duplex grade families such as S31803, S32205, S32750, and S32760, and they share identical chemical composition and mechanical property requirements. The main difference between ASTM A790 and ASTM A789 lies in the product form and dimensional designation, not in material performance.
ASTM A790 Duplex Stainless Steel Pipe Standard
ASTM A790 is a standard published by the American Society for Testing and Materials (ASTM) that covers the manufacturing and quality requirements for seamless and welded ferritic/austenitic (duplex) stainless steel pipes. This standard applies to pipe grades like UNS S31803 (2205), UNS S32750 (2507), and UNS S32760 that combine an austenite-ferrite dual-phase microstructure for high strength and superior corrosion resistance.
It defines key criteria, including chemical composition, mechanical properties, heat treatment conditions, dimensional tolerances, and testing requirements. The specification covers both seamless and straight-seam welded pipe intended for general corrosive service, with emphasis on resistance to stress corrosion cracking and pitting.
ASTM A790 duplex steel pipe is widely used in offshore oil and gas platforms, subsea flowlines, chemical processing plants, desalination systems, pulp and paper mills, and marine-related piping. This guide covers every section of ASTM A790 grades, chemical composition, mechanical properties, tolerances, heat treatment, testing requirements, and common questions, giving engineers and procurement teams the data they need to select the right duplex steel pipe for demanding service conditions.
ASTM A790 PDF Free Download
ASTM A790 PDF Free Download
What is ASTM A790?
ASTM A790 is the standard specification for seamless and welded ferritic/austenitic stainless steel pipe. The specification is published by ASTM International, a globally recognised organisation that develops technical standards for materials, products, systems, and services. It is maintained under the jurisdiction of ASTM Committee A01 on Steel, Stainless Steel, and Related Alloys, Subcommittee A01.10 on Stainless and Alloy Steel Tubular Products.
Duplex steels get their name from their two-phase microstructure: roughly 50% austenite and 50% ferrite. This balanced structure provides roughly twice the yield strength of 316L or more, compared to 316L, for accuracy, while also resisting chloride stress corrosion cracking far better.
A key distinction between ASTM A790 and ASTM A789 is that ASTM A790 covers pipes (dimensioned by NPS and schedule), while ASTM A789 covers tubes (dimensioned by OD and wall thickness). Both standards apply to the same duplex/super duplex grade families, but A790 pipes are designed for pressure-containing fluid conveyance, whereas A789 tubes target heat exchangers and general service.
Chemical Composition of ASTM A790 Pipe
The chemical composition of ASTM A790 duplex pipe grades is tightly controlled to maintain the correct austenite-ferrite balance. Chromium (Cr), molybdenum (Mo), and nitrogen (N) are the primary elements that drive corrosion resistance and PREN. Nickel (Ni) stabilises the austenite phase. Carbon is very low (0.03% max for most grades) to prevent carbide precipitation and sensitisation during welding.
| GRADE | UNS Designation |
ASTM A790 Chemical Requirement (Max) | ||||||||||
| C | Mn | P | S | Si | Ni | Cr | Mo | N | Cu | Other | ||
| S31500 | 0.03 | 1.2-2.0 | 0.03 | 0.03 | 1.4-2.0 | 4.2-5.2 | 18.0-19.0 | 2.5-3.0 | 0.05-0.1 | |||
| S31803 | 0.03 | 2.0 | 0.03 | 0.02 | 1.0 | 4.5-6.5 | 21.0-23.0 | 2.5-3.5 | 0.08-0.2 | |||
| 2205 | S32205 | 0.03 | 2.0 | 0.03 | 0.02 | 1.0 | 4.5-6.5 | 22.0-23.0 | 3.0-3.5 | 0.14-0.2 | ||
| S32520 | 0.03 | 1.5 | 0.035 | 0.02 | 0.8 | 5.5-8.0 | 24.0-26.0 | 3.0-5.0 | 0.2-0.35 | 0.5-3.0 | ||
| 255 | S32550 | 0.04 | 1.5 | 0.04 | 0.03 | 1.0 | 4.5-6.5 | 24.0-27.0 | 2.9-3.9 | 0.1-0.25 | 1.5-2.5 | |
| S32707 | 0.03 | 1.5 | 0.035 | 0.01 | 0.5 | 5.5-9.5 | 26.0-29.0 | 4.5-5.0 | 0.3-0.5 | 1.0 | Co:0.5-2.0 | |
| 2507 | S32750¹ | 0.03 | 1.2 | 0.035 | 0.02 | 0.8 | 6.0-8.0 | 24.0-26.0 | 3.0-5.0 | 0.24-0.32 | 0.5 | |
| S32760² | 0.03 | 1.0 | 0.03 | 0.01 | 1.0 | 6.0-8.0 | 24.0-26.0 | 3.0-4.0 | 0.2-0.3 | 0.5-1.0 | W:0.5-1.0 | |
Note: All values are maximum percentages unless a range is specified.
Mechanical Properties of ASTM A790 Pipe
The mechanical properties of ASTM A790 pipes define their strength, ductility, and resistance to harsh environments. These properties include tensile strength, yield strength, and elongation, which define the structural performance of the pipe under load. The table below presents the key mechanical requirements for different duplex and super duplex stainless steel grades in the annealed condition.
| Grade | Standard | Heat Treating Temp. : min | Tensile Strength Min. MPa |
Yield Strength Min. MPa |
Elongation min. % |
| S31500 | A790 | 980–1040°C | 630 | 440 | 30 |
| S31803 | A790 | 1020–1100°C | 620 | 450 | 25 |
| S32205 | A790 | 1020–1100°C | 655 | 450 | 25 |
| S32520 | A790 | 1080–1120°C | 770 | 550 | 25 |
| S32550 | A790 | 1040°C min | 760 | 550 | 15 |
| S32707 | A790 | 1080–1120°C | 920 | 700 | 25 |
| S32750 | A790 | 1025–1125°C | 800 | 550 | 15 |
| S32760 | A790 | 1070–1140°C | 750 | 550 | 25 |
All duplex grades under ASTM A790 must be supplied in the solution-annealed and quenched condition. Hardness limits also apply. Most grades are limited to 293 HB / 31 HRC maximum. These steels are susceptible to embrittlement if exposed to prolonged service above 572°F (300°C), so ASTM A790 is intended for general corrosive service, not high-temperature duty.
ASTM A790 Tolerances
ASTM A790 specifies dimensional tolerances for outside diameter, wall thickness, and length of duplex stainless steel pipes. These limits help ensure consistent quality and accurate sizing across applications. Seamless pipes generally allow slightly wider OD tolerances compared to welded pipes due to manufacturing processes. The table below shows the standard ASTM A790 tolerance ranges.
| STANDARD | OD(D) | TOLERANCE(MM) | THICKNESS(S) | TOLERANCE | LENGTH | TOLERANCE |
| MM | COMMON HIGH | MM | MM | MM | ||
| A790 | 10.3≤D≤48.3 | +0.4/-0.8 | 10.3≤D≤73.0 | +20%/-12.5% | For all sizes | +6.0/0 |
| 48.3<D≤114.3 | +0.8/-0.8 | 88.9≤D≤457.2, ID<5% | +22.5%/-12.5% | |||
| 114.3<D≤219.1 | +1.6/-0.8 | 88.9≤D≤457.2, ID>5% | +15.0%/-12.5% | |||
| 219.1<D≤457.2 | +2.4/-0.8 | D≥508.0,tD≤5%,SMLS | +22.5%/-12.5% | |||
| 457.2<D≤660.4 | +3.2/-0.8 | D≥508.0,tD>5%,SMLS | +16.0%/-12.5% | |||
| 660.4<D≤762.0 | +4.0/-0.8 | D≥508.0,WLD | +17.5%/-12.5% |
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ASTM A790 Duplex Stainless Steel Pipe Grades
ASTM A790 covers two families of duplex stainless steel: standard duplex (such as 2205) and super duplex (such as 2507 and 2760). Standard duplex grades offer Pitting Resistance Equivalent Number (PREN) values above 30, while super duplex grades exceed PREN 40. The higher the PREN, the better the resistance to pitting and crevice corrosion in chloride-rich environments.
UNS S31803 / Duplex 2205
UNS S31803 is one of the most commonly used duplex stainless steel pipe grades. With 21–23% chromium, 4.5–6.5% nickel, and 2.5–3.5% molybdenum, it provides roughly twice the yield strength of 316L, depending on the chloride concentration and the applied stress. It is preferred for offshore platforms, chemical tankers, and process piping.
More About UNS S31803 / Duplex 2205UNS S32205 / Duplex 2205
UNS S32205 is the tighter-composition version of the 2205 family with a controlled nitrogen of 0.14%. This nitrogen raises the PREN above 34 and improves both pitting resistance and weld-zone toughness compared to S31803. Most new projects specify S32205 instead of S31803 for critical service.
More About UNS S32205 / Duplex 2205UNS S32550 / Alloy 255
UNS S32550 contains 24-27% chromium, 4.50-6.50% nickel, 2.90-3.90% molybdenum, and 1.5-2.5% copper. The copper addition improves resistance to sulfuric acid environments. It performs well in flue gas desulfurisation (FGD) systems, phosphoric acid processing, and chemical storage piping.
More About UNS S32550 / Alloy 255UNS S32750 / Super Duplex 2507
UNS S32750 is the most specified super duplex grade with a PREN above 41. Its 25% chromium, 7% nickel, 4% molybdenum, and 0.24–0.32% nitrogen give it high resistance to pitting, crevice corrosion, and stress corrosion cracking in hot seawater. Applications include subsea manifolds, topside piping, and desalination systems.
More About UNS S32750 / Super Duplex 2507UNS S32760 / Super Duplex 2760
UNS S32760 adds tungsten and copper to the super duplex base for extra corrosion resistance in aggressive chloride and acidic media. The tungsten addition raises the modified PREN≥40. It is specified for hot seawater cooling systems, sour gas service, and high-pressure subsea flowlines.
More About UNS S32760 / Super Duplex 2760Heat Treatment Requirements for ASTM A790
Every ASTM A790 pipe must be delivered in the solution-annealed and quenched condition. The heat treatment process dissolves intermetallic phases (sigma, chi) and carbide precipitates that form during hot working, then locks in the correct austenite-ferrite balance through rapid cooling.
Solution annealing temperatures vary by grade. Standard duplex 2205 (S31803/S32205) requires a minimum of 1900°F (1040°C). Super duplex grades like S32750 (2507) are typically solution annealed in the range of 1880–2060°F (1025–1125°C), depending on the specific alloy. The steel must then be quenched in water or rapidly cooled to prevent the reformation of the sigma phase.
Without proper solution annealing, intermetallic phases like sigma and chi form at grain boundaries. These phases are hard and brittle. They destroy toughness and corrosion resistance. A correctly annealed duplex pipe should have 40–60% ferrite content with no detectable sigma phase.
Ferrite Content and Microstructure
The ferrite-austenite phase balance is the single most critical quality factor for duplex stainless steel pipe. ASTM A790 requires that the microstructure consist of approximately equal proportions of ferrite and austenite after solution annealing. Most project specifications require 35–65% ferrite, measured by magnetic methods (such as Fischer FERITSCOPE®) or metallographic point counting (ASTM E562).
Too much ferrite (above 70%) reduces toughness and corrosion resistance. Too little ferrite (below 30%) weakens resistance to stress corrosion cracking, which is the very property that sets duplex apart from austenitic steels. Welding heat input, interpass temperature, and cooling rate all affect the final ferrite content, so proper welding procedures are just as important as the base metal heat treatment.
Testing and Inspection Requirements
ASTM A790 requires several tests to verify pipe quality. Every lot must pass chemical analysis, mechanical testing, and either a hydrostatic test or a nondestructive electric test (NDE). Additional tests apply to welded pipes. Here is a summary of each required test.
Tension Test
One tension test per lot. The test measures tensile strength, yield strength (0.2% offset), and elongation in 2 in. (50 mm) gauge length. Longitudinal or transverse specimens may be used, depending on pipe size.
Flattening Test (Welded Pipe)
For welded pipes, a flattening test is required on specimens from each end of one pipe per lot. The pipe section is flattened between parallel plates with the weld at the point of maximum bending. No cracks or openings at the weld should appear until the pipe is flattened to two-thirds of its original OD
Hardness Test
Brinell or Rockwell hardness tests on 2 pipes per lot. Most duplex grades under ASTM A790 must not exceed 293 HB / 31 HRC / 310 HV. Hardness limits verify that no harmful intermetallic phases remain after heat treatment.
Hydrostatic or NDE Test
Every pipe must pass either a hydrostatic pressure test or a nondestructive electric test (eddy current or ultrasonic per ASTM E213 / E309 / E426). For the hydrostatic test, the test pressure is calculated using: P = 2St/D, where S = 64,000 psi (441 MPa) for duplex grades, t = wall thickness, and D = outside diameter.
Corrosion Test (Supplementary)
ASTM A790 references ASTM A923 (Practice B or C) as a supplementary requirement for detecting detrimental intermetallic phases in duplex stainless steel. Many end users, especially in oil and gas, require ASTM A923 testing in addition to the standard mechanical tests. This test uses either a Charpy V-notch impact method or a ferric chloride corrosion test to confirm the absence of sigma phase.
Frequently Asked Questions
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What is the Difference Between ASTM A790 and ASTM A789?
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Difference Between UNS S31803 and UNS S32205?Both are Duplex 2205, but S32205 has a tighter composition with a higher nitrogen minimum of 0.14%. S31803 allows nitrogen as low as 0.08%. The higher nitrogen in S32205 raises the PREN above 34, improves weld-zone properties, and reduces the risk of sigma phase formation. Most modern specifications now prefer S32205.
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What is PREN and Why Does It Matter?PREN (Pitting Resistance Equivalent Number) is a formula that predicts pitting corrosion resistance: PREN = %Cr + 3.3(%Mo) + 16(%N). A higher PREN means better resistance to pitting and crevice attack in chloride environments. Standard duplex 2205 has a PREN of about 34; super duplex 2507 reaches 41+. For seawater service, most specifications require a minimum PREN of 40.
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What Heat Treatment Is Required for ASTM A790 Pipes?All grades require solution annealing followed by rapid water quenching. Duplex 2205 (S32205) is annealed at 1900°F (1040°C) minimum. Super duplex 2507 (S32750) is typically annealed at 1025–1125°C (1880–2060°F). Rapid quenching prevents sigma phase formation and locks in the correct ferrite-austenite balance.
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What is the Difference Between ASTM A790 and ASTM A312?A790 specifies ferritic/austenitic (duplex) stainless steel pipes, and A312 specifies austenitic stainless steel pipes. Duplex A790 pipes offer roughly double the yield strength and superior chloride SCC resistance compared to A312 grades like TP316L. A312 is the right choice for general-purpose austenitic service; A790 is specified when higher strength and chloride resistance are required.
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Can ASTM A790 Pipes Be Used in ASME Boiler and Pressure Vessel Code Applications?Yes. ASTM A790 has a corresponding ASME specification: SA-790. SA-790 is listed in ASME Boiler and Pressure Vessel Code, Section II, Part A. Pipes produced to ASTM A790 that also meet ASME requirements can carry the SA-790 designation for code-stamped pressure equipment.