ASTM A789 vs A790 Duplex Stainless Steel Tube and Pipe: Standards, Testing and Selection

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

Introduction

When a buyer orders "duplex stainless steel," the single most common source of confusion is the difference between ASTM A789 and ASTM A790. Both cover ferritic/austenitic — duplex — stainless steel, both allow seamless and welded product, and both are frequently requested for the same grades, such as 2205. Yet they govern two different product forms, and mixing them up produces the wrong product, the wrong dimensional system, and the wrong acceptance requirements.

The core distinction is simple but consequential: ASTM A789 covers duplex stainless steel tube; ASTM A790 covers duplex stainless steel pipe. The rest of this article explains what that difference means in practice — for dimensions, for testing, for applications, and for the purchase specification itself.

A key point to establish early: the ASTM standard and the alloy grade are two different things. A789 and A790 are product specifications; 2205, 2507, and other duplex grades are the alloys. The same duplex grade can be supplied under either standard — the standard is chosen by the product form, not by the alloy.

1. What ASTM A789 Covers

ASTM A789/A789M covers seamless and welded ferritic/austenitic stainless steel tubing for general service. The word to focus on is tubing — A789 is the duplex stainless steel tube specification.

In practice, tube is ordered by its actual outside diameter and wall thickness, and it is commonly used where a precisely controlled outside dimension or a specific wall thickness matters — for example, in heat exchangers, instrumentation, and mechanical applications. Because A789 covers both seamless and welded tube, the buyer must state which manufacturing method is required.

The scope of A789 is "general service" — it is the tube specification for duplex stainless steel where the application does not carry the additional requirements of a heat-exchanger or boiler tube specification.

2. What ASTM A790 Covers

ASTM A790/A790M covers seamless and welded ferritic/austenitic stainless steel pipe. The word to focus on is pipe — A790 is the duplex stainless steel pipe specification.

Pipe is ordered through the nominal pipe size (NPS) and schedule system, where a nominal size and schedule designation correspond to standardized outside diameters and wall thicknesses. Pipe is the product form used for fluid transport and process piping, where the pipe must integrate with standard fittings, flanges, and valves.

Like A789, A790 covers both seamless and welded product, so the manufacturing method must be stated on the order.

3. A789 vs A790 Quick Comparison

4. Tube vs Pipe: Why the Product Form Matters

The difference between tube and pipe is not just vocabulary — it changes how the product is specified, measured, and used.

• Pipe is associated with the NPS and schedule system. Its outside diameter is standardized by NPS, and its wall thickness is designated by schedule. Pipe is designed to connect to standard fittings and is the natural choice for process and transport piping.
• Tube is described by its actual outside diameter and wall thickness. Tube is commonly used where a precise outside dimension or a specific wall thickness is needed, such as heat-exchanger tubing or instrumentation lines.

The practical consequence for procurement: a "2-inch" pipe and a "2-inch" tube are not the same thing. Pipe is ordered by NPS (whose actual OD differs from the nominal number for many sizes), while tube is ordered by a stated outside diameter. Confusing the two systems — or comparing only the OD while ignoring the wall thickness and dimensional system — is a common and costly error.

The specific dimensions, tolerances, and standard tables must be confirmed against the current applicable ASTM standard and the related general-requirements standard; they should not be estimated from memory.

5. Manufacturing: Seamless vs Welded Duplex

Both A789 and A790 cover seamless and welded product, so the manufacturing method is a buyer decision, not something the standard resolves for you.

• Seamless duplex tube or pipe is produced without a longitudinal weld seam, typically by piercing and working a solid billet. It is commonly specified where the design or code requires the absence of a weld seam.
• Welded duplex product is formed from strip or plate and joined by a longitudinal weld. It is generally more economical, and the weld seam is subject to the standard's requirements.

Duplex grades are more demanding to weld than standard austenitic grades because the weld and its heat-affected zone must retain the correct ferrite–austenite balance. For welded duplex product, the weld quality and phase balance are therefore key acceptance concerns, and the applicable requirements must be confirmed against the standard.

Because both standards cover both forms, the buyer must always state seamless or welded on the order; the two are not interchangeable by default and differ in cost, availability, and testing.

6. Duplex Grades Commonly Ordered

The duplex (ferritic/austenitic) stainless grades commonly ordered under these standards include 2205, which is the workhorse duplex grade — balanced corrosion resistance, strength, and cost. Super duplex grades such as 2507 are also frequently specified where higher pitting resistance is required. Lean duplex grades, with lower alloy content, are used where cost and a reduced alloying level are acceptable.

The choice between these grades is driven primarily by corrosion environment and strength requirement. 2205 provides a strong, general-purpose duplex option; 2507 provides higher pitting and chloride resistance for more aggressive service. The exact grade suitability is an engineering decision that the standard does not make for you.

The critical point for buyers: the grade and the standard are independent choices. 2205 can be supplied as tube under A789 or as pipe under A790, depending on the product form and application. Do not assume that 2205 "belongs to" one standard — it is available under whichever standard matches the product form you are buying.

Always confirm the grade and its UNS designation against the current standard edition. Coverage of a specific grade should be verified rather than assumed, particularly for less-common duplex and super duplex grades.

7. Dimensions, OD/Wall Thickness and Ordering Considerations

The dimensional system is where A789 and A790 differ most concretely in daily purchasing:

• A789 tube — specify the outside diameter and wall thickness directly, along with length and tolerance requirements.
• A790 pipe — specify the NPS and schedule (or wall thickness), recognizing that the actual outside diameter and wall thickness follow the standard pipe-size tables.

In both cases, the buyer should confirm the length (random or specified cut length) and the applicable tolerances against the current standard edition. Do not assume a single universal tolerance, and do not carry a dimension from the tube system into the pipe system or vice versa.

Beyond dimensions, the buyer should also confirm surface finish and end finish where they matter for the application. Tube and pipe can be supplied with different surface finishes, and pipe ends may need to be plain or beveled for welding. These are specification items that affect fabrication and cost, and they should be stated explicitly rather than assumed.

8. Mechanical Properties and Heat Treatment

Duplex stainless steel achieves its combination of high strength and corrosion resistance through a balanced ferrite–austenite microstructure, which is produced and controlled by solution annealing — heating to dissolve phases and then controlled cooling to establish the correct phase balance.

The standards specify the required heat treatment and the mechanical property requirements — including strength and hardness — for each grade. These values are grade- and condition-specific and must be confirmed against the current standard edition. Duplex grades generally provide higher strength than standard austenitic grades, which is part of their appeal for weight- and wall-thickness-sensitive designs, but the exact values should be read from the standard, not estimated.

Because duplex microstructures are sensitive to thermal history, the heat treatment and its verification are a central acceptance concern — a duplex product with an incorrect phase balance can lose both strength and corrosion resistance even if its chemistry is correct.

9. Testing and Inspection Requirements

Testing under A789 and A790 must be understood as three distinct categories, and confusing them is a common error:

• Standard requirements — the tests the standard requires for the product, such as chemical analysis, mechanical testing, and a pressure or nondestructive electric test, per the standard's requirements.
• Conditional requirements — tests that apply depending on the grade, product form, or manufacturing method.
• Supplementary requirements — tests added by the buyer, such as PMI, third-party inspection, corrosion testing, or impact testing, when the project or code requires them.

The core standard-required tests for these duplex products generally include chemical analysis to confirm the grade, mechanical testing (tensile and, where applicable, hardness), and a hydrostatic test or nondestructive electric test to confirm integrity. For tube, mechanical tests such as flattening or flaring may apply to demonstrate ductility and, for welded product, the integrity of the weld seam. The exact list depends on the grade, product form, and standard edition, and must be read from the standard.

Because duplex stainless steel depends on a correct ferrite–austenite phase balance, verification of that balance — where required by the specification or project — is a relevant supplementary check. This is not a default test on every order, but it is a meaningful one for welded duplex product or for service where phase balance affects performance.

PMI, third-party inspection, corrosion testing, and impact testing are not default requirements of every A789 or A790 order — they are added deliberately when the application or specification justifies them. The exact testing requirements depend on the standard edition, the grade, the product form, and any supplementary requirements the buyer writes into the order.

10. When to Choose A789

Choose ASTM A789 when you are buying duplex stainless steel tube — product ordered by actual outside diameter and wall thickness, typically for heat exchangers, instrumentation, or mechanical applications where a precise tube dimension is required.

A typical example (hypothetical, for illustration only): a buyer sourcing duplex 2205 tube for a heat-exchanger bundle would specify the tube by OD and wall thickness under A789.

11. When to Choose A790

Choose ASTM A790 when you are buying duplex stainless steel pipe — product ordered by NPS and schedule, for process piping and fluid transport that must connect to standard fittings, flanges, and valves.

A typical example (hypothetical, for illustration only): a project specifying duplex 2205 process piping would order pipe by NPS and schedule under A790.

12. Common Purchasing Mistakes

• Treating A789 or A790 as a material grade. They are product standards; 2205, 2507, and others are the alloys.
• Writing only "Duplex 2205" without an ASTM standard. The standard determines the product form and acceptance requirements.
• Confusing tube and pipe. Ordering "2-inch" without stating whether it is tube (OD-based) or pipe (NPS-based) produces the wrong product.
• Comparing only OD without confirming wall thickness and the dimensional system. Tube and pipe dimensions are not interchangeable.
• Confusing standard requirements with supplementary testing. PMI and corrosion testing are not default requirements unless specified.
• Not confirming the UNS and grade. The grade and its UNS designation must be verified.
• Not confirming heat treatment and MTC. Duplex products depend on correct heat treatment; the MTC documents it.
• Referencing an outdated ASTM edition. Confirm the current edition or the contract-specified edition.

13. How to Write an A789/A790 Purchase Specification

The following are example purchase specifications (not ASTM standard text):

A complete specification states: grade + UNS, ASTM standard and edition, seamless or welded, dimensions (OD × wall for tube, or NPS and schedule for pipe), heat treatment, required testing, MTC type, and any supplementary inspection.

Frequently Asked Questions

Q1: What is the difference between ASTM A789 and A790?
ASTM A789 covers seamless and welded duplex (ferritic/austenitic) stainless steel tube; ASTM A790 covers seamless and welded duplex stainless steel pipe. The difference is the product form and its dimensional system.

Q2: Is 2205 available under ASTM A789 and A790?
Yes. 2205 is commonly available under both standards. It is supplied under A789 as tube and under A790 as pipe, depending on the product form you need.

Q3: Is A789 for tube or pipe?
ASTM A789 is for tube — duplex stainless steel tubing ordered by outside diameter and wall thickness.

Q4: Is A790 for tube or pipe?
ASTM A790 is for pipe — duplex stainless steel pipe ordered by NPS and schedule.

Q5: What is the difference between duplex tube and pipe?
Tube is ordered by actual OD and wall thickness and is used for precise-dimension applications such as heat exchangers. Pipe is ordered by NPS and schedule and is used for process and transport piping. They use different dimensional systems.

Q6: What information should be included in an A789/A790 purchase order?
Include the grade and UNS, the ASTM standard and edition, seamless or welded, dimensions (OD × wall for tube, NPS and schedule for pipe), heat treatment, required testing, MTC type, and any supplementary inspection.

Q7: What tests are required for A789/A790?
The standard requires chemical analysis, mechanical testing, and a pressure or nondestructive electric test, per its requirements. Supplementary tests such as PMI, corrosion testing, or impact testing are added when the project or specification requires them.

Q8: Can the same duplex grade be supplied under both standards?
Yes. A duplex grade such as 2205 can be supplied under A789 as tube or under A790 as pipe. The standard is chosen by the product form, not the alloy.

Q9: Which standard should I specify for a heat exchanger tube?
For duplex heat-exchanger tubing, specify ASTM A789, ordered by OD and wall thickness.

Q10: Which standard should I specify for process piping?
For duplex process piping, specify ASTM A790, ordered by NPS and schedule.