Mechanical Properties of Duplex and Super Duplex Stainless Steel Pipe

ASTM A790 Mechanical Properties: 2304 vs. 2205 vs. 2507

The following values are specified minimum tensile properties and maximum hardness limits from ASTM A790/A790M-24, Table 3.

¹ Elongation is specified over a 2 in. or 50 mm gauge length.

How to use this table:

• These are specification limits, rather than typical mill results or design allowable stresses.
• S31803 and S32205 have different minimum tensile strengths. Specify the required UNS number instead of relying on “2205” alone.
• ASTM treats SI and inch-pound values as separate standard systems. Use the unit system specified in the purchase order.
• Confirm compliance against the contract edition and the material test report.

What Do These Mechanical Properties Mean?

Mill Test Reports in Practice: 2205 vs 2507 Tensile Results

The specified minimums above describe the floor a heat must meet before it can be certified. The two tensile test reports below, from our own production of seamless duplex pipe, show how actual mill results compare with those limits.

Why Is Duplex Stainless Steel Strong?

Duplex stainless steel contains both ferrite and austenite. Its balanced microstructure and alloying additions, including nitrogen, provide high strength while retaining useful ductility.

The structure is often described as approximately 50/50, but this is a general description rather than an exact acceptance criterion for every base metal and weld. Phase balance depends on composition, thermal history and processing — see the IMOA fabrication guide, Sections 3–4 and 12.1.8.

Can Duplex Pipe Reduce Wall Thickness?

Duplex pipe can permit a thinner wall when pressure design governs and the applicable code allows a higher design stress. A material substitution still requires a complete design review. Check the following before selecting a lower pipe schedule:

Where a thinner wall is feasible, potential benefits include lower pipe weight, easier handling and reduced weld volume.

Twice the yield strength does not automatically mean half the wall thickness. Evaluate the complete piping system, including fittings, flanges, fabrication and inspection costs. A first screening can be started with our pipe wall thickness calculator.

Fatigue and Corrosion Fatigue Performance

Duplex and super duplex stainless steels are used in cyclically loaded systems where strength and corrosion resistance are both important. Alleima identifies corrosion fatigue resistance as a characteristic of its SAF® 2507 seamless tube and pipe — see the Alleima SAF® 2507 datasheet.

There is no single fatigue-strength percentage that applies to every duplex piping application. Fatigue performance depends on:

• Stress range, mean stress and cycle count.
• Weld geometry, defects and residual stress.
• Surface finish, notches and local stress concentrations.
• Temperature, corrosive medium and exposure conditions.
• Loading frequency and the selected fatigue assessment method.

For pressure cycling, pump vibration or offshore service, use fatigue data and design curves appropriate to the material, environment and joint configuration. ASTM A790 tensile requirements do not establish a fatigue design curve.

Temperature and Impact Toughness

Room-temperature strength values are insufficient for assessing elevated-temperature or low-temperature service.

Prolonged exposure at elevated temperatures can cause duplex stainless steels to lose toughness through microstructural changes. ASTM A790/A790M-24 explicitly identifies susceptibility to embrittlement under prolonged elevated-temperature exposure.

For low-temperature service, establish the required impact-test temperature, specimen size and acceptance criteria under the governing code and project specification.

High tensile strength does not guarantee adequate impact toughness. Review the base metal, weld metal and heat-affected zone requirements separately.

Fabrication and Welding Considerations

Cold Forming and Machining

High duplex yield strength increases forming loads and springback. Select equipment capacity, tooling and machining parameters for the specific grade and product condition — see the IMOA fabrication guide, Sections 10–11.

Welding

Use a qualified welding procedure that controls heat input, interpass temperature, consumables and shielding.

IMOA gives general maximum interpass guidance of 150 °C for lean and standard duplex and 100 °C for super duplex. These are guidance values; the qualified procedure and project requirements govern production welding — see IMOA fabrication guide, Section 12.1.6.

Both insufficient and excessive heat input can impair the weld microstructure. The objective is acceptable phase balance, toughness and corrosion performance, rather than an exact 50/50 phase ratio — see IMOA fabrication guide, Sections 12.1.6–12.1.8.

What to Specify When Ordering Duplex Pipe

A clear purchase specification helps prevent grade ambiguity and incomplete documentation. Include:

• Material specification and edition: ASTM A790/A790M and the required UNS grade.
• Manufacturing route: seamless or welded pipe, with any project restrictions.
• Dimensions: NPS or outside diameter, schedule or wall thickness, length and tolerances.
• Service conditions: design temperature, pressure and relevant corrosive exposure.
• Inspection requirements: applicable standard tests and explicitly agreed supplementary tests.
• Certification: required material test report, heat identification and traceability.

Review reported chemistry, tensile properties, hardness and heat treatment against the specified requirements. When impact, corrosion or metallurgical testing is required, define the method and acceptance criteria in the order.

Actual mill strength above the specification minimum should not be used to increase design allowable stress unless the governing code expressly permits it.