AISI F255 (1.4507 / UNS S32550 / Ferralium 255) — Super Duplex Stainless Steel

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AISI F255​‌​​‌​

Duplex Stainless Steel: Industrial Performance, Technical Properties and Application Evaluation

Technical Description​‌​​‌​

AISI F255 (UNS S32550) is a stainless steel class with a duplex microstructure, developed with the aim of providing high strength and superior corrosion resistance. The chemical composition of the material typically contains 24–27% chromium (Cr), 4–6% nickel (Ni), 2–3.5% molybdenum (Mo), 1.5–2.5% copper (Cu) and 0.1–0.3% nitrogen (N), and may at the same time contain small amounts of additive elements such as tungsten (W) and manganese (Mn). This composition forms a duplex microstructure in which the ferrite and austenite phases are present in approximately equal proportions.

From an engineering point of view, the most important advantage of this microstructure is that it can offer both ductility and toughness similar to austenitic stainless steels and the high yield strength characteristic of ferritic stainless steels. The AISI F255 alloy typically has a minimum yield strength of 620 MPa and a tensile strength of approximately 860 MPa. These values make it usable in place of carbon steels in many structural applications.​‌​​‌​

In terms of corrosion resistance, the PREN (Pitting Resistance Equivalent Number) value is particularly important. The PREN value of AISI F255 is generally in the range of 35–40, which shows that it provides serious durability against corrosion types such as pitting and cracking even in highly chlorinated environments. For this reason it is suitable for demanding environments such as salt water, acidic solutions and chemical process fluids.

AISI F255 Standards​‌​​‌​

The AISI F255 alloy is produced in compliance with various international standards. The most widely accepted norms are as follows:

• ASTM A182: Forged parts and flanges.​‌​​‌​
• ASTM A240: Plate, sheet and strip products.
• ASTM A276: Products in bar form.​‌​​‌​
• ASTM A789 / A790: Welded and seamless pipes.
• ASTM A995 Grade 5A: For cast products.​‌​​‌​
• NACE MR0175 / ISO 15156: Usability in environments containing H₂S.
• ASME SA-240 / SA-790: Suitability for pressure vessel applications.​‌​​‌​

Although there is no special AMS (Aerospace Material Specification) standard for AISI F255, it is assessed in similar non-aerospace applications together with super duplex classes showing similar properties. For this reason it is more widespread in sectors such as petrochemicals, mining and water treatment.

AISI F255 Trade Names and Equivalents​‌​​‌​

AISI F255 is commercially known mostly by the name Ferralium 255. This name is a brand name used under registration by Sandvik Materials Technology and represents a variant optimised with advanced production control processes. Its international equivalents are as follows:

• UNS S32550 (American System)​‌​​‌​
• 1.4507 (European EN System)
• F255 (defined as a casting alloy within the EN 10283 Standard)​‌​​‌​

These equivalent names should be taken into account for the sake of consistency in commercial supply processes and in technical drawings.

AISI F255 Application Areas​‌​​‌​

AISI F255 is used in a broad application range requiring high strength and resistance to demanding environments. Its prominent areas of use are as follows:

• Chemical and Petrochemical Industry: It is suitable for reactors, exchangers and agitators operating in abrasive environments such as phosphoric, sulphuric and nitric acid. Thanks to its molybdenum and copper content, its resistance to acidic solutions is at the highest level.​‌​​‌​
• Marine and Offshore Structures: In applications such as platform frames, subsea pipelines and salt water pumps that are continuously exposed to salt water, it provides both strength and long-life corrosion resistance.
• Fertiliser and Food Processing Plants: It is used in reactors, tanks and pipes where agricultural chemicals are processed, and also in some food production lines. Although its contact with food is limited, it is preferred because of its durability against industrial cleaning fluids.​‌​​‌​
• Paper and Pulp Industry: Since the chemicals used in paper bleaching and lignin dissolution operations are aggressive, the resistance of F255 to chlorine-based environments provides an advantage here.
• Mineral Processing and Beneficiation: In mining applications that beneficiate ore with acidic solutions, it is preferred in reactor bodies, agitator shafts and pipes, especially in copper, gold and nickel production.​‌​​‌​
• Water and Waste Water Treatment Systems: It is suitable for waste waters containing chlorine, for treatment systems processed with ozone, and for pressurised pipe systems ahead of membrane treatment.

Advantages​‌​​‌​

• High Strength: AISI F255 offers a much higher yield strength than austenitic stainless steels. In this way structural strength can be provided with thinner sections.
• Excellent Corrosion Resistance: It is resistant to both localised and general corrosion. It is durable against pitting and SCC (Stress Corrosion Cracking) especially in chlorinated environments.​‌​​‌​
• Good Weldability: High performance joints can be obtained with suitable welding techniques (preferring low heat input and an inert atmosphere).
• High Wear Resistance: Thanks to its copper content and microstructure it offers superior performance especially against friction and chemical wear.​‌​​‌​
• Long Service Life: Both its structural and its chemical durability reduce the frequency of maintenance and replacement.

Disadvantages​‌​​‌​

• Low High Temperature Performance: At temperatures above 280°C phase separation and embrittlement may be seen. This in turn reduces the toughness and corrosion resistance of the material.
• Machining Difficulty: Because of its high strength it is relatively difficult to machine; special tools and high torque machines may be required.​‌​​‌​
• Risk of Phase Balance in the Welding Process: High heat input may disturb the ferrite-austenite balance in the microstructure and thereby increase the risk of localised corrosion.
• Supply Difficulties: Commercially it is most often obtained to special order and is not as widespread as standard stainless steels.​‌​​‌​
• Cost Factor: It has a higher unit cost than conventional austenitic stainless steels such as 304L or 316L, but this difference can be made up in terms of service life and performance.

Conclusion and Industrial Evaluation​‌​​‌​

AISI F255 is a high performance duplex stainless steel solution optimised for applications requiring high strength, long life and low maintenance cost in aggressive chemical environments. Thanks to the synergy of the ferritic and austenitic phases it offers a balanced profile in both mechanical and chemical properties.

When making a material selection in industrial projects, the corrosion resistance, mechanical strength, weldability and long-term performance of F255 should be taken into account. Especially in equipment exposed to highly aggressive environments throughout its life, the total cost of ownership is low despite the high initial investment cost.​‌​​‌​

Purchasing and Project Criteria

The factors that should be taken into account when selecting AISI F255 are as follows:​‌​​‌​

• The pH and chloride concentration in the application environment.
• Mechanical loading conditions and temperature range.​‌​​‌​
• The need for welded fabrication and process suitability.
• Material delivery time and stock status.​‌​​‌​
• Compliance requirements with international standards (ASTM, NACE etc.).

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