What is duplex stainless steel and when is it the right choice?

Duplex stainless steel takes its name from its two-phase microstructure. It consists of approximately equal proportions of an austenitic phase and a ferritic phase. This microstructural combination produces a material that is significantly stronger than conventional chromium-nickel stainless steel. While the austenitic phase provides good formability and corrosion resistance, the ferritic phase delivers high yield strength and resistance to stress corrosion cracking. In modern materials engineering, duplex stainless steel is considered a highly efficient solution for reducing component weight through higher strength while significantly extending service life in aggressive media such as salt water or acids.

Designations and synonyms

Duplex grades are identified according to various international standards. To avoid incorrect orders and ensure precise compliance with technical specifications, familiarity with common synonyms and standard designations is essential.

Standard Duplex (Material No. 1.4462)

The most commonly used grade is material 1.4462. Internationally, we offer it under the designation AISI 2205 or UNS S31803/S32205. It is considered the industry standard for applications requiring a balance between cost and exceptional resistance.

Lean Duplex (Grade 1.4162)

Lean duplex has become established as a cost-effective alternative, for example grade 1.4162 (UNS S32101). This grade contains less nickel and molybdenum, yet offers higher strength than conventional stainless steels such as 1.4301, making it particularly attractive for structural applications.

Super Duplex (grades 1.4410 / 1.4501)

For the most extreme conditions, we use super duplex grades such as 1.4410 (AISI 2507) or 1.4501. These materials have a PREN (Pitting Resistance Equivalent Number) above 40, making them ideal for use in offshore engineering and chemical tanker construction.

Chemical composition

The performance of duplex stainless steel is based on a precisely balanced alloy composition. We process materials whose chemical analysis shows the following main constituents:

• Chromium (18% to 28%): Forms the protective passive layer and ensures resistance to oxidation and corrosion.
• Nickel (3.5% to 8%): Stabilizes the austenitic phase and improves the material’s toughness.
• Molybdenum (up to 5%): Significantly increases resistance to pitting corrosion in chloride-containing environments.
• Nitrogen (0.1% to 0.3%): Increases strength and improves resistance to crevice corrosion by strengthening the microstructure.
• Manganese and copper: Added depending on the grade to optimize workability and corrosion resistance in specific acids.
• Tungsten: Found in some super duplex grades (e.g. 1.4501) to further enhance resistance to localized corrosion.

Properties of Duplex Stainless Steel

Its technical properties make this material a superior choice to standard austenitic grades such as 1.4404 (V4A) where mechanical loads and chemical exposure are more severe.

Mechanical Properties

Duplex stainless steels have approximately twice the yield strength of austenitic stainless steels. This high strength allows designers to reduce wall thicknesses, resulting in substantial material savings. Despite its hardness, the material offers good energy absorption and impact toughness, even at low temperatures.

Physical Properties

A key advantage is its low thermal expansion, which is closer to that of unalloyed structural steel than to that of austenitic stainless steel. This minimises thermal stresses in structures. In addition, duplex stainless steel is ferromagnetic due to its ferrite content, which must be taken into account when planning magnetically neutral zones.

Corrosion Resistance

Resistance to stress corrosion cracking is the key advantage of duplex stainless steel. While standard austenitic grades can fail in chloride-containing environments at elevated temperatures, duplex remains stable. Its high chromium and molybdenum content also provides effective protection against pitting and crevice corrosion in coastal or industrial atmospheres.

Applications and Industries

Thanks to the combination of durability and aesthetics achieved through finishing by mirrorINOX, we deliver projects across a wide range of sectors:

• Architecture and façade construction: We supply surface-finished duplex stainless steel for buildings in coastal regions or areas with high levels of air pollution, ensuring lasting brilliance without corrosion damage.
• Chemical industry: Used in tanks and piping exposed to aggressive media, acids and alkalis.
• Seawater desalination and offshore: Super duplex grades withstand continuous contact with saltwater and high pressures on drilling platforms or aboard ships.
• Food and pharmaceutical industries: Here, we leverage the low Ra values and cleanability of our polished surfaces for tanks with stringent hygiene requirements.
• Oil and gas industry: Pipelines and heat exchangers benefit from the erosion and abrasion resistance of the duplex microstructure.

Overview of international standard designations

For global sourcing and specification, we use the following equivalent standards:

• Material No. 1.4462: Equivalent to AISI 2205, UNS S31803 / S32205 and AFNOR Z3 CND 22-05.
• Material No. 1.4162: Known as UNS S32101, often marketed under the trade name LDX 2101®.
• Material No. 1.4410: Corresponds to AISI 2507, UNS S32750, the standard grade for super duplex.
• Material No. 1.4501: Corresponds to UNS S32760, often used in naval defence applications.
• Material No. 1.4362: Corresponds to UNS S32304, an alternative to 1.4404 with higher strength.

Finishing duplex stainless steel – mirrorINOX expertise

As supplied, duplex grades often have a visually unappealing, matt and rough surface (e.g. 2E or 1D). At mirrorINOX, we transform this “ugly duckling” into aesthetically refined masterpieces. Since our founding in 2016, we have polished and coated duplex stainless steel in compliance with stringent ISO 9001 quality standards.

We apply our proprietary finishes specifically to duplex substrates: Our Perfect Mirror No. 10® is a perfectly reflective finish that retains its brilliance even under extreme conditions. With the nanoINOX® sol-gel coating, we give the material additional functional properties such as anti-fingerprint and easy-to-clean performance. Our processes, including grinding and glass bead blasting, create structural depth. We guarantee exact reproducibility from batch to batch, enabling large-scale projects to be completed with a consistent appearance. mirrorINOX thus offers a globally unique combination of maximum material performance and high-end surface aesthetics.