What is Duplex Stainless Steel?

Duplex Stainless Steel is named for its unique two-phase microstructure, which consists of roughly 50% austenite and 50% ferrite. This combination of phases gives duplex stainless steels many of the advantages of both austenitic and ferritic stainless steels, while also minimizing their individual weaknesses.

The exact composition of duplex stainless steel can vary depending on the specific grade, but it typically includes components such as:

• Carbon
• Manganese
• Silicon
• Chromium
• Nickel
• Phosphorous
• Sulphur

Additionally, elements like molybdenum, nitrogen, and copper might be added to enhance certain properties. This blend of elements allows duplex material to exhibit high strength, excellent corrosion resistance, and good weldability.

A Brief History of Duplex Stainless Steel

The concept of duplex stainless steels began to take shape in the 1920s, but it wasn’t until the 1930s that the first duplex alloys were produced. Initially, these early duplex steels were mainly cast and had high carbon content, which limited their applications.

By the late 1960s, advancements in de-carburation techniques allowed for the creation of low-carbon duplex steels with high levels of chromium and nickel, leading to more balanced and effective duplex alloys. This breakthrough resulted in improved duplex stainless steel grades, including Duplex 2205 (UNS S31803/32205), which remains a popular choice for industries needing high corrosion resistance.

Overview of Duplex Stainless Steel (DSS)

Duplex stainless steel (DSS) has seen significant growth with the development of many new grades. While this variety offers more material options, the availability of some newer grades in all product forms can be limited. Duplex grades are often less expensive than austenitic grades with similar corrosion resistance due to their reduced nickel and molybdenum content.

Duplex stainless steels are favored for their high yield strength, which allows for reduced section thickness in tanks and pressure vessels, leading to cost savings. To avoid the precipitation of undesirable alpha prime in the ferrite phase, most duplex grades have a maximum service temperature of 315°C (600°F). These grades are used widely across various industries, driven by their improved resistance to stress corrosion cracking and the potential for reducing cross sections in high-stress or high-pressure applications.

• Lean Duplex Stainless Steels : Lean Duplex Stainless Steels  Examples include LDX 2101 (S32101), UR 2202 (S32202), ATI 2102® (S82011), and 2304 (S32304). These grades have lower levels of nickel and molybdenum, compensated by higher nitrogen and manganese levels. They have high yield strength and good pitting and crevice corrosion resistance, making them suitable for tank construction and structural applications.
• Standard Duplex Stainless Steels : Standard Duplex Stainless Steels  A common example is 2205 (S32205). These grades typically contain 21-25% chromium, 2-3% molybdenum, and 0.15% nitrogen. Widely used across industries, 2205 has twice the yield strength of Type 316 and similar pitting and crevice corrosion resistance to Type 904L. Applications include tanks, piping, process vessels, and structural applications.
• Super duplex stainless steels : Super duplex stainless steels contain 25% chromium, 3.5-4.0% molybdenum, and 0.25-0.27% nitrogen. They offer excellent pitting and crevice corrosion resistance, similar to 6% Mo super austenitic steels. Common grades are 2507 (S32750) and Z100 (S32760), used in piping, heat exchangers, tanks, and process vessels for chemical and marine applications.
• Hyper Duplex Stainless Steels : Hyper Duplex Stainless Steels Examples are S33207 and SAF S32707. These are the most highly alloyed in the duplex family, containing 26-30% chromium, 3.5-5.0% molybdenum, and 0.30-0.50% nitrogen. 

Duplex stainless steels are an excellent choice for demanding applications due to their combination of strength and corrosion resistance.

Chemical Composition duplex stainless steels

How the Austenite/Ferrite Balance is Achieved

To understand how duplex steels work, first compare the composition of two familiar steels: austenitic 304, (1.4301), and ferritic 430, (1.4016).

• Ferritisers – Cr (chromium), Si (silicon), Mo (molybdenum), W (tungsten), Ti (titanium), Nb (niobium)
• Austenitisers – C (carbon), Ni (nickel), Mn (manganese), N (nitrogen), Cu (copper)
• Grade 430
• Grade 304
• Duplex Stainless Steel
• Duplex Steel

This content highlights how ferritisers and austenitisers influence the structure of stainless steel and explains the significance of these elements in achieving the balanced properties of duplex stainless steels.

Here are some typical compositions of duplex stainless steels:

In some of the recently developed grades, manganese and nitrogen are used together to bring the nickel content to very low levels. This has a beneficial effect on price stability.

Corrosion Resistance of Duplex Stainless Steels

Duplex stainless steels offer excellent corrosion resistance comparable to austenitic and ferritic grades. To rank their resistance, the Pitting Resistance Equivalent Number (PREN) is used. The formula for PREN is:

PREN = %Cr + 3.3 × %Mo + 16 × %N

The following table shows how the duplex steels compare with some austenitic and ferritic grades.

It must be emphasized that this table is only a guide to material selection. It is always important to assess the suitability of a particular with a full knowledge of the corrosive environment.

Benefits of Duplex Stainless Steel

Duplex Stainless Steel offers several notable benefits compared to traditional austenitic and ferritic stainless steels:

1. Improved Strength

Many duplex stainless steel grades are up to twice as strong as standard austenitic and ferritic stainless steel. This superior strength makes duplex stainless steels ideal for demanding structural applications.

2. High Toughness and Ductility

Despite being more formable under pressure than ferritic steels, duplex stainless steels offer impressive toughness and ductility. Although they might not match the toughness of austenitic stainless steels, their unique microstructure provides a balanced combination of strength and flexibility.

3. High Corrosion Resistance

Depending on the specific grade, duplex stainless steels offer exceptional resistance to corrosion. With higher levels of nitrogen, molybdenum, and chromium, these steels are well-suited for resisting crevice corrosion, chloride pitting, and other forms of localized corrosion.

4. Cost Effectiveness

Duplex stainless steel is generally more cost-effective than many traditional austenitic grades. Its lower molybdenum and nickel content helps keep prices stable, and its superior properties mean that parts can often be made thinner without sacrificing performance, reducing overall costs.

Best Uses for Duplex Stainless Steel

Thanks to its strength, corrosion resistance, and cost-effectiveness, duplex stainless steel is a popular choice in various industries:

• Offshore and Near-shore Operations: Ideal for oil drilling, desalination, and water treatment due to its high resistance to seawater and other harsh conditions.
• Chemical and Liquid Processing: Perfect for environments with aggressive chemicals and corrosive substances.
• Naval Parts and Components: Used in the construction of marine equipment and naval vessels.
• Pollution Control Equipment: Employed in systems designed to manage emissions and pollutants.
• Pulp and Paper Production: Effective for equipment used in the paper manufacturing process.
• Construction: Suitable for building structures and components exposed to the elements.

Hot Water and Brewing Tanks: Used in tanks and vessels that handle hot water and brewing processes.

Conclusion

In summary, duplex stainless steel stands out for its unique two-phase microstructure which combines the best qualities of both austenitic and ferritic stainless steels. Its superior strength, high corrosion resistance, and cost-effectiveness make it a versatile material for many challenging applications. Understanding the benefits and applications of duplex stainless steels can help you choose the right material for your next project, ensuring both performance and value.

FAQs