Duplex Stainless Steel: Properties, Advantages, and Characteristics

Key Takeaways

• Duplex stainless steel combines the strengths of austenitic and ferritic stainless steels, delivering high strength and excellent corrosion resistance.

• Its superior resistance to chloride-induced corrosion and stress corrosion cracking makes it ideal for harsh marine and chemical environments.

• Duplex stainless steel provides stable long-term value by reducing dependence on costly nickel while maintaining excellent performance.

Overview

Duplex stainless steel offers a balance of strength, corrosion resistance, and stress corrosion cracking resistance by combining a roughly 50/50 mix of austenitic and ferritic structures, making it a highly valued material in demanding environments.

Austenitic stainless steel is known for corrosion resistance, strength, and weldability, while ferritic stainless steel provides cost-effectiveness and strong resistance to stress corrosion cracking.

Composition

In addition to iron, duplex stainless steel typically contains 20 to 30% chromium and about 1 to 10% nickel, with some grades also containing molybdenum, nitrogen, or copper. Compared to austenitic stainless steels, duplex stainless steel use less nickel, but unlike ferritic stainless steel, it does contain some nickel.

Per JIS standards (Japanese Industrial Standards), SUS329J1, SUS329J3L, and SUS329J4L are classified as duplex stainless steels.

Key Properties

High Strength

Duplex stainless steel has roughly twice the strength of common austenitic grades such as SUS304 and SUS316.

• Minimum tensile strength of SUS304/SUS316: 520 N/mm²

• Minimum tensile strength of SUS329J4L (duplex): 620 N/mm²

At temperatures of 500 °C (932 °F) and above, duplex stainless steel loses strength rapidly and becomes brittle, limiting its use in high-heat applications.

Excellent Corrosion and Pitting Resistance

Duplex stainless steel provides better corrosion resistance than austenitic grades, especially against chloride-induced stress corrosion cracking and pitting. This is due to its higher chromium and molybdenum content, which enhances the corrosion resistance of its passive film.

While SUS304 and SUS316 can experience stress corrosion cracking in neutral aqueous solutions containing chloride ions, duplex stainless steel tends to have far fewer occurrences. This is because the critical temperature for cracking is higher.

• Critical cracking temperature for SUS304: ~50 °C (122 °F)

• For duplex stainless steel: ~180–200 °C (356–392 °F)

This makes duplex stainless steel well-suited for environments exposed to seawater, chlorides, and harsh chemicals.

Price Stability

Although generally more expensive than SUS304 and SUS316, duplex stainless steel contains less nickel and molybdenum, reducing the impact of raw material price fluctuations. This makes it a more stable option compared to alloys with higher rare-metal content.

Applications of Duplex Stainless Steel

Thanks to its strength and corrosion resistance, duplex stainless steel is used in:

• Floodgates and tunnels

• Sewage facilities

• Seawater desalination plants

• Condensers and heat exchangers

• Flue gas desulfurization systems

• Chemical tankers and plant equipment

• Machinery exposed to chloride-rich environments

Metal CNC Machining at Yumoto Electric

Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals using milling, turning, and specialized metal processing. We have experience with over 200 materials including aluminum alloys, stainless steel, molybdenum, and titanium. 

If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.