Development of Duplex Stainless Steel

Duplex stainless steel originated in the 1930s, with the first French patent obtained in 1935. It is generally acknowledged that by the 1980s, duplex stainless steel had undergone three generations of development.

The research on first-generation duplex stainless steel began in the 1940s with the development of 329 steel in the United States. This steel was characterized by high Cr and Mo content, offering improved corrosion resistance. However, due to its relatively high carbon content (≤0.1%), carbide precipitation along grain boundaries during welding reduced corrosion resistance, limiting its use primarily to castings. During this period, extensive efforts were made worldwide to reduce carbon content to enhance weldability, leading to the development of new grades such as:

The Soviet Union’s titanium-stabilized 1Cr21Ni5Ti,

Germany’s 1.4582,

France’s UR50 (Cr21-Ni7.5-Mo2.5-Cu1.5),

Japan’s 329J1 (C≤0.08%, Cr25-Ni4.5-Mo2).

In 1960, Sweden introduced the renowned 3RE60 (00Cr18Ni5Mo3Si2), a representative of the first-generation duplex stainless steels. It exhibited excellent weldability and formability and was widely used as a substitute for 304L and 316L in applications requiring resistance to chloride-induced stress corrosion cracking. However, its drawback was the tendency for single-phase ferrite formation in the weld heat-affected zone (HAZ), which reduced joint toughness and corrosion resistance.

The second generation of duplex stainless steels emerged in the 1970s. With the advancement of secondary refining technologies such as AOD (Argon Oxygen Decarburization) and VOD (Vacuum Oxygen Decarburization), the widespread adoption of ultra-low carbon (C ≤ 0.03%) smelting became possible. Concurrently, a deeper understanding of the unique role of nitrogen (N) in stainless steels significantly promoted the development and application of N-containing, ultra-low carbon duplex stainless steels. Building on this foundation, the addition of corrosion-resistant elements such as molybdenum (Mo), copper (Cu), and silicon (Si) led to the creation of numerous high-performance, widely used duplex stainless steels.

Representative grades include Sweden’s SAF 2205, France’s UR 45N, and Japan’s 329J3L and 329J4L. Among these, SAF 2205 currently accounts for over 70% of all duplex stainless steel production.

The third generation of duplex stainless steels, known as Super DSS (Super Duplex Stainless Steels), was developed in the late 1980s. These steels are characterized by ultra-low carbon content, 3–4% Mo, approximately 0.3% N, and some grades also incorporate Cu or tungsten (W). They exhibit exceptional resistance to chloride-induced pitting corrosion, with a Pitting Resistance Equivalent Number (PREN = %Cr + 3.3 × (%Mo + 0.5%W) + 16 × %N) exceeding 40. Examples include S32550, S32906, DP3, and UR52N.

Compared to the first and second generations, third-generation duplex stainless steels offer significantly improved corrosion resistance, not only resisting stress corrosion cracking (SCC) but also being suitable for use in seawater environments.

Entering the 1990s, due to global nickel resource shortages and rising nickel prices, stainless steel manufacturers were compelled to seek cost-effective alternatives, leading to the development of new economical nickel-containing duplex stainless steels. Examples include Sweden’s LDX2101 (Ni content: 1.35–1.70%), the U.S.’s Allegheny AL2003, France’s UR2202, and Japan’s Nippon Steel NSSC2120. These alloys achieved an optimal balance between cost and performance, gaining successful international applications and significant attention.

Building on the SuperDSS platform, SANDVIK enhanced Cr, Mo, and N content (up to 5000 ppm) to develop super duplex stainless steels SAF2707HD and SAF3207HD, with a PREN exceeding 50 and tensile strength reaching 1000 MPa. Their corrosion resistance surpasses that of 6% Mo steels and rivals nickel-based corrosion-resistant alloys.

In summary, driven by advancements in metallurgical technology and growing awareness of resource efficiency, duplex stainless steels have evolved through distinct developmental phases. Today, they form a well-established family represented by traditional grades like 2205, 2304, and 2507, with mature measures in composition, performance, design, and fabrication.

Looking ahead, duplex stainless steels will continue to develop along two trends:

Low-nickel duplex stainless steels for cost optimization.

Super and hyper duplex stainless steels with ultra-high corrosion resistance.