Read it in one article｜What is duplex stainless steel? It’s too simple

Read it in one article｜What is duplex stainless steel? It's too simple

Duplex stainless steel is a type of steel that combines many excellent properties such as excellent corrosion resistance, high strength, and ease of manufacturing and processing. Its physical properties are between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel.

Duplex Stainless Steel History
Duplex stainless steel has a history of nearly 80 years. Its microstructure consists of approximately half ferrite phase and half austenite phase. Early grades were alloys of chromium, nickel and molybdenum. The first wrought duplex stainless steels were produced in Sweden in 1930 and were used in the sulphite paper industry. These steel grades are high carbon duplex stainless steels that were originally developed to reduce intergranular corrosion problems. In 1930, Finland produced duplex stainless steel castings. In 1936, France obtained a patented steel type, which later became the famous Uranus50. After World War II, AISI329 stainless steel became a mature grade and was widely used in heat exchanger pipes in nitric acid plants. 3RE60 was one of the first generation duplex stainless steel grades developed specifically to improve resistance to chloride stress corrosion cracking (SCC); later, both forged and cast duplex stainless steel grades were used in a variety of process industries including vessels, heat exchange devices and pumps.

The first generation of duplex stainless steels had good properties but had limitations in the welded state. The heat-affected zone (HAZ) of the weld has low toughness due to excessive ferrite, and its corrosion resistance is significantly lower than that of the base metal. These limitations limited the applications of first-generation duplex stainless steels, which were generally limited to non-welded use. The invention of stainless steel refining and argon oxygen decarburization (AOD) processes in 1968 made it possible to produce a series of new stainless steel grades. One of the many advances brought about by AOD is the deliberate addition of nitrogen as an alloying element. Nitrogen alloying of duplex stainless steel makes it possible for the toughness and corrosion resistance of HAZ in the welded state to be close to those of the base metal. Along with increasing austenite stability, nitrogen also reduces the rate of formation of harmful intermetallic phases.

Second-generation duplex stainless steels feature nitrogen alloying. This new commercialization development began in the late 1970s, coinciding with the development of offshore oil and gas fields in the North Sea and the need for stainless steels with excellent chloride corrosion resistance, good processability and high strength. 2205 has become the main grade of second-generation duplex stainless steel and is widely used in gas gathering pipelines and processing facilities on offshore oil platforms. The high strength of this type of steel allows for reduced wall thickness and weight reduction in platforms, making its application very attractive.

Duplex stainless steels are constantly evolving and comprise a range of grades with varying corrosion properties, depending on their alloy composition. Modern duplex stainless steels can be divided into five types:

* Economical duplex stainless steel without added molybdenum such as 2304;
* Standard duplex stainless steel, such as 2205, is the main steel type, accounting for more than 80% of duplex steel consumption;
* 25Cr duplex stainless steel such as alloy 255, PREN value is less than 40;
* Super duplex stainless steel (PREN value 40 ~ 45), containing 25% ~ 26% Cr, higher molybdenum and nitrogen content than 25Cr duplex stainless steel, such as 2507;
* Special super duplex stainless steel, a highly alloyed duplex stainless steel with a PREN value exceeding 45.

Applications of Duplex Stainless Steel
Flue gas desulfurization
In order to further reduce SO2 emissions from coal-fired power facilities, flue gas desulfurization (FGD) is a method to achieve low SO2 emissions. "Wet scrubbing" SO2 from flue gases using lime or limestone slurry is a mature technology that has been used as a basic method in utility boiler systems since the 1970s.

Modern scrubbers can currently remove 90% of SO2 from exhaust gases. Modern FGD units consist of several zones with different temperatures, chloride contents and pH. 2205 stainless steel (S32205) is already used in FGD units in Europe and Asia due to its lower cost and better corrosion resistance compared to austenitic stainless steel. Recently, North America has embraced the use of duplex stainless steel, which has become the most popular material of choice for FGD absorber towers due to its high post-weld strength, good corrosion resistance and high toughness.