One Read | What Is Duplex Stainless Steel, Too Simple

One read | What is duplex stainless steel, too simple

Duplex stainless steel is a kind of steel with excellent corrosion resistance, high strength and easy manufacturing. Its physical properties are between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel.

Duplex stainless steel has a history of nearly 80 years, and its microstructure is about half of the ferrite phase and austenite phase. Early grades were alloys of chromium, nickel and molybdenum. The first forged rolled duplex stainless steels were produced in Sweden in 1930 and used in the sulfite paper industry. These grades are high-carbon duplex stainless steels, which were originally developed to reduce intergranular corrosion problems. In 1930 Finland produced duplex stainless steel castings, and in 1936 France obtained a patent steel grade, which was later known as Uranus50. After World War II, AISI329 stainless steel became a mature grade and was widely used in heat exchanger pipes for nitric acid installations. 3RE60 is one of the first generation duplex stainless steel grades developed specifically to improve resistance to chloride stress corrosion fracture (SCC). Later, forged and cast duplex stainless steel grades were used in a variety of processing industries, including containers, heat exchangers and pumps.

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

The second generation duplex stainless steel has the characteristics of nitrogen alloying. This new commercialisation began in the late 1970s, coincecting with the development of offshore oil and gas fields in the North Sea and the demand for stainless steel with excellent chloride ion corrosion resistance, good workability and high strength. 2205 became the main grade of the second generation duplex stainless steel and is widely used in offshore oil platform gas collection lines and processing facilities. Due to the high strength of this type of steel, wall thickness reduction and weight reduction are allowed on the platform, making its application very attractive.

Duplex stainless steel has been continuously developed, duplex stainless steel contains a range of corrosion characteristics of different grades, their corrosion performance depends on their alloy composition. Modern duplex stainless steel can be divided into five types:

* Economic duplex stainless steel without molybdenum such as 2304;
* Standard duplex stainless steel, such as 2205, is the main steel grade, accounting for more than 80% of the duplex steel consumption;
* 25Cr duplex stainless steel such as alloy 255, PREN value 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;
* Extra super duplex stainless steel, high alloyed duplex stainless steel with PREN value over 45.