Duplex pipe is a combination of excellent corrosion resistance, high strength and easy to manufacture and process many of the excellent properties of the steel. Its physical properties between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel.
History of Duplex pipes
Duplex pipe has a history of nearly 80 years, its microstructure in the ferrite phase and austenite phase about half each. Early grades were alloys of chromium, nickel and molybdenum. The first wrought duplex stainless steels were produced in Sweden in 1930 and used in the sulfite paper industry. These grades were high-carbon duplex pipes originally developed to minimize the problem of intergranular corrosion. 1930 saw the production of duplex stainless steel castings in Finland, and 1936 saw the patenting of a grade in France that would later become known as Uranus 50. After World War II, the AISI 329 stainless steel became a well-established grade and was widely used in heat exchanger piping for nitric acid plants. 3RE60 was developed specifically to improve resistance to chloride stress corrosion fracture (chloride stress corrosion fracture), which can occur in a wide range of materials. 3RE60 is one of the first generation of 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 exchangers and pumps.
First-generation duplex pipes have good properties but have limitations in the welded condition. The heat affected zone (HAZ) of the weld is less ductile due to excess ferrite and has significantly lower corrosion resistance than the base material. These limitations restricted the application of the first generation of duplex pipes, which were usually limited to non-welded use. the invention of the stainless steel refining and argon oxygen decarburization (AOD) process in 1968 made it possible to produce a series of new stainless steel grades. one of the many advances brought about by AOD was the deliberate addition of nitrogen as an alloying element. The nitrogen alloying of duplex pipes made it possible to achieve toughness and corrosion resistance of HAZ in the welded condition close to that of the parent material. Along with increased austenite stability, nitrogen also reduces the rate of formation of harmful intermetallic phases.
The second generation of duplex pipes is characterized by nitrogen alloying. This new commercialization progression began in the late 1970s, coinciding with the development of offshore oil and gas fields in the North Sea and the need for duplex pipes with excellent resistance to chloride ions, good machinability, and high strength. 2205 became the main grade of second-generation duplex stainless steels and is widely used in offshore oil platforms for gas gathering lines and processing facilities. The high strength of these steels allows for reduced wall thicknesses and weight reductions on platforms, making their application very attractive.
Duplex pipes have been under constant development. Duplex stainless steels contain a range of grades with varying corrosion properties, which depend on their alloy composition.
Duplex Pipe Applications
- Flue gas desulfurization:Flue gas desulphurization (FGD) is a method for achieving low SO2 emissions in coal-fired power facilities to further reduce SO2 emissions. “Wet scrubbing” of SO2 from flue gas using lime or limestone slurry is a mature technology that has been used as a basic method in power station boiler systems since the 1970s. Modern scrubbers are now capable of removing up to 90% of the SO2 in the exhaust gas. modern FGD plants consist of several zones with different temperatures, chloride content and pH. Duplex pipes have been used in FGD plants in Europe and Asia due to their lower cost and better corrosion resistance compared to austenitic stainless steels. More recently, duplex stainless steels have been accepted for use in North America and have become the most popular choice of material for FGD absorption towers due to their high strength after welding, good corrosion resistance and high toughness.
- Desalination：Desalination puts materials through one of the toughest tests due to its high chloride content and high temperature corrosive process environment. The history of desalination is essentially the history of materials development, as customers in the desalination industry balance the need for corrosion resistance with the need to keep investments affordable. In the early days of desalination, carbon steel was used for the evaporators of multi-stage flash (MSF) and multi-effect (MED) desalination plants. Later, MSF evaporators were typically clad in 316L (EN1.4404) austenitic stainless steel.MED evaporation chambers were first epoxy-coated, and later clad in duplex pipe. The advantage of using duplex pipe is that its high strength is twice that of conventional austenitic grades combined with high corrosion resistance. As a result, duplex stainless steel evaporators can be fabricated from thinner steel sheets, requiring less material and welding. Further benefits include easy disposal and less overall impact on the environment.
- Food and beverages：Economical duplex pipes have also proved their worth in the food and beverage industry. The material was used in two projects in Spain, a food storage tank and a wine storage tank. In the Port of Barcelona, Emypro SA constructed the food storage tanks entirely in S32101, replacing EN 1.4301 / 1.4307 (304/304L). In Demijer, in the south of Spain, duplex pipes were used for the first time in the wine storage tank for Garcia Carrion, built by the Spanish tank manufacturer Martinez Sole.
- Building industry：Duplex pipes continue to play an important role in the construction of bridges, which are used in corrosive and salty environments and require high load-bearing strength. Two recent examples, both from Asia, are the Stonecutters Bridge in Hong Kong and the Twin Helix Pedestrian Bridge in Singapore, both of which used 2205 duplex pipes. 2,000 tons of 2205 duplex steel plates and tubes were used for the Stonecutters Bridge in 2006, with the surface sections constructed from custom-sized plates by a Chinese fabricator. The plates were polished and shot-peened for optimum reflectivity both during the day and at night.