Duplex Flanges is an alloy composed of Fe, Ni, and Cr with two distinct phases that produce an austenitic ferritic microstructure at room temperature.
Combining austenitic grades and ferritic microstructures provides several advantages, one being an enhanced metallurgical structure that benefits from both microstructures. Duplex steel's enhanced properties have made it extremely popular within heavy industries such as chemical processing, gas, nuclear power, and oilfield services.
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Duplex Steel Flanges
Super Duplex Flanges can be identified by molybdenum levels of up to 5%, high chromium (19-28%), and reduced nickel content than austenitic stainless steel. Two common duplex stainless steel grades used today are 2205 (5%) Nickel 22% Chromium 25% Chromium) and 2507 (7% Nickel 25% Chromium); with 2507 known for its superior corrosion resistance properties.
Duplex steels date back to the 1920s with the first cast made in Sweden in 1930. But as steelmaking technology advanced further over time, its popularity began increasing significantly around 30 years ago.
Ferritic and austenitic metal structures used to form duplex are both applicable, yet each has certain limitations that limit their use for all applications.
Austenitic grades have lower resistance/strength to reduce stress corrosion cracking while ferritic grades offer poorer low-temperature toughness, weldability issues and lower strength ratings.
Combining ferritic and austenitic metals into one chemical composition can produce metal with resistance to stress corrosion cracking, good toughness, weldability, and higher strength.
Super Duplex Flanges feature high strength; with current grades offering 0.2% PS ranges spanning 400-500 MPa. This allows them to significantly reduce section thickness and weight - an advantage especially apparent for structural applications, such as bridges, storage tanks and pressure vessels.
One downside of duplexes is their tendency to be fragile at extreme temperatures, which means their use should be restricted between 300 degrees and 50 degrees.
Advantages of Duplex Steel
Duplex stainless steel provides numerous benefits. These include:
Corrosion Resistance
Corrosion resistance of stainless steel depends on its composition; nitrogen, molybdenum, and chromium being crucial components. Duplex stainless steel exhibits exceptional resistance to corrosion even in environments containing chlorides or sulfides due to high levels of nitrogen and molybdenum content.
It even resists stress corrosion cracking (SCC), an aggressive form of corrosion caused by exposure to conditions with strong acids or bases combined with tension stress and temperature increase. SCC occurs under specific conditions when these factors come together: an acid environment plus tension stress plus sufficient temperatures are present resulting in SCC's formation.
Strength
Stainless Steel Pipe Fittings boast nearly double the strength of regular ferritic or austenitic stainless steels. It exceeds ferritic grades for both ductility and toughness; however, their touchiness does not match those of austenitic grades.
Weldability
Duplex stainless steel variants generally possess excellent weldability. Standard welding processes may be employed when welding duplex stainless steel grades; however, their weldability may not compare favorably to austenitic grades.
Heat Resistance
Duplex stainless steel offers lower thermal expansion and greater heat conductivity compared to austenitic steel grades, making them an excellent choice for use down to temperatures as low as 50 C - giving duplex grades greater ductility than ferritic grades of steel at those temperatures.
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Cost
Duplex stainless steels contain significantly lower concentrations of molybdenum and nickel than their austenitic counterparts, leading to significantly reduced alloying content, leading to cost savings as well as possible thickness reduction. Thinner products mean significant weight savings.
Stainless Steel Pipe Fittings surpass ferritic grades when it comes to their ductility and toughness, yet remain more flexible than austenitic varieties.