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Properties and classification of duplex stainless steel

The biphase in the name of the duplex stainless steel is mainly derived from the microstructure of the alloy. The duplex stainless steel usually contains about 50/50 austenite and delta ferrite phase mixture. Compared to austenitic stainless steel (for example, 304 or 316 stainless steel), duplex stainless steel has better corrosion resistance, especially chloride stress corrosion and chloride pitting, and higher strength.
Compared with ordinary austenitic stainless steel, the main difference is that the dual phase stainless steel has higher chromium content, 20-28%, and high content of molybdenum, up to 5%, the nickel content is low, about 9%, and the nitrogen content is about 0.05-0.5%.
The low nickel content and high strength (dual thinner part) of duplex stainless steel have obvious cost effectiveness. Therefore, they are widely used in the pipeline system of offshore oil and natural gas industry, and are becoming more and more popular in the pipeline and pressure vessel industry of petrochemical industry.
Duplex stainless steel has higher corrosion resistance and higher strength than 300 stainless steel. For example, 304 stainless steel has a 0.2% yield strength in the 280N / mm 2 region, and the minimum 0.2% yield strength of 22% chromium duplex stainless steel is about 450N / mm 2, and the minimum grade of super duplex stainless steel is 550N / mm 2.
Although duplex stainless steel has strong corrosion resistance and oxidation resistance, it can not be used at high temperature. This is because the ferrite phase will form brittle phase at lower temperature, which has a disastrous effect on the toughness of the duplex stainless steel. Therefore, the ASME pressure vessel specification limits the use temperature of all grades to less than 315 C, other specifications and even lower use temperatures, for super dual phase steel may be as low as 250 C.
The biphase alloys can be divided into three major categories: poor biphase, 22%Cr biphase and 25%Cr super double phase, and even higher alloying super biphase steel grades. This division is mainly based on the alloying level of the duplex stainless steel, such as the resistance to pitting, and the resistance to pitting resistance of the alloy. PREN is calculated by a simple formula: PREN =%Cr + 3.3%Mo + 16%N, and sometimes the factor of W can be considered 1.65. The PREN of the biphase steel is less than 40; the PREN of the super complex is between 40-45, the super complex PREN is over 45, and the rarefied grade usually has the lower nickel, so the price is lower.
Source: China Stainless Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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