Double phase stainless steel
Double phase stainless steel is a kind of steel that combines excellent corrosion resistance, high strength and easy manufacturing and processing. Its physical properties are between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel. The chloride pitting and crevice corrosion resistance of double phase stainless steel is a function of chromium, molybdenum, tungsten, and nitrogen content and can be similar to 316 stainless steels or higher than seawater stainless steels such as 6% Mo austenitic stainless steels.
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Double phase stainless steel are twice as strong as conventional austenitic stainless steels and have better corrosion resistance. Due to these advantages, it has recently been considered for use in the oil and gas industry as a material for gas (dry and wet carbon dioxide gas, acid gas) oil pipelines. It can also be used in chemical, electric power and other industries to manufacture seamless steel pipes, generally used as heat exchangers, transporting hot brine, and transporting cooling water containing chlorides of various organic and inorganic chemicals. In addition, dual-phase steels are often used in petroleum refining heat exchangers.
The most typical is used to prevent the occurrence of stress corrosion cracking and localized corrosion caused by chlorides containing conveying steam, cooling water or deposits, with good corrosion resistance to compounds such as hydrogen sulfide, sulfuric acid, naphthenic acid , amines and ammonium sulfate, can also be used in environments where corrosion is caused by these compounds. Double phase stainless steel is also used in the case of corrosion during processing. Many factories are more corrosive than actual processing when dealing with cooling water containing chlorides. In this case, duplex stainless steel can fully play its advantages.
Advantages: high strength, light weight, can reduce manufacturing cost, can be used as high-strength structural parts; compared with standard stainless steel (304L/316L), it has good corrosion resistance, especially resistance to stress corrosion cracking, so it has a longer service life.
Disadvantages: Welding is not difficult, but to ensure dual-phase balanced welding, heat input and process temperature control must be well controlled. Otherwise, the advantages of double phase stainless steel will not be reflected. It is also necessary to control the residence time at a temperature of 700 to 980 °C. The heat affected zone (HAZ) time is a key factor in controlling the metal intermediate phase. In addition, the formability of rolling and bending is not good, and the tensile strength, yield strength and work hardenability of duplex stainless steel make the workpiece spring back during processing, and even need to be reworked. Of course this is not always the case, but it is common in workpieces with a thickness of more than 20mm. Each material has its advantages and disadvantages, and different methods can be used to overcome the corresponding disadvantages. In conclusion, the advantages of duplex stainless steel materials make them stand out in a growing variety of materials.
