Duplex stainless steels are a family of stainless steels that called duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite and ferrite in roughly equal proportions. They are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as Type 304 or 316.
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Thanks to Mohammad Fouladi, his suggestions and comments have improved this presentation