Duplex Stainless Steels-An overview

Stainless steel is one of the most important materials in the engineering world.The material"s wide applications in chemical, petrochemical, off-shore, and power generation plants prove that it is one of the most reliable materials.The Newest fast growing family of stainless steels is duplex alloys.The ferritic-austenitic grades have a ferrite matrix intermix with austenite and in other words island of austenite in a continuous matrix of highly alloyed ferrite commonly called "Duplex" stainless steel.Duplex stainless steel covers ferritic/austenitic Fe-Cr-Ni alloy with between 30% to 70 % Ferrite .Due to high level of Cr, Mo, and N steels shows high pitting & stress corrosion cracking resistance in chloride-containing environments.Hence it is frequently used in oilrefinery heat exchangers & typical applications where there is a risk for SCC and localized corrosion as a result of chloride-containing process streams, cooling waters or deposits.Modern duplex stainless steels have generally good Weldability.Due to a balanced composition, where nitrogen plays an important role, austenite formation in the heat affected zone (HAZ) and weld metal is rapid.Under normal welding conditions a sufficient amount of austenite is formed to maintain good resistance to localized corrosion where as too rapid cooling may result in excessive amounts of ferrite, reducing the toughness.Therefore, welding with low heat input in thick walled materials should be avoided.Welding methods, such as resistance welding, laser welding and electron beam welding, which cause extremely rapid cooling should also be avoided or used with extreme caution.Too slow cooling can in the higher alloyed duplex grades cause formation of inter-metallic phases detrimental to corrosion resistance and toughness.