Determiniation of the formation mechanisim of sigma and chi phases with the different heat treatment parameters in duplex stainless steel

Duplex stainless steels that have approximately equal amount of ferrite and austenite in the microstructure and have good mechanical and corrosion properties with higher alloying elements content. Duplex steels are widely used in piping, chemical and offshore industries because of their higher mechanical and corrosion properties. Duplex stainless steels have great amount of Nickel and Chrome content. One of the most important problem that encountered in the duplex stainless steel is formed secondary phases (most tpycally of these are σ and χ phases) during the heat treatments. These phases occur between 450-950 ˚C and comprising of σ and χ phases are dramatically deteriorated mechanical properties and corrosion resistance. Moreover, secondary austenite can occur during the comprising of σ and χ phases and desired austenite and ferrite fractions cannot be provide with secondary austenite formation. Elimination of σ and χ phases or minimizing the negative effects of σ and χ phases on the properties of alloy can be provided by heat treatments. In this study, heat treatment map of SAF 2205 duplex stainless steel was created. Aim of this study is to investigate the temperatures conditions of formation of secondary phases, effects of the heating and cooling mediums and lastly effects of distributions and sizes of secondary phases on mechanical properties and corrosion resistance. For this purpose, solution, aging and isothermal aging heat treatments were applied with different temperatures and times to the duplex stainless steel.