Welding Processes, Microstructural Evolution and Final Properties Of Duplex and Superduplex Stainless Steels

The study of weldability of Duplex and Super Duplex Stainless Steels (DSS and SDSS, respectively) is a fundamental task for their proper industrial application. During welding, two main problems may arise, due to the evolution of microstructure during cooling: the achievement of a not well-balanced austenite-to-ferrite ratio and the precipitation, in the Fusion Zone (FZ) and Heat Affected Zone (HAZ), of secondary phases which generally decrease the corrosion resistance and the toughness of the alloy. The FZ and HAZ microstructure could be very different from that of the base material as a function of both chemical composition and thermal history. In this chapter, the problems relating to the microstructural evolution of DSS and SDSS will be presented • by describing the kinetics of δ → γ phase transformation, • by analysing the isothermal and anisothermal transformations leading to the formation of phases and precipitates which are generally detrimental for the mechanical and corrosion properties, • by examining with some examples the effect of the thermal history induced by the welding process on the austenite-to-ferrite ratio and on precipitation phenomena, • by reviewing the welding solutions (in terms of materials choice, processes and filler metals) which can lead to optimized microstructures in welding joints.