Heat-affected zone ferrite content control of a duplex stainless steel grade to enhance weldability

Abstract

Duplex stainless steels (DSS) have a microstructure consisting of 50% ferrite α and 50% austenite γ. This two-phase microstructure provides the duplex stainless steels with attractive mechanical and corrosion properties. However, the microstructure is very dependent on the heat treatment and chemical composition. During the welding process, the temperature reached in the heat-affected zone (HAZ) retransforms austenite into ferrite. Due to the high cooling rates, only a part of the ferrite is retransformed into austenite. This tends to modify the 50/50 phase equilibrium. The cooling rate is linked not only to heat input but also to the thickness of the welded plates. Most of the standards limit the ferrite content in the range from 30 to 70%. However, the 70% limit for ferrite content in HAZ may be difficult to obtain especially for heavy plate thicknesses. To limit the maximum ferrite content in HAZ, a duplex stainless grade with high nitrogen and nickel within the 2205 composition range, called Arctic 2205, was developed. This paper will provide details on ferrite content measurement by metallographic method and welding results obtained on industrial welds showing very low and stable ferrite content in HAZ. Mechanical properties obtained on these welds at low temperature will also be reported, pushing the limits of use of duplex from − 50 °C (− 58 °F) down to − 100 °C (− 148 °F).