Heat affected zone ferrite content control of a new duplex stainless steel grade with enhanced weldability

Abstract

Duplex stainless steels 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 and is strongly 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 phases equilibrium. Cooling rate is linked to heat input but also to parts thickness and increases with the thickness of the plates. Most of the actual standards limits the ferrite content in the range from 30% to 70% in HAZ. However, the 70% limit for ferrite content in HAZ could be difficult to reach on high thickness plate. To avoid this risk, a new duplex stainless grade, called 2205 Arctic, 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).