ESSC & DUPLEX 2019

Duplex stainless steels consist of almost equal amounts of austenite and ferrite due to their chemical composition and thermal history. The balanced microstructure of austenitic islands embedded in the ferritic matrix is achieved by solution annealing followed by water quenching to room temperature. Thick-walled plates have locally fairly wide austenite spacing as compared to thin sheets. This can have a negative effect on the mechanical properties and make the material more sensitive to hydrogen induced stress corrosion cracking (HISC) in subsea applications or pressure vessels. As sigma (σ), chi (χ) and R phases, secondary austenite (γ2), nitrides and carbides form at elevated temperatures; the long-time service temperature for these alloys is limited to maximum 250°C (TÜV) or 315°C (ASME BPVC) depending on which standard is used. These alloys may, however, also be subject to elevated temperatures for longer time during processing, particularly in the heat-affected zone while multipass welding. The sensitivity to precipitation of carbides or nitride and formation of intermetallic phases is thus of vital importance for optimizing the fabrication. A newly reported arc heat treatment method was employed to thermally age discs extracted from a 30 mm thick 1.4462 / UNS S32205 plate. The discs were heat treated for different times using a stationary tungsten inert gas (TIG) arc to generate a steady-state temperature field from ambient to melting temperature. The resulting microstructure was investigated using light optical microscopy (LOM), scanning electron microscopy (SEM) and micro-hardness testing. Sensitization of the microstructure was confirmed by etching in oxalic acid. The experimental results were compared to thermodynamic calculations using JMatPro. Finally, time-temperature-precipitation and property diagrams were created.