Annealing temperature effects on super duplex stainless steel UNS s32750 welded joints. I: microstructure and partitioning of elements

Welding of austenitic-ferritic stainless steels is a crucial operation and all the materials and parameters used in this process must be optimized in order to obtain the suitable corrosion and mechanical properties. Since a great part of super duplex stainless steels is used in very aggressive environment, their corrosion resistance, referred in particular to pitting and crevice corrosion, is an all-important facet in production and processing of this type of steels. Pitting corrosion resistance of super duplex stainless steels welded joints depends on several aspects: microstructure of the bead, elemental partitioning between ferrite and austenite, and the possible presence of secondary phases. For these reasons, a post-weld annealing is generally performed to homogenize the microstructure. The annealing temperature is the most important parameter to be optimized in this heat treatment. In the present work, a comparison between the as-welded and solution-treated joints is carried out. An effort has been made to correlate the main factors that affect pitting corrosion of the welded joints (microstructure, secondary phases, chemical composition of single phases) with the experimental data obtained from corrosion tests. In this first part of the work the results regarding microstructure and partitioning of elements are presented. The phase balance and the austenite morphology are locally upset during submerged-arc welding of UNS S32750. In the fusion zone, the two phases (ferrite and austenite) result to have approximately the same composition regarding Cr, Mo, and Ni content, while nitrogen is heavily concentrated in austenite. After annealing treatment, the austenite volume fraction increases and the partitioning ratios of elements reach the equilibrium values. The base material results to be less sensitive to annealing treatment than the fusion zone, and the partitioning of elements in the base material is in agreement with previous works reported in the literature.