Abstract
Two annealing heat treatments were investigated regarding localised corrosion behaviour on Nb and Mn-bearing super austenitic stainless steel. The material was designed based on physical metallurgical principles supported by Thermo-Calc modelling and was produced in an open-air atmosphere based on pre-sorted SS scrap plus ferroalloys elements. The production method aims for metal waste valorisation considering small foundries without an atmosphere-controlled process. Secondary phases at annealing temperatures were experimentally analysed by different microstructural characterisation methods and correlated by the Thermo-Calc modelling. In addition, electrochemical techniques and SKPFM were used to study the relationship between observed phases and localised corrosion performance. As the main results, a good correlation was observed between Thermo-Calc calculations and the microstructural characterisation. In particular, M23C6 carbides, σ phase, and Nb-MX were identified for the sample treated at 1120 °C, while for the sample treated at 1180 °C, σ phase and Nb-MX were identified, no M23C6 were detected. As regards localised corrosion behaviour, the M23C6 carbides and σ phase for sample 1120 °C generated Cr and Mo depletion zones and diminished the corrosion resistance in corrosive aqueous solutions. In the case of sample 1180 °C, the σ-phase with a smaller size and volume fraction than sample 1120 °C was observed, leading to a more uniform Cr and Mo distribution through the microstructure, obtaining high corrosion resistance and showing promising corrosion behaviour, similar to commercial SS 254smo considering its production in open-air.
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