Bragança, Camilla dos Santos Motta; Bott, Ivani de Souza (Advisor). The Effect of Different Atmospheres on the Austenitic Transformation and Its Corrosion Resistance in Hyper Duplex Steel UNS S33207. Rio de Janeiro, 2020. 125p. Hyper duplex stainless steel UNS S33207 is used in the oil and gas industry in ultra-deep well operations due to its excellent mechanical properties and corrosion resistance. When stainless steels are subjected to thermal cycles reaching temperatures close to the

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... ation temperature, then followed by fast cooling, the microstructure transforms. The ferritic matrix and different austenite morphologies will occur during cooling, such as Widmanstätten, allotriomorphic, and intragranular, in addition to chromium nitride precipitation. It is essential to highlight that for duplex stainless steels, among the chemical elements in solid solution, nitrogen is vital in the austenitic phase precipitation. It is also responsible for the pitting corrosion resistance. Therefore, the atmospheres in which heat treatments are carried out or when thermal cycles are applied to these steels can affect nitrogen desorption or absorption, thus influencing corrosion resistance. In this dissertation, UNS S33207 hyper duplex stainless steel was analyzed in the asreceived condition and as heat-treated at 1380°C and 1390°C in four different atmospheres: atmospheric air, argon, nitrogen and a mixture of argon and 5% nitrogen. Microstructural characterization was performed by optical microscopy and scanning electron microscopy (SEM) in addition to electron backscatter diffraction (EBSD). Hardness, microhardness, and ASTM G48 corrosion tests were also performed. After the heat treatment, the balance between the ferrite and austenite phases changed. Due to the fast cooling rate, the ferritic phase became more abundant in the heat-treated condition. The atmosphere influence was more evident on the primary austenite after the heat treatment. The samples heat treated with nitrogen atmospheres presented a higher percentage of primary austenite than the samples heat-treated in atmospheric air, or even in pure argon, the latter gave the lowest fraction of remaining austenite. The ASTM G48 corrosion test indicated a more significant mass loss in the heat-treated samples. Among these samples, those that were heat-treated in atmospheres containing nitrogen presented better corrosion resistance. The pitting in all samples nucleated at the γ/δ interface. The pitting then propagated into the ferritic grains. This dissertation PUC-Rio -Certificação Digital Nº 1812693/CA main objective is to correlate the influence of the different atmospheres on the percentage of phases transformed as a function of heat treatments and the effect on corrosion properties.