Name: Leonardo Belichi Vieira
Type: MSc dissertation
Publication date: 14/07/2017

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Examining board:

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Adonias Ribeiro Franco Júnior Co advisor *
Cherlio Scandian Internal Examiner *

Summary: The application of cobalt alloys, such as Co-30Cr-19Fe, in components used in the production of 2nd generation ethanol has been studied. In this process, about 8% of the load worked in the reactor is composed by hard particles, mainly silica. Thus, it’s necessary to increase the reactor components wear resistance. In this context, samples of the cast superalloy Co-30Cr-19Fe were subjected to a solution treatment and then low temperature plasma nitrided, aiming at the formation of a hard, wear resistant and precipitate free layer known in the literature as S-phase or expanded austenite. The alloy presents predominantly homogeneous microstructure and mixed crystalline structure, with cobalt α (CFC) and cobalt ε (HC) phases. The solution treatment was carried out at 1200 °C and treatment time of 8 hours. Three plasma nitriding conditions were chosen, the temperatures used were 325, 350 and 400 °C, in all cases the gas mixture was 75% H2 and 25% N2, treatment time of 20 hours and work pressure of 2.5 Torr. The samples were characterized before and after each treatment using optical microscopy, microhardness measurements and X-ray diffraction. The micro-scale abrasive wear tests were carried out using a Plint TE-66 equipment, three 0,1 g/cm3 slurries composed by SiO2, Al2O3 and SiC particles, in suspension in distilled water, were used. The CETR/Bruker APEX Universal Micro Tester was used in the scratch tests. The solution treatment resulted in a more homogeneous microstructure, with reduction of the average size of the precipitates and only partial dissolution of these. The solution did not promote major changes in the hardness and wear resistance of the material. S-phase was obtained in all conditions, however, at a temperature of 400 ° C, precipitation of chromium nitrides occurred. Thickness and hardness of the layers increased as the nitriding temperature increased. In general, the wear resistance increased after the layers formation. The solution nitrided at 350 ° C samples showed, even slightly, lower wear coefficients. The abrasive that provided the highest wear coefficient was Al2O3, followed by SiC and SiO2. The dominant wear micromechanism observed in all tests was two-body abrasion, grooving wear. When small loads were applied, S-phase promoted increase in the scratch hardness of the Co-30Cr-19Fe superalloy.

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