Name: Daniela Bahiense de Oliveira
Type: MSc dissertation
Publication date: 18/03/2019

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Examining board:

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FLAVIO JOSE DA SILVA External Examiner *
Temistocles de Sousa Luz External Examiner *

Summary: When subjected to fluid action, operating equipment are subject to wear by cavitation erosion. To mitigate the effects of this wear, it can be used superalloys and thermochemical treatments, such as plasma nitriding. Therefore, it is necessary to promote the increase of resistance to wear by cavitational erosion of the materials used in these applications. The objective of this work was to evaluate the resistance to cavitation erosion of the plasma nitrided Co-30Cr-19Fe alloy at temperatures of 380°C, 400°C and 420°C and to determine the main wear mechanisms in each of these conditions. The samples were subjected to a solution treatment, carried out at 1200 °C for 12 hours and subsequent quenching in water. Subsequently, the thermochemical treatment of nitriding was carried out at temperatures of 380°C, 400°C and 420°C. The gas mixture used was 75% N2 and 25% H2, the treatment time was 20 hours and the working pressure was 2.5 Torr for all nitriding conditions. The characterization, before and after each treatment, was done by means of optical microscopy, microhardness measurements and X-ray diffraction. The vibration cavitation tests were performed on the ultrasonic equipment (Telsonic DG-2000) with periodically interruptions and the evolution of the mass loss and to record the evolution of the wear mechanism by scanning electron microscopy. Before the thermochemical treatment a mixed microstructure was observed whose phases are cobalt α (CFC) and ε (HC), with few carbides and precipitates. And after the solution treatment there were partial dissolution of these few carbides and precipitates that existed. S phase and chromium nitrides were obtained under all nitrided conditions. The condition that presented the greatest wear resistance due to cavitational erosion was the condition nitrided in 380°C from the analysis of the accumulated mass loss curves with the time of exposure and the evaluation of the wear mechanisms. However, the thermochemical treatment was not effective to improve the wear resistance by cavitation erosion of the nitrided conditions at 400°C and 420°C in relation to the solution treated condition. In general, the wear mechanism observed for all nitrided conditions was the absence of significant plastic deformation, with pites and craters formation, brittle detachment of the material due to fatigue and consequently absence of incubation time. Keywords: Co-30Cr-19Fe alloy; Plasma Nitriding; Expanded austenite; Cavitation Erosion.

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