Erosão, corrosão, erosão-corrosão e cavitação do aço ABNT 8550 nitretado a plasma

• Autor(a) principal: Silva, Flávio José da
• Data de Publicação: 2008
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da UFU
• Texto Completo: https://repositorio.ufu.br/handle/123456789/14679
• topic: Aço ABNT 8550; Nitretação; Erosão; Corrosão; Erosão-corrosão; Cavitação; Tribologia; Desgaste mecânico; Aço; Materiais - Erosão; ABNT 8550 steel; Nitriding; Erosion; Corrosion; Erosion-corrosion; Cavitation; CNPQ::ENGENHARIAS::ENGENHARIA MECANICA

Multiphase pump rotors for deepwater applications are usually made of ABNT 8550. This justifies a study aiming an improvement of its wear and corrosion resistance. The main purpose of this work was to evaluate erosion, corrosion, erosion-corrosion and cavitation behavior of nitrided ABNT8550 steel. Additionally, nitrided surfaces combined with PVD applied coatings were investigated. The experimental procedure included: a) production and characterization of different types of samples, varying the microstructure of base material (quenched and tempered martensite and ferrite-perlite) and the main nitriding parameters; b) corrosion tests in stagnant synthetic seawater; c) construction and evaluation of an erosion, corrosion, and erosion-corrosion simulation apparatus, and d) ultrasonic cavitation tests. The sample characterization showed that a combination of long nitriding period of time with high nitrogen content leads to thicker layers. Considering the maximum layer hardness, it was observed that it decreases with time when nitriding with 5% N2 and remains constant for samples heat treated in an atmosphere with 76% N2. The base material microstructure influenced neither the thickness nor the hardness profile of the nitrided samples. The combination of nitriding and PVD coatings lead to a combination of good coating adhesion with high hardness in samples without white layer. Nitriding and application of CrN and AlCrN coatings improved significantly the electrochemical characteristics of ABNT 8550 steel in sea water, reducing its potential and corrosion current. The compound layer in nitrided samples slightly improves the corrosion performance, in comparison to the corrosion behavior of the diffusion zone. The designed and built infrastructure for erosion and erosion-corrosion tests allowed the evaluation of different materials. Quenched and tempered and annealed samples of ABNT 8550 steel showed the same erosion rate and the erosion mechanism was the ductile mode. The wear rate of nitrided samples was systematically smaller than that observed at non-nitrided samples. The brittleness of white layer was observed by its higher wear rate at normal impingement of erosive particles. At small impingement angles, the white layer showed higher erosion resistance than that observed at the diffusion zone. The wear rates (in g/g) of CrN and AlCrN coatings were significantly lower than those observed in nitrided samples. Considering the wear rate/coating thickness ratio of the studied layers, the nitrided samples with white layer showed similar performance in comparison to those observed at the CrN and AlCrN coatings. The best performance was observed at the sample with the thickest diffusion zone. Under the present test conditions, no synergy in the erosion-corrosion results was observed. Nitriding improved the cavitation resistance of ABNT 8550 steel, though, according to the present results, it can be suggested that the brittle white layer should be avoided. The AlCrN coating presented the best performance with respect to cavitation, when compared with both CrN and nitrided steel. In general, the nitrided or coated ferrite-perlite microstructure presents an indication of a better performance in respect to cavitation, when compared to that of quenched and tempered martensite.