Comportamento em desgaste por erosão-cavitação e em ensaios por esclerometria linear da liga Stellite 250 submetida a tratamentos térmicos e termoquímico de carbonitreção a plasma

• Autor(a) principal: Oliveira, Daniela Bahiense de
• Data de Publicação: 2023
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da Universidade Federal do Espírito Santo (riUfes)
• Texto Completo: http://repositorio.ufes.br/handle/10/12401
• topic: Engenharia Mecânica; Stellite 250; Tratamento térmico; Tratamento termoquímico; Esclerometria linear; Erosão-cavitação

Cobalt superalloys are widely used due to their ability to maintain their mechanical, physical and chemical properties when used at high temperatures. However, concerns about the increasing demand for cobalt, unstable prices and few suppliers, make it essential to assess the efficiency of the materials applied. In this context, the present work investigated the effect of heat treatments (solution and aging treatments) and thermochemical plasma carbonitriding on the tribological behavior of the Stellite 250 alloy. The solution treatment was carried out at 1200°C for 1,5 hours and aging at 850°C for 4,5 hours. Thermochemical treatments were carried out at 360°C and 380°C for 9 hours. For tribological characterization, scratch tests were carried out with a progressive load (2 to 20N) and cavitation erosions tests were performed during 15 hours of exposure. The cross-sectional area of the scratches was verified by 3D profilometry. The wear mechanisms of the tribological tests were analyzed by scanning electron microscopy (SEM). The result of the microstructural characterization showed that for the cast alloy, the structure is formed by coarse grains, consisting of the α and ε phases. From the heat treatments, the structure remained mixed. Under all carbonitring conditions there was formation of a double layer of S-phase, composed of an N-enriched phase (SN) and another C-enriched layer (SC). The surface hardness values observed under carbonitrided conditions are in the order of 1100-1500 HV0.01. The scratch tests results showed that plasma carbonitriding condition enabled a considerable reduction in the coefficient of friction in relation to that observed for the Stellite 250 cast alloy. The wear mechanisms were tensile cracks in carbonitrided conditions; and in heat-treated conditions: formation of slip bands on the sides and cracks. The solution treated condition and subsequently carbonitrided at 380°C (SC380) showed greater resistance to cavitation erosion wear than the Stellite 250 cast alloy. Wear mechanisms include premature damage with formation and coalescence of pits, crater formation and brittle behavior of the material due to fatigue.