Cinética de transformação de fases em novos aços inoxidáveis superferríticos com alto molibdênio

• Ano de defesa: 2015
• Autor(a) principal: Moura, Lorena Braga
• Orientador(a): Abreu, Hamilton Ferreira Gomes de
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Palavras-chave em Português: Ciência dos materiais; Molibdênio; Aço inoxidável
• Link de acesso: http://www.repositorio.ufc.br/handle/riufc/16999

Previous research on experimental ferritic steel with high Mo content showed that Mo increases the resistance to naphthenic corrosion and sulfur complexes. However, Mo content above 5 wt% favored the formation of deleterious phases and reduced the toughness of the steel. To improve the toughness of these alloys and keep the ferrite phase stable, was added Ni, increased Cr content to 25% and maintained high Mo content. These new alloys belong to a family of steels known as superferritic stainless steels. They were originally developed for use in heat exchangers and marine environments. There is a current trend to use these alloys in the oil industry driving the research on the effect of the increase of Mo content on the microstructure of these steels. The kinetics of phase precipitation in experimental compositions (Fe25%Cr 5-7%Mo 2-4%Ni) with addition of Nb and Ti will be investigated for temperatures from 400 ° C to 900 ° C for different treatment times. In this first stage, Thermo-Calc software was used to determine the temperature stability of the ferritic phase and to identify possible intermetallic phases precipitated at thermodynamic equibrium. The alloys were aged at 400° and 475°C for study the alfa prime phase precipitation and their effects on the mechanical, magnetic and corrosion properties. Isothermal treatments were carried out 600 °C to 900 ° C to study the kinetics of precipitation of intermetallic phases. The microstructural changes on the mechanical and corrosion properties due to variation in composition and heat treatment were studied. The results obtained in the experimental alloys treated at 400° and 475°C indicated an increase in hardness, while wt% of ferrite had decreased, accompanied by increased susceptibility to pitting corrosion, the best performance observed for 5Mo4Ni alloy these conditions. For samples treated 600° to 900°C the 7Mo2Ni alloy showed the smallest amount deleterious phase precipitated and less susceptibility to pitting corrosion. Austenite phase precipitation occurred for the alloys containing 4%Ni treated at 800°C and 900°C. The 7Mo4Ni alloy showed worse performance compared to other experimental alloys in all conditions studied