Estudo da estrutura local dos sítios de ferro em ligas Fe-Cr-Mo por espectroscopia mössbauer e difração de raios-X

• Ano de defesa: 2011
• Autor(a) principal: Oliveira Júnior, Francisco das Chagas
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Palavras-chave em Português: Ciência dos materiais; Ligas de aço; Metalurgia; Ligas Cr-Mo; Campo magnético
• Link de acesso: http://www.repositorio.ufc.br/handle/riufc/15935

Fe-Cr-Mo alloys are widely used in the manufacture of piping for the petrochemical industry in oil transport and refining processes due to a combination of properties such as hardness, toughness, creep resistance and corrosion resistance at high temperatures. Nowadays the Fe-9Cr-1Mo alloy is the most common alloy in use. However, in some refineries with higher acidity levels, strong corrosion processes have been observed, specially in processes related to naphthenic acid corrosion. An increasing in the Molybdenum content has been pointed out as a possible solution to this problem, since the addition of this alloying element results in a better corrosion resistance. In this context, Fe-9Cr-XMo alloys with three different Molydbenum contents (X = 5, 7, 9 wt%) have been developed at LACAM - Laboratório de Caracterização de Materiais at the Universidade Federal do Ceará. The characterization of these alloys, as well as the precipitates that appear on them due to heat treatment and studies of their corrosion resistance were made through various techniques. Specifically in this work, Transmission Mössbauer Spectroscopy and X-Ray Diffraction were used in order to achieve a better understanding of the structure changes of the alloys when they are subjected to different service conditions in the temperature range from 450°C to 650°C. The results led to the conclusion that the distribuition of the alloy elements in the matrix is influenced by the molibdenum content, the temperature and the treatment time. The precipitates present in the ASTM A213 grade T91, used for comparation with E1, E2 and E3 alloys, have been extracted and characterized as well. The presence of this precipitates changes the distribuition of the alloy elements in the matrix. The analysis of the influence of the impurity elements in the structure of the alloys was investigated by a model of first and second neighbors. The changes in the magnetic hyperfine field is proportional to the number of impurity atoms in the neighborhood of the iron sites. The distribuition of the alloy elements in the matrix is direct related to the distribution of the magnetic hyperfine field. The probabilities associated to the values of hiperfine field allow for a better understanding of the structure of the alloys.