Projeto de aço inoxidável lean dúplex com adição de nióbio
Carregando...
Arquivos
Citações na Scopus
Tipo de produção
Dissertação
Data
2015
Autores
Alves, J. R. O.
Orientador
Magnabosco, R.
Periódico
Título da Revista
ISSN da Revista
Título de Volume
Citação
ALVES, J. R. O. Projeto de aço inoxidável lean dúplex com adição de nióbio. 2015. 201 f. Dissertação (Mestrado em Engenharia Mecânica) - Centro Universitário da FEI, São Bernardo do Campo, 2015 Disponível em: . Acesso em: 14 maio 2015.
Texto completo (DOI)
Palavras-chave
Aço inoxidável,Nióbio,Simulações termodinâmicas
Resumo
O presente trabalho teve como objetivo o projeto de liga de aço lean dúplex, com adição de nióbio, para compreensão das transformações de fase envolvidas nas solidificação, e durante solubilização, comparando os resultados obtidos com os estimados pelas simulações termodinâmicas de equilíbrio de fases e de Scheil por meio do software Thermo-Calc©. A identificação das fases foi realizada utilizando-se microscopia óptica, microscopia eletrônica de varredura (MEV), análises por espectroscopia por Energia Dispersiva (EDS), e difração de raios X. Já sua quantificação foi realizada via medidas magnéticas por meio de ferritoscópio. Segundo as simulações termodinâmicas em equilíbrio não foi possível obter uma estrutura dúplex composta apenas por 50% de ferrita e 50 % de austenita uma vez que ocorreram precipitações de nitretos a partir de líquido. Porém, diferentemente das simulações de equilíbrio de fases, as simulações de Scheil não indicaram a formação de fase Z. Um dos possíveis motivos para esta ausência da fase Z pode estar relacionado à formação de nitretos a partir do líquido e, portanto, pode ser uma evidência de que o consumo de elementos como cromo, nitrogênio e nióbio para a formação destas fases não permitirá a formação de fase Z no estado sólido. Porém, como as simulações de Scheil não indicam a precipitação de fase Z, as fases ricas em nitrogênio evidenciadas na análise microestrutural e de EDS provavelmente são nitretos de Nb. Assim como foi observado nas simulações exploratórias, o Nb tendeu a promover efeitos de redução da primeira temperatura de solubilização, partindo de 1280ºC para uma liga sem adição de Nb até 1130ºC para uma liga com 1% Nb. Porém, trata-se de temperaturas de solubilização elevadas para este caso e, portanto, mesmo com a redução desta temperatura de solubilização, afirmar que somente o nióbio evitará um crescimento de grão excessivo pode não ser conclusivo. Porém, não houvel alterações da segunda temperatura de solubilização em função do teor de Nb, mantendo-se em 730ºC. Com relação a fase sigma (ð), amplamente estudada pela literatura, a adição de nióbio não promoveu alterações quanto ao seu início de precipitação. O modo de solidificação encontrado nas simulações com teor de Nb de 0 a 4% (fração mássica), indicaram a sequência de solidificação como modo FA pela formação inicial de dendritas de ferrita, na qual a formação de austenita se dará a partir do líquido enriquecido com nitrogênio, já que esse nitrogênio foi segregado para o líquido quando da formação da ferrita. Já as simulações para 0,5 e 1% Nb indicara o modo F (ferrítico), típico dos aços inoxidáveis dúplex onde a austenita se formaria em transformação no estado sólido.
The main purpose of this work is the design of a lean duplex steel alloy with addition of nioubium, for the understanding of the phase transformations during solidification and solution annealing treatment, comparing the results obtained with the estimated results by thermodynamic simulations using Thermo-Calc® software. Phase identification was perfomed using optical microscopy, scanning electron microscopy (SEM), analysis by energy dispersive spectroscopy (EDS) and X-ray diffraction, and ferrite quantification was performed by magnetic measurements. According to the thermodynamic equilibrium simulation it was not possible to obtain a duplex structure composed of only 50% ferrite and 50% austenite, since the precipitation of nitrides and Z-phase from the liquid is predicted. However, unlike the simulations of phase equilibrium, Scheil simulations has not indicate the formation of Z-phase. One of the possible reasons for this absence of Z-phase may be related to the formation of nitrides and carbonitrides from the liquid, and therefore it can be an evidence that the use of such elements as chromium, nitrogen and niobium for the formation of these phases do not allow the formation of Z-phase in the solid state. Cosidering that Scheil simulations did not indicate Z-phase precipitation, it was expected that nitrogen-rich phases evidenced in the microstructural and EDS analysis were nitrides of Nb and not phase Z. As noted in exploratory simulations, Nb tended to reduce the first solution treatment temperature from 1280 °C for an alloy without the addition of Nb to 1130 °C for an alloy with 1% Nb. However, it is still a high annealing temperatures for this case, and therefore, even with this reduction in annealing temperature, the assumption that only the addition of niobium will prevent excessive grain growth by reduction of annealing temperature was not conclusive. However, there was no change in the second annealing temperature of the Nb content remained at 730 ° C. With respect to sigma phase, widely studied in the literature, the addition of niobium did not change the temperature for the beginning of sigma precipitation. The solidification mode found in simulations with Nb from 0 to 0.4% (mass fraction) showed solidification mode FA, with the initial formation of ferrite dendrites and the formation of austenite at the liquid enriched in nitrogen, as this nitrogen was segregated into the liquid when the formation of ferrite, while simulations of Nb alloys from 0.5 to 1% Nb indicated the F mode, typical of duplex stainless steels, in which only ferrite was formed from liquid, and austenite would be formed by transformation in the solid state.
The main purpose of this work is the design of a lean duplex steel alloy with addition of nioubium, for the understanding of the phase transformations during solidification and solution annealing treatment, comparing the results obtained with the estimated results by thermodynamic simulations using Thermo-Calc® software. Phase identification was perfomed using optical microscopy, scanning electron microscopy (SEM), analysis by energy dispersive spectroscopy (EDS) and X-ray diffraction, and ferrite quantification was performed by magnetic measurements. According to the thermodynamic equilibrium simulation it was not possible to obtain a duplex structure composed of only 50% ferrite and 50% austenite, since the precipitation of nitrides and Z-phase from the liquid is predicted. However, unlike the simulations of phase equilibrium, Scheil simulations has not indicate the formation of Z-phase. One of the possible reasons for this absence of Z-phase may be related to the formation of nitrides and carbonitrides from the liquid, and therefore it can be an evidence that the use of such elements as chromium, nitrogen and niobium for the formation of these phases do not allow the formation of Z-phase in the solid state. Cosidering that Scheil simulations did not indicate Z-phase precipitation, it was expected that nitrogen-rich phases evidenced in the microstructural and EDS analysis were nitrides of Nb and not phase Z. As noted in exploratory simulations, Nb tended to reduce the first solution treatment temperature from 1280 °C for an alloy without the addition of Nb to 1130 °C for an alloy with 1% Nb. However, it is still a high annealing temperatures for this case, and therefore, even with this reduction in annealing temperature, the assumption that only the addition of niobium will prevent excessive grain growth by reduction of annealing temperature was not conclusive. However, there was no change in the second annealing temperature of the Nb content remained at 730 ° C. With respect to sigma phase, widely studied in the literature, the addition of niobium did not change the temperature for the beginning of sigma precipitation. The solidification mode found in simulations with Nb from 0 to 0.4% (mass fraction) showed solidification mode FA, with the initial formation of ferrite dendrites and the formation of austenite at the liquid enriched in nitrogen, as this nitrogen was segregated into the liquid when the formation of ferrite, while simulations of Nb alloys from 0.5 to 1% Nb indicated the F mode, typical of duplex stainless steels, in which only ferrite was formed from liquid, and austenite would be formed by transformation in the solid state.