Cinética de crescimento de grão na solubilização de um aço inoxidável dúplex
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Tipo de produção
Dissertação
Data
2016
Autores
Leandro, Rafael Malagutti
Orientador
Magnabosco, R.
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Citação
LEANDRO, Rafael Malagutti. Cinética de crescimento de grão na solubilização de um aço inoxidável dúplex. 2016. 113 f. Dissertação (Mestrado em Engenharia Mecânica) - Centro Universitário FEI, São Bernardo do Campo, 2016 Disponível em: . Acesso em: 11 out. 2018.
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Palavras-chave
Aço inoxidável,Transformação de fases,Cinética de crescimento de grão
Resumo
A motivação principal desta dissertação é compreender as transformações de fase que ocorrem durante a solubilização de um aço inoxidável dúplex, avaliando a cinética de obtenção de fases e de crescimento de grão, que são as principais variáveis para a obtenção da desejada microestrutura dúplex. A princípio, o aço foi conformado mecanicamente em laminador de planos para no mínimo 50% de redução de espessura, conferindo microestrutura intensamente deformada, com grande número de locais para nucleação heterogênea das fases ferrita e austenita durante os tratamentos de solubilização subsequentes. Foram produzidas, após a laminação, três séries de amostras a 1050°C, 1100°C e 1150°C, por tempos de 15 minutos a 96 horas, visando a obtenção de diferentes tamanhos de grão de austenita e ferrita, permitindo a avaliação da temperatura e do tempo de solubilização na formação da estrutura dúplex ferritaaustenita. O tamanho de grão das amostras foi determinado por um método manual de medição do tamanho médio de intercepto. Para auxiliar na caracterização da microestrutura, as áreas de interface (Sv) entre ferrita/ferrita, ferrita/austenita e austenita/austenita foram também determinadas por método manual. Obteve-se valores médios de intercepto da estrutura bifásica
como um todo e das fases austenita e ferrita separadamente. Nos menores tempos e temperaturas de solubilização, o tamanho de grão encontrado é semelhante ao da amostra antes da deformação. Portanto, em todos os tempos e temperaturas estudadas completaram-se os processos de recuperação e recristalização. Os valores de expoente de crescimento de grão (n) obtidos foram menores que 0,5, sendo diferentes entre si nas equações que descrevem a cinética de crescimento de grão da estrutura geral, da austenita e da ferrita. A medição do tamanho médio de intercepto permitiu a detecção de diferenças entre as fases quanto à cinética de
crescimento de grão, pois o valor de n apresentou tendência de aumento para austenita e redução para a ferrita quando se elevou a temperatura de tratamento, além de ter sido determinado um tempo de transição onde a cinética de crescimento de grão da austenita se torna maior que a da ferrita. As medidas de intercepto também levaram a diferenças morfológicas entre as amostras (evidenciadas por diferentes razões de aspecto, parâmetros dúplex, de dispersão e razões de contiguidade). As medidas de Sv demonstraram que a redução do número de interfaces é causada pelo aumento do tamanho médio de grão da estrutura dúplex, ou seja, a área de superfície diminui com o aumento da temperatura e do tempo de solubilização. As energias de ativação obtidas para o crescimento de grão da estrutura geral, austenítica e ferrítica foram respectivamente 65 kJ/mol; 55,4 kJ/mol e 77,8 kJ/mol, o que permitiu concluir que a ferrita é a fase que comanda o crescimento de grão global da estrutura bifásica, devido à maior energia de ativação requisitada para o crescimento do grão ferrítico.
The main motivation of this dissertation is the understanding of the phase transformation involved during a solution heat treatment of a duplex stainless steel, evaluating the kinetics of phase formation and grain growth, which are the key variables to achieve the duplex microstructure. The alloy was cold rolled to at least a reduction in thickness of 50%, resulting in an intensely deformed microstructure with a large number of sites for heterogeneous nucleation of ferrite and austenite phases during later solution treatments. After cold-rolling, three series of samples were subjected to solution treatment at 1050°C, 1100°C and 1150°C for periods varying from 15 minutes to 96 hours, seeking for different phase content and grain sizes of austenite and ferrite. Hence, the influence of temperature and time on the formation of duplex microstructure could be verified. The grain size of the specimen was obtained by manual measurement of the mean intercept length. To provide a better characterization of the microstructure, the surface areas (Sv) between ferrite/ferrite, ferrite/austenite and austenite/austenite were determined by a manual evaluation method. Intercept mean values were determined for the two-phase structure as well as for austenite and ferrite phases separately. In the lowest times and temperatures of solution heat treatment, the obtained grain size was similar to the one found in the material prior to cold-rolling. Therefore, all the times and temperatures studied in this work led to the processes of recovery and recrystallization. The values of grain growth exponent (n) obtained were smaller than 0.5 and possess different values among each other, considering the equations which describe the grain growth kinetics of duplex, austenitic and ferritic structures. The mean intercept length measurement has also allowed the detection of differences between the phases in respect to the grain growth kinetics, because n tended to increase for austenite and reduce for ferrite when the temperature got higher. Moreover, it was determined a transition time where the grain growth kinetics of austenite becomes higher than the one found for ferrite. The intercept length measurements either conducted to morphological differences among specimen (evidenced by different aspect ratio, duplex parameter, dispersion parameter and contiguity ratio). The Sv measurements have shown that the reduction in the number of interfaces is due to the increase of the mean grain size of the duplex structure, that is, the surface area diminishes with the increase in the temperature and time of the solution heat treatment. The activation energies determined for the global, austenitic and ferritic grain growth were respectively 65 kJ/mol; 55.4 kJ/mol and 77.8 kJ/mol, which has led to the conclusion that ferrite is the phase that commands the global grain growth of the two-phase structure, taking into account the highest activation energy required to the ferrite grain growth.
The main motivation of this dissertation is the understanding of the phase transformation involved during a solution heat treatment of a duplex stainless steel, evaluating the kinetics of phase formation and grain growth, which are the key variables to achieve the duplex microstructure. The alloy was cold rolled to at least a reduction in thickness of 50%, resulting in an intensely deformed microstructure with a large number of sites for heterogeneous nucleation of ferrite and austenite phases during later solution treatments. After cold-rolling, three series of samples were subjected to solution treatment at 1050°C, 1100°C and 1150°C for periods varying from 15 minutes to 96 hours, seeking for different phase content and grain sizes of austenite and ferrite. Hence, the influence of temperature and time on the formation of duplex microstructure could be verified. The grain size of the specimen was obtained by manual measurement of the mean intercept length. To provide a better characterization of the microstructure, the surface areas (Sv) between ferrite/ferrite, ferrite/austenite and austenite/austenite were determined by a manual evaluation method. Intercept mean values were determined for the two-phase structure as well as for austenite and ferrite phases separately. In the lowest times and temperatures of solution heat treatment, the obtained grain size was similar to the one found in the material prior to cold-rolling. Therefore, all the times and temperatures studied in this work led to the processes of recovery and recrystallization. The values of grain growth exponent (n) obtained were smaller than 0.5 and possess different values among each other, considering the equations which describe the grain growth kinetics of duplex, austenitic and ferritic structures. The mean intercept length measurement has also allowed the detection of differences between the phases in respect to the grain growth kinetics, because n tended to increase for austenite and reduce for ferrite when the temperature got higher. Moreover, it was determined a transition time where the grain growth kinetics of austenite becomes higher than the one found for ferrite. The intercept length measurements either conducted to morphological differences among specimen (evidenced by different aspect ratio, duplex parameter, dispersion parameter and contiguity ratio). The Sv measurements have shown that the reduction in the number of interfaces is due to the increase of the mean grain size of the duplex structure, that is, the surface area diminishes with the increase in the temperature and time of the solution heat treatment. The activation energies determined for the global, austenitic and ferritic grain growth were respectively 65 kJ/mol; 55.4 kJ/mol and 77.8 kJ/mol, which has led to the conclusion that ferrite is the phase that commands the global grain growth of the two-phase structure, taking into account the highest activation energy required to the ferrite grain growth.