Estudo de caso da utilização de aços inoxidáveis para aplicação em recuperadores de calor na indústria siderúrgica
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Tipo de produção
Trabalho de Conclusão de Curso
Data
2021-12-08
Autores
Basso, Guilherme Arruda
Orientador
Santos, Daniella Caluscio dos
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Texto completo (DOI)
Palavras-chave
aços inoxidáveis,recuperadores de calor,fases deletérias,precipitados,tainless steels,heat recuperators,detrimental phases,precipitates
Resumo
Dentre as aplicações de aços inoxidáveis, existe a fabricação de tubos sem costura para trocadores de calor, como em recuperadores em siderúrgicas. Em uma planta siderúrgica, por exemplo, processos que envolvem temperaturas acima de 500ºC de operação favorecem a corrosão e, por esse motivo, utilizam-se aços inoxidáveis contendo, principalmente, maiores teores de Cr em sua composição química. Entretanto, em altas temperaturas, algumas transformações de fase podem levar à formação de fases deletérias capazes de comprometer o desempenho da liga desenvolvida para tal aplicação.
Dessa forma, a motivação deste trabalho é caracterizar a microestrutura das ligas AISI 446 e 253 MA após tratamentos isotérmicos de 15 dias, 2, 4 e 5 meses, a 600ºC e 700ºC, com foco na formação de fases secundárias, quantificadas por meio análises de imagens de microscopia óptica, utilizando o software Image J. A liga AISI 446 já é utilizada em recuperadores de calor, enquanto a liga 253 MA ainda é estudada para a mesma aplicação. Os materiais foram analisados por microscopia óptica e difração de raios-X. Acredita-se que, em ambas as ligas, as partículas de tamanhos maiores correspondam à fase sigma, enquanto as partículas menores correspondem a carbonetos do tipo M23C6 e nitretos do tipo Cr2N. Estes últimos, especialmente na liga AISI 446, são encontrados principalmente nos contornos de grãos. Também foi caracterizada a microestrutura das ligas AISI 446 e AISI 409 após anos de uso, e das ligas AISI 446 e 253 MA como fornecidas. Na liga AISI 446, provavelmente houve formação de sigma, juntamente com carbonetos e nitretos de Cr precipitados em matriz ferrítica. Essas fases secundárias já estavam presentes nessa liga como fornecida, mas em menores frações volumétricas e tamanhos. Por outro ado, a liga 253 MA, como fornecida, aparenta ser completamente austenítica, sem fases secundárias precipitadas. Na liga AISI 409, após anos de operação em recuperador de calor, foram encontrados apenas o que se acredita serem carbonetos (e, possivelmente, nitretos) de Ti em matriz ferrítica.
A 600ºC a formação de fases secundárias foi superior para a liga AISI 446, enquanto a 700ºC, é a liga 253 MA que apresenta mais fases secundárias formadas. Entretanto, as simulações termodinâmicas não se mostraram eficientes na descrição do comportamento de precipitação para as duas ligas, especialmente para a liga 253 MA a 700ºC, pois a quantidade de fases secundárias precipitadas foi mais expressiva que o previsto.
Após 5 meses, acredita-se que, para a liga 253 MA, a formação de fases secundárias atingiu um platô, sem precipitação adicional esperada. De forma contrária, para a liga AISI 446, após 4 meses, foi observada tendência de evolução de precipitação.
A formação de fases secundárias para ambas as ligas foi superior ao esperado e sugerido pela literatura. Após 5 meses, os valores de 19% e 35% foram obtidos para as ligas AISI 446 e 253 MA, respectivamente.
Among the applications of stainless steels, there is the fabrication of seamless tubes for heat exchangers, as in recuperators in steel plants. In a steel plant, for example, processes which involve working temperatures above 500ºC favors corrosion and, for this reason, stainless steels containing, especially, higher levels of Cr in its chemical composition are used. However, in high temperatures, some phase transformations can lead to the formation of detrimental phases capable of compromising the performance of the alloys designed for those applications. Therefore, the motivation for this work is to characterize the microstructure of AISI 446 and 253 MA alloys after isothermal heat treatments of 15 days, 2, 4 and 5 months, at 600ºC and 700ºC, focusing on the formation of secondary phases, evaluated by contrast, using Image J software. Alloy AISI 446 is already used in recuperators, while 253 MA alloy is still studied for the same application. The materials were analyzed by optical microscopy and X-ray diffraction. It is believed that, in both alloys, particles of bigger sizes correspond to sigma phase, while the small particles correspond to M23C6 carbides and Cr2N nitrides. The last ones, especially in AISI 446 alloy, are found mainly in grain boundaries. It was also characterized the microstructure of AISI 446 and AISI 409 alloys after years of use, and AISI 446 and 253 MA alloys as supplied. In AISI 446 alloy after years of operation in a recuperator, it was found what is believed to be sigma phase, alongside with Cr carbides and nitrides precipitated in ferritic matrix. These secondary phases were already found in this alloy as supplied, but in less volumetric fraction and smaller sizes. On the other hand, 253 MA alloy, as supplied, appears to be completely austenitic, with no secondary phases precipitated. In AISI 409 alloy, after years of operation in a recuperator, it was found only what is believed to be Ti carbides (and, possibly, nitrides) in ferritic matrix. At 600ºC the formation of secondary phases was higher for AISI 446 alloy, while at 700ºC, 253 MA presents more secondary phases formed. However, thermodynamical simulations were not efficient in describing the precipitation behavior for both alloys, especially for 253 MA alloy at 700ºC, since the amount of secondary phases precipitated were far more expressive than it was predicted. After 5 months, it is believed that, apparently, for 253 MA alloy, the formation of secondary phases has reached a plateau, with no more precipitation expected. On the other hand, for AISI 446 alloy, after 4 months, it was observed a tendency for precipitation evolution. The formation of secondary phases for both alloys was higher than expected and suggested by literature. After 5 months, the values of 19,03±1,53% and 34,98±2,74% were obtained for AISI 446 and 253 MA alloys, respectively.
Among the applications of stainless steels, there is the fabrication of seamless tubes for heat exchangers, as in recuperators in steel plants. In a steel plant, for example, processes which involve working temperatures above 500ºC favors corrosion and, for this reason, stainless steels containing, especially, higher levels of Cr in its chemical composition are used. However, in high temperatures, some phase transformations can lead to the formation of detrimental phases capable of compromising the performance of the alloys designed for those applications. Therefore, the motivation for this work is to characterize the microstructure of AISI 446 and 253 MA alloys after isothermal heat treatments of 15 days, 2, 4 and 5 months, at 600ºC and 700ºC, focusing on the formation of secondary phases, evaluated by contrast, using Image J software. Alloy AISI 446 is already used in recuperators, while 253 MA alloy is still studied for the same application. The materials were analyzed by optical microscopy and X-ray diffraction. It is believed that, in both alloys, particles of bigger sizes correspond to sigma phase, while the small particles correspond to M23C6 carbides and Cr2N nitrides. The last ones, especially in AISI 446 alloy, are found mainly in grain boundaries. It was also characterized the microstructure of AISI 446 and AISI 409 alloys after years of use, and AISI 446 and 253 MA alloys as supplied. In AISI 446 alloy after years of operation in a recuperator, it was found what is believed to be sigma phase, alongside with Cr carbides and nitrides precipitated in ferritic matrix. These secondary phases were already found in this alloy as supplied, but in less volumetric fraction and smaller sizes. On the other hand, 253 MA alloy, as supplied, appears to be completely austenitic, with no secondary phases precipitated. In AISI 409 alloy, after years of operation in a recuperator, it was found only what is believed to be Ti carbides (and, possibly, nitrides) in ferritic matrix. At 600ºC the formation of secondary phases was higher for AISI 446 alloy, while at 700ºC, 253 MA presents more secondary phases formed. However, thermodynamical simulations were not efficient in describing the precipitation behavior for both alloys, especially for 253 MA alloy at 700ºC, since the amount of secondary phases precipitated were far more expressive than it was predicted. After 5 months, it is believed that, apparently, for 253 MA alloy, the formation of secondary phases has reached a plateau, with no more precipitation expected. On the other hand, for AISI 446 alloy, after 4 months, it was observed a tendency for precipitation evolution. The formation of secondary phases for both alloys was higher than expected and suggested by literature. After 5 months, the values of 19,03±1,53% and 34,98±2,74% were obtained for AISI 446 and 253 MA alloys, respectively.