Avaliação microestrutural e do comportamento termomecânico do aço inoxidável HK-30 conformado por gelcasting
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Dissertação
Data
2015
Autores
Oliveira, L. F. R.
Orientador
Ortega, Fernando dos Santos
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OLIVEIRA, L. F. R. Avaliação microestrutural e do comportamento termomecânico do aço inoxidável HK-30 conformado por gelcasting. 2015. 86 f. Dissertação (Mestrado em Engenharia Mecânica) - Centro Universitário da FEI, São Bernardo do Campo, 2015 Disponível em: . Acesso em: 24 set. 2015.
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Palavras-chave
Aço inoxidável,Gelcasting de metais,Tensão mecânica
Resumo
O processo gelcasting consiste em uma técnica de conformação originalmente desenvolvido para materiais cerâmicos, baseado em uma suspensão concentrada de material particulado em uma solução aquosa que pode ser moldada e posteriormente, através de uma reação química, gelifica no interior do molde, resultando em uma peça rígida e com boa resistência mecânica. Embora tenha sido desenvolvido há mais de 20 anos, há poucos registros do processo gelcasting
aplicado à metalurgia do pó. Além disso, ao ser adaptado para sistemas de partículas metálicas, torna-se necessário lidar com a maior tendência à sedimentação desses pós, uma vez que se tratam de partículas maiores e mais densas que partículas de materiais cerâmicos. Nesse sentido, este trabalho verifica a eventual ocorrência de sedimentação no período que precede a gelificação, o que poderia gerar um gradiente de compactação nas peças com efeitos indesejados sobre suas propriedades mecânicas. Para isso, amostras cilíndricas foram moldadas com uma altura de 10 cm, utilizando um pó de liga de aço inoxidável para aplicações em alta temperatura (HK-30) com duas granulometrias distintas (10F, D50=7?m e 20F, D50=12 ?m). Após sinterização em forno a vácuo, as peças foram seccionadas e amostras foram extraídas das regiões mais alta (topo) e mais baixa (base) para avaliação de densidade e caracterização microestrutral. Adicionalmente, a tensão de escoamento foi avaliada através de ensaio de compressão em temperatura ambiente e a 800oC. Os resultados mostraram que houve excelente densificação de todas as amostras. Entretanto, a avaliação microestrutural revelou a existência de certa porosidade residual para as amostras processadas a partir do pó com granulometria 10F. As propriedades mecânicas medidas em temperatura ambiente e a 800oC foram
compatíveis às disponíveis na literatura para a mesma liga processada por fundição, para as amostras produzidas a partir do pó com granulometria 20F. No entanto, para as amostras produzidas com o pó 10F verificaram-se valores de tensão de escoamento 50% menores, apenas para os testes conduzidos a 800oC, o que foi atribuído à maior presença de poros na microestrutura. Em todos os casos, não foi identificada qualquer diferença significativa de densidade ou de tensão de escoamento entre as amostras extraídas do topo e da base do cilindro, comprovando a hipótese de que foi possível processar os pós de aço HK-30 através da técnica gelcasting evitando-se de maneira satisfatória a ocorrência de segregação de partículas e heterogeneidade microestrutural.
The gelcasting process is a forming technique originally developed for ceramic materials, based on a highly concentrated suspension of particulate material in an aqueous solution that could be mold and then, through a chemical reaction, gels inside the mold, resulting in a rigid part with good mechanical resistance. Although it had been developed for over 20 years ago, there are a few records of the gelcasting process applied to powder metallurgy. Furthermore, the adaptation to metallic particles, it is necessary to deal with the increased tendency for the particles settling, since these ones have higher density and size than ceramic materials particles. In this sense, this study verifies the possible occurrence of settling in the period before the gelation step, what could generate a compaction gradient on the parts with undesired effects on their mechanical properties. For this, cylindrical samples with 10 cm height were molded, using a stainless steel alloy powder for high temperature application (HK-30) with two different particle sizes (10F, D50=7?m e 20F, D50=12 ?m). After the sintering step on a vacuum atmosphere furnace, the parts were cut and samples were taken from the top and bottom of it for density evaluation and microstructural characterization. Additionally, the yield stress was measured using compression tests under room temperature and 800°C. The results has showed that the densification was well reached on all the samples. However, the microstructural evaluation pointed out the existence of residual porosity on samples made by the 10F particle size powder. The mechanical properties measured at room temperature and at 800°C were similar to those reported in literature for the same alloy processed by casting, for the samples made by the 20F particle size powder. However, for the samples made by the 10F particle size powder, the yield stress values were 50% fewer only on the 800°C tests, which was attributed to the increased presence of pores in the microstructure. In all cases, it was not identified any significant difference of density or yield stress between the samples taken from the top and the bottom of the parts, proving the hypothesis that the HK-30 stainless steel gelcasting process was possible avoiding the particles segregation occurrence satisfactorily and both the microstructural heterogeneity.
The gelcasting process is a forming technique originally developed for ceramic materials, based on a highly concentrated suspension of particulate material in an aqueous solution that could be mold and then, through a chemical reaction, gels inside the mold, resulting in a rigid part with good mechanical resistance. Although it had been developed for over 20 years ago, there are a few records of the gelcasting process applied to powder metallurgy. Furthermore, the adaptation to metallic particles, it is necessary to deal with the increased tendency for the particles settling, since these ones have higher density and size than ceramic materials particles. In this sense, this study verifies the possible occurrence of settling in the period before the gelation step, what could generate a compaction gradient on the parts with undesired effects on their mechanical properties. For this, cylindrical samples with 10 cm height were molded, using a stainless steel alloy powder for high temperature application (HK-30) with two different particle sizes (10F, D50=7?m e 20F, D50=12 ?m). After the sintering step on a vacuum atmosphere furnace, the parts were cut and samples were taken from the top and bottom of it for density evaluation and microstructural characterization. Additionally, the yield stress was measured using compression tests under room temperature and 800°C. The results has showed that the densification was well reached on all the samples. However, the microstructural evaluation pointed out the existence of residual porosity on samples made by the 10F particle size powder. The mechanical properties measured at room temperature and at 800°C were similar to those reported in literature for the same alloy processed by casting, for the samples made by the 20F particle size powder. However, for the samples made by the 10F particle size powder, the yield stress values were 50% fewer only on the 800°C tests, which was attributed to the increased presence of pores in the microstructure. In all cases, it was not identified any significant difference of density or yield stress between the samples taken from the top and the bottom of the parts, proving the hypothesis that the HK-30 stainless steel gelcasting process was possible avoiding the particles segregation occurrence satisfactorily and both the microstructural heterogeneity.