Avaliação da possibilidade de aplicação de tratamentos térmicos de múltiplas normalizações como mecanismo de refino microestrutural e incremento de propriedades mecânicas em aços API-OCTG
Evaluation of multiple normalizing heat treatments as mechanism of microstructural refinement and mechanical properties improvement in API-OCTG steels
José Márcio da Rocha, Junia Ananias, Geraldo Lúcio de Faria
Resumo
Os aços API-OCTG graus K55 e T95 são importantes materiais de engenharia atualmente especificados para fabricação de tubos sem costura para aplicação nos processos de extração de óleo e gás. Considerando a crescente demanda destes produtos com cada vez melhores relações entre resistência mecânica e tenacidade ao impacto, este trabalho se propôs a investigar os efeitos de tratamentos térmicos de múltiplas normalizações sobre a evolução microestrutural e as propriedades mecânicas de dois aços com composições químicas que atendem os requisitos dos referidos graus. Simulações termodinâmicas computacionais e ensaios dilatométricos foram utilizados para se determinar temperaturas críticas de austenitização e a susceptibilidade à formação de carbonetos nos aços estudados. Sucessivos tratamentos térmicos de normalização foram planejados e executados. Por meio da aplicação de técnicas de caracterização microestrutural (MEV-STEM) e de ensaios mecânicos de dureza, tração e impacto Charpy, mostrou-se que os tratamentos térmicos de múltiplas normalizações tiveram efeito positivo sobre o refino microestrutural dos dois aços. Os efeitos mais significativos foram observados até o segundo ciclo de normalização, sendo pequenas as diferenças microestruturais e de propriedades mecânicas entre o segundo e o terceiro ciclo. O efeito das múltiplas normalizações foi mais pronunciado para o grau T95, onde a diminuição do tamanho de grão austenítico prévio, as redistribuições de soluto e de precipitados propiciaram melhoria significativa das propriedades mecânicas.
Palavras-chave
Abstract
The API-OCTG steels grades K55 and T95 are important engineering materials specified, nowadays, as raw-material for the manufacturing of seamless pipes used in oil and gas extraction processes. Considering the growing demand for these products with increasingly better mechanical strength and impact toughness ratio, this work proposed to investigate the effects of multiple normalizing heat treatments on the microstructural evolution and mechanical properties of two steels with chemical compositions that meet the requirements of the abovementioned grades. Computational thermodynamic simulations and dilatometric tests were carried out to determine critical austenitizing temperatures and the steel susceptibility to carbide formation. Successive normalizing heat treatments were planned and executed. Applying microstructural characterization techniques (MEV-STEM) and mechanical tests (hardness, tensile test, Charpy impact test), it was shown that multiple normalizing heat treatments had a positive effect on the microstructural refining of the both studied steels. The most significant effects were observed until to the second normalizing cycle. There were not great differences between microstructure and mechanical properties measured for the second and the third cycles. This effect was more pronounced for grade T95, where the decrease of the previous austenitic grain size, solute and precipitate redistributions provided significant improvements in mechanical properties.
Keywords
Referências
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Submetido em:
22/11/2021
Aceito em:
15/06/2022
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