ResumoA utilização de tubos de aço C-Mn na indústria de óleo e gás é muito comum desde o nascimento desta indústria. Mesmo com a grande evolução da metalurgia destes aços ao longo das últimas décadas, o desafio de desempenho em ambientes severos de corrosão devido às características reativas (H 2 S, CO 2 ) do petróleo e seus derivados, impõem uma nova era para a fabricação de tubos. A solução tecnológica mais direta seria a utilização de aços com alta liga (Ni, Cr) e/ou superligas de níquel. No entanto, os custos de produção tornariam inviáveis pelo alto custo destes materiais. Neste contexto, é crescente iniciativas de pesquisas para o desenvolvimento de overlay e/ou cladding, ou seja, revestimentos de aços C-Mn usando superligas de níquel e aços inoxidáveis. Neste trabalho, foi utilizada a deposição de uma camada da superliga de níquel Inconel 625 através do processo laser. Uma das desvantagens da deposição a laser ou qualquer outro processo de fusão é a formação de uma região de descontinuidade microestrutural no aço, chamada de ZTA (zona termicamente afetada pelo calor). Neste estudo, diferentes rotas de tratamentos térmicos foram investigadas com o objetivo de eliminar a ZTA e preservar as características originais do substrato e do revestimento. Palavras-chave: Revestimento inconel 625/aço C-Mn; Perfil de microdureza; Transformação de fase; Martensita revenida; ZTA; Tratamentos térmicos. EFFECT OF DIFFERENT HEAT TREATMENTS ON MICROSTRUCTURE AND MICROHARDNESS OF A C-Mn STEEL/INCONEL 625 COATING SYSTEM AbstractThe application of C-Mn steel pipe in the oil & gas industry is quite common since starting of this industry. Even with the great evolution of the metallurgy of this type of steel over the last decades, the challenge is to overcome the corrosion severity caused by sour (H 2 S, CO 2 ) species present in the petroleum and its derivates. The direct technological solution would be to replace the C-Mn steel by Ni-based superalloys and/or stainless steels. However, the high costs of these materials would make several projects impracticable. In this context, there are several initiatives in order to develop overlay and/or cladding, that means, coatings of C-Mn steels using superalloys and stainless steels. In this work, it was used an overlay deposition of Inconel 625 on the surface of a C-Mn steel using the laser process. A disadvantage of laser deposition, as it is for any deposition melting process, is the formation of a microstructural discontinuity in the steel substrate caused by the thermal cycle (HAZ = heat affected zone). In this study, different heat treatment routes were investigated aiming to eliminate the HAZ preserving the original characteristics of the substrate and of the coating.
The superficial coatings in micro-alloyed steel pipes has been a pointed way to decrease the corrosion problems in oil and gas industry. However, little emphasis has been given to the substrate. The effects of the deposition method on the steel microstructure and properties are still not well described. In this context, this work studied the effect of Ni superalloys clads on the phase transformations, microstructure and hardness of the heat-affected zone (HAZ) of an API steel. The underestimate of the HAZ might be dangerous, since, despite the coating good corrosion performance, the substrate HAZ may present a weak region, which may lead to an in-service coating tearing. In this work, Inconel 625 and Hastelloy C276 superalloys were clad on the steel surface by a laser deposition. Dilatometry, optical and scanning electron microscopy, and computational simulation were applied. The studied steel was originally constituted by tempered martensite. The austentizing temperature had a strong influence on the austenite grain size and on the steel CCT diagram. Due to that, the laser superalloy deposits promoted a complex HAZ, where grain growth occurred. A post-cladding heat treatment was proposed to homogenize the steel microstructure and to decrease the hardness gradient at the superalloy-steel interface.
ResumoA aplicação de materiais metálicos exige que os mesmos sejam submetidos a etapas de conformação mecânica. Estes procedimentos elevam significativamente a densidade de descontinuidades, alterando a morfologia dos grãos, assim como o estado de tensão induzida na estrutura. Os materiais submetidos a processos de conformação, em especial aos de conformação a frio, são submetidos, após a deformação, a um tratamento térmico denominado de recozimento de recristalização, cujo objetivo é recuperar a estrutura e recristalizar os grãos. Neste contexto, estudou-se a viabilidade de aplicação da técnica de calorimetria diferencial (DSC), como ferramenta para a determinação da temperatura de início de recristalização de aços. Para a execução deste trabalho, foram utilizados amostras de aço SAE1010 que foram deformadas a frio e em seguida submetidas a rampas de aquecimento em um equipamento DSC. A técnica de DSC evidenciou apenas o início do fenômeno de recuperação do material. Algumas amostras deformadas foram submetidas a tratamentos térmicos de recozimento de recristalização a 500°C, 600°C e 700°C. A partir da caracterização microestrutural e de ensaios de microdureza, calculou-se os valores de k e n do modelo de Avrami para as amostras tratadas a 600°C e 700°C, obtendo-se após isto a energia de ativação para as deformações de 69,6% (199,5KJ) e 87% (178,4KJ). À 500°C, as amostras não se recristalizaram, confirmando esta temperatura abaixo da crítica para as deformações estudadas. Palavras-chave: Deformação plástica; Recozimento de recristalização; DSC. EVALUATION OF DSC APPLICATION AIMING TO DETERMINE THE RECRYSTALLIZATION TEMPERATURE IN A LOW CARBON STEEL AbstractMany applications of metallic materials require their submission to mechanical forming steps in order to adapt their geometries. The mechanical forming of metals consists in their plastic strain, increasing discontinuity densities, changing grain morphology as well inducing an internal stress state. The materials submitted to forming processes, particularly cold forming, are heat treated after cold strain. This heat treatment is named annealing and its main objective is to recrystallize the structure. In this context, this work studied the application of differential calorimeter (DSC) aiming to determine the temperature in which the steel recrystallization starts. Samples of steel SAE1010 were cold rolled and then submitted to slow heating in a DSC machine. The DSC technic was not able to present the start of recrystallization in steels, but showed only a phenomenon related to recovery. Other deformed samples were submitted to heat treatments of recrystallization annealing, at 500°C, 600°C and 700°C. From the microstructural characterization and microhardness testing, it was possible to study the recrystallization kinetics for samples heat treated at 600°C and 700°C. The k and n values of Avrami's model were calculated, obtaining after that the activation energy for 69.6% (199,5KJ) and 87% (178.4KJ) strains. At 500°C, the samples were not recrystallized, conf...
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