Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Pouco se sabe a respeito das consequências da operação de reparo nas propriedades mecânicas e metalúrgicas das juntas do aço ASTM A335 grau P91 soldadas por Arame Tubular. Neste trabalho foi realizado um estudo para analisar e caracterizar juntas de um tubo do aço P91 soldadas pelo processo Arame Tubular com proteção gasosa, antes e após situação de duplo reparo. Foram utilizadas técnicas de microscopia eletrônica de varredura (MEV), análise química semiquantitativa por EDS (Energy Dispersive Spectroscopy), microscopia óptica (MO), além da obtenção de medidas de microdureza Vickers e realização de ensaios de impacto e tração à temperatura ambiente. A macrografia revelou a existência de uma nova zona termicamente afetada formada no centro da junta reparada e a análise micrográfica mostrou que esta zona apresentou uma pequena tendência a formação de grãos menores do que na zona de fusão que sofreu reparo, além disso, a região tinha grãos colunares que não são característicos de uma zona termicamente afetada e quantidade de carbonetos menor comparativamente. Não houve variações significativas nos valores de tração e microdureza. Os ensaios de impacto revelaram baixos valores de energia absorvida na zona de fusão do 2 reparo, no entanto, todas as outras zonas avaliadas das juntas com e sem reparo apresentaram valores dentro das especificações. / Little is known about the consequences of the repair operation on the mechanical and metallurgical properties of ASTM A335 grade P91 steel joints welded by Flux-Cored Arc Welding (FCAW). In this work, a study was made to analyze and characterize joints of P91 steel tube, welded by FCAW with shielding gas, before and after double repair situation. Techniques of scanning electron microscopy (SEM), chemical analysis by semi-quantitative EDS (Energy Dispersive Spectroscopy) and optical microscopy (OM) were used, in addition, Vickers microhardness measurements, impact and tensile tests at room temperature, were carried out. The macrography revealed the existence of a new heat affected zone formed in the center of the repaired joint and the micrographic analysis showed that this zone has a tendency to formation of smaller grains compared to the repaired fusion zone, furthermore, the region has columnar grains which are not characteristics of a heat affected zone and comparatively lesser amounts of carbides. There were no significant variations in the tensile and hardness values. Impact tests showed low values of absorbed energy in the fusion zone of the second repair; however, all other evaluated zones, with and without repair, had values within specifications.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Pouco se sabe a respeito das consequências da operação de reparo nas propriedades mecânicas e metalúrgicas das juntas do aço ASTM A335 grau P91 soldadas por Arame Tubular. Neste trabalho foi realizado um estudo para analisar e caracterizar juntas de um tubo do aço P91 soldadas pelo processo Arame Tubular com proteção gasosa, antes e após situação de duplo reparo. Foram utilizadas técnicas de microscopia eletrônica de varredura (MEV), análise química semiquantitativa por EDS (Energy Dispersive Spectroscopy), microscopia óptica (MO), além da obtenção de medidas de microdureza Vickers e realização de ensaios de impacto e tração à temperatura ambiente. A macrografia revelou a existência de uma nova zona termicamente afetada formada no centro da junta reparada e a análise micrográfica mostrou que esta zona apresentou uma pequena tendência a formação de grãos menores do que na zona de fusão que sofreu reparo, além disso, a região tinha grãos colunares que não são característicos de uma zona termicamente afetada e quantidade de carbonetos menor comparativamente. Não houve variações significativas nos valores de tração e microdureza. Os ensaios de impacto revelaram baixos valores de energia absorvida na zona de fusão do 2 reparo, no entanto, todas as outras zonas avaliadas das juntas com e sem reparo apresentaram valores dentro das especificações. / Little is known about the consequences of the repair operation on the mechanical and metallurgical properties of ASTM A335 grade P91 steel joints welded by Flux-Cored Arc Welding (FCAW). In this work, a study was made to analyze and characterize joints of P91 steel tube, welded by FCAW with shielding gas, before and after double repair situation. Techniques of scanning electron microscopy (SEM), chemical analysis by semi-quantitative EDS (Energy Dispersive Spectroscopy) and optical microscopy (OM) were used, in addition, Vickers microhardness measurements, impact and tensile tests at room temperature, were carried out. The macrography revealed the existence of a new heat affected zone formed in the center of the repaired joint and the micrographic analysis showed that this zone has a tendency to formation of smaller grains compared to the repaired fusion zone, furthermore, the region has columnar grains which are not characteristics of a heat affected zone and comparatively lesser amounts of carbides. There were no significant variations in the tensile and hardness values. Impact tests showed low values of absorbed energy in the fusion zone of the second repair; however, all other evaluated zones, with and without repair, had values within specifications.

This thesis deals with a high temperature testing of fracture toughness and studies the size effect on measured values using miniature size CT specimen. Two types of specimen geometry were manufactured from P91 steel, the standard size and the quarter size specimen. J-R curves were obtained in the temperature range from 23°C to 600°C. No specimen size effect was observed at room temperature tests. The realized experiments together with fractography analysis demonstrated the drop of toughness at 400°C caused by the effect of dynamic strain aging.

The thesis deals with the determination of fracture toughness using compact tension (CT) test specimens at elevated and high temperatures. The experimental material steel P91 designated for application at temperatures 550650°C was used. The fracture toughness in the ductile fracture region of the steel was characterized by the R curve, which characterises the resistance against crack propagation depending on the crack length. The effect of temperature on the R curve at range 23600°C was evaluated. Next the specimen size effect using three sizes of CT specimens at 23°C and the effect of loading rate (2, 0,2 a 0,02 mm/min) at 600°C was examined. The results showed that the temperature has distinct effect on the R-curve, which yields minimal values at 400°C. Only the smallest test specimen size with thickness 6.25mm showed the specimen size effect giving about 10% lower values of toughness comparing to larger specimens. The effect of loading rate was clearly distinguishable. The values of toughness varied about 20% of the toughness value comparing individual loading rate.

The thesis is focused on the prediction of the fracture toughness temperature dependence through a universal curve of fracture toughness (also known as the master curve). To determine the parameters of the universal curve of fracture toughness, values acquired from the measurement results of fracture toughness and tensile tests of structural steel P91 are used. The theoretical part is based on a summary of the relevant information from the field of fracture mechanics and brittle-ductile fracture behavior of steels that are important for the understanding of fracture-strain response of materials depending on load conditions. The experimental part of the thesis contains the results from practical measurements and analyses, which were used for determining the parameters of a universal curve of fracture toughness as well as for the evaluation of fracture behavior and description of the impact of structural parameters on this behavior in case of steel P91.