京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21434号 / エネ博第376号 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 今谷 勝次 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

<p>Materials to be used in thermal power plants have to resist creep deformation for time periods up to 30 years. The role of alloying elements for creep strength of 9-12% Cr steels is analysed. The creep strength in these steels relies on minor additions of alloying elements. Precipitates give rise to the main strengthening and remaining elements produce solid solution hardening. Nucleation, growth and coarsening of particles are predicted by thermodynamic modelling. Phase fractions and size distributions of M₂₃C₆ carbides, MX carbonitrides and Laves phase are presented. The size distributions are needed in order to determine the particle hardening during creep. At elevated temperatures the climb mobility is so high that the dislocations can climb across particles instead of passing by making Orowan loops.</p><p>By solving Fick's second law the concentration profile around a moving dislocation can be determined. The results show an accumulation of solutes around the dislocation that slows down dislocation movement. When Laves phase grows a decrease in creep strength is observed due to a larger loss in solid solution hardening than strength increase by particle hardening. Solid solution hardening also gives an explanation of the low dislocation climb mobility in 9-12% Cr steels.</p><p>Three different dislocation types are distinguished, free dislocations, immobile dislocation and immobile boundary dislocations. This distinction between types of dislocations is essential in understanding the decreasing creep with strain during primary creep. The empirical relation with subgrain size inversely proportional to stress has been possible to predict. The total creep strength can be predicted by adding the contribution from individual mechanisms.</p>

Materials used in thermal power plants have to resist creep deformation for time periods up to 30 years. Material evaluation is typically based on creep testing with a maximum duration of a few years. This information is used as input when empirically deriving models for creep. These kinds of models are of limited use when considering service conditions or compositions different from those in the experiments. In order to provide a more general model for creep, the mechanisms that give creep strength have to be identified and fundamentally described. By combining tools for thermodynamic modelling and modern dislocation theory the microstructure evolution during creep can be predicted and used as input in creep rate modelling. The model for creep has been utilised to clarify the influence of aluminium on creep strength as a part of the European COST538 action. The results show how AlN is formed at the expense of MX carbonitrides. The role of heat treatment during welding has been analysed. It has been shown that particles start to dissolve already at 800ºC, which is believed to be the main cause of Type IV cracking in commercial alloys. The creep strength of these steels relies on minor additions of alloying elements. Precipitates such as M23C6 carbides and MX carbonitrides give rise to the main strengthening, and remaining elements produce solid solution hardening. Particle growth, coarsening and dissolution have been evaluated. By considering dislocation climb it is possible to determine particle strengthening at high temperatures and long-term service. Transient creep is predicted by considering different types of dislocations. Through the generation and recovery of dislocation densities an increase in work hardening during primary creep is achieved. The role of substructure is included through the composite model. Cavity nucleation and growth are analysed in order to explain the intergranular fracture and to estimate the ductility. / QC20100616

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Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Os aÃos Fe-Cr-Mo sÃo amplamente usados em tubulaÃÃes e componentes que estÃo constantemente sujeitos a processos de degradaÃÃo nas indÃstrias petroquÃmicas. A exposiÃÃo desses aÃos em ambientes agressivos por um perÃodo prolongado quando submetidos a elevadas temperaturas tem como consequÃncias alteraÃÃes microestruturais, precipitaÃÃes na forma de carbonetos que atuam nos aÃos afetando sua resistÃncia a fluÃncia e a corrosÃo. Os aÃos Fe-Cr-Mo possuem propriedades ferromagnÃticas que podem ser exploradas com o objetivo de fornecer informaÃÃes sobre essas alteraÃÃes microestruturais. Nesse trabalho foram analisadas as propriedades magnÃticas das ligas Fe-9Cr-XMo (X = 1%, 5%, 7% e 9%) e a relaÃÃo dessas propriedades com a formaÃÃo de precipitados. Para isso as amostras foram submetidas a tratamentos tÃrmicos de solubilizaÃÃo (com exceÃÃo da liga contendo 1% de Mo) e envelhecimento. As anÃlises das propriedades magnÃticas foram feitas usando ciclos de histerese obtidos pela tÃcnica de magnetometria de amostra vibrante. Os parÃmetros magnÃticos analisados foram o campo coercivo, a magnetizaÃÃo residual e a magnetizaÃÃo de saturaÃÃo. A anÃlise dos precipitados foi feita usando micrografias obtidas por microscopia eletrÃnica de varredura e espectros de EDS, alÃm de histogramas obtidos com auxÃlio do software ImageJ que mostram a distribuiÃÃo e o tamanho mÃdio de precipitados. Os resultados obtidos permitem concluir que existe uma relaÃÃo entre a distribuiÃÃo e tamanho mÃdio dos precipitados com o campo coercivo. Em geral, o aumento na densidade de precipitados contribui de forma significativa para o aumento na dureza magnÃtica do material, esse mecanismo està relacionado a um processo de ancoragem das paredes de domÃnio magnÃtico. A magnetizaÃÃo de saturaÃÃo à afetada tanto pela formaÃÃo de precipitados quanto pelo teor de molibdÃnio na liga. / Cr-Mo steels are widely used in pipes and components constantly subjected to degradation processes in the petrochemical industries. The exposure of these steels in harsh environments for extended periods when subjected to elevated temperatures lead to microstructural changes, such as carbide precipitation which affect its creep and corrosion resistance. Fe-Cr-Mo steels have ferromagnetic properties that can be investigated in order to provide information on these microstructural changes. In this work we analyzed the magnetic properties of Fe-9Cr-Xmo (X = 1%, 5%, 7% and 9%) alloys and the relation between these properties and related to the formation of precipitates. For this, the samples were first subjected to solution heat treatments (except alloy containing 1% Mo) and then aged. The analysis of the magnetic properties was carried out by hysteresis loops obtained by vibrating sample magnetometry. The magnetic parameters analyzed were the coercive field, the residual magnetization and the saturation magnetization. The precipitates were analyzed by scanning electron microscopy and EDS spectra, and their distribution and average size were calculated through histograms obtained by the ImageJ software. The results indicate that the distribution and average size of the precipitates are related to the coercive field. In general, the increase in the density of precipitates contributes signicantly to the increase in the magnetic hardness of the material. This mechanism is related to a process of pinning the magnetic domain walls. The saturation magnetization is affected both by the formation of precipitates and by the molybdenum content in the alloy.

Made available in DSpace on 2014-10-09T12:52:01Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:53Z (GMT). No. of bitstreams: 0 / A difração de raios X e de elétrons retroespalhados, em escalas distintas, foram aplicadas para acompanhar mudanças microestruturais em dois aços ferríticos baixo carbono (2%Si e ABNT 1006), observando-se o desenvolvimento da textura nas etapas de laminação a frio (skin-pass) e recozimento a 760 oC. Para ambos os aços, os resultados permitiram afirmar que o fenômeno que ocorre após a laminação por skin-pass e tratamento térmico, nas condições deste trabalho, é a recristalização primária. A aplicação do skin-pass criou mais discordâncias em grãos com baixo fator de Taylor por sofrerem mais deformações. Observou-se a nucleação e crescimento de grãos em regiões de grande densidade de células de discordâncias. A presença do silício atrasou a recuperação favorecendo o aumento de subcontornos. Não foi observado o crescimento anormal de grãos, nem em função da presença de grãos Goss. Os contornos CSL não garantiram o crescimento do grão. Núcleos de crescimento de grãos deram origem a grãos com orientações distintas, indicando que o crescimento de grãos não dependeu da existência prévia de grãos com as orientações desenvolvidas, demonstrando que o crescimento anormal de grãos não está relacionado necessariamente com os grãos Goss. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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