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

Diffusion couples between nickel and S.A. 212 ferritic steel and between nickel and AISI 304 austenitic stainless steel were studied to determine the effect of nickel on the structure of these steels after diffusion anneals at 1300 ℉. Diffusion times varied from 50 to 4000 hours. The migration of nickel resulted in the formation of a martensitic band between nickel and S.A. 212 ferritic steel and an austenitic band between nickel and AISI 304 austenitic stainless steel. The width of the bands increased exponentially with the time of annealing. The band width increased faster in nickel-S.A. 212 couple than in nickel-AISI 304 couple. Hardness values were obtained within the band of both diffusion couples and varied across the band. Generally, the hardness was greatest in the band. In the nickel-AISI 304 diffusion couple, chromium carbides were observed in the nickel plate after diffusion. / M.S.

Dans le cadre du développement des réacteurs nucléaires de 4ème génération et plus particulièrement du réacteur à neutrons rapides refroidi au sodium (SFR), le CO2 supercritique, dans un cycle de Brayton, a été identifié comme fluide potentiel en remplacement de la vapeur d'eau dans le cycle de conversion de l'énergie. Les aciers ferrito-martensitiques contenant 9 à 12 % en poids de Cr sont de bons candidats pour la réalisation d'échangeurs thermiques car ils présentent de bonnes propriétés mécaniques jusqu'à une température de 600°C, une forte conductivité thermique, un faible coefficient d'expansion thermique ainsi qu'un coût plus faible que celui des aciers austénitiques. Cependant, il a été montré que ces aciers forment une couche d’oxyde à croissance rapide et carburent fortement dans les conditions du circuit de conversion d'énergie (550°C, 250 bar).Cette étude a pour objectif d’étudier l’influence de différents paramètres (les impuretés présentes dans le CO2, les vitesses de rampe thermique ainsi que l’état de surface) sur le mécanisme d’oxydation d’un acier Fe-12Cr dans le CO2 à 550°C. Il est montré qu’en fonction de la valeur de ces paramètres, il est possible de former une couche d’oxyde fine protectrice en surface sans carburation. Un modèle permettant de rendre compte de l’ensemble des résultats expérimentaux est proposé. / In the framework of the development of Sodium Fast Reactors in France, supercritical carbon dioxide integrated in the Brayton cycle is proposed as new cycle energy conversion system to replace current steam generators. Ferritic-Martensitic steels with 9-12 wt% Cr are good candidates for heat exchanger application because they have good mechanical properties up to a temperature of 600°C, a high thermal conductivity, a low coefficient of thermal expansion and a lower cost than that of austenitic steels. However, it has been found that these steels present a high parabolic oxide growth rate and a strong carburization in the temperature and pressure conditions of the SC-CO2 cycle (550°C, 250 bar).This study aims to investigate the influence of different parameters (impurities present in CO2, thermal ramp rate and surface state) on the oxidation mechanism of a Fe-12 Cr steel in CO2 at 550°C. It has been shown that depending on these parameters, a thin protective oxide scale without any strong carburization can be obtained. A model is proposed to explain the experimental results.

Orientador: Vicente Afonso Ventrella / Resumo: Incluído na família dos Ferríticos, o aço Inox AISI 430 é um material importante sob os aspectos econômico e tecnológico, onde, atualmente passaram a ser utilizados cada vez mais nas indústrias de diversas áreas, como: automobilística, química e de aparelhos domésticos, perante a grande possibilidade de aplicação deste material. Em geral, esse material apresenta uma baixa sᴏldabilidade, particularmente se cᴏmparados cᴏm os austeniticos, pᴏis a sᴜa sᴏlda é caracterizada pᴏr dutilidade e tenacidade baixas além de sensibilidade à cᴏrrosão intergranᴜlar. A soldagem a laser desse material faz com que ocorra um crescimento do grão, tendo, entretanto, grãos colunares, com uma estrutura de granᴜlação grᴏsseira, apresentando uma rede martensita jᴜnto aᴏs contorno de grão, apresentando uma ZAT (zona afetada termicamente) perdendo sua resistência nas propriedades mecânicas. Este trabalho tem por objetivo avaliar as características do cordão de solda, realizando a análise da influência do processo de soldagem utilizando o laser pulsado Nd:YAG aplicado em soldas de chapas de aço inoxidável ferrítico AISI 430, com 1,0 mm de espessura, observando as características mecânicas e microestruturais da junta soldada. Por meio da variação da taxa de sobreposição do cordão de solda se fez uma soldagem em junta de topo, com seis condições de amostras, sendo estas de valor fixo de energia de soldagem em 10 Joules, com largura temporal em 5 ms, velocidade de soldagem em 1 mm/s, tendo alteração apenas ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Included in the Ferritic family, AISI 430 stainless steel is an important material in terms of economics and technology, where, today, they are increasingly used in industries such as: automobile, chemical and household appliances, possibility of applying this material. In general, such material has a low solvency, particularly if cured as austenitic, because the latter is characterized by low ductility and toughness in addition to sensitivity to intergranular corrosion. The laser welding of this material causes a growth of the grain to occur, however, having columnar grains with a gross granulation structure, presenting a martensite network along the grain boundary, presenting a ZAT (thermally affected zone), losing its resistance mechanical properties. This work aims to evaluate the characteristics of the weld bead, performing the analysis of the influence of the welding process using the pulsed Nd: YAG laser applied in welds of AISI 430 ferritic stainless steel sheets, 1.0 mm thick, observing the mechanical and microstructural characteristics of the welded joint. By means of the variation of the overlapping rate of the weld bead was made a top joint welding with six sample conditions, these being of fixed value of welding energy in 10 Joules, with time width in 5 ms, welding speed in 1 mm / s, with only frequency changes ranging from 1.8 to 10 Hz, thus giving rise to different overlapping rates, in which argon gas protection was obtained, with a flow rate of 15 l / min. Th... (Complete abstract click electronic access below) / Mestre

O aço ferrítico-martensítico ODS Eurofer com 9%pCr (ODS - do inglês oxide dispersion strengthened), objeto de estudo dessa dissertação, é um potencial candidato para fins estruturais em reatores de fusão nuclear. Este material foi produzido via metalurgia do pó e consolidado por prensagem isostática. Em seguida sofreu laminação cruzada a quente e revenimento em 750°C por 2h. Esta foi a condição como recebida desse aço, o qual foi cedido pelo KIT (Karlsruher Institut für Technologie - Alemanha). Este aço possui 0,3%p de partículas de ítria (Y2O3) com diâmetro entre 10 e 30 nm. Uma das finalidades dessa dispersão de partículas de óxido é impedir a livre movimentação de contornos de grão no material, de modo a garantir a estabilidade microestrutural do mesmo sob recozimento. O aço ODS Eurofer como recebido foi laminado a frio com reduções de 20, 40, 60 e 80% da espessura e, posteriormente, foi recozido em diversas temperaturas entre 300 e 1350°C por 1h. Como o enfoque desse trabalho é sobre o aço ODS Eurofer recozido em altas temperaturas, para as temperaturas de 1250, 1300 e 1350°C foram feitos recozimentos adicionais (para o material com 80% de redução) variando-se o tempo de recozimento de 1 a 8 h. Para todos os recozimentos, com exceção dos realizados em 1350°C, o resfriamento das amostras se deu ao ar. Para a temperatura de 1350°C isso não foi possível e o resfriamento das amostras se deu no interior do forno. As amostras foram caracterizadas utilizando-se de medidas de dureza, medidas magnéticas e microscopia eletrônica de varredura (MEV). Amostras representativas também foram analisadas utilizando-se de difração de elétrons retroespalhados (EBSD) e espectroscopia por energia dispersiva (EDS). Para recozimentos em temperaturas acima de 800°C seguidos de resfriamento ao ar o material sofreu uma transformação martensítica. Na faixa de temperatura entre 800°C e 1300°C verificou-se um ligeiro decréscimo na dureza do material. Para as amostras com 80% de redução e recozidas em 1250 e 1300°C por diversos tempos até 8 h, seguido de resfriamento ao ar, não ocorreu uma variação significativa tanto nos valores de dureza e de campo coercivo das amostras com o tempo de recozimento. Estes valores se mantiveram em um patamar bem superior ao verificado para as amostras sem recozimento. Para as amostras recozidas em 1350°C, devido às características do resfriamento a microestrutura resultou em grãos ferríticos, aproximadamente equiaxiais e com tamanho de grão médio da ordem de 15 ?m. Observou-se uma notável queda tanto no valor de dureza como de campo coercivo dessas amostras. A observação mais importante nesse caso foi a observação de partículas da ordem de 100 nm ricas em ítrio no interior dos grãos, uma evidência de que ocorre o engrossamento das partículas de ítria nessa temperatura. Em virtude disso, a capacidade dessa dispersão de óxidos em impedir a livre movimentação de contornos de grãos no material fica prejudicada em 1350°C. / The object of this study is Eurofer 9% Cr Oxide Dispersion Strengthened (ODS) steel. This ferritic/martensitic steel is a potential candidate for structural applications in nuclear fusion reactors. It is produced through powder metallurgy and consolidated by hot isostatic pressing. The material undergoes hot cross lamination and is tempered at 760 °C. This was the condition of the steel as received, which was provided by KIT (Karlsruher Institut für Technologie, Germany). This steel contains 0.3 wt% yttria particles (Y2O3) with a diameter in the range 10-30nm. The main purpose of this oxide particle dispersion is to prevent the free movement of the grain boundaries in the material, so as to ensure stability of the microstructure during annealing. The material as received was cold rolled to reduce thickness by 20, 40, 60 and 80%. It was annealed at different temperatures from 300 to 1350 °C for 1 h. The focus of this study is the effects of high temperature annealing on the microstructure of ODS Eurofer. For this purpose, additional heat treatments were carried out on the steel that had been rolled to reduce thickness by 80% at temperatures of 1250, 1300 and 1350 °C. Annealing time varied between 15 min and 8 h. For all annealing conditions, except those carried out at 1350 °C, the samples were air cooled. For the temperature of 1350 °C, this was not possible. These samples were cooled in the oven. The samples were characterized using hardness testing, magnetic testing, and scanning electron microscopy (SEM). Representative samples were also analyzed using electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS). For annealing at temperatures above 800 °C, the material underwent a martensitic transformation after air cooling. Between 800 and 1300 °C, there was a slight decrease in the hardness of the material. For samples with 80% reduction annealed at 1250 and 1300 °C followed by air cooling, annealing time up to 8h didn\'t lead to a significant variation in either the hardness or the coercive field. Both hardness and coercive field of these samples were at a level well above the samples without annealing. For samples annealed at 1350 °C, due to the cooling characteristics of the samples, the microstructure took on a ferritic matrix with equiaxed grains with an average grain size of 15 um. There was a remarkable decrease in hardness and coercive field values of these samples. The most important result in this case was the observation of yttria-rich particles of the order of 100nm inside the grains. This is an evidence of the coarsening of the yttria particles at this temperature. As a result, the capacity of oxide dispersion to prevent the free movement of grain boundaries in the material is impaired at 1350°C.

AgÃncia Nacional do PetrÃleo / Os efeitos do envelhecimento a 400C e 475C na resistÃncia a corrosÃo, nas propriedades mecÃnica e magnÃtica do aÃo inoxidÃvel ferrÃtico AISI 444 foram investigados. O endurecimento causado pelo envelhecimento em ambas temperaturas foi medido, tendo sido observado que à mais intenso a 475ÂC. A susceptibilidade de corrosÃo localizada aumentou com o tempo de envelhecimento nas duas temperaturas, sendo tambÃm mais significativa a 475ÂC do que a 400ÂC. Com o tratamento de dissoluÃÃo a 570ÂC e 675ÂC ocorre a recuperaÃÃo dos valores de dureza e da resistÃncia à corrosÃo do aÃo. Diferente do aÃo inoxidÃvel duplex, o aÃo AISI 444 nÃo apresentou variaÃÃo na forÃa coercitiva e na temperatura de Curie com o aumento do tempo de envelhecimento. Os efeitos nos espectros de MÃssbauer tambÃm foram analisados. / The effects of aging at 400 C and 475   C in corrosion resistance, mechanical and magnetic properties of the ferritic stainless steel 444 were investigated. The hardening caused by aging in both temperatures was measured, it was observed that is more intense at 475  C. Susceptibility to localized corrosion, increased with aging time at the two temperatures, is also significantly more than 475  C to 400  C. In the treatment of dissolution at 570  C and 675  C is the recovery of hardness and corrosion resistance of steel. Unlike the duplex stainless steel, steel AISI 444 did not change in coercive force and the Curie temperature with increasing aging time. The effects in the MÃssbauer spectra were also analyzed.

<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>

No description available.

Ferritic stainless steels are the most commonly used materials for solid oxide fuel cell interconnect application. Although these alloys may meet the criteria for interconnect application for short periods of service, their application is limited for long-term use (i.e., 40,000 h) due to poor oxidation behaviour that results in a rapid increase in contact resistance. In addition, volatile Cr species migrating from the chromia scale can poison the cathode resulting in a considerable drop in performance of the cell. Coatings and surface modifications have been developed in order to mitigate the abovementioned problems. In this study, composite electrodeposition of reactive element containing particles in a metal matrix was considered as a solution to the interconnect problems. Nickel and Co were used as the metal matrix and LaCrO3 particles as the reactive element containing particles. The role of the particles was to improve the oxidation resistance and oxide scale adhesion, while the role of Ni or Co was to provide a matrix for embedding of the particles. Also, oxidation of the Ni or Co matrix led to the formation of conductive oxides. Moreover, as another part of this study, the effect of substrate composition on performance of steel interconnects was investigated. Numerous experimental techniques were used to study and characterise the oxidation behaviour of the composite coatings, as well as the metal-oxide scale interface properties. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), as well as surface analysis techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS), were used for the purpose of characterization. The substrate used for coating was AISI-SAE 430 stainless steel that is considered as a typical, formerly used interconnect material. Also, for the purpose of the metal-oxide scale interfacial study, ZMG232 stainless steel that is a specially designed interconnect alloy was used. It is shown that the composite coatings greatly reduce the contact resistance and effectively inhibit Cr outward migration. In addition, it was determined that the presence of impurities in the steel, especially Si, and the absence of reactive elements drastically contribute to interconnect degradation. / Materials Science and Engineering

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