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Slurry coating technique, as one of the most popular deposition methods, is widely used to produce various material coatings. This method includes two processes: spraying, brushing or dipping of slurry, and sintering heat treatment of the coated specimen. Superalloys and stainless steels are the most common materials used as either coating materials or substrate materials because of their excellent corrosion, wear, high-temperature and mechanical properties. This research is aimed at investigating the influence of the sintering heat treatment in the slurry coating process developed at Kennametal Stellite Inc. on the microstructure, hardness and wear behavior of superalloys and stainless steels. Low-carbon Stellite 22, cobalt-based Tribaloy T-400C, martensitic AISI 420 and AISI 440C stainless steels are studied in this research. The microstructure, hardness and wear resistance of these alloys before and after the heat treatment are investigated, stressing the influence of the heat treatment on these material characteristics. The hardness and wear tested are conducted on these alloys at both room temperature and at elevated temperatures. The worn surfaces of each specimen are analyzed using a Scanning Electron Microscope (SEM) with backscatter electron imaging (BEI) and energy dispersive X ray (EDX) spectrum. It is demonstrated that the heat treatment alters the microstructures of these alloys differently; it increases the hardness but affects the wear resistance more complexly than hardness. At room temperature, the wear resistance of these alloys is governed by their microstructures. However, at high temperatures, oxidation, resulting in formation of oxide films on the specimen surface, influences the wear resistance significantly.

Radiation-induced changes in microstructures often lead to significant changes in mechanical properties of alloys used in the construction of nuclear reactors. It is desirable to test small specimens to make efficient use of the small volumes available in test and commercial reactor cores and also because small specimens are less affected by the sometimes steep flux gradients experienced in reactor cores and the sometimes large temperature gradients developed in the specimens from gamma heating. (Continues...).

An increasing number of engineering components are expected to have fatigue life in the range of 107 - 1010 load cycles. Some examples of such components are found in airplanes, automobiles and high speed trains. For many materials fatigue failures have lately been reported to occur well after 107 load cycles, namely in the Very High Cycle Fatigue (VHCF) range. This finding contradicts the established concept of a fatigue limit, which postulates that having sustained around 107 load cycles the material is capable of enduring an infinite number of cycles provided that the service conditions are unchanged. With the development of modern ultrasonic fatigue testing equipment it became possible to experimentally establish VHCF behaviour of various materials. For many of them the existence of the fatigue limit at 107 load cycles has been proved wrong and their fatigue strength continues to decrease with increasing number of load cycles. High performance steels is an important group of materials used for the components subjected to VHCF. This study explores the VHCF phenomenon using experimental data generated by ultrasonic fatigue testing of selected tool steels. The overall aim is to gain knowledge of VHCF behaviour of some common tool steel grades, while establishing a fundamental understanding of mechanisms for crack development in the very long life regime. The study demonstrates that VHCF cracks in tested steels initiate from microstructural defects like slag inclusions, large carbides or voids. It is established that VHCF life is almost exclusively spent during crack formation at below threshold stress intensity values which results in a unique for VHCF morphology on the fracture surface. Significant attention is devoted in the thesis to the ultrasonic fatigue testing technique, i.e. the validity and applicability of its results. FEM is employed to give an additional perspective to the study. It was used to calculate local stresses at fatigue initiating defects; examine the effect of material damping on ultrasonic stresses; and to evaluate various specimen geometries with respect to resulting stress gradient and maximum stressed material volume.

Increasing demand from flourishing construction markets led to the successful development of high strength steels (HSS). The new structural steel has exceptional high strength, high fracture toughness, long fatigue life, high corrosion resistance, and better weldability making the material attractive for structural design applications in the modern steel buildings and bridges. With their high strength, typically in the range of 500~700 MPa, and reduced weight/dimensions, it frees imaginations of modern designers and opens up new possibilities. Although HSS cost more, this is more than offset by reduced fabrication and erection costs. The advantage of the intrinsic properties of the HSS makes it possible to achieve successful applications in a cost-effective manner. At present, the Australian steel design standard, AS 4100 (SA, 1998), is limited to conventional low strength steels (LSS) with yield stress less than 450 MPa, (i.e. fy . 450 MPa). As a result steel structural members fabricated from HSS in Australia are usually designed according to overseas specifications, such as AISC-LRFD (AISC, 2003) which allows the design for structures fabricated from HSS materials. However, the design provisions of AISC-LRFD were mainly based on experimental and analytical studies on standard LSS. HSS exhibits mechanical properties that are quite different from conventional LSS. On the other hand, the design procedure and approach of the American specifications (AISC, 2003) are unfamiliar with Australian design engineers, which explains why practising engineers in Australia are reluctant to use AISC-LRFD specification in the design of HSS members. Therefore research into the behaviour of HSS members is essential to address this shortcoming. However, since the use of HSS often leads to smaller sections, hence thinner plates, the elastic and inelastic instability of these thin-walled and HSS members become highly critical. Conservatively, the local instabilities of the constituent plate element interactions in the cross-section have been ignored in the current steel practices. Increasing the slenderness of either plate elements within a cross-section leads to a significant reduction in the section capacity of the structural member. Therefore, the interactive effects between flange and web plate elements have to be considered in the strength, stability and deformation studies of HSS members. Furthermore, the current definitions and values of the plate slenderness limits also vary among major steel design codes (AS4100, 1998; AISC, 2003; EN1993, 2003; BS5950, 2000). The main aim of this research project is to investigate the structural behaviour of Ishaped HSS members subjected to local buckling effects in the elastic and inelastic ranges. For this purpose, it will use advanced numerical analyses and laboratory experiments to study the structural behaviour of these HSS members in compression and bending, respectively. The critical review has found that various inconsistencies among the major steel design specifications (AS4100, 1998; AISC, 2003; EN1993, 2003, BS5950, 2000) in the current practice produce conflicting design predictions of section capacities. The experimental measurements of residual stress distributions have confirmed that the ECCS recommendation (1984) is inappropriate for crosssections fabricated from typical HSS materials (i.e. BISPLATE80). The experimental measurements and numerical studies carried out in this project have produced a better understanding of the structural behaviour of HSS members subjected to local instabilities. The study has enabled to provide a series of proposals for proper assessment of plate slenderness limits for structural members made of HSS materials. It may also enable the inclusion of future version of the AS4100 code for HSS materials to be used in the design of steel building and bridge constructions. It is believed that the use of HSS in building and bridge constructions will increase significantly in the very near future, and to fully-facilitate this, the future versions of national and international steel design specifications must include rational and reliable design rules for members made of all steel grades by including the effects of HSS special characteristics and true interactive local buckling behaviour of HSS members. This research project has contributed towards this.

Made available in DSpace on 2014-06-11T19:27:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-08-14Bitstream added on 2014-06-13T20:16:11Z : No. of bitstreams: 1 silva_rc_me_guara.pdf: 9001097 bytes, checksum: 6f1a3bc8edbfd3fd88699167a64182bc (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Na exploração e produção de petróleo e gás existe uma demanda crescente por aços especiais e de alta resistência. Com os campos mais fáceis e de melhor relação custo benefício já em produção, as empresas estão se movendo para exploração e produção de áreas não convencionais. O aumento da perfuração nessas áreas críticas traz um impacto direto no consumo de produtos de alta tecnologia, como tubulações fabricadas com aços de alta resistência. Para obtenção de aços de alta resistência, o mercado tem investido no desenvolvimento de aços, com progresso considerável nos processamentos térmicos e termomecânicos. Neste contexto, a martensita revenida, obtida através de têmpera e revenimento, representa uma alternativa importante, tendo em vista que esta estrutura pode aumentar a resistência, mantendo um bom nível de tenacidade. Os aços ligados ao boro são de baixo custo, boa temperabilidade e baixa dureza do material na condição após laminado, o que facilita sua conformação, e o torna um boa opção para fabricação de tubos. Com o tratamento térmico de têmpera e revenimento, há a possibilidade de obtenção de distintas propriedades mecânicas em função da variação das temperaturas de revenimento. Neste projeto, objetivou-se otimizar os parâmetros de tratamento térmico (têmpera e revenimento), para obtenção de uma microestrutura martensítica revenida, a partir de um aço de baixo carbono ligado ao boro utilizado na fabricação de tubos. Foram realizadas caracterizações mecânicas e microestruturais do material tratado termicamente em distintas faixas de temperatura. O material apresentou alta temperabilidade após têmpera, e foi possível verificar a mudança das propriedades em função das distintas temperaturas de revenimento. As características microestruturais e mecânicas (resistência à tração, limite de escoamento e ductilidade) são apresentadas neste trabalho. / The exploration and production of oil and gas has increased its demand for special steels and high resistance steels. With the easiest, most cost-effective fields already in production, oil companies are now moving to non-traditional areas and fields that are more difficult to develop. Increased drilling in those critical environments will have a direct impact in the consumption of high-steel grade tubulars. In oder to attend this demand, the market has invested in R&D of high resistance steels, focusing on heat treatment and thermomechanical processes. In this sense, tempered martensite microstructure represents a good steel design alternative, since that microstructure can increase steel resistance maintaining its toughness properties. Boron bearing steels are low cost, have excellent hardenability and low hardness on the as-rolled condition, which makes it a good alternative for pipe manufacturing. This characteristic has put the tempered martensite as one of the microstructures with high potential in terms of utilization for pipe manufacturing. The quench and tempering heat treatments gives the possibility of obtaining different mechanical properties as a result of changes in the tempering temperatures. This project aimed to optimize quench and tempering heat treatment processes in order to obtain tempered martensite microstructures utilizing a boron microalloyed steel. The material presented high hardenability after quenching and it was verified mechanical properties changes after tempering at different temperatures. The microstructural characteristics and mechanical properties (tensile strength, yield strength and ductility) are presented in this work.

Made available in DSpace on 2014-10-09T12:43:36Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:20Z (GMT). No. of bitstreams: 1 06531.pdf: 4466032 bytes, checksum: a6e28ab0187aa788035c1d0e3ed8764f (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:96/08958-1

Des industriels du domaine aéronautique se sont regroupés autour du projet MEKINOX (Mécanique Inoxydable) visant à développer l'utilisation de différentes nuances d'aciers inoxydables en raison de leur haute résistance mécanique et de leur résistance à la corrosion. Cependant ces aciers sont réputés sensibles au frottement. Dans ce contexte, ce travail de thèse est dédié dans un premier temps à l'étude de l'effet des différents traitements thermiques et de surface sur la microstructure des aciers inoxydables visant à améliorer leur aptitude au frottement. Nous avons mis en évidence différents mécanismes de durcissement : par précipitation, par changement de phase et par solution solide. Dans un deuxième temps, nous avons mis en place une démarche expérimentale permettant de comparer la réponse des différents couples de matériaux sous contact roulant et glissant extrêmes. Ces essais nous ont permis de caractériser l'évolution du coefficient de traction et de classer les différents états métallurgiques selon leur résistance à l'usure. Dans un troisième temps, nous avons caractérisé les différents types d'endommagements se produisant en surface et en sous-couche. Enfin, l'utilisation de l'EBSD nous a permis caractériser la déformation plastique en sous couche et de tenter de corréler la valeur de la densité de HAGB (Angles à forte désorientation) avec la résistance à l'usure des différents états métallurgiques des aciers étudiés. / Aeronautics industrials gathered thought MEKINOX project ("Mécanique Inoxydable") in order to develop the use of different stainless steels grades, because both their high strength and corrosion resistance. However, these steels are deemed sensitive to friction. In this context, this thesis is dedicated firstly to study the effect of different thermal and surface treatments on the microstructure of stainless steels to improve their ability to friction. Different hardening mechanisms were observed: precipitation, phase change and solid solution. Secondly, we have implemented an experimental approach for comparing the response of the various materials pairs under extreme sliding and rolling contact. These tests have allowed us to characterize the friction coefficient evolution and classify the different metallurgical materials and treatments according to their wear resistance. Thirdly we have characterized the different types of damage occurring at surface and in the subsurface. Finally, the use of EBSD allowed us to characterize the plastic deformation in the subsurface and to correlate the value HAGB (High Angle Grains Boundaries) density with the wear resistance of the different metallurgical materials and treatments of the examined steels.

Made available in DSpace on 2014-06-11T19:32:50Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-04-13Bitstream added on 2014-06-13T20:24:12Z : No. of bitstreams: 1 delsant_r_dr_guara.pdf: 3721030 bytes, checksum: 88232a756e66c084fa98967c48ab9853 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Os aços multifásicos constituídos de ferrita, bainita, austenita retida e martensita apresentam combinações muito atrativas de resistência e tenacidade. Há ainda um potencial adicional de melhorias de propriedades mecânicas quando a fração de austenita retida for alta, conferindo alta conformabilidade pelo efeito TRIP. Neste contexto, é fundamental a análise qualitativa e quantitativa das fases, especialmente de austenita retida e sua transformação em martensita induzida por deformação. Este trabalho enfoca a transformação da austenita retida em martensita por deformação em tração monotônica em um aço AISI 4340 com estrutura multifásica. Os resultados confirmam a transformação da austenita retida em martensita atingindo cerca de 80% de transformação. As frações volumétricas de austenita retida antes e após a deformação foram estimadas por duas técnicas. A primeira foi feita por análise de imagens em microscopia óptica e a segunda por magnetização de saturação, tendo em vista o caráter paramagnético desta fase. As frações estimadas pelas duas técnicas foram comparadas, concluindo-se que o método magnético deve ser reavaliado, tendo sido proposto um fator de correção na equação básica deste processo e presenta na literatura. / The multiphase steels made of ferrite, bainite, retained austenite and martensite present very attractive combinations and toughness. There is still an additional potential of improvement of the mechanical properties when the fraction of retained austenite is high allowing high conformability by the TRIP effect. In this context the qualitative and quantitative analysis of the phases is essentual specially from retained austenite and its transformation in martensite induced by deformation. This work focus on the transformation of retained austenite in martensite by the deformation in monotonic traction in one steel AISI 4340 with multiphasic structure. The results confirm the transformation of retained in martensite reaching about 80% of transformation. The volumetric of retained austenite before and after the deformation were estimated by two technique: the first was made by the anllysis of images in optic microscopy and the second by magnetization of saturation taking into consideration the paramagnetica character of this phase. The estimated fraction by the two technique were compared leading to the conclusion that the magnetic method must be evaluated again using a proposed factor of correction in the basic equation of this process and present in the literature.

Made available in DSpace on 2014-06-11T19:34:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-12Bitstream added on 2014-06-13T19:44:43Z : No. of bitstreams: 1 silva_ap_dr_guara.pdf: 3551125 bytes, checksum: 3f75e8c0e99826910556f8cd772c1403 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho analisa a transformação da austenita retida induzida por deformação, em um aço AISI 4340 com estruturas multifásicas. O aço foi submetido a duas rotas de tratamentos isotérmicos selecionadas e foi analisado quanto à fração volumétrica das fases e características mecânicas através da valiação dos limites de escoamento. Os corpos de prova foram submetidos a três diferentes níveis de tensões de tração, acima do limite de escoamento, para que se produzissem deformações plásticas determinadas. A influência destas deformações, na microestrutura e nos novos limites de escoamento, foram avaliadas. Mostrando variações significativas nas frações volumétricas das fases, com diminuição das quantidades de austenita, indicando uma transformação, por efeito TRIP (Transformation Induced Plasticity), em martensita o que provocou um aumento nos valores dos limites de escoamento e envelhecimento por deformação. / The Transformation induced plasticity of austenite on the multiphase microstructure in a AISI 4340 steel was studied. Two sequences of isothermal treatments were performed. The volume fractions of the phases were analyzed and evaluation of mechanical characteristics through the yield point measurements. The samples were submitted at three different levels of traction above of the yield strength, so that definitive plastic deformations were produced. The influence of these deformations in the microstructure and in the new yield strength was evaluated. Significant variations were showed in the phase volume fraction, with reduction of the amounts of austenite indicating a martensitic transformation, by effect TRIP (Transformation Induced Plasticity) which results increases in the values of the yield strength and deformation ageing.