Thesis (Ph. D.)--Ohio State University, 2008. / Title from first page of PDF file. Includes bibliographical references (p. 152-158).

Este trabalho apresenta um estudo dos mecanismos de fadiga em mola para válvula automotiva, realizado através da análise do comportamento mecânico correlacionado ao aspecto microestrutural. As amostras foram fabricadas em aço de alta resistência Si-Cr, submetidas a tratamentos térmicos convencionais (têmpera e revenimento) e não convencionais (austêmpera), obtendo um mesmo nível de dureza (aproximadamente 610 HV). Os corpos de prova foram submetidos a ensaios de bancada com diversos níveis de carregamentos, controlados através de variação da amplitude de tensão cisalhante. Esta variação promoveu diferentes regimes de fadiga, conhecidos como alto e ultra-alto ciclo, para maiores e menores níveis de amplitudes de tensões, respectivamente. O desempenho foi medido através do cálculo da vida total em fadiga, probabilidade de sobrevivência, além da verificação do relaxamento da força elástica e da tensão residual. As amostras foram caracterizadas através de métodos metalográficos, determinação da composição química, ensaios de dureza e resistência à tração, além de técnicas difratométricas de raios-X. As superfícies de fratura foram investigadas utilizando técnicas de microscopia eletrônica de varredura Os mecanismos de início e propagação das trincas, de ambas as amostras, foram semelhantes, apesar do tratamento térmico proposto de austêmpera produzir uma microestrutura mista, com predominância de bainita inferior, contendo regiões esparsas de austenita retida. A microestrutura bainítica, resultante da austêmpera, quando comparada com as amostras martensíticas, geradas pela têmpera e revenimento, propiciou um aumento da vida total em fadiga nos maiores níveis de amplitudes das tensões, tendendo à equalização com a diminuição desta amplitude, além de apresentarem um maior relaxamento elástico, gerando uma discreta diminuição de magnitude da tensão média. Por outro lado, não houve relaxamento das tensões residuais, em ambas as amostras, após uma quantidade significativa de ciclos, portanto este fenômeno não influenciou em sua vida total. / This paper presents a study on the valve spring fatigue mechanisms, carried out through analysis of mechanical behavior correlated to microstructural aspect. The samples were made of high strength steel Si-Cr, obtained by conventional (quenching and tempering) and unconventional (austempering) heat treatments, generating a single level of hardness (approximately 610 HV). The samples were subjected to bench test with various loading levels, controlled through shear stress amplitude fluctuation. This variation has promoted distinct fatigue regimes known as high and ultra-high cycle, for greater and smaller levels of stress amplitudes, respectively. The performance was measured by calculating the fatigue total life, survival probability, verification of the elastic force and residual stress relaxation. The samples were characterized by means of metallographic methods, chemical composition analysis, hardness/tensile strength testing and X-ray diffractometry techniques. The fracture surfaces were investigated using scanning electron microscopy techniques. The cracks initiation and propagation mechanisms were similar in both samples; despite the proposed austempering heat treatment produced a mixed microstructure with predominance of lower bainite, containing sparse regions of retained austenite. The bainitic microstructure, resulting of austempering, when compared with martensitic samples, generated by quenching and tempering, led to an increase in fatigue total life under higher levels of stress amplitudes, but tending to be the same as the stress amplitude deacresases and the number of cycles increases. On the other hand, there was no residual stresses relaxation in both samples, after a significant amount of cycles, so this phenomenon did not influence the total life.

Orientador: Célia Marina de Alvarenga Freire / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-22T11:43:34Z (GMT). No. of bitstreams: 1 Costa_GiselleBarata_D.pdf: 6195639 bytes, checksum: 7b424607db5a3372771d157f6bac717c (MD5) Previous issue date: 2013 / Resumo: O hidrogênio é reconhecido como sendo um elemento residual sempre prejudicial ao aço. Proveniente de reações químicas e eletroquímicas, tais como, corrosão, proteção catódica e reações de processo, é absorvido pelo aço e retido nos defeitos da rede cristalina podendo causar a fragilização do aço e a propagação de trincas e fraturas. O presente trabalho visa analisar a influência do hidrogênio na estrutura cristalina e no comportamento magnético do aço AISI 4340 obtido pelas normas AMS 6414K e 6415S, nas condições como recebido, temperado e revenido. Para tanto corpos-de-prova foram retirados da posição transversal dos lingotes, temperados a 845°C e revenidos a três diferentes temperaturas (350°C, 400°C e 500°C). Para a caracterização do material foram feitas micrografias, ensaios de dureza, difração de raios-X, voltametria cíclica e análise de difusão. As propriedades magnéticas foram obtidas utilizando-se um magnetômetro de amostra vibrante (VSM) onde a magnetização de saturação foi obtida e foram analisadas amostras antes da hidrogenação e após hidrogenação. O carregamento de hidrogênio foi feito em solução de H2SO4 a 0,05 M em temperatura ambiente. Os resultados obtidos nos ensaios de voltametria cíclica e difusão foram correlacionados às difrações de raios-x e magnetização de saturação em função da contaminação por hidrogênio e as diferentes condições de tratamento térmico. A comparação das curvas antes e após hidrogenação para as difrações de raios-X mostraram que a hidrogenação provoca uma diminuição das curvas em todas as condições de tratamento térmico para os dois tipos de aço. As curvas de difusão mostraram que os coeficientes de difusão da amostra como recebida é menor que as tratadas com revenimento para o aço AISI 4340 AMS 6415S e maior para o aço AISI6414K. A magnetização de saturação mostrou que os aços AISI 6414K e 6415S antes da hidrogenação apresentaram magnetização de saturação maior que após hidrogenação / Abstract: Hydrogen is recognized as a residual element always harmful to steel. From chemical and electrochemical reactions, such as corrosion, cathodic protection and reaction process, the steel is absorbed and retained in the crystal lattice defects may cause embrittlement of steel and propagation of cracks and fractures. This study aims to analyze the influence of hydrogen in the crystal structure and magnetic behavior of AISI 4340 obtained by the rules AMS 6414K 6415S and the conditions as received, quenched and tempered. For this purpose the test piece bodies were removed from the transverse position of the ingots, quenched to 845 ° C and tempered at three different temperatures (350 ° C, 400 ° C and 500 ° C). For the characterization of the micrographs were taken, hardness tests, X-ray diffraction, cyclic voltammetry analysis and dissemination. The magnetic properties were obtained using a vibrating sample magnetometer (VSM), where the saturation magnetization was obtained and analyzed samples before hydrogenation and after hydrogenation. The hydrogen loading was done in a solution of 0.05M H2SO4 at room temperature. The results obtained in the test and cyclic voltammetry were correlated to diffusion x-ray diffraction and saturation magnetization due to hydrogen contamination and different heat treatment conditions. A comparison of the curves before and after hydrogenation for X-ray diffraction showed that the hydrogenation causes a decrease of the curves in all heat treatment conditions for both types of steel. The curves show that the diffusion coefficient of diffusion of the sample as received is lower than those treated with tempering for the steel AISI 6415S AMS 4340 steel and higher for AISI6414K. The saturation magnetization showed that AISI 6415S 6414K and exhibited before the hydrogenation higher than saturation magnetization after hydrogenation / Doutorado / Materiais e Processos de Fabricação / Doutora em Engenharia Mecânica

No description available.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第17154号 / 工博第3644号 / 新制||工||1553(附属図書館) / 29893 / 京都大学大学院工学研究科建築学専攻 / (主査）教授 中島 正愛, 教授 吹田 啓一郎, 教授 金子 佳生 / 学位規則第4条第1項該当

This diploma thesis deals with weldability of Domex 460 MC material. There are two welding methods and three other welded materials in heterogenous connection which will be researched in this diploma thesis. The welding methods are fiber solid-state laser and a new method of hybrid welding called laser – TIG. The other materials are Domex 700 MC, S355J2 and DC01. Domex 460 MC is a high-strenght steel with fine-grained structure. During the welding process the imput heat decreases mechanical properties of the materials. This effect is caused by material seeds growing. From these samples (which were produced by welding) were created metallographic cuts. These metallographic cuts were analysed focusing on their makrostructure, microstructure, mikrohardnest and Mn arrangement in welded heterogenous samples using electron microscope and EDX analysis.

The project elaborated within the engineering studies of the branch Manufacturing technology presents an experiment of welding of high strength steel by fiber laser. The project includes a theoretical study, which deals with the welding material properties, principles and types of lasers and laser welding technology. The practical part contains a verification of the welding parameters according to the standard ČSN EN ISO 15614-11. The final evaluation results of the experiment at the close of the project are presented.

The use of high-strength steel is more and more frequent in the car industry. I focused on USIBOR® 1500 steel welded by Nd-YAG laser. For my experiment, there were used various welding parameters and shielding gases. To compare the weld samples, I used the tensile, bending, macrostructure, microstructure and Vickers hardness test with different combinations of thickness as well as welded material.

Although high strength steels represent yield strength up to 1300 MPa, welded structures reveal lowerstrength values. The strongest commercially available electrode provides the yield strength of about900 MPa. Therefore, in welded steels with strength above this type of filler metal, achieving anacceptable global strength is a crucial issue. In this master thesis, affects of different welding procedures on static strength of welded jointsof Weldox 960 and Weldox 1100 steels, were studied. These steels are produced by SSAB inOxelösund. Meanwhile, finite element method analyses were applied in order to investigatethe static strength behavior of such weldments under uniaxial tension. The welding parameters which were selected as variables are:  Heat input Weld joint geometry Filler metal When weld metal is undermatching in strength levels than the base material, by applyingtension the soft weld metal begins to deform before parent metal. At that point thedeformation of resulted soft zone, including the weld metal and the heat affected zone, ishindered by high strength parent metal. Thus, uniaxial stress caused by uniaxial load isconverted to multiaxial stress. This conversion in tension results in increase in the staticstrength of weldment. The increase in strength is emphasized by increase in the width of thewelded joint while the thickness of the plate is kept as constant. After experiments and performing FEM studies, it was revealed that the static strength ofWeldox 960 welded joints approaches towards the tensile strength of parent metal by increasein the width of the weldment. In Weldox 1100 joints; a slight increase in tensile properties ofthe weldments, when the width of the sample increases, was observed.

This research is about modelling and simulation of how the hydrogen diffuses in high strength steels. The hydrogen diffusion in the material was examined by using finite element software with the help of material properties and some existing data. For modelling and simulating the diffusion analysis in finite element software, a cylindrical type dog-bone shaped specimen was chosen. To determine the diffusion at the centre of specimen, a cross-sectional area of the material was selected to proceed for the analysis. Abaqus software was considered as finite element software to progress the hydrogen diffusion and tensile testing of the specimen. Diffusion analysis was studied under the analogy of heat transfer and also, diffusion analysis with the addition of mechanical load was studied under the analogy of coupled temperature displacement in the Abaqus software. This process has executed for two types of high strength steels 316L and 304L stainless steels. The crack is also considered for analysis to check how it affects the specimen. Further, The 316L and 304L stainless steel results were compared to review that which steel is better to withstand the hydrogen diffusion rate and mechanical load on the material.