Dans le cadre de ces travaux, il a clairement été établi que l’hydrogène piégé ou diffusible pouvait avoir une forte influence sur les propriétés mécaniques des matériaux. Cependant, cet effet varie de façon importante en fonction de leur microstructure, leur composition chimique et leur traitement thermique. En effet, les aciers martensitiques trempés/revenus dédiés à des tubes pour des milieux sous-service présentent, de par leur structure, différents types de pièges tels que les dislocations, les joints de grains, les précipités, les inclusions, les lacunes et d’autres interfaces qui jouent un rôle important dans les mécanismes endommageants. Ces aciers de haute résistance mécanique, lorsqu’ils sont soumis à des contraintes mécaniques et à un environnement agressif (qui dépend de la pression en H2S et du pH de la solution) peuvent rompre à cause du phénomène de Sulphide Stress Cracking (SSC). Ce dernier est une forme de fragilisation par l’hydrogène (FPH) qui inclut un amorçage de fissure suivi d’une étape de propagation conduisant à la rupture, dont la contribution de l’hydrogène reste encore mal comprise. En parallèle de l’impact de la microstructure de l’acier, les champs de contrainte et déformation subis par le matériau modifient les effets induits par l’hydrogène. C’est pourquoi un montage de perméation sous contrainte a été utilisé afin de pouvoir réaliser des essais mécaniques jusqu’à rupture sous flux d’hydrogène et les comparer au comportement du matériau lorsque celui-ci est sollicité à l’air ou dans un environnement H2S. Ainsi, l’impact sur le comportement mécanique du flux d’hydrogène mais également de son piégeage peut être étudié. Dans ce cadre, des éprouvettes plates et axisymétriques, lisses et entaillées ont été employées. Les informations expérimentales obtenues dans ce travail ont servi à alimenter un modèle numérique qui a permis de caractériser localement l’état mécanique et les concentrations d’hydrogène piégé et diffusible dans le matériau. Ceci a rendu possible la définition d’un critère local de rupture. / The findings of this work established that the diffusible and trapped hydrogen could have a strong influence on the mechanical properties of materials. However, this effect varies significantly with the materials’ microstructure, chemical composition, and heat treatment. Due to their structure, quenched and tempered martensitic steels (developed for tubes suitable for sour service environments) have different types of traps such as dislocations, grain boundaries, precipitates, inclusions, vacancies and other interfaces that play an important role in the damage mechanisms. These high strength steels may break due to Sulphide Stress Cracking (SSC) if subjected to mechanical stress and an aggressive environment (which depends on the H2S partial pressure and pH solution). This phenomenon is a form of hydrogen embrittlement (HE) that includes a crack initiation followed by a propagation step leading to failure. However the hydrogen contribution is still insufficiently understood. In addition to the impact of the microstructure on the steel, the stress and the deformation fields in the material also modify the effects induced by hydrogen. To investigate this event, electrochemical permeation tests under stress were used to perform mechanical tests under hydrogen flux until failure is reached. The results were compared to those mechanically loaded in air or in a H2S environment. This enabled the examination of the impact of the hydrogen flux and trapping on the mechanical behavior of martensitic steel. In this framework, flat and axisymmetric, smooth and notched specimens were employed. Experimental data obtained in this work were used to provide a numerical model that enables the locally characterization of the mechanical condition and the concentrations of trapped and diffusible hydrogen in the material. These outcomes enabled us to determine a local failure criterion.

Orientador: Itamar Ferreira / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-18T17:02:04Z (GMT). No. of bitstreams: 1 Uehara_AndreYugou_M.pdf: 4445631 bytes, checksum: ba69e40eee3a8de5233ab826f19564d7 (MD5) Previous issue date: 2011 / Resumo: Este trabalho tem por objetivo investigar a etapa de decapagem ácida e os processos de desidrogenação e retrabalho realizados durante a zincagem de parafusos quanto à fragilização por hidrogênio. Parafusos fabricados conforme a norma JIS B 1127 (1995), cementados, de 6 mm de diâmetro e 16 mm de comprimento de rosca foram organizados em 39 diferentes grupos. A decapagem foi avaliada quanto ao volume de ácido clorídrico (700 ml, 900 ml e 1000 ml) e presença, ou ausência, de inibidor para este ácido, utilizando tempos de decapagem de 15, 30, 45 e 60 minutos para cada condição de volume de ácido e inibidor utilizados. A desidrogenação foi avaliada utilizando parafusos decapados (solução: 1000 ml de ácido, ausência de inibidor e decapados por 15, 30, 45 e 60 minutos) e desidrogenados a uma temperatura de 1000C e tempo total de processo de 1, 2 e 3,5 horas. O retrabalho foi avaliado utilizando parafusos de 8 ?m de espessura de camada zincada, retrabalhados por 4 minutos em 700 ml de ácido, ausência de inibidor e 300 ml de água, avaliando as hipóteses do retrabalho único e duplo sem desidrogenação e do retrabalho único seguido de desidrogenação (1000C - 2 horas). Ensaios de pré-carregamento para a detecção da fragilização por hidrogênio foram realizados em parafusos que sofreram as preparações citadas, sendo o torque de ruptura avaliado nestes parafusos, assim como naqueles obtidos após o tratamento térmico. Análises química e metalográfica e ensaios de microdureza Vickers e tração, além de análise fratográfica por microscopia eletrônica de varredura (MEV) também foram realizados. A análise metalográfica revelou uma matriz ferrítica composta por grãos equiaxiais com carbonetos esferoidizados para o fio máquina, enquanto que o parafuso possui estrutura ferrítico-perlítica no núcleo e martensita revenida na camada cementada. Os ensaios de microdureza e de tração revelaram que o fio máquina e o parafuso possuem níveis de resistência mecânica dentro do esperado conforme as condições utilizadas, enquanto que não houve diferenças significativas entre os valores de torque de ruptura obtidos em ambas as situações analisadas. A etapa de decapagem ácida revelou que, nos grupos que não utilizaram inibidor, o número de falhas aumenta num primeiro momento com o aumento do tempo de decapagem, reduzindo após um determinado tempo de decapagem relacionado ao volume de ácido utilizado. O número de falhas sofre grande redução com a utilização do inibidor, porém mesmo a baixas concentrações de ferro, a utilização de maiores volumes de ácido associado a maiores tempos de decapagem aumentam o risco de fragilização. Apenas houve falha para a desidrogenação realizada a 1000C, por 1 hora, revelando a importância de adequados controles de temperatura, procedimentos de homogeneização e parâmetros de temperatura e tempo. Não houve falhas para o retrabalho revelando a importância de adequados procedimentos de retrabalho (tempo e solução) e de desidrogenação, enquanto que a análise da superfície de fratura revelou apenas os micromecanismos de fratura intergranular e dimples, sendo este último mais freqüentemente associado a regiões mais próximas ao núcleo, além da presença de trincas secundárias / Abstract: The main aim of this work is to investigate the effects on hydrogen embrittlement of bolts due to acid pickling, baking, and strip processes performed during zinc plating. Carburized bolts type "hexagon flange head tapping screws", with 6 mm of diameter and 16 mm of thread length were organized into 39 different groups. Acid pickling was evaluated using volumes of hydrochloric acid of 700 ml, 900 ml, and 1000 ml, presence, or absence, of acid inhibitor, and pickling periods of 15, 30, 45, and 60 minutes for each condition of acid volume and inhibitor used. Baking was evaluated using 1000C, and periods of 1, 2, and 3.5 hours for bolts that were subjected to acid pickling with a solution of 1000 ml of acid, absence of inhibitor, and pickling periods of 15, 30, 45, and 60 minutes. Strip was evaluated using bolts with zinc layer thickness of 8 ?m, stripped for 4 minutes in a solution of 700 ml of acid, absence of inhibitor, and 300 ml of water, performing the hypotheses of single and double strip without baking, and single strip followed by baking (1000C - 2 hours). Preloading tests for the detection of hydrogen embrittlement were conducted in bolts that were subjected to the preparations mentioned, while torsional tests were also conducted at these bolts, as well as in those obtained after heat treatment. Chemical and metallographic analysis, Vickers microhardness and tensile tests, and fractographic analysis using scanning electron microscopy (SEM) were also conducted. Metallographic analysis revealed a ferritic matrix composed of equiaxed grains with spheroidized carbides for the wire, while the bolts showed a ferritic-pearlitic microstructure at the center and tempered martensite at the hardened layer. Microhardness and tensile tests revealed that wire and bolts have strength levels as expected according to the conditions used, while no significant differences between the breaking torque values were obtained in both situations analyzed. Acid pickling revealed that in the groups, which did not use inhibitor, the number of failures increases at a first stage with increasing pickling periods, however it starts to decrease after a certain pickling period related to the volume of acid used. The number of failures is greatly reduced with the use of the inhibitor, but even at low concentrations of iron, the use of larger amounts of acid associated with longer pickling periods increases the risk of hydrogen embrittlement. Failures were observed only at 1000C - 1 hour as baking parameters, showing the importance of proper temperature controls, homogenization procedures, and temperature and time parameters. There were no failures related to strip, revealing the importance of adequate procedures for strip (period and solution used) and baking procedures as observed. The fracture surface analysis revealed only intergranular and dimples micromechanisms of fracture, where the latter being more often associated with regions closer to the core of the bolts, also showing the presence of secondary cracks / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Nos processos de fabricação de parafusos, a fosfatização é necessária para servir de ancoradouro aos lubrificantes, e outros redutores de atrito, por facilitarem o processo de deformação a frio. No entanto, antes do início do tratamento térmico de têmpera e revenimento, é importante que o banho de desfosfatização seja eficiente para impedir que, durante a austenitização, o fósforo residual presente na superfície do parafuso se difunda para o aço e forme uma fase frágil, rica em fósforo, denominada ferrita delta (&#948). Acredita-se que esta fase, uma vez presente, promove não apenas a diminuição da vida em fadiga mas, também, a fragilização do parafuso. Neste sentido, o presente trabalho objetivou comprovar a influência negativa dessa fase, devidamente caracterizada por MEV, EDS e análise fractográfica, quanto ao desempenho à fadiga, através da comparação das curvas S-N em corpos de prova com e sem ferrita &#948. Os resultados obtidos claramente evidenciaram que a presença da fase &#948 promoveu uma redução de até 40% na vida em fadiga. / In the process of manufacturing bolts, a coat phosphating is required to serve as anchorage for lubricants and other friction-reducer, facilitating the cold forming process. However, before the beginning heat treatment of quenching and tempering, it is important that the alkaline bath for cleaning be efficient in order to prevent that during austenitizing, the residual phosphorus spread on the steel surface and forms a brittle phase, rich in phosphorus, called delta ferrite (&#948). In the presence of this phase, is credited with a decrease in fatigue life of the bolt and embrittlement. The intention in this work was to show the negative influence of this phase on fatigue performance, comparing the S-N curves for specimens with and without ferrite &#948. Trials have shown a reduction of up to 40% in life fatigue. The ferrite &#948 was characterized by SEM, EDS and also performed fractographic analysis.

<p>One of the major degradation mechanisms in a nuclear power plant structural or mechanical component is the neutron embrittlement of the irradiated steel component. High energy neutrons change the microstructure of the steel, so the steel loses its fracture toughness. This neutron embrittlement increases the risk of the brittle fracture. Meanwhile, the reactor pressure vessel support is exposed in low temperature with high neutron irradiation environment which is an unfavorable condition for the fracture failure. In this study, the failure assessment of a reactor pressure vessel support was conducted using the fitness-for-service failure assessment diagram of API 579-1/ASME FFS-1(2016, API) with quantifying the structural margin under the maximum irradiation and extreme load events. </p> <p>Two interrelated studies were conducted. For the first investigation, the current analytical methods were reviewed to estimate the embrittled properties, such as fracture toughness and the yield strength incorporates the low irradiation temperature. The analytical results indicated that the reactor pressure vessel support may experience substantial fracture toughness decrease during the operation near the lower bound of the fracture toughness. A three-dimensional (3D) solid element finite element model was built for the linear stress analysis. Postulated cracks were located in the maximum stress region to compute the stress intensity and the reference stress ratio. Based on the stress result and the estimated physical properties, the structural margin of the reactor pressure vessel support was analyzed in the failure assessment diagram with respect to the types of the cracks, level of the applied load and the level of the neutron influence. </p> <p>The second study explored the structural stress analysis approaches at hot-spot which was found to be key parameter in failure analysis. Depending on the methods to remove the non-linear peak stress and the stress singularities, the accuracy of the failure assessment result varies. As an alternative proposal to evaluate the structural stress in 3D finite element analysis (FEA), the 3D model was divided into two-dimensional (2D) plane models. Five structural stress determination approaches were applied in 2D FEA for a comparison study, the stress linearization, the single point away approach, the stress extrapolation and the stress equilibrium method and the nodal force method. Reconstructing the structural stress in 3D was carried by the 3x3 stress matrix and compared to the 3D FEA results. The difference in 2D FEA structural stress results were eliminated by the constructing the stress in 3D. </p> <p>This study provides the failure assessment analysis of irradiated steel with prediction of the failure modes and safety margin. Through the failure assessment diagram, we could understand the effects of different levels of irradiation and loadings. Also, this study provides an alternative structural stress determination method, dividing the 3D solid element model into two 2D models, using the finite element analysis. </p><br>

The present paper studied the influence of hydrogen on the mechanical properties and microstructure of the materials. This study is done by doing hydrogen charging of steel bar specimens. The two types of steel are used for study, structural steel S235 and stainless steel 316L. The tensile test has been performed to get the data of the charged and uncharged specimens. Different strain rates were used to study the effect of the material after charging. The charging time ranged from 24 hours to 144 hours. The specimens are cylindrical shape. Digital image analysis is performed for the uncharged specimens to correctly analyse the strain field and compare it with the results from tensile test. For DIC analysis a GOM correlate software was used to study the 2D strain analysis of the specimen. The results show that hydrogen influences the mechanical properties of the material and microstructure.

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

The carburizing atmosphere during the case hardening process contains a large proportion of hydrogen. Due to the rapid diffusion of hydrogen a high amount of hydrogen can be absorbed by the carburizing component. The amount of absorbed hydrogen is dependent on some factors such as for example the carburizing time and component dimensions. Hydrogen diffused in material can then cause hydrogen embrittlement and in some cases cause cracking under a static load. This hydrogen must therefore be removed. High amounts of hydrogen diffuse out spontaneously at room temperature. Tempering accelerates the process. The aim of this study was to experimentally measure the amount of absorbed hydrogen after case hardening and hydrogen content after storage at room temperature and also after tempering. The effect of the enriching gas in carburizing furnace on hydrogen absorption was investigated in this study. Three steel grades with different content of alloying elements were used in this investigation. Steel samples were case hardened by gas carburizing and tempering. The hydrogen content analyses included the measurement of hydrogen content before case hardening, after case hardening and after tempering using Leco-RHEN602. Based on the results in this study it was concluded that all steel grades used in this investigation absorb hydrogen during case hardening by gas carburizing. A major part of the absorbed hydrogen is then released by effusion after being stored at room temperature and during tempering. Around 50% of the absorbed hydrogen content during gas carburizing is due to the presence of the enriching gas in the carburizing atmosphere. Around 50 % of hydrogen diffuses out of the steel specimens after one day. It is likely that all of free diffusible) hydrogen has diffused out of the specimens of two steel grades after one week at room temperature or after tempering. / Vid sätthärdning består den uppkolande atmosfären till stor del av vätgas och p.g.a. vätets snabba diffusion kan stora mängder av väte absorberas i komponenten. Halten av absorberade väte beror bl.a. på sätthärdningstid och komponentens dimensioner. Väte i materialet kan sedan leda till sprickbildning vid statisk belastning. Detta väte måste därför avlägsnas. En stor del av väte diffunderar ut spontant vid rumstemperatur. Vid anlöpning går processen fortare. Syftet med denna studie var att experimentellt mäta halten av väte som absorberas under sätthärdning, samt efter att metallen har lagrats i luft vid rumstemperatur. Dessutom mättes vätehalten efter anlöpning. Dessutom undersöktes effekten av ugnsatmosfärens tillsatsgas på mängden absorberad väte efter uppkolning. Tre olika höghållfasta och låg legerade stål sorter sätthärdades genom gas uppkolning. Mängden väte analyserades innan sätthärdning, efter sätthärdning, efter lagring i rumstemperatur och efter anlöpning med hjälp av Leco-RHEN602. Enligt resultaten i denna studie, absorberar alla av de tre undersökta stålsorterna väte under sätthärdning. En stor del av det absorberade vätet diffunderar ut efter att stålet har lagrats i luft vid rumstemperatur och under anlöpning. Omkring 50 % av den absorberade vätehalten under uppkolningen är på grund av reaktionen med tillsatsgasen i ugnsatmosfären. Omkring 50 % av vätet diffunderar ut ur proverna efter en dag. Möjligen all fritt (diffunderbart) väte har diffunderat ut ur proverna i två av stålsorterna efter en vecka i rumstemperatur eller efter anlöpning.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第19093号 / エネ博第317号 / 新制||エネ||64(附属図書館) / 32044 / 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 / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19308号 / 工博第4105号 / 新制||工||1633(附属図書館) / 32310 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 白井 泰治, 教授 乾 晴行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM