A fragilização por hidrogênio é uma potencial causa de falha em diversos componentes industriais. Este tipo de fragilização ocorre principalmente em aços ao carbono e também em aços inoxidáveis martensíticos, que apresentam alta resistência mecânica. Neste trabalho, avaliou-se a influência das temperaturas de revenimento de 300 ºC, 400 ºC e 500 ºC na fragilização por hidrogênio do aço ABNT 10B22-Modificado, após o processo de carbonitretação, o qual é muito utilizado na fabricação de parafusos e fixadores. Foram confeccionados corpos de prova convencionais segundo a Norma NACE TM 177-90 e também foram usados parafusos M4 X50 fabricados conforme a Norma DIN 13 para realização dos ensaios. As amostras foram submetidas a três níveis de introdução forçada de hidrogênio (10mA/cm2, 20mA/cm2 e 30mA/cm2) por controle galvanostático, para as diferentes temperaturas de revenimento. Para efeito comparativo, para cada temperatura de revenido foram produzidas amostras isentas de hidrogênio, caracterizando a condição inicial do material. Todas as amostras foram submetidas ao ensaio de tração com baixa taxa de deformação. Os corpos de prova convencionais foram também ensaiados em tração com carregamento simultâneo de hidrogênio, enquanto os parafusos foram submetidos ao ensaio de torque de fragilização com carregamento prévio. Foram caracterizados os micromecanismos de fratura em todas as condições das amostras confeccionadas. Os resultados obtidos evidenciaram que para menores temperaturas de revenimento as amostras carregadas com hidrogênio apresentaram uma maior perda das propriedades mecânicas como resistência à tração, tenacidade e alongamento quando comparadas a condição inicial, sendo que a geometria do parafuso foi mais propensa aos efeitos deletérios do hidrogênio que os corpos de prova convencionais. O micromecanismo de fratura na camada carbonitretada foi predominantemente intergranular nas amostras carregadas com hidrogênio e o núcleo apresentou coalescimento de microcavidades na maioria dos casos, com microcavidades rasas nas amostras carregadas com hidrogênio. / Hydrogen embrittlement is a potential cause of failure in various industrial components. This type of embrittlement occurs mainly in carbon steels and in martensitic stainless steels, which have high mechanical strength. In this study, it was evaluated the influence of the tempering temperatures of 300 ºC, 400 ºC and 500 ºC on the hydrogen embrittlement effect, for the ABNT 10B22-modified steel, after the process of carbonitriding, which is widely used in the manufacture of screws and fasteners. The specimens were machined according to the NACE TM 177-90 standard and were also employed as specimens in the tests, screws M4 X50 manufactured according to the DIN 13 standard. The samples were subjected to three levels of hydrogen charge (10mA/cm2, 20mA/cm2 and 30mA/cm2) by galvanostatic control for the different tempering temperatures. For comparison, each tempering temperature sample was produced free of hydrogen, characterizing the initial condition of the material. All samples were subjected to slow strain rate tensile test. The specimens were also analyzed in conventional tensile tests with simultaneous charging of hydrogen, while the screws were subjected to the embrittlement torque testing with precharged specimens. Micromechanisms of fracture were characterized in all conditions for each of the samples. The results showed that lower tempering temperatures caused greater losses of mechanical properties such as tensile strength, elongation and toughness for the hydrogen charged samples when compared to the initial condition, and the geometry of the screw was more prone to the deleterious effects of hydrogen that conventional standardized specimens. The case fracture micromechanism was predominantly intergranular in samples charged with hydrogen and the core showed coalescence of microcavities micromechanism, in most cases, with shallow microvoids in the samples charged with hydrogen.

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

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 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(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

No description available.

Hydrogen embrittlement is a well-known source of cracking in metallic mechanical elements. This work studies the influence of hydrogen and carbides on the crack formation in a castnickel-alloy-component in a selective catalytic reduction system. It was found in literature that the alloy is prone to hydrogen embrittlement, and could therefore be a key factor in the crack formation. Cracks were found in the component, which is subjected to urea vapor (which contains a lot of hydrogen) at high temperatures. The propagation path of the crack seems to follow the carbide network in the material. From these observations two objectives for this project was formulated. The first objective of the work is to determine if hydrogen embrittlementis a plausible theory as a cause of the cracking. The second objective of this work is to investigate whether local stress levels in the vicinity of carbides is further raised by hydrogen. A modified boundary formulation of a crack tip is coupled with hydrogen diffusion, in order to realize the first objective. The coupled mechanical and diffusion problem is solved with a finite element model. The finite element model approximates the concentration of hydrogen that is diffused into the body during the working time, by the process of hydrogen diffusion for various parameters: hydrogen concentration in the crack, carbide trap density, carbide trap energy and more. A literature study is carried out and relevant intervals of such parameters is determined. It is found from the FE model that the concentration of hydrogen, 0.5mm ahead of the crack tip, can be approximately 1000 appm for smaller carbide trap energies (weak traps). For largercarbide trap energies (strong traps), the hydrogen concentration 0.5mm ahead of the crack tipcan be as high as 10000 appm and above. As a range of feasible hydrogen concentrations has been established, the second objective can be considered. The second objective is determined by considering dislocations and hydrogen in the vicinity of a carbide with a discrete dislocation dynamics (DDD) model. A conservative hydrogen concentration of 1000 appm, as well as hydrogen free setting is considered in the DDD simulation. The presence of hydrogen is shown to elevate the local dislocation density byapproximately 20%, which in turn elevates the local stress levels. It is highly plausible that stress levels may be further elevated by hydrogen as hydrogen concentrations from the first objective may be much higher than 1000 appm.

[pt] As tubulações de aços utilizadas pela indústria do petróleo estão constantemente expostas à ação do ácido sulfídrico (H2S) que é o veículo para a difusão do hidrogênio no aço, provocando o surgimento de trincas induzidas pela fragilização por hidrogênio e/ou trincas de corrosão sob tensão. A junta soldada, que é considerada a região crítica dos dutos, foi o objeto de avaliação neste trabalho mediante ensaios de tração sob baixa taxa de deformação (BTD) e através da norma NACE TM0177/96 - Método A (Standard Tensile Test), verificando o seu comportamento quanto à fragilização por hidrogênio e à corrosão sob tensão em aços da classe API grau X70 e X80. Uma solução de tiossulfato de sódio foi utilizada no ensaio BTD para avaliação da fragilização por hidrogênio e corrosão sob tensão, reduzindo custos e atendendo aos ítens de segurança. Os resultados mostraram que os aços API 5L X70 e X80 são susceptíveis à fragilização por hidrogênio e à corrosão sob tensão. / [en] The steel tubes used in the Oil Industry are constantly exposed to hydrogen sulphide (H2S) which leads to the diffusion of hydrogen into the steel, provoking hydrogen induced embrittlement cracks and/or stress corrosion cracking. Welded joints, generally considered to be the critical region in pipelines, were evaluated in this study, using slow strain rate tensile testing (SSRT) and NACE TMO177/96 - Method A (standard tensile test) norm, verifying the behaviour of joints in API X-70 and X-80 grade steels, with regard to hydrogen embrittlement and stress corrosion cracking. Sodium thiosulphate was used in the slow strain rate tests permit the evaluation of hydrogen embrittlement and stress corrosion cracking while reducing testing costs and maintaining safety standards. The results show that the API X-70 and X-80 grade steels are susceptible to hydrogen embrittlement and stress corrosion cracking.

Diverse hypotheses are behind the strength degradation in metals due to hydrogen diffusion, leading to a severe, sudden failure. These diverse hypotheses of hydrogen embrittlement include various mechanisms that are responsible for the embrittlement of metals due to hydrogen exposure in their microstructures. This research study focuses on one hydrogen embrittlement mechanism: Hydrogen-Enhanced-Localized-Plasticity (HELP). The HELP is the only single mechanism characterized by promoting localization of plastic flow ahead of the crack by increasing dislocation motion in that region. The current state of the art is a development of a numerical model representing a fully diffusion-mechanical coupled model. This fully coupled model attempts to gain valuable insights into hydrogen's influence on the mechanical properties and the fatigue life of metals, in general. First, detailed development of a numerical approach is illustrated describing how to fully couple the hydrogen diffusion and stresses using a finite element method. The formulation is based on a coupled temperature-displacement procedure using Abaqus. This coupled computational model, described in this first part, is novel because the mechanical part is based on an isotropic-kinematic hardening law. Furthermore, this fully coupled numerical model can capture both a hardening and softening effect of the stress-strain curve when the solution of the plastic properties is dependent on hydrogen. This can also contribute in a complementary way to the results previously shown by other researchers. Though these previous studies used the same hydrogen diffusion model, their mechanical part was based on a power law. Second, this research attempts to delve into the hydrogen effect on the constitutive response of metals undergoing a cyclic load. Hence, based on the HELP theory, this constitutive coupled model can capture different cyclic hardening behaviors. This study can largely contribute to understanding the degradation of the mechanical properties of materials before crack propagation, which has been heavily covered in the literature.