Submitted by Pedro Silva Filho (pfsilva@ipen.br) on 2017-10-09T19:31:24Z No. of bitstreams: 0 / Made available in DSpace on 2017-10-09T19:31:24Z (GMT). No. of bitstreams: 0 / A marcação por feixe laser em superfícies é utilizada para assegurar a identificação e a rastreabilidade em biomateriais. A texturização via feixe laser confere maior aderência às superfícies dos dispositivos médicos implantáveis. Este trabalho teve o objetivo de avaliar o comportamento da superfície do aço inoxidável austenítico ABNT NBR ISO 5832-1 submetido à marcação e texturização com feixe laser de fibra óptica, alterando-se a frequência dos pulsos; frente ao seu comportamento biomecânico, por meio de: ensaios de resistência à tração, fadiga e desgaste; verificar a susceptibilidade à corrosão localizada, por ensaios eletroquímicos em solução que simula os fluidos corpóreos; e caracterizar a sua microestrutura. Os tratamentos alteraram a rugosidade e a dureza dos biomateriais em função do aumento da frequência dos pulsos. A microestrutura e composição química das superfícies sofreram mudanças que afetaram diretamente a camada passiva dos aços inoxidáveis. Este efeito foi comprovado com o uso de SVET, XPS e caracterização de propriedades eletrônicas do filme passivo. Os dois tipos de tratamentos implicaram em aumento de susceptibilidade magnética das superfícies. Os parâmetros utilizados para as marcações e texturizações não causaram diminuição na viabilidade celular, de modo que não apresentou citotoxicidade mesmo após incubação prolongada. Este biomaterial mostrou-se adequado perante os ensaios biomecânicos, uma vez que os tratamentos a laser, nas condições utilizadas, não induziram a formação de tensões superficiais de magnitude capaz de levar à fratura por fadiga, indicando vida em fadiga infinita; tampouco se pôde relacionar a região de fratura por tração com as marcações a laser. O volume de desgaste diminuiu em função do aumento da dureza produzido pela elevação da frequência do pulso nas texturizações. O caráter visual das marcações e texturizações por feixe laser foi assegurado após a maioria dos ensaios realizados. / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP

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No. of bitstreams: 1 elisondafonsecaesilva.pdf: 5310395 bytes, checksum: c89f2c8b3846fc7ab92440accf66f534 (MD5) Previous issue date: 2010-11-26 / Este trabalho estudou a composição química e metalográfica (microestrutura, tamanho de grão, teor de inclusões) de aços inoxidáveis austeníticos, desenvolvidos para aplicações como biomateriais, utilizados na fabricação de implantes ortopédicos, removidos de pacientes afetados por quadro inflamatório, e comparou as superfícies das amostras, através da Microscopia Eletrônica de Varredura (MEV) após ataque eletroquímico. Os resultados da análise química, por Espectrometria de Emissão Ótica e Microanálise por Dispersão de Energia (EDS), mostraram que todos os grupos apresentaram conformidade com as normas ASTM F138-92 e ABNT NBR ISO 5832-9:2008. O tamanho de grão foi determinado por Microscopia Ótica e Microscopia Eletrônica de Varredura (MEV). As superfícies das amostras foram analisadas por Microscopia Eletrônica de Varredura (MEV), depois de serem submetidos a ensaio de polarização cíclica potenciodinâmica em potencial de 50 mV após a região passiva em meio de solução de Ringer Lactato e solução de NaCl 0,9 mol.L-1na temperatura de 36,5 oC. Essas análises por MEV foram conduzidas nas amostras para identificar o tipo de corrosão apresentada. Concluiu-se que todos os implantes apresentavam tamanho de grão superior ao recomendado pela norma. Constatou-se também a presença de ferrita delta em oito dos doze implantes removidos, que de acordo com a norma ASTM 138-92, não deveria ser percebida microscopicamente com um aumento de 100 vezes. Verificou-se ainda que em meio de NaCl, o aço ISO 5832-9 ao contrário do aço F138, não apresentou nenhuma forma de corrosão localizada. Em solução de Ringer lactato, após ataque por 15 min. no potencial de 1000 mV/ECS foi notada a presença de pites no aço ISO 5832-9. O ensaio de polarização cíclica revelou que um dos aços, identificado como ISO 5832 9 apresentou resistência à corrosão localizada muito superior ao outro identificado como F 138, além de não liberar íons metálicos nas soluções eletrolíticas. Essa propriedade é principalmente atribuída ao aumento da estabilidade do filme passivo, que por sua vez, é favorecida pela presença do nitrogênio em solução sólida intersticial na austenita do aço ISO 5832-9. Os ensaios de polarização revelaram-se úteis para indicar a baixa resistência à corrosão por pites exibida “in-vitro” pelas próteses. As amostras do aço F 138 após serem submetidas ao ensaio eletroquímico em solução de Ringer lactato apresentaram perdas de 63% de níquel e 26% de ferro em relação à composição inicial. Tais elementos foram deslocados eletroquimicamente na forma iônica para a solução de Ringer lactato, onde formaram precipitados sendo, portanto, um forte indicador de que as reações sofridas pelos portadores das próteses foram motivadas por esses íons metálicos, que são posteriormente incorporados no organismo. Pretende-se com os resultados dessa pesquisa propor aos órgãos governamentais reguladores de normas técnicas e de vigilância sanitária, bem como a médicos e hospitais que exijam dos fabricantes de próteses de aços inoxidáveis laudos técnicos que atestem a qualidade dos implantes, onde cada lote de fabricação das peças seja acompanhado por certificados garantindo a composição química e as características metalográficas especificadas pelas norma existentes no Brasil. / This work has studied the chemical and metallographic composition (microstructure, grain size, inclusion rate) of austenitic stainless steels which were developed to be applied as biomaterials, and used in the production of orthopedical implants, removed from patients which had been affected by inflammation; it has also been compared the sample surfaces, by means of scanning electronic microscopy (SEM) after electrochemical attack. The results of chemical analysis, performed by optical emission spectroscopy and Energy Dispersive Spectroscopy (EDS) have shown that all samples are in agreement with the ASTM F138-92 e ABNT NBR ISO 5832-9:2008 regulations, whereas the grain size has been determined by optical microscopy and SEM. The surfaces of the samples were analyzed by SEM and EDS techniques; samples were submitted to potentiodynamic cyclic polarization tests in 50 mV potential after the passivation region in Ringer lactate and NaCl 0,9 mol.L-1 solutions at 36,5C; such SEM measurements were done to identify the type of corrosion over the samples. It has been concluded that all implants showed average grain sizes above the recommended by the regulations; also the presence of delta ferrite in eight of the twelve investigated implants has been seen, which according to the ASTM 138-92 regulation this would not been possible even with a 100-fold increase at the optical microscope. It has also been verified that in NaCl medium the ISO 5832-9 steel did not present any kind of localized corrosion, instead of F 138 steel; in Ringer lactate solution, after 15 minutes attack at 1000mV/SCE, has been noted the presence of pits for ISO 5832-9 steel. The cyclic polarization measurements have revealed that one of the investigated steels, named ISO 5832-9, presented a very high resistance to localized corrosion when compared to the F 138 samples, besides they did not release metallic ions to the electrolytic solutions. This characteristic can be mainly attributed to the increasing of the passivation film stability, which is also favored by the presence of nitrogen atoms in the solid solution composing the structure of ISO 5832-9 steel. The pit corrosion tests have shown very useful to indicate the low resistance to pit corrosion, present “in-vitro” in all the studied samples. The EDS mapping analysis of F 138 samples, submitted to electrochemical attack in Ringer lactate medium, have shown a loss of 63% of nickel and 26% of iron, when compared to the initial composition; such metals were electrochemically solubilized to the Ringer lactate solution, where they can probably precipitate from the solution, being a strong indication that the reactions taking place in the patients have been motivated by these metal ions, which can be incorporated by the human body. The obtained results of this work are the basis of a tentative proposition to the governmental organisms which are the responsible for the regulation of the technical specifications for the prosthesis producers, requiring from them technical reports assuring the quality for the implant pieces, i.e., for each manufacturing lot a description or a special certificate containing the chemical composition and the metallographic characteristics specified by the Brazilian laws.

Lors de l'usinage des alliages métalliques, les outils de coupe sont soumis à un chargement thermomécanique intense conduisant à une réduction considérable de leur durée de vie. L'utilisation d'outils revêtus s'avère alors bénéfique, en particulier lors de l'usinage à sec des aciers inoxydables considérés comme difficiles à usiner. Ce travail de thèse porte sur l'effet des revêtements en abordant les aspects de modélisation, de simulation numérique et expérimentaux de l'usinage à sec, avec des outils à géométries complexes. Des essais de chariotage ont été réalisés sur l'acier AISI 304L avec des outils revêtus et non revêtus. Une attention particulière a été apportée à la température, aux efforts et à la rugosité. Une large gamme de conditions de coupe a été considérée pour une compréhension avancée des phénomènes physiques mis en jeu. Cela a permis l'identification des conditions de coupe optimales pour le couple outil/pièce considéré, et la mise en évidence de l'importance des revêtements lors de l'usinage à sec des aciers inoxydables. Par ailleurs, une modélisation hybride analytique/numérique a été développée et mise en oeuvre. Fondée sur la direction d'écoulement du copeau, elle permet de déduire les efforts 3D à partir d'une simulation numérique 2D de l'usinage. Une procédure d'extraction de profils réels de l'outil à été introduite en utilisant un système de numérisation 3D Breuckmann. Cette procédure permet la prise en compte de la géométrie réelle de l'outil lors des simulations numériques. Enfin, la comparaison des résultats numériques et expérimentaux a permis la validation de la modélisation proposée / When machining metal alloys, cutting tools are subjected to intense thermomechanical loading, which can lead to a significant reduction of their lifetime. The use of coated tools is then beneficial, in particular during dry machining of stainless steels which are considered as difficult to cut materials. This phD thesis is focused on the effect of coatings addressing aspects of modeling, simulation and experimental tests using tools with complex geometries. Experimental tests under dry turning configuration were performed on an AISI 304L stainless steel with coated and uncoated tools. Particular attention was paid to the temperature evolution, cutting forces and roughness. A wide range of cutting conditions was considered for an advanced understanding of the physical phenomena involved in machining. Experimental results allowed the identification of optimum cutting conditions for the considered tool/workpiece couple and highlighted the importance of coatings in dry machining of stainless steels. In addition, a hybrid analytical/numerical modeling was developed and implemented in DEFORM code. Based on the chip flow direction, 3D forces can be deduced from a 2D numerical simulation of machining. An extraction procedure of real profiles of the tool was introduced using a 3D scanning Breuckmann system. This procedure allows taking into account the real geometry of the tool in numerical simulations. Finally, the comparison between numerical and experimental results allowed the validation of the proposed model

L’endommagement de fluage des aciers 316L(N) a été étudié expérimentalement et théoriquement à des températures élevées et des temps à rupture jusqu'à dix-neuf ans. Pour le fluage à court terme, les durées de vie sont correctement prédites par le modèle de striction en tenant compte de la dispersion expérimentale. Le modèle de Riedel couplant croissance de cavités par diffusion lacunaire et germination continue est utilisé afin de prédire l’effet de l’endommagement intergranulaire sur la durée de vie des aciers 316L(N). Les durées de vie sont correctement prédites par ce modèle pour le fluage à long terme quelle que soit l'acier austénitique étudié et la température appliquée (525°C-700°C). En tenant compte du régime de vitesse basse contrainte de la loi de Norton, le modèle de Riedel permet de prédire la durée de vie de fluage jusqu'à 25 ans. Aucun paramètre ajusté n’a été utilisé dans le modèle de Riedel. Mais le taux de nucléation de cavités doit être déduit des mesures de densité de cavités à partir des observations MEB-FEG. La cavitation se produit principalement aux interfaces carbures M23C6 /matrice austénitique. L'effet de l'hétérogénéité de la microstructure sur les concentrations de contraintes à l’interface matrice/précipité est simulé par la méthode des éléments finis (logiciel Cast3M). Elle vise à déterminer la distribution des champs de contraintes normales autour de précipités et à prédire numériquement le taux de nucléation de cavités. Les caractéristiques des précipités et le comportement en fluage de la matrice austénitique sont conjointement pris en compte. Les simulations numériques sont en accord avec les observations de sites préférentiels de micro-cavitation. / The creep fracture of 316L(N) austenitic SSs has been studied both experimentally and theoretically for high temperatures and lifetimes up to nineteen years. For short term creep, experimental lifetimes are predicted by the necking model taking into account scatter in input parameters. The Riedel modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation is then carried out. Lifetimes are predicted fairly well using this model for long term creep failure whatever the considered austenitic SSs and the applied temperature (525°C - 700°C). Taking into account low and high stress regimes of the Norton-power law, the Riedel model allows us to predict the creep lifetimes in agreement with literature results up to 25 years. No fitted parameter has been used as applying the Riedel model. But the cavity nucleation rate should be deduced from cavity density measurements using FEG-SEM observations. The intergranular cavitation occurs mainly at M23C6 carbides / austenitic matrix interfaces. That is why the effect of the heterogeneity of the microstructure at the matrix/precipitate interface stress concentrations is simulated by the finite element method (Cast3M software). It aims to determine the distribution of normal stress fields around precipitates and to predict the cavity nucleation rate. The features of the precipitates and the creep behavior of the austenitic matrix are both taking into account. Numerical simulations are in agreement with the observations of preferential sites cavitation.

[pt] Atualmente, pesquisas envolvendo metais e suas ligas têm como um de seus objetivos o controle microestrutural, como forma de aprimorar as propriedades de interesse, para as aplicações desejadas. Um fator importante que possui influência nas propriedades dos materiais policristalinos são os tipos, e distribuições de contornos de grãos e/ou de fase. A aplicação de processos que envolvam altas temperaturas, como tratamentos térmicos ou processos de soldagem, podem levar a transformações de fase, as quais modificam a cristalografia, composição química, distribuição e tamanho dos grãos. Os aços inoxidáveis duplex são ligas bifásicas constituídas de proporções aproximadamente iguais de ferrita e austenita pertencentes ao sistema Fe-Cr-Ni. Essa estrutura bifásica combina elevada resistência, boa tenacidade e excelente resistência à corrosão. Devido à essas excelentes propriedades são utilizadas em vários setores industriais como: indústrias químicas, petroquímicas, de petróleo e gás, e de construção naval. Incrementos na concentração de Cr e Ni nestes aços levam a formação de duas novas classes, os aços super e hiper duplex, cujas propriedades são semelhantes aos aços duplex, porém superiores. Neste caso, devido ao aumento no teor dos elementos de liga os processos de soldagem podem levar a formação de fases intermetálicas e/ou precipitados, as quais são deletérias as propriedades dos aços, facilitando o processo de corrosão. Muitas das transformações no estado sólido que ocorrem nos aços, seguem determinadas relações de orientação cristalográfica. Essas relações descrevem as interfaces de baixa energia que determinam os processos de nucleação e crescimento de novas fases. Portanto a determinação das relações de orientação presente nos aços duplex, super-duplex e hiper-duplex, a partir de transformações de fase obtidas em condições de equilíbrio e paraequilíbrio foram estudadas com a intenção de correlacionar com a susceptibilidade à corrosão por pites. Foram estudados os aços UNS S32205, UNS S32750 e UNS S33207, respectivamente pertencendo as classes duplex, super e hiperduplex A determinação das relações de orientação e fração de fases foi obtida por difração de elétrons retroespalhados (Electron Backscattering Diffraction - EBSD). A composição química das fases foi obtida por Espectroscopia de Energia Dispersiva (Energy Dispersion Spectroscopy - EDS). Estes dados foram correlacionados com os resultados obtidos após testes de corrosão ASTM G-48. Para todos os aços se observa a tendência do aumento da relação de orientação KS após submissão aos ciclos térmicos. No entanto, para as condições estudadas não foi encontrada uma correlação clara entre a presença da orientação KS com a formação de pites na ferrita. / [en] Currently, research involving metals and their alloys has microstructural control as one of its objectives, to improve the properties of interest, for the desired applications. An important factor that influences the properties of polycrystalline materials are the types and distributions of grain and/or phase boundaries. The application of processes involving high temperatures, such as heat treatments or welding processes, can lead to phase transformations, which modify the crystallography, chemical composition, distribution and size of the grains. Duplex stainless steels are biphasic alloys made up of approximately equal proportions of ferrite and austenite belonging to the Fe-Cr-Ni system. This two-phase structure combines high strength, good toughness and excellent corrosion resistance. Due to these excellent properties, they are used in various industrial sectors such as: chemical, petrochemical, oil and gas, and shipbuilding industries. Increases in the concentration of Cr and Ni in these steels lead to the formation of two new classes, super and hyper duplex steels, whose properties are like duplex steels, but superior. In this case, due to the increase in the content of alloying elements, the welding processes can lead to the formation of intermetallic phases and/or precipitates, which are deleterious to the properties of the steels, facilitating the corrosion process. Many of the solid-state transformations that occur in steels follow certain crystallographic orientation relationships. These relationships describe the lowenergy interfaces that determine the processes of nucleation and growth of new phases. Therefore, the determination of orientation relationships presents in duplex, super-duplex and hyper-duplex steels, from phase transformations obtained under equilibrium and paraequilibrium conditions, were studied with the intention of correlating with the susceptibility to pitting corrosion. The UNS S32205, UNS S32750 and UNS S33207 steels were studied, respectively belonging to the duplex, super and hyperduplex classes. of the phases was obtained by Energy Dispersion Spectroscopy (EDS). These data were correlated with the results obtained after ASTM G-48 corrosion tests. For all steels, there is a trend towards an increase in the KS orientation ratio after submission to thermal cycles, however no clear correlation between the presence of the KS orientation and the formation of pits in the ferrite was found.

Duplex stainless steels (DSSs), consisting of an equal amount of ferrite and austenite phases, have wide applications in e.g. vehicles, chemical engineering as well as nuclear plant  because  of  the  combination  of  excellent  mechanical properties  and  corrosion resistance.  However,  low  temperature  embrittlement has  existed  for  a  few  decades restricting the application of DSSs over about 250 ºC. When the service temperature ranges from around 250 to 500 °C, DSSs would become brittle because of the phase separation in the ferrite  phase. The phase separation is the  main reason  for the  low temperature embrittlement, and the kinetic of this phase separation achieves the fastest rate at about 475°C, so it is also termed as ‘475°C embrittlement’. Plenty of research has been performed in this field, but the problem remains. The mechanism of phase separation is due to the existence of a miscibility gap in the iron  chromium  binary  system,  and  previous  research  has  reported  some alloying elements can have the potency to delay the phase separation and the goal of my thesis is to investigate the influence of different alloying elements and select one which could be a plausible one to retard phase separation, and subsequently try to mitigate the low temperature embrittlement problem of DSSs. This  work  includes  the  literature survey  of  different  alloying  elements  which could influence   the   microstructure   and   mechanical   properties   of   DSSs   in general. Subsequently the thermodynamic calculation was performed to identify the effect of the  selected  alloying  elements  addition  on  phase  formation during  heat  treatment. Vanadium was selected to be a potentially suitable alloying element to be added into DSSs and experimental investigations were performed on the heat treatment process and the effect of V alloying. The main conclusions of the proposed thesis can be drawn as follow: The elemental addition  of Al,  Si, V,  Nb  and Ti  are  calculated  by Thermo-Calc,  they are  all  ferrite stabilisers and V addition seems most likely to be effective due its combination with the  interstitial  elements  C  and  N.  In  the  experimental  part, vanadium  additions combined with intermediate temperature solution treatment could be effective to retard the age hardening effect and the impact toughness test has a consistent tendency. Also, according to Thermo-Calc calculations and experimental results, the more interstitial elements that combine with V and precipitate from the ferrite phase the better was the performance of the duplex stainless steels. / Duplexa rostfria stål (DSS), bestående av lika stor mängd ferrit- och austenit, har bredtillämpning inom t.ex. fordon, kemiteknik samt kraftverk på grund av kombinationen av utmärkta mekaniska egenskaper och korrosionsbeständighet. Försprödning vid låg temperatur   begränsar emellertid tillämpningen   av   DSS   över   250   °C.   När driftstemperaturen varierar från cirka 250 till 500 ° C så kan DSS bli spröda på grund av   fasseparation   i   ferritfasen.   Fasseparationen   är   den    främsta   orsaken   till lågtemperaturförstöring, och kinetiken för denna fasseparation uppnår den snabbaste hastigheten  vid  cirka  475  °C,  så  den  kallas  också  '475  °C-försprödning'.  Mycket forskning har utförts inom detta område, men problemet kvarstår. Mekanismen för fasseparation beror på förekomsten av ett blandningslucka i det binära järn-krom-systemet, och tidigare forskning har rapporterat att vissa legeringselement kan  ha  förmågan  att  fördröja  fasseparationen.  Målet med mitt  examensarbete  är  att undersöka påverkan av olika legeringselement och välja ett som kan användas för att fördröja fasseparationen och därav mildra problemet med lågtemperaturförsprödning hos DSS. Detta  arbete  inkluderar  litteraturundersökning  av  olika  legeringselement som  kan påverka  mikrostrukturen  och  mekaniska  egenskaper  hos  DSS  i allmänhet.  Därefter termodynamiska beräkningar för att identifiera effekten av de valda legeringselementen på fasjämvikt under värmebehandlingen. Vanadin valdes som ett potentiellt lämpligt legeringselement som kan tillsättas i DSS. Experimentella undersökningar utfördes på värmebehandlingsprocessen och effekten av V-legering. De huvudsakliga slutsatserna av examensarbetetär: tillsatsen av Al, Si, V, Nb och Ti beräknades av Thermo-Calc, de är alla ferritstabiliserande och V-tillsats verkar kunna vara effektiv på grund av dess kombination med de interstitiella elementen C och N. I den    experimentella    delen    studerades vanadintillsatser    i    kombination    med värmebehandling för att fördröja den hårdnandet och försprödningen. Enligt Thermo- Calc-beräkningar  och experimentella  resultat  så  kan  prestanda  förbättras  desto  merinterstitiella element som kombineras med V.

Die Bildung von Anlauffarben durch molekularen Sauerstoff im Wurzelschutzgas ist bereits gut erforscht. Weitere Sauerstoffquellen für die Bildung von Anlauffarben sind Feuchte und CO2. Besonders Feuchte ist in diesem Zusammenhang kritisch, da sie sich wesentlich langsamer mit den etablierten Mitteln entfernen lässt. Im Rahmen dieser Arbeit konnte für durch Feuchte verursachte Anlauffarben im Wurzelschutzgas ein Grenzwert von 300 ppm bei 23 °C ermittelt werden. Die Farbausprägung der durch Feuchte verursachten Anlauffarben tendiert besonders bei schwachen bis mittleren Ausprägungen (450-800 ppm) stärker ins Bräunliche als die durch molekularen Sauerstoff verursachten Anlauffarben. Mit Hilfe des an der TU Dresden entwickelten Algorithmus für die Auswahl von Gaskomponenten für Prozessgase, wurde für das gepulste WIG-Orbitalschweißverfahren von austenitischen CrNi-Stahlrohren das Gasgemisch Ar/He/H2 20/70/10 ausgewählt. Gegenüber dem ursprünglich als Prozessgas verwendeten Argon und einem Ar/He 70/30 Gemisch, zeigte sich eine mögliche Steigerung der Vorschubgeschwindigkeit um den Faktor 3,5. Bei Fragen und Anmerkungen zur Arbeit sowie beim Verfassen einer weiterführenden Arbeit bitte Kontakt mit dem Autor aufnehmen (gerd.hauser@gmx.de). Dieser ist an einer Weiterführung des Themas sowie einer unabhängigen Überprüfung der Messwerte sehr interessiert. / The formation of annealing colors by molecular oxygen in the backing gas is already well understood. Other sources of oxygen for the formation of annealing colors are humidity and CO2. Moisture is especially critical in this context, since they can be removed much more slowly with the established agents. As part of this work can be set a limit for annealing colors (caused by moisture in the root protection gas) of 300 ppm moisture with a environment temparatur of 23 ° C. The annealing colors caused by humidity tends especially in low to moderate conzentrtions (450-800 ppm) mor to a brownish than the annealing colors caused by molecular oxygen. Using the algorithm developed at the TU Dresden for the selection of gas components for process gases,the gas mixture Ar/He/H2 20/70/10 was chosen for the pulsed TIG orbital welding of austenitic stainless steel tubes. Compared to the originally used process gas argon and Ar/He-mixture 70/30, showed the Ar/He/H2 mixtur a possible increase in feed rate by a factor of 3.5.

Les aciers inoxydables austénitiques métastables sont le siège de différents mécanismes de déformation qui sont à l'origine des propriétés mécaniques qui distinguent ce type d’alliages. Cependant, ces dernières, dépendant de la microstructure locale, sont fortement anisotropes. Par ailleurs, la déformation d'un échantillon massif serait différente de celle obtenue en surface. De ce fait, une étude détaillée trouve tout son intérêt. Le présent travail vise donc à identifier les principaux mécanismes de déformation et de leur évolution progressive, en se basant sur une déformation contrôlée de grains austénitiques individuels par des tests mécaniques de nanoindentation monotoniques et cycliques. Les courbes correspondantes au chargement-déchargement révèlent des informations détaillées sur les propriétés mécaniques sous-jacentes qui pourraient être liées à une étude complète de la structure de déformation en surface et en volume par différentes techniques de caractérisation à une échelle très fine. La déformation en fonction du temps, les phénomènes de transformation de phase réversible sous charge, l'anisotropie cristalline, l'influences de la taille des grains, la transmission de la plasticité et la tenue en fatigue ont été mis en évidence et étudiés / Metastable austenitic stainless steels feature an abundance of different deformation mechanisms, which contribute to the distinguished mechanical properties of these alloys. However, these properties are known to depend on the local microstructure and also are highly anisotropic. Furthermore, deformation is expected to be different for the bulk and the surface of a sample. In this sense, a discrete study is not trivial. The present work aims at investigation of the main deformation mechanisms and their gradual evolution, by employing controlled deformation of individual austenite grains via monotonic and cyclic nanoindentation. The corresponding loading–unloading curves have given extensive information about underlying mechanical properties, which could be related to an exhaustive reconstruction of the deformation substructure, both in surface and bulk, by different small scale characterization techniques. Amongst others, features such as time-dependent deformation, reversible phase transformation under load, crystalline anisotropy and grain size influences, besides plasticity transmission and fatigue behavior have been found and analyzed

Les matériaux métalliques sont utilisés dans des conditions de plus en plus sévères et doivent présenter une parfaite intégrité sur des périodes de plus en plus longues. L’objectif de ce travail de thèse est d’évaluer le potentiel d'un traitement de refusion laser pour améliorer la résistance à la corrosion d'un acier inoxydable de type 304L ; l’utilisation du laser dans le domaine des traitements de surface constituant un procédé en pleine évolution à cause des changements récents dans la technologie des lasers. Dans le cadre de ce travail, le choix du laser s’est porté sur un laser nano-impulsionnel à fibre dopée ytterbium dont les caractéristiques permettent la fusion quasi-instantanée sur quelques microns de la surface traitée, immédiatement suivie d'une solidification ultra-rapide avec des vitesses de refroidissement pouvant atteindre 1011 K/s. La combinaison de ces processus favorise l'élimination des défauts surfaciques, la formation de phases hors équilibre, la ségrégation d’éléments chimiques et la formation d’une nouvelle couche d’oxyde dont les propriétés sont gouvernées par les paramètres laser. Afin de les corréler avec la réactivité électrochimique de la surface, l’influence de deux paramètres laser sur les propriétés physico-chimiques de la surface a été étudiée : la puissance du laser et le taux de recouvrement des impacts laser. Pour clarifier ces relations, la résistance à la corrosion par piqûration des surfaces traitées a été déterminée par des tests électrochimiques. Pour des paramètres laser spécifiques, le potentiel de piqûration d'un acier inoxydable de type 304L augmente de plus de 500 mV traduisant ainsi une meilleure tenue à la corrosion localisée en milieu chloruré. L’interdépendance des différents phénomènes résultant du traitement laser a rendu complexe la hiérarchisation de leur effet sur la sensibilité de l’alliage testé. Cependant, il a été montré que la nature de l’oxyde thermique formé au cours de la refusion laser et ses défauts sont du premier ordre pour l’amorçage des piqûres. / Metallic materials are more and more used in severe conditions with particularly strong request for improving their behavior in aggressive environment and especially over long periods. The objective of this PhD work is to estimate the potentiality of a laser surface melting treatment on the improvement of the stainless steel 304L corrosion resistance, surface treatments by laser can be revisited on the basis of a recent change in the laser technology. In the frame of this work, a nano-pulsed laser fiber was chosen : it allows the treated surface to be melted for few microns in depth, followed by an ultra-fast solidification occuring with cooling rates up to 1011 K/s. The combination of these processes leads to the elimination of the surface defects, the formation (trapping) of metastable phases, the segregation of chemical elements and the growth of a new oxide layer which properties are governed by the laser parameters. To correlate these latter to the electrochemical reactivity of the surface, the influence of two laser parameters on the physico-chemical properties of the surface was studied : the laser power and the overlap of the laser impacts. To support this approach, the pitting corrosion resistance of the samples was determined by standard electrochemical tests. For specific laser parameters, the pitting potential of a 304L stainless steel was increased by more than 500 mV corresponding to an important enhancement in localized corrosion resistance in chloride environment. The interdependence of the different phenomena resulting from the laser treatment lead to a quite complex prioritization of their role on the sensibility of the 304L. However, it was demonstrated that the nature of the thermal oxide formed during the laser surface melting and the induced defects are first-order parameters for the initiation of pits.

stainless steel, duplex steel, vakuum metallurgy, decarburization