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De nos jours, il est crucial pour les industries de réduire leur consommation énergétique. Pour les industries du transport, cela peut se faire par le biais de l’allègement des pièces de structure. Dans ce contexte, les structures cellulaires représentent une des solutions les plus prometteuses. Grâce au développement de la fabrication additive, l’élaboration de telles géométries complexes n’est plus un frein à leur utilisation. Néanmoins, cette dernière restera limitée tant que les propriétés mécaniques – et plus particulièrement la résistance en fatigue pour les pièces aéronautiques – ne seront pas maîtrisées. Ce travail de thèse a pour objectif de déterminer les mécanismes qui gouvernent le comportement en fatigue de ces structures cellulaires. Pour cela, le travail s’est focalisé sur l’élement unitaire les constituant : la poutre. Des éprouvettes minces représentatives de la poutre ont été élaborées par Electron Beam Melting puis caractérisées à l’état brut de fabrication à l’aide de différentes techniques (tomographie aux rayons X, microscopie optique et électronique, …). Leurs propriétés statique et cyclique en traction ont ensuite été évaluées. L’état de surface et en particulier les défauts d’entaille ont été identifiés comme responsable de la perte de résistance. L’impact de ces défauts sur la résistance en fatigue a été prédit avec succès par le biais de diagrammes de Kitagawa. L’impact de l’orientation de fabrication a également été observé et prédit. Différents post-traitements ont ensuite été utilisés afin d’améliorer ces propriétés. Le polissage chimique et le grenaillage ultrasonique ont réduit de manière significative la criticité des défauts de surface ce qui a grandement amélioré les propriétés mécaniques des éprouvettes, jusqu’à se rapprocher de celles obtenues après usinage. Par ailleurs, la compression isostatique à chaud a provoqué la fermeture de l’entièreté des défauts internes ainsi qu’un grossissement de la microstructure. Ce dernier point permet une amélioration supplémentaire de la performance en fatigue une fois combiné avec un traitement de surface. Enfin, une méthode permettant de détecter automatiquement tous les défauts d’entailles et de déterminer leur criticité et leur influence sur la résistance en fatigue a été proposée et discutée. Elle a ensuite été appliquée avec succès aux échantillons attaqués chimiquement mais des modifications demeurent nécessaire pour l’appliquer à d’autres états de surface. / Nowadays, reducing the energy consumption is crucial for most of the industries. For transportation industries, it can be achieved through weight reduction. In this context, cellular structures turn out to be one of the most efficient solution. Thanks to the development of additive manufacturing, producing such complex geometries is no longer an issue. However, their use will remain limited as long as their fatigue performances are not known. This PhD work aimed at understanding the mechanisms that govern the fatigue behaviour of such cellular structures. It was first decided to focus on their unitary element, i.e. a single strut. Single struts samples were manufactured by Electron Beam Melting and then characterized in as-built conditions using different experimental techniques (X-ray tomography, optical and electron microscopy, etc.). Their static and cyclic tensile properties were then evaluated. The rough surface and in particular notch-like defects were found to be responsible for the knockdown of the mechanical properties. Regarding the fatigue resistance, their detrimental impact was predicted using Kitagawa diagrams. It also enabled to explain the impact of the build orientation. Different post-treatments were used in order to improve these mechanical properties. Chemical etching and ultrasonic shot peening (USP) significantly reduced the severity of surface defects of as-built thin struts and thus increased their mechanical properties. After USP, the fatigue properties of machined samples were almost matched. Hot Isostatic Pressing lead to the closure of all internal defects and to the coarsening of the microstructure. When combined with one of the surface treatments, the fatigue properties were further improved. Finally, a method enabling to systematically and automatically extract from the surface the most critical defects and quantitatively analyze their influence on fatigue life was proposed and discussed. It was successfully applied to chemical etched samples but improvements are mandatory for other surface conditions.

O Jateamento com granalha (shot peening, em inglês) é um processo de fabricação amplamente utilizado em indústrias mecânicas, automotivas, navais, aeronáuticas e outras indústrias metalúrgicas. O objetivo principal é induzir tensões residuais de compressão para melhorar a vida à fadiga das peças e estruturas. A modelagem e simulação do processo são muito difíceis, pois envolvem a consideração de muitos e complexos aspectos. Este trabalho propõe um método de modelagem tridimensional utilizando o método dos elementos finitos para a simulação numérica desse processo assim como a sua influência na propagação de trinca, considerando efeitos pouco estudados, como o amolecimento do material devido ao aumento da temperatura na peça durante os impactos, ao comportamento elasto-plástico das granalhas e a rugosidade da peça tratada nos parâmetros da trinca. A modelagem proposta para simular o processo de jateamento com granalhas inclui: (i) análise dinâmica explícita, (ii) modelagem tridimensional, (iii) cálculo da velocidade real das granalhas, (iv) modelo de contato com coeficientes de atrito estático e dinâmico entre as áreas em contato, (v) o modelo constitutivo de Johnson-Cook para o comportamento do material alvo, (vi) um comportamento elasto-plástico multilinear para as granalhas, (vii) a consideração da cobertura real de uma área dada da superfície jateada, (viii) a consideração do aumento da temperatura no material durante o processo e (ix) o relaxamento de tensões residuais devido a este aumento de temperatura. As simulações numérico-computacionais para investigar a influência do processo em fenômenos, tais como fadiga e propagação de trinca, necessitou uma nova abordagem para a incorporação das tensões residuais num modelo de elementos finitos. Além disso, foram realizados vários ensaios quase-estáticos e dinâmicos para a determinação dos parâmetros do modelo constitutivo de Johnson-Cook para o aço AISI 5160. Para a avaliação das metodologias propostas, as tensões residuais foram introduzidas e avaliadas em uma mola parabólica automotiva, sendo que as tensões residuais foram avaliadas através da técnica de difração de raios-X, a intensidade e a cobertura do processo de jateamento com granalha na mola foram avaliadas com Lupa e rugosímetro. Também, dados experimentais de ensaios de fadiga relatados na literatura, realizados em corpos de prova tipo CT foram utilizados. A modelagem numérica do processo de jateamento com granalha desenvolvida neste trabalho consegue prever com sucesso os perfis das tensões residuais, com valores bem próximos aos obtidos experimentalmente. A investigação numérica do efeito do processo na vida à fadiga e fratura das peças comprovou a sua influência nos parâmetros de propagação de trinca nas regiões afetadas pelo processo, ocasionando um amplo aumento da vida à fadiga, e confirmou o relaxamento das tensões residuais devido aos ciclos de carregamento conforme as observações experimentais na literatura. / Shot peening is a manufacturing process widely used in the mechanical, automotive, marine, aeronautical and other metallurgical industries. The main purpose is to induce residual compression stresses to improve fatigue life. The modeling and simulation of the process are very difficult as they involve the consideration of many and complex aspects. This work proposes a three-dimensional modelling method using the finite element method for the numerical simulation of this process as well as its influence on crack propagation, considering little studied effects such as softening of the material due to the increase in temperature in the part during impacts , the elastic-plastic behavior of the shot and the roughness of the treated part in the crack parameters. The proposed modelling approach to simulate the shot peening process includes: (i) explicit dynamic analysis, (ii) three-dimensional modeling, (iii) calculation of the real velocity of the shot, (iv) contact model with static and dynamic friction coefficients between the contact areas, (v) the Johnson-Cook constitutive model for target material behavior, (vi) a multilinear elasto-plastic behavior for the shot, (vii) consideration of real coverage in a given area of the peened surface, (viii) consideration of temperature increase in the material during the process and (ix) relaxation of residual stresses due to this rise in temperature. Numerical-computational simulations to investigate the influence of the shot peening process on phenomena, such as fatigue and crack propagation, required a new approach for the incorporation of residual stress in a finite element model. In addition, several quasi-static and dynamic tests were carried out to determine the parameters of the constitutive Johnson-Cook model for AISI 5160 steel. To assess the proposed methodologies, the residual stresses were introduced and evaluated in an automotive parabolic spring, with the residual stresses being evaluated by the X-ray diffraction technique, the intensity and coverage of the shot peening process for the spring were evaluated with magnifying glass and roughness gauge. Also, fatigue tests available in the literature, carried out on types of CT specimens, were used. The numerical modeling of the shot peening process developed in this work can successfully predict the residual stress profiles, with values close to those obtained experimentally. The numerical investigation of the effect of the process in the fatigue life and fracture of the pieces proved its influence in the parameters of crack propagation in the regions affected by the process, causing a wide increase of the fatigue life, and confirmed the relaxation of the residual stresses due to the cycles loading according to experimental observations in the literature.

Fretting fatigue is a damage process that may arise in engineering applications where small cyclic relative displacements develop inside contacts leading to detrimental effects on the material fatigue properties. Fretting is located in regions not easily accessible, which makes it a dangerous phenomenon. It is therefore important to be able to make reliable predictions of the fretting fatigue lives. The work presented in this thesis has its focus on different aspects related to fretting fatigue in the titanium alloy Ti-17. A fretting experiment was developed which allowed for separate control of the three main fretting loads. Initially, the evolution of the coefficient of friction inside the slip region was investigated experimentally and analytically. Subsequently, 28 fretting tests were performed in which large fatigue cracks developed. The fretting tests were firstly evaluated with respect to fatigue crack initiation through five multiaxial fatigue criteria. The criteria predicted a too high fretting fatigue limit. A possible clue to the discrepancy was found in the fretting induced surface roughness with the asperity-pit interactions. The fatigue growth of the large fretting cracks was numerically modelled through a parametric crack growth procedure. The predicted lives were compared to the experimental outcome. The numerical simulations showed that linear elastic fracture mechanics was an appropriate tool for the prediction of fretting fatigue propagation lives in the long crack regime. Fatigue cracks spend most of their propagation life in the small crack regime. The possibility of modelling the small crack behaviour is therefore very important from the engineering point of view. The fatigue growth of through thickness short cracks was studied experimentally and numerically in the four-point bend configuration. It was found that linear elastic fracture mechanics and closure-free material growth data furnished conservative estimates for cracks longer than 50 μm. One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental results on fretting life with or without shot peening were simulated. The fatigue life enhancement in shot peened specimens could be explained by slower crack growth in the surface material layer with residual compressive stresses. / QC 20100827

Η σφυρηλάτηση με βολή σωματιδίων (shot peening) είναι μία επιφανειακή κατεργασία που πραγματοποιείται με σκοπό την αύξηση της αντοχής μεταλλικών υλικών και εφαρμόζεται στο τελευταίο στάδιο της γραμμής παραγωγής. Η αύξηση της αντοχής επιτυγχάνεται με την ανάπτυξη θλιπτικών παραμενουσών τάσεων κοντά στην επιφάνεια του υλικού έπειτα από την κρούση σωματιδίων με υψηλές ταχύτητες. Η ανάπτυξη θλιπτικών παραμενουσών τάσεων αυξάνει την αντοχή σε κόπωση, σε εργοδιάβρωση, καθώς και σε άλλες μηχανικές καταπονήσεις και επιτρέπει την μείωση του βάρους σχεδιάζοντας διατομές με μικρότερο πάχος. Στην παρούσα Διδακτορική Διατριβή παρουσιάζεται μια ολοκληρωμένη αριθμητική προσομοίωση της κατεργασίας και εξετάζεται η μηχανική συμπεριφορά των υπό κατεργασία υλικών σε υψηλούς ρυθμούς καταπόνησης. Συγκεκριμένα η μεθοδολογία που αναπτύσσεται περιλαμβάνει την ανάπτυξη ενός αριθμητικού προτύπου για την προσομοίωση της κατεργασίας της σφυρηλάτησης με βολή σωματιδίων και τον υπολογισμό των αποτελεσμάτων της στο υλικό. Τα βήματα που ακολουθηθήκαν για την ανάπτυξη του αριθμητικού προτύπου είναι, α) ο χαρακτηρισμός του κράματος αλουμινίου 7449-Τ7651 σε υψηλούς ρυθμούς καταπόνησης μέσω της πειραματικής διάταξης Split Hopkinson Bar που σχεδιάστηκε και κατασκευάστηκε στο Εργαστήριο Τεχνολογίας και Αντοχής Υλικών, β) η ανάπτυξη βοηθητικών επιμέρους αριθμητικών μοντέλων, γ) η ανάπτυξη κινηματικών μοντέλων προσομοίωσης της ροής των σωματιδίων, δ) η ανάπτυξη κριτηρίων και η εφαρμογή τους για τον υπολογισμό του ελαχίστου απαιτούμενου αριθμού σωματιδίων για την προσομοίωση, καθώς και των θέσεων κρούσης, ε) η ανάπτυξη ενός αριθμητικού προτύπου πλήρους γεωμετρίας της πλάκας για την κρούση του απαιτούμενου αριθμού σωματιδίων και στ) η πειραματική επαλήθευση του αριθμητικού προτύπου. Με το αριθμητικό πρότυπο που αναπτύχτηκε υπολογίστηκαν τα αποτελέσματα της κατεργασίας της σφυρηλάτησης με βολή σωματιδίων στο υλικό και επιβεβαιώθηκαν μέσω συγκρίσεων με αντίστοιχα πειραματικά αποτελέσματα. Αποτελέσματα της κατεργασίας εκτός από τις παραμένουσες τάσεις αποτελούν και η πλαστική παραμόρφωση, η σκληρότητα, η επιφανειακή τραχύτητα και κατ' επέκταση ο συντελεστής έντασης τάσης. Στη συνέχεια, πραγματοποιήθηκε μια παραμετρική μελέτη για την επίδραση της διαμέτρου, της ταχύτητας και της γωνίας κρούσης στην ανάπτυξη των παραμενουσών τάσεων. Επίσης το αριθμητικό πρότυπο επαληθεύτηκε και για άλλα μεταλλικά υλικά. / Shot peening is a surface treatment process that is performed to increase the strength of metallic materials and is applied to the last stage of the production line (post manufacturing process). The increase in strength is achieved by the developed compressive residual stresses near the surface and the subsurface of the treated material after the impact of small diameter particles with high speeds. The developed compressive residual stresses increases the fatigue strength, the mechanical performance of the component under stress corrosion cracking (SCC), under higher stresses and allows lighter structure design. This PhD thesis presents a comprehensive numerical simulation of the Shot peening process and includes a comprehensive study of the mechanical behaviour of treated materials under high strain rates of deformation. Specifically, the methodology developed includes the development of a comprehensive numerical model to simulate Shot peening treatment and calculate the results on the treated material. The steps followed for the development of the numerical model are: a) the characterization of the Aluminium alloy 7449-T7651 at high strain rates using a Split Hopkinson Bar apparatus designed and built at the Laboratory of Technology and Strength of Materials, b) the development of auxiliary partial numerical models, c) the development of a kinematic simulation model for the analysis of the flow particles, d) the development and the application of two criteria for the successful calculation of the minimum number of particles that required for the simulation, and the impact positions e) the development of a numerical model describing the full plate geometry for the impact of the minimum number of particles required and f) the experimental verification of the numerical model. The process outcomes and results on the treated material were calculated by the numerical model developed. The numerical results that were calculated for the threaded material were confirmed by comparison with experimental results. Treatment results include the residual stresses, the plastic deformation, hardness, surface roughness, and hence the stress concentration factor. A parametric study on the effect of the diameter, speed and angle of impact to the development of residual stresses was performed. The numerical model was also verified for a number of other metallic materials.

La surface des pièces mécaniques est une zone sensible soumise à des conditions de sollicitations particulières, tant mécaniquement (frottement, contrainte maximale) que chimiquement (atmosphère ambiante, corrosion). Ainsi, la ruine des pièces de service est généralement initiée en surface ; les grands secteurs industriels sont donc à la recherche de solutions technologiques permettant une amélioration des propriétés mécaniques globales par une modification des propriétés de surface. De nombreuses techniques ont été développées dans ce but, notamment les traitements de surface mécaniques. Parmi ceux-ci, le grenaillage ultrasonique permet de déformer sévèrement et superficiellement les pièces par de nombreux impacts de billes ayant des trajectoires aléatoires au sein de la chambre de traitement. Le propos de cette étude repose sur l'analyse et la compréhension des microstructures de déformation induites par un traitement de grenaillage ultrasonique, particulièrement sous conditions cryogéniques ; sujet très peu exploré à ce jour voir nouveau concernant i) des métaux susceptibles de subir une transformation martensitique et ii) l'influence d'un tel traitement sur la tenue en fatigue cyclique. Pour ce faire, plusieurs nuances d'aciers inoxydables austénitiques présentant des stabilités différentes vis-à-vis de la transformation de phase ont été traitées à très basses températures et les propriétés obtenues ont été comparées à celles mesurées sur les échantillons traités à température ambiante. Les premières observations ont montré que, suite à un traitement sous condition cryogénique (-130 °C), une baisse de dureté intervient en sous-couche de l'alliage 310S stable, associée à une hausse des propriétés mécaniques sous basse température rendant le matériau plus difficile à écrouir. Ce phénomène est complètement supprimé au sein de l'alliage métastable 304L par une transformation martensitique facilitée, intervenant plus profondément qu'à température ambiante et entrainant une augmentation de la dureté de sous-couche. Deux alliages métastables (304L et 316L) ont donc été sélectionnés afin de détailler l'influence des paramètres de traitement sur le durcissement de sous-couche par une étude paramétrique comprenant l'amplitude de vibration (40 et 60 µm), la durée (3 et 20 min) ainsi que la température de traitement (ambiante, -80 et -130 °C). Il en ressort qu'augmenter l'énergie de traitent par une hausse de l'amplitude et/ou de la durée de grenaillage entraine une augmentation des duretés de surface et de sous-couche, accompagnée par la production de couches durcies plus épaisses. L'utilisation de températures cryogéniques permet une augmentation du potentiel de durcissement, et ce principalement en sous-couche. En associant les gradients de dureté aux distributions de martensite le long des épaisseurs affectées, il a été montré que la fraction de martensite était directement liée au potentielle de durcissement en profondeur. La fraction de martensite produite étant dépendante de la température de déformation et, afin de prendre en compte la stabilité initiale de l'alliage comme paramètre additionnel, des mesures complémentaires ont été faites sur l'alliage 316L plus stable. Les résultats ont alors montré qu'il est primordial d'adapter la température de traitement à la stabilité de l'échantillon afin d'optimiser l'efficacité du durcissement de sous-couche et éviter ainsi une baisse de la dureté en profondeur. Finalement, les structures de déformation obtenues sous condition cryogénique ont été reliées à la tenue mécanique sous sollicitations cycliques en flexion rotative. Comparé à un traitement réalisé à température ambiante, un grenaillage cryogénique permet une baisse la rugosité de surface et la production de contraintes résiduelles de compression plus élevées par la présence de martensite. Cependant, une plus grande relaxation de ces dernières associée à une réduction de l'épaisseur [...] / The surface of mechanical components is a sensitive zone subjected to particular mechanical (friction, maximum stress) and chemical (ambient atmosphere, corrosion) interactions. Hence, the rupture is generally initiated on the surface. In order to increase the global integrity of the working parts, the industrial groups are still seeking technological solutions allowing the modifications of the surface properties. Nodaway, plenty of surface modification techniques have been developed like the mechanical surface treatments. Among them, the ultrasonic shot peening (or surface mechanical attrition treatment) focus on superficially deform the mechanical parts through numerous collisions of peening medias having random trajectories inside a confined chamber. The purpose of this study is based on the analysis and the comprehension of the deformed microstructures induced by the ultrasonic shot peening treatment, especially under cryogenic temperatures. To do so, several austenitic stainless steel grades having different stabilities regarding the martensitic transformation have been treated under cryogenic conditions and compared to the properties obtained under room temperature. The first observations have shown that, after a cryogenic peening, a decrease of the subsurface hardness takes place in the stable 310S alloy which was attributed to an increase of the mechanical properties under cryogenic temperature. This phenomenon is suppressed in the metastable 304L by triggering a martensitic phase transformation promoted under low temperature and happening deeper compared to room temperature, increasing substantially the subsurface hardness. Two metastable alloys (304L and 316L) were then selected to conduct an ultrasonic shot peening parametric study including the vibration amplitude (40 and 60 µm), the treatment duration (3 and 20 min) and temperature (room temperature, -80 and -130 °C). It has been shown that increasing the treatment energy by raising the vibration amplitude and/or the duration leads to an increase of the surface and subsurface hardnesses as well as the affected layer thickness. The use of cryogenic temperatures allows an additional increase of the hardness, especially in subsurface. By comparing the different hardness gradients with the martensite distributions along the hardened layers, a direct correlation with the hardening rate and the martensite fraction was observed. The initial stability of the treated material was also taken in account by carried out additional observations on the 316L having a higher stability. The results have indicated that the deformation temperature needs to be wisely chosen regarding the stability of the processed material in order to avoid a decrease of the subsurface hardness. Finally, the deformed microstructures generated under cryogenic ultrasonic shot peening were associated to the mechanical behaviors of cylindrical specimens using rotating bending fatigue tests. Compared to a room temperature treatment, a cryogenic peening allows a decrease of the surface roughness and the generation of higher surface compressive residual stresses by the formation of martensite. However, compared to a room temperature treatment, the fatigue behavior was not increased after a cryogenic peening because of a more pronounced surface residual stress relaxation and a reduction of the affected layer. However, the potential increase of the fatigue life after a cryogenic surface deformation was depicted by the study of the rupture surfaces. It was observed that, if the involvement of the surface defects introduced by the high surface roughness can be lowered, a single subsurface crack initiation can be produced increasing considerably the fatigue behavior of the processed material

Submitted by Marco Antonio Oliveira da Silva (maosilva@ipen.br) on 2017-05-25T11:30:49Z No. of bitstreams: 0 / Made available in DSpace on 2017-05-25T11:30:49Z (GMT). No. of bitstreams: 0 / No presente trabalho foram estudados alguns efeitos causados pelo jateamento por granalhas no aço Cr-Si-V, processo que tem por objetivo aumentar a resistência à fadiga. Para este estudo a variação de parâmetros no processo são ferramentas para melhor compreender os mecanismos que influenciam esta propriedade. Os parâmetros utilizados neste trabalho foram a variação da granalha e o pré tensionamento das amostras aplicados em lâminas utilizadas em feixe de molas automobilísticas de aço de liga cromo silício vanádio (SAE 9254+V). Inicialmente foi realizada a avaliação do perfil de tensão residual, efetuada por difração de raios-x pelo método do sen2 ao longo da espessura na região onde a tensão é compressiva. Nos resultados nota-se um efeito anômalo em relação ao perfil característico da distribuição de tensão residual com a perda de compressão nas camadas iniciais em relação à superfície jateada. Com o uso da microscopia eletrônica de varredura foi observado a região afetada pelo jateamento por granalhas notando que as regiões plasticamente deformadas se encontram nas mesmas regiões onde ocorre a diminuição da tensão residual compressiva. O perfil obtido pela difração de raios-x fornece as informações necessárias com o propósito de conjugar os efeitos que a microtensão (microdeformação) influenciam na macrotensão (tensão residual). Esta relação foi comprovada pela sobreposição dos resultados encontrados na distribuição da microdeformação da rede cristalina com a tensão residual ao longo da espessura na região plasticamente deformada. Os resultados dos perfis das difrações de raios-x mostraram a existência de anisotropia de tensões entre os planos, geradas por defeitos de empilhamento e pela densidade de discordâncias. Assim sendo, para obter os valores das microtensões devem ser considerados os fatores das constantes elásticas (conforme o módulo das direções) e dos planos cristalográficos. O método aplicado foi o Williamson-Hall modificado. Além deste método outros também foram utilizados, tais como: método de Warren-Averbach e o método Single Line, este aplicando a série de Fourier, porém, ao observar os resultados, nota-se a diferença entre os valores obtidos nas deformações, tanto aos métodos quanto aos planos cristalográficos, porém a ênfase deste trabalho foi dada às características dos perfis da distribuição e não aos valores absolutos. Somando a isto, foi proporcionado o estudo da distribuição do tamanho médio de cristalito ao longo da espessura no perfil das difrações de raios-x e os resultados mostraram que esta distribuição varia de forma inversa a microdeformação. A relação entre o tamanho médio de cristalito e a variação das distâncias interplanares corresponde diretamente a alterações das densidades de discordâncias ocorridas no material que por sua vez estão ligadas às microdeformações da rede cristalina. Como complemento ao trabalho foram analisados o fator de concentração de tensão e ensaios de fadiga do material. O fator de concentração de tensão não variou conforme a utilização das granalhas S330 e S390, ambas sendo aplicadas no mesmo regime de jateamento. Os ensaios de fadiga indicaram que o pré tensionamento aumenta a vida útil nas tensões cíclicas do material estudado. / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Corrosion fatigue is a common failure mechanism in rock drilling components and many othermechanical parts subjected to cyclic loads in corrosive environments. A crucial part in the design ofsuch components resides in the selection of the right materials for the application, which ideallyinvolves testing and comparison of their performance under working conditions. The present work was performed with the purpose of designing a corrosion-fatigue testing methodthat would allow the designer to compare the performance of different materials exposed to corrosionfatigue, permitting also the comparison with results from dry fatigue testing. The method was designedfor rotating-bending machines. Two different steel grades were used during the work, one throughhardened and one case hardened. The effect of these heat treatments and of shot peening overcorrosion-fatigue behaviour were studied using the proposed method. It was proven that the testing speed has a strong impact on the fatigue life of steel. It was found that,at a fixed stress level, the case hardened and shot peened steel reached 3X10^6 cycles at 2300 rpm,while it failed at only 5X10^5 cycles with a testing speed of 500 rpm. A large beneficial influence of theshot peening was demonstrated. It was also observed that, at fixed testing speed, the shot peening onthe through hardened steel can increase its fatigue strength from 190 MPa to 600 MPa under corrosionfatigue. Many cracks were found at the surface of the shot peened parts, which are arrested near thesurface by the compressive stress layer from the shot peening. It was also found that, for the non-shotpeened parts, case hardening had a slightly higher corrosion-fatigue strength than the throughhardened. This might be a result of the compressive stresses from carburization, or due to the highercore toughness of this steel grade.

Des travaux scientifiques récents ont ouvert de nouveaux champs d’application aux traitements mécaniques tels que le grenaillage. Il a été montré que de tels traitements, réalisés avant un traitement de nitruration à la surface d’alliage ferreux, permettaient d’abaisser les températures de traitement et d’augmenter significativement les cinétiques de diffusion. Nous avons entrepris de tester les performances de cette combinaison de traitements mécanique et thermochimique sur des aciers inoxydables et des alliages à base nickel austénitiques. Des essais ont été réalisés à partir d’une technique de grenaillage mécanique appelée « Surface Mechanical Attrition Treatment » (SMAT). Des billes en métal ou en céramique sont introduites dans l’enceinte et mises en mouvement par la sonotrode. Les billes percutent et introduisent donc une déformation plastique à la surface des échantillons. Après cette étape, les échantillons subissent un traitement thermochimique de nitruration assisté plasma. La comparaison des résultats obtenus après nitruration sur des échantillons traités mécaniquement avec ceux n’ayant pas été pré-traités mécaniquement a permis de quantifier les effets des traitements combinés. Les analyses par diffraction des rayons X, les mesures de microdureté, les observations au microscope optique/électronique à balayage/électronique en transmission, les analyses de texture par EBSD (Electron BackScatered Diffraction) et la mesure des profils de concentration en azote par SIMS (Secondary Ion Mass Spectrometry) et SDL (Spectroscopie à Décharge Luminescente) ont montré l’importance de la nature de la couche transformée mécaniquement sur la diffusion de l’azote / Recent scientific work has opened new fields of application to mechanical treatments such as shot blasting or peening. Indeed, it has been shown that this treatment, performed before a nitriding treatment on the surface of ferrous alloy, lowers processing temperatures and significantly increases the diffusion kinetics. We undertook to test this combination of mechanical and thermochemical treatments on stainless steels and nickel-based alloys. The mechanical treatments were done by Surface Mechanical Attrition Treatment (SMAT). This method is implemented in a box where metal or ceramic balls were introduced and set in motion by an ultrasound system in order to impact the surface of the pieces. The treated samples were then nitrided at low temperature by using a remote plasma. The comparison of the results obtained after nitriding treatments on mechanically treated samples and those not mechanically treated allows quantifying the effects of the combined treatments. Analyses by X-ray diffraction, microhardness measurement, observations by optical and scanning and transmission electron microscopy, texture analysis by EBSD (Electron Diffraction BackScatered) and measurement of nitrogen concentration profiles by SIMS (Secondary Ion Mass Spectrometry) show the importance of the nature of the deformed layer

This thesis work was conducted at Luleå University of Technology in collaboration with Sandvik Rock Tools. The aim of the work was to determine the influence of carbon content on the wear performance in carburized steel in the dry contact interface of threaded connections between drill rods. In order to investigate this, samples of drill rod steel were carburized to three different carbon concentrations and shot peened to replicate the production process of a drill rod. The samples were wear tested by utilizing a twin-disc wear tester with one disc rotating at 100 RPM and the other at 3000 RPM to mimic the operating conditions in the threaded connection between drill rods. The results was evaluated by wear rate, surface topography, hardness as well as optical analysis by light optical microscopy and scanning electron microscopy. The results show that an increased surface carbon content provide a decrease in wear rate and an increase in hardness in the surface layer that undergo microstructural changes due to the frictional heat and contact pressure during wear testing. The primary wear mechanisms were identified as plastic deformation, adhesive scratching and material removal through delamination.