No description available.

No description available.

A austenita retida está sempre presente na microestrutura de camada cementada de aços, em maior ou menor quantidade. Como é uma fase dúctil comparada à martensita, sua presença tem sido alvo de muita controvérsia. Este trabalho apresenta um estudo sobre a influência da austenita retida na propagação de trincas curtas por fadiga em camada cementada de aço SAE 8620. Foram feitos ensaios de fadiga por flexão em quatro pontos, a temperatura ambiente, em corpos de prova sem entalhe com três níveis de amplitude de tensão e razão de tensões de 0,1. Através de diferentes ciclos de cementação e tratamentos térmicos, foram obtidas camadas cementadas com quatro níveis de austenita retida na microestrutura. O teor de austenita retida foi medido através da técnica de difração de Raios-X. Trincas superficiais foram monitoradas por meio da técnica de réplicas de acetato. Como resultados foram obtidos tamanho de trinca em função do número de ciclos e taxa de crescimento de trincas curtas. Corpos de prova com maiores níveis de austenita retida apresentaram maior vida em fadiga. / The retained austenite is always present in case carburized steel microstructure in small or high percentages. Since it is a ductile phase, its presence has long been a controversial subject. The influence of retained austenite on short fatigue crack propagation in case carburized SAE 8620 steel was studied in this work. Four-point-bend fatigue tests were carried out at room temperature in specimens without notch using three levels of stress range and a stress ratio of 0.1. Four different amount of retained austenite in the case carburized microstructure were obtained through different cycles of carburizing and heat treating. The retained austenite content was measured by X-ray technique, and the surface short crack growth was monitored by means of acetate replication technique. Crack length versus number of cycles and crack growth rate versus mean crack length were obtained as results. Specimens with higher levels of retained austenite in the carburized case showed longer fatigue life.

A austenita retida está sempre presente na microestrutura de camada cementada de aços, em maior ou menor quantidade. Como é uma fase dúctil comparada à martensita, sua presença tem sido alvo de muita controvérsia. Este trabalho apresenta um estudo sobre a influência da austenita retida na propagação de trincas curtas por fadiga em camada cementada de aço SAE 8620. Foram feitos ensaios de fadiga por flexão em quatro pontos, a temperatura ambiente, em corpos de prova sem entalhe com três níveis de amplitude de tensão e razão de tensões de 0,1. Através de diferentes ciclos de cementação e tratamentos térmicos, foram obtidas camadas cementadas com quatro níveis de austenita retida na microestrutura. O teor de austenita retida foi medido através da técnica de difração de Raios-X. Trincas superficiais foram monitoradas por meio da técnica de réplicas de acetato. Como resultados foram obtidos tamanho de trinca em função do número de ciclos e taxa de crescimento de trincas curtas. Corpos de prova com maiores níveis de austenita retida apresentaram maior vida em fadiga. / The retained austenite is always present in case carburized steel microstructure in small or high percentages. Since it is a ductile phase, its presence has long been a controversial subject. The influence of retained austenite on short fatigue crack propagation in case carburized SAE 8620 steel was studied in this work. Four-point-bend fatigue tests were carried out at room temperature in specimens without notch using three levels of stress range and a stress ratio of 0.1. Four different amount of retained austenite in the case carburized microstructure were obtained through different cycles of carburizing and heat treating. The retained austenite content was measured by X-ray technique, and the surface short crack growth was monitored by means of acetate replication technique. Crack length versus number of cycles and crack growth rate versus mean crack length were obtained as results. Specimens with higher levels of retained austenite in the carburized case showed longer fatigue life.

No description available.

Les liaisons bimétalliques en acier inoxydable (LBM inox) permettent, au sein des réacteurs nucléaires français actuels, de connecter les gros composants en acier ferritique faiblement allié (cuve, pressuriseur, générateur de vapeur) à la tuyauterie du circuit primaire en acier austénitique inoxydable. De par leurs microstructure et propriétés mécaniques hétérogènes, ces liaisons sont des zones dites "sensibles" pour l'intégrité des structures et il est donc indispensable de caractériser leur tenue mécanique dans les situations de fonctionnement nominal et accidentelles. Ce travail de thèse a pour objectif d'évaluer le risque d'amorçage fragile de la LBM inox dans le haut de la transition fragile-ductile à l'aide d'un critère adapté. Les microstructures au voisinage de l'interface entre l'acier ferritique et le beurrage austénitique ont tout d’abord été caractérisées, et un liseré martensitique d’épaisseur variable ainsi qu’une couche entièrement austénitique ont été observés. Ces deux couches, qui sont le siège d’une intense précipitation de carbures pendant le traitement thermique de détensionnement, forment ensemble une couche dure de martensite et d’austénite carburées potentiellement fragile. Le comportement mécanique de l’ensemble de la LBM inox a ensuite été étudié à 20°C et à -175°C, et des lois de comportement élasto-plastiques isotropes ont été identifiées pour les différentes couches macroscopiques à partir d’essais de traction sur des éprouvettes multi-matériaux travers-joint à diamètre variable. Le comportement mécanique de la couche dure a, quant à lui, été caractérisé à partir d’essais in-situ sur des micro-éprouvettes usinées au FIB et testées à l’aide d’une micro-machine de traction développée dans cette thèse. Une étude des mécanismes de rupture de la LBM inox dans le domaine de la transition fragile-ductile a par ailleurs été réalisée à partir d’essais sur éprouvettes CT et a mis en évidence une fragilité de l’interface MA (entre martensite et austénite) liée à un mécanisme de rupture intergranulaire amorcée sur les carbures et systématiquement activé pour des fronts de préfissure traversant la couche dure. Une modélisation par éléments finis des essais a permis d’analyser les champs de contrainte sur l’interface MA et d’identifier un modèle de Weibull linéique à 3 paramètres basé sur une contrainte seuil et une distance seuil pour les éprouvettes CT. Finalement, l’effet du vieillissement thermique sur les LBM inox a été étudié à partir d’un traitement thermique de 10 000h à 400°C et un durcissement des couches austénitiques résultant d’un mécanisme de décomposition spinodale de la ferrite résiduelle a été mis en évidence à partir d’essais de traction. L’analyse des mécanismes de rupture à l’état vieilli a également montré que ce durcissement provoque une augmentation d’environ 30°C de la température de transition associée à la rupture intergranulaire de l’interface MA. / Stainless steel dissimilar metal welds (SS DMW) are widely used within the French nuclear power plants where they connect the main components (pressure vessel, pressurisor, steam generator) made of low-alloy ferritic steel to the primary circuit pipes made of austenitic stainless steel. Because of their heterogeneous microstructure and mechanical properties, these junctions are critical components for the structure integrity and their fracture resistance has to be demonstrated for all the nominal or accidental operating conditions. This PhD work aims at building a model to evaluate the risk of brittle fracture of the SS DMW in the upper shelf of the brittle-to-ductile transition range. The observation of the microstructures around the fusion line revealed a martensitic layer and a fully austenitic zone, which undergo an important carbides precipitation during the post-weld heat treatment and form a narrow hard layer of carburized martensite and austenite. The mechanical behavior of the SS DMW was then characterized at 20°C and -175°C and isotropic elastoplastic constitutive laws were determined for each macro/mesoscopic layer of the weld from tensile tests on crossweld specimens with variable diameters. The mechanical behavior of the narrow hard layer was also studied with micro tensile tests on specimens extracted by FIB micro processing and tested using an in-situ tensile testing device developed during the PhD. Furthermore, fracture toughness tests were carried out on CT specimens in the brittle-to-ductile temperature range and helped identify the MA interface (between martensite and austenite) as the weakest region in the SS DMW because of an intergranular fracture mechanism initiated at the carbides-rich interface. This mechanism was consistently observed for specimens with fatigue precrack fronts in the hard layer. The stress distributions on the MA interface calculated from the FE numerical simulation of these tests were then analysed and a 1D 3 parameters Weibull model based on a threshold stress and a threshold length was identified for the CT specimens. Finally, the effect of thermal ageing on the SS DMW was explored with a thermal ageing treatment of 10000h at 400°C and a hardening of the austenitic layers was measured by tensile tests and was associated to a spinodal decomposition mechanism of the residual ferrite. The fracture mechanisms of the SS DMW were also analysed in the aged state and showed that this hardening caused an increase of the transition temperature associated with the intergranular fracture of the MA interface by about 30°C.

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.