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Les propriétés en fatigue à grand et très grand nombre de cycles des superalliages base Ni monogranulaires ont été étudiées à 1000°C sous trois fréquences (0.5, 70 et 20000 Hz) et quatre rapports de charge (-1, 0.05, 0.3, 0.8), en prenant le CMSX-4 comme alliage de référence.Dans un premier temps, le régime de fatigue à très grand nombre de cycles a été étudié à 20 kHz. A R = -1, la fissure fatale s’amorce à chaque fois depuis un pore de fonderie en interne et se propage suivant les plans cristallographiques {111}, pour des durées de vie entre 106 et 109 cycles, et ce malgré la présence d’autres défauts tels que les eutectiques ou carbures. Le procédé d’élaboration, en impactant directement la taille des pores de fonderie, contrôle la durée de vie alors que la microstructure a peu d’influence. A R = 0.8, le fluage contrôle la durée de vie en fatigue gigacyclique, et rend alors ce régime sensible à la microstructure et à la composition chimique de l’alliage. Les essais à très longues durées de vie (> 109 cycles) ont également été caractérisés et ont permis de proposer un mécanisme d’amorçage dans ce régime de fatigue.Dans un second temps, l’influence de la fréquence d’essai sur les endommagements dépendants du temps tels que l’oxydation et le fluage, et leur interaction, a été étudiée. A R ≤ 0, diminuer la fréquence d’essai revient à inhiber la criticité des défauts internes en favorisant l’endommagement en surface par oxydation. A R ≥ 0.8, le fluage est l’endommagement conduisant à la rupture des éprouvettes quelle que soit la fréquence de sollicitation ou la durée de vie. Les rapports de charge intermédiaires présentent une interaction importante entre les endommagements en oxydation, fatigue et fluage en fonction de la fréquence de sollicitation et des conditions de chargement. A partir de ces observations, un modèle d’endommagement a été proposé afin de prédire la durée de vie du CMSX-4 et présente des résultats satisfaisants par rapport aux données expérimentales. / High and very high cycle fatigue properties of Ni-based single crystal superalloys have been studied at 1000°C using three frequencies (0.5, 70, 20000 Hz), four stress ratios (-1, 0.05, 0.3, 0.8) and CMSX-4 alloy as reference.Firstly, the very high cycle fatigue regime (VHCF) has been studied at 20 kHz. At R = -1, fatal crack always initiates between 106 and 109 cycles from an internal casting pore et then propagates in a crystallographic mode, despite the presence of other metallurgical defects such as eutectics and carbides. The elaboration process controls VHCF life under these conditions as it directly affects casting pore size, whereas microstructure has no relevant influence. At R = 0.8, creep controls VHCF life and makes this regime sensitive to microstructure and alloy’s chemical composition. Long term tests (> 109 cycles) have also been characterized and have enabled to propose a crack initiation mechanism operating in the VHCF regime.Secondly, the influence of frequency on time-dependent damages such as oxidation and creep, and their interaction, has been studied. At R ≤ 0, reducing frequency inhibits the harmfulness of internal critical defects by promoting oxidation surface damage. At R ≥ 0.8, creep damage leads to specimens rupture for all frequency test and loading conditions. Intermediate stress ratios present an important interaction between oxidation, fatigue and creep damage according to the frequency and loading conditions. Based on these observations, a damage model that predicts CMSX-4 fatigue life has been proposed and presents satisfying results in comparison to experimental data.

Classical fatigue models teach that there is an intrinsic fatigue limit for steels, representing a level of stress that is too low for regular crack growth where every cyclic load propagates a fatigue crack through the material. Modern application with extreme lifetimes has shown that fatigue will still take place in steels with stress levels well below the expected fatigue limit. This relatively new area of study has been named Very High Cycle Fatigue, or VHCF, and describes fatigue failures with a number of load cycles exceeding 107. Fractography of steels that has suffered VHCF tends to reveal an especially rough crack surface adjacent to where the fatigue crack originates, which is typically some form of defect in the bulk of the steel. This area is believed to be critical for VHCF and has been referred to in a number of ways by different studies, but will herein be called Fine Granular Area, or FGA. The aim of this study is to try and get a better understanding of VHCF. This was done by fractography analysis of test specimens of high strength tool steel that suffered fatigue failure at lifetimes ranging from about 106 cycles to 1,9x109 cycles. The lower lifetimes were achieved using hydraulic testing equipment, while the specimens in the VHCF range suffered fatigue failure in ultrasonic testing equipment allowing the application of a cyclic stress at a rate of 20 000 Hz. The resulting fracture surfaces were then investigated using a scanning electron microscope, or SEM, taking special note of the fatigue initiating defects and, in the case of VHCF, the rough area found adjacent to it. In combination with the SEM an elemental analysis of the fatigue initiating defects as well as the bulk of the material was done using energy-dispersive X-ray spectroscopy, or EDS. This was done to find out what the defects consisted of; confirming that they were slags and checking that the composition of the material of the bulk of the specimen matches what was expected. Using light optical microscopy in combination with acid etching of the surface of samples cut out of the test specimens the structure of the steel was investigated. Calculating the local stresses at the location of the fatigue initiating defect was done using FEM in combination with displacement amplitude gathered from the ultrasonic testing equipment. The data gathered was then measured and compared to that of previous studies, using models of prediction and seeing how they match the experimental results. The results suggest that the stress intensity factor at the internal slags is critical for VHCF and that with lower stress intensity factors one can expect longer lifetimes. Another observation is a relatively consistent stress intensity factor at the edge of the FGA combined with the original defect, likely signifying the transition from the creation of FGA to traditional crack propagation. There also seems to be a connection between the size of the FGA and the number of cycles to failure, with larger FGA with increasing lifetimes. The most glaring shortcoming of this study is the amount satisfactory tests conducted, and thus amount of data points, is very low due to the majority of specimens suffered failure at the threading used to connect them to the ultrasonic testing equipment at lifetimes far too low to be relevant. / Klassiska utmattningsmodeller lär ut att det finns en utmattningsgräns för stål, vilket representerar en spänningsnivå som är för låg för regelbunden sprickväxt där varje cyklisk belastning sprider en utmattningsspricka genom materialet. Moderna applikation med extrema livstider har visat att utmattning fortfarande äger rum i stål med spänningsnivåer långt under den förväntade utmattningsgränsen. Detta relativt nya studieområde har fått namnet Very High Cycle Fatigue, eller VHCF, och beskriver utmattningsfall med ett antal belastningscykler som överstiger 107. Fraktografi av stål som har drabbats av VHCF tenderar att ha en särskilt gropig sprickyta som ligger intill där utmattningssprickan har sitt ursprung, vilket typiskt är någon form av defekt i stålets bulk. Detta område tros vara kritiskt för VHCF och har hänvisats till på ett antal sätt av olika studier, men kommer här att kallas Fine Granular Area eller FGA. Syftet med denna studie är att försöka få en bättre förståelse för VHCF. Detta gjordes genom fraktografianalys av testprover av verktygsstål med hög hållfasthet som drabbades av utmattningsbrott vid livstider från cirka 106 cykler till 1,9x109 cykler. De lägre livslängderna uppnåddes med hjälp av hydraulisk testutrustning, medan proverna i VHCF-området drabbades av utmattningsbrott i ultraljudstestutrustning som klarar att applicera en cyklisk stress med en frekvens på 20 kHz. De resulterande sprickytorna undersöktes sedan med hjälp av ett svepelektronmikroskop, eller SEM, med särskild fokus på utmattningsinitierande defekter och, i fallet med VHCF, det grova området som hittades intill det, FGA. I kombination med SEM utfördes en elementanalys av utmattningsinitierande defekter liksom huvuddelen av materialet med energidispersiv röntgenspektroskopi, eller EDS. Detta gjordes för att ta reda på vad inneslutningarna bestod av för att bekräfta att de var slagg samt kontrollera att sammansättningen av materialet i huvuddelen av provet matchar det som förväntades. Med användning av optisk ljusmikroskopi i kombination med syraetsning av ytan på prover som skars ut ur testproverna undersöktes stålets struktur. Beräkning av de lokala spänningarna på platsen för den utmattningsinitierande defekten gjordes med hjälp av FEM i kombination med förskjutningsamplituden som samlats från ultraljudsutrustningen. De insamlade uppgifterna mättes sedan och jämfördes med tidigare studier genom att använda diverse modeller och se hur de matchar de experimentella resultaten. Resultaten antyder att stressintensitetsfaktorn vid inneslutningarna är kritisk för VHCF och att man med lägre stressintensitetsfaktorer kan förvänta sig längre livstid. En annan observation är en relativt konsekvent stressintensitetsfaktor vid kanten av FGA, vilket sannolikt markerar övergången från skapandet eller utbredning av FGA till traditionell sprickutbredning. Det verkar också finnas en koppling mellan storleken på FGA och antalet cykler till fel, med större FGA med ökande livslängd. Den mest uppenbara bristen i denna studie är mängden tillfredsställande tester som genomförts. Därmed är mängden datapunkter mycket låg, detta på grund av att majoriteten av proverna misslyckades vid gängningen som användes för att ansluta dem till ultraljudstestutrustningen vid livstider alltför låga för att vara relevanta.

This work were requested by Uddeholms AB. Uddeholm is the worldś leading manufacturer of high performance tool steel for industrial tools. The company exists worldwide and is present in over 100 countries. One of the company’s main production processes is manufacturing powder steels with extremely high quality. In this work, one of these steels has been analyzed to evaluate the presence and distribution of non-metallic inclusions. Higher demands on quality and more global competition worldwide means that you always have to strive towards perfection in the manufacturing processes. Non-metallic inclusions have severe effects on the mechanical properties of steels. That's why it's of utmost importance to investigate their presence in the tool steel and especially their size. The reason for this is because it's the largest inclusions that's the most dangerous for the material. To obtain a statistically number of certainty of the largest of inclusions, very vast areas of steel have to be examined. Therefore a theoretical method of extrapolation is often used instead to approximate the distribution of the largest inclusions. The precision of this method is still very uncertain. Different methods of this kind of analysis do exist. The method applied in this work of degree is extreme values analysis by statistics of extreme values (SEV). To ensure the pre-conditions of the extreme values analysis, a large quantity of powder steel was analyzed. This was done by light-optic microscopy (LOM) and exclusively performed by Uddeholms AB at their R&D-facility (Research and development-facility) in Hagfors. At Karlstad Universityś test-facility specimens were subjected to ultrasonic fatigue testing at 20 kHz. The specimens derived from the same material as the ones examined in LOM. The result from the ultrasonic fatigue testing were that the largest inclusions in the material were found. This was done by applying very high cycle fatigue (VHCF). To calculate the maximum theoretical inclusion size, SEV was used. The extreme values analysis was performed on gathered data from both LOM and VHCF. All fracture surfaces from the ultrasonic fatigue testing were then examined in scanning electron microscope (SEM) at Karlstad University. In SEM, the largest inclusion in every fractured surface was identified and measured. The maximum real inclusion size from the fractured surfaces was then compared to the maximum theoretically calculated inclusion size from the extreme values analysis. As it turned out the real inclusion size proved to be slightly larger than the theoretical. The difference between them was found to be 3,25 µm. Conclusions drawn were that Uddeholms powder steel exhibits very high purity and that extreme values analysis as an analytical method is recommended. However, the analysis should be repeated to underline the chosen solution methodology.