In this master thesis different overlay welding methods suitable for boiler application has been investigated. The purpose of this project is to define advantages and disadvantages for each overlay welding methods and suggest some evaluation criteria on some commercial and experimental alloys aimed for overlay welding material. Many components in a boiler are made of low alloy steel and the atmosphere in the furnace region can be very complex; therefore many different types of corrosion can occur. Weld overlay is a process where one or multiple layers of corrosion resistant material are applied to a base material. The two overlay welding methods investigated in this study were Tungsten Inert Gas welding and Metal Inert/Active Gas welding. Tests were performed with FeCrAl alloys (Kanthal A, Kanthal D and some experimental alloys). FeCrAl alloys in general are ferritic iron-based steels with a typical concentration of 20-23 wt. % chromium and ~5 wt.% aluminum. Different overlay welding evaluation was made; visual examination, dye penetrant inspection, macro/micro examination, side bend test and short term corrosion test (~50hours). Conclusion of this thesis is that MIG welding is a more productive method than TIG, but more defects such cracks and lack of fusion can be produced for MIG welding. These defects can be “fixed” if welding parameters is optimized. If repairing a certain place TIG welding is a better option. A conclusion about number of layers; one layer with MIG welding is almost as thick as three layers with TIG welding with welding wire Ø 1mm. Three welding evaluation that is really important is visual examination, dye penetrant testing and corrosion test in order to choose which overlay welding method is suitable in boiler application. / I detta examensarbete har olika påsvetsningsmetoder som är lämpliga för en pannapplikation undersökts. Syftet med denna studie var att undersöka både för- och nackdelar med påsvetsningsmetoderna och föreslå några utvärderingskriterier på vissa kommersiella och experimentella legeringar som lämpar sig för påsvetsningsmaterial. Många komponenter i en panna är tillverkade av låg legerat stål och miljön där förbränningen sker kan vara väldigt komplex, därför kan det ske korrosion. Påsvetsning är en process där ett eller flera skikt av ett mer korrosionsbeständigt material appliceras på ett basmaterial. De två olika svetsmetoderna som undersöktes i denna studie var TIG-svetsning och MIG-svetsning. Testerna utfördes med FeCrAl legeringar (Kanthal A, Kanthal D och vissa experimentella legeringar). FeCrAl legeringar är i allmänhet ferritiska järnbaserade stål med 20-23% krom och ~5% aluminium. De utvärderingsmetoderna som undersöktes var: visuell undersökning, penetrantprovning, makro/ mikroundersökning, sidobockprovning och korttidskorrosionstest (~50 timmar). Slutsatsen av detta examensarbete är att MIG-svetsning är en mer produktiv metod än TIG, men fler defekter såsom bindfel och sprickor uppkom för MIG-svetsning. Dessa defekter kan ”fixas” genom optimering av svetsparametrar. Om man bara ska reparera ett specifikt område är TIG-svetsning ett bättre alternativ. En slutsats om antal lager är att ett lager med MIG-svetsning är nästan lika tjockt som tre lager med TIG-svetsning med Ø 1mm svetstråd. Tre svetsutvärderingar som är viktiga är visuell undersökning, penetrantprovning och korrosionstest för att välja vilken påsvetsmetod som är lämpligast i pannapplikation.

Silicon Carbide (SiC) is increasingly gaining attention as a potential fuel cladding material, on account of its favorable thermo-mechanical and neutronic properties. The major limitations of such a cladding is currently associated with joining and hermetic sealing. The work presented here investigated the use of Al, Cr and Fe metals and a specialized alloy (FeCrAl) to achieve hermetic sealing of SiC tubes as well as a joining technology of SiC. Major part of solving this issue requires addressing joining of ceramic and metallic components, which are largely dissimilar in both thermal and mechanical properties. Preliminary experiments to bond SiC with FeCrAl resulted in adverse separation partially attributed to the differences in thermal expansion mismatch. To alleviate these problems, thin and thick coatings of the metals and alloys were applied to SiC. Qualitative microstructural characterization of the final product indicated satisfactory bonding between the materials.

ELECTRA is a Swedish training reactor concept that will use fourth generation nuclear reactor technology, and is cooled by natural circulation of liquid lead. During a fuel inspection the reactor vessel has to be emptied on metal coolant and the power must be shutoff, this can lead to a meltdown. By calculating the produced residual heat in the fuel pins during the power shutdown and coolant drainage, the cladding temperature could be calculated. The potential usage of argon, helium or nitrogen gas as an emergency coolant during loss of lead was investigated. The gas coolant will unavoidably contain small amounts of water vapour which can have a negative impact on the core. This matter was examined with a literature study on the effect of water vapour on the cladding surface; FeCrAl, an iron- base alloy with high aluminium and chromium content.  The burnup simulation code Serpent was used for calculating the residual heat. The results indicated that just after five days of shutdown the residual heat decreased to 31 W/pin and continued to progressively decrease in the coming days. It was evaluated that the residual heat developed in the core would not exceed 600°C.  Calculations on the emergency gas coolant showed that nitrogen gas offered the best solution on terms of thermal decrease and cost-effectiveness. While helium cooling was the recommended option due to its inertness.  The literature study done on the oxidizing effect of water vapour on FeCrAl, shows little impact due to the high oxidation resistance of this alloy as a result of the high aluminium and chromium percentage.

ELECTRA is a Swedish training reactor concept that will use fourth generation nuclear reactor technology, and is cooled by natural circulation of liquid lead. During a fuel inspection the reactor vessel has to be emptied on metal coolant and the power must be shutoff, this can lead to a meltdown. By calculating the produced residual heat in the fuel pins during the power shutdown and coolant drainage, the cladding temperature could be calculated. The potential usage of argon, helium or nitrogen gas as an emergency coolant during loss of lead was investigated. The gas coolant will unavoidably contain small amounts of water vapour which can have a negative impact on the core. This matter was examined with a literature study on the effect of water vapour on the cladding surface; FeCrAl, an iron- base alloy with high aluminium and chromium content.  The burnup simulation code Serpent was used for calculating the residual heat. The results indicated that just after five days of shutdown the residual heat decreased to 31 W/pin and continued to progressively decrease in the coming days. It was evaluated that the residual heat developed in the core would not exceed 600°C.  Calculations on the emergency gas coolant showed that nitrogen gas offered the best solution on terms of thermal decrease and cost-effectiveness. While helium cooling was the recommended option due to its inertness.  The literature study done on the oxidizing effect of water vapour on FeCrAl, shows little impact due to the high oxidation resistance of this alloy as a result of the high aluminium and chromium percentage.

ELECTRA is a Swedish training reactor concept that will use fourth generation nuclear reactor technology, and is cooled by natural circulation of liquid lead. During a fuel inspection the reactor vessel has to be emptied on metal coolant and the power must be shutoff, this can lead to a meltdown. By calculating the produced residual heat in the fuel pins during the power shutdown and coolant drainage, the cladding temperature could be calculated. The potential usage of argon, helium or nitrogen gas as an emergency coolant during loss of lead was investigated. The gas coolant will unavoidably contain small amounts of water vapour which can have a negative impact on the core. This matter was examined with a literature study on the effect of water vapour on the cladding surface; FeCrAl, an iron- base alloy with high aluminium and chromium content.  The burnup simulation code Serpent was used for calculating the residual heat. The results indicated that just after five days of shutdown the residual heat decreased to 31 W/pin and continued to progressively decrease in the coming days. It was evaluated that the residual heat developed in the core would not exceed 600°C.  Calculations on the emergency gas coolant showed that nitrogen gas offered the best solution on terms of thermal decrease and cost-effectiveness. While helium cooling was the recommended option due to its inertness.  The literature study done on the oxidizing effect of water vapour on FeCrAl, shows little impact due to the high oxidation resistance of this alloy as a result of the high aluminium and chromium percentage.

<p> The question of how the plastic strength of alloys depends on composition is critical to alloy design. Numerous classical works have tackled this question in the past. Yet, the models available to date primarily focus on the strength of alloys at the onset of yielding and seldom address the role of alloy composition in the hardening and dislocation microstructure evolution regime. The above question becomes even more important in situations in which the alloys are compositionally nonuniform at the mesoscale, as in spinodally decomposed alloys, irradiated alloys, high entropy alloys, and additive-manufactured alloys. In this work, the interaction between alloy plasticity and compositional inhomogeneity is addressed from a discrete dislocation dynamics (DDD) perspective. A framework comprising of three components: (1) analysis of the 3D composition morphology in inhomogeneous alloys with tendency to undergo spinodal/spinodal-like instability, (2) atomistic simulations of the dislocation mobility as a function of the local composition, and (3) dislocation dynamics simulations, has been utilized to understand the collective dynamics of dislocations and mesoscale plasticity in inhomogeneous alloys. Irradiated FeCrAl has been used as a model alloy for the implementation of the current framework and subsequent investigations. The investigation reveals that the composition inhomogeneity plays a crucial role in influencing microplasticity and macroscopic plasticity in inhomogeneous alloys. This happens due to the motion of dislocations taking place in a wavy fashion due to coherency stresses and locally varying dislocation velocities. </p> <p>To further understand alloy microplasticity from a single dislocation perspective, Cahn’s theory of hardening in compositionally modulated alloys based on coherency stresses has been modified to account for superposition of solid solution strengthening on spinodal strengthening due to the composition modulation. A new definition for the CRSS in compositionally modulated alloys is provided. Subsequently, CRSS is determined as a function of dislocation line direction, amplitude, and wavelength of the composition fluctuations.</p> <p>Lastly, an application of the developed framework is demonstrated where plasticity in irradiated FeCrAl nanopillars is investigated using DDD, with a comparison to transmission electron microscopic in situ tensile tests of ion- and neutron-irradiated commercial FeCrAl C35M alloy.</p>

<p>Nuclear fission energy has developed for more than five decades and become one of the most important low-carbon energy forms. The extreme environment in the advanced reactors, including high operating temperature and high neutron radiation dose, raises new challenges for structural materials. To date, no materials are immune to radiation damage. Bombardment by energetic particles displaces atoms from their original sites, leaves various forms of defect aggregates after cascade, and degrades the properties of the irradiated materials. FeCrAl alloys, known for their excellent high-temperature oxidation resistance, were developed under the accident tolerant fuel program in hope to replace the Zr cladding alloys in future reactors. The radiation response and mechanical properties of FeCrAl alloy have attracted great attention. The objective of this thesis is two-fold. First, investigate the high temperature mechanical behavior of coarse-grained FeCrAl alloys with and without irradiation from the perspective of small-scale testing. Second, develop a fine-grained FeCrAl alloy variant and evaluate its mechanical properties and radiation tolerance.</p> <p>Critical resolved shear stress of pristine and proton irradiated CG FeCrAl alloy was quantified at elevated temperatures. {112}<111> slip system exhibited higher irradiation induced hardening compared with the {110}<111> slip system. Gradient FeCrAl alloy was fabricated through surface mechanical grinding treatment. In situ pillar compression tests revealed an excellent combination of strength and deformability of ultra-fine-grained (UFG) FeCrAl alloys. The activation energy for plastic deformation of a nanolaminate (NL) FeCrAl alloy was determined through strain rate jump tests. Ex situ Fe-ion irradiation showed the interplay between dislocation loops and grain coarsening and their contributions to the mechanical properties of the irradiated UFG FeCrAl alloys. In situ Kr ion irradiation studies on the helium pre-injected NL FeCrAl and CG FeCrAl show that the helium induced swelling was effectively reduced in NL alloy due to their abundant grain boundaries serving as defect sinks. The findings in this thesis may provide innovative perspectives on the design and manufacture of novel FeCrAl alloys with outstanding mechanical properties and radiation tolerance.</p> <p><br></p>

L'Electrolyse de la Vapeur d'eau à Haute Température (EVHT) est l'un des procédésde production d'hydrogène les plus prometteurs. Dans l'optique d'une économie del'hydrogène produit par EVHT, de nombreux verrous restent à lever. L'un d'entre euxporte sur l'étanchéité. En effet, dans un EVHT, la gestion des gaz est primordiale. Ilfaut pouvoir gérer et prévoir dans le temps le comportement des joints afin d'éviter unedégradation des performances. Or, en EVHT, les températures de fonctionnement sontélevées (classiquement autour de 800 °C), des phénomènes de fluage ou de relaxationapparaissent, le différentiel de dilatation thermique entre les cellules électrochimiques encéramique et les interconnecteurs métalliques doit être pris en compte. Enfin, il convientde maintenir l'étanchéité de l'empilement à faible niveau d'effort pour ne pas risquerd'endommager la partie céramique.L'objet du travail de cette thèse démarre par un constat simple : nous ne disposons pasd'outils de prédimensionnement des joints à haute température permettant de prévoirun débit de fuite. Dès lors que l'on est amené à changer un paramètre de fonctionnement,comme la température, la pression, la stratégie de chargement, la géométrie ou la naturedu joint, une nouvelle expérience doit être menée.A partir d'essais d'étanchéité et de simulations numériques aux éléments finis, un modèleoriginal est proposé. Ce modèle qui a été validé en fonction de différents paramètresexpérimentaux permet d'estimer le débit de fuite associé à un joint en Fecralloy (Fe-CrAl) selon sa forme, ses conditions de serrage et du temps de maintien. Offrant ainsila possibilité de concevoir à moindre coût des joints spécifiques pour l'application visée. / High Temperature Steam Electrolysis (HTSE), is one of the most promising processfor hydrogen production. In a hydrogen economy produced via HTSE, many problemshave to be overcome. One of them is related to sealing. Actually, in a HTSE, gasmanagement is very important. The behavior of the seal has to be predicted in time toavoid a deterioration of the performances. But, in a HTSE, the fuctioning temperaturesare important (typically around 800 °C), creep or relaxation may occur, the differencebetween the thermal expansion of the ceramic cells and the metallic interconnectorsmust be taken into account. Finally, the sealing has to be maintain with low effortsprotect the ceramic.This thesis started from the noticing that we don't have any designing tool for hightemperature seals that may help to foresee a leak rate. Since we have to change anyexperimental parameter, such as the temperature, the pressure, the loading strategy, thegeometry or the material of the seal, another experiment has to be done.From sealing tests and finite element modelisation, an original model is presented.This model that has been validated for different experimental parameters allows toestimate the leak rate of a Fecralloy (FeCrAl) seal depending on its shape, the loadingconditions and tightening time. This may help to design specific low cost seals for thedesired applications.

L'Electrolyse de la Vapeur d'eau à Haute Température (EVHT) est l'un des procédésde production d'hydrogène les plus prometteurs. Dans l'optique d'une économie del'hydrogène produit par EVHT, de nombreux verrous restent à lever. L'un d'entre euxporte sur l'étanchéité. En effet, dans un EVHT, la gestion des gaz est primordiale. Ilfaut pouvoir gérer et prévoir dans le temps le comportement des joints afin d'éviter unedégradation des performances. Or, en EVHT, les températures de fonctionnement sontélevées (classiquement autour de 800 °C), des phénomènes de fluage ou de relaxationapparaissent, le différentiel de dilatation thermique entre les cellules électrochimiques encéramique et les interconnecteurs métalliques doit être pris en compte. Enfin, il convientde maintenir l'étanchéité de l'empilement à faible niveau d'effort pour ne pas risquerd'endommager la partie céramique.L'objet du travail de cette thèse démarre par un constat simple : nous ne disposons pasd'outils de prédimensionnement des joints à haute température permettant de prévoirun débit de fuite. Dès lors que l'on est amené à changer un paramètre de fonctionnement,comme la température, la pression, la stratégie de chargement, la géométrie ou la naturedu joint, une nouvelle expérience doit être menée.A partir d'essais d'étanchéité et de simulations numériques aux éléments finis, un modèleoriginal est proposé. Ce modèle qui a été validé en fonction de différents paramètresexpérimentaux permet d'estimer le débit de fuite associé à un joint en Fecralloy (Fe-CrAl) selon sa forme, ses conditions de serrage et du temps de maintien. Offrant ainsila possibilité de concevoir à moindre coût des joints spécifiques pour l'application visée.

The primary intention with this Doctoral thesis is to fillin the knowledge gaps and raise the level of understandingregarding the different metal dusting mechanisms in general andexplain the process in detail for high alloyed materials.Considerable effort is put into identifying the driving forcesand elucidating the diffusional processes in metal dusting. The results are based on a series of long-term laboratory exposures of stainless steels and high-performance commercial alumina-forming Fe- and Ni-base alloys in synthesis gasmixtures, plus a separate shorter study on ultra pure iron. ANi-base alloy was also investigated after a two years field exposure in a methanol plant. Post exposure metallographic examinations and analysis aswell as thermodynamic calculations were made in order toidentify and describe the operating metal dusting mechanisms.Two main mechanisms were previously used to explain metaldusting, one on the basis of decomposition of metastable carbides (Type I) the other on graphite formation (TypeII). A new metal dusting mechanism has been identified in this Thesis which appears on high alloyed steels and Ni-base alloys,an active corrosion induced by carbon and oxygen, denoted TypeIII. Both the mechanisms and the type of corrosion products were consistent with the thermodynamic conditions of the material under the influence of a carbon and oxygen gradient.It was shown that this mechanism not only accelerates the metaldusting process, in fact, it determines the overall metaldusting kinetics on stainless steels and Ni-base alloys. Another feature, which may occur at temperatures where metalbulk and even static grain boundary diffusion is too slow forexplaining the metal dusting corrosion process, was identifiedon a Ni-base alloy. It involves a fast growing cellular structure with discontinuous precipitated carbides whichprovides fast metal dusting kinetics by the Type IIImechanism. A Type IV metal dusting mechanism, continued fragmentationby graphitization until nano-sized particles are formed andcatalyse carbon nanotube formation is also described. Thesteady state process and the driving force for metal dusting onpure iron was identified and described. With these additional processes it is now possible to extendour understanding of the metal dusting processes to a widerange of engineering alloys. / QC 20100825