No description available.

Duplex stainless steels (DSSs) are important engineering materials due to their combination of good mechanical properties and corrosion resistance. However, as a consequence of their ferrite content, DSSs are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation, namely, the ferrite decomposed into Fe-rich ferrite (α) and Cr-rich ferrite (α'), respectively. The phase separation is accompanied with a severe loss of toughness. Thus, the ‘475°C embrittlement’ phenomenon limits DSSs’ upper service temperature to around 250°C. In the present work, Fe-Cr binary model alloys and commercial DSSs from weldments were investigated for the study of phase separation in ferrite. Different techniques were employed to study the phase separation in model alloys and commercial DSSs, including atom probe tomography, transmission electron microscopy and micro-hardness test. Three different model alloys, Fe-25Cr, Fe-30Cr and Fe-35Cr (wt. %) were analyzed by atom probe tomography after different aging times. A new method based on radial distribution function was developed to evaluate the wavelength and amplitude of phase separation in these Fe-Cr binary alloys. The results were compared with the wavelengths obtained from 1D auto-correlation function and amplitudes from Langer-Bar-On-Miller method. It was found that the wavelengths from 1D auto-correlation function cannot reflect the 3D nano-scaled structures as accurate as those obtained by radial distribution function. Furthermore, the Langer-Bar-On-Miller method underestimates the amplitudes of phase separation. Commercial DSSs of SAF2205, 2304, 2507 and 25.10.4L were employed to investigate the connections between phase separation and mechanical properties from different microstructures (base metal, heat-affected-zone and welding bead) in welding. Moreover, the effect of external tensile stress during aging on phase separation of ferrite was also investigated. It was found that atom probe tomography is very useful for the analysis of phase separation in ferrite and the radial distribution function (RDF) is an effective method to compare the extent of phase separation at the very early stages. RDF is even more sensitive than frequency diagrams. In addition, the results indicate that the mechanical properties are highly connected with the phase separation in ferrite and other phenomena, such as Ni-Mn-Si-Cu clusters, that can also deteriorate the mechanical properties. / <p>QC 20130308</p>

Irradiation of a reactor pressure vessel (RPV) causes a shift of the ductile to brittle transition region towards higher temperature regions. In the event of a pressurized thermal shock (PTS), where the temperature drops drastically, the ductile to brittle transition region might be entered for irradiated ferritic steel. Hence, there is a risk of brittle cleavage fracture. Cleavage fracture is a transgranular unstable fracture initiated by cracked second phase particles and rapidly propagated over grain boundaries. The warm pre-stressing (WPS) effect can be helpful as it increases the apparent fracture toughness of ferritic steel pre-loaded in the ductile temperature region, which is the case for a PTS. This effect has been proven effective for virgin material, but the impact of residual stress fields on the WPS effect have not been investigated thoroughly. Utilizing a finite element model of notched three-point bending specimens and a non-local probabilistic model for fracture prediction the effect of residual stresses on the WPS effect was investigated in this thesis. Regarding the crack tip state, expressed as J, the probability of fracture was alike for both material with and without residual stresses, however a significant loss of load bearing capacity was found comparing them two. The magnitude of this loss depends on pre-load level as well as specimen size. This loss however, was also found when not considering the WPS effect. / Bestrålning av ferritiska reaktortankar orakar en förskjutning av den duktil-spröda omslagstemperaturen till högre temperaturer. Vid en trycksatt termisk shock (TTS) sjunker temperaturen drastiskt i reaktorn och omslagstemperaturen kan nås. Därför uppstår en risk för klyvbrott. Klyvbrott är en transgranulär ostabil spricktillväxt initierad av sprickor i sekundärfaspartiklar som propagerar över korngränserna om spänningstillståndet är gynnsamt. Varm förbelastning, eller warm pre-stressing (WPS) kan vara fördelaktig eftersom den höjer den effektiva brottsegheten hos ferritiskt stål som förbelstats i den duktila temperaturregionen, som är fallet för TTS. Den här effekten har visats effektiv för material utan restspänningar, men för fallet med restspäninningar saknas utförliga undersökningar av WPS-effekten. Med hjälp av en finita element model av tre-punkt böjprovstavar och en icke-lokal sannolikhetsmodell för prediktion av brott så undersöktes vilken effekt restspänningar har på WPS-effekten. Brottsannolikheten visade sig vara lika för både material med och utan restspännigar om man syftar på sprickspetstillståndet, uttryckt som J. Däremot syns en tydlig förlust i lastbärande förmåga mellan de två fallen. Storleksordningen på förlustern beror både på förbelastningsnivå och provstavsstorlek. Den förlusten kunde dock finnas även för fall utan någon WPS effekt.

No description available.

This master thesis was carried out at Scania CV AB. The focus for this thesis is the prediction of welding distortions that can cause problems in the manufacturing process of Scania's after-treatment system. The after-treatment system is mainly assembled by sheet metal plates of the ferritic stainless steel EN 1.4509. The plates are welded together. When welding, distortions and residual stresses occur, and they also depend on the sequence in the component was welded together. The distortions and residual stresses can cause tolerance related issues and a lower lifetime for the welded components. Experiments are expensive and therefore it is desirable to simulate the welding process, thereby controlling distortions and optimizing welding sequences. To simulate the welding process and predict the welding distortions a thermo-mechanical FE-model was created for two typical welds found on the after-treatment system. The first scenario was two thin plates welded onto each other in an overlap weld joint and the second scenario was a thin plate welded onto a thick plate in a overlap weld joint. After the FE-model was compared to the experiments. An optimization of the welding sequences was also made on a larger component typically found on the after-treatment system. The FE-model can predict the distortion shape with good accuracy for the T-fillet weld, while the model predicted a more symmetric distortion shape on the overlap weld compared to a more asymmetric shape found on the experiments, but the error is still not very large. The Fe-model can also be used to optimize the welding sequence for bigger components on the after-treatment system within a reasonable time span compared to doing the opimization manually in an experiment. / Detta examensarbete gjordes för Scania CV AB. Fokus for detta examensarbete har varit kvarvarande deformationer efter svetsning som kan skapa problem vid tillverkningen av Scanias avgasefterbehandlingssystem. Avgasefterbehandlingssystemet är till mesta dels konstruerat av stålplåtar av det ferritiska rostfria stålet EN 1.4509, plåtarna är svetsade ihop och då uppstår kvarvarande deformationer. När komponenter svetsas samman uppstår deformationer och restspänningar. Dessa deformationer och restspänningar är också beroende på i vilken sekvens komponenterna har svetsats ihop. Deformationerna och restspänningarna kan skapa problem med toleranser och sänka livslängden för komponenterna som sammanfogats. Experiment är kostsamma och därför är det önskvärt att simulera svetsprocessen, och därav kontrollera deformationerna som uppstår och optimera i vilken sekvens som komponenterna ska svetsas ihop. För att simulera svetsprocessen och prediktera de kvarvarande deformationerna efter svetsning så gjordes termo-mekanisk FE-model för två vanliga svetsscenarion för avgasefterbehandlingssystemet. Det ena scenariot är två tunna plåtar som svetsas ihop i en överlappande position och det andra var en tunn plåt som svetsas på en tjockare plåt. Ett experiment gjordes sedan för båda svetstyperna. Efter att svetstyperna hade jämförts med experimentet så gjordes en optimering av svetssekvensen för en större komponent likt komponenter funna på avgasefterbehandlingssystemet. Den termomekaniska FE-modelen kunde prediktera de kvarnvarande deformationerna och deras form med bra noggrannhet jämfört med experimentet med undantag för en deformationsform på de tunna plåtarna som var mer symmetrisk i FE-modellen jämfört med den asymmetriska formen i experimentet. FE-modellen kunde också användas för att optimera svetssekvensen för den större komponenten inom en rimlig tidsrymd.

[pt] Esta pesquisa teve como objetivo avaliar a vida-fadiga de dois ferros fundidos nodulares modificados metalurgicamente, sendo uma classe predominantemente ferrítica e outra.perlítica. Inicialmente, amostra dos dois ferros fundidos nodulares ferrítico e perlítico foram fundidas adotando-se moldação em areia na geometria padrão Y-block. Em seqüência, corpos de prova para ensaios mecânicos e de fadiga foram usinados das amostras ferríticas e perlíticas. Após os ensaios de tração e dureza, realizaram-se análises metalográficas qualitativas e quantitativas em ambos os materiais, com o intuito de se determinar suas características metalúrgicas, tais como contagem, distribuição e classe dos nódulos de grafita, bem como quantidade da matriz ferrítica e perlítica. Dando continuidade a etapa experimental, as curvas tensão versus número de ciclos para a falha do ferro fundido nodular ferrítico e do ferro fundido nodular perlítico foram levantadas por meio de ensaios de flexão rotativa. A vida útil em fadiga dos dois materiais foi relacionada com as suas características metalúrgicas. Quanto a resistência à fadiga, as amostras do ferro fundido nodular perlítico tiveram um melhor comportamento sob carregamento cíclico do que as amostras do ferro fundido nodular ferrítico. Tal comportamento superior foi atribuído a maior microdureza da matriz e a presença da estrutura olho-de-boi. Finalmente, as curvas experimentais tensão versus número de ciclos para a falha dos ferros fundidos nodulares ferrítico e perlítico foram modeladas pela equação de Coffin-Manson, que se mostrou eficiente no tratamento de dados experimentais da vida em fadiga de ambos os materiais. / [en] The objective of the present work was to evaluate the fatigue life of two nodular cast irons with metallurgical modifications and resulting in ferritic and perlitic different classes of material. Initially, samples of both materials were cast in sand moulds adopting the standard Y-block geometry. In the sequence, tensile and fatigue specimens were machined from the ferritic and perlitic samples. After the tensile and hardness tests, the microstructure of the both materials were analyzed by qualitative and quantitative metallography, aiming to characterize their metallurgical aspects as content, distribution and class of graphite nodules, as well as the contents of the ferritic and perlitic matrix. Following the metallurgical characterization, rolating bend fatigue tests were performed in order to estabilish the stress-life curves of the ferritic and perlitic nodular cast irons. Regarding the fatigue resistance, the specimens machined from the perlitic nodular sample showed a longer fatigue life than that related to the ferritic nodular specimens. The longer fatigue life of the perlitic nodular specimens was associated with a higher microhardness of the perlitic matrix and the preserve of the bull`s-eye structure. Finally, the experimental stress-life curves of the ferritic and perlitic nodular cast irons were modeled adopting the Coffin-Manson law, which was considered efficient in fitting experimental fatigue life data of both materials.

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Integration of AHSS into the automotive architecture has brought renewed challenges for achieving acceptable welds. The varying alloying content and processing techniques has further complicated this initiative. The current study examines resistance spot welding of high strength and advance high strength steels including high strength low alloy (HSLA), dual phase (DP) and a ferritic-bainitic steel (590R). The mechanical properties and microstructure of these RSW welded steel alloys are detailed. Furthermore a relationship between chemistries and hardness is produced. The effect of strain rate on the joint strength and failure mode is also an important consideration in the design of welded structures. Current literature, however, does not explain the effects of weld microstructure and there are no comprehensive comparisons of steels. This work details the relationship between the joint microstructure and impact performance of spot welded AHSS. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower, respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes are detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties are developed using observed microstructures in the fusion zone and heat-affected-zone. Friction stir spot welding (FSSW) has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has also been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn-coated DP600 AHSS (1.2mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process.

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Integration of AHSS into the automotive architecture has brought renewed challenges for achieving acceptable welds. The varying alloying content and processing techniques has further complicated this initiative. The current study examines resistance spot welding of high strength and advance high strength steels including high strength low alloy (HSLA), dual phase (DP) and a ferritic-bainitic steel (590R). The mechanical properties and microstructure of these RSW welded steel alloys are detailed. Furthermore a relationship between chemistries and hardness is produced. The effect of strain rate on the joint strength and failure mode is also an important consideration in the design of welded structures. Current literature, however, does not explain the effects of weld microstructure and there are no comprehensive comparisons of steels. This work details the relationship between the joint microstructure and impact performance of spot welded AHSS. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower, respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes are detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties are developed using observed microstructures in the fusion zone and heat-affected-zone. Friction stir spot welding (FSSW) has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has also been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn-coated DP600 AHSS (1.2mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process.

Made available in DSpace on 2014-12-17T14:07:10Z (GMT). No. of bitstreams: 1 JoseFSJ_TESE.pdf: 4970258 bytes, checksum: fcd6ec64fc7d0cdced502c9b8b4dd5f9 (MD5) Previous issue date: 2012-11-01 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The 15Kh2MFA steel is a kind of Cr-Mo-V family steels and can be used in turbines for energy generation, pressure vessels, nuclear reactors or applications where the range of temperature that the material works is between 250 to 450?C. To improve the properties of these steels increasing the service temperature and the thermal stability is add a second particle phase. These particles can be oxides, carbides, nitrites or even solid solution of some chemical elements. On this way, this work aim to study the effect of addition of 3wt% of niobium carbide in the metallic matrix of 15Kh2MFA steel. Powder metallurgy was the route employed to produce this metallic matrix composite. Two different milling conditions were performed. Condition 1: milling of pure 15Kh2MFA steel and condition 2: milling of 15Kh2MFA steel with addition of niobium carbide. A high energy milling was carried out during 5 hours. Then, these two powders were sintered in a vacuum furnace (10-4torr) at 1150 and 1250?C during 60 minutes. After sintering the samples were normalized at 950?C per 3 minutes followed by air cooling to obtain a desired microstructure. Results show that the addition of niobium carbide helps to mill faster the particles during the milling when compared with that steel without carbide. At the sintering, the niobium carbide helps to sinter increasing the density of the samples reaching a maximum density of 7.86g/cm?, better than the melted steel as received that was 7,81g/cm?. In spite this good densification, after normalizing, the niobium carbide don t contributed to increase the microhardness. The best microhardness obtained to the steel with niobium carbide was 156HV and to pure 15Kh2MFA steel was 212HV. It happened due when the niobium carbide is added to the steel a pearlitic structure was formed, and the steel without niobium carbide submitted to the same conditions reached a bainitic structure / O a?o 15Kh2MFA, da fam?lia dos a?os CrMoV, pode ser utilizado em turbinas para gera??o de energia, vasos de press?o, reatores nuclear ou aplica??es, onde o material ? submetido a temperaturas de servi?o entre 250 e 450?C. Uma forma de melhorar as propriedades do a?o, para que ele trabalhe a temperaturas mais altas ou que se torne mais est?vel ? adicionar part?culas de segunda fase na sua matriz. Estas part?culas podem estar na forma de ?xidos, carbetos, nitretos ou at? mesmo em solu??o s?lida quando alguns elementos qu?micos s?o adicionados ao material. Neste contexto, este trabalho objetiva estudar o efeito da adi??o de 3% de carbeto de ni?bio na matriz met?lica do a?o 15Kh2MFA. Para isto a metalurgia do p? foi a rota empregada para a produ??o deste comp?sito de matriz met?lica. Para tal, duas moagens distintas foram realizadas. A primeira com o a?o 15Kh2MFA e a segunda com o a?o 15Kh2MFA com adi??o de 3% de carbeto de ni?bio. A moagem de alta energia foi realizada durante 5 horas. Em seguida, os dois p?s produzidos foram sinterizados em um forno a v?cuo (10-4torr) a temperaturas de 1150?C e 1250?C durante 60 minutos. Ap?s a sinteriza??o as amostras foram submetidas ao tratamento t?rmico de normaliza??o a 950?C. Os resultados mostraram que a adi??o do carbeto de ni?bio ajuda o processo de cominui??o das part?culas, quando comparado com o a?o sem o carbeto de ni?bio. O carbeto de ni?bio tem um papel fundamental na densifica??o das amostras durante a sinteriza??o, levando a densidade 7,86g/cm?, que ? maior do que a densidade do a?o fundido recebido que era de 7,81g/cm?. Apesar desta boa densifica??o, ap?s a normaliza??o, o NbC n?o contribuiu de forma significativa para aumento da dureza, onde a melhor dureza obtida para o a?o com NbC foi de 156HV e para o a?o puro foi de 212HV. Isto se deve ao fato de que, quando o NbC foi adicionado ao a?o, formou-se uma estrutura perl?tica, enquanto que, com o a?o sem adi??o de NbC, submetido as mesmas condi??es, obteve-se uma estrutura bain?tica

Le développement à grande échelle des projets de véhicules électriques à pile àcombustible nécessite le déploiement d’infrastructures de transport et de stockaged’Hydrogène gazeux. La conception de ces structures et la sélection des matériaux nécessitede s’affranchir des risques liés à la fragilisation par l’Hydrogène des alliages métalliques. Cephénomène est bien décrit depuis plusieurs décennies, mais les mécanismes élémentaires àl’origine de ce mode d’endommagement restent controversés, notamment par manque demodèles quantitatifs. Plus précisément, le rôle de la déformation (micro-)plastique en pointede défaut sur le piégeage et l’endommagement par l’hydrogène, s’il est bien démontréexpérimentalement dans de nombreux systèmes, reste mal pris en compte dans les modèlesmicro-mécaniques. Le centre SMS de l’ENSM.SE a proposé des approches originales demodélisation des interactions hydrogène – dislocations, qui ont pu être validéesexpérimentalement dans des matériaux modèles de structure cubique à faces centrées. Cette thèsese propose d’appliquer une démarche semblable dans des alliages de structure cubiquecentrée. On mettra en oeuvre des essais de déformation sur des matériaux modèles pré-chargésen hydrogène, des modèles semi-analytiques et des observations des structures de déformationen microscopie électronique à transmission. / The development of electrical vehicles powered by hydrogen fuel cells requires the large scaledeployment of hydrogen storage and transport infrastructures. This in turn requires theassessment of the sensitivity of structural materials to hydrogen embrittlement phenomena.These damage modes, while being well described experimentally for since several decades,are still highly debated when it comes to elementary physical processes, mainly because of thelack of quantitative models for these elementary processes. More precisely, the role of the(micro-)plasticity developing at the tip of structural defects, while being well establishedexperimentally, is still poorly accounted for in the available micro-mechanical models. TheScience of Materials and Structures division of ENSM.SE already proposed originalmodelling approaches for hydrogen – dislocation interactions, that have been experimentallyvalidated in face-centred cubic materials. This project aims at applying the same type ofapproach to body-centred cubic metals. This will be achieved by means ofdeformation tests on hydrogen-charged model body centred cubic alloys, investigations of thedislocation microstructures by transmission electron microscopy and the development ofsemi-analytical models of hydrogen-dislocation interactions.