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The stress corrosion cracking (SCC) of components which are fabricated from austenitic stainless steel has been observed in the primary circuit of pressurised water reactors (PWR). In recent years it has become an increasing concern that cold work can induce susceptibility to SCC in these materials, even when exposed to good-quality flowing coolant. Laboratory studies which were launched in response to this observation have confirmed that SCC susceptibility is enhanced by cold work. The intention of this study is therefore to investigate the link between the effects of cold work on the material and the susceptibility to SCC. The investigation has been conducted on a grade 304 austenitic stainless steel. Characterisation of the microstructure and mechanical properties has been carried out in the annealed condition, and following cold rolling to a reduction in thickness of 20 %. The cold rolled material has then been subjected to SCC tests in simulated PWR primary circuit coolant. Two types of test were utilised: slow strain rate tests (SSRTs) were carried out in order to investigate the initiation of cracks from a smooth surface and constant load tests using pre-cracked specimens were used to investigate the crack propagation behaviour. In both types of test the SCC produced was predominantly intergranular. The SSRTs revealed that the most susceptible grain boundaries separated grains which had dissimilar deformation microstructures (one grain deformed heavily by planar bands, the other more homogenously). It was also observed that initiation could occur on a grain boundary which is adjacent to an annealing twin. In both microstructural configurations the susceptibility is likely to be due to the deformation incompatibility across the failed boundary, possible indicating that shear at the boundary is important for the initiation of cracking. The crack propagation behaviour of the rolled material was particularly anisotropic; regardless of the loading direction (specimens were manufactured to allow loading along the rolling, transverse and normal plate directions) cracking was observed to occur parallel to the rolling-transverse plane. The origin of this behaviour was explored in terms of preferential alignment of the deformation microstructure and the anisotropic mechanical properties of the rolled plate. Limited transgranular cracking was also observed, which occurred along oxidised deformation bands. The results overall indicate that heterogeneous deformation between different regions of the material, and preferential alignment of the deformation microstructure are important with respect to the SCC susceptibility of the rolled material.

Stainless steel is one of the most commonly used materials in most industries. Excellent corrosion resistance of stainless steel is due to the formation of an oxide film on the surface (passive film), which protects the material from continuous corrosion attacks. When subjected to an attack combining corrosion and erosion, the passive film is damaged and thus, higher and unpredictable degradation rates are observed, which may result in costly consequences. In the first part of this study a model was developed for erosion enhanced corrosion of 304 stainless steel. A new device was designed and constructed, which made possible the impingement of single particles on the surface of sample material at different impact velocities and angles. Based on the electrochemical response of material to the impact of single particles, a model was proposed that considered the number of the impacting particles on the surface. The predictions made by this model were later compared with the results of a slurry jet experiment, which was used to simulate the service conditions. The second part of the research included the basic mechanical and electrochemical studies of the interactions occurring between the particle and material surface during the particle impact. This included the effects of different impact parameters such as coefficient of friction, impact angle, impact energy and particle angular velocity on depassivation of 304 stainless steel and its erosion-corrosion. A depassivation mechanism was proposed that considered a combined effect of the friction force and its effective path of action on the surface. In the last part improving the erosion-corrosion properties of 304 stainless steel was tried based on the results of the second part of the study. Samples were cold rolled and the effect of hardness on the coefficient of friction was investigated, which in the second part was proven responsible for the depassivation of the surface. It was found that the coefficient of friction between the particles and the surface remains unchanged in different applied percentages of cold work. Also it was shown that work hardening is an effective method for increasing the resistance of the material to erosion-corrosion. / Materials Engineering

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Made available in DSpace on 2016-03-15T19:37:01Z (GMT). No. of bitstreams: 1 Eva Paganini Martins.pdf: 2852717 bytes, checksum: 4c8bb41c27d4434bc48bef3b6484ab0f (MD5) Previous issue date: 2010-03-05 / This paper deals with the effects of cold work on the nitriding kinetics and surface micro-hardness of the 3XX family of austenitic stainless steels. Nitriding is a thermo-chemical treatment based on the nitrogen diffusion and, under some specific circumstances, carbon in lower quantities, in to a ferrous matrix. The nitriding process improves particularly high cycle fatigue resistance, wear resistance, and corrosion resistance for carbon steels. To study the cold work effect on the material, samples of 304 and 316 were strained in tension tests to conventional deformation of 10%, 25% and up to rupture point prior to the nitriding process. A sample without deformation was kept for reference. It is possible to verify, based on the test performed, that the AISI 304 has smaller deformation capacity when compared to the 316 in terms of elongation on strain test presenting a higher degree of cold work deformation. Also the 304 presents a reduction on nitrided layer thickness and a smaller percentage of nitrogen in the nitrided layer after the cold work (difference of 1% point versus 0,5 for the 316) It is also possible to observe the smaller density on the 304 after the cold work which can be attributed, according to the literature, to the formation of cold work martensite during the cold work process. / O presente trabalho apresenta uma avaliação dos efeitos de encruamento na cinética do processo de nitretação das ligas de aço inoxidável austeníticos da série 3XX e no perfil de dureza das superfícies nitretadas. Nitretação é um tratamento termoquímico baseado na difusão de nitrogênio, em algumas circunstancias carbono em pequenas quantidades, na matriz ferrosa. Para estudar os efeitos do encruamento no material, amostras de 304 e 316 foram encruadas em ensaio de tração convencional, em 10%, 25% e até a ruptura e posteriormente nitretadas. Ainda manteve-se uma a mostra sem deformação para fins de comparação. Com base nos ensaios avaliados é possível verificar que o AISI 304 tem menor capacidade de deformação que o AISI 316 em termos de alongamento no ensaio de tração apresentando maior taxa de encruamento em relação ao 316. Além disso o 304 apresenta redução na camada nitretada proporcional ao grau de deformação plástica a frio e menor percentual de nitrogênio na camada após deformação (queda de aproximadamente 1 ponto percentual enquanto no 316 a diferença mantém-se em torno de 0,5 pontos). Ainda é possível observar a diminuição da densidade no 304, o que pode ser atribuído, segundo a literatura, a formação de martensita e deformação durante o processo de deformação plástica a frio.

Additive manufacturing (AM) is a promising manufacturing process that provides that ability to fabricate components with complex geometries with relatively low lead times compared to other manufacturing processes. This allows for more freedom of design, as prototypes can easily be produced throughout the development process. AM is also especially beneficial in tooling applications, where internal geometries such as cooling channels are required in order to improve the quality of the manufactured parts. These geometries are more difficult to produce with more conventional manufacturing methods such as forging or casting. Currently, Laser Powder Bed Fusion (LPBF) shows the most promise in the field of Additive Manufacturing (AM) of metals, as it offers the freedom to produce complex components with little post processing required. Additionally, post processing with Hot Isostatic Pressing (HIP) can be implemented to significantly enhance the final properties of the material.  The LPBF-process can produce many different defects within the parts such as: part porosity and lack of fusion. This is mainly due to the layer-by-layer configuration of the process. Parts can also experience large thermal fluctuations and rapid cooling rates which can generate large residual stresses. This can result in significant cracking in certain high alloyed materials which can impact part quality and  material properties. If the cracking is severe enough, it will result in failure of the entire component and render the entire parts completely useless. Post processing with HIP may remove some of these defects and reduce the residual stresses in the material and thus produce a material with properties that are satisfactory. The purpose of this thesis is to investigate the processability of a high alloy cold work tool steel with LPBF. The main focus is the influence of the processing parameters and part geometry on the quality of the produced parts. Furthermore, the influence of the processing parameters on defects and microstructure will also be investigated. The aim is to produce parts that can be enhanced with HIP as a post processing treatment. Additionally, the impact of HIP on the properties of the part will also be investigated in order to determine if the there are any improvements in terms ofreduction in part defects and the removal of any undesired microstructural features which are produced from the process. The experimental results showed that the processability of the tool steel is difficult. Several sample volumes were produced with varying processing parameters and scanning strategies, and all the specimens from all sample volumes exhibited some cracking. Parts produced with a combination of contouring and hatching strategy, where there is an internal structure showed the most promise, as these parts exhibited the least amount of severe cracking. However, additional research of the processing parameters and scanning strategies is required in order to reduce the amount of cracking of the external shell structure and thus, achieve proper densification of the parts when post processing with HIP. / Additiv tillverkning (AM) är en lovande tillverkningsprocess som ger möjligheten att tillverka komponenter med komplexa geometrier med relativt låga ledtider jämfört med andra tillverkningsprocesser. Detta ger större frihet i under designprocessen eftersom prototyper enkelt kan produceras under hela utvecklingsprocessen. AM är också särskilt fördelaktigt i verktygstillämpningar, där interna geometrier såsom kylkanaler krävs för att förbättra kvaliteten på de tillverkade delarna. Dessa geometrier är svårare att tillverka med mer konventionella tillverkningsmetoder som smidning eller gjutning. För närvarande visar det sig att Laser Powder Bed Fusion (LPBF) är det mest lovande inom området additiv tillverkning av metaller, eftersom processen erbjuder friheten att producera komplexa komponenter samt att efterbearbetning som krävs blir mindre. Dessutom kan efterbearbetning med Hot Isostatic Pressing (HIP) implementeras för att avsevärt förbättra materialets slutliga egenskaper. LPBF-processen kan ge upphov till många olika defekter i delarna såsom: delporositet och lack of fusion. Detta beror främst på att processen sker lagervis vilket kan ge upphov att många småfel. Delar kan också uppleva stora termiska fluktuationer och snabba kylningshastigheter som kan generera stora restspänningar. Det kan resultera i stor sprickbildning i vissa höglegerade material vilket kan påverka delarnas kvalitet och materialegenskaper. Om sprickorna som bildas är stora eller djupa nog kommer detta att resultera i att hela komponenten blir oanvändbar. Efterbearbetning med HIP kan ta bort en del av dessa defekter och minska restspänningarna i materialet och därmed producera ett material med goda egenskaper. Syftet med detta arbete är att undersöka bearbetbarheten hos ett höglegerat kallbearbetningsstål med som produceras med LPBF. Huvudfokus är påverkan av processparametrar och detaljgeometrin på kvaliteten på de producerade delarna. Vidare kommer också processparametrarnas inverkan på defekter och mikrostruktur att undersökas. Syftet är att producera delar som kan förbättras med HIP som efterbehandlingsbehandling. Dessutom kommer effekterna av HIP på delens egenskaper också att undersökas för att avgöra om det finns några förbättringar i termer av minskning av deldefekter och avlägsnande av alla oönskade mikrostrukturella egenskaper som produceras från processen. De experimentella resultaten visade att verktygsstålets bearbetbarhet är svår. Flera provvolymer producerades med varierande processparametrar och skanningsstrategier, och alla prover från alla provvolymer uppvisade viss sprickbildning. Delar som tillverkats med en kombination av kontur och hatch, där det finns en inre struktur visade sig mest lovande, eftersom dessa delar uppvisade minst sprickbildning. Ytterligare arbete av processparametrarna och skanningsstrategier krävs dock för att minska mängden sprickbildning i den yttre skalstrukturen och därmed uppnå korrekt förtätning av delarna vid efterbearbetning med HIP.

The demand for Advanced High Strength Steel (AHSS) in the automotive industry is increasing day by day. It is mainly motivated by the fact that AHSS can be used as thin sheets while having high strengths. It enables weight reduction of the automobiles which consequently increases the fuel efficiency and has proven to be less harmful to the environment. It is also expected that AHSS will have even higher strength in the near future. Cold work tools steels with 5 wt.% Cr are commonly used to process AHSS. Therefore, the tool steel must meet the challenges in the future, i.e. have even higher hardness, compressive strength and toughness. One way of increasing the mechanical properties of the tool steel is by improving the heat treatment parameters. However, it is not possible without a deeper understanding of the heat treatment process. Therefore, this work presents investigations related to phase transformations occurring in a 5 wt.% Cr cold work tool steel during heat treatment. Furthermore, the influence of austenitisation and tempering temperatures on the microstructure and mechanical properties were investigated. The studies revealed that a higher austenitisation temperature can be used to achieve a higher hardness, good compressive strength and adequate toughnessof the steel. However, too high austenitisation temperature may result inexcessive coarsening of prior austenite grains which reduced the impact toughness. It was also found that retained austenite can transform during tempering by two different mechanisms. Firstly, when tempering at 525°C, carbides precipitate in retained austenite lowering its stability and permitting a transformation to marten site on cooling. Secondly, when tempering at 600°Cfor extended holding time retained austenite isothermally transforms to ferrite and carbides. This occurs by precipitation of carbides in retained austenite followed by a final transformation to ferrite and carbides.These results were used to understand the standard tempering procedure of the 5 wt.% Cr cold work tool steel. Furthermore, alternative heat treatment procedures are discussed based on the important findings presented in this thesis.

The purpose of this report is to evaluate the hardness and microstructure in bent and welded samples of Alloy 718. The results will be used by GKN aerospace to evaluate the simulated values of the production process of vines in a jet engine. In total, eleven samples from three different production chains are evaluated. All samples are bent and go through different stages within the production including bending, solution treatment and welding. The samples are cut and mounted in bakelite then polished and etched in order for the microstructure to be seen and evaluated. Hardness tests were made on the mounts to evaluate how bending, solution treatment and welding affects the hardness. The bent samples without the solution treatment became harder depending on the amount of cold deformation. The amount of cold deformation controls how fast the material recrystallizes during subsequent solution treatment. During the solution treatment, the δ-phase (Ni3Nb) is precipitated in the grain boundaries which prevents a coarser grain size and therefore promotes a smaller grain size.  The laser weld creates a small HAZ (heat affected zone) that becomes softer because the heat dissolves the δ-phase which therefore triggers the grains to grow. The weld consists of eutectic γ-dendrites with interdendritic pools of alloying elements. After the solution treatment, many needle shaped δ-phases arise from the pools and HAZ. The hardness measurements were tested on a manual machine which makes potential human error important to consider when the measurements are evaluated. When the grain size measurements are done on the solution treated samples, the grain boundaries can be difficult to determine because the δ-phases and twins create wide and incorrect boundaries. The results will be used by GKN Aerospace in order to verify their simulations. The heterogenic material after solution treatment will probably be studied further. / Syftet med projektet är att undersöka hårdhet och mikrostruktur i bockade och svetsade prover tillverkade av Alloy 718. Totalt undersöks elva stycken prover som är tillverkade på olika sätt enligt tre tillverkningskedjor. Alla provbitar är bockade och har sedan genomgått olika många steg i tillverkningen som består utav bockning, upplösningsbehandling och svets. Genom att kapa upp provbitarna i mindre bitar, baka in de i bakelit och etsa de studerades mikrostrukturen och hur den påverkas av bockning, upplösningsbehandling och svets. Vidare gjordes hårdhetsmätningar över olika delar av proven för att undersöka hur mycket hårdare materialet blir vid kalldeformation och hur upplösningsbehandling och svets påverkar hårdheten i Alloy 718. Endast bockade prov blir hårdare beroende på hur mycket kalldeformation provbiten har utsatts för. Hur mycket provet har deformerats styr hur snabbt rekristallisationen sker vid en efterföljande upplösningsbehandling. Under upplösningsbehandlingen utskiljs även δ-fas (Ni3Nb) i korngränser vilket främjar en mindre kornstorlek. Lasersvetsen ger ett litet värmepåverkat område men värmen från svetsen bidrar till att δ-fasen löses upp och större korn bildas. Svetsen består av långa eutektiska γ-dendriter med interdendritiska poler av mycket legeringsämnen. Vid upplösningsbehandling efter utförd svets bildas det δ-fas i de värmepåverkade och i svetsen. Hårdhetsmätningarna har utförts på en manuell hårdhetsmaskin vilket innebär att felmarginalen blir större då den mänskliga felfaktorn spelar en stor roll. Vid beräkning av kornstorlek har det varit svårt att utskilja vad som är en korngräns i upplösningsbehandlade prov då mängden δ-fas efter upplösningsbehandling ger tjocka korngränser.  Resultaten kommer användas av GKN Aerospace för att verifiera sina simulationer. Det heterogena materialet efter upplösningsbehandlingen kommer troligtvis studeras vidare.

Pour assurer une bonne résistance à l’oxydation à haute température, les couches d’oxydes thermiques formée sur les aciers inoxydables, doivent rester fines, riches en chrome et adhérentes à leur substrat métallique. Lorsque les aciers inoxydables sont soumis à des conditions sévères de températures ou d’atmosphères, l’oxydation catastrophique entraîne la croissance rapide de nodules d’oxydes de fer non protecteurs au détriment de la couche d’oxyde riche en Cr. Cette étude s’est intéressée aux différents mécanismes mis en jeu dans la perte du caractère protecteur des couches d’oxydes, dans le développement des nodules d’oxydes de fer, ainsi que dans la formation de zones d’oxydation interne. L’étude de la morphologie et de la composition des oxydes formés à l’aide de cartographie spectrale Raman ainsi que de cartographies MET et EBSD ont permis de proposer un mécanisme de formation de la zone d’oxydation interne, qui repose en partie sur l’évolution de la composition d’une couche d’oxyde « bordure » qui se forme le long des joints de grains du métal sous-jacent lors de l’oxydation. Cette étude a également démontré qu’une prédéformation avant oxydation améliore la durabilité des aciers en favorisant la formation d’une couche protectrice dès les premiers instants de l’oxydation. Lorsqu’elle survient, l’oxydation catastrophique reste localisée alors que sans prédéformation un régime protecteur n’est jamais atteint à 850 °C pour l’acier austénitique 304L. / To provide good resistance to oxidation at high temperature, the oxide layers formed on stainless steels must stay thin, rich in chromium and adhere to their metallic substrate. When the stainless steels operate at atmospheres or temperatures that are too severe, breakaway oxidation triggers the quick growth of Fe-rich oxide nodules, which are non-protective, instead of the Cr-rich layer. This study focuses on the different mechanisms that lead to the loss of the protective characteristic of the oxide layer, to the growth of the iron oxides, and in the formation of internal oxidation zones. The study of the morphology and composition of the oxides formed, along with Raman spectroscopy and TEM and EBSD mappings, allowed to propose a mechanism for the formation of the internal oxidation zone. This mechanism relies in part on the formation of a "boundary" oxide layer, that forms along the grain boundaries of the underlying metal during oxidation. This study also showed that a deformation prior to oxidation improves the durability of the steels by encouraging the formation of a protective layer during the first stages of the oxidation. When it starts, breakaway oxidation stays localized while with no deformation, a protective regime is never reached at 850 °C for austenitic stainless steel 304L.