The widespread use of renewable energy sources for power generation leads to the limited use of conventional power plants. They are only used when the renewable sources are unavailable. Therefore, they must be able to guarantee a high level of flexibility. From a mechanical point of view, this translates into a greater impact of fatigue damage combined with high temperature creep damage that needs to be studied and investigated for a reliable forecast of the service life. This thesis deals with the investigation of the interaction of creep-fatigue in several materials for power plant components. The main examined materials are the P91 steel used in heat exchangers as well as in steam piping and Inconel 718 superalloy mainly used in gas turbine components such as turbine discs. The damage mechanism of creep-fatigue and its interaction in these materials is discussed for various test conditions with observations of the fracture surfaces. Furthermore, the microstructural modifications of the considered materials are analyzed with the help of SEM and TEM images. In order to study the interaction of creep-fatigue in these materials, a concise and binomial model for the creep-fatigue crack growth has been studied. The model is able to predict the interaction of creep-fatigue and also the dominant factors of creep-fatigue influencing the crack growth behavior. The advantages and limitations of the model and the results obtained for both the materials are explained. On the other hand, the results obtained from the model are supported by the observations of microstructural degradation in the considered materials.
L’utilizzo diffuso di fonti energetiche rinnovabili, per la produzione di energia, porta ad un regime di servizio diverso delle centrali elettriche convenzionali. Esse vengono utilizzate solo quando le fonti rinnovabili non sono disponibili. Per cui devono essere in grado di garantire un elevato livello di flessibilità. Dal punto di vista meccanico, questo si traduce in un maggiore impatto del danneggiamento a fatica unito al danneggiamento a creep ad alta temperatura che necessità di essere studiato e approfondito per una affidabile previsione della vita di esercizio.. Questa tesi si occupa dello studio dell'interazione di creep-fatica in diversi materiali per componenti operanti ad alata temperatura. I principali materiali esaminati sono l'acciaio P91 utilizzato negli scambiatori di calore e nelle tubazioni del vapore e la superlega Inconel 718 utilizzata principalmente nei componenti delle turbine a gas come i dischi delle turbine. Il meccanismo di danneggiamento della creep-fatica e la sua interazione in questi materiali viene discusso per varie condizioni di prova con osservazioni delle superfici di frattura. Inoltre, le modifiche microstrutturali dei materiali considerati vengono analizzate con l'ausilio di immagini SEM e TEM. Al fine di studiare in questi materiali l'interazione di creep-fatica, è stato approfondito un modello conciso e binomiale per la crescita della cricca da scorrimento-fatica. Il modello è in grado di prevedere l'interazione di creep-fatica e anche i fattori dominanti di creep-fatica che influenzano il comportamento di crescita della cricca. Vengono spiegati i vantaggi e i limiti del modello e dei risultati ottenuti per entrambi i materiali. D'altra parte, i risultati ottenuti dal modello sono supportati dalle osservazioni del degrado microstrutturale nei materiali considerati.
Application of a creep-fatigue growth model on materials for power plant components
Fernandes, Rolson
2019/2020
Abstract
The widespread use of renewable energy sources for power generation leads to the limited use of conventional power plants. They are only used when the renewable sources are unavailable. Therefore, they must be able to guarantee a high level of flexibility. From a mechanical point of view, this translates into a greater impact of fatigue damage combined with high temperature creep damage that needs to be studied and investigated for a reliable forecast of the service life. This thesis deals with the investigation of the interaction of creep-fatigue in several materials for power plant components. The main examined materials are the P91 steel used in heat exchangers as well as in steam piping and Inconel 718 superalloy mainly used in gas turbine components such as turbine discs. The damage mechanism of creep-fatigue and its interaction in these materials is discussed for various test conditions with observations of the fracture surfaces. Furthermore, the microstructural modifications of the considered materials are analyzed with the help of SEM and TEM images. In order to study the interaction of creep-fatigue in these materials, a concise and binomial model for the creep-fatigue crack growth has been studied. The model is able to predict the interaction of creep-fatigue and also the dominant factors of creep-fatigue influencing the crack growth behavior. The advantages and limitations of the model and the results obtained for both the materials are explained. On the other hand, the results obtained from the model are supported by the observations of microstructural degradation in the considered materials.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/166029