dc.contributor.advisor | Leen, Sean | |
dc.contributor.author | Li, Ming | |
dc.date.accessioned | 2019-02-15T11:49:55Z | |
dc.date.available | 2019-02-15T11:49:55Z | |
dc.date.issued | 2019-02-15 | |
dc.identifier.uri | http://hdl.handle.net/10379/14960 | |
dc.description.abstract | Chromium-molybdenum steel pipe (9Cr-1Mo steel) is an important high temperature construction material for welded steam piping networks in gas-fired power plants. A major global problem of such materials is the premature failure which commonly occurs in the weld regions at elevated temperature. The primary objective of this thesis is the development of a macro- and micro-mechanical framework to predict this premature failure in the 9Cr-1Mo weldments. At the macro-scale level, a computational methodology for cyclic plasticity in multi-material welded components including laboratory scale test specimens and real plant is developed. At the micro-scale level, mixed-phase crystal plasticity models are developed to evaluate the mechanical performance of welding-induced micro-structure textures providing guidance into optimised heat treatments for welded components and material design for welded 9Cr-1Mo steel.
For the work at the macro-scale level, a cyclic plasticity model including non-linear kinematic and isotropic hardening was adopted for the finite element analysis of welded 9Cr-1Mo steel in both cross-weld uniaxial test specimens and a welded T-piece power plant connection. The identified parameters are successfully calibrated and validated against uniaxial high temperature low cycle fatigue data from test data and the finite-element modelling of the cross-weld configuration, including three different material zones: parent metal, weld metal and heat-affected zone (HAZ). The predicted plastic strain in the cross-weld test is shown to concentrate in the HAZ at the interface region with parent metal, where the premature failure location is observed. The predicted critical location of the 9Cr-1Mo welded T-piece header-branch connection, as predicted by a three-dimensional finite element model under combined thermo-mechanical fatigue loading histories from Aghada power plant, is consistent with the location of observed premature cracking for in-service conditions. The results highlight the significant importance of incorporating multi-material cyclic plasticity parameters for prediction of plasticity and stress evolution in such welded connections.
A key contribution of the present thesis is the development of a physically-based micro-mechanical framework for the modelling the ferrite-martensite inhomogeneity in the premature failure region of 9Cr-1Mo weldments. Previously-published monotonic tensile test data of IC-HAZ at 20 °C and 625 °C are adopted to calibrate the crystal plasticity finite element model of IC-HAZ. This demonstrated the importance of crystallographic orientation for localized stress and strain distributions, micro-crack initiation and subsequent damage evolution. Small volume fractions of ferrite are shown to have a clear detrimental effect on the mechanical behaviour of strength and ductility. The effect is significantly larger on both ductility and strength at 625 °C than at 20 °C. The micro-mechanical methodology demonstrates the key role of a small amount of ferrite on micro-crack nucleation and accelerated material degradation in IC-HAZ at 625 °C, leading to reduced life of 9Cr-1Mo welded joints at high temperature.
The methodology for the macro-scale will specifically allow assessment of the impact of more flexible plant operation to allow for renewable energy uptake and energy cost fluctuations and provides a framework for extension to future ultra-supercritical operation scenarios. The micro-structure sensitive material model developed in this thesis will allow for more detailed consideration of the constitutive behaviour of welded 9Cr-1Mo material in power plant components. This will, in turn, enable power plant designers to carry out more reliable material design from a micro-structural perspective. The macro and micro computational methodologies, in direct collaboration with a power plant operator, provideguidance for life assessment and material optimization of existing, retrofitted and proposed new plant. | en_IE |
dc.publisher | NUI Galway | |
dc.rights | Attribution-NonCommercial-NoDerivs 3.0 Ireland | |
dc.rights.uri | https://creativecommons.org/licenses/by-nc-nd/3.0/ie/ | |
dc.subject | 9Cr welds, HTLCF, crystal plasticity, material inhomogeneity | en_IE |
dc.subject | 9Cr welds | en_IE |
dc.subject | HTLCF | en_IE |
dc.subject | Crystal plasticity | en_IE |
dc.subject | Material inhomogeneity | en_IE |
dc.title | Macro- and micro-scale modelling of material inhomogeneity for the premature failure of welded 9Cr steels in power plants | en_IE |
dc.type | Thesis | en |
dc.contributor.funder | Science Foundation Ireland | en_IE |
dc.local.note | Ming Li is awarded a PhD in Mechanical Engineering for research supervised by Prof. Sean Leen and Prof. Padraic O'Donoghue on modelling of material inhomogeneity for the premature failure of welded 9Cr Steels for ultra-super critical power plant applications.
This thesis investigates the premature failure of welded 9Cr-1Mo steel for next-generation power plant. A programme of macro-scale modelling is developed to characterise the material inhomogeneity effects to the premature failure of 9Cr-1Mo welded joints under high temperature low cycle fatigue with application to a T-piece in power plant. Another key contribution of the present thesis is the development of a physically-based micro-mechanical framework for the modelling the ferrite-martensite inhomogeneity in the premature failure region of 9Cr-1Mo weldments. | en_IE |
dc.local.final | Yes | en_IE |
dcterms.project | info:eu-repo/grantAgreement/SFI/SFI Principal Investigator Programme (PI)/10/IN.1/I3015/IE/Materials for Energy: Multiscale Thermomechanical Characterisation of Advanced high temperature Materials for Power generation (METCAMP)/ | en_IE |
dcterms.project | info:eu-repo/grantAgreement/SFI/SFI Investigator Programme/14/IA/2604/IE/Multi-scale, Through-process Chracterisation for Innovative Manufacture of Next-generation Welded Connections (MECHANNICS)/ | en_IE |
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