Macro- and micro-scale modelling of material inhomogeneity for the premature failure of welded 9Cr steels in power plants
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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.
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