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

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.

Funder

Science Foundation Ireland

Publisher

NUI Galway

Collections

University of Galway Theses (PhD Theses)