Thermomechanical Analysis of P91 Power Plant Components

Abstract

This thesis presents a combined experimental and computational study on the thermo-mechanical fatigue performance of welded P91 material to (i) characterise the thermomechanical behaviour, and (ii) predict the number of cycles to fatigue crack initiation, for power plant P91 material (including welded material) and components, operating under realistic loading conditions. A programme of isothermal high temperature low cycle fatigue (HTLCF) strain- and stress-controlled and thermo-mechanical fatigue tests on service-aged (SA) P91 base metal, weld metal and welded specimens was conducted to characterise the thermo-mechanical performance of the P91 parent, weld metal and heat-affected zone materials. Novel cross-weld test specimens, as well as weld and parent metal specimens, were manufactured here from a specially-fabricated P91 weld repair header pipe, in collaboration with the plant operator. A sequentially coupled thermomechanical simulation methodology, using realistic temperature and pressure loading histories, was developed within a general-purpose, non-linear, finite element code to predict and analyse the thermomechanical behaviour of P91 power plant components. An anisothermal cyclic viscoplasticity material model for P91 parent material was developed and calibrated for both service-aged and as-new material, using the tests conducted within this thesis and published data. This constitutive model including isotropic softening, non-linear kinematic hardening and viscoplasticity (Norton power-law creep) terms to simulate the complex evolution of material hysteresis response. This calibrated material model formed the basis for a sequential thermo-mechanical methodology of power plant components, namely, a plain pipe and a branched connection from a fossil fuel plant in Ireland. This methodology also includes a transient heat transfer phase with sequential cyclic thermo-mechanical phase to simulate (i) a plant start-up, and (ii) a load-following scenario, based on measured data to identify key damaging events caused during plant operation. The transient heat transfer model was calibrated and validated against the measured plant start-up cycle data. For the branched connection, a global-sub-modelling framework was employed for detailed mesh refinement to identify the local thermomechanical stress-strain response at critical locations. A multiaxial, rain flow cycle counting methodology was developed for thermo-mechanical fatigue prediction using a critical-plane approach and applied to predict thermomechanical fatigue crack initiation at critical locations of the branched pipe sub-models. The results were shown to be consistent with plant operator experience and previously published experimental findings for similar plant connections. A key contribution of the present thesis is the development of a methodology for identification of the high temperature cyclic viscoplasticity parameters for the weld material and heat-affected zone, including isotropic softening, non-linear kinematic hardening and viscoplasticity terms. Theoretical and finite element models of the parent, weld and cross-weld test specimens were employed for identification of the cyclic viscoplasticity material parameters, via direct comparison with the measured hysteresis evolutions for the parent, weld and cross-weld specimens. The methodology developed, in direct collaboration with a power plant operator, provides a comprehensive framework for life assessment of existing, retrofitted and proposed new plant, for coal-fired and gas-fired operation. It 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. This work also provides a more rational basis for experimental thermomechanical fatigue characterisation of candidate materials, as well as, design of tests for welded connections under power plant loading conditions.