Noel P. O’Dowd

Deformation Characteristics of a High Chromium, Power Plant Steel at Elevated Temperatures

The changing face of power generation requires an improved understanding of the deformation and failure response of materials that are employed in power plants. Important insights can be obtained through microstructurally motivated modelling studies. With the drive for increased efficiency, there is a corresponding drive towards increasing operating temperatures in conventional power plant. With these increasing temperatures, and with the increased flexibility required of modern power plant working in a mixed energy economy, more robust material testing and modelling tools are required to accurately predict the response of power plant steels. This works deals with the development of a material model for a martensitic steel, P91, relevant to the range of temperatures typically seen in a modern power plant. High temperature (20, 400, 500, 600°C) tensile testing at various strain rates was carried out the steel. Tests were taken to failure and the stress strain response recorded. Electron backscatter diffraction (EBSD) is employed to determine the complex microstructure of the P91 material. This information is incorporated within a representative volume element (RVE) and a nonlinear, rate dependent, finite strain crystal plasticity model used to represent the deformation of the material. The material model was calibrated to each temperature and strain rate to give a robust physically based model that has been fully validated through experimental data.Copyright

The Role of Plasticity in the Transverse Lattice Strain Evolution of a Martensitic Steel

In this work, in-situ neutron diffraction measurements are performed for a martensitic steel in conjunction with crystal plasticity analysis. The results indicate that heterogeneous plastic contraction on transverse {200} grains is responsible for the observed nonlinear lattice strain evolution. The effect of slip properties on plastic contraction is computationally identified.

Modelling of Micro-Plasticity Evolution in Crystalline Materials

In this work, a micromechanical finite element model is presented to investigate micro-plasticity evolution in crystalline materials, with a comprehensive consideration of microstructural interactions, including morphology-based intragranular stress-strain response and the strain gradient induced scale effect. A dislocation-mechanics based crystal plasticity formulation has been employed to account for slip based inelastic deformation. A polycrystalline model has been constructed using the Voronoi tessellation technique to represent the microstructure of a martensitic power plant steel, P91. The model has been validated through a uniaxial tensile test. The effects of strain gradient have been examined at both macroscopic and microscopic levels and the importance of accounting for strain gradient effects in the prediction of local deformation states is discussed for P91.Copyright

Thermomechanical Analysis of a Pressurised Pipe Under Plant Conditions

This paper is concerned with the development of a methodology for thermo-mechanical analysis of high temperature, steam-pressurised P91 pipes in electrical power generation plant under realistic (measured) temperature and pressure cycles. In particular, these data encompass key thermal events, such as ‘load-following’ temperature variations and sudden, significant fluctuations in steam temperatures associated with attemperation events and ‘trips’ (sudden plant shut-down), likely to induce thermo-mechanical fatigue damage. An anisothermal elastic-plastic-creep material model for cyclic behaviour of P91 is employed in the transient FE model to predict the stress-strain-temperature cycles and the associated strain-rates. The results permit characterisation of the behaviour of pressurised P91 pipes for identification of the thermo-mechanical loading histories relevant to such components, for realistic, customised testing; this type of capability is relevant to design and analysis with respect to the evolving nature of power plant operating cycles, e.g. associated with more flexible use of fossil fuel plant to complement renewable energy sources.Copyright

Investigating Ductile Failure at the Microscale in Engineering Steels: A Micromechanical Finite Element Model

In this study, we present a microstructure-based micromechanical model to quantify failure mechanisms in engineering steels. Crystal plasticity at the microscale, governed by crystallographic slip, is explicitly taken into account in the frame-work of continuum mechanics. Furthermore, it is assumed that material damage at the microscale is controlled by the accumulated equivalent plastic strain, such that failure occurs once this strain exceeds a threshold. Both single- and poly-crystalline materials containing sufficient numbers of grains are investigated under a representative macroscopic loading. The calibration of the present model relies on uniaxial tensile test data. Both austenitic stainless steels (such as 316H) and martensitic steels (such as P91) are examined to illustrate the application of the method. The micromechanical modelling provides insights into understanding of the mechanical response at the microscale in engineering steels.Copyright

Prediction of Transient Creep Response Under Combined Primary and Secondary Loading

Residual stress effects on creep deformation and fracture play a significant role in structural integrity assessments of engineering components. The focus of the current work is to investigate creep behaviour of mechanically loaded cracked structures in the presence of residual stress fields. Finite-element analyses have been carried out on single edge notch bend, SEN(B) , and tension, SEN(T) , specimens at different residual stress and mechanical stress levels. The redistribution time and associated stress relaxation for combined primary and secondary (residual) stresses have been determined (from the finite-element analysis) and interpreted using the transient fracture mechanics parameter, C(t). The observed trends are consistent with earlier studies involving combined thermal and mechanical stress.Copyright

Combined finite element and phase field method for simulation of austenite grain growth in the heat-affected zone of a martensitic steel weld

Engineering components operating at high temperature often fail due to the initiation and growth of cracks in the heat-affected zone adjacent to a weld. Understanding the effects of microstructural evolution in the heat-affected zone is important in order to predict and control the final properties of welded joints. This study presents a combined finite element method and phase field method for simulation of austenite grain growth in the heat-affected zone of a tempered martensite (P91) steel weld. The finite element method is used to determine the thermal history of the heat-affected zone during gas tungsten arc welding of a P91 steel plate. Then, the calculated thermal history is included in a phase field model to simulate grain growth at various positions in the heat-affected zone. The predicted mean grain size and grain distribution match well with experimental data for simulated welds from the literature. The work lays the foundation for optimising the process parameters in welding of P91 and other ferritic/martensitic steels in order to control the final heat-affected zone microstructure.

Influence of material inhomogeneity on the mechanical response of a tempered martensite steel

Failure in steel weldments operating at high temperatures often occurs in the heat-affected zone adjacent to the weld. Such failures can be a result of material inhomogeneity within the heat-affected zone and in the case of tempered martensite steels have been linked with regions of untransformed α (ferrite) phase or over-tempered martensite within the intercritical region of the heat-affected zone. In this work, two-dimensional Voronoi tessellation is used to construct polygonal Voronoi cells to represent the microstructure of the heat-affected zone of a weld in a tempered martensite steel. The Voronoi construction is treated as a representative volume element of the material and is discretised by 8-node linear brick elements, with periodic boundary conditions. The lattice orientation at each material point is specified by three Euler angles, which are assumed to be randomly distributed, to represent the initial lack of texture in the intercritical region of the heat-affected zone. The constitutive response is represented by a nonlinear, rate-dependent, finite-strain crystal plasticity model. The results indicate that small amounts of ferrite can induce significant enhancements in stress and inelastic deformation at the interface of the ferrite and martensite grains. This localisation of stress and strain may be critical for microcrack and/or void formation and may be a contributory factor to Type IV cracking.

Limit Load Solution and Crack Driving Force Estimation Scheme for Embedded Flaws in Pipeline Girth Welds

The acceptability of flaws in a subsea rigid pipeline is usually sanctioned based on the results of an engineering criticality assessment (ECA), carried out considering all loads seen by the pipeline from fabrication to the end of service life. Reel-lay is an efficient installation method, frequently used for installing subsea pipelines. Unlike surface breaking flaws, embedded flaws are not directly assessed in a reeling ECA because the available assessment solutions are too conservative. A work around approach is often used, whereby acceptable surface breaking flaw sizes are deemed acceptable beneath the surface, provided that the embedment depth is equal to or greater than half of the flaw height. However, the results of more recent research work suggest that this approach could be non-conservative in some cases.In this work, a parametric finite-element (FE) study was carried out to assess the effect of the embedment depth, the crack length and the crack height on the load required to cause collapse of the shorter ligament of an embedded flaw. Subsequently, a closed form limit load solution was developed, and compared against available solutions for pipes subjected to tension. A J-based crack driving force (CDF) estimation scheme was developed for a selected material behaviour. Finally, recommendations were made for the direct reeling ECA of subsea pipelines with embedded flaws.Copyright

Cyclic Visco-Plasticity Testing and Modelling of a Service-Aged P91 Steel

A service-aged P91 steel was used to perform an experimental programme of cyclic mechanical testing in the temperature range of 400°C to 600°C, under isothermal conditions, using both saw-tooth and dwell (inclusion of a constant strain dwell period at the maximum (tensile) strain within the cycle) waveforms. The results of this testing were used to identify the material constants for a modified Chaboche, unified visco-plasticity model, which can deal with rate-dependant cyclic effects, such as combined isotropic and kinematic hardening, and time-dependent effects, such as creep, associated with visco-plasticity. The model has been modified in order that the two-stage (non-linear primary and linear secondary) softening which occurs within the cyclic response of the service-aged P91 material is accounted for and accurately predicted. The characterisation of the cyclic visco-plasticity behaviour of the service-aged P91 material at 500°C is presented and compared to experimental stress-strain loops, cyclic softening and creep relaxation, obtained from the cyclic isothermal tests.Copyright