A design tool for high temperature power-generation materials
13 June 2013
Authors: R A Barrett 1,2; N.P. O’Dowd 3; S B Leen 1,2
1 Mechanical Engineering, College of Engineering and Informatics, NUI Galway
2 Ryan Institute for Environmental, Marine and Energy Research, NUI Galway
3 Department of Mechanical, Aeronautical & Biomedical Engineering, Materials & Surface Science Institute, University of Limerick
From the Parsons steam turbine of 1884 to the Whittle gas turbine of 1930 and more recent state-of-the-art combined cycle gas turbines (CCGT), the load-temperature capacity of engineering materials has been a key limiting factor. To meet the environmental requirements set out under Directive 2009/28/EC, by 2020, Ireland must increase its energy output from renewable sources to 16% and reduce emissions by 20%.
To achieve these requirements, as well as more sustainable and efficient power generation, the next major steps in power plant evolution require (i) a shift to an ultra-supercritical (USC) cycle, (ii) more widespread use of biomass co-fired and CCGT plant and (iii) increased operational flexibility.
These demands require a significant step change in advanced high temperature materials capability. The key difference, nowadays, is the availability of advanced computational methods for materials design and characterisation, to complement (typically limited) experimental data and facilitate reliable extrapolation from accelerated laboratory tests to realistic plant time-scales.
Currently, power plant are designed upon a creep-based failure criteria, with a design life of approximately 200,000 hours (more than 20 years). However, due to the flexible nature of modern power generation, thermal cracking in some components has been observed after as little as 10% of design life. These cracks are typically observed in the heat-affected zone (HAZ) of welded connections, where Type IV cracking is the main failure mechanism.
Thus, there is an urgent need to develop an advanced materials capability to (i) characterise the current generation of power plant materials, i.e. the 9-12Cr family of martensitic steels, under flexible loading conditions and (ii) design optimum materials for flexible loading conditions at higher temperatures and pressures.
To achieve these goals, a Materials Design Tool, as illustrated in Fig 1, is under development as part of a collaborative research project involving NUI Galway, the University of Limerick, ESB Energy International and the University of Nottingham. The project, which is funded by Science Foundation Ireland (SFI), is named METCAM: Materials for Energy: Multi-scale, Thermo-mechanical Characterisation of Advanced High Temperature Materials for Power Generation.
The kernel of the Materials Design Tool is based on advanced computer models for representing the microstructural and mechanical behaviour of advanced alloys across a range of length-scales, from the nano-scale to the macro-scale. The big challenge with computer models is to ensure accurate calibration and validation against measured material data. This is particularly challenging at small length scales.
However, advanced materials characterisation methods are being adopted within the project to achieve this. Neutron/X-ray diffraction, in-situ scanning electron microscopy (SEM) and electron back-scatter diffraction (EBSD) tests of the materials have been carried out in the UK, Switzerland and University of Limerick for micro-scale characterisation and validation of the micro-scale material models.
Novel macro-scale material models have been developed which allow modelling of realistic power plant steam headers and branched pipe components (Fig 1) including the effects of high temperature, cyclic plastic deformation and microstructural changes. The materials of interest are essentially composite materials (Fig 2), where the metal matrix is reinforced by the addition of a distribution of very fine precipitates (about 20-300 nanometres) of hard carbides. In order to ensure accuracy of the macro-scale computer models for the material, an advanced high temperature, low cycle fatigue test capability, with temperature control up to 1000 degrees Celsius, has been developed at NUI Galway.
Furthermore, due to the extreme nature and complexity of the temperature (up to 700 degrees Celsius) and steam pressures (up to 25 MPa) required for next generation plant, the METCAM project also involves the University of Nottingham (group of Professor Thomas Hyde). The work at Nottingham involves measurement of the material performance under realistic power plant temperature and pressure conditions using an advanced thermo-mechanical fatigue (TMF) test rig (Figs 3a and 3b).
A key aim of the NUI Galway and Nottingham tests is to characterise the creep and fatigue cracking behaviour of the candidate materials.
The weakest part of the complex networks of pipework in power generation plant tends to be the weldments connecting one pipe to another. The welding process, even though carefully controlled, leads to a mixture of material properties and behaviour in the welds (Fig 1).
The differences in microstructural properties of the weldments have led to early cracking of high pressure steam piping in modern alloys. Therefore, in order to design against such cracking and to ensure safe and reliable operation of future power plant, under even more arduous conditions, METCAM is currently developing computer models which represent this mixture of microstructures in the welded joints.
The tests will also measure the performance of welded joints under realistic thermo-mechanical fatigue and creep conditions. One of the unique aspects of the METCAM project is the development of computer models of real ESB power plant components (Fig 1) with representation of the measured steam and temperature histories. These models were used to help design the material test conditions. Another exciting aspect of the METCAM collaboration with ESB International is the manufacture of a weld-repair pipe section, to simulate a realistic weld repair operation in a power station pipe or steam header after 35,000 hours services (about five years’ service).
More than fifty small-scale test specimens were extracted from this weld repair pipe for testing at NUI Galway. The results of these tests have given unique insight into the thermal fatigue cracking behaviour of such weld repair scenarios, which are commonplace in plant throughout the world and represents an important factor of a modern plant life-cycle. This will also allow calibration and validation of the multi-scale computer models for design and damage assessment of next generation USC plant.
Richard A Barrett is a PhD student in Mechanical Engineering at NUI Galway funded by SFI under the PI Award programme. He has a 1st class honours degree in Mechanical Engineering from NUI Galway.
Noel P O’Dowd is Professor of Mechanical at the University of Limerick and Director of the Materials and Surface Science Institute. His research interests include fracture mechanics, computational mechanics of materials and structural integrity.
Sean B Leen is Professor of Mechanical Engineering at NUI Galway. His research interests include computational solid mechanics, structural integrity (fatigue, creep, wear), tribology and manufacturing processes.
This publication has emanated from research conducted with the financial support of Science Foundation Ireland under Grant Number SFI/10/IN.1/I3015.http://www.engineersjournal.ie/2013/06/13/a-design-tool-for-high-temperature-power-generation-materials/http://www.engineersjournal.ie/wp-content/uploads/2013/06/power-plant-elec-1024x683.jpghttp://www.engineersjournal.ie/wp-content/uploads/2013/06/power-plant-elec-300x300.jpgMechenergy,NUI Galway,UL