Improved Modelling of Material Properties for Higher Efficiency Power Plant (TP/5/MAT/6/I/H0101B)

Transcription

1 Improved Modelling of Material Properties for Higher Efficiency Power Plant (TP/5/MAT/6/I/H0101B) A Case Study The efficiency of power stations depends on the temperature at which they operate. The higher the temperature, the more efficient the turbine and the less fuel is needed to produce a given amount of energy. One of the key aims of this project was to find out how the materials used to construct the turbine behave over long time periods at higher temperatures. In addition, to make sure the lights don t go off in the UK in a few years time (the energy gap ) we will have to run some power stations for longer than they were originally designed (perhaps for forty years rather than twenty five). This takes us into rather unknown territory. We can t do a test on a material for forty years before we actually give the go ahead for a power station to keep going for that long. So we need to be able to predict how materials will behave over very long time periods and at higher temperatures to make sure they are safe. If we can extend the life of equipment in power stations then there will be a very considerable saving in cost and environmental terms - building new power stations consumes a lot of resources. A significant part of the project has been dedicated to understanding whether turbine blades (which start to deteriorate after they have been in use for a long time) can be rejuvenated and if so, what is the best treatment to achieve that. This is important because, if there is an unplanned shutdown to repair a blade, this can cost 1M per day, and if a new set of blades has to be purchased this has a 1M price tag. There is thus a major financial incentive to keep blades going. In addition, welds are critical to plant integrity. A reduction in the amount of weld repair will save millions of pounds every year. This project, therefore, aimed to give more clarity and understanding of the material behaviour over long times at relatively high temperatures; to investigate turbine blade rejuvenation; and to improve understanding of welding in order to reduce weld repair. The partners were: three companies closely involved with power station build and operation (Doosan Babcock, Alstom and E.ON), NPL (who bring expertise in testing miniature samples of material and in prediction of the thermodynamics of the materials), QinetiQ (who are experts in predicting how materials behave over long periods of time under stress) and two universities, Leicester and Loughborough, who have world-class knowledge of the techniques for characterising materials and of predicting how the structure at a micro-scale evolves with time. We estimate that the economic benefit from the public investment in this project is over 10M per annum. Key Technical Outcomes 1. Model for creep life prediction of power plant steels using an innovative probabilistic approach. 2. A quantitative description of how the microstructure of nickel alloys changes during conditions typically experienced by components in power plants, including knowledge of the formation of inclusions which are detrimental to the material properties. These microstructure models can then be linked to creep behaviour models to predict component lives. 3. Model for creep life prediction of singlecrystal nickel-based alloys used for CMSX4 turbine blades. 4. Reheat cracking susceptibility of nickel superalloys from cyclic notch testing 5. A finite element model to simulate post weld heat-treatment in P91 steel. 6. A hardness-based model for creep rupture life assessment of plant components. 7. A hardness-based model of the effect of multiple repair and heat treatment on P91 steel creep life. 8. Increased accuracy of remanant life assessment of ½CrMoV pipework 9. Development of a multi-step heat treatment schedule capable of microstructurally rejuvenating degraded CMSX4 superalloy. 10. Development of tests on miniature samples ( small punch testing ).

2 Project Structure The project was divided into four tasks. To achieve better prediction of the behaviour of materials over very long periods at high temperatures and with applied stress (i.e. creep conditions), the first task was focussed on modelling of the way the microstructure changes over time and then linking the outputs to a continuum model which predicts the creep behaviour. In the second task, real data on creep of a particular type of nickel alloy ( solid solution strengthened ), which is used for advanced steam generation plant, was obtained in sophisticated long-running tests. The third task dealt with steels for high temperature steam plant and their weld repair. The fourth task was concerned with the single crystal nickel alloys which are precipitation strengthened and which are used for turbine blades. These blades have potential for rejuvenation but the precise treatment details were not clear at the beginning of the project. It is expected that the models developed in this work will facilitate design and application of new alloys and accelerate the time to market the product by reducing the amount of testing required (saving millions of pounds in costs). The future of UK and EU power generation industries is thus critically dependent upon developing novel materials and advanced modelling technology. In this work, it has been aimed to develop an integrated materials modelling approach for predicting long term service behaviour of high efficiency advanced power plant materials, such as creep resistant ferritic steels and Ni-based superalloys, with validation obtained from data generated by complex laboratory testing and detailed characterisation of ex-service components. Task 1: Integration of the Models The underlying philosophy behind the modelling approach in this programme was that creep deformation of the materials should be considered entirely in terms of microstructural mechanisms and parameters which can be observed or measured. An approach which is only phenomenological can sometimes develop a successful model for a specific alloy under a limited range of conditions but cannot then predict what happens under different conditions. In contrast, an approach which exploits the understanding of mechanisms and the effects of microstructure is much more powerful and has the potential to be applied more generally over a wider range of service conditions and to a wider range of alloys, including the ability to make predictions for novel and even theoretical alloys. Fig 1 Dislocations Interacting with Precipitates in Nickel Superalloy For high alloy ferritic steels a microstructual evolution model was developed at Loughborough using an innovative Monte Carlo procedure. This was integrated with a damage mechanics model to predict creep behaviour. Both microstructure and deformation behaviour predictions agree fairly well with available experimental data. Modelling of a single-crystal nickel-based alloy at QinetiQ has shown excellent agreement between model predictions and experimental results under conditions where assumptions about the microstructure and deformation mechanisms are satisfied. Deviations from the model are understood where the assumptions are not met and this provides a clear strategy for further model development to introduce aspects which account for these. A microstructural evolution model for lowgamma prime, solid solution-strengthened nickel-based alloys has been developed at Leicester based on extensive metallographic observation and analysis. This considers precipitation, growth and interaction of multiple phases. Initial steps have been taken to integrate this with damage mechanics models to predict

3 creep curves. The results of the microstructural modelling will inform future development of the creep model. The long-term microstructural state of creepexposed IN617 has been characterised and quantified at Leicester in terms of the evolution of grain size, dislocation density, and the precipitation of four important particulate phases. The observations have been found to be consistent with the predictions of the kinetic and thermodynamic model developed at Leicester. This information has been communicated to QinetiQ for direct input into their microstructure-based creep model in order to determine the creep rupture life of components under realistic in-service conditions. Task 2: Advanced Steam Plant Nickel Superalloys Alstom have carried out extensive creep strain/rupture tests on advanced power plant superalloys IN617, IN625, Nimonic 263. The data and samples have been provided to modellers to validate the modelling predictions and to enable microstructural characterisation and hence much deeper understanding. Cyclic notched relaxation tests on superalloy weldments have been performed to assess the reheat cracking susceptibility of these superalloys. Task 3: Steels for High Temperature Steam Plant Doosan Babock investigated the effects of thermal treatments on creep strain/ rupture properties of P/T91, P/T92 parent, weld and heat affected zone (HAZ) material at plant operating conditions (650 C) and supplied samples to Loughborough for characterisation. The resulting mechanical test data including stress relaxation and small punch tests (at NPL) was used to develop a finite element (FE) model predicting post weld heat treatment in a typical P91 HSC boiler component. The degradation of mechanical properties and microstructure during long term service ageing of ½CrMoV steel have also been investigated by microstructural characterisation, mechanical testing and data analysis. The creep rupture strength data (at 650 C) obtained from Grades 91 and 92 steels, weld and HAZ have shown a short tempering of 1h is better than 16 h. Stress Figure 2 Creep Testing Laboratory. relaxation data in P91 gave an insight into residual stresses in this alloy in the temperature range C. With data obtained, a FE simulation model based on continuum damage mechanics (CDM) was developed to predict the risk of Type IV cracking and forming undesirable phases in weld and in HAZ areas in P91. A model has been developed and optimised to enable cast-specific creep rupture properties of P91 steel to be estimated from hardness, combined with an analytical model relating the creep stress to generic high temperature tensile properties. This was shown to provide reasonable life estimates for soft mis-manufactured items to determine safe plant operation timescales. The model was validated by test data on the effects of repair and repeat post-weld heat treatment on creep life. A method developed through microstructural characterisation, mechanical testing and onsite hardness monitoring of ½CrMoV steel allowed increased accuracy of remanent life assessment in pipework and other power plant components towards an improved replacement strategy and reduced risk of failure. Task (4) : Advanced Gas Turbine Superalloys CMSX4 single crystal superalloy was thermally degraded by E.ON to produce a rafted structure analogous to gas turbine blades during service. The mechanical properties with and without a refurbishment heat treatment were investigated along with microstructural assessments. In addition to the mechanical tests conducted on virgin, degraded and rejuvenated material, the

4 microstructural and mechanical behaviour of rejuvenated material that had undergone a simulated return to service was also investigated. A multi-step heat treatment schedule has been developed which is capable of microstructurally rejuvenating thermally degraded CMSX4 superalloy. Test results show that, whilst the mechanical properties of degraded CMSX4 are restored (or in fact increased) by rejuvenation heat treatment, these are subject to more rapid decline than those of its as-received equivalent during subsequent degradation treatments. The mechanisms underpinning the mechanical response of virgin, degraded and rejuvenated CMSX4 superalloy have been established. Refurbishment practices involving relatively short thermal treatments at 1140 ºC were found to be inappropriate. Future Activities The improved materials modelling tools will enhance understanding of service behaviour of advanced power plant materials (ferritics and nickel base) while enabling the user to predict failure mode and remanent life in critical components at high temperatures and stresses. Such increased capabilities will strengthen the excellent reputation of the two academic institutions in attracting new funding and students. It is thought future modelling activities should be directed toward the goal of quantitative high temperature performance modelling for component design, metallurgical damage evaluation, and life assessment. In terms of future nickel alloy developments, the rate of degradation in the rejuvenated material should be empirically determined. This will help to quantify the life extension potential of the rejuvenation process. Development of FE modelling of welding technology in ferritics will find its usage with the fabricators of power plant boilers, and this will lead to significant commercial impact in the energy generation industry. The novel experimental techniques and long term test data that have been produced will provide new business opportunities and strengthen capabilities for carrying out advanced materials research. Cost:The total cost of this project was 2,243,896, with the Technology Strategy Board contributing 1,156,648. Duration:48 months Jan 2007-Dec Contractor Doosan Babcock Ltd, Porterfield Road Renfrew PA4 8DJ; Tel: Fax: E-m: schakravorty@doosanbabcock.com Web: Quotes from contractors The TSB funding made valuable interaction possible between partners from academia, research institutions and industry to greatly benefit from the development of novel predictive modelling. These models must now be employed to enhance our knowledge of advanced HSC power plant materials and their service behaviour to bring commercial benefits. Dr L W Buchanan, Chief Metallurgist Doosan Babcock Ltd. This has been a great project, and one of those long term R&D Projects that can have an real influence on how we manage and operate our stations. Dr Scott Lockyer, Technical Head of Materials, E.ON New Build and Technology Collaborators: Alstom Power Ltd, E.ON UK, Leicester University, Loughborough University, NPL, QinetiQ and Doosan Babcock. Collaborative Research and Development Collaborative Research and Development is one of the primary means by which the Government invests in innovation. It is designed to enable industry and research communities to work together in strategically important areas of technology to develop successful new products, processes and services. It is managed by the, a business-led organisation established to accelerate research into, and development and exploitation of, technology and innovation for the benefit of UK business building economic growth and quality of life. Further Information If you have any queries about the Low Carbon Energy Technologies programme please contact the Helpline on or info@technologyprogramme.org.uk For further information about the :

5 TSB Reference : Improved Modelling of Material Properties for Higher Efficiency Power Plant (IMMP3)