Final Report Summary - Z-ULTRA (Z phase strengthened steels for ultra-supercritical power plants)
Executive Summary:
Thermal power generation will be the backbone of Europe’s electric power supply for many years, until sufficient renewable capacities will be available, and it will remain indispensable thereafter due to the fluctuating nature of wind and solar power. In the emerging economies thermal power is expected to grow fast in the coming decades in order to provide affordable electrification.
To minimize the fuel consumption and the CO2 emissions of fossil fired power plants, the thermal efficiency, and therefore the steam inlet temperatures, must be as high as possible. However, higher temperature means accelerated material degradation. Hence, in order to enable a safe and economic operation of highly efficient power plants it is necessary to control and retard the microstructural degradation. This goal is pursued in the present project by developing a new 12% chromium steel.
The strength of the 9% chromium steels developed in the past 20-30 years is based on fine nitride particles. Further progress beyond the currently possible 615 °C steam temperature requires chromium contents higher than 9% to achieve better corrosion resistance. However, previous attempts showed that higher Cr content inevitably leads to the transformation of some of the nitrides into the thermodynamically stable Z phase in service. Usually, the Z-phase particles are coarse and brittle and grow at the expense of the fine nitride particles, so that the material properties degrade.
The idea in the proposed project is to exploit the Z phase as a thermodynamically stable strengthening agent rather than trying to suppress its occurrence. This means that chemical compositions and heat treatments must be identified which lead to a controlled nucleation of finely dispersed Z phase particles.
Starting from an available test alloy seven new alloys were developed by experience-based alloy design supported by thermodynamic modelling tools. From a practical point of view the most important result is that the creep strength of the latest alloys is clearly better than that of the best existing 9%Cr steel, Grade P92. The creep rupture strength exceeds that of P92 by 30%. The oxidation resistance is excellent.
Further improvements of the creep strength appear to be possible in the future due to a comprehensive tool kit of nanoscale experiments developed in the project. These investigations revealed the relevance of different alloying elements for the occurrence and morphology of precipitates, such as Laves phase, nitrides and especially the Z phase.
There are world-wide efforts to complement the classical methods of material development by Integrated Computational Material Science and Engineering. The Z-Ultra project is intended to contribute substantially to the progress of this field. In particular, the multiscale modelling toolbox established in Z-Ultra supports the development of Z-phase strengthened alloys by understanding the pathways to Z-phase formation.
It is crucial for the value of a novel material that welding procedures are available, which conserve the outstanding properties of the base material. Welding procedures were developed for the new steels, including multipass welding of a large ring cut from a forging. No hot or cold cracks were detected. The welding activities were supported by extensive finite-element simulations.
The production processes was scaled-up to a large 12 ton forging and to large-section weldments, as they occur in welded turbine rotors. No severe cracks occurred in the forging. At the end of the manufacturing process, the material exhibited excellent impact toughness, but the yield stress was a little low with regard to turbine rotor applications.
Finally, the acceptance of a novel material increases, if it has demonstrated its capabilities in operating power plants. Therefore demonstration tubes were welded into boilers of two Ukrainian power plants for demonstration purposes. Also the fire-side and steam-side corrosion of superheater tubes was investigated in a special test rig. These tests confirmed that the oxidation resistance on the steam side of the tube wall is very slow, and the oxide layer is thin and homogeneous. On the flue gas side of the tube surface, the corrosion layers are substantially thicker. Up to 647°C the corrosion behaviour on the flue-gas side is still acceptable.
In summary, the project reached its goal to develop a 12% Cr steel with excellent corrosion resistance and superior long-time creep strength.
Project Context and Objectives:
Background
Thermal power generation will be the backbone of Europe’s electric power supply for many years, until sufficient renewable capacities will be available, and it will remain indispensable thereafter due to the fluctuating nature of wind and solar power. In the emerging economies thermal power is expected to grow fast in the coming decades in order to provide affordable electrification.
To minimize the fuel consumption and the CO2 emissions of fossil fired power plants, the thermal efficiency, and therefore the steam inlet temperatures, must be as high as possible. However, higher temperature means accelerated degradation of the microstructure of materials and a concomitant loss of creep and fatigue strength. Hence, in order to enable a safe and economic operation of highly efficient power plants it is necessary to control and retard the microstructural degradation.
In the past 20-30 years stronger 9% chromium steels had been developed, which, after a long period of stagnation, allowed increasing the steam temperature from 540 °C up to 615 °C. The increased strength was mainly obtained by precipitation of fine nitride particles with typical compositions (V,Nb)N. Further progress beyond 615 °C requires chromium contents higher than 9% to achieve better corrosion and oxidation resistance. However, it has been found that in 11-12% chromium ferritic-martensitic steels strengthened by fine (V,Nb)N particles, formation of the thermodynamically stable Z phase, Cr(V,Nb)N, in long-time service is unavoidable and detrimental. Usually, the Z-phase particles are coarse and brittle and grow at the expense of the desired fine nitride particles. This means that the material properties degrade. To delay the occurrence of the Z phase, compromises in the alloy composition, such as reduced Cr content, and hence reduced corrosion resistance, were necessary.
Objectives
The goal of the Z-Ultra project is to obtain a 12% Cr steel, which has significantly higher 100 000 h creep rupture strength than the best current 9% Cr steels in the temperature range 600 to 650 °C. In addition the oxidation resistance will be improved considerably, so that the oxidation rate is reduced typically by 80% at 650°C.
The idea in the proposed project is to exploit the Z phase as a thermodynamically stable strengthening agent rather than trying to suppress its occurrence. This means that chemical compositions and heat treatments must be identified which lead to a controlled nucleation of finely dispersed Z phase particles and to reduced coarsening rates.
A secondary objective pursued in the project is to contribute to the general progress of Integrated Computational Material Science and Engineering (ICMSE). There are world-wide efforts to complement the classical methods of material development by enhanced multiscale computational modelling; the Materials Genome Initiative in the U.S. is an example. Computer based methods will greatly accelerate the development of new materials, such as steels or all kinds of functional materials. The task in Z-Ultra is particularly demanding, since advanced steels may have more than ten constituents forming a great variety of precipitate particles. Hence, Z-Ultra is expected to contribute substantially to the progress of Integrated Computational Material Science and Engineering. The computational methods need to be validated by the best available nanoscale experiments
Methods
Starting from an alloy, which was available from a preceding research project, new alloys were developed by experience-based alloy design supported by thermodynamic modelling tools. The steelmaker participating in the project produced the new alloys using state-of-the-art production methods. Appropriate heat treatment procedures were worked out. The new alloys were distributed to the partners where the following experiments were carried out:
1. Welding. It is crucial for the value of a novel material that welding and post-weld heat treatment procedures are available, which conserve the outstanding properties of the base material. Welding methods were developed for creep test specimens, for relatively thin-walled tubes and for thick-section forgings. The activities were supported by extensive numerical welding simulations based on the finite-element method.
2. Nanoscale experiments such as Transmission Electron Microscopy (TEM) with atomic resolution and Atom Probe Tomography (APT) were applied and adjusted to the special requirements of Z-phase strengthened steels. The goal is to explore the morphology and composition of precipitates, e.g. nitrides, Laves phase, carbides and the Z phase. Of special interest is the evolution of these precipitates during high-temperature exposure.
3. Macroscale experiments (aging, creep, internal friction, oxidation) were carried out to measure the technologically important data of the new alloys, and to compare them with the best commercial alloys. Internal friction tests allow measuring diffusion coefficients, which are important for the particle evolution kinetics and which are otherwise difficult to measure in the relevant temperature range.
In addition to the standard laboratory tests, a special test rig was developed in order to test tubes of one of the new materials under conditions which are close to those in a superheater of a power plant boiler. The outer surface of the tube was exposed to flue gas and the typical ashes of a hard-coal fired power plant, while the inner surface was exposed to pressurized water vapor. In another test, a circumferentially welded tube was tested under internal pressure.
Further, the production processes was scaled-up from test melts of typically 80 kg to forgings of 12 tonnes and to large-section weldments, as they occur in welded turbine rotors. The question is whether a sufficiently homogeneous microstructure can be obtained in the large forging, whether the mechanical properties (yield stress and impact toughness) lie in the desired range and whether the detectability of possible defects by ultrasonic inspection is good enough.
Finally, the acceptance of a novel material increases, if it has demonstrated its capabilities in operating power plants. Therefore demonstration tubes were welded into boilers of two Ukrainian power plants for demonstration purposes.
The modelling methods applied in Z-Ultra were
1. Density Functional Theory (DFT) is an approximate method to solve the quantum-mechanical many-electron Schrödinger equation. The method has a sound theoretical basis and usually provides accurate results for material properties, but it is restricted to relatively small numbers of atoms.
2. The Tight Binding (TB) method is a less accurate approximation with parameters that need to be adjusted using DFT results. It allows modelling greater ensembles of atoms.
3. Thermodynamic/kinetic models describe the evolution of precipitates in an alloy depending on its composition and the temperature history. An extension of the existing models is needed to account for the large volume change during Z-phase formation and for the necessary accommodation mechanism.
4. A constitutive model for the mechanical behaviour was developed.
5. The finite element method was applied to demonstrate the behaviour of parts from power plants under creep conditions.
The models, especially the DFT calculations are expected to yield insight into the chemistry of the Z phase in different alloys and into the pathways which lead to the transformation of the nitrides into Z phase.
Implementation
The consortium of the Z-Ultra project combined the expertise of a steelmaker, a utility company, an engineering consultant company and eight research organizations and universities from the European Union and from three Eastern Partnership Countries.
The work was organized in eight work packages:
WP1 Material development and welding
WP2 Nanoscale experiments
WP3 Ageing, corrosion, creep and internal friction tests: macroscale experiments
WP 4 Nano- and microscale modelling
WP 5 Meso- and macroscale modelling
WP 6 Demonstration, Upscaling
WP 7 Project management
WP 8 Dissemination and Exploitation
Relevance for the European industry
The know-how to produce steel with superior creep and oxidation resistance is a great competitive advantage for European steelmakers. The steelmaker who participated in the project is primarily interested in marketing large turbine rotors. For manufacturers of boilers it is very attractive to use a more creep and corrosion resistant steel, which would allow higher steam temperatures or would replace the more expensive austenitic materials. Utilities will use stronger alloys for retrofit of their facilities, since higher strength allows smaller wall thickness, which means smaller thermal stresses and eventually greater flexibility.
Project Results:
Attachment Z-Ultra_S&T_results_final report.pdf
Potential Impact:
Impact
The aim of Z-Ultra was to develop steels that would allow higher steam temperatures and pressures in fossil-fired power plants. The concomitant progress in thermal efficiency will have a strong impact on the world-wide efforts to reduce the CO2 emissions. Higher efficiency also reduces the fuel cost, and therefore makes new coal and gas fired plants more competitive compared to nuclear power plants and compared to older plants with lower efficiency.
The decision whether new power plants will be built depends on economic and political factors, which vary from country to country. In some countries of Western Europe only few additional coal-fired power plants will be built. Apart from political factors, the oversupply on the some of the national current markets due to the extension of renewable sources makes it unattractive to invest in new plants.
In other parts of the world, new coal fired plants were built at a high rate, and this will continue, though probably at a reduced rate. For these new plants higher thermal efficiency remains an economic and environmental advantage. European steelmakers and component manufacturers who can provide the necessary materials and components will benefit.
In many countries of the European Community, the increasing share of renewable energies makes great demands on the flexibility of thermal power plants, which have to compensate the fluctuating input from wind and solar power. In this connection, the advantage of stronger materials is that the wall thickness of thick-walled components can be reduced, which reduces thermal stresses during start-up and shut-down cycles, and therefore the thermal fatigue of the components. Current power plants were usually designed for base load with only few load changes, and not against frequent thermal fatigue cycles. The possibility to reduce the wall thickness by using a stronger material is especially interesting for the retrofit of critical components in existing power plants that have already developed fatigue damage or have reached their end of life for other reasons.
Some European countries make efforts to reduce their CO2 emissions by using renewable biomass, as well as waste in their power plants. Usually these fuels are chemically more aggressive than coal or natural gas and require steels with a higher Cr content than the current 9%Cr steels. The Z-phase strengthened steels with their excellent corrosion resistance will here be an option.
The oxidation and corrosion resistance of the new ferritic-martensitic steels renders them competitive with the more expensive austenitic steels, not only in superheaters of power plants, but also in the chemical and petrochemical industry. The replacement of an austenitic component by a ferritic-martensitic part allows saving rare and expensive alloying elements.
Bringing a new material into service in power plant components requires much longer testing times than the limited time of a three- or four-year project. The reliability of power plants needs to be guaranteed for at least 100,000 h. It will therefore be necessary to initiate projects for long-term creep and oxidation tests of the most successful Z-Ultra materials. The excellent properties of the ZU1 and ZU4 materials justify the cost and time of such a test program and the upscaling activities, which are necessary to qualify the material for use in power plants.
These activities will be initiated in European working groups on creep resistant steels in collaboration with steel and component manufacturers (e.g. Saarschmiede, Vallourec & Mannesmann) and equipment manufacturers. End users of the steels (Siemens, and Air Liquide) have already expressed their interest in the Z-Ultra steels and cooperations are being established. Professor John Hald, who is active in European working groups and a well-recognized expert in power plant technology and creep resistant steels, is organizing a follow-up project involving several of the Z-Ultra participants and additional industrial partners.
A secondary, but also very important goal of Z-Ultra was to advance Integrated Computational Material Science and Engineering. This is a long-term, world-wide effort to reduce the time and cost for developing new materials. In Z-Ultra a great number of precipitate phases with variable compositions had to be analysed. Modelling the formation of the Z phase was a particular challenge. The enhanced know-how of the Z-Ultra participants will be useful in future projects, not only for Z-phase strengthened steels, but also for other material classes.
The same can be said of the nano- and microscale experimental methods used and developed in Z-Ultra. The methods to identify different, very small phases and their chemical composition were substantially refined.
Since the design lifetime of power plants is usually longer than the longest creep test times, reliable models are needed to extrapolate the test results for establishing safety and maintenance concepts to prevent failures of potentially hazardous high temperature/high pressure equipment. Unexpectedly fast microstructural degradation, as it some¬times occurred in previous steels (e.g. ½Cr-½Mo-¼V), causes a great loss of invested capital and high mainten¬ance costs.
Last but not least the demonstration activities in Z-Ultra will promote the interest in the new steels. The participants showed that they were able to produce large parts from a new material without trial and error cycles, and that the first attempt to weld large parts would lead to crack-free welds. The novel test rigs for investigating the material behaviour in corrosive atmospheres like in real superheaters will be extremely useful for utilities.
Dissemination activities
The objectives of the dissemination activities are to communicate and deliver knowledge about the Z-Ultra project by using the appropriate channels to the various interested stakeholders. The web site (http://www.z-ultra.eu) is continuously updated. A complete list of dissemination activities is appended.
Workshops: The Z-ultra project organized workshops in Tbilisi, Kiev and Graz.
Workshop in Tbilisi: A short course for students and faculty was held in conjunction with the Consortium meeting in Tbilisi from September 29 to October 1, 2014. A flyer was distributed three months ahead of the event. It is shown on the project website www.z-ultra.eu. Sixteen scientific/technical lectures were presented by the Z-Ultra partners and by the Project Technical Advisor. Another presentation informed the Georgian students about European exchange programs. About 25 students and 10 staff members from the Georgian Technical University as well as a PhD student from Jülich in Germany subscribed for the workshop. The students attending the workshop study energetics, engineering physics, and metallurgy at GTU. The local organization was excellent. After each lecture, enough time was planned for questions from the students, and also extensive coffee breaks were used for personal discussions and direct exchange with the students. After the workshop, all presentations were made available online for download for the students.
The impact of the workshop has been regarded by all participants as a success. The students were interested in the lectures and good mutual exchange was given between the students and the Z-Ultra members giving the lectures. The excellent spoken English of the students, their curiosity and their previous exchange experiences with the Forschungszentrum in Jülich, Germany, helped in creating an excellent basis for communication. Exchange programs with Europe and Germany in particular were presented, in order to help outstanding Georgian students to find opportunities to study or to perform a PhD thesis in Europe. The email addresses of the Z-Ultra members giving the lectures were made available, thus creating a further contact possibility for the students. Overall, the workshop promoted communication between the Georgian and the European scientists and students and created a sound basis for a smoother and barrierless collaboration.
Workshop in Kiev: The workshop in Kiev took place on 12/3/2015 in the same week as the SICA meeting, which was held in Kiev on 10 to 11/3/2015. The title of the workshop was Science Based Material Development for Power Engineering. The workshop was announced in Kiev by the flyer shown on the project website www.z-ultra.eu. The program comprised 8 lectures from the Z-Ultra consortium, 6 from Paton Institute and one from the Project Technical Advisor. The list of participants contains 47. The presentations of the Z-ultra partners are available on the Content Management Server of the Fraunhofer Society, https://dms-prext.fraunhofer.de.
After each lecture, enough time was planned for questions, and also the coffee and lunch breaks were used for personal discussions and direct exchange with the participants from Kiev. Especially PhD students were interested in publications of Z-Ultra members. In summary the workshop will strengthen the long-lasting relations between members of Z-Ultra and members of the Paton Institute and the Academy of Science in Kiev. The local hosts from Paton Institute made the visit for the participants a very pleasant experience.
Workshop in Graz: The Z-Ultra workshop “Novel Materials for Power Generation” was included in the THERMEC conference organised by the Graz University of Technology (TUG). Thermec 2016, an international conference on processing and manufacturing of advanced materials, took place from the 29th of May till the 3rd of June 2016 in Graz, Austria. About 1300 researchers from all around the globe presented their recent advances in the field of development, fabrication, processing, performance and applications of future-oriented materials. This conference provided an appropriate context and a unique opportunity to present the concept and disseminate results obtained within the Z-ultra project to the international scientific and industrial community. The workshop was extended to three days parallel to the conference, instead of 2 days initially planned.
Twelve oral presentations, including 2 keynote presentations, were held by Z-ultra partners and 2 posters were presented. Speakers benefited from a much larger audience than one could expect from an isolated workshop. The presentations aroused strong interest from scientists as well as from industrial researchers and led to fruitful discussions. Potential partners for future collaborations were identified.
Publications
A list of publications is appended. Between 2013 and 2016 the project partners published 30 papers in peer reviewed journals. Several others are in preparation. Two theses were completed during the project.
Among the more recent publications by DTU and Chalmers are some which address Z-Ultra specific problems and solutions that emerged during the project. Several papers published by TUG (with S. Yadav as a co-author) deal with experiments and models for creep deformation and fracture of 9-12%Cr steels. The publications from IPM (with J. Svoboda as a co-author) treat various problems of material science using the thermodynamic extremal principle. This very powerful method is also used and refined in Z-Ultra. The most recent of these publications address the problem of volumetric misfit of growing particles and its accommodation mechanisms, which is especially important not only for the Z phase, but also for other kinds of particles. In previous models this aspect was not properly taken into account. Finally a group of publications by Paton Institute describe the welding problems occurring in creep resistant steels and the welding procedures to avoid them.
Lectures
Z-Ultra participants gave 42 lectures at conferences on Z-Ultra related subjects. The topics are the same as in the publications, but obviously the lectures are closer to the frontline of research than the publications. For example, the important density functional calculations on Z-phase alloys are described in several lectures, but the publications are only in preparation.
Website
The web site http://www.z-ultra.eu has been continuously updated.
Exploitation
A list of exploitable foreground is appended.
Although the alloy compositions worked out in Z-Ultra already lead to very good high-temperature properties, it will be necessary to initiate projects for long-term creep and oxidation tests of the most successful Z-Ultra materials. The excellent properties of the ZU1, ZU4 and ZU5 materials justify the cost and time of such a test program and the upscaling activities, which are necessary to qualify the material for use in 625 to 650°C power plants. End users of the steels (Siemens, and Air Liquide) have already expressed their interest in the Z-Ultra steels and cooperations are being established.
The actual compositions and the scientific arguments leading to these compositions are valuable and exploitable know-how for these future steps. Primarily SSF and DTU, but other partners as well, will benefit from the progress made in alloy development. New steel grades have always displa¬ced the previous generation in the most demanding applications. For example, the ‘COST E’ steel (X10 CrMoVNbN 1011) developed in joint European projects accomplished a dominant position on the world market for large forgings for 600°C within only a few years and contributed substantially to the turnover of SSF. European steelmakers can sustain a leading position on the growing international market of steam power plants only if they can offer materials with superior properties. The new creep and corrosion resistant Z-phase strengthened 12%Cr steel would open up new opportunities, not only in the European Union and in the Eastern Partnership countries, but also in the fast growing markets of the emerging economies.
Heat treatment and welding procedures are closely related to material development. Here, Paton, SSF and DTU are the main beneficiaries.
The know-how developed during the Z-Ultra project on Integrated Computational Material Science and Engineering enhances the competitiveness of IWM and IPM in a subject area which experiences world-wide attention with large amounts of funding. Since IWM and IPM gain a great fraction of their revenues from theoretical modelling activities, outstanding competence in this area is of paramount importance. Enhanced competence on steels opens an avenue to one of the biggest markets.
Chalmers greatly improved its capabilities in nanoscale experiments. Similarly to the modelling competence, and in conjunction with it, this competitive advantage can be offered in future project proposals to industries or public funding agencies.
RWE together with its subcontractor StandZeit GmbH developed unique testing methods to investigate the material behavior under conditions which are close to those in real power plants. This is the basis for future projects with the electricity generating industry worldwide.
List of Websites:
www.z-ultra.eu
List of participants with contact data attached
Thermal power generation will be the backbone of Europe’s electric power supply for many years, until sufficient renewable capacities will be available, and it will remain indispensable thereafter due to the fluctuating nature of wind and solar power. In the emerging economies thermal power is expected to grow fast in the coming decades in order to provide affordable electrification.
To minimize the fuel consumption and the CO2 emissions of fossil fired power plants, the thermal efficiency, and therefore the steam inlet temperatures, must be as high as possible. However, higher temperature means accelerated material degradation. Hence, in order to enable a safe and economic operation of highly efficient power plants it is necessary to control and retard the microstructural degradation. This goal is pursued in the present project by developing a new 12% chromium steel.
The strength of the 9% chromium steels developed in the past 20-30 years is based on fine nitride particles. Further progress beyond the currently possible 615 °C steam temperature requires chromium contents higher than 9% to achieve better corrosion resistance. However, previous attempts showed that higher Cr content inevitably leads to the transformation of some of the nitrides into the thermodynamically stable Z phase in service. Usually, the Z-phase particles are coarse and brittle and grow at the expense of the fine nitride particles, so that the material properties degrade.
The idea in the proposed project is to exploit the Z phase as a thermodynamically stable strengthening agent rather than trying to suppress its occurrence. This means that chemical compositions and heat treatments must be identified which lead to a controlled nucleation of finely dispersed Z phase particles.
Starting from an available test alloy seven new alloys were developed by experience-based alloy design supported by thermodynamic modelling tools. From a practical point of view the most important result is that the creep strength of the latest alloys is clearly better than that of the best existing 9%Cr steel, Grade P92. The creep rupture strength exceeds that of P92 by 30%. The oxidation resistance is excellent.
Further improvements of the creep strength appear to be possible in the future due to a comprehensive tool kit of nanoscale experiments developed in the project. These investigations revealed the relevance of different alloying elements for the occurrence and morphology of precipitates, such as Laves phase, nitrides and especially the Z phase.
There are world-wide efforts to complement the classical methods of material development by Integrated Computational Material Science and Engineering. The Z-Ultra project is intended to contribute substantially to the progress of this field. In particular, the multiscale modelling toolbox established in Z-Ultra supports the development of Z-phase strengthened alloys by understanding the pathways to Z-phase formation.
It is crucial for the value of a novel material that welding procedures are available, which conserve the outstanding properties of the base material. Welding procedures were developed for the new steels, including multipass welding of a large ring cut from a forging. No hot or cold cracks were detected. The welding activities were supported by extensive finite-element simulations.
The production processes was scaled-up to a large 12 ton forging and to large-section weldments, as they occur in welded turbine rotors. No severe cracks occurred in the forging. At the end of the manufacturing process, the material exhibited excellent impact toughness, but the yield stress was a little low with regard to turbine rotor applications.
Finally, the acceptance of a novel material increases, if it has demonstrated its capabilities in operating power plants. Therefore demonstration tubes were welded into boilers of two Ukrainian power plants for demonstration purposes. Also the fire-side and steam-side corrosion of superheater tubes was investigated in a special test rig. These tests confirmed that the oxidation resistance on the steam side of the tube wall is very slow, and the oxide layer is thin and homogeneous. On the flue gas side of the tube surface, the corrosion layers are substantially thicker. Up to 647°C the corrosion behaviour on the flue-gas side is still acceptable.
In summary, the project reached its goal to develop a 12% Cr steel with excellent corrosion resistance and superior long-time creep strength.
Project Context and Objectives:
Background
Thermal power generation will be the backbone of Europe’s electric power supply for many years, until sufficient renewable capacities will be available, and it will remain indispensable thereafter due to the fluctuating nature of wind and solar power. In the emerging economies thermal power is expected to grow fast in the coming decades in order to provide affordable electrification.
To minimize the fuel consumption and the CO2 emissions of fossil fired power plants, the thermal efficiency, and therefore the steam inlet temperatures, must be as high as possible. However, higher temperature means accelerated degradation of the microstructure of materials and a concomitant loss of creep and fatigue strength. Hence, in order to enable a safe and economic operation of highly efficient power plants it is necessary to control and retard the microstructural degradation.
In the past 20-30 years stronger 9% chromium steels had been developed, which, after a long period of stagnation, allowed increasing the steam temperature from 540 °C up to 615 °C. The increased strength was mainly obtained by precipitation of fine nitride particles with typical compositions (V,Nb)N. Further progress beyond 615 °C requires chromium contents higher than 9% to achieve better corrosion and oxidation resistance. However, it has been found that in 11-12% chromium ferritic-martensitic steels strengthened by fine (V,Nb)N particles, formation of the thermodynamically stable Z phase, Cr(V,Nb)N, in long-time service is unavoidable and detrimental. Usually, the Z-phase particles are coarse and brittle and grow at the expense of the desired fine nitride particles. This means that the material properties degrade. To delay the occurrence of the Z phase, compromises in the alloy composition, such as reduced Cr content, and hence reduced corrosion resistance, were necessary.
Objectives
The goal of the Z-Ultra project is to obtain a 12% Cr steel, which has significantly higher 100 000 h creep rupture strength than the best current 9% Cr steels in the temperature range 600 to 650 °C. In addition the oxidation resistance will be improved considerably, so that the oxidation rate is reduced typically by 80% at 650°C.
The idea in the proposed project is to exploit the Z phase as a thermodynamically stable strengthening agent rather than trying to suppress its occurrence. This means that chemical compositions and heat treatments must be identified which lead to a controlled nucleation of finely dispersed Z phase particles and to reduced coarsening rates.
A secondary objective pursued in the project is to contribute to the general progress of Integrated Computational Material Science and Engineering (ICMSE). There are world-wide efforts to complement the classical methods of material development by enhanced multiscale computational modelling; the Materials Genome Initiative in the U.S. is an example. Computer based methods will greatly accelerate the development of new materials, such as steels or all kinds of functional materials. The task in Z-Ultra is particularly demanding, since advanced steels may have more than ten constituents forming a great variety of precipitate particles. Hence, Z-Ultra is expected to contribute substantially to the progress of Integrated Computational Material Science and Engineering. The computational methods need to be validated by the best available nanoscale experiments
Methods
Starting from an alloy, which was available from a preceding research project, new alloys were developed by experience-based alloy design supported by thermodynamic modelling tools. The steelmaker participating in the project produced the new alloys using state-of-the-art production methods. Appropriate heat treatment procedures were worked out. The new alloys were distributed to the partners where the following experiments were carried out:
1. Welding. It is crucial for the value of a novel material that welding and post-weld heat treatment procedures are available, which conserve the outstanding properties of the base material. Welding methods were developed for creep test specimens, for relatively thin-walled tubes and for thick-section forgings. The activities were supported by extensive numerical welding simulations based on the finite-element method.
2. Nanoscale experiments such as Transmission Electron Microscopy (TEM) with atomic resolution and Atom Probe Tomography (APT) were applied and adjusted to the special requirements of Z-phase strengthened steels. The goal is to explore the morphology and composition of precipitates, e.g. nitrides, Laves phase, carbides and the Z phase. Of special interest is the evolution of these precipitates during high-temperature exposure.
3. Macroscale experiments (aging, creep, internal friction, oxidation) were carried out to measure the technologically important data of the new alloys, and to compare them with the best commercial alloys. Internal friction tests allow measuring diffusion coefficients, which are important for the particle evolution kinetics and which are otherwise difficult to measure in the relevant temperature range.
In addition to the standard laboratory tests, a special test rig was developed in order to test tubes of one of the new materials under conditions which are close to those in a superheater of a power plant boiler. The outer surface of the tube was exposed to flue gas and the typical ashes of a hard-coal fired power plant, while the inner surface was exposed to pressurized water vapor. In another test, a circumferentially welded tube was tested under internal pressure.
Further, the production processes was scaled-up from test melts of typically 80 kg to forgings of 12 tonnes and to large-section weldments, as they occur in welded turbine rotors. The question is whether a sufficiently homogeneous microstructure can be obtained in the large forging, whether the mechanical properties (yield stress and impact toughness) lie in the desired range and whether the detectability of possible defects by ultrasonic inspection is good enough.
Finally, the acceptance of a novel material increases, if it has demonstrated its capabilities in operating power plants. Therefore demonstration tubes were welded into boilers of two Ukrainian power plants for demonstration purposes.
The modelling methods applied in Z-Ultra were
1. Density Functional Theory (DFT) is an approximate method to solve the quantum-mechanical many-electron Schrödinger equation. The method has a sound theoretical basis and usually provides accurate results for material properties, but it is restricted to relatively small numbers of atoms.
2. The Tight Binding (TB) method is a less accurate approximation with parameters that need to be adjusted using DFT results. It allows modelling greater ensembles of atoms.
3. Thermodynamic/kinetic models describe the evolution of precipitates in an alloy depending on its composition and the temperature history. An extension of the existing models is needed to account for the large volume change during Z-phase formation and for the necessary accommodation mechanism.
4. A constitutive model for the mechanical behaviour was developed.
5. The finite element method was applied to demonstrate the behaviour of parts from power plants under creep conditions.
The models, especially the DFT calculations are expected to yield insight into the chemistry of the Z phase in different alloys and into the pathways which lead to the transformation of the nitrides into Z phase.
Implementation
The consortium of the Z-Ultra project combined the expertise of a steelmaker, a utility company, an engineering consultant company and eight research organizations and universities from the European Union and from three Eastern Partnership Countries.
The work was organized in eight work packages:
WP1 Material development and welding
WP2 Nanoscale experiments
WP3 Ageing, corrosion, creep and internal friction tests: macroscale experiments
WP 4 Nano- and microscale modelling
WP 5 Meso- and macroscale modelling
WP 6 Demonstration, Upscaling
WP 7 Project management
WP 8 Dissemination and Exploitation
Relevance for the European industry
The know-how to produce steel with superior creep and oxidation resistance is a great competitive advantage for European steelmakers. The steelmaker who participated in the project is primarily interested in marketing large turbine rotors. For manufacturers of boilers it is very attractive to use a more creep and corrosion resistant steel, which would allow higher steam temperatures or would replace the more expensive austenitic materials. Utilities will use stronger alloys for retrofit of their facilities, since higher strength allows smaller wall thickness, which means smaller thermal stresses and eventually greater flexibility.
Project Results:
Attachment Z-Ultra_S&T_results_final report.pdf
Potential Impact:
Impact
The aim of Z-Ultra was to develop steels that would allow higher steam temperatures and pressures in fossil-fired power plants. The concomitant progress in thermal efficiency will have a strong impact on the world-wide efforts to reduce the CO2 emissions. Higher efficiency also reduces the fuel cost, and therefore makes new coal and gas fired plants more competitive compared to nuclear power plants and compared to older plants with lower efficiency.
The decision whether new power plants will be built depends on economic and political factors, which vary from country to country. In some countries of Western Europe only few additional coal-fired power plants will be built. Apart from political factors, the oversupply on the some of the national current markets due to the extension of renewable sources makes it unattractive to invest in new plants.
In other parts of the world, new coal fired plants were built at a high rate, and this will continue, though probably at a reduced rate. For these new plants higher thermal efficiency remains an economic and environmental advantage. European steelmakers and component manufacturers who can provide the necessary materials and components will benefit.
In many countries of the European Community, the increasing share of renewable energies makes great demands on the flexibility of thermal power plants, which have to compensate the fluctuating input from wind and solar power. In this connection, the advantage of stronger materials is that the wall thickness of thick-walled components can be reduced, which reduces thermal stresses during start-up and shut-down cycles, and therefore the thermal fatigue of the components. Current power plants were usually designed for base load with only few load changes, and not against frequent thermal fatigue cycles. The possibility to reduce the wall thickness by using a stronger material is especially interesting for the retrofit of critical components in existing power plants that have already developed fatigue damage or have reached their end of life for other reasons.
Some European countries make efforts to reduce their CO2 emissions by using renewable biomass, as well as waste in their power plants. Usually these fuels are chemically more aggressive than coal or natural gas and require steels with a higher Cr content than the current 9%Cr steels. The Z-phase strengthened steels with their excellent corrosion resistance will here be an option.
The oxidation and corrosion resistance of the new ferritic-martensitic steels renders them competitive with the more expensive austenitic steels, not only in superheaters of power plants, but also in the chemical and petrochemical industry. The replacement of an austenitic component by a ferritic-martensitic part allows saving rare and expensive alloying elements.
Bringing a new material into service in power plant components requires much longer testing times than the limited time of a three- or four-year project. The reliability of power plants needs to be guaranteed for at least 100,000 h. It will therefore be necessary to initiate projects for long-term creep and oxidation tests of the most successful Z-Ultra materials. The excellent properties of the ZU1 and ZU4 materials justify the cost and time of such a test program and the upscaling activities, which are necessary to qualify the material for use in power plants.
These activities will be initiated in European working groups on creep resistant steels in collaboration with steel and component manufacturers (e.g. Saarschmiede, Vallourec & Mannesmann) and equipment manufacturers. End users of the steels (Siemens, and Air Liquide) have already expressed their interest in the Z-Ultra steels and cooperations are being established. Professor John Hald, who is active in European working groups and a well-recognized expert in power plant technology and creep resistant steels, is organizing a follow-up project involving several of the Z-Ultra participants and additional industrial partners.
A secondary, but also very important goal of Z-Ultra was to advance Integrated Computational Material Science and Engineering. This is a long-term, world-wide effort to reduce the time and cost for developing new materials. In Z-Ultra a great number of precipitate phases with variable compositions had to be analysed. Modelling the formation of the Z phase was a particular challenge. The enhanced know-how of the Z-Ultra participants will be useful in future projects, not only for Z-phase strengthened steels, but also for other material classes.
The same can be said of the nano- and microscale experimental methods used and developed in Z-Ultra. The methods to identify different, very small phases and their chemical composition were substantially refined.
Since the design lifetime of power plants is usually longer than the longest creep test times, reliable models are needed to extrapolate the test results for establishing safety and maintenance concepts to prevent failures of potentially hazardous high temperature/high pressure equipment. Unexpectedly fast microstructural degradation, as it some¬times occurred in previous steels (e.g. ½Cr-½Mo-¼V), causes a great loss of invested capital and high mainten¬ance costs.
Last but not least the demonstration activities in Z-Ultra will promote the interest in the new steels. The participants showed that they were able to produce large parts from a new material without trial and error cycles, and that the first attempt to weld large parts would lead to crack-free welds. The novel test rigs for investigating the material behaviour in corrosive atmospheres like in real superheaters will be extremely useful for utilities.
Dissemination activities
The objectives of the dissemination activities are to communicate and deliver knowledge about the Z-Ultra project by using the appropriate channels to the various interested stakeholders. The web site (http://www.z-ultra.eu) is continuously updated. A complete list of dissemination activities is appended.
Workshops: The Z-ultra project organized workshops in Tbilisi, Kiev and Graz.
Workshop in Tbilisi: A short course for students and faculty was held in conjunction with the Consortium meeting in Tbilisi from September 29 to October 1, 2014. A flyer was distributed three months ahead of the event. It is shown on the project website www.z-ultra.eu. Sixteen scientific/technical lectures were presented by the Z-Ultra partners and by the Project Technical Advisor. Another presentation informed the Georgian students about European exchange programs. About 25 students and 10 staff members from the Georgian Technical University as well as a PhD student from Jülich in Germany subscribed for the workshop. The students attending the workshop study energetics, engineering physics, and metallurgy at GTU. The local organization was excellent. After each lecture, enough time was planned for questions from the students, and also extensive coffee breaks were used for personal discussions and direct exchange with the students. After the workshop, all presentations were made available online for download for the students.
The impact of the workshop has been regarded by all participants as a success. The students were interested in the lectures and good mutual exchange was given between the students and the Z-Ultra members giving the lectures. The excellent spoken English of the students, their curiosity and their previous exchange experiences with the Forschungszentrum in Jülich, Germany, helped in creating an excellent basis for communication. Exchange programs with Europe and Germany in particular were presented, in order to help outstanding Georgian students to find opportunities to study or to perform a PhD thesis in Europe. The email addresses of the Z-Ultra members giving the lectures were made available, thus creating a further contact possibility for the students. Overall, the workshop promoted communication between the Georgian and the European scientists and students and created a sound basis for a smoother and barrierless collaboration.
Workshop in Kiev: The workshop in Kiev took place on 12/3/2015 in the same week as the SICA meeting, which was held in Kiev on 10 to 11/3/2015. The title of the workshop was Science Based Material Development for Power Engineering. The workshop was announced in Kiev by the flyer shown on the project website www.z-ultra.eu. The program comprised 8 lectures from the Z-Ultra consortium, 6 from Paton Institute and one from the Project Technical Advisor. The list of participants contains 47. The presentations of the Z-ultra partners are available on the Content Management Server of the Fraunhofer Society, https://dms-prext.fraunhofer.de.
After each lecture, enough time was planned for questions, and also the coffee and lunch breaks were used for personal discussions and direct exchange with the participants from Kiev. Especially PhD students were interested in publications of Z-Ultra members. In summary the workshop will strengthen the long-lasting relations between members of Z-Ultra and members of the Paton Institute and the Academy of Science in Kiev. The local hosts from Paton Institute made the visit for the participants a very pleasant experience.
Workshop in Graz: The Z-Ultra workshop “Novel Materials for Power Generation” was included in the THERMEC conference organised by the Graz University of Technology (TUG). Thermec 2016, an international conference on processing and manufacturing of advanced materials, took place from the 29th of May till the 3rd of June 2016 in Graz, Austria. About 1300 researchers from all around the globe presented their recent advances in the field of development, fabrication, processing, performance and applications of future-oriented materials. This conference provided an appropriate context and a unique opportunity to present the concept and disseminate results obtained within the Z-ultra project to the international scientific and industrial community. The workshop was extended to three days parallel to the conference, instead of 2 days initially planned.
Twelve oral presentations, including 2 keynote presentations, were held by Z-ultra partners and 2 posters were presented. Speakers benefited from a much larger audience than one could expect from an isolated workshop. The presentations aroused strong interest from scientists as well as from industrial researchers and led to fruitful discussions. Potential partners for future collaborations were identified.
Publications
A list of publications is appended. Between 2013 and 2016 the project partners published 30 papers in peer reviewed journals. Several others are in preparation. Two theses were completed during the project.
Among the more recent publications by DTU and Chalmers are some which address Z-Ultra specific problems and solutions that emerged during the project. Several papers published by TUG (with S. Yadav as a co-author) deal with experiments and models for creep deformation and fracture of 9-12%Cr steels. The publications from IPM (with J. Svoboda as a co-author) treat various problems of material science using the thermodynamic extremal principle. This very powerful method is also used and refined in Z-Ultra. The most recent of these publications address the problem of volumetric misfit of growing particles and its accommodation mechanisms, which is especially important not only for the Z phase, but also for other kinds of particles. In previous models this aspect was not properly taken into account. Finally a group of publications by Paton Institute describe the welding problems occurring in creep resistant steels and the welding procedures to avoid them.
Lectures
Z-Ultra participants gave 42 lectures at conferences on Z-Ultra related subjects. The topics are the same as in the publications, but obviously the lectures are closer to the frontline of research than the publications. For example, the important density functional calculations on Z-phase alloys are described in several lectures, but the publications are only in preparation.
Website
The web site http://www.z-ultra.eu has been continuously updated.
Exploitation
A list of exploitable foreground is appended.
Although the alloy compositions worked out in Z-Ultra already lead to very good high-temperature properties, it will be necessary to initiate projects for long-term creep and oxidation tests of the most successful Z-Ultra materials. The excellent properties of the ZU1, ZU4 and ZU5 materials justify the cost and time of such a test program and the upscaling activities, which are necessary to qualify the material for use in 625 to 650°C power plants. End users of the steels (Siemens, and Air Liquide) have already expressed their interest in the Z-Ultra steels and cooperations are being established.
The actual compositions and the scientific arguments leading to these compositions are valuable and exploitable know-how for these future steps. Primarily SSF and DTU, but other partners as well, will benefit from the progress made in alloy development. New steel grades have always displa¬ced the previous generation in the most demanding applications. For example, the ‘COST E’ steel (X10 CrMoVNbN 1011) developed in joint European projects accomplished a dominant position on the world market for large forgings for 600°C within only a few years and contributed substantially to the turnover of SSF. European steelmakers can sustain a leading position on the growing international market of steam power plants only if they can offer materials with superior properties. The new creep and corrosion resistant Z-phase strengthened 12%Cr steel would open up new opportunities, not only in the European Union and in the Eastern Partnership countries, but also in the fast growing markets of the emerging economies.
Heat treatment and welding procedures are closely related to material development. Here, Paton, SSF and DTU are the main beneficiaries.
The know-how developed during the Z-Ultra project on Integrated Computational Material Science and Engineering enhances the competitiveness of IWM and IPM in a subject area which experiences world-wide attention with large amounts of funding. Since IWM and IPM gain a great fraction of their revenues from theoretical modelling activities, outstanding competence in this area is of paramount importance. Enhanced competence on steels opens an avenue to one of the biggest markets.
Chalmers greatly improved its capabilities in nanoscale experiments. Similarly to the modelling competence, and in conjunction with it, this competitive advantage can be offered in future project proposals to industries or public funding agencies.
RWE together with its subcontractor StandZeit GmbH developed unique testing methods to investigate the material behavior under conditions which are close to those in real power plants. This is the basis for future projects with the electricity generating industry worldwide.
List of Websites:
www.z-ultra.eu
List of participants with contact data attached
