th Draft for Comment

Mat UK Energy Materials Review Materials R&D Priorities For Gas Turbine

Based Power Generation.

10th July 2007 Draft for Comment

Authors: J Hannis - Siemens Industrial Turbomachinery G McColvin - Siemens Industrial Turbomachinery C J Small – Rolls-Royce plc J Wells - RWE npower Responses to C J Small < [email protected]> or G.McColvin [email protected]

Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

Contents 1. EXECUTIVE SUMMARY. .................................................................................................................3 2. INTRODUCTION................................................................................................................................4 3. GAS TURBINE MATERIALS. ...........................................................................................................5

3.1 Generic technology.........................................................................................................................5 3.2 Compressors. ..................................................................................................................................5 3.3 Combustors.....................................................................................................................................7 3.4 Turbines..........................................................................................................................................9

3.4.1 Turbine Blades.........................................................................................................................9 3.4.2 Turbine Discs.........................................................................................................................11 3.4.3 Bolted Joints. .........................................................................................................................12 3.4.4 Sealing. ..................................................................................................................................13

4. FUTURE R&D PRIORITIES. ...........................................................................................................14 5. ANNEX 1 – COATINGS. ..................................................................................................................15 PICTURE CREDITS..............................................................................................................................16

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Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

1. EXECUTIVE SUMMARY. The following report summarises the current materials technology for land based gas turbine components, and reviews the commercial and technology drivers. The current trends and future prospects for component design and operating conditions (e.g. pressure ratios, turbine inlet temperatures etc) are described. The following key recommendations are made for materials technologies for gas turbine applications:

• Ongoing incremental development of existing advanced high temperature material systems for industrial gas turbine applications to serve the short and medium term needs.

• Step change disruptive materials system development to meet the medium and long term needs.

• Advanced manufacturing development for cost reduction, increased materials performance and integrity (including materials and process modelling).

• Develop of robust accurate NDE and lifing methods. • Overhaul and repair capability coupled with remnant life

assessment

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2. INTRODUCTION. Gas turbines are key components in the most efficient forms of advanced power generation plants available. Their high versatility and flexibility enables gas turbines to be used as a means of generating electrical energy using operational cycles such as conventional simple cycle, combined cycle, and combined heat and power generation systems.

The success of the gas turbine power generation industry in the UK is dependent upon satisfying customer and operator requirements, while simultaneously optimising the complex, and often interacting, technical challenges arising from market and legislative factors. These have effects on capital and operating costs, efficiency, power output, fuels and emissions. A range of fuels are used, including liquid fuels, natural gas, synthetic gas produced from coal or other feedstocks

and biomass. However, the properties of lower calorific value gas fuels produce several problems such as:

• Increased fuel mass flow for a given heat input requirement; • Presence of fuel-borne nitrogen leading to higher NOx

emissions; • Increased fuel gas temperature and higher levels of

contaminants, where gas cooling/cleaning is restricted to maximise overall cycle efficiency.

More complex cycles that incorporate hardware such as intercoolers, recuperators and heat exchangers and reformers are being developed that aim to provide improvements in thermal efficiency, but consequently lead to more complex running conditions for core components and more complex plant design and operation. Other issues include CO2 reduction and sequestration, use of hydrogen fuel and catalytic combustion systems The emphasis placed on each of these factors by organisations within the power generation industry depends on the particular role played within the market, i.e. manufacturer, supplier, operator, etc. The following sections aim to describe the technologies and developmental drivers for land-based gas

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turbine materials and components. The trends and future prospects for component design and operating conditions are described.

3. GAS TURBINE MATERIALS.

3.1 Generic technology The following sections review the materials used in the different parts of modern industrial gas turbines, however underpinning any materials development are 4 key technologies that must be developed in parallel. These are:

• Coatings. • NDE • Lifing • Repair

Each of these technologies are considered to be part of the individual materials solutions in each section, however where possible specific reference to some or all of them is made as appropriate. In addition, given their visibility and importance to the gas turbine, a summary of coatings developments is given in annex 1.

3.2 Compressors. The challenge facing the compressor is to provide improved cycle efficiency, operability and reduced costs by optimising the work done by each stage.

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This can be achieved by better control of the pressure ratio and mass flow through the compressor, improved component reliability and reduced parts count. As well as the materials developments that support these requirements advanced manufacturing methods are being introduced to enable cost reduction and improved integrity. For example, welded structures are being considered to replace bolted joints. Future developments are aimed at increasing pressure ratios from the current 15:1 level to 40:1 and beyond, while continuing to target efficiency and surge margin gains. The need to maintain compressor performance and integrity through life, while reducing parts costs and the use of more effective manufacturing processes is paramount, as is the need to achieve operational lifetimes in excess of 100,000 hours. Many of these targets are dependent upon improved design and aerothermal analysis methods in conjunction with test and validation procedures; however without suitable high temperature materials these cannot be achieved. For small to intermediate gas turbine compressors, the temperature loadings experienced currently range from -50 to about 500ºC. In the short to medium term the continued use of improved low-alloy and ferritic stainless steels will be adequate. This situation will continue until significant increases in compressor temperatures are needed because of much higher-pressure ratios and rotor speeds. In such a situation it is assumed that aero-derivative technology such as titanium alloys, nickel alloys and composites will be employed. This would, however, present a significant increase in cost and manufacturing complexity (forgings, machining, joining, component lifing) as well as operational difficulties (component handling, overhaul, repair, cleaning) and may introduce additional problems associated with thermal mismatch and fretting fatigue from adjoining ferritic alloys. Consideration has also been given towards lightweight materials such as aluminium matrix composites, polymer composite blading and vanes, as well as intermetallic TiAl-based alloys to provide reduced rotor and overall engine mass, and lower disc stresses to enable higher rotational speeds. In addition, design and materials concepts have evaluated the application of integrally

Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

bladed discs (bliscs) based on steel, titanium or nickel alloy technology using friction welding. Issues associated with rotor corrosion are largely operator dependent, being influenced by the specific nature of the fuel, compressor washing and cleaning practices. These are currently addressed by use of protective coatings. Likewise, commercially available abradable tip sealing coatings are currently used to provide and maintain efficiency and currently present little technical risk. For large utility power generation engines the temperature and strength limitations of the rotor steels used are currently limiting the performance. The development and demonstration of high-nitrogen, nano-precipitate strengthened steels for high pressure compressor disc applications that offer equivalent strength and temperature capabilities to some nickel alloys with much reduced cost is crucial. Application of these high strength creep-resistant steels necessitates the development of improved large-scale melting (up to 100 tonnes) and forging capabilities (up to 18 tonnes) and the development of suitable welding technologies, non-destructive testing methods for large-scale rotors and validated life assessment and risk analysis methods. Successful development of this technology would negate the need to introduce much more expensive (by a factor of 5) nickel alloy technologies. To achieve this the materials and process developments required will be heavily dependant on successful integrated process modelling that links the materials and process developments and enables an optimised, affordable, manufacturing route to be developed. An added complication for the compressor is the introduction of water/fogging at the intake to improve performance. The presence of water droplets leads to erosion issues on compressor blading. In the short term this will require the development of erosion resistant coatings for existing materials but in the medium to longer term an erosion resistant materials system solution will be required

3.3 Combustors. The combustor experiences the highest gas temperatures in a gas turbine and is subject to a combination of creep, pressure loading, high cycle and thermal fatigue. The materials used presently are generally wrought, sheet-formed nickel-based superalloys. These provide good thermomechanical fatigue; creep and oxidation resistance for static parts and are formable to fairly complex shapes such as combustor barrels and transition ducts. Equally of importance is their weldability, enabling design flexibility and the potential for successive repair and overhaul operations, which is crucial to reducing life cycle costs. The high thermal loadings imposed often mean that large portions of the combustor hardware need to be protected using thermal barrier coatings.

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Current temperature loadings experienced by combustors range from 1250 to 1375ºC, depending on the engine size and duty cycle. Future developments aim to reduce CO2, NOx and SO2 emissions to meet anticipated environmental legislation and customer demands for cleaner running by optimising the distribution of fuel during firing and by using catalytic combustion systems. The significance of this requirement is to place a limit on the anticipated future turbine entry temperature levels, placing more emphasis on controlling peak flame temperatures within the combustor. This should also provide more air for cooling the combustor liner; however, as a consequence other components may be required to run hotter as the demand for combustion flame temperature control increases. Materials technology acquisition programmes for future combustor designs are aimed at replacement of conventional wrought nickel-based products with:

• More capable Ni-based alloys. • Oxide dispersion strengthened metallic systems • Ceramic matrix composites.

The programmes are primarily aimed at addressing the limitation in temperature capability and coating compatibility of current alloys. Candidate

materials have been identified and demonstration hardware has been manufactured and engine tested. However, there are limitations to these technologies that need to be developed before they can be deployed on products, e.g. joining methods, environmental barrier coating systems, robust design, inspection, lifing and repair capabilities. These materials have also been identified as candidates for efficient, high temperature heat exchangers for a range of externally fired combined cycle

systems that separate the turbine working fluid from the aggressive combustion gases generated by poor quality fuels. This limits the damage incurred by hot section hardware during engine running and enables the use of a range of low calorific value and biomass fuels combined with combined heat and power recovery systems. The current thermal barrier coatings technology for metallic combustor applications is based exclusively on multi-layered systems comprising of an MCrAlY bondcoat and a ceramic topcoat applied using plasma spray deposition techniques. Application of this technology generally aims to limit peak metal temperatures to 900 to 950°C. Future developments are aimed at

Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

applying thicker coatings to enable higher flame temperatures and/or reduce metal temperatures further. Other programmes are aimed at increasing the phase stability and resistance to sintering of the ceramic topcoat at temperatures above 1250°C and to the inclusion of diagnostic sensor layers within the coating that enable the plant and component condition to be actively monitored. New materials will however require new coatings systems that can only be specified and developed in parallel with the substrate development. Finally, the use of coal gasification cycles may lead to much higher particulate loadings than for other fuels. The development of high temperature erosion and corrosion resistant coatings and substrate materials, as well as improved gas cleaning facilities will be required. The increased operating temperatures and corrosive/erosive fuel gas will require the further development of coating technologies.

3.4 Turbines.

3.4.1 Turbine Blades. Turbine blades are subjected to significant rotational and gas bending stresses at extremely high temperatures, as well as severe thermomechanical loading cycles as a consequence of normal start-up and shutdown operation and unexpected trips. The turbine entry temperature is typically in excess of 1375ºC, with base metal temperatures ranging from 850 to >1050ºC. The target lifetime under these conditions is dependent on engine type and duty cycle, but can be in excess of 50,000 operating hours. The blades pass through the wake of the combustor and nozzles and are subject to high frequency excitations, which can lead to high cycle fatigue failure. The high-pressure stages are cooled to withstand the hot gas temperatures and, depending on the type of fuel, corrosion and erosion of the blade structure is limited by the use of protective coatings. For many years the primary consideration in the design of blades has been to avoid the possibility of creep failure due to the combination of high stresses, temperatures and the expected length of running time. To meet this requirement and increase the efficiency by running a higher turbine temperatures more advanced materials have been continuously introduced. For vanes and blades there has been a gradual move away from conventionally cast nickel-based superalloys towards directional solidification. The introduction of these alloys, manufactured using near-net shape investment casting has provided significant benefits in terms of much improved creep and thermal fatigue properties. Further significant benefits have been gained by the use of single crystal technology for both blades and vanes. A number of issues are, however, still to be resolved. The increased cost of manufacture needs to be mitigated by the use of revert materials and increased casting yields, and offset against improved component lifetimes and more efficient running (through the higher turbine entry temperatures they make possible). Alloys with greater defect tolerance, in particular to low –

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medium angle boundaries, need to be developed and proved with advanced modelling of the behaviour of defects under load. To achieve increased creep strength, successively higher levels of alloying additions (Al, Ti, Ta, Re, W) have been used to increase the level of precipitate and substitutional strengthening. However, as the level of alloying has increased the chromium additions have had to be significantly reduced to offset the increased tendency to form TCP phases, which limit ductility and reduced strength. Reduced chromium levels also significantly reduce the corrosion resistance of the alloys. This has necessitated the development of a series of protective coating systems. The coatings are applied to provide increased component lifetimes, but they often demonstrate low strain-to-failure properties that can impact upon the thermomechanical fatigue endurance. The development of industrial gas turbine specific turbine blade alloys continues to be a difficult problem to resolve. Much dependence has been placed by the land-based sector on the transfer of advanced technologies from the aero sector and this has not always provided the necessary solutions. The key issues associated with this dependence are as:

• Development of a succession of alloys with reducing corrosion resistance, despite increasing requirements for the use of differing poor quality fuels and a range of running conditions to satisfy the power generation market requirements.

• Limited castability of large-scale components due to recrystallisation and microstructural defects such as freckles, large angle grain boundaries and coarse dendritic structures leading to reduced property levels.

• Over-emphasis on high stress and high temperature creep strength.

Efforts have been made to address these issues with the development of a number of alloys having improved castability, higher corrosion resistance and reduced heat treatment times. Alloys have been developed with varying degrees of success; however significant work is needed in this field to develop alloy systems that address not only the alloy, but its coating, lifing and repair as an entity not as a series of unrelated steps. There is a large scope in industrial gas turbines for continued incremental development of Ni-based alloys and coatings for the short and medium term.

Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

The issue of coatings development for turbine blades and vanes is currently treated almost as a separate topic from the substrate that will carry it into service. They are applied for oxidation/hot corrosion protection reasons (as described above) and include aluminising, chromising and MCrAlY. They are also applied to reduce metal temperatures by acting as a thermal barrier protection system, usually as a ceramic plus bond coat system. They are usually applied over the top of existing materials and a variety of similar proprietary solutions exist and are being further refined. This “classical” additive approach will doubtless continue but future developments will have to treat the substrate, the bond coat, the coating and the environment as inherent components of the materials system solution that is required. Continued development of multilayer coatings and application methods are therefore required to improve reliability and reduce cost but with this holistic approach in mind.

3.4.2 Turbine Discs. The main functions of a turbine disc are to locate the rotor blades within the hot gas path and to transmit the power generated to the drive shaft. To avoid excessive wear, vibration and poor efficiency this must be achieved with great accuracy, whilst withstanding the thermal, vibrational and centrifugal stresses imposed during operation, as well as axial loadings arising from the blade set. Under steady-state conditions, current turbine disc temperatures can vary from approximately 450°C in the cob to in excess of 650°C close to the rim with a requirement for >50,000h operating life. These temperature loadings are set to increase further across the disc as the demand for improved efficiencies continues. Creep and low cycle fatigue resistance are the principal properties controlling turbine disc life and to meet the operational parameters requires high integrity advanced materials having a balance of key properties:

• High stiffness and tensile strength to ensure accurate blade location and resistance to overspeed burst failure;

• High fatigue strength and resistance to crack propagation to prevent crack initiation and subsequent growth during repeated engine cycling;

• Creep strength to avoid distortion and growth at high temperature regions of the disc;

• Resistance to high temperature oxidation and hot corrosion attack and the ability to withstand fretting damage at mechanical fixings.

To meet the highest operating temperatures and the component stress levels demanded, it has been necessary to develop a series of progressively higher strength steel and Ni-based superalloys. These are generally manufactured using cast and wrought processing. However, the complex chemistry of these alloys makes production of segregation-free ingots very difficult. A triple melt

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Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

process is necessary, involving vacuum induction melting, electroslag refining and vacuum arc remelting to limit macrosegregation and defect inclusion. Manufacture of larger components, or more complex alloys, would necessitate a change to atomised powder processing to limit segregation, while dual alloy processing offers the potential for overcoming the variability in strain

distribution across the section of large forged turbine wheels. Many of these developments are being pursued by the aero-gas turbine industry to meet their more demanding conditions and it is assumed that much of this aero-derived technology can be inserted into these applications. However the cost may preclude this in which case a completely new approach to the materials, design, manufacture and repair of such components will be required.

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The issue of coatings development

for turbine discs is currently treated in the same way as coating for blades but is a much less mature technology. The same holistic systems approach is therefore required.

3.4.3 Bolted Joints. To provide serviceability of hot section components, bolted joints are employed on both rotor and stator components. On rotors, such bolts are safety critical and should ensure that rotor integrity is maintained in the event of a blade failure. In rotors, bolted joints are used with in conjunction with features such as Spline, Hirth, Curvic or Spigot couplings to ensure component concentricity and balance are maintained. High strength wrought superalloys are used for bolting in high temperature areas where resistance to creep is essential to ensure that bolt tension is maintained over the life of the product. Low cycle fatigue strength is required to ensure adequate cyclic life in both bolt shanks and threads. In general, turbine disc alloys are suitable for bolting applications with the additional requirement that materials which are not subject to thread seizure are needed to permit disassembly after extended service. To avoid the possibility of thread damage in high cost complex components, the use of captive nuts or thread inserts is common.

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Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

3.4.4 Sealing. (i) Rotating Seals. Turbine gas path seals include rotor tip seals and disc rim seals. On unshrouded rotor tips and rim seals, coatings such as MCrAlY and Nickel-Graphite can be employed as an abradable coating to avoid damage in the event of a rub. Erosion resistance and the ability to provide abradability after long-term exposure at high temperature are required for such coatings. On shrouded rotor blading, superalloy honeycomb foil materials are employed as abradable seals and high temperature oxidation resistance is needed for foil materials to achieve long life at high temperature. Research is ongoing to develop improved seal materials and novel methods of construction and application for the above seal types. The labyrinth seal is the most common form of air system seal, usually with an abradable stator material. Positive contact carbon seals are often used for bearing chamber seals. The development of brush seals has provided improved sealing efficiency and is displacing the use of labyrinth seals in critical locations. Both metallic and non-metallic bristles are employed depending on temperature levels and wear resistance and fatigue resistance key parameters. Film riding face seals have the potential for even higher sealing efficiencies but place extreme demands on manufacturing technology if they are to be practicable. (ii) Static Seals. Static seals are used to seal gaps and joints between components where there are small relative movements due to thermal expansions and they can be employed on both turbine rotor and turbine and combustor stator components. On cylindrical joints, piston rings and E seals are used and for sealing gaps between adjacent blades and vanes strip seals are employed. On rotor blades it is common for sealing features and blade locking features to be combined. Seal strips and E seals used in high temperature locations employ wrought sheet materials as used for combustor components. Wear and fretting can be experienced on such seals and the development of hard coating systems can alleviate this. For this type of seal, brush seals, used in a static application, can allow larger thermal movements with low leakage.

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Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

4. FUTURE R&D PRIORITIES. Based on the preceding sections the following items have been identified as the key materials enabling technologies for gas turbine applications:

• Ongoing incremental development of existing advanced high

temperature material systems for industrial gas turbine applications to serve the short and medium term needs.

• Step change disruptive materials system development to meet the medium and long term needs.

• Advanced manufacturing development for cost reduction, increased materials performance and integrity (including materials and process modelling).

• Develop of robust accurate NDE and lifing methods. • Overhaul and repair capability coupled with remnant life

assessment

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Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

5. ANNEX 1 – COATINGS. Surface engineering and coatings technology plays a crucial role in the operation of all high temperature plant, particularly for GT engines. The desire for higher operating temperatures, improved performance, extended component lives and cleaner, more fuel-efficient power generation places severe demands on the structural materials used and many components operating at high temperature are coated or surface-treated to enable cost-effective component lifetimes to be achieved. Coatings are applied to provide wear, erosion, oxidation, corrosion or thermal protection depending on the nature of the operating environment and the thermal loads to be endured. Coatings are also used for improving and maintaining surface finish, anti-fouling, anti-fretting, abrasive seal tips and seal materials. Any coating should possess the requisite mechanical properties, adhesion and metallurgical stability in contact with the substrate to withstand the thermomechanical cyclic loadings imposed. The main types of protective coatings used for gas turbine components can be defined as follows:

• Diffusion Coatings: Formed by the surface enrichment of an

alloy with aluminium (aluminides), chromium (chromised) or silicon (siliconised). In some systems combinations of these elements are possible i.e. chrome-aluminised or silicon-aluminised.

• Overlay Coatings: Formed by applying a layer to the component surface. This type forms the bulk of the coatings used in GT engines. They are applied by a variety of methods including thermal and slurry spraying, physical vapour deposition and welding. Examples include:

Simple paints Corrosion resistant coatings such as MCrAlY (where M is

the base metal, normally Ni or Co or a combination of the two; Cr is chromium, Al is aluminium and Y is yttrium).

Thermal barrier coating (based on a ceramic topcoat, usually partially stabilised cubic Zirconia, attached to the metal substrate by means of an oxidation-resistant bond coat (typically a MCrAlY or a diffusion aluminide coating)).

In the short to medium term continued development of new coatings to be applied to existing materials will be required – however in the long term as new materials are introduced coatings and their associated technologies will have to be developed as an integral part of the delivery of the overall materials system solution.

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Mat UK - Materials R&D Priorities For Gas Turbine Based Power Generation

PICTURE CREDITS Page 6 – Alstom Power Front Cover and pages 4, 8 and 10 – Rolls-Royce plc Page 5, 12 (both) – Siemens Industrial Turbomachinery

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