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BWD TURBINES LIMITED |
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ADVANCED TURBINE TECHNOLOGIES FOR THE INDUSTRY WORLDWIDE |
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A NEW METHOD OF METAL TEMPERATURE ESTIMATION FOR SERVICE-RUN BLADES AND VANES
K. A. Ellison, J. A. Daleo, K. Hussain, Superalloys 2004, proceedings of the 10th International Symposium on Superalloys, Champion, Pennsylvania September 19-23,2004, edited by K. A. Green et. al., ISBN 0-87339-576-X, pages 759-768, TMS, 184 Thorn Hill Road, Warrendale, Pennsylvania, 15086-7528.
ABSTRACT A NiCoCrAlYRe overlay coating, SICOAT 2453* exhibited a series of solid state transformations involving five equilibrium phase fields between room temperature and 1200ºC. The largest changes in γ, γ' and β phase fraction (over 50 v/o for γ and γ') occurred in the temperature range 815ºC to 970ºC. These changes are potentially useful for metallurgical service temperature estimation, as they occur within the operating range of many combustion turbine blades and vanes. A phase fraction vs. temperature diagram was developed for SICOAT 2453 and used to estimate the local metal operating temperatures of a service-run, single crystal PWA 1483 alloy first stage blade and a polycrystalline first stage MAR M 509 turbine vane coated with the NiCoCrAlYRe alloy. The strengths and limitations of the new temperature estimation method are discussed and compared to older methods of obtaining metal temperature estimates from service-run blades and vanes.
METALLURGICAL CONSIDERATIONS FOR LIFE ASSESSMENT AND THE SAFE REFURBISHMENT AND REQUALIFICATION OF GAS TURBINE BLADES J. A. Daleo, K. A. Ellison, D. H. Boone, Journal of Engineering for Gas Turbines and Power, July, 2002, Vol. 124, pp. 571-579.
ABSTRACT Metallurgical analysis of rotating blades operating in advanced gas turbine engines is important in establishing actual operating conditions, degradation modes, remaining life, and most importantly, the proper repair and rejuvenation techniques to be used in developing optimum component life strategies. The elevated firing temperatures used in the latest engine designs result not only in very high metal surface temperatures but also in very high temperature gradients and concomitant thermal strains induced in part by the complex and efficient cooling systems. This has changed the primary function of today’s superalloy-coating systems from one of hot corrosion protection to moderating high temperature oxidation reactions. Furthermore, as a result of the high thermal strains induced by the cooling systems, long-term metallurgical structural stability issues now revolve around optimizing both thermal mechanical fatigue (TMF) resistance and creep life. Thus the gradual change to Directionally Solidified (DS) and Single Crystal (SC) alloys throughout the industry. The use of DS and SC alloys coated with state of the art TBC, platinum modified aluminide and MCrAlY coatings with or without subsequent aluminizing applied by vacuum plasma spray (VPS), high velocity oxygen fuel (HVOF), physical vapor deposition (PVD), air plasma spray (APS) and by chemical vapor deposition (CVD) methods along with the wide spread use of internal aluminide coatings have made today’s rotating components prohibitively expensive to replace after only one cycle of operation. It is therefore, or should now be a high priority for all cost conscience gas turbine users to help develop reliable repair and rejuvenation strategies and techniques to minimize their operating cost. Traditional metallurgical considerations required for life assessment and the reliable refurbishment and re-qualification of gas turbine blades are reviewed along with some new exciting techniques. Examples of component degradation modes are presented. Appropriate attention to metallurgical issues allows turbine users to more successfully and economically operate their turbines.
K. A. Ellison, J. A. Daleo and D. H. Boone, Superalloys 2000, proceedings of the 9th International Symposium on Superalloys, Champion, Pennsylvania September 17-21,2000, edited by T.M Pollock et. al., ISBN 0-87339-477-1, pages 649-654, TMS, 184 Thorn Hill Road, Warrendale, Pennsylvania, 15086-7528.
ABSTRACT Inter-diffusion between a NiCoCrAlYRe coating and an IN-738 blade alloy substrate was investigated. Tests were conducted by exposing coated blade specimens in still air at 940° C and 1050° C for times of up to 9720 h. The inter-diffusion zone microstructures were characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy. It was found that at 1050° , the central NiCoCrAlYRe coating was comprised of g , b and a phases and that a continuous a -Cr layer formed along the original substrate interface. At 940° C a more complex sequence of inter-diffusion microstructures was observed, including the formation of a continuous g ¢ + a zone adjacent to the blade alloy substrate. The NiCoCrAlYRe coating was also comprised of four phases at the lower exposure temperature: g , b , a and s ). It is shown that the complex inter-diffusion behavior requires the use of diffusion path analysis for proper interpretation. The implications for development of service temperature estimation models based on coating/substrate inter-diffusion in this system are discussed.
APPLICATION OF STRESS RELAXATION TESTING IN METALLURGICAL LIFE ASSESSMENT EVALUATIONS OF GTD111 ALLOY TURBINE BUCKETS J. A. Daleo, K. A. Ellison and D. A. Woodford, Journal of Engineering for Gas Turbines and Power, January, 1999, Vol. 121, pp. 129-137.
ABSTRACT: Stress relaxation and constant displacement rate tensile tests were performed on poly-crystalline GTD111 alloy material removed from General Electric MS6001B first stage combustion turbine buckets. Samples were examined in the standard heat treated condition, thermally exposed at 900°C for 5000 hours and from service run buckets. Creep rates of the material were measured and evaluated directly in terms of temperature capability at 850°C and 900°C. Stress relaxation tests done at 0.8% total strain indicated that the creep rate properties in the service exposed airfoil were an order of magnitude higher than the material properties in the standard heat treated condition measured in the root form. In terms of temperature capability, the creep rate properties of the service run airfoil material had decreased by the equivalent of almost 40°C. The stress relaxation test method was demonstrated to be a very useful tool in quantifying the degradation of creep properties in service run components. Creep data that would require years to gather using conventional creep tests was generated in a few days . This now makes realistic life assessment and repair / replace decisions possible during turbine overhauls. The test method’s unique ability to measure changes in creep rate over a large stress range, enabled the technique to distinguish between changes in creep strength due to (normal) microstructural evolution from the combined effects of microstructural evolution and strain related creep damage. A method for estimating standard constant load creep rupture life from the stress relaxation creep rate data is also presented along with time-temperature parameter correlations. The data sets examined in this study indicate that creep rupture lives can be estimated within a factor of three from the stress relaxation data. The information and analysis techniques described in this paper are directly applicable to metallurgical life assessment evaluations and the re-qualification of repaired General Electric buckets in Frame 3, 5, 6,7 and 9 engine models.
MICROSTRUCTURAL EVALUATION OF MCrAlY / SUPERALLOY INTER-DIFFUSION ZONES K. A. Ellison, J. A. Daleo and D. H. Boone, Life Assessment of Hot Section Gas Turbine Components, proceedings of a conference held at Heriot Watt University, Edinburgh, UK , 5-7 October 1999, edited by R. Townsend et al, ISBN 1-86125-108-4, pages 311-326, Book B731, 2000, IOM Communications Ltd., 1 Carlton House Terrace, London SW1Y 5DB.
ABSTRACT Overlay coatings of the MCrAlY type (M = Ni, Co, Fe) are used extensively for protection of hot section components in gas turbine engines. Recently, the rates of interdiffusion between a CoCrAlY coating and a DS GTD-111 alloy substrate were quantified and used to predict the surface operating temperatures of a rotating blade airfoil. The metallurgical temperature estimates are used to calibrate finite element heat transfer models that support component life and repair calculations. A qualitative understanding of the diffusional interactions and microstructural features is needed before this type of analysis can be extended to other commercial coating / substrate systems. In this paper, the interdiffusion zones between CoNiCrAlY and NiCoCrAlYRe coatings and nickel-base superalloys were evaluated after isothermal exposures at 1000° C (1832° F). The multi-phase diffusion zones that formed in each case were characterized by optical and scanning electron microscopy. This information, coupled with previously published and more recent experimental work on similar coatings, was used to construct schematic 1000° C isothermal sections for the Ni-Co-Al-Cr quaternary system. The interdiffusion zone microstructures were qualitatively interpreted by constructing schematic diffusion paths on existing Ni-Cr-Al and Ni-Co-Al ternary phase diagrams.
STRESS RELAXATION AS A BASIS FOR BLADE CREEP LIFE ASSESSMENT D. A. Woodford and J. A. Daleo, Life Assessment of Hot Section Gas Turbine Components, proceedings of a conference held at Heriot Watt University, Edinburgh, UK, 5-7 October 1999, edited by R. Townsend et al, ISBN 1-86125-108-4, pages 293-310, Book B731, 2000, IOM Communications Ltd., 1 Carlton House Terrace, London SW1Y 5DB.
ABSTRACT An integrated approach to life management incorporating a new methodology for evaluating the creep strength and fracture resistance of serviced blades is described. Creep strength is determined from a stress relaxation test (SRT) which is capable of generating data covering five decades in creep rate in less than one day. The SRT test may be augmented with a separate constant displacement rate (CDR) test to evaluate the current fracture resistance. There are five basis steps involved in the blade life assessment. Metallographic evaluations identify the effects of engine specific operating conditions on coating life, structural stability and the state of the components cooling system. Finite element analysis allows a three dimensional centrifugal stress distribution to be established. Next, temperature profiles are estimated using heat transfer analysis in conjunction with microstructural based temperature estimations. Allowable stresses may then be estimated as a function of temperature using conventional design procedures and compared with the computed values. Finally, the properties of the material using the SRT (and CDR) data allow for condition assessment or life prediction. This paper will outline the testing methodology and present results relative to current CC, DS and MX blade alloys.
ANALYSIS OF HOT SECTION FAILURES ON GAS TURBINES IN PROCESS PLANT SERVICE I. H. Hunter, J. A. Daleo, J. R. Wilson and K. A. Ellison, Proceedings of the 28th Turbomachinery Symposium sponsored by The Turbomachinery Laboratory and The Texas A&M University, September 14 to 16, 1999, Houston Texas.
ABSTRACT In-service failures of process plant gas turbines can have major economic consequences in terms of repairs and downtime. Following such an incident, steps need to be taken to avoid a recurrence. This is best accomplished through a formal analysis of the failure and this paper discusses the key aspects of the procedure. Several case histories pertaining to hot section failures on mechanical drive and generator drive gas turbines in a variety of process plant applications are also presented. These provide a representative cross-section of the different failure mechanisms and range of causes that can be encountered as well as demonstrating the multi-disciplinary approaches used to investigate the incidents. Lessons learned are highlighted.
THE THERMAL INERTIA ANALYSIS TECHNIQUE IN GAS TURBINE COMPONENT RELIABILITY ASSESSMENT J. J. Stiglich, Jr., C. C. Bishop, J. A. Daleo, D. H. Boone and T. E. Eelkema, Gas Turbine Materials Technology 99, proceedings of Materials Solutions 98, Rosemont Illinois, 12-15 October 1998, pages 138 to 144, edited by P. H. Maziasz, et al., ISBN;0-87170-628-8, ASM International, Metals Park, OH, 1999.
ABSTRACT Critical components in today’s advanced turbines place greater demands on materials, particularly their temperature capabilities. To lower and decrease temperature variations, mechanical and cooling technologies have been integrated into one reliable, cost-effective unit. Requirements for thermal efficiencies have resulted in very complex designs of components with thin walls and equally complex cooling systems. These materials and designs are expensive to produce and require testing, computer simulation and modeling, as well as trial and error techniques with appropriate analysis and feedback. Thermal analysis based on the use of sensitive infrared (IR) detectors and related equipment has been used with success in a number of applications including the inspection of gas turbine components and the design evaluation of materials systems. Until recently, systems have been capable only of basic analysis and simple interpretation of data. Increased system capabilities are required to assure the reliability demanded of advanced gas turbines. An innovative, integrated thermal-analysis technique, based on an advanced-design imaging system combined with new software algorithms, has been developed to analyze an expanded spectrum of both human/biological and mechanical systems including gas turbine components. The use of the Bales Thermal Inertia Analysis System (BTIAS) for the evaluation of gas turbine component design and NDE will be described. Examples include: · cooling system design substantiation · casting porosity and wall thickness determination and QC · coating quality, thickness, adherence, and thermal property characterization · core removal and proper cooling-channel geometry substantiation · proper cooling hole and system flow quantification · post-operation cooling system condition assessment, e.g., to determine a potentially compromised operation and the suitability of a component for repair and rejuvenation · determination of repair integrity and cooling-system suitability, e.g., impingement tube placement
GTD-111 ALLOY MATERIAL STUDY J. A. Daleo and J. R. Wilson, Journal of Engineering for Gas Turbines and Power, April, 1998, Vol. 120, pp. 375-382.
ABSTRACT Very little property data on this common turbine blade alloy has been published. As longer hours of service are accumulated, maintenance considerations such as developing optimum component life strategies and repair processes become important. The lack of specific material data hampers the effort of users and repair facilities to achieve optimum service from this alloy. This study measured some of the basic mechanical and metallurgical characteristics of this poly-crystalline nickel base superalloy. Tensile and short term creep rupture properties as well as microstructural and fracture characteristics are presented. Both the as-heat-treated and thermally exposed characteristics at two different temperatures are examined.
METALLURGICAL TEMPERATURE ESTIMATES BASED ON INTER-DIFFUSION BETWEEN CoCrAlY OVERLAY COATINGS AND A DIRECTIONALLY SOLIDIFIED NICKEL-BASE SUPERALLOY SUBSTRATE K. A. Ellison, J. A. Daleo and D. H. Boone, in Materials for Advanced Power Engineering 1998, edited by J. Lecomte-Beckers et al., proceedings of the 6th Liege Conference, Volume 5, Part III, P.1523, Forschungszentrum Jhlich GmbH, Central Library, D-52425, Germany, ISSN1433-5522, ISBN 3-89336-228-2.
ABSTRACT The interdiffusion reactions between stand-alone Co-29Cr-6Al-0.5Y (GT-29) and over-aluminized (GT-29 In-Plus) vacuum plasma sprayed coatings and a nickel-base superalloy (DS GTD-111) were calibrated and used to estimate the operating temperatures of service-exposed combustion turbine buckets. The rates of interdiffusion were quantified by exposing laboratory test specimens at temperatures between 850o and 1050oC for times of up to 8000 hours. The bulk CoCrAlY coatings were comprised of four major phases: the (Co,Ni) solid solution matrix in its hexagonal eCo (HCP) and/or g (FCC) forms, the b-CoAl ordered compound and smaller amounts (£15%) of a globular s-(CrxCoy) precipitate. Layered, multi-phase diffusion zones developed between the CoCrAlY coatings and the g+g’ substrate alloy. The phase relations at the various stages of coating degradation were qualitatively represented at 850oC by composition paths mapped onto schematic Ni-Co-Cr-Al pseudo-ternary sections. The growth kinetics of the interdiffusion zones were measured and the temperature dependance of the interdiffusion rate constants was determined. At temperatures of 940oC and above, the calibrated growth zones were two-phase, g+b layers. At 850oC, the measured growth layers reverted to a four-phase, g+g’+b+s zone and a three-phase, g+g’+s zone. A computer model was written which allows the surface temperatures of service-exposed gas turbine components to be estimated based on the observed and calibrated rates of coating/substrate interdiffusion. The model was used to estimate surface temperatures of GE MS7001EA and MS7001FA first stage turbine buckets. The metallurgical temperature estimates based on MCrAlY coating interdiffusion were in reasonable agreement with those derived from existing models of gamma prime precipitate coarsening in g+g’ nickel-base substrate alloys.
FAILURE MECHANISMS OF COATING SYSTEMS APPLIED TO ADVANCED TURBINE COMPONENTS J. A. Daleo and D. H. Boone, ASME Paper 97-GT-486, presented at the International Gas Turbine & Aeroengine Congress & Exhibition, Orlando, Florida – June 2 – 5, 1997.
ABSTRACT: The maintenance period of critical gas turbine components operating in advanced D, E and F technology industrial turbines is often determined by the life of the coating system applied to protect the base alloy substrate. The progressively higher firing temperatures used in all of the advanced engine designs results not only in very high metal surface temperatures but also in very high temperature gradients and concomitant thermal stresses induced in part by the complex cooling systems. In order to develop optimum component life strategies, it is important to establish the actual operating conditions of each component, and, to define the predominant degradation modes. Metallurgical life assessment of these advanced component designs has identified several distinct coating/base metal failure mechanisms not generally encountered in earlier generation turbines, in addition to the more usual degradation modes, accelerated by the increases in temperature. A review and examples of these degradation mechanisms encountered on service exposed coating systems currently used by some of today’s major manufactures are presented.
J. A. Daleo and D. H. Boone, in Risk, Economy and Safety, Failure Minimization and Analysis, edited by R. K. Penny, ISBN 90-5410-8231, pp. 187 – 201, 1996 A. A. Balkema, PO Box 1675, Rotterdam, Netherlands.
ABSTRACT The proper characterization of critical components operating in advanced gas turbine engines is important in establishing actual operating conditions, degradation modes, remaining lives, and optimum component life strategies. Metallurgical analysis of components operating in the actual turbines using these techniques provides an important and often only insight into actual turbine conditions. These include response and actual state of the materials and, probably most importantly, proper repair and rejuvenation techniques to be used. Advanced turbine components are too expensive to be "replaced" after only one cycle of operation. These evaluation techniques will be reviewed in detail together with examples of their use in the diagnoses and life assessment of particular gas turbine components. Proper use of these powerful techniques allows the turbine users to more successfully and economically operate their turbines.
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Practical Applications of Temperature Estimation Models Based on Superalloy Base Metal and MCrAlY Coating Microstructure Evolution In Service Run Gas Turbine Components
K. Hussain, J.A. Daleo, Materials for Advanced Power Engineering 2006, Proceedings of the 8th Liege Conference, Volume 53, Part II, P.737, Forschungszentrum Julich GmbH, Zentralbibliothek, Verlag, D-52425 Julich, Germany, ISSN1433-5522, ISBN 3-89336-436-6.
ABSTRACT The level of service damage accumulated in gas turbine hot section components is directly proportional to the peak and steady state metal temperatures and stress experienced in service. It is now possible to accurately estimate the actual operating metal temperature by characterizing the evolution of the base metal and coating microstructures. The gamma prime precipitate coarsening kinetics of the single crystal alloy PWA-1483 were examined over the temperature range of 845°C to 1010°C and used to construct a temperature estimation model. The model was successfully used in conjunction with temperature estimates obtained from previously developed coating/base alloy inter-diffusion models and/or solid state phase changes that occur in MCrAlY coating systems to improve the accuracy of our finite element heat transfer models that support component life and repair limit calculations. The limitations of using temperature estimation models based on gamma prime growth/rafting kinetics for single crystal alloys are also discussed. |
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Technical Publications |