COATINGS FOR THE PROTECTION OF TURBINE BLADES FROM EROSION

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y. 117 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is pub lished in an ASME Journal. Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright 1993 by ASME 93-GT-291 COATINGS FOR THE PROTECTION OF TURBINE BLADES FROM EROSION P. N. Walsh, J. M. Quets, and R. C. Tucker, Jr. Praxair Surface Technologies, Inc. Indianapolis, Indiana TABLE I. Characteristics of Coatings ABSTRACT Many types of turbines, including aircraft gas turbines, steam turbines, and power recovery turbines, suffer from solid particle erosion caused by a variety of materials ingested into the machines. Utilization of various laboratory erosion tests tailored to the specific application by using various erodents, temperatures, velocities, and angles of impact, have been shown to be effective in the development and selection of coatings for the erosion protection of turbine blades and other components. Detonation gun coatings have demonstrated their efficacy in providing substantial protection in many situations. It has now been shown that several tungsten carbide and chromium carbide Super D-GunTMl coatings not only have better erosion resistance than their D-Gun analogs, but cause little or no degradation of the fatigue properties of the blade alloys. Nonetheless, caution should be employed in the application of any laboratory data to a specific situation and additional testing done as warranted by the turbine designer. 1Super D-GunT is a trademark of Praxair S. T. Technology, Inc. Deposition Density % Strain Coating Method Composition g/cc to fractur UCAR WT-1 D-Gun 17 Ni [(W,Ti)C] SDG 2 Super 86W-12Co-2C D-Gun UCAR LC-1H D-Gun [8Ni2OCr] [Cr 7 C 3 ] SDG 27 Super 15 [IN 718] D-Gun + 85 [Cr 5C 3 ] Introduction Erosion by solid particles is a problem common to the operation of many types of turbines. For example, aircraft gas turbine compressors ingest a variety of debris from runways,' the initial stages of steam turbines suffer from scale that spalls from the heat exchanger tubing,' and power recovery turbines are subjected to entrained process materials such as catalyst particles or blast furnace effluent 3 The resulting erosion causes a loss in efficiency due to a reduction in chord width, tip recession, or surface roughening. It may also reduce service life, increase down time, cause unscheduled shut downs, and increase replacement or repair costs. The overall cost to industry is extremely large. Typical blade and vane alloys, whether they are aluminum, titanium, steel, or superalloys, do not have adequate erosion resistance. Moreover, because of structural, mechanical and thermal property requirements, it is unlikely they they ever will have. A solution to the problem for many applications has been the use ofcoatings. For a coating to be useful, however, Presented at the International Gas Turbine and Aeroengine Congress and Exposition Cincinnati, Ohio May 24-27, 1993 This paper has been accepted for publication in the Transactions of the ASME Downloaded From: Discussion of on it 11/1/18 will be accepted Terms of at Use: ASME Headquarters until September 3,1993

2 I.8 * v v.6 * -> N * -' W.5 N.4 ^ M.3 O Figure 1: Microstructures of coatings: Clockwise from top left UCAR WT-1, SDG 2, SDG 27, UCAR LC-1H. The SDG 27 is 3 µm thick. Q.2 ***** Ti with SDG 2 coating with UCAR WT-1 coating. 1 4 s s 1 s z s 1 e z s 1 r : it must not only be substantially more erosion resistant than the bare blade alloy, but it must also be stable in the service environment (i.e., be thermally stable, corrosion resistant, etc.) and not detrimentally affect the performance of the substrate (e.g., not significantly reduce the fatigue strength of the blade). Detonation gun and Super D-Gun coatings based on tungsten and chromium carbides have been shown to satisfy these requirements for a variety of applications. Several of these coatings and applications are disscribed in this paper..8 Figure 2: High cycle tension-tension (R =.1) fatigue S/N curves for stress relieved Ti-6-4 specimens coated with SDG 2 and UCAR WT-1 coatings. Coatings Detonation gun coatings 4 are produced by introducing.6 a small amount of powder as a pulse into an explosive gas Oa' mixture of oxygen and acetylene in a long gun barrel and then N detonating the gas. The resulting detonation wave moving 1 down the barrel heats the powder particles to near their melting W point and accelerates them to a velocity of about 75 m/s. The.4 molten or nearly molten particles impacting on the surface to be coated form a very dense, well bonded coating. For more than three decades detonation gun (D-Gun) coatings have been the hallmark of thermal spray coatings, largely because of the Q extremely high velocity.2 and well controlled temperature ^' imparted to the powder particles. Recently, a new family of ***** Ti coatings has been developed using Super D-Gun4 technology. with SDG 4 coating By changing the gases used in the detonation process and other with UCAR LW-1N4 coating modifications, the powder particle velocity has been increased. to about 1 m/s with a concommitant near doubling of the z s s e s 1' kinetic energy. The result is coatings that are even more dense ' and more strongly bonded. An additional benefit achieved with Super D-Gun coatings is a unique control of the residual stress in the coatings. While most thermal spray coatings have an asdeposited residual tensile stress as the result of the thermal contraction of the cooling particles on a cold substrate, well Figure 3: High cycle tension-tension (R =.1) fatigue S/N curves for stress controlled compressive residual stresses can be developed in relieved Ti specimens coated with SDG 4 and UCAR LW-1N4 coatings. Super D-Gun coatings. 2

3 (. V).8 W TABLE II. Characteristics of Erosion Tests Temp Velocity Test Target Erodent "C m/s Site Application Ref µm Univ. of Steam 8 chromite Cincinnati Turbine 5µm alumina PSTI Lab Aircraft Compressor -> - 27µm alumina PSTI Lab Power Recovery M_.4 '2. 1' z ***** 434 steel, peened with SDG 4 coating with UCAR LW -1 N4 coating s 1s z s 1s z s 1z Figure 4: High cycle tension-tension (R =.1) fatigue S/N curves for peened 434 steel specimens coated with SDG 4 and UCAR LW-1N4 coatings. A wide variety of ceramic, metallic and cermet coatings have been made; those of primary interest here are tungsten carbide - cobalt, (tungsten,titanium) carbide - nickel, chromium carbide - nickel chromium, and chromium carbide - Alloy 718. The compositions and some selected properties of these coatings are shown in Table I. Representative micrographs are shown in Figure 1. As noted earlier, of particular concern are the effects of a coating on the properties of the substrate, especially the fatigue properties when rotating turbine components are to be coated. The effects of several D-Gun coatings are compared with their Super D-Gun counterparts in Figures In these figures, the number of cycles to failure in a tension-tension fatigue test is plotted against the maximum tensile stress applied, in comventional S/N plots. It can readily be seen that while the D-Gun coatings caused a significant reduction in the fatigue properties of the titanium and steel substrates tested, a characteristic that had to be taken into account in the use of these coatings, the Super D-Gun coatings caused no such degradation, in fact improving performance in a few instances. (It should be kept in mind that coatings on test specimens may differ somewhat from those on actual parts due to geometry and other effects so additional testing by users may be needed.) noted by Gill and Quets (1992) with regard to aircraft gas turbine compressors. Table II contains the parameters for the erosion testing used here as well as the target applications. Greater detail on the test procedures can be found in the references listed in Table II. Steam Turbine Solid Particle Erosion --- The most relevant testing to simulate the erosion caused by the exfolliation of magnetite boiler scale in steam turbines was found' to be that using sub-1 micron iron chromite (Fe2Cr2O4) particles at impact angles of 15 to 3 degrees. Tests at 55C and 35 m/s (close to the turbine conditions) were run in a test apparatus at the University of Cincinnati under the direction of Prof. W. Tabakoff. Coatings consisting of a mixture of chromium carbide with a nickel chromium alloy (D-Gun) or Alloy 718 (Super D-Gun) were compared with bare Type 422 stainless 2C Ratio to 422 c. 1E 1s Testing for Specific Erosion Applications A major problem in any practical materials development program is the simulation of the service environment in the laboratory with sufficient accuracy to at least relatively rank materials in a meaningful manner. This problem seems to be particularly acute in the field of erosion. Materials react differently, not only as a function of the angle of impact, but also as a function of temperature and the composition, size and shape of the erodent. This issue was explored extensively by Walsh (1992) with regard to the erosion of steam turbines and 15 degrees 3' Figure 5: Erosion resistance of heat-treated coatings relative to that of tune 422 stainless steel at two impingement angles. Measured at 55 C, with -1 micron chromite impinging. 'Plasma" indicates the Cr 3C2 + FeCrAIY plasma coating. In this view, higher values correspond to greater erosion resistance.

4 Ratio to 422 ss 3 ErOSion. uccicn heat treated as coated degrees 9 degrees Figure 6: Erosion resistance of coatings relative to that of We 422 stainless steel at impingement angle. Measured at room temperature with -1 micron chromite impinging. "Plasma" indicates the Cr3C2 + FeCrA1Y plasma coating. Figure 7: Erosion rates of coatings for aircraft gas turbine compressor protection. Measured at room temperature with 5 micron alumina impinging. In this view, lower values correspond to greater erosion resistance. steel, the blade alloy, and a plasma-deposited composition developed by General Electric Company under contract to the Electric Power Research Institute (EPRI). 1 The coatings were tested as-deposited and after heat treatment in air for 72 hr at 55C, a temperature close to the turbine operating temperature and required in the EPRI coating prescription. The results for the heat-treated coatings are shown in Figure 5. In-house room temperature tests at 49 m/s with the same erodent were also run, Figure 6. The lower erosion rates measured for the D-Gun coating, UCAR LC-1H, is consistent with service experience of its effectiveness. The even better performance indicated for the Super D-Gun coating, SDG 27, is currently being evaluated in field tests. Aircraft Gas Turbine Compressor Erosion --- The erosion caused by runway debris in the compressor section of aircraft gas turbine engines can be reasonably simulated using nominally 5 micron diameter alumina particles at low angles of impact. A D-Gun coating of tungsten,titanium carbide - nickel (UCAR WT-1) has been extensively used on titanium compressor blades in the lower temperature stages of compressors and was compared with a new Super D-Gun coating of tungsten carbide - cobalt (SDG 2) in a laboratory test at 49 m/s at room temperature. The results are shown in Figure 7. Based on these results and similar results in engine testing, combined with the very favorable fatigue benefits described previously, the new coating has supplanted the old in some applications. In the higher temperature stages of compressors, the oxidation resistance of the tungsten carbide based coatings may be inadequate. The chromium-carbide-based coating UCAR LC-1H, discussed earlier in connection with steam turbines, has been shown to be very effective in this regime. Power Recovery Turbine Erosion --- The coating required for a given power recovery turbiine is, of course, a function of the erodent and the environment. An example is the use of UCAR LC-1C, a D-Gun chromium carbide - nickel,chromium analog of LC-1H, already described. Here, however, the erodent is better simulated by a fine alumina, Figure 8. Even though this coating continues to provide excellent service, an improvement is being sought with a Super D-Gun analog, also shown in Figure 8. In another power recovery turbine application, turbines extracting energy from blast furnace off-gas, a tungsten carbide - cobalt coating (UCAR LW-1N4) has proven effective. Nonetheless, Figure 9 indicates that further improvement might be obtained by using SDG 4, a Super D-Gun analog. Conclusions The utilization of various laboratory erosion tests tailored to the specific application by using various erodents, temperatures, velocities, and angles of impact, have been shown to be effective in the development and selection of coatings 4

5 1 Erosion, Ogrn 1 Erosion, imlgrn degrees 9 degrees 3 degrees 9 degrees Figure 8: Erosion rates of chromium carbide coatings for power recovery turbine protection. Measured at two angles at room temperature with 27 micron alumina impinging. Figure 9: Erosion rates of tungsten carbide coatings for power recovery turbine protection. Measured at two angles at room temperature with 27 micron alumina impinging. for the erosion protection of turbine components. Examples have included coatings for blades in steam turbines, the compressor sections of aircraft gas turbines, and power recovery turbines. It has been shown that several tungsten carbide and chromium carbide Super D-Gun coatings not only have better erosion resistance than their D-Gun analogs, but cause little or no degradation of the fatigue properties of the blade alloys. Caution should be used in the application of any laboratory data to specific hardware designs and additional testing done if warranted by the turbine designer. REFERENCES 1. Tucker, R. C., Jr. and Nitta H., 1988, "Detonation Gun Coatings - Coating Characteristics and Applications,' ATTAC'88, Osaka, Japan, May McCloskey, T. H., and Bellanca, C., 1989, "Minimizing the Effects of Solid Particle Erosion in Utility Steam Turbines," in Solid Particle Erosion of Steam Turbine Components: 1989 Workshop, EPRI GS Wolektarski, W., 1982, Proceedings of the 11th Turbomachinery Symposium, Texas A&M university, College Station, Texas 4. Tucker, R. C., Jr. and Nitta H., 1992, "Detonation Gun and Super D-Gun Coating Processes," J. High temp. oc. Japan, Vol. 18 Supplement, pp 4 5. Cox, L. C., 1988, "The Four Point Bend Test as a Tool for Coating Characterization," Surface and Coatings Technology, 36, Gill, B. J., and Quets, J. M., 1992, "Wear and Fatigue Resistant Coatings from the New Super Detonation Gun Process," presented at SURFAIR, France 7. Walsh, P. N., 1992, "Erosion Resistance of Coatings at Steam Turbine Temperatures," in Steam Turbine-Generator Developments for the Power Turbine Industry, Vol. PWR 18, ASME, New York 8. Tabakoff, W., and Wakeman, T., 1979, "Test Facility for Material Erosion at High Temperature," ASTM Special Publication 664, pp ASTM G 76-83, "Practice for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets" 1. Wlodek, S. T., 1987, "Erosion Resistant Coatings for Steam Turbines," EPRI CS-5415, Electric Power Research Institute, Palo Alto, CA 5