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Rejuvenation of Used Turbine Blades by Hot Isostatic Processing 57

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Rejuvenation of Used Turbine Blades by Hot Isostatic Processing 57 View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Texas A&M University REJUVENATION OF USED TURBINE BLADES BY HOT ISOSTATIC PROCESSING by G. VanDrunen, Ph.D. Material Development Engineer and J. Liburdi Manager, Metallurgical Section Product Assurance Laboratory Turbine and Generator Division Westinghouse Canada Limited Hamilton, Ontario, Canada Dr. Gary VanDrunen received his calculating blade life, the designer has to rely on extrapolation Ph. D. in Metallurgical Engineering in of relatively short term (100 to 10,000 hours) test data and the 1972 from Queen's University in Kings­ application of a suitable safety factor both of which are subject ton, Ontario. His thesis work was con­ to considerable variance. Given these inherent design limita­ u cerned with the deformation of nickel­ tions, it is difficult to predict the exact life of a t rbine blade r cobalt alloys. He was awarded a National without periodic monitoring of the creep p operties to deter­ Research Council of Canada post­ mine the extent of service damage and the·r,emaining life. As a doctorate fellowship at the University of result of such test programs, extensive data has been generated on the service behaviour of gas turbine blading which permits a British Columbia during 1973 for con­ l ducting research on heat-flow and prod- realistic assessment of remaining blade ife. Almost all blade • uct quality in continuously cast steels. alloys deteriorate during service; howeve�:, there is generally He joined the Turbine and Generator Division of Westinghouse considerable scatter among the various units, making it difficult Canada Limited in 1974 as materials development engineer. to establish any definitivelife limits without beiqg unduly con­ servative. Usually, when the creep ruphire )ife of a serviced Dr. VanDrunen is a member of ASM and AIME and is a blade falls below the specified minimum life for a new blade registered professional engineer (Ontario). the blades become suspect and recommendations are made t� Joseph Liburdi graduated in 1967 from either replace or reheat treat. However, neither of these so­ i the University ofWindsor with a BASe in lutions is completely satisfactory because �;ep acement blades Engineering Materials. After graduation, are expensive and reheat treatment has not always proved to be he joined Westinghouse Canada Limited satisfactory. Therefore, a development was undertaken to and in 1974 was appointed manager of apply hot isostatic press (HIP) technology to the recovery of creep damage in used turbine blades. Actual serviced blades the Metallurgical Section, Product As­ � surance Laboratory, Turbine and were used in the program which culminated by pro essing a Generator Division. complete set of blades for return to engine service. Mr. Liburdi is a member ofASM and is a registered professional engineer (On­ Nature of Service Damage in Turbine Blades tario). Two distinct types of damage are recognized, surface dam­ age and internal degradation. Surface damage may he due to either mechanical impact or corrosion and is generally confined ABSTRACT to the blade airfoil. In both cases, light damage can be removed The creep properties of gas turbine blades will deteriorate by blending or dressing. Blades with severe surface damage or with prolonged service exposure. This deterioration is primar­ cracks are scrapped. ily due to internal microstructural changes and the formation of The primary concern of the present work is internal dam­ creep voids or cavitation. Development work has shown that age. Microstructural changes, resulting from extended expo­ HIP processing is the best available treatment for recovery of sure at high temperature under stress, are responsible for a properties in used blades. The results obtained on a complete reduction in mechanical properties. Three forms of internal engine set oflnconel X-750 blades indicate that HIP processing degradation have been verified; precipitate. coarsening or over­ is capable of restoring new or near new creep resistance to used aging, changes in grain boundary carbides and cavitation or blades. void formation. A considerable fraction of the intermediate temperature INTRODUCTION strength of nickel-base turbine blade alloys results from the ' The hot gas path components in land base gas turbines are fine y' precipitate Ni3 (AI, Ti). The y particles generally coar­ generally designed for a theoretical life of 100,000 hours with sen according to a time to the 1;3 power growth law with a creep resistance being the primary limitation for blades. In corresponding decrease in strength. Also grain boundary car- 55 .56 PROCEEDINGS OF THE SLXTH TURBO!IIACHINERY SYMPOSIUM bide morphology and amount can change with time. Since the lowed hy controlled precipitation at loiVer temperatures. \\'ith alloy heat treatments which cause carbide fiJrmation arc gener­ regard to creep voids, it is not dear if cavitation damage can he . ally optimized f(l!· short term properties, long term changes in removed lw com·cntional heat treatment. Davies l't a! [.'5, 8] cm·hicle structure are usually detrimental. particularly with re­ report that in Nimonic 80A test bars, cavitation can be ''sin­ spect to such propertivs as ductility and notch sensitivity. Cavi­ tcrecl ont" at 750°C if the test is interrupted dnring secondary tation represents the initial step in creep Llilure. It mnsists of c-reep; a test har reheat treated in early tertiary creep htiled the nucleation and growth of voids on grain boundaries. \ Vith "prematurely" after three recoverv treatments. All of the � cracks. Cavitation has specimens tested bv Davies et al exhibited tmnstwllv low duc­ time, the isolated v Jids link 11p to f(mn · been reported in test specimens tested for long tinws as inter­ tility (appro'>imatd}· 2'X) ftn· a forged N i-hase allov . This mav mediate stress levels [ 1 ,2.:3,4]; i.e., typical turbine bl.ade have been clue to the usc of a non-standard initiai heat treat­ operating conditions. In somv hut not all cases [:'5], the voids ment for their material. Hart and Gayter [7] studied the dfcct have been observed directly, although their presence has lwcn of complete rclwat treatment cycles on spcciml'ns of Nimonic i 75. Nimonic 90, l\'imonic ](),")and lncolov inferred hom experimenta evidence. Jt appears that material - 800. Some Nimonic has to be near or in tertiary creep hd(H·e cavitation can rcadil) 75 test bars exhibited incn'ascd stress nr]Jtnrc life as a result of be detected by optical microscopy. reheat treatment, while in other bars the total life was appreci­ ably decreased. In the c·ase of Ninwnic YO, reheat treatment It is not known to what extent each of the above mecha­ resulted in considerably inneased total life; however, ductility nisms contributes to turbine blade degradation during service. was generally reduced. Test bars gin'n repeated rclwat treat­ It is also probable that each alloy will respond difl('rcntly to a ments indicatecl that life improvements could not bt' achieved particular tempnaturc stress combination. Figure l shows the iJ](leflnite]y; each successive cycle resulted in a smaller im­ typical variation in stress mpturc life determined at 73.'5oC provement. The response of Nimonic HlS was silllilar to (l3.'50°F) with scrvic·e time for {(Jrgecllnconcl X-7'50 blades. The N imonic 90. He beat treated I ncoloy 800 test hars vxhibited data hH· the machines 11as taken Jrorn ivlc(\tll [6]. It should he only slightly increased life over specimens tc'sted directly to noted that the tests conducted by us vvere conducted at .34!'5 his standard reheat M Pa (50 Ksi), while McCall's data resulted from tests at :3!0 f�1ilure. 'dcCall [6] reports experic•nc<' \l·ith trcatmt'nt of used Inconcl X-750 turbim' blades. His results arc \JPa ( 4:) KsiL Both data sourcc·s display l'Onsidcrablc scattn; Sllpcrimposed ou Figure I. Although thnc is a distinct im­ some is umlonhtccllv due to variations in initial (new) material provement, there is a continual decrease in stress mptnre life properties and difl<,n'nCTS in machinv operating conditions. li·mn new blad." properties. Additional reasons f(>r large scatter are that the data inch1dt,S values from both \VG2 row :) blades and \\'82 rows 4 and 5 The above results suggest that normal reheat treatment blades. Figure 1 indicates that. after 40,000 hours service. can partially reston' blade propntics; howcvn, it docs not ahout half of thv blades cvalua!<'d could not nwt'l the new appear to be ('apahl<:of fiJI\ pmpcrty recovery even though thv blade stress ruptmc specification value of l 00 hours at :H5 microstructures arc comparable to new blades. This implies MPa/7:3!'5''C. The blades which were sclt,ctcd l<n thl' HIP re­ that cavitation may lw present and was not removed by con­ juvenation program lwei !wen in service for approxinwtcly V<-'ntional rdwat treatment. Au alternative to cnsun' \'oid re­ 4!'5,000 hours and cxhibitt'd the lowest life of all blades tested nwval is hot isostatic press (HIP) processing which has dcmon­ (r'igure II (L'S to :35 hours as rcceiv('d). slratc·d its ahili ty to rcmm•e even gross internal shrinkag c porosity in investment castings. as illustrated i11 Figure 2. To evaluate the potential of HIP processing I(Jr used blades. a Bladr Hestoration Altcnwtiu.\· preliminary studv was conducted on sampks removed from the Of the three types of internal Sl'rvicc damage discussed in same blades and snbjedcd to either cmiVt'ntional reheat treat­ the previous section, precipitate coarsening and changes in !l1('llt or HIP treatment. grain boundary carbit!cs should gem'rallv he reversible by con­ Tlw results of this work, presented in Figmc :1, dearly ventional heat trcatmt'nt involving complete solutioning hJI- show that the HIP proet,sscd material is superior to both com­ mercial and laboratory conventional reheat treated material.
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