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1Fretting Fatigue of Slot-dovetails in Turbo-generator Rotor(From O&M Issues Discussed in Recent EPRI Meetings)

H. Ito Toshiba Corporation

1-1-1, Shibaura, Minato-Ku, Tokyo, 105-8001 Japan

Abstract-This paper describes the fretting fatigue of slot-dovetails in turbo-generator rotor including typical examples andrepairs of fretting fatigue cracks, results of fretting fatigue tests,factors affecting fretting fatigue strength, fretting fatiguepreventive technologies, and UT inspection methods.

I INTRODUCTION

In the 70s, two 660 MW turbo-generators in Europeexperienced shaft cracking due to fretting fatigue of the slot-dovetails. The authors started the basic study of frettingfatigue of the shaft slot-dovetails in the late 70s. Inspectionresults showed that generator shafts, manufactured by theauthors, also cracked in the slot-dovetails. High-speed fatiguetest equipment was introduced to perform comprehensivefretting fatigue tests simulating actual machines. The authorssucceeded in quantitatively defining dominant factors affectingfretting fatigue strength of slot-dovetails of turbo-generatorrotor.

A fretting fatigue preventive technology was established forthe slot-dovetails based on the test results and review. Anondestructive inspection technology using UT was alsodeveloped. The fretting fatigue preventive technology hasadopted in the new machines as a standard design application.It is also used in the installed machines as refurbishment. Themachines with applied by these technologies are successfullyoperating for more than 20 years since application.

This paper introduces examples and repairs of slot-dovetailfretting fatigue cracks, results of fretting fatigue tests, factorsaffecting fretting fatigue strength, fretting fatigue preventivetechnologies, UT inspection technology, and application offretting fatigue preventive technology to the installed machines.

II EXPERIENCES OF INSPECTION, CRACKED ROTOR SLOT-DOVETAIL AND REPAIR

Fig. 1 shows experience of inspections and observed crackedslot-dovetails since 1979 by year. Nine out of 601 inspectedrotors were found to have cracks. Table 1 shows operatinghistory, description of cracks and repairs, and countermeasures.As a severe example, machine A* is described below in detailincluding crack profile, repair and countermeasure.

0

5

10

15

20

25

30

35

40

Inspection YearN

umbe

r of I

nspe

cted

Rot

or1980 1985 1990 1995 2000

; Cracked Rotor Slot-dovetail

Fig. 1 Experience of Inspection and Cracked Rotor Slot-dovetail

Table 1. Experiences of Cracked Rotor Slot-dovetail

Unit Year ofInspectionOut-put(MW)

Year inservice Description of Cracking Repair / Modification

A 1981 220 1963

Crack size: L10 x D3.2 mmCrack position: About the core centerNo. of cracks: 1Wedge hardness: Equal to shaftOperation hours: 100,000hNo. of starts and stops: 766

Grind off cracks.Increase corner radius at wedge shoulder.Replace with wedges of a lower hardness.

B 1982 350 1972Crack size (max.): L3.5 x D0.98 mmCrack position: About the core centerNo. of cracks: 10Wedge hardness: Harder than shaft


C 1982 250 1971Crack size: L2.5 mmCrack position: About the core centerNo. of cracks: 1

Clearing of Cracking with Grainder.Make taper with radius at end of wedgeshoulder.

D 1982 500 1968Crack size (max.): L6.6 x 1.5 mmNo. cracks: 55Wedge hardness: Equal to or harder than shaft


E 1983 600 1973Crack size: L4.5 x D1.4 mmNo. of cracks: 9Wedge hardness: Equal to shaft


F 1987 350 1972Crack size: L4.0 mmNo. of cracks: 3Wedge hardness: Shaft is harder


G 1990 500 1973Crack size: L4.0 mmNo. of cracks: 28Wedge hardness: Shaft is harder

Grind off cracks.Increase corner radius at wedge shoulder.

A* 1990 220 1963Crack size: L23 x D8.5 mmNo. of cracks: 2

Grind off cracks.Replace with aluminum alloy wedges.Change wedge length and arrangement.

H 1991 375 1973Crack size: L5.5 mmNo. of cracks: 3

Grind off cracks.Increase corner radius at wedge shoulder.

Two cracks were observed at the core center of the slots bythe pole on the joint of steel wedges. A larger one measured 23mm in length and 8.5 mm in depth. Fig. 2 shows the crack indetail. The cracks were ground off to suitable shapes to preventstress concentration and the surface was smoothly finished (Fig.3). Wedges were bridged over the ground off area. The wedgematerial was changed from steel to aluminum alloy.Concentration of contact pressure was avoided by increasing thecorner radius on the area where the wedge edge contacts the slotdovetails. The unit is running for 10 years since then withoutproblems.

210

8.5

Cracking

8 5

Fig 2. Detailed Cracked Rotor Slot-dovetail of A* Unit

(10)

(16)

Fig 3. Repair and Correction for A* Unit

III EXPERIMENTAL STUDIES

Fretting fatigue of turbine and turbo-generator rotors havebeen studied by many researchers for many years. Areas ofconcern are blade roots and coupling-to-shaft shrinkage fit. Theconventional fretting fatigue tests used only one pad andadjacent wedges such as in the slot-dovetails of a turbo-generator shaft were not simulated.

We developed a high-speed fatigue testing equipmentincorporating two double-pads that simulate actual arrangementsof wedges in slot-dovetails. We quantitatively evaluated factorsaffecting fretting fatigue strength of slot-dovetails. Theevaluated factors included proximity effect of pads (wedges),contact pressure, relative slippage (wedge length), pad (wedge)materials (hardness, Youngs modulus, rigidity) and repeatedstress (shaft bending stress).

We studied the fretting fatigue preventive measures andfatigue life evaluation method for slot-dovetails based on the testresults and review.Test Equipment and Method

Fig. 4 shows a schematic test arrangement of the frettingfatigue testing system. Fig. 4-(a) and 4-(b) are conventionaltesting systems. Fig. 4-(c) is the double-pad type test system,which is newly devised to define the proximity effects ofwedges. Fig. 5 shows a specimen and a pad. Fig. 6 illustratesthe testing equipment. Fig. 7 is a photograph of the test

equipment. The specimen is made of Ni-Cr-Mo-V steel, which isthe same as the shaft material. The pad is made of S55C(quenched and tempered) (AISI-1055), which is the same as thematerial of steel wedge. Table 2 and 3 show chemistry andmechanical properties of materials of specimen and pad,respectively.

Fig. 4 Schematic Illustration of Fretting Fatigue Test Method

Fig. 5 Specimen and Pad

Fig. 6 Fretting Fatigue Test Equipment

3Fig. 7 Photograph of Setup of Fretting Fatigue Testing

Table 2 Chemical Composition of Specimen and PadC Si Mn P S Ni Cr Mo V

Specimen

0.24 0.10 0.32 0.007

0.005

3.71 1.60 0.27 0.12

Pad 0.56 0.21 0.75 0.01 0.01 - - - -

Table 3 Mechanical Properties of Specimen and Pad

0.2% proofstress MPa

Tensilestress MPa

Elongation%

Reductionof area

%

HardnessHv (1.0kg)

Specimen 790 895 25.3 71.3 269Pad 634(*) 811 19.5 39.6 265

(*) Yield stress

Fretting fatigue tests were conducted in the atmospheric air atroom temperature with load control of pulsating tension at 15Hz. Pad clearance C was 0.25 and 4 mm and infinity (singlepad). Nominal contact pressure was 50, 100 and 200(equivalentto actual machines) MPa. Relative slippage of the pad edge was5 to 6 m (equivalent to actual machines).

Fatigue tests to evaluate the effect of pad materials wereconducted using the specimens and pads shown in Fig. 8 and thetesting equipment shown in Fig. 9. The specimen materials wereNi-Cr-Mo-V steels or the same as the above specimens. The padmaterials tested were 5 each ferrous and nonferrous materials asshown in Table 4.

Fatigue tests were conducted in the atmospheric air at roomtemperature with load control of pulsating tension at 100 Hz.Nominal contact pressure was 200 MPa constant.

Specimen

PadFig. 8 Specimen and Pad for Pad Materials Effect Test

Fig. 9 Test Equipment for Pad Materials Effect Test

Table 4 Mechanical Properties of Pads for Pad Materials Effect Test

Experimental Results and ReviewFig. 10 shows the effect of pad-to-pad clearance on S-N curve.

N represents the number of repetitions at the initiation of a

Contact pad 0.2 u E wfmaterial MPa MPa % % GPa MPa

3.5NiCrMoV-QT 790 895 25.3 71.3 269 205 0.7 753.5NiCrMoV-Q 1103 1660 13.7 46.4 403 207 0.7 80S55C-QT 561 933 21.5 45 255 201 0.68 75S25C-N 231 430 27.7 58.1 195 201 0.7 90SUS304 247 663 69.9 81 200 190 0.7 80Cu-Be-Ni 507 760 15.5 31.5 239 130 0.55 100Cu-Cr 380 450 26 51.2 150 120 0.65 130Cu 245 264 30 85 102 110 0.65 140A2024-T351 296 477 13.7 17.2 141 75 0.7 130Ti-6Al-4V 941 1035 15 35.5 365 115 0.68 100

Hv

0.2 = 0.2% proof Hv = Hardness u = UTS E = Youngs modulus =Elongation = Coefficient of friction = Reduction of areawf = Fretting fatigue limit (amplitude at 2x107)

4crack. Fig. 11 indicates the relationship between pad-to-padclearance and fretting fatigue limit. As Fig. 11 shows, fatiguelimit decreases sharply with the decreasing pad-to-pad clearance. Fretting fatigue limit by single pad was 40 MPa, which is 1/8.8of the ordinary fatigue limit of 350 MPa. Fig. 12 shows theinitiation and the propagation of cracks. A number of smallcracks(A) were observed on the pad edges at the initial stage offatigue. Along with the increase in the number of repetitions, thepad edges were worn out, and then the major cracks(B) initiatedand propagated at the inside of the pad edges.

Fig. 10 Effect of Pad-to-Pad Clearance on S-N Curves

Fig. 11 Relationship between Pad-to-Pad Clearance and Fretting Fatigue Limit

C=0.25mm, a=15MpaFig. 12 Fretting Fatigue Cracks Observed in Specimen

Fig. 13 shows the effect of contact pressure on S-N curve.Fig. 14 indicates the relationship between fretting fatigue limitand contact pressure. For the contact pressure up to 100 MPa,fretting fatigue limit decreases rapidly with increasing contactpressure but for the contact pressure above 100 MPa, decrease infretting fatigue limit is very small.

Fig. 13 Effect of Contact Pressure on S-N Curves

5Fig. 14 Relationship between Contact Pressure and Fretting Fatigue Limit

Fig. 15 illustrates the effect of pad contact length on S-Ncurve. Fretting fatigue life decreases with increasing contactlength.

Fig. 15 Effect of Contact Length of Pad on S-N Curves

Figs. 16 and 17 show the result of fretting fatigue tests ofvarious pad materials. Fig. 18 shows the relationship betweenfretting fatigue limit and hardness of pad materials. Generally,fretting fatigue limit decreases rapidly with increasing hardnessof pad materials. In the high hardness region, however, padmaterials of the same hardness do not have an identical frettingfatigue limit. Youngs modulus seems to be another factorsaffecting fretting fatigue limit in addition to hardness.

Fig. 16 Effect of Pad Materials (Ferrous) on S-N Curves

Fig. 17 Effect of Pad Materials (Non-Ferrous) on S-N Curves

Fig. 18 Relationship Between Fretting Fatigue Limit and Pad Hardness

6IV IMPROVEMENTS IN DESIGN AND PRODUCTION

Fretting fatigue preventive technologies have beenestablished based on the results and review of the extensivefretting fatigue tests simulating actual machines as described inChapter III. The preventive technologies are summarized below.

(1) Fretting fatigue strength decreases rapidly with decreasingpad-to-pad clearance. As a countermeasure, the area of thewedge edge that contacts slot-dovetails was tapered topractically increase the wedge-to-wedge clearance and thecorner radius of the wedge edge was enlarged to prevent theconcentration of contact pressure.

(2) Fatigue life decreases with increasing wedge contact length.As a countermeasure, several different wedge lengths in usewere unified into the shortest length. Furthermore, longwedges of a whole length or several split lengths wereadopted with wedge joints to avoid meeting the rotor corecenter.

(3) Fatigue life decreases with increasing hardness, rigidity andYoung modulus of the wedge. As a countermeasure, wedgematerials of a lower hardness than shaft materials wereselected when using ferrous wedges. Analysis of main fluxdistribution indicated that no operational problem occurswhen using nonmagnetic wedges in the slot by the pole. Inview of this finding, aluminum alloy wedges may be used inall slots in some cases.

Residual stress in the shaft is one of the known factorsaffecting initiation and propagation of fretting fatigue cracks inthe slot-dovetails. Residual stress originates at the stage ofshaft forging production and in the process of slots making.The former is controlled to a level of several kg/mm2 or belowby improving steel making process in the shaft forgingproduction or by stress relieving annealing after steel making.For the latter, thickness of the residual stress layer is controlledat several tens m or less by using carbide cutter and improvingthe slots making process.

V INSPECTION AND REFURBISHMENTInspection

Fretting fatigue cracks in the slot-dovetails are easily detectedby magnetic particle or eddy current inspection when thewedges are removed. When the wedges are in place into slots,ultrasonic tests are used but testing accuracy with standardprobes is often insufficient because of the short distancebetween the rotor surface (inspection surface) and the crack andinclination of slot-dovetails relative to the rotor surface.

We developed a special probe featuring an optimum angle ofrefraction and frequency. Model teeth simulating actual shaft

teeth were prepared and artificially flaws to confirm theaccuracy of detection. It was confirmed that the probe detects aminimum of 1.0mm deep defect. Fig. 19 shows the special probeand the test using the model slot-dovetail.

We recommend UT inspection of slot-dovetails at majoroutage (with the rotor taken out). 361 turbo-generator rotors ,asretaining rings and wedges installed, have been inspected by UTinspection at site so far.

Fig. 19 UT Probe for Shaft-Dovetail

RefurbishmentPermanent countermeasures against fretting fatigue cracking

introduced in Chapter IV were applied to approximately 150installed generators so far. No crack event is reported, in thefollow-up research, since early 1990s when the refurbishmentwas almost completed. This means that the validity ofprevention measures has been confirmed.

VI CONCLUSION

In the 1970s, it was reported that generator shafts werecracked due to fretting fatigue in the slot-dovetails. We studiedthe countermeasures and have developed fretting fatiguepreventive technologies and UT inspection technology. Thesetechnologies were applied to new and existing machines as astandard design application and refurbishment. The machinesare running for more than 20 years successfully since then.

Thermal power turbo-generators are, however, running underincreasingly diverse and severe operating conditions in recentyears. It is not rare that units are operated over the originaldesign conditions. Fretting fatigue life of slot-dovetails may beaffected also by operating conditions. We will keep close contactwith users to prevent fatigue damage and contribute to stablepower supply.

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