ML20086B331
| ML20086B331 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 10/31/1974 |
| From: | Brass E, Darrell Roberts, Rosenberg R PUBLIC SERVICE CO. OF COLORADO |
| To: | |
| Shared Package | |
| ML20086B283 | List: |
| References | |
| NUDOCS 8311170167 | |
| Download: ML20086B331 (26) | |
Text
{{#Wiki_filter:,. . ., . _ __ -. - _ _ . . _ _ _ _ . i O O. I . ..- , i t l l l ' FORGED'INCONEL 718 PELTON WHEELS t i FOR THE FORT ST. VRAIN CIRCULATORS l 1 1 i i i 1
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i 4 PREPARED BY: i . EDWARD A. BRASS
- 1 i DAVID I. ROBERTS -
l : 4 RICHARD ROSENBERG
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1 l O O I. SUMMRY , The use of wrought material for the manufacture of the FSV Pelton wheels will avoid the predilection of the cast alloy to develop i incipient fatigue cracks of the type found on two Pelton wheels. (See Reference 1.) The Fort St. Vrain (FSV) circulator Pelton wheels, now provided as Inconel 718 precision castings, can be successfully machined from forged Inconel 718 billets. In comparison with the cast counterpart of the alloy, wrought Inconel 718 offers both higher mechanical properties and improved fatigue fracture resistance. Fabrication of machined-from-forged-billet Inconel 718 Pelton wheels to be used as replacements for the precision cast units now installed in the plant, is now proceeding. II. INTRODUCTION As reported in Reference 1, flourescent penetrant inspection of an in-service Pelton wheel uncovered the presence of heretofore un-detected incipient fractures in several of the Pelton buckets and ) in the roots of the Curvic coupling teeth (see Figure 1). A check of Pelton wheels used in the development programs found ont other , occurrence. In this case, the incipient fractures were in the curvic coupling area or.ly. - A tho'.ough program of analysis, experimental testing and material l characterization developed the following conclusions: I 2
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4 P R T o A.f B U C I G t S a 9 F/GURE' /
I 0 0 l 1 The cast version of the Inconel 718 alloy has low l
. fatigue fracture res'istance, particularly in the presence of a high steady-state stress;
- 2. The cracks were propagated by high cycle fatigue loading;
- 3. The cracks developed and propagated in areas of high tensile stress loading, but quickly became self-arresting as the fractures progressed to less highly stressed areas;
- 4. The cracks in the Pelton buckets were caused by the bending stresses imposed by the force of the water jet amplified by the dynamic effects of super-imposed loading pulses;
- 5. The cracks in the curvic coupling area were pro-pagated by a low amplitude alternating stress. The source of the excitation is most probably due to disc vibrations caused by the abnormal steam conditions associated with the operation at the Valmont test facility.
After identifying the low fatigue resistance predilection of the cast Inconel 718 alloy, a survey was made of potential sources of machined-from-wrought-billet Pelton wheels. The requirement was
- established that the final surfaces were not to be produced by either Electrical Discharge Machining (EDM) or Electro-Polishing to avoid adversely affecting the fatigue resistance of the wrought material. '
l As a result of the survey, the Contura Corporation of Brea,- California, was selected to machine the Pelton wheels from forged l
1 O O Inconel 718 billets obtained from Airco-Viking of Verde, Nevada. The selection of Contura was based on their prior accomplishments with similar fabrications and confidence in their being able to succeed with the FSV Pelton wheel machining task. The overall fabrication plan covering forging procurement, non-destructive material testing, general machining, Pelton bucket profiling, Curvic coupling machining, and balancing and final inspect-ion is targeted at having four replacement units available by January 3, 1975. III. DESCRIPTION The detail drawings of the machined-from-forged-billet Pelton wheel are shown in Figures 2 and 3. In addition to the material change from precision-cast Inconel 718 per AMS 5383 to Consumable Electrode Vacuum Melted (CEVM) Inconel 718 forged material per AMS 5662, the following changes have been made:
- 1. The non-functional material in the base of the Pelton bucket splitter (site of the previously observed cracks) has been removed as shown in the indicated area in Figure 3. This minor alteration to the -
geometry in the most highly stressed Pelton bucket , cross-section can be expected to improve the overall fatigue fracture resistance of the structure by eliminating a probable site of crack initiation. ~
- 2. The Pelton buckets will be shot peened after fir:al heat treatment and machining. The shot peening is done to further improve the fatigue fracture resistance of the part by developing favorable ccmpressive stresses
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4 IV . ' FORGING DESCRIPTION AND PROPEP, TIES The forgings, from which the Pelton wheels are machined are 7" dia. x 7" long cylindrical pieces made individually by hot cross-forging larger stock. The forgings are purchased to meet the requirements of #ts 5662C. The starting stock for the forgings is obtained by the forging vendor in the form of 16" diameter, consumable-electrode vacuum-arc remelted ingots. The ingots are then heated to 2000*F and hot forged, by means of a 1500 ton open die press, to 12" octagonal billets. These billets are then cut to produce pieces weighing 115 pounds $ which is the weight of the finished forging. Fol-lowing this operation, the billets are reheated to 2000 F and reduced in diameter to 10" by further hot forging. At this point, the forgings are inspected for cracks. .The 10" diameter billet is then hot upset to a 25" to 30" diameter. It is then rotated 90 degrees to the disc axis and pressed to its final diameter of 7". A schematic representation of the forging process is shown in Figure 4. The forgings are then solution heat treated for one hour at 1750 F. , Test coupons representing the forgings are aged and mechanically tested in accordance with the requirements of AMS 5662C. - Subsequent to machining of the Pelton wheels from the forgings, they are aged. At this time additional samples, cut from the forgings and aged along with the Pelton wheels, are mechanically tested. In addition, samples cut from the forgings are aged and
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U O O mechanically tested at GA to spot check material properties and heat treatment adequacy. The results of vendor mechanical property tests and spot check tests performed at GA on the early forgings are shown in Table 1. In addition, metallographic specimens from the forgings have been examined and have confirmed that the forging processes used are producing average grair, sizes of four or finer as required by AftS 5662C.
- v. WROUGHT If!CONEL 718 MECHAflICAL PROPERTIES The fatigue properties of wrought Inconel 718 have been the subject of considerable study over the years and appreciable amounts of data are available (References 2 and 3). Much of the data is
~ based on thin sheet-material. Since it is known (Figure 5) that the fatigue behavior of Inconel 718 is grain size sensitive, cau-tion must be used in relating the properties of the Pelton wheel forgings to the behavior of thin sheet.
Based on data obtained from Huntington Alloys shown in Figure 6, it is conservatively expected that the -forged Pelton wheel material 8 will exhibit a 10 cycles endurance limit of approximately 45,000 PSI. I a l The Modified Goodman Diagram for the wrought Inc'onel 718 material, I based on the 45,000 PSI endurance limit and 185,000 PSI ultimate tensile stress, is shown in Figure 7. '. The fracture toughness of wrought Inconel 718 forgings in relevant I heat treatment conditions is indicated in Figure 8 (Reference 2). t
C TABLE 1
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' TYPICAL MECHANICAL PROPERTY TEST RESULTS ON PELTON WHEEL FORGINGS .
Heat No. Specimen No. Tested By UTS(ksi) YS(ksi) %,Elong. % R/A Av' Grain' Size (f -'4) 9381 1 Vendor 203.3 160.9 20 29.5 2 GA* 200.4 167.7 16.5 20.2 5-6 3 GA* 186.6 156.8 20 26.2 4-5 8-3938 1 Vendor 207.3 161.0 21 27.3 ,- [, AMS 5662C - Longitudinal 185 min. 150 min. 12 min. 15 min 4 5 Long transverse 180 min 150 min. 10 min. 12 min. O Specimens in short trhnsverse orientation. W
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- 0. O A comparison of the expected or measured properties of the wrought Pelton wheel forgings to 'the measured properties of the previously used castings is shown in Table 2.
VI . STRESS Af!ALYSIS ,
- 1. Pelton Buckets - The extensive program of analysis and test performed as part of the investigation into the cause of the
~
incipient fractures discovered in the base of the S/fl 4 Pelton buckets led to the following conclusions (Reference 1): j e The fractures were caused by the high cyclical fatigue stress produced by the impinging water loads. e The actual stress level produced by the water jets depends on dynamic considerations (superposition of
. pulses on the . vibrating bucket) which become in-creasingly important at higher rotational speeds, e Although the S/N 4 Pelton wheel had been used at high speeds during the plant check-out program, actual water operation could be limited to a maximum speed of 7000 RPM and meet all safety and operational requirements.
At the 7000 RPM (depressurized reactor) design point, the , stresses in the most highly stressed bucket cross-section are as follows: e Steady-state uniform centrifugal stress of 2500 PSI. e A bending stress of 8500 PSI increased by a. dynamic factor of 1.10 for a total (1/2 amplitude) 7.lternating stress of 9400 PSI. 0 0
, TABLE 2
'COMPARIS0N OF PROPERTIES OF INCONEL 718: WROUGitT'VS. CAST i
Property . Wrought Cast Ultimate strength (ksi) 180(min)II) 125(min)(4) 185-203(2) 115-119(2) Yield strength (ksi) 150(min)(I) , 110(min)(4) i 156-168(2) 88-97(2) Endurance limit (ksi),108cycles 45(I') 23(5) Elongation % 10(min)(I) 5 (min) (4)
, 16.5-21(2) 7-10(2)
Allowable stress t' 60 ksi i31(6) 7.5(5)' meanstress (ksi) Fracture toughness: Kic (ksi (iii) 90(3) 60(2) 6 AMS 5662C Measured values ., Reference 2 - AMS 5383 Extrapolated from measured data Based on Figure 7
{ O O NOTE: After the load pulse produced by the water
. impingement is removed, the bucket is assumed to go into a free vibratory mode corresponding to-the first flexural resorant
, frequency. The half amplitude of the vibra-tion is conservative.ly assumed to correspond to the maximum bending stress, with a mean valve equal to the steady-state stress. The 9400 PSI alternating stress /2500 PSI steady-state stress is shown on a modified Goodman Diagram for both the cast and wrought Inconel 718 in Figure 9. The safety factor, i.e., allowable alternating stress divided by the actual alternating stress, is 4.7 for the forged material in com-parison to the safety factor of 2.4 available with the cast alloy. The improved margin afforded by the wrought alloy will provide additional assurance of consistently trouble-free service life for the Pelton wheels and increased tolerance for any unexpected occurrences that might other-wise adversely affect the durability of the unit.
.2. Curvic Couplina - The analysis in Referer.ce 1 indicates that the cracks developed in the curvic coupling fillet radii on wheels 3 and 4 were very likely caused by a relatively low anplitude vibratory stress superimposed.on s high 2,teady-state stress. ,
Figure 10 shows the results of the finite element analysis
~
of the basic loads that exist in the curvic coupling area. Both the bearing load on the tooth flanks (53), and the. - hoopload-(32), produce a tensile stress in the fillet area (a). f
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m 0 0 The normal loading on the tooth flanks due to the tension- .
, ing of the clamping bhlt (see Figure 11) is 42,500 PSI.
The primary source of hoop load in the curvic coupling is due to the forced strain induced by the centrifugal load in the turbine disc (see Figure 12). The fully interlocking behavior of the curvic coupling teeth prevents relative radial motion of the joined parts so that the disc strain is transferred directly to the Pelton wheel side of the joint. At the maximum normal circulator speed of 10,000 RPM, the hoop stress is the curvic coupling is 11,100 PSI. The fillet stress produced by the combination of tooth flank and hoop load is, therefore, 59,000 PSI. The source of the vibratory disturbance is suspected to be the steam disc vibrating in the first (zero circle) flexural mode, probably excited by the atypical high velocity steam conditions that were used during the testing program at the Valmont facility (see Reference 1). Referring to the Modified Goodman Diagram in Figure 13, the cast Inconci 718 permissible half-amplitude vibratory stress at 59,000 PSI mean stress without facture is 4000 PSI. In contrast, the forged Inconel 718 would be expected to withstand a half-amplitude vibratory stress to 31,000 PSI without fracture at b the 59,000 PSI mean stress condition. The interlocking characteristic of the curvic coupling teeth, - acting to set equal stresses in the joined parts, can be used to establish an upper bound for the stresses that could have occurred during operation. The same stress exposure that developed the fractures in the cast Inconel 718 Pelton
l k l WATER SEAL REVERSE FLEXIBLE . COMPRESSOR THRUST TOROIDAL ROTOR BRAKE BEARING SHELL
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o o 1 wheel was not sufficient to initiate fractures in the
. wrought AISI 422 (ASTM A565 GR 616) steam turbine disc.
The Modified Goodman Diagrams for a typical wrought 422 tempered at 1200*F (developed from data obtained from Reference 5) and the cast Inconel 718 alloy shown in Figure 13 indicate the combination of alternating and mean stresses that could fracture the cast Inconel without damaging the wrought 422. The Modified Goodman Diagram for the wrought Inconel 718 is also shown. All the probable combinations lof alternating stress and mean stress fall within the allowable limits for the wrought 718 material. VI. CONCLUSION
- Replacing the existing cast'Inconel '718 Pelton wheels with parts 9
machined-from-forged billets will: e Increase the Pelton bucket fatigue fracture safety factor from approximately 2.4 to approximately 4.7. e ' Provide fatigue fracture resistance in the curvic coupling area sufficient to withstand any combina-tion of alternating and mean stresses that could have been imposed during operation. , P
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REFEREflCES l '. " Investigation Into the Cause and Consequences of the Fort St. Vrain Pelton Wheel Incipient Fractures" - General Atomic Company Report tio. GA-A13179.
- 2. "flASA Tech Brief" - B73-10396, October,1973.
- 3. " Aerospace Structural Materials Handbook" - 1974 Publication.
- 4. " Private Communication from R. Moeller" - Huntington Alloys.
- 5. " Private Communication from Allis Chalmers" - October,1969.
I O b
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