ML20040G506

Speech Entitled Exam of Turbine Discs from Nuclear Power Plants, Presented at 820322-26 Intl Corrosion Forum in Houston,Tx

ML20040G506
Person / Time
Site:	Yankee Rowe, Cooper
Issue date:	03/22/1982
From:	Czajkowski C, Weeks J BROOKHAVEN NATIONAL LABORATORY
To:
References
BNL-NUREG-30647, NUDOCS 8202160182
Download: ML20040G506 (22)

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i APER NUMBER 220 BNL-NUREG-30647 p)k$. ktStudh On Gt 'I'$C eu ca $ l U IS1'aus Q forf Tho International Corro,lon Forum Sponsorod by the National Assoc,btion of Corrosion Engineers / March 22 26,10821 Albert Thornas C1nvention Contor, Houston, Texas EXAMINATION OF TURBINE DISCS FROM NUCLEAR POWER PLANTS CARL J. CZAJK0WSKI p \\to /

JOHN R. WEEKS

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ABSTRACT it Investigations were perforined on a cra.cked turbine, disc from the Cooper Nuclear Power Station, and on two failed turbine discs (governor and generator ends) from the Yankee-Rowe Nuclear Power Station. Cooper is a boiling water reactor (BWR) which went into commercial operation in July 1974, and Yankee-Rowe is a pressur-ized water reactor (PWR) which went into commercial operation in June 1961. Cracks were identified in the bore of the Cooper disc after 41,913 hours0.0106 days <br />0.254 hours <br />0.00151 weeks <br />3.473965e-4 months <br /> of operation, and the disc removed for repair.

At Yankee-Rowe two discs failed af ter 100,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation.

Samples of the Cooper disc and both Yankee-Rowe discs (one from the governor and one from the generator end of the LP turbine) were sent to Brookhaven National Laboratory (BNL) for failure analysis.

The fracture face on the Cooper disc was observed to have both intergranular and transgranular cracks. The governor end disc at Yankee-Rowe showed no indication of intergranular attack, and failed as a result of fast fracture, probably following impact by one or more of the fragments produced by failure of the generator di sc. The generator end disc had numerous intergranular axial cracks in the bore area. MoS2 was observed on the bore of all three discs. The Yankee-Rowe discs were subjected to SEM/EDAX, uniaxial tension tests, hardness testing, Charpy "V" notch test-ing, and environmental notched tensile tests. The notched tensile tests of the generator discs performed with MoSg had the effect of reducing the notch tensile strength of the disc by a factor of 3.5.

This result was consistent with a similar test conducted with H S at ambient temperature and pressure. On the governor 2

disc, the H S reduced the notched tensile strength by only a 2

Publication Right Copyright by the author (s) where copyright is applicable. Reproduced by the National Association of Corrosion Engineers with per.

mission of th3 author (s). NACE has been given first r'ghts of publication of this manuscript. Requests for permission to publish this manuscript in any form,in part or in whole, must be made in writing to NACE, Publications Dept., P. O. Box 218340, Houston, Texas 77218. The manuscript has not yet been reviewed by NACE, and accordingly, the material presented and the views expressed are -

solely thost oftha author (s) and are not necessariff endorsed by the Association.

Printed in USA 8202160182 820322 PDR RES 82021601EE2 PDR 9

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factor of 2.1, suggesting a reason for the ' absence of intergran-ular cracks on this disc. Laboratory data indicate that floS2 can hydrolyze in a steam environment to form H S, causing stress 2

corrosion cracks to initiate. The results of this investigation support a model that the cracks initiated at startup of the tur-bine, probably from H S produced by hydrolysis of itS, and 2

2 grew at a rate consistent with the published British data for propagation of cracks in pure steam.

INTRODUCTION The possibility of inservice failures of steam turbines, especially-at nu-clear power stations, has become the object of increasing concern in the past few years. This concern arises out of not only the non-profits derived by plant shutdowns which can amount to hundreds of thousands of dollars a day, but

, also of the possibility of significant damage 'to safety systems in the event of a catastrophic-type failure.

The most notable instance of this type of failure occurred in 1969 at the British Hinckley Point Nuclear Power Station.Ll? The principal cause of the failure was determined to be intergranular stress corrosion cracking of a low pressure disc in the areas of the keyways, attributed to a concentration of NaOH in the keyways combined with a low fracture toughness of the steel caused by temper embrittling during manufacture.

As a result of this incident, investigationg)were initiated to determine if any U.S. turbines had similar flaws. Week s,('

in a review of stress cor-rosion cracking in various nuclear turbines, e.g., Rancho Seco, Arkansas Nuclear-1, Hinckley Point-A, and Shippingport, determined that caustic was-present in all instances and that sufficient evidence was available to conclude stress corrosion cracking as the primary mode of cracking.

(Weeks considered that any portion of the fracture face which exhibited an intergranular surface evidenced the fact that the environment played a part in the propagation of the fracture by either stress corrosion cracking or corrosion fatigue.) Since stress corrosion cracks normally occur at stresses substantially less than those required to cause a failure in a benign environment and can produce critical size flaws, the type of failures which result can be quite cataclysmic in nature, e.g., Hinckley Point-A.

However t.'e growth rate of these stress corrosion cracks is relatively small (10-7,o 104 mm sec-1), and their t

presence can usually be detected before catastrophic failure occurs.

The United States Nuclear Regulatory Commission (USNRC) has been conducting ongoing investigations into these phenomena for some time.-

At the request of the NRC, a section of a turbine disc from the Nebraska Public Power's Cooper Nuclear Station, which was found to be cracked by in-service inspections, was sent to Brookhaven National Laboratory for examina-tion.

On February 14, 1980, the low pressure turbine at the Yankee-Rowe Nuclear Power Station failed catastrophically.

The USNRC requested BNL personnel to 2

visit the Yankee-Rowe site to examine the faileo fragments, and, subsequently, pieces of both the failed governor and generator end discs were sent to BNL for analysis.

In this paper are presented the results of the BNL examinations of the turbine disc fragments submitted for examination, and the results of exper-iments perfonned to suggest the cause of the cracking.

EXAMINATIONS A.

Cooper Disc Fragment The following information regarding the unit's. operating chracteristics and material history was obtained from discussions with representatives of both Nebraska Public Power and the USNRC:

a) The disc was manufactured of ASTM A471 steel, Class unknown.

b) The disc was the #1 disc (generator end) and contained two cracks.

c) The Station is a General Electric Boiling Water Reactor and the turbine was manufactured by Westinghouse Corporation.

d) The low pressure turbine did not have a prior reheating of the steam.

e) The station went into commercial operation July 1,1974.

f) The disc received 41,913 hours0.0106 days <br />0.254 hours <br />0.00151 weeks <br />3.473965e-4 months <br /> of operation (on line).

g) The keyway temperature during operation was approximately 350*F (177'C).

h) Prior to shipment to BNL the disc had undergone both ultrasonic test-ing and liquid penetrant examination and had been cleaned with acetone before crating for shipment.

The disc section weighed approximately 110 pounds and its dimensicns were recorded.

Initial examination detected a crack in the keyway traversing its length measuring approximately 5 inches (12.5 cm) long. There was a second crack which seemed to initiate at the toe of the keyway and ran almost parallel to the main crack.

Further examination of the section disclosed some evidence of pitting in the keyway. There was also a coating of a greyish graphite-like substance cov-ering the hub / keyway surfaces of the disc. Figure 1 shows the cracked area af ter a liquid penetrant examination to delineate the cracks. The crack ex-tends into the hub surface from the tip of the keyway and a second crack branches into the keyway, also seening to initiate at the keyway tip. The depth of the crack on the outer surface was estimated to be 2".

A section was cut from the turbine disc to facilitate opening the frac-ture face. The section was then immersed in liquid nitrogen for almost one hour and the crack opened by applying force on the notch in a compression l

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tes ter. The resulting fracture face is shown in Figure 2.

The remainder:of the turbine disc was then liquid penetrant tested again, to determine if the entire crack had been removed.

This examination showed tha'.t some of the cracks still ; remained. The-disc was then cut approximately ohe, inch further along the crack direction, and the above steps repeated. The fracture faces were then reassembled and measurements taken of the crack dimensions (Figure 3).. The crack's length measured 13.45 cm (approximately 5,.3") at its 1ongest dir'ansion, and 5.564 cm (approximately 2.1") at its greatest depth.

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1 The crack was characterized by a black adherent film covering the fracture -

face and appeared to be mixed intergranular and transgranular in nature as viewed through a stereo microscope.

Visual examination iif sclosed no arrest lines or beach marks, suggesting that once the crack iriitiated,91t progressed with no apparent cyclic frequency.

' '4 s

Various chips of the fracture face were brokeh off for SEM/EDS examina-tion. These were taken near the crack tip in hopes of determining if any corrodent species were present and to pick areas where the prior liquid pene-trant examinations had not contaminated the fracture face.

The cracking ap-peared to be of a mixed mode, both intergranular and transgranular in nature, with the intergranular appearance more predominant in the chips taken from areas closer to the keyway (probable area of initiation).

(See Figs. 4 and 5.1 Various particulates were investigated by EDS-on the varying chips. All showed peaks of C1 and evidence of both Si and K in varying degrees from traces to peaks; one scan showed a trace of S and V with ar. abnormally high Mn peak with the C1 and K present. These particulates were widely dispersed among the chips.

Several pr.rtially opened cracks were scanned, showing peaks of Na, Mg, St, Mo, C1, K and Ca, with a trace of Ti in one case, only Al and Ti in one. case, 7 s

and C1, Si, S and Al in three others.

D An EDS scan of the graphite-like substance found on tha hub surface -ofsthe disc was also performed, and identified peaks of S and Mo with a backgrouna

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peak of Fe (possibly from the tool used to remove substance from the disc sur-

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face).

This is indicative that MoS2 was used as a lubricant during assembly of the turbine rotor.

B.

Yankee-Rowe Fragments During the site visit it was observed that only the generator disc frag-ments showed evidence of stress corrosion cracking on the fractured surfaces; the governor disc appeared to have failed upon impact from fragments of the generator disc. The turbine housing and stator were not breached, so that fragment ricochet is quite possible.

On neither turbine disc was there any visual evidence that the cracks initiated from the keyways.

This investigation was two-fold in objective:

1) to detennine, if pos-sible, the cause'of the intergranular attack on the bore surface of the gener-ator end disc, and 2) to ascertain if the generator end disc differed in any way from its uncracked (no intergranular attack) sister disc on the governor end.

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9 The following infomation regarding the unit's operation was obtained from the Nuclear Regulatory Commission:

1) The first commercial operation for the Yankee-Rowe unit was in June 1961.

2) The failbre occurred during startup af ter the turbine had reached 3 operating speed, but before the generator was placed in line; con-ditions were 1800 rpm and 2% steam.

3) s The disc material was forged in 1958.

4) The inlet steam temperature to the LP turbine was approximately 300*F (149'C).

5) 7

- 'bine had operated more than 100,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> on line.

-6)

. Power plant was a Westinghouse Pressurized Water. Reactor (PWR) with a Westinghouse Staam Turbine.

Visual Ir.specticn/ Photography / Optical Metallography The crated weighkot' the two disc sections received at BNL was approxi-mately 700 pounds. The two sections are shown in Figs. 6 and 7.

After being photographed, the two sections were cut to allow the bore sur-faces to be examined more readily. The section of the generator disc was char-acterized as having numerous axial cracks running perpendicular to the circum-ferential direction of the hub. The fracture faces contained thumbnail cor-roded areas, which appeared to be mostly intergranular as viewed through a stereo microscope.

(Prior to photographing this section, the visible thumbnail cracks were outlined with chalk.) The fracture faces were also characterized as having a tight black adherent oxide film on them in the thumbnail' areas, similar to the film observed on the Cooper turbine disc. The balance of the fracture surface was clean and appears to have failed by cleavage af ter the stress corrosion crack reached a critical size. There was evidence of a gray-ish graphite-like substance on the hub which appeared to have been applied by brushing, as evidenced by the marked brush strokes in the photograph. These brush strokes were applied in the circumferential direction of the hub and were generally perpendicular to the cracks. There was also evidence of secondary cracking not necessarily associated with the intergranular cracks shown in Fig.

8, which is assjmed to be a result of the fast fracture mode of the failure.

The governor end disc had a large rubbed area'(Fig. 7) near the bore which was probably the result of its location during rundown after the failure. The

~area of rubbed metal 'which extended into the bore area was removed and the bore surface inspected by 11guid penetrant examination. The bore surface of this disc was also coated with a gray substance. There was no evidence of bore cracking on this disc, nor were there any " thumbnail" cracks on any of the fracture surfaces of the governor disc, as observed at the Yankee-Rowe site and in the BNL examination.

e 5

Two cracks fran the generator end disc were sectioned and mounted, with the polished face perpendicular to the growth direction of the cracks.

In the photonicrographs (Figs. 9 and 10), the cracks appear to be generally inter-granular in nature with very little crack branching.

The metal matrix appeared to be tempered martensite (bainite) with areas of proeutectoid ferrite. This structure would be consistent with that expected by quenching and tempering a low alloy high strength steel. There was no significant difference in the mi-crostructures of the two discs.

Various axial cracks from the generator disc were broken open after cool-ing the cut sections in liquid nitrogen.

The fracture faces appeared to be generally intergranular in nature (Fig.11) and had traces of Si, S, C1, Mn with the Fe, Cr and Ni, as shown in a representative EDS scan (Fig.12).

Samples from each of the discs were sent to an outside laboratory for chemical analysis. Table 1 depicts the results of the analysis. There are two items worth noting:

1) the carbon analysis reported exceeds the limits of the tec specifications listed in the table, and 2) there appears to be some segre-gation in the heavier elements analyzed (e.g., Mo, V, Cr).

This segregation may be the result of the relative locations of both discs in their original i ngot. There is, however, no significant difference in the chemistry of the two discs.

To determine if there were any detectable differences in the mechanical properties of the two discs that might explain the differences in their prop-erties, tensile tests, hardness tests and Charpy "V" notch tests were performed i

on specimens cut from each disc.

No significant differences were observed.

DISCUSSION l

Although previous investigators, cited in Ref. 2, had suggested that caus-tic deposition in the LP turbines was responsible for much of the cracking, there was no concrete evidence that caustic played a role in the cracking at either unit investigated.

Indeed, Cooper is a BWR, in which no attempts are made to raise the pH of the coolant, so that caustic deposition would be less likely to occur than in a PWR turbine, where steam generator pH is usually greater than 9.0.

Consequently, the common factor that might have led to crack initiation appeared to be the fioS2 lubricant identified from this investiga-tion as having been used in assembling both turbines.

To this end, notch ten-sile tests were performed on specimens cut from both Yankee-Rowe discs, both in air and in clean steam. Since, when MoS2 was sprayed on the specimens, a distinct odor of H S was observed during exposure to steam, comparative tests 2

were also performed in an H S environment. The results are shown in Fig.13.

2 Whereas steam appears to have little effect on the notched tensile strength of the material, the presence of steam plus tbs 2 and the presence of H S caused a 3.5-fold reduction in this property on the generator disc..The 2governor disc specimens appeared to be less sensitive to H 5 attack, since 2

the reduction in notched tensile strength was only 2.1-fold for this disc.

High strength low alloy steels have been known to be sensitive to many varied environments.

Since the notched tensile tests had suggested that there 6

_ = _

was probably some interaction between the MoS, the turbine disc steel and f

2 the steam, a further review of the effects of this lubricant was made.

Clauss(3) has recorded that tbs begtn to oxidize in dry air at approximately 750*F with Mo0is resistant to most ac 2

3 and S02 as the products of oxidation.

He also recorded, however, that MoS2 will perceptibly oxidize in moist air at room temperature. This oxidation in moist air at room temperature is quite probably)the characteristic most applicable to turbine di sc s.

Haltner and Oliver,W in their studies on the frictional behavior of MoS2 in flowing nitrogen / water mixtures when in contact with a rotating disc, detennined in part that 1) increasing the water content increased the friction, and 2) this increase in friction was associated with gg subsequent relea H 5.

2 This reaction is apparently reversible, as Rowe I was able to regen-erata-an oxidized MoS pressure of H S gas. 2 film by heating it to a temperature of 850*C in a low 2

Whether the MoS2 itself or some subsequent species produced durin mation of H 5 interacts with the metal surface is currently not known.g for-2 fonnation of metallic sulfides on the metal /MoS The 2 interface have been ob-served, and H S-initiated cracking of high strength steels is well known.(6) 2 with either moisture or steam on various metals.Other investigations ha 2

Calhoun(73 detemined that MoS2 in greases increased the corrosive tendency of all greases tested.

postulates in part that th He c)is caused by the fonnation of corrosive products, through hydrolysis. Perna concluded that MoS rosion of various metals when in a steam enviroment.2 accelerates galvanic cor-Coating turbine disc steels wi th MoS2 and subjecting them to a steam environment has also beer. ob-served (9 < to reduce the crack initiation time by a factor of three.

It can be argued that hydrogen attack cannot occur in the temperature range of turbine operation (over 100*C).

This would be true if the turbine operation was isothennal; this is not the case, as turbines are periodically shut down for fuel loads, etc.

H2 gas embrittlement, not H S attack.Even so, the 100*C tempgrature limit is for 2

Clark has showntoi that H S em-brittlement is relatively insensitive to temperature up to 100*C.2 Since H S can be liberated by MoS 2

2 in a steam environment, it is nec-essary to see how this reaction could cause disc cracking in either the key-ways or the bore.

Bore Cracking ufacturing sequence of turbine assembly. Turbine discs are normally sh high as 60% of the design yield strength. This shrunk on pre-load can be as Various investigators have shown a correlation between applied stress and its proportional increase of hydrogen assisted cracking.

If the hub of a turbine had been coated with MoS to the disc being shrunk fit on it, then this initial pre-load may be signif-2 prior icantly increased due to the tremendous load carrying ability of MoS -

Clausst3? has recorded a load bearing capacity for MoS 2

2 of over 400,000 psi.

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I It can be safely stated, therefore, that the original pre-load can be sig-nificantly increased if itS 2 is applied to the hub of a turbine disc, at least in areas of heavier deposits of brushed.on MoS.

This increase in pre-2 load might cause one disc to fail in a given turbine with an adjacent disc (with less MoS ) showing no evidence of attack.

The mode of intergranular 2

attack is assumed to be caused by a hydrogen or H S assisted mechanism 2

arising from the MoS / water / metal interaction.

i 2

Keyway Cracking Keyway cracking may result from a slightly different mechanism due to the manufacturing processes involved. The keyways are normally drilled after the disc has been shrunk fit on the shaft, and unless MoS2 has been used to in-sert the key, very little if any MoS2 would normally be present in the key-l way.

Since the tip of the keyway, however, would be at the disc / shaft /MoS l

interface, there is a possible explanation.

As previously discussed, MoS2 2

can hydrolyze to H S in steam.

The interaction of H S with the clean metal 2

2 of the keyway accelerates corrosion by a hydrogen assisted mechanism. This is j

probably the result of FeS formation on the clean surface of the metal, allow-ino H l

(10) 2 entry at a faster rate. This layer of FeS is considered by Steradzki to be only a few monolayers thick, which would not be in the detectable l

range of EDS analysis.

The 9moynt of H S needed would also not have to be 2

great because Hudgins et al.tlli observed failures in a quenched and tempered alloy steel with as little as 0.10 ppm H S in solution.

2 i

Postulated Cracking Mechanism l

Any effect of the H S genered by hydrolysis of the MoS2 would be 2

l likely to be of short duration compared to the thousands of hours the turbines have operated. However, cracks, once initiated, are known to grow in this material in a clean steam environment at a rate that is relatively insensitive m g. If we assume, therefore, that on the affected discs, hydrolysis ofstr to the MoS2 produced an environment that led to crack initiation early in the operating life of the turbine disc, then these cracks would be expected to propagate in the steam environment throughout the life of the unit.

To test this hypothesis, the crack propagation rates were estimated from the maximum size of the " thumbnail" marks at Yankee-Rowe and the deepest pene-tration measured at These were 3.7 x 10 gooper, divided by the operating times of the twn units.

mm/sec for the Cooper disc and 1.04 x 10-7 mm/seg for the Yankee-R wg and McIntyret b) disc. These values are compared with the data of Ford (12) 9 in Fig. 14. Clearly, the measured crack depths are consis-tent with this hypothesis. Further this figure and hypothesis are consistent with the work of Thornton et al.,19I which showed that MoS2 has little ef-fect on the crack growth rate.

ACKu0WLEDGEfENTS The authors wish to thank Dr. K. Sieradzki for his invaluable help in the area of mechanical testing; K. Sutter and R. Graeser for their help in sample preparation and mechanical testing, and R. Sabatini for the SEM work.

This work was performed under the auspices of the U. S. Nuclear Regulatory Commission, Of fice of Nuclear Reactor Regulation.

8

REFERENCES

1. ' Gray, J. L.. Proc. Inst. Mech. Eng. 186, 379 (1972).

2.

Weeks, J. R., BNL-NUREG-22689-R (June 1978).

3.

Clauss, F. J., Solid Lubricants and Self-Lubricating Solids, Academic Press (1972).

4.

Haltner, A. J. and Oliver, C.

S., Proc. ACS Pet. Div. Symp. on Chemistry of Friction and Wear, 3,, No. 4, pp. A77-84 (1958).

5.

Rowe, G. W., Sci. Lubn.,11,, No. 10, pp. 12-15 (1959).

6.

Clark, W. G., Jr., Journal of Materials for Energy Systems, 1, 33 (1979).

7.

Calhoun, S. F., Rock Island Arsenal Report #62-2752 ( August 15, 1962).

8.

Perna, C., Picatinny Arsenal' Report #DC3-1 (January 1961).

9.

Thornton, D. V., Fbul d, P. B. and Patrick E. C., Conf. on Grain Bound-aries (1976) (London: Institution of Meta 11urgists).

10. Sieradzki, K., Scripta Met., lji (February 1981).

11. Hudgins, C.

M., McGlasson, R. L., Mehdizadeh, P. and Rosborough, W. M.,

Corrosion, 21 (August 1966).

12. Ford, F.

P., General Electric Report No. 79CRDD119 (Phy 1979).

13. McIntyre, P., Central Electrici ty Research Laboratory (unpublished),

private communication (1980).

t 1

1 9

TABLE 1 CHEMICAL ANAL YSIS Generator End Disc ASTM A294 ASTM A471 Results, % by wt.

Class C Class 5 8

D F

Carbon 0.52 0.54 0.53 0.35 max 0.35 max lianganese 0.42 0.41 0.41 0.60 - 0.90 0.70 max Phosphorus 0.002 0.001 0.002

.0.035 max 0.015 max Sulfur 0.017 0.016 0.018 0.035 max 0.015 max Nickel 2.08 2.15 2.08 1.50 - 3.50 2.00 - 4.00 Chromium 0.65 0.68 0.60 0.70 max 0.75 - 2.00 Molybdenum 0.31 0.48 0.41 0.20 min 0.20 - 0.70 Yanadium 0.03 0.07 0.02 0.03 - 0.12 0.05 min Antimony

< 0.01

< 0.01

< 0.01 Silicon 0.21 0.21 0.20 0.1 5 - 0. 35

< None detected, less than Governor End Disc Results, % by wt.

A C

E Carbon 0.53 0.53 0.54

.035 max 0.35 max Manganese 0.41 0.41 0.42 0.60 - 0.90 0.70 max Phosphorus 0.001 0.001 0.001 0.035 0.015 max Sul fur 0.018 0.018 0.019 0.035 0.015 max Nickel 2.17 2.16 2.15 1.50 - 3.50 2.00 - 4.00 Chromium 0.64 0.69 0.64 0.70 max 0.075 - 2.00 Molybdenum 0.52 0.48 0.45 0.20 min 0.20 - 0.70 Vanadium 0.07 0.03 0.07 0.03 - 0.12 0.05 m'n Antimony

< 0.01

< 0.01

< 0.01 Silicon 0.20 0.20 0.20 0.15 -'O.35

<.None detected, less than 10

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FIGURE 13 RESULTS OF tiOTCHED TEllSILE TESTS YAftKEE-ROWE TURBIllE DISCS 255 236.3 231,4 O

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1 Materials Engineering Branch, ONRR i

c/o S. S. Pawlicki 1

Materials Engineering Branch, ONRR

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c/o C. D. Sellers (10) i I

1.

W. Y. Kato, Deputy Chairman Corrosion Science Group Files (50)

P

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j lowe Power Plant 1

Plant Manager (2)

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