ML18139A294

Metallurgical Investigation of Cracks in Steam Generator Feedwater Lines of Surry Station 1.

ML18139A294
Person / Time
Site:	Surry
Issue date:	12/31/1979
From:	Albertin L, Enrietto J, Rao G WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML18139A293	List: ML18139A292 ML18139A294
References
WCAP-9638, NUDOCS 8006090396
Download: ML18139A294 (46)

Text

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METALLURGICAL INVESTIGATION OF CRACKS IN THE STEAM GENERATO.R FEEDWATER LINES OF SURRY STATION NO. 1 L. Albertin G. V. Rao T. R. Mager December 1979

. AP ROVEO~. i;.<4-E. . .;~ ;: ;. rie. .;. t_to-,-M-a-n-ag-e-r_,

. .-/

Material and Mechanical Technology

, Work Performed Under Shop Order No. RMN-66374 WESTINGHOUSE ELECTRIC CORPORATION I .;,. Nuclear Energy Systems

~oof£/jo 3 96 P. 0. Box 355 Dm:~rnt# 5 () - z. 8 Pittsburgh, Pennsylvania 15230 Con*trnl # 8 t>t> ~~'I 0 ~ & 8

° Date ~ o 3 &.o of Document:

.REGULATORY DOCKET HlE COPY REGULAT -RY DOC~ET Pill

1 ABSTRACT.

This report describes the results of a ml!!talll!rgicaf investigation of *the cracking ~f reducer sections that were part of* Loop 8 and C steam generator .feedwater pipes at the Surry No. 1 Nuclear

• Generating Station of the Virginia Electric and Power Company; The cracks were discovered during ultrasonic and radiographic nondestructive examinations. The metallurgical evaluations

• included. chemistry determinations, tensHe testing, .*impact.testing, mefallogr~phy, and fractography ..

Tests showed that the material met the chemistry and strength specification requirements. The ductile-brittle transition ~emperature for the mat~rial was near 25° F with typical Charpy impact values at operating tE!mper~ture on the order of 190 ft. lb. Many cracks were found near the nozzle-to~reducer joint* in* a machined and tapered section. These. cracks generally started from *

.* machining. grooves and progressed to various depths in "the reducers. The largest* crac

k *in the*

Loop B reducer measured

. 0.080 inches,

. . while. . . the crack depth in the

. Loop C reducer

. reached 0.070 inches. Metallographic and fractographic observations suggest that. cracking was initi~lly caused by general corrosion at stre~s concentrations; with subsequent crack growth most likely caused by *corrosion fatigue. . .

ii.i

TABLE OF CONTENTS Section Title Page.*.

INTRODUCTION 2 EXAMINATIONS AND TESTS 2-1

• • 2-1. Surface and Metallurgical Ex~minations of Sections Containing Pits and Cracks 2-1 2-2. Metallography 2-1 2-3. Morphology and Distributiori of Cracks 2-1 2-4. Microstn.ictural Characterization Studies 2-2 2-5. Fractography . 2-2 2-6. Scanning Electron Microscope (SEM)

Fractography * .2-2 2~7. TransmissJon Electron Replica Microscopy (TEM) 2-2 2~a. Chemical Analysis 2-3 2-9.. Base Metal . 2-3 .*

• 2-10 ..

• Corrosion Deposits on Crack Surface 2~3 2-11. Mechanical .property* Tasks*. 2-3 2-12. Tensile Tests ** **. 2-3 2-13.

• Charpy Impact Tests .* 2-4 3 . DISCUSSION 3-1 .*.

4 CONCLUSIONS . 4-1 v

. I

_J

\""'

LIST OF ILLUSTRATIONS

. Figure Title *Page 2-1 Cross Section and Inside Diameter Surface Appearance of Reducer B Near the Nozzle-to-Pipe ~oint at 121°. 2-5

• 2-2 Inside Diameter* Surface and Cross Section of Reduce( B Near Crack Location 64°. 2-6 2-3 . Inside Diameter Surface and Cross Section of Reducer B at 227°. *

• 2-7 2-4 ., Inside Diameter Surface Appearance and Cross Section of Reducer B at 294°.

• 2-8 2-5 Location and Shape of. Cracks at Various Locations in Reducer C (Mag 2x)

• 2-9 2-6 .. Metallographic Section Containing the Longest Crack at 121° in Reducer B. 2-10 2-7 Section at 121° Showing Location and Shape of Smaller Cracks Near Weld in Reducer B. 2-11 2-8 Section at 121° Sho~ing Details of Smaller Cracks i~

• Reducer B Near the Nozzle-to~Weld.Joint

• 2~12 2-9 MetallographiC Cross *section of Reducer c Showing Several Small Cracks and the Deepest Crack (at 108°) 2-13 2-10 Details of one of the *crack!i'Shown in Figure 2-9* 2-14

. 2-11 Further Detail of a Second Crack Shown in. Figure* 2-9 2-15 2-12 . Surface Oxidation and Small Oxide Protrusions Found in Reducer B, Setion. 108° . .

2-16 2-13 Light Optic -iVlicrographs Showing the Size and Distribution of Ferrite. and Pearlite Phases. (Reducer Materiat Loop B) 2-17 .

2-14 Thin Foil Transmission. Electron Micrographs of the Reducer Material (Loop B) Showing the Fine Structure .

• in Ferrite Grains 2-18 2-15 Thin Foil Transmission Micrographs of the Re.ducer (Loop B)

Material Showing the. Fine Structure and lnterlath Spacing

• of the Pearl ite Phase* *

• 2-19 2-16 Appearance of the Crack Surface at 64° in Reduceir B *

.. Before and After Cleaning 2-20 vii

• LIST OF ILLUSTRATIONS (Cont)

Figure Title Page 2-17 Appearance of Crack Surface at 108° in Reducer C Before Cleaning 2-21 2-18 Crack Surface at 64° After Cleaning, Reducer B 2-22 2-19 Fractographs Showing the. Oxidized and Corroded Surface of Crack at 64° ih Reducer B

• 2-23.

2-20 Fractographic Features of Opened Crack at 64° Near Crac.k tip in Reducer B . 2-24 2-21 Fractographs Showing the Oxidized and Corroded Surface of Crack at 108° in Reducer C 2-25 2-22 Fractographic Features of Cleaned Crack at 64°. for

• Reducer B 2-26 2-23 Topographic Features of Crack Surface at 64°, Reducer. B, Showing Clusters of Holes Caused* by Corrosion Deoxidized by Electrolytic Cleaning 2-27 2-24 Fractographs Showing the Corroded Nature of the Fracture Surface at 64°, Reducer B, Surface Deoxidized by Cleaning 2-28 2-25 Fractographs. of Reducer B Crack at 64° Showing a**

Partially lntergranular Nature of the Fracture; Deoxidized Surface *

• 2-29 Fractograph Showing the Corroded Nature of the Crack Surface at 108° for Reducer C ** 2-30 2-27 Additional Fractographic Features S~en on Crack Surface at 108° in Reducer C 2-31 2-28 . Fractographi.c Features Seen at the Crack-Overload Area Transition

• 2-32

• 2-29 TEM Fractographs Showing Topographic Features of Crack and Overload* Fracture Surfaces in~ Reducer B at 64° 2-33 2-30 Fractographic Features on Reducer B Crack Surface Resembling Fatigue Stri.ations * . 2-34 2-31 Crack Deposits Analyzed, and Output Charts of Energy-Dispersive X"rays * * *

• 2-35

• 2-32 Charpy V-notch Properties of Reducer B Material in the Test Temperature Range of 100 to 440° F 2-36 viii

SECTION 1

• INTRODUCTION This report describes_ metallurgical _investigations to determine the nature. and cause of cracking in the steam generator feedwater lines in the* Surry No. 1 Nuclear Generating Station of the Virginia Electric and Power Company. The cracks were found during a nondestructive examination of pipe sections near the nozzle-to~pipe joint where- similar cracks were detected in other nuclear power plants. The Surry No. 1 Station is a three-loop plant with a generating capacity of 822

• MWe. It went into commercial service* in 1972.

The feedwater lines A, B, and C c_onnect to the steam generator nozzle via a 1.6-inch-diameter reducer. The piping is made of Schedule 80 ASTM A106. Gr. B .steel, and the nozzle is construe-

. ted from.

A 105 steel. .

Of the three. cracked reducers received. for. investigation, the.

one from*. L.oop A

was too radioactive to be hcindled in conventional shops and laboratories and was therefore not used in this investiga-tion. The Loop. C reducer was cut too close to the defective area, so that some .*

crac.ked sections.

were missing. However, other areas on the reducer were useful so that apartial evaluation was possible.

The main emphasis of the examination was then centered on Loop Bandincluded the following tasks:

• Surface and nietallographic examination of various sections containing pits or cracks . * * * . * *

• Microstructural characterization studies by light optic and thin foil electron microscopy techniques

• Fractographic exarninations ofthe fra~ture faces of the cracks *

• *Chemical analysis of the base material and* of the deposits

• • Mechanical property tests

.Th~:*results of these tasks are presented in the following sections.

1-1

SECTION 2 EXAMINATIONS AND, TESTS 2-1. SURFACE AND METALLOGRAPHIC EXAMINATIONS OF SECTIONS CONTAINING PITS AND CRACKS Prior to cutting various specimens for nietallurgicai tests, all three reducers were subjected to a nondestructive examination to determine the location and degree of cracking. This examination

. was done at the Westinghouse Waltz .

Mill facility . using ultrasonic techniques on the cut periphery *

• surface near the nozzle~to-pipe joint. Cracks were reported to be preserit in all. three loops. .

Looking* down from the nozzle towards. the red.ucer and measured clockwise from the top of the reducer at the nozzle end (0°), cracks were found at locations between 15° and 30° and*

between 90° and 120° in Reducer A, and between 75° arid 180° in Reducer B.. Crack positions in Reducer. C we~e between 330° and 15°, between 45° .and 90°, and between 21 Q ~nd 0

• 315° locations. The examination indicated th~t the cracks were not running continuou~ arbund the periphery of the reducer but l,(aried in depth and shape. Cut sections froni crack iocations showed*. the deepest cracks* are located at .the ;'knee" ~f a machinedtapered section belo\IV the

• weld. These deepest cracks measured 0:080 inches in* depth at 121° In Reducer B and 0.070 in; in depth at position 108° in. Reducer C. Smaller cracks were found along the tapered section:*.

The profile of th.e rriost pronounced cracked sections of Reducer ir at* 121° and 64° positions together with the inside diameter. surfa~e appearance at the crack. area is shown in figur.es 2~ 1 and 2-2. Other cross sections of .cracked areas in Reducers B and C are shown in figures 2-3 .

• to 2-5.

2-2. . METALLOGRAPHY 2-3~ Morphology and Distril;>ution of. Cracks

• Samples containing the largest cr*acks in *Reducers B and C were* further evaluated by metallo-graphy. Detai Is of these cracks are shown in figures 2-6 to. 2-12. Most cracks tend to initiate at .stress concentrations such as machining grooves. : *

. 2-1 .

2-4.

• Microstructural. Characterization Studies

. 1) Light Microscopy '-

Light microscopy was conducted .to study the size, in~rphology and distribution of ferrite and pearlite phases. Figure 2-13 illustrates the typical light micrographs and shows the distribution of pro-eutectoid ferrite and pearlite phases with some banding <;>f pearlite in. the major

. working direction. * *

2) Fine Structure Studies by Thin. Foil Electron Microscopy*

The fine structure of the ferrite and peai"lite phases WC!S .studied by thin foil transmission eiectron microscopy. Figures 2-14 and 2-15 illustrate the substructures within ferrite and pearlite phases, respectively. No abnormal features are seen. The interlath spacing of pearlite lamaelle is estin:iateq

• to be approximately 0.20 m.icrons ..* * *

• 2-5 .. FRACTOGRAPHY

.. 2~6 . Scanning Electron Microscope (SEM) Frai:tography

.Sections. containing some deep. cracks (at 64° in Reducer B *and 108° in Reducer C) were. cut .

' ~nd ~peried in the laboratory. The appearance of the crack su.rface from Reducer B before anc;t .,.

after cle~ning with an electrolytic solution is $hqwn in figures 2~ 16 and of. Reducer C i~ .the

. as-received .condition in figure 2-17. The cleaned s~rface. of Reducer B is sh~wn at higher magni-fication in figure 2, 18. Both the .as-received and cleaned crack surfaces were further examined in detail~ Fractographic features observed on the as-received surface of .the crack in Reducer B are shown in figures 2-19 and 2-20, and those in Reducer C in figure 2-21. Typical fractographs of the cleaned surface (oxide removed) are shown in figures 2~22 to 2-28.

2-7. Transmission Electron Replica Microscopy :(TEM)

• Transmission electron replica examination.~ were carried out on one Reducer* B cracked surface in order to find any indicaticms of metal fatigue. These indications would be: in the form of striations left behincj an advancing cr;;ick growing t:>v the application of an alternating .stress. For

• this purpose. two-stage cellulose acetate-carbon. repli~as shadowed with plati.num carbon .were.. .

prepared from an opened and cleaned. fr~cture surfac~ ~t the 64~ location; The shado~ed carbon replicas were examined with the transmission electron microscope which allows for a better

• resolution of fracture details at high magnifications.* Representative fractographs are shc:iwn in

• figures 2-29 and 2-30. The results. are not conclusive. Although some fracture. patches resembling

• *fatigue striations were noted, it could not be conelusively shown that these were not produced by the fractur~ of the pearlite colonies, and hence are not structure related.*

2-2

.. 2-8.

• CHEMICAL ANALYSIS

• 2-9. * * * .Base Metal

. A sample of the. Loop B reducer was submitted for a chemical analysis to verifY that the material composition met the ASTM A 106 Gr. B chemical requirements: Th.e res*ults are shown in the table 2-1.

• .TABLE 2-1 CHEMICAL ANALYSIS OF LOOP B RE_DUCER SAMPLE Elements, Wt. % *

• c Mn Si. .S .p.

• Reducer B 0.25 1.03 0~17 0.013 . 0.006 '.

ASTM A106 Gr. B I Requirement  ; 0;3 max o.29-1.06 0.10 min 1 0.058 max.* 0.048 inax * .

. 2-10. Corrosion Deposits on Crack :Surface

• The corrosion deposits shown in figure 2-31 were analyzed using energy dispersive x-rays. The

. output of the semiquantitative analysis also shown in this figure indicates that; beside irori oxide, the deposits. contained measurable quantities ~f sulfur and. copper.

. 2-11. MECHANICAL PROPERTY TESTS 2-12. Tensile Tests Specimenblanks removed from location 12° and representing the long axis ~fReducer B were rri~chined into 0.250-inch round tensile specimens having a 1-inch-gage section .. Two spedmens

• each were tested at 75° and 440° F. The results> are sh~wn i~ table 2-2; . ..

. *.TABLE 2-2 TENSILE PROPERTIES OF REDUCER B

• Test 0.2% Offset Ultimate. o/o Elongation *  % Red. of Temp., ° F Yield Strength, psi Tensile Strength, psi *in 1 Inch Area 75 40,910 . 65,660 36.9 71.5 75 41,410 66, 160 36.8 70.8 440 31,310 61,620 . 32.6. 70.8

.. 440 31,820. 63,130 32.2 68.2 2-3

2-13. Charpy Impact Tests Ten Charpy impact specimen blanks were cut from Reducer B, location 121°~ in such a way that the notch plane in* the Charpy specimens woulo corresp()hd to the crack plane of the field

. fracture~ Machined specimens were theri tested at various temperatures to determine the ductile-brittle behavior of* the st~eL The. results are shown ih figure 2-32. .

2-4 J

15,801-1 Weld Polished Cross Section Mag. 2x As-received ID surface at crack Mag. 2.5x Cleaned ID surface at crack Mag. 2.5x Figure 2-1. Cross Section and Inside Diameter Surface Appearance of Reducer B Near the Nozzle-to-Pipe Joint at .121°, Note machining grooves and heavy pitting at crack.

2-5

15,801-2 ID Mag. 2x ID Mag. 2x (cleaned)

Figure 2-2. Inside Diameter Surface and Cross Section of Reducer B Near Crack Location 64 Degrees 2-6

'~ ~. { ..

15,801-3 ID Mag, 2x ID Mag. 2,x (cleaned)

Figure 2-3. Inside Diamet~r .Surface and Cros~ Sectibn of Reducer B at 227° 2-7

(

15,801-4

',

• I ID Mag. 2x ID Mag. 2x (cleaned)

Figure 2-4. Inside Diameter Surface Appearance and Cross Section. of Reducer B at 294° 2-8

15,801-5 22° 40° 277° Figure 2-5. Location and Shape of Cracks at Various Locations in Reducer C (Mag. 2x) 2-9

'15,801-6 Un etched Mag. 50x Etched (2% Nital) Mag. 50x Un etched Mag. 200x Etched (2% Nital) Mag. 200x Figure 2-6. Metallographic Section Containing the Longest Crack at 121° in Reducer B. Lower photographs are details of the crack tip.

2-10

15,801-7 Unetched Mag. 50x Etched (2% Nital} Mag. 50x Un etched Mag. 200x Etched Mag. 200x Figure 2-7. Section at 121° Showing Location and Shape of Smaller Cracks Near Weld in Reducer B 2-11

15,801-8 Un etched Mag. 200x Etched (2% Nital) Mag. 200x Un etched Mag. 500x. Etched Mag. 500x Figure 2-8. Section at 121° Showing .Details of Smaller Cracks in Reducer B Near the Nozzle-to-Weld Joint 2-12

15,801-9 Mag. 50x Mag. 50x Figu~ 2-9. Metallographic. Cross Section of Reducer C Showing Several Small Cracks and the Deepest Crack (at 108°)

2-13

15,801-10 Mag. 200x Figure 2-10. Details of One of the Cracks Shown in Figure 2-9

15,801-11

• -;.

• Mag. 200x Figure 2-11. Further Detail of a Second Crack Shown in Figure 2-9 2-15

15,801-12 Mag. 200x Mag. 500x Mag. 500x Figure 2-12. Surface Oxidation and Small Oxide Protrusions Found in Reducer B, Section 108 Degrees 2-16

Mag .. 200x Mag. 500x (11 00 0

Figure 2-13. Light Optic Micrographs Showing the Size and Distribution of Ferrite and .....

Pearlite Phases. (Reducer Material, Loop B~ .....'

w

I\.)

I CX)

Mag. 12,000x Mag. 12,000x (11 00 0

Figure 2-14. Thin Foil Transmission Electron Micrographs of the Reducer Material (Loop B) .....

I

~

Showing the Fine Structure in Ferrite Grains

Mag. 12~000x Mag. 12,000x

' .fl

~ -.,. ..

01 00 0_.

Figure 2-15. Thin Foil Transmission Electron Micrographs of the Reducer (Loop B) Material _.

I Showing the Fine Structure and lnterlath Spacing of the Pearlite Phase CTI

15,801-16

} Service. crack Laboratory fracture

• Before cleaning Service crack Laboratory fracture After cleaning Figure 2-16. Appearance ~f Crack Surface at 64° in Reducer B Before and After Cleaning 2-20

Crack Surface

} ID Surface 01 Co 0_.

Figure 2-17. Appearance of Crack Surface at 108° in Reducer C Before Cleaning -.J

}Field crack Lab.

Fracture Mag. 20x 01 Co 0

Figure 2-18. Crack Surface at 64 Degrees After Cleaning, Reducer B ....

I 00

15,801-19 Mag. 500x Mag. 1000x Figure 2-f9. .* Fractographs Showing the Oxidized and Corroded Surface of Crack at 64 Degrees in Reducer B 2-23

15,801-20 Mag. 500x Mag. 2000x Figure 2-20. Fractographic Features of Opened* Crack at 64 Degrees Near Crack Tip in Reducer B. Upper photograph shows cleavage fracture produced in laboratory when the crack was

• opened. Note the crystalline nature of oxide.

2-24

.15,801-21 a) Oxide deposit

• }* Laboratory Fracture Corroded surface Figure 2-21. Fractographs Showing.the Oxidized and Corroded Surface of Crack at 108 Degrees in Reducer C

• 2-25

15,801-~2 Mag. 500x Mag. 500x Figure 2-22. Fractographic Features of Cleaned Crack Surface at 64 Degrees for Re~ucer B *

• 2-26

1'5,801-23

' ;,

I I

Mag. 100x Mag. 1000x Figure 2-23. Topographic Features of Crack Surface at 64 Degrees,

Reducer B, Showing Clusters of Holes Caused by Corrosion Deoxidized by Electrolytic Cleaning

• 2-27

15;801-24

\/

l Mag. 500x Mag. 2000x Figure 2-24. Fractographs Showing the Corroded Nature of the Fracture Surface at 64 Degrees, Reducer B, Surface Deoxidized by Cle~ning f

2-28

15,801-25

.. '. ':'~ .,-:: -

Mag. 2000x Mag. 2000x

• Figure 2-25. Fractographs of Reducer B Crack at 64 Degrees Showing a Partially Intergranular Nature of the Fracture; Deoxidized Surface 2-29

15,801-26 '"

Figure 2-26. Fractographs Showing the Corroded Nature of the Crack Surface at 108 Degrees for Reducer C 2-30

15,801-27 Clea rage Overload

. Fracture Crack surface Figure 2-27. Additional Fractographic Features Seen on Crack Surface at 108 Degrees in Reducer C 2-31

1s,ao1-2a Figure 2-28. Fractographic Features Seen at the Crack-Overload Area Transition. Fracture of pearlite islands is clearly visible.

• f 2-32 J

.15,801-29 a) Fracture area Mag. 2300x b) Overload fracture produced Mag. 2850x in laboratory Figure 2-29. TEM Fractographs Showing Topographic Features of. Crack (a)" and Overload (b) Fracture Surfaces in Reducer B at 64 Degrees.

• Fractograph a) depicts the corroded surface of the crack with

• markings produced by the fracture of pearlite colonies, while b) shows a typical brittle cleavage fracture produced at low temper-ature in the laboratories during opening of the crack

• 2-33 .

15;801-30 Mag. 27 ,OOOx Mag. 27 ,OOOx Figure 2-30. Fractographic Features on Reducer B Crack Surface Resembling Fatigue Striations.

r f

2-34

15,801-31

.,,.' ~

, ~- .

Mag. 1000x Mag. 1000x Figure 2-31. Crack Deposits Analyzed, and Output Charts of Energy I

Dispersive X-rays

" 2-35

220 110 200 100 180 90 IMPACT STRENGTH 160 80 en al .% BRITTLE

...J I-u.

140 FRACTURE 70 -~w a:

c I-I- 120 60 (.)

~ <(

2 .. a:

w u.

a: 100 50 w l'J I- .....

w m

Cl)

I-I-

I-(.) 80 40 cc

<(

c. ca

2:

60 30 40 20 20 10 0 0

-300 ~100 0 100 200 300 400 500 TEMPERATURE (°F) 01 "co Figure 2c32 ..

• Charpy V~Notch Properties of. Reducer B Material in the Test Temperature Range ...

Q) -

. c:.,,

• of 100 to 440° F * *

• N y_ ... I r .. ,.

SECTION 3 DISCUSSION An examination of the chemical composition and strength properties of the Loop B reducer showed that the material comformed t~ the ASTM A 106 Gr. B specification. No microstructural abnormalities contributing to cracking were found. The material can be .considered as notch .

insensitive,* having Charpy impact values of about *.190 ft. lb at room ahd :operating temperature; and a ductile-brittle transition temperature of about 25° F:

• Visual and microscopic examination of the insi.de surfaces of *Reducer* B and C sections .near the nozzle-to-pipe joint revealed several cracks and pits (figures 2-1 to 2-5). The cracks tended to initiate from machining grooves or grinding ~arks below the 0 knee" of a redu.ced section. A

• piece containing the largest .crack h Re.ducer B (figure 2-1) showed severe*; continuous. pitting

.. along the crack. This crack, when opened, mea~u.red 0.080* inches ~nd was. located 121° away J' . from the top (0°) position. The deepest crack in ReducerC measured 0.070 inches and was located at 108°. Smaller cracks were found at other* 1ocations in the reduced sections of both pipes. The cracks ran along the circumference of the pipes and grew in a relatively straight .*

manner from the ID surface towards the OD surface (figur~s 2-6 to 2-11) with very little branch-ing. The. cracks were transgranular in nature and were not influenced by the banded micro-structure. The walls of the cracks were severly oxidized. Bulging. and narrowing of the oxide along the crack path (figures 2-5 and 2-9) suggest a crack~arre~t and growth mechanism. A few beach ~arks, seen in figure 2-16; further point to an *intermittant growth mechanism. Some of the oxidation was in the form of intrusions rather than cracks (figurns 2-8 and 2~ 12). The relative~.

ly straight .nature of cracking* in the radial direction suggests that aqueous. corrosion was assisted by stress. Opened cracks showed a dark corrosion product on the surface~ Higher magnification .

fract~graphs showed the product to be mostly crystailine (figure 2-21) presumably. Fe304.* This.

I oxide is more voluminous than the parent m~tal and *could exert an additional stress for crack

• propagation. The cracking process appears to be relatively slow and perhaps self-limitin~.

Although electron replica fractography indicated some regions of fracture surface resempling fatigue striations, it cou.ld not be conclusively shown that these were not the result of*

fracture of peailite colonies and hence are not structure related. It is also recognized however, that any substantial evidence of fatigue. striations could have been lost by the presence of

corrosion. The character of the cracks found. in the Surry reducers is similar to those s~en

• in other cracked. teedwaier pipes (notably the b.C. Cook Unit 1 and Palisades plants[ 1 *2 1) wtiere fatigue striations were cleafiy seen oh the fracture faces. Based on the evidence noted above and the similarity of the character of Surry cracks to those of the Cook and Palisades cracks, it is believed that the cracking iri the sufrY pipes is most likely caused by corrosion fatigue;

.. 1 \;.

3-2

SECTION 4 CONCLUSIONS The cause of cracking in the feedwater line reducers of Loqp B and Loop C at the Surry No. 1 Station appears to be a combination of aqu~ous corrosion and corrosion. fatigue. Contributing to crack initiation were a sharp geometrical discontinuity, machining, and grinding marks.

The material met the chemistry and strength requirements of ASTl\ll A 106 Gr. B specification.

Microstructural. studies indicated no abnormalities in the fine structure .

i "

4-1

REFERENCES

1. A. Madeyski, G. V. Rao, and T. R. Mager, "Metallurgical Investigation of Cracking in the Steam Generator Feedwater Pipes of the* D. C. Cook Units 1 and 2, WCAP-9581,

• Westinghouse Nuclear Energy Systems, August 1979.

• 2. A. Madeyski, G. V. Rao, and T. R. Mager, "Metallurgical Investigation of Cracks in the Steam Generator Feedwater Pipe of the Palisades Nuclear Generating Station,"

WCAP-9669, Westinghouse Nuclear Energy Systems, *February* 1980 .

I

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