ML20112K059

Unit 2, Metallurgical Defects in Steam Generator Tubes

ML20112K059
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Issue date:	04/03/1985
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SAN ON0FRE NUCLEAR GENERATING STATION - UNIT 2 METALLURGICAL DEFECTS IN STEAM GENERATOR TUBES SOUTHERN CALIFORNIA EDISON COMPANY April 3, 1985

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METALLURGICAL DEFECTS IN STEAM GENERATOR TUBES 3

TABLE OF CONTENTS Section Page

1.0 INTRODUCTION

1

2.0 BACKGROUND

2 3.0 METALL0 GRAPHY EXAMINATION RESULTS 7

4.0 CORRELATION OF EDDY-CURRENT RESULTS WITH METALL0 GRAPHY 9

5.0 PLUGGING CRITERIA 10 6.0

SUMMARY

AND CONCLUSIONS 11 Appendices A.

San Onofre Unit #2 Steam Generator Tube R89-C151 HL (Westinghouse Report) 8.

Destructive Examination of Tube R89-L151 from San Onofre Nuclear Generating Station - Unit 2 Steam Generator 88 (Combustion Engineering Report)

C.

Destructive Examination of Tube R96-L118 from San Onofre Nuclear Generating Station - Unit 2 Steam Generator 88 (Combustion Engineering Report)

D.

Fabrication and Testing of an As-Drawn / Mill Annealed Eddy Current Standard (Westinghouse Report)

E.

Laboratory Induced Eddy Current Transition in a Steam Generator Tube (Combustion Engineering Report)

D O

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METALLURGICAL DEFECTS IN STEAM GENERATOR TUBES

1.0 INTRODUCTION

The purpose of this report is to document discussions held with NRC staff on January 17 and February 7,1985 regarding the status of San Onofre Nuclear Generating Station (SONGS) Unit 2 steam generators.

The purposes of these meetings were to review the history of steam generator tube leakage, inform the NRC of results from tube examinations and analyses performed and discuss the corrective actions taken.

The subject of this report is the metallurgical defects which resulted in steam generator tube leaks at both SONGS 2 and 3 during Cycle 1 operation. The following infor-mation substantiates the conclusion that the metallurgical defect has been identified and isolated.

Hence, this defect is not considered capable of adversely affecting the health and safety of the public as a result of SONGS 2 operation throughout the balance of plant life.

A total of three steam generator tube leaks developed during the first cycle of operation of SONGS 2 and 3; one leak occurred in SONGS 2, two leaks occurred in SONGS 3.

Each of the leaks developed slowly over approximately a four-week period. None of the leaks exceeded the Tech-nical Specification leakage limit of 720 gallons per day prior to unit shutdown. As part of the follow-on action, a multifrequency eddy-current 4

examination was performed on 100 percent of the Unit 2 steam generator tubes during the refueling outage.

This report describes the details of each of the three steam generator tube leaks experienced during commercial operation and the results of nondestructive and destructive tube examinations performed. A discussion is also provided of the common characteristics identified from the eddy-current test (ECT) results of each of the affected tubes. These char-acteristics are correlated with ECT results from laboratory samples and a tube failure mechanism is postulated.

The conservative tube plugging criteria developed from these evaluations are presented.

Finally as a result of implementing this criteria, it is concluded that there is little likelihood of additional steam generator tube leaks resulting from this type of metallurgical defect.

o

2.0 BACKGROUND

On May 8, 1984 the activity measured in the blowdown of Unit 2 steam generator E-088 increased; subsequently, a steam generator tube leak was confirmed. On June 19, with the leak rate measured at 300 gallons per day (the Technical Specification limit is 720 gallons per day), Unit 2 was shut down to locate and repair the leak. On June 26 the leak was located and on June 27 it was confirmed to be a small flaw in tube 89-151. The flaw was located 9 to 9 1/2 inches above the top of the tubesheet, on the inlet (hot leg) side of the steam generator. Based on eddy-current testing (ECT), the flaw was less than 0.5 inches long (axially) and less than 0.3 inches in the circumferential direction. This flaw had not been present during the 1979 100% baseline ECT examination.

The tube was then plugged and 62 tubes surrounding the leaking tube were inspected using ECT. Only one other tube (84-154) had an indication of a flaw (24%

through-wall, located 6 inches above'the 6th tube support). However, this indication had been present during the 1979 baseline ECT examination. On June 28, 1984, tube 89-151 was plugged and the unit was returned to service. On October 20, 1984, Unit 2 was shutdown for the first refueling outage and a section of this tube was removed for metallo-graphic examination (Section 3.0 of this report).

i On June 10, 1984, a, leak was confirmed by activity in the blowdown of i

steam generator E-089 in Unit 3.

On July 19 the leak rate was about-120 gallons per day and Unit 3 was shut down to locate and repair the leak. On July 26 the leak was located (tube 66-64) and on July 27 it was confirmed. Based on ECT, the flaw was located about 3 inches below the third tube support on the hot leg side.

Based on ECT results, it was

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smaller than the Unit 2 defect, less than 0.3 inches long and less than 0.3 inches in the circumferential direction. Additionally, two other ECT indications (79% and 73% through-wall) were identified and located 10.5 i

and 16.9 inches, respectively, above the third support on the same tube.

None of these flaws were present during the 1979 baseline ECT examination.

In all, 61 tubes were inspected in the immediate vicinity of the leaking tube. No other indications in other tubes were found. On July 28 tube 66-64 was plugged and Unit 3 was returned to service.

l 4 -

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2.0 BACKGROUND

(Continued)

On September 26, 1984 the activity measured in the blowdown of Unit 3 steam generator E-089 again increased.

Subsequently, a tube leak was confirmed and on October 27, with the leak rate measured at 130 gallons per day, Unit 3 was shut down to locate and repair the leak. On November 7 the leak was located and confirmed to be a flaw in tube 79-15.

ECT of 110 tubes, including the affected tube, was performed.

No other tube exhibited flaw indications.

The leaking tube had a total of six indications near the second tube support on the outlet (cold leg) side of the steam generator.

All of the indications were greater than 80% through-wall.

Four indica-tions were located below the second tube support (3 1/2 inches, 5 inches, 11 inches and 12 1/2 inches from the support) and two were located above the second support (3 inches and 12 inches from the support). All six indications were detected using 400 KHz differential ECT; none of them were present during the baseline ECT done in 1979 using the same frequency.

In addition to the six indications, two areas of variation in the 100 KHz absolute ECT signal were identified. These were located near the second tube support in the same region. One area extended for 12 inches just above the first support and the other area extended for 15 inches below the third tube support.

A review of the 100 KHz absolute data (vertical channel) associated with each of the three tubes which leaked showed three-characteristic indica-tions. These characteristics are: (1) a negative conductivity drift, (2) a sharp negative to positive transition, and (3) a length of tube exhi-biting relatively constant positive conductivity. These characteristics occurred in the sequence listed and are depicted in the sketched trace shown in Figure 2-1.

Figure 2-2 shows the actual strip charts in the area of interest.

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UNIT 2 (88)

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UNIT 3 (89) l TUBE 89-151 TUBE 66-64 TUBE 79-15 3

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Actual 100 KHz Absolute ECT Signals i

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t 2.0 BACKGR^UND (Continued)

A solitary negative conductivity trace was common in the ECT results at the 100 KHz frequency.

The negative traces ranged from small, irregular sig-nals to changing conductivity over the entire steam generator tube length.

More than 70% of~these indications were less than 1 1/2 feet long and had less than a 1-volt signal amplitude.

Tube 96-118 which was removed and destructively examined, had one of the largest of these small negative.

amplitude signals.

The leaking tubes, on the other hand,. demonstrated a drift of approximately one-half to one-volt amplitude over a length of tube ranging from about one to three feet followed immediately by the distinct transition from negative I

to positive conductivity.

The transitions ranged from one to two volts in amplitude and occurred over approximately one. foot of tube length.

In each of the leaking tubes, the transition was immediately followed by a rela-tively constant positive signal. The constant positive signal of the ECT trace ran from the transition zone for at least four feet. All three of the through-wall flaws occurred in the area of positive conductivity.

The flaw locations along the tube appeared to be independent of the distance from the negative drift and transition zone.

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• 3.0 METALL0 GRAPHIC EXAMINATION RESULTS s-This section addresses the results of destructive metallographic examina-tions performed on Unit 2 steam generator tubes89-151 and 96-118.

Tube 89-151, the defective tube, was examined by two organizations.

Westinghouse performed the destructive testing on the length containing the flaw.

CE performed testing on the length of tube containing the 100 Kliz absolute ECT transition region.

The reports of the results of these e.<aminations are provided as Appendices A and B, respectively.

Tube 96-118 was found by ECT to contain an area of negative conductivity which regis-tered approximately half way between a normal trace and the trace char-

-acteristic of the leaking tubes.

A destruct.tve examination was performed by CE on this area of the removed tube to determine whether such an ECT trace was indicative of a susceptible cold-worked microstructure. The test report from the evaluation of tube 95-118 is provided as Appendix C.

The report demonstrates that the tube contains a normal microstructure.

In Appendix A Westinghouse characterizes the flaw in tube 89-151 as an axf al intergranular crack. The crack measured 0.7 inches along the tube interior diameter and 0.9 inches along the tube outer diameter. The crack was less than 0.3 inches across. Additionally, fractography indicated that ductile failure had occurred along the tube outer diameter.

Deposits on the tube were of limited quantity and appeared normal. No aggressive species were identified. Metallographic analysis indicated that the flawed tube contained a cold-worked microstructure with a hardness of 400 KN0OP; normal Inconel tubing has an equiaxial grain structure and a hardness of 200 KN0OP.

i 3.0 METALL0 GRAPHIC EXAMINATION RESULTS (Continued)

The section of tube 89-151 which did not contain the flaw was examined by o

Combustion Engineering. The section had been removed from the tube between the first and second supports (the defect was located between the tubesheet and the first support). Based on microstructural examination, a modified Huey test, bulk chemical analysis, X-ray diffraction and fluorescence and microhardness measurements, the tube was determined to

'have an abnormal microstructure. The tube section had a region in which the 100 KHz absolute ECT showed a transition from a negative to a positive signal. As discussed in Appendix B, the examination showed a cold worked high hardness microstructure in the region of positive 100 KHz absolute signal, an equiaxial but slightly small grain structure in the region of negative 100 KHz absolute signal and a mixture of the two microstructures within the transition region. No defects were observed in the tube section during any of the examinations (i.e., ECT, visual, radiographic and destructive).

The following conclusions were drawn based upon the metallographic examination of the 89-151. The presence of limited amount of normal deposits on the tube and absence of any aggressive species indicates that the flaw was not likely related to secondary chemistry control.

The ductile failure along the tube outer diameter indicates that the crack was initiated at the tube interior (the reactor coolant side).

Inter-granular stress-corrosion cracking of the highly susceptible cold-worked microstructure is the most likely failure mechanism.

Steam generator tube 96-118, which as indicated in Section 2.0, exhibited a small 100 KHz ECT negative conductivity indication, was also removed and examined by Combustion Engineering. As described in_ Appendix C the results of that examination indicated that the tube contained normal equi-axial grain structure and normal hardness.

These results were consistent with what was expected from properly annealed Inconel steam generator tubing.

f.

e 4.0 CORRELATION OF EDDY-CURRENT RESULTS WITH METALLOGRAPHY In addition to the removed tubes, two laboratory samples were utilized to complete the correlation of ECT results with susceptible grain structure.

Tubing manufactured by Westinghouse Specialty Metals Division and Noranda were provided by Westinghouse and CE respectively. The results of the work done to correlate ECT results with microstructure in these two samples are provided in Appendices D and E.

4 Both samples were initially in a cold-worked condition.

Subsequently, half of each sample was annealed by placing one end of the tube in a furnace at the proper annealing temperature. The samples were then eddy-current tested using a 100 KHz absolute signal.

Finally, a metallographic j

. examination of each tube was performed.

4 Both of the sample tubes exhibited areas of normal and abnormal con-4 ductivity from the ECT strip charts.

The abnormal signals were very similar to the strip chart from the three leaking steam generator tubes:

a negative conductivity drift followed by a sharp transition and a rela-tively constant positive conductivity. As with tube 89-151 (Appendix B) these changes in conductivity correlated with distinct microstructural and hardness changes in the tube sample.

Small, equiaxial grain structure was exhibited in the area of negative conductivity drift.

The transition zone and adjacent areas exhibited mixed microstructure, varying from small, equiaxial grains to slender, elongated grains. The slender, elongated grains, characteristic of a cold-worked microstructure, continued through-l out the area of positive conductivity.

It should be noted that only the three tubes which developed leaks and the two test samples exhibited all three of the common characteristics.

The tests demonstrated that the presence of cold-worked microstructure in an otherwise properly manufactured tube can be detected by 100 KHz absolute ECT.

It was also shown that tubing containing only a small magnitude negative conductivity signal has a normal microstructure.

f6.

4.0 CORRELATION OF EDDY-CURRENT RESULTS WITH METALLOGRAPHY (Continued)

Based on Appendices A through E, it was concluded that a reproducible and verifiable correlation between.100 KHz absolute ECT signals and changes in

.Inconel tube microstructure exists.

It was further concluded that all tubes susceptible to defects such as those found in the steam generator

-tubes which developed leaks-have been positively identified from the 100 percent ECT inspection.

5.0 PtVGGING CRITERIA During the 100 percent Multifrequency Eddy-Current examination (MFEC) performed on the Unit 2 steam generator tubes, all tubes which_ exhibited 100 KHz absolute signal variations in the negative direction, transitions, or positive conductivity signals were identified for further review. A limited number of tubes were found to be affected (approximately 220 tubes or 0.5 percent). No tubes, other than the three tubes which leaked in service, were found to exhibit all three characteristics of negative drift, conductivity transition followed by a constant, positive.100 KHz absolute ECT signal. ' Evaluation of the metallography and correlation of data from steam generator-tube 96-118 placed a lower bound on the metallurgical defect.

Although none of the Unit 2 steam generator tubes were found to be defective or degraded as a result of this metallurgical defect, tubes exhibiting the following 100 KHz absolute ECT indications were preventively plugged:

-(1) all tubes with a positive conductivity signal, 9

.(2) all tubes coataining a sharp conductivity transition (negative to positive or positive to negative),

5.0 PLUGGING CRITERIA (Continued)

(3) all tubes which contained a high amplitude ( > 1 volt) negative conductivity signal and

.(4) all tubes exhibiting a long (extended) or drif ting negative conductivity signal.

Implementation of these criteria resulted in 60 tubes being preventively plugged to preclude leaks attributable to this metallurgical defect.

1 6.0

SUMMARY

AND CONCLUSIONS Evaluation of the 100 KHz absolute ECT signals identified three character-istics common to the three steam generator tubes which developed leaks during operation. All three of these common, distinct characteristics were found only in the three tubes that leaked. Tubing flaws were found only in lengths of tube exhibiting a relatively constant positive conduct-ivity on 100 KHz absolute ECT. Based upon the sample from tube 89-151, this positive signal was correlated with the presence of a cold-worked microstructure. This cold-worked microstructure is believed to have resulted from an incomplete final anneal of the steam generator tubes during manufacture.

3 Two steam generator tube samples were prepared in the laboratory in order to confirm the ECT metallography correlation seen in tube 89-151.

These samples were initially cold-worked and then partially annealed. The signals from the 100 KHz absolute ECT signal and the metallographic analysis confirmed those correlations developed from tube 89-151.

It was therefore concluded that the 100 KHz absolute ECT data could positively identify tube conditions associated with the phenomenon causing the leakage.

t 6

6.0

SUMMARY

AND CONCLUSIONS (Continued)

The metallography of tube 89-151 indicated no abnormal deposits, no aggres-sive species and that the flaw was most likely initiated from the reactor coolant side.

The postulated failure mechanism is intergranular stress-corrosion cracking occurring in a highly susceptible microstructure. Based upon the metallography, the ECT results were reviewed to identify any signals which could be conservatively associated with the presence of a susceptible microstructure.

This process resulted in development of criteria utilized to preventively plug 13 tubes in steam generator. E-088 and 47 tubes in steam generator E-089.

There were no degraded or defective tubes in the Unit 2 steam generators at the end of Cycle 1 which were associated with the metallurgical defect.

Based upon the evaluations completed, the metallographic conditions associated with the leaks have been directly and reliably identified by 100 KHz absolute ECT testing. All Unit 2 steam generator tubes considered capable of developing leakage from the metallurgical defect have been preventively plugged.

It is concluded that the metallurgical defect has been identified ~ and. isolated.

Hence, this defect is not considered to pose a hazard to the public health and safety for the operating life of l

San Onofre Unit 2.

Therefore, this issue is considered closed.

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APPENDIX A San Onofre Unit #2 Steam Generator Tube R89-C151 HL (Westinghouse Report) 9

8 5-5 D2-S ANG F-R1 SG-85-01066 3AN ONOFRE UNIT #2 STEAM GENERATOR TU3E R89-C1'i HL l

R. P. Shogan and P. J. Kuchirka-SGTD

, January 23, 1985 APPROVED:

,d 5f.,c6cA,=x G.

P. Sabol, Manager Nuclear Materials Department APPROVED:

D. E. Harrison, Manager Materials Science Division Westinghouse R&D Center 1310 Beulah Road Pittsburgh, Pennsylvania 15235

e Research Report 85-SD2-SANGF-R1 January 23, 1985 SG-85-01066 s

SfR ONOFRE UNIT #2 STEAM GENERATOR TUBE R89-C151 HL R. P. Shogan, P. J. Kuchirka" Westinghouse Electric Corporation I

ABSTRACT A section of hot leg steam generator Tube R89-C151 from San Onofre Unit #2 was examined by nondestructive and destructive means.

l The tube was found to have an axial through-wall crack approximately 0.9 inches long at a location about 9 inches above the tubesheet top. The j

intergranular crack probably originated at the I.D.

Metallographic examination revealed very fine axially oriented grains that appeared heavily cold worked. Hardness measurements predicted a yield strength of 150 kai and a 70% cold worked condition for the material. Most likely the tube was in the as-drawn condition and may never have been heat treated.

d

'SGTD

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I 1.

INTRODUCTION In December 1984, a segment of hot leg tube, R89-C151 HL, was pulled from steam generator E-088 of SONGS Unit 2 and shipped to the Westinghouse Research and Development Hot Cells for non-destructive and destructive examination.

Previously, on May 9, 1984, this tube had developed a leak which by June 19, 1984 measured 300 gallons per day.

The Tube R89-C151 was then plugged and the unit returned to power.

i SONGS Unit 2, a Combustion Engineering desi6ned plant, had been in operation approximately one year prior to the leak.

Field eddy current examination during the June shutdown revealed an indication, reported to be located between 9 and 9.5 inches above the tubesheet top, which had not been present during the baseline examination of the steam generator.

The received tube segment was photographed, dimensioned, visually examined, and characterized nondestructively by X-ray radiography and laboratory eddy current techniques.

Detailed metallography, scanning electron microscopy, and microanalytical examinations followed. This y

report contains the significant observations of these examinations as well as an analysis'of the findings.

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2 2.

NONDESTRUCTIVE EXAMINATION 2.1 Visual Examination Two sections of Tube R89C151 were received for examination.

These were labeled Piece #2 and Piece #3 Figure 1 shows the cutting diagram as received from SCE. Visual examination revealed a clean tube with minimal 0.D. deposits above the tubesheet and heavy deposits within the tubesheet region. An axial crack was observed 8.0 to 8.9 inches above the secondary face of the tubesheet as determined by the SCE cutting diagram. The crack was in the form of a single straight line with some branching occurring at both crack tips.

Figure 2 shows the as-received tube sections.

2.2 Eddy Current Examinatien A laboratory eddy current examination of Pieces 2 and 3 of Tube R89-C151 HL was performed using a 0.610 inch bobbin-type probe manufactured by Zetec. The inspection used frequencies of 200 and 400 kHz differential mode and 100 and 300 kHz absolute mode. Figure 3 shows-the calibration standard eddy current signals used to simulate the SONGS Unit 2 steam generator tubing.

From approximately 3.1 to 4.35 inches from the bottom of piece 3 (3.0 to 4.25 inches from the bottom after 0.1 inches of tubing was removed from the bottom of piece 3 to facilitate eddy current inspection) a large volume axial indication, 90 to 100 percent through-wall, was observed.

Figure 3 also shows the laboratory eddy current signals for this-indication.

Within the tubesheet region of piece 2, two dent-like signals were observed at approximately 0.9 inches below the tubesheet top and at approximately 13.9 inches below the tubesheet top.

Later radiographic data showed that these dent-like signals represented small 2

t

i circumferential islands of I.D. surface material which had not been deformed by the Combustion Engineering tube pulling operation. Figure 4 shows strip chart eddy current data for the overall tubesheet region along with the calibration standard. No obvious baseline drift was observed which could be interpreted as representing IGA (intergranular attack), nor was an abrupt change in the baseline noted which could have been interpreted as representing a conductivity change.

2.3 Radiograohy Radiographs of the suspect area and the tubesheet region are shown in Figure 5.

Radiography supported the visual examination description _of the crack and showed that the two eddy current dent-like signals from the tubesheet region were caused by the I.D.

tube pulling grippers.

- 2.4 Macrophotography A section of tubing containing the crack was removed and sectioned longitudinally to allow I.D. crack examination and macrophotography.

Photographs of the crack on the 0.D. and I.D.

surfaces are shown in Figures 6 and 7.

On the 0.D. the crack measured 0.85 inches long. The crack was located in a narrow depressed region. The I.D. crack was 0.71 inches long. On the I.D. the crack was surrounded by a lighter colored deposit than the surrounding deposit.

2.5 Dimensional Measurements Heasurements of the tube lengths showed that piece 2 was 21.7 inches long and that piece 3 was 17.8 inches long. These values were slightly less than those presented in Figure 1.

Outside diameter and wall thickness measurements were taken at three locations along the tubing.

Results are tabulated in Table 1.

The diameter measured 0.742 l

2 0.002 inch and the wall was a uniform 0.049 1 0.001 inch.

3

4 0

3.

DESTRUCTIVE EXAMINATION Figure 8 shows the cutting plan used to obtain test specimens for the various destructive examinations.

l 3.1 Scanning Electron Microscooy (SEM) of Fracture Face A section containing the lower half of the crack was broken open to reveal the crack surface for SEM analysis.

After an initial examina-tion, the crack surface was cleaned using an Endox chemical treatment to remove a heavy oxide layer. The three general topographical regions observed are shown in Figure 9.

These include an intergranular region (the bulk of the fracture face), a dimpled rupture region formed by breaking open the remaining ligament in the cracked section, and an I

intermittent dimpled rupture region along much of the 0.D. surface.

This latter region was believed to have been cracked by stress overload l

during service as evidenced by a heavy oxide coating.

All OD surface areas for which the oxide layer was sufficiently removed for observation exhibited a dimpled rupture morphology. The crack tip was observed to be growing preferentially at mid-wall locatious (see Figure 9). The l

dimpled ruptured region extended for only a short distance along the l

I.D. but intermittently along the entire O.D.

Most of the evidence l

points to an I.D. origin for the crack.

l l

Typical SEM micrographs of the intergranular region are shown in Figure 10.

The metal grains are very elongated in the longitudinal i

I direction. No evidence of fatigue is observed on the fracture face.

Figure 11 shows a typical overload area along the 0.D. surface.

The depressed or slumped region along the 0.D. surface, observed in the visual examination, is clearly visible. Multiple surface cracks can be seen above a thin layer of dimpled rupture failure.

Most likely the 4

e i

i crack was growing outwardly in this region, and the final web of l

material was plastically deformed, causing the depression, before final overload failure.

3.2 Metallography A transverse metallographic cross-section of the upper half of l

the crack was made approximately 0.3 inches in from the visible 0.D.

crack tip. Additional metallographic sections were made progressively i

towards the crack tip to show the growth of the defect. Typical micrographs are shown in Figure 12 throug': 17.

In the initial planes the crack penetrated only the I.D. surface.

In the 0.020 inch depth section from the original plane, the crack penetrated neither surface.

This verified the SEM observation of a mid-wall preferential growth direction at,the opposite crack tip.

The transverse micrographs show an intergranular crack although the crack tip near the 0.D. surface on the first plane appears blunted.

The 0.D. depression along the crack is visible and secondary 0.D.

surface cracks can be seen. The grain size in the transverse direction is very fine.

To better quantify the elongated grain structure observed in the SEM, a longitudinal metallographic mount was also prepared. The plane chosen was perpendicular to the tube diameter so that the crack near the 0 D. would be included in the view. Typical micrographs are shown in Figures 18 and 19. The grain size in this direction was also fine but showed a longitudinal to transverse size ratio of between 5 and 10.

The grains appear twinned and heavily cold worked. The primary crack appears blunt and may show a transgranular component. The secondary cracks may also be mixed mode and are very blunt which would be typical of over-load ductile failure. These are likely the 0.D. surface cracks visible in Figures 11 and 13.

I i

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3.3 Microhardness To quantify metallographically observed evidence for cold work, 4

i micro-hardness traces were taken on the transverse and longitudinal metallographic mounts. 500g Knoop indentations were used. The results I

are listed in Table 2.

Typical values were 400 KHN for the bulk material. This value is about twice as high as expected for typical mill annealed Inconel 600

{

tubing.

A 400 Knoop hardness roughly corresponds to a yield strength of 150 ksi and indicates approximately 70% cold work.

i To verify this high hardness value, a series of R hardnesses c

f were also performed.

An average R 39 was obtained which verifies the e

high' hardness of the material.

Microhardness measurements taken adjacent to the crack show no evidence of hardness increase relative to the bulk mater'ial indicating that fatigue was not a degradation mechanism.

,I l

3.4 Huev Testing i

Modified Huey tests were conducted on sections of tube removed from the top of Section 3, the crack region, and the bottom of Section 2 l

(near the tubesheet bottom). Weight losses from the HNO3 exposure for 2

48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> were 13.3. 10.0 and 145 mg/dm / day respectively. Westinghouse 2

i uses a greater than 200 mg/dm / day weight loss to indicate tube i

sensitiration. None of the specimens tested were determined to be sensitized.

i 3.5 Energy Otspersive Spectroscopy (EDS) of Crack Face i

EDS was used on the crack surface to verify the composition of the alloy and to analyze for gross composition of the crack oxide coating. The approximate composition of the tubing material was determined to be:

j 6

i l-

. - = _ _

i Si 0.49 w/o Cr 16.35 Fe 8.29 Ni 74.97 This analysis verifies the composition as Inconel 600.

The EDS analysis of the oxide coating on the crack surface is as i

follows:

j Si 0.24 w/o 1

Cr 4.94 Hn 0.71 Fe 62.88 Ni 22.39 Cu 5.51 Zn 2.88 Zr 0.46 j

1 EDS cannot detect low atomic number elements such as oxygen, hydrogen or i

carbon.

1 3.6 SEM-EDS of 0.D. Surface SEM of the crack on the 0.D. surface is shown in Figures 20 thru

22. The crack is basically a single straight crack which shows

{

extensive branching near the crack tips. The mid-portions of the crack show jagged components which connect to form the single straight crack.

l As observed in Section 3.1 and 3.2, the 0.D. surface deformed and i

slumped down to meet the growing intergranular crack.

It is believed that the jagged components represent local areas of this tensile slumping. EDS analyses of surface deposits at the crack edge were obtained in a number of areas. Other than the base alloy elements, I

significant concentrations of Si and Ng were noted along with lesser l

concentrations of S and Cu. Tabic 3 shows representative chemical l

7 t

1

~

~

compositions from three separate areas adjacent to the crack edge - the first one comes from the deposit shown in Figure 20 while the latter two come from the deposits shown in Figure 21.

3.7 SEM-EDS of I.D. Surface SEM of the crack on the I.D. surface is shown in Figures 23-24.

The appearance of the -crack on the I.D. surface is similar to that

^

observed on the O.D. surface in that some branching was observed at crack tips and in that the basically straight single crack appeared to be composed of jagged and inter-connected segments. Minimal deposits were observed adjacent to the I.D. crack. Table 4 shows an example of the deposit composition adjacent to the crack on the I.D. surface. This region was rich in Si and Zr.

l 3.8 Auger Auger surface analysis was performed on the 0.D. and I.D.

surface deposits, the crack fracture face deposit, and the fresh dimpled i

ruptured (clean) area near the crack tip produced in tearing open the crack. The latter is a measure of the environmental contamination i

produced prior to inserting the specimen in the test chamber vacuum.

Results are shown in Table 5.

Ignoring changes in the base metal elements, the crack deposit film was relatively high in S, Cu, 2n, and Sn compared to the clean metal. The potentially deleterious element S was present in a lesser amount than in the surface deposits.

3.9 Microprobe Electron microprobe elemental analyses traces were made across the 0.D., I.D., and crack deposits using the transverse metallography mount.

Elements analyzed were Na, S, Cu, C1, Zn, Pb, Ni, Fe, Cr, 0, Sn and C.

Results were as follows:

8

.!.D. Deposits C1 and 0 concentration Ni depletion 0.D. Deposit Zn, Fe and 0 concentration Na, Ci slight concentration Ni depletion Crack Deposit 0, C1 and C concentration Na and S slight concentration Cr, Fe, Ni depletion 3.10 Electron Spectrometry for Chemical Analysis (ESCA)

ESCA was performed on the tube and fracture face deposits.

Due to the small sample size and due to surface contamination (primarily l'

carbon) caused by the necessity of breaking open the fracture face outside of the ESCA vacuum chamber, no meaningful results were obtained.

1 i

4 D

-l 1

i i

i 9

i i

1

0 4.

DISCUSSION AND CONCLUSIONS An axial crack O.85 inches long on the 0.0., 0.71 long on the I.D., and slightly in excess of 1.0 inches long at mid-wall locations was found approximately 9 inches above the tubesheet on the hot leg side of steam generator E-088 of Tube R89 C151 from SONGS Unit 2.

The intergranular corrosion portion of the defect intersected a longer length of the I.D. surface than of the 0.D. surface. The O.D. surface crack did not show intergranular corrosion, but rather ductilely torn material (wherever the oxide coating could be sufficiently removed) which had slumped down to meet the intergranular corrosion growing towards the 0.D. surface. To a lesser extent (near the crack tips only), the I.D, surface also showed tensile tearing.

It is believed.that the crack originated on the I.D. by primary water stress corrosion cracking (PWSCC). The reasoning behind this hypothesis is three fold.

First there is the observation that more intergranular corrosion intersected the !.D. surface than the 0.D.

surface. Second, there is no supporting data for an 0.D. origin to the crack. The crack location was well away from any crevice which could have concentrated deleterious impurity elements wh'ich might have caused caustic-originated intergranular corrosion.

Further, there was no evidence of a sludge pile in this region which might have concentrated deleterious element.s.

Huey testing showed that the tubing was not sensitized, therefore reduced sulfur attack of sensitized tubin6, which does not require a concentrating mechanism, was ruled out.

The third factor behind the hypothesis of PWSCC is the metallurgical condition of the tube.

PWSCC in the field has occurred only in mill annealed Inconel 600 tubing which has experienced geometrical deformation which resulted in high stress or strain 10

I i

j gradients. Laboratory work,III however, has shown that as-drawn tubing i

i is significantly more susceptible to PWSCC than is mill annealed f

tubing. Meta 11ography and hardness measurements on tube R89 CIS) yielded results which were more typical of as-drawn tubing than mill-i i

t

(

annealed tubing. Mill annealed tubing is observed to have an equiaxial i

grain structure with a grain size larger than that observed for Tube R89 l

C151. The grains in tube R89 C151 were highly elongated, small, and l

showed evidence of cold work and extensive twinning.

Hardness measurements also showed that the tubing was highly cold worked and not as soft as mill annealed tubing would be expected to be. Consequently, it is hypothesized that the unusual microstructure of this tube caused a high susceptibility to PWSCC which resulted in the through-wall axial

crack, i

t i

11

[

e

d i

1 5.

RECOMMENDATIONS i

It is recommended that autoclave corrosion testing of tubing remaining from this investigation be initiated to confirm r. hat Tube R89 j

C151 is highly susceptible to PWSCC.

Reverse U-bend corrosion specimens would be fabricated from Tube R89 C151, as would controls from tubing with a known PWSCC response. They would then be tested in 680*F pure water with a hydrogen overpressure.

Results from this test could then

[

I be used to evaluate the hypothesis of a PWSCC mechanism.

Additionally, it is recommended that any further tubing, which j

may be pulled and examined, should be selected on the basis of it having I

a partially through-wall crack in order to definitively confirm that the 1

degradation is !.D. originated.

s d

t 4

e i

1 I

e I

12 s

O

6.

REFERENCES 1.

G. P. Airey, " Optimization of Metallurgical Variables to improve Corrosion Resistance of Inconel Alloy 600," EPRI NP-3051, July 1983.

13

Table 1 e

Dimensional Measurements of Supplied Tubing Outside Axial Angular Diameter Wall i

Location Location (inch)

(inch)

I Bottom of 0*

0.742 0.048 Piece 2 90*

0.745 0.049 4

180*

0.049 1

270*

0.050 1

i Bottom of O'

O.746 0.049 i

Piece 3 90*

O 743 0.048 i

l 180*

0.049 270*

0.049 i

Top of O'

O.742 0.048 i

Piece 3 90*

0.745 0.049 180*

0.049

{

l 270*

0.050 4

f I

i 4

i l

I 4

I l

1 l

l i

t i

I i

t l

I r

j l

. ~. - -. - - - -..

i

[

Table 2 Microhardness Results Orientation Location KHN Transverse Near 0.D.

418, 415 j

0.D..near crack 388, 404 Bulk, midwall 399, 418 i

Near crack, 388, 383, 384, 380 Midwall 291, 380 Near I.D.

363, 363 I.D. near crack 378, 391 i

Longitudinal Near crack 312 378 l

Successively away 375

]

from crack 396 399 i

356 343 e

375 s

i 427 361 I

d d

i

'S t

15 i

t

I 3

t I-Table 3 EDS of 0.D. Surface Deposits Standardless EDS Analysis (ZAF Corrections via Magic V)

Element Weight Atomic l

' Location

& Line

- Percent Percent i

Area 1 MG KA 22.21 33.85 (Figure 20)

SI KA 21.32 28.14 S KA 3.59 4.15 CA KA 0.65 0.60 CR KA 5.32 3.79 FE KA 20.06 13.31 e NI KA 11.55 7.29 CU KA 12.02 7.01 i

ZN KA 3.28 1.86 i

Area 2 HG KA 19.20 31.57 l

(Figure 21)

SI KA 14.49 20.62 S KA 1.82 2.27 CA KA 0.26 0.26 TI'KA 0.22 0.19 CR KA 13.35.

10.27 i

FE KA 18.02 12.90 NI KA 26.79 18.24 CU KA 5.84 3.67 Area 3 HG KA 25.25 32.15 (Figure 21)

SI KA 42.78 47.14 S KA 6.76 6.52 i

CA KA-1.20 0.92-TI KA 1.62 1.04 CR KA 1.85 1.10 FE KA 14.68 8.14 NI KA 3.57 1.88 CU KA 2.20 1.07 HO LA 0.09 0.03 l

16 w

rw y

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e.

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Table 4 1-j; EDS of I.D. Surface Deposit Standardless EDS Analysis i

(ZAF Corrections via Magic V) t, Element Weight Atomic

& Line Percent Percent 1

i-SI KA 0.76 1.53 i

CA KA 0.05 0.07 TI KA 0.42 0.50 i

CR KA 9.18 10.02 FE KA 48.03-48.79 i

NI KA 38.46 37.16 I

ZR'LA 3.11 1.93 i

1-1 4

4 1

9 1

t 3

J i

2 I

t j

e 17 n

l l

. Table 5 3

Auger Results w/o s

./

Element I.D. Deposits 0.D. Deposits Crack Deposits Clean Metal S-1.73 6.03 0.62 0.25 C1 1.52 1.11 0.43 1.39 C

10.48 18.45 3.18.

2.96 N

1.47 1.38 0.22 0

Sn 0

0 0.76 0

0 34.39 31.28 29.07 38.89 Cr 10.77 0

0 7.37 Fe 13.14 2.12-22.40 3.38 Ni 21.58 1.49 14.76 45.01

)'

Cu O

O 17.94 0

N 1.16 6.51 0-0 2

2n 1.89 8.89 9.56 0

SiO 1.77 17.97 1.04 0.76 x

M0 0.09.

4.76 0

0 6

I

)

i i,

's.

i.

-18

[

1.

Tube Sketcht I = 2" OF TUBING

==================I============

BOTTOM OF 1ST I

EGGCRATE (49.25")

>I< 3rd Cut (46")

I I

I Piece-III==>

I I

I i

====_________I FLAW LOCATION (32")

=__-- ---

I

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I I

I===============

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==

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--More--

I Piece-II==>

I I

I I

I

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I

================I=============

BOTTOM OF T/S (O")

)

Figure 1.

Tube R89C151 Cutting Diagram as Received from the Site 1

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)

1 Figure 3.

Laboratory Eddy Current Signals from the Axial Crack in Piece 3 of Tube R89C151 HL, as well as Calibration Data RM-5812

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V

~%>:Lv%:*r z; Figure 21. SEM of Upper Tip of Crack on O.D. Surf ace Showing Branching at Crack Tip l l l l l RM-5829 i I I i 1 1 i i i w: 17 -'. m 1 ,t 4-t.. a -~ ~ _. - .-.m . s c.. r

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• TTri' Q.-

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'Q "4 % m ~ 4' - e Y' h. '.$, f.,,, )j.x.p$'. f'l..ft $lO-950X fy .- 4 [ '.g......: -.. g$,jf v.,w<*q. y - .2 3,< h'f .i 9 *ji.<gt . %p..g-7 ~.' ,f,. i- ~, )t..' y ^ t 4% , J, Figure 24. SEM of Crack on I.D. Surface Showing Upper Fracture Tip and Branching I RM-5832 APPENDIX B Destructive Examination of Tube R89-L151 from San Onofre Nuclear Generating Station - Unit 2 Steam Generator 88 (Combustion Engineering Report) G 9 Destructive Examination of Tube R89 L151 from San Onofre Nuclear Generating Station - Unit 2 Steam Generator 88 January 1985 E. P. Kurdziel C. M. Owens D. E. Powell E Reviewed by:. [d M _ + ~ J. F. Hall Supervisor, Corrosion Engineering Technology-(L/ Q Approved by: K. R. Qraig Manager, Syster: ChamAs and Corrosion COMBUSTION ENGINEERING, INC. NUCLEAR POWER SYSTEMS 1000 PROSPECT HILL ROAD WINDSOR, CONNECTICUT 06095 v. N J r LEGAL NOTICE THIS REPORT WAS PREPARED AS AN ACCOUNT OF WORK SPONSORED BY COMBUSTION ENGINEERING, INC. NEITHER COMBUSTION ENGINEERING NOR ANY PERSON ACTING ON ITS BEHALF: 4 A. MAKES ANY WARRANTY OR REPRESENTATION, EXPRESS OR IMPLIED INCLUDING THE WARRANTIES OF FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY, WITH RESPECT TO THE ACCURACY, COMPLETENESS, OR USEFULNESS OF THE. INFORMATION CON-TAINED IN THIS REPORT, OR THAT THE USE OF ANY INFORMATION, APPARATUS, METHOD, OR PROCESS DISCLOSED IN THIS REPORT MAY NOT INFRINGE PRIyATELY OWNED RIGHTS: OR i B. ASSUMES ANY LIABILITIES WITH RESPECT TO THE USE OF, OR FOR DAMAGES RESULTING FROM THE USE ' 0F, ~ ANY INFORMATION, APPARATUS, METHOD OR PROCESS DIS-CLOSED IN THIS REPORT. I i l l-TABLE OF' CONTENTS SECTION TITLE PAGE NO. List of Tables iii List of Figures iv 1.0'

SUMMARY

1,

2.0 INTRODUCTION

1 3.0 VISUAL EXAMINATION 2 4 4.0 DIMENSIONAL MEASUREMENTS. 2 5.0 EDDY-CURRENT TESTING. 3 3' 6.0 RADIOGRAPHY 't 7.0 LIGHT MICROSCOPY EXAMINATIONS 7 8.0 CHEMICAL ANALYSIS ,15 c 9.0 MODIFIED HUEY TESTS 15 10.0 MICR0HARONESS MEASUREMENiS -18 j 11.0 X-RAY DIFFRACTION /X-RAY FLUORESCENCE 19 e -i- \\

. ~ -. -. -.. -. ~. -. TABLE OF CONTENTS (continued) j ~ i SECTION TITLE PAGE'NO. 12.0 OISCUSSION 19 13.0' CONCLUSIONS 21 t- -14.0 RECOMENDATIONS 22 f I' 4 1 1 4-l I is l i 4 i 4 1 1 f f e i ' l l f' -ii- _,_..,.-i. , ~. - ,<ve- ..--.y, ,,.--.,,a

1 LIST OF TABLES t l. TABLE NUMBER. TITLE PAGE N0. l I 7-1 Dual Etch Procedure for NiCrFe Alloy 600 8 e l 8-1 Chemical Analysis of Tube Sections 17 1 11-1 X-Ray Fluorescence Results 20 i i I 4i 6 4 t t i i i-l- 2 5 3 j. i f 4 l -iii- 't t 8 'h ^ a.

LIST OF-FIGURES . FIGURE NUMBER TITLE PAGE NO. 4 5-1 1984 Inservice -4 5-2 Windsor - Prior to Descaling 4 3 Windsor - Sectioned & Descaled 4 5-4 Conductivity Locus 5 6-1 Photograph of Radiograph 6 7-1 Tube Sectioning Diagram 9 't 0 7-2 ECT Transition Section-10 7-3 Microstructure of Tube 11 7-4 Microstructure of Upper Transition Region 12 7-5 Microstructure of Midpoint of Transition Region 13 1 7-6 Microstructure of Lower Transition Region. 14 8-1 SEM/EDS Analysis of Tube Sample 16 1 i l -iv- .,e.w-e

i 1.0

SUMMARY

i Steam generator tube R89 L151 from the San Onofre Nuclear Generating Sta-tion (SONGS) Unit 2 f ailed in service in 1984, resulting in an unscheduled outage, during which the tube was plugged. Eddy Current Testing (ECT) by the absolute mode detected an area where the test data was offset from the normal signal baseline for this tube. A section of tube R89 L151 was pulled during the-first refueling outage and examined to characterize the ECT indications. Destructive examination revealed that the microstructure of the tube varied along its length from a fine grained, annealed struc-ture to a cold worked structure in the ECT transition region. Additional-ly, the cold worked structure showed signs of significant sensitization. The metallurgical condition of the tube appeared to be the result of a breakdown in the manufacturing process.

2.0 INTRODUCTION

During 1984, one tube in SONGS Unit 2 and two tubes in Unit 3 steam generators developed leaks resulting in unscheduled outages. At the time of these f ailures, the ' tubes were plugged and the stations brought back up to full power. The leakage was due to through-wall defects on the hot legs at mid-span locations. ECT on the f ailed tubes revealed an area where the absolute mode test data deviated from the normal signal baseline between the first and second eggcrates. (This area will be identified as the ECT transition area.) One of the tubes, R89 L151, was pulled from steam generator 88 at SONGS - 2 during the outage in December 1984. Another vendor performed a destructive examination on the portion of the tube that contained the through-wall defect to determine the cause of the defect. Combustion Engineering examined the section of the tube which contained the offset ECT indication. The information available from visual examination was not sufficient to -identify the cause of the the ECT indication. Since the only effective way to identify the cause was to destructively examine the tube, C-E developed a program that included the following:

\\ 1. Visual examination and documentation of the condition of the as-re-ceived tube segment. 2. Dimensional measurements of the tube section 3. Laboratory eddy-current testing 4. Radiographic examination 5. Light microscopy examination 6. Chemical Analysis 7. Modified Huey tests to detect sensitization 8. Microhardness measurements. 9. X-ray diffraction /X-ray fluorescence analyses of deposit samples ~ 3.0 VISUAL EXAMINATION The steam generator tube, as-received, was labeled to identify the lower, i hot leg side and the upper, cold leg side. Videography and photography documented the as-received condition of the tube which was 17 11/16 inches in length. The surface was covered by the black tenacious oxide typically found on Allny 600 exposed to high temperature water, with no apparent defects on the surface of the tube. 4.0 OIMENSIONAL MEASUREMENTS Internal and external diameter measurements were taken along a twelve inch length of the tube (5 inches were removed for Modified Huey testing and bulk chemical analysis). The inside diameter measured 0.647"+.001" and the outside diameter measured 0.7441 001" along the length. Subsequent sectioning showed that-the wall thickness was -0.048"1 001". These measurements are within the as-ordered size specifications of 0.750"+.000" 1 .0075" 0.D. and 0.048"+10t (average wall thickness). I 1 l t b

5.0 E00Y-CURRENT TESTING \\ The defect occurred in an area where the absolute test data at 300 and 100 KHz was offset from a normal signal baseline (Figure 5-1). Thi s offset - was prevalent in the two other failed tubes (SONGS - 3) and was not evidenced in any other tubes examined to date (12/21/84). The possible causative factors which could create such an offset are: Conductive deposits on the tube outside diameter Conductivity increase in the tubing Wall thickness variation Increase in the tube inside diameter Combinations of the above. The section of the tubing containing the transition of the absolute offset was tested prior to sectioning or descaling (Figure 5-2). The test setup replicated the field test and the results closely resembled that of the inservice examination. Testing of the tube after sectioning and descal-ing (Figure 5-3) ruled out conductive deposits as the primary cause of the data shift. A conductivity locus determination was performed on the tube sections using an outside diameter surf ace pancake probe at 400 KHz. The sections above and the section below the transition had the same conductivity loci, although small wall thickness variations were noted. The transition area of the sectioned tube showed a conductivity decrease as well as a wall thickness variation (Figure 5-4). 6.0 RADIOGRAPHY The transition region was examined by radiography to attempt to help characterize the ECT transition region. The radiography was _ performed using a 10 curie iridium-192 isotope source and Custom Pac cassettes containing OR 54 film. Double walled radiography was conducted at 0 0 and 90 orientations. No defects were identified on the radiographs, which are shown in Figure 6-1. 3 -y

FIGURE 5-1 1984 INSERVICE ! : i l j l t ! l l j i l: :1 ::j !l :l - 1 i ^ .;_.!_if!- ' ! boo MHa 19 -;;&i -i.100 KR 1

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7.0 LIGHT MICROSCOPY EXAMINATIONS Longitudinal sections of tube R89 L151 were mounted, ground on sic papers, polished and etched using a dual etch procedure to characterize the tube microstructures. The electrolytic oxalic etchant generally used on Alloy 600 may give misleading information on carbide presence and distribution. As'a result, a dual electrolytic etch procedure using nital to etch grain boundaries and orthophosphoric acid to etch carbides was used, as des-cribed in Table 7-1. Figures 7-1 and 7-2 illustrate the areas of the tube from which the sec-tions were removed for analysis. The most significant specimen was that which extended along the axis of the tube section containing the ECT transition region. The other specimen was removed from the upper end of the tube (cold leg) and used for comparison to the ECT transition region. 1 Photomicrographs of the upper piece af ter the nital etch and the ortho-phosphoric etch are shown in Figure 7-3. The microstructure of this piece, as' shown after the nital etch, was a fine grained, annealed structure (ASTM Grain Size 8-9), which contained numerous intragranular carbides (orthophosphoric etch). This tube structure was atypical of mill annealed steam generator tubing due to the fineness of the grain size, which should be ASTM 5-6,. and the location of the carbides. Addi-tionally, ID side " carbide banding" was present in the microstructure. Figures 7-4 through 7-6 are photomicrographs of the upper, lower and center of the ECT transition region. The' nital etched microstructure of the upper end (Figure 7-4) was similar to that in Figure 7-3, and con-sisted of a fine grained recrystallized material. However, the orthophos-phoric etch indicated carbide 'di stribution along prior-hi story grain boundaries (grain boundaries present during processing prior to the final anneal).

Table 7-1 DUAL ETCH PROCEDURE FOR NiCrFe ALLOY 600

1..

Etch electrolytically in 5T nital j 2 1/4 - 3 1/4 volts approximately 10 seconds 2. Locate tic particle or other surface feature which can be used to relocate the same area 3. Photograph - the nital etch reveals grain boundaries i 4. Lightly polish the specimen to remove the nital etch 5. Etch: electrolytically in orthophosphoric acid / water, 8:1 2 1/2 - 3 1/2 volts approximately 10 seconds 4 4 6. Photograph - orthophosphoric etch reveals carbides-s i I

r -17 11/16" i 4 7/8" =l = i i l 2 1/8"

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s 1

.. ;'t. lM.%...t..... :..*b.:,'ls.?}n. +,p3 a,. i y .l a. '1 Orthophosphoric Etch l l FIGURE 7-5 l Microstructure of Midpoint of Transition Region, Longitudinal View, 500X -

a. W ~~~R.'. y -l. ;- f. A. -~ n~U &W

~ ~" ). 7.% rh.... ) y:-Q... . EW---L ^

• s +, W

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u..

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~.:... Qw. 9 2.,(c.,b.~ &.~ W. A ~...., m. n.; 3 ~ v ,.4... m;; . - r.fes..m2:; % - u ~~ w, t~- .,e, ~F... ...c >. -.,.,<. %s >+ + c',,..v.% c.%. p: ~*,,.... t . s.. .. ?5 '.... a:c. 'J'-.y.Q, ,.p,a -- .+ s t R;.swM:~t*;D=_r.;%.;-s.s.iECg Orthophosphoric Etch FIGURE 7-6 Microstructure of Lower Transition Region, Longitudinal View, 500X

1 The microstructure of the lower end of the ECT transition region (Figure 7-6) was a cold worked structure that contained elongated grains. The carbide distribution coincided with the grain boundaries. Figure 7-5, which is the area in the middle of the ECT transition region, shows a mi crostructure that is approximately 50% recrystallized. The carbide distribution in this area also coincided with prior-hi story grain boundaries (cold worked grains). Microstructures of this type are not typical of mill annealed Alloy 600 tubing in C-E-supplied units. The presence of a cold worked microstructure and the fine, equiaxed grain structure with numerous carbides on prior-history grain boundaries indicates that the tube was not uniformly and sufficiently annealed during processing (i.e. the proper processing to acquire the specified mill annealed condition). 8.0 CHEMICAL ANALYSIS Two areas of the tube were sectioned to determine the chemical composi-tion of the base metal as shown in Figures 7-1 and 7-2. One piece was from the upper (recrystallized) end and another from the ECT transition region. The oxide layer was removed and the specimens decontaminated by immersing them in a nitric / hydrofluoric acid solution. A SEM/EDS qualifi-cation analysis showed that the material composition was consistent with that of Alloy 600 (Figure 8-1). The results of the chemical analyses of the tube samples are given in Table 8-1. The results are also consistent with standard Alloy 600 compositions. 9.0 MODIFIED HUEY TESTS The Modified Huey test was performed to determine the degree of sensitiza-tion in the tube. Sensitization is defined as the formation of chromium-deficient areas adjacent to grain boundaries, susceptible to corrosion in specified environments, as the result of precipitation of grain boundary carbides. I

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i l L 1 Table 8-1 CHEMICAL ANALYSIS OF TUBE SECTIONS

• Element ASME**

Transition Area Cold Leg End Fe 6.0-10.0 7.35. 7.38 i Si 0.5 Max. .134 .145 Cu 0.5 Max. .037 .040 Mn 1.0 Max. .177 .178 Ni 72.0 Min. 76.74 76.66 Cr 14.0-17.0 15.20 15.23 l Co .029 .027 l 5 0.015 Max. .002 .002 C 0.15 Max. .052 .037 i

• Analysis performed by J. Dirats and Co., Inc., Westfied, MA.
  • ASME Boiler and Pressure Vessel Code, Section II, Part B. 58-163, July 1, 1983
    • Cobalt counts as Nickel i

1 i l l

.. ~ r The test consists of placing a pre-weighed sample into a solution of boil-ing 25% nitric acid solution for 48 hours. At the end of this period of time, the sample is removed, dried and weighed. Typical mill annealed material has a weight loss of up to 0.51, with 0.2 to 0.34 typical. Figures 7-1 and 7-2 illustrate the areas of the tube from which the speci-i mens were removed. One specimen was taken from the upper end away from the transition area; one specimen was taken from the ECT' transition area and one was taken from the cold worked end (lower) below the ECT transi-tion region. The results of the tests are shown below: Specimen Location 4 Wt. Loss Recrystallized Section (Upper) 0.5 % ECT Transition Region 8.0 % Cold Worked Section (Lower) 19.4 % The high percentage weight losses indicate a substantial amount of sensi-tization due to the manufacturing process anomalies discussed in Section 7.0. The lower end of the tube was exposed to temperatures high enough to cause precipitation of the carbides and thus sensitization (800 F - 1450 F), while the upper end was exposed to temperatures high enough to cause recrystallization (1500 F - 1600 F). 10.0 MICR0 HARDNESS MEASUREMENTS Microhardness was measured throughout the ECT transition region. The recrystallized end had a hardness reading of HRC (15), which is normal for annealed materials with a fine grain structure. The cold worked end of' the ECT transition region had a hardness of HRC 33, indicative of ~ a cold worked material. These measurements were expected based upon the microstructure of the region. I 6 -la-

O 11.0 X-RAY O!FFRACTION/X-RAY FLUORESCENCE Oxide deposits were removed from two areas of the tube in order to perform for analyses. One set of scrapings was from the 00 of the transition region, and the other set of scrapings was taken from the 00 of the upper end. X-ray diffraction showed the major phase (30% or greater) to be Fe 0 and the minor phase (10% or greater) to be Cu. The results of 34 the X-ray fluorescence are given in Table 11-1. 12.0 DISCUSSION The microstructure of steam generator tube R89 L151 from SONGS Unit 2 was atypical of mill annealed tubes used in C-E supplied steam generators. The presence of a significant change in microstructure indicates a pro-cessing problem at the manufacturer's facility. I In the 2 3/4" long transition region, the microstructure varied from a cold worked structure at the lower end to a fine-grained annealed struc-ture at the upper end. The middle section was approximately 50% recry-stallized. The dual etch examinations revealed chromium carbides pre-cipitated in the prior-history grain boundaries. This type of microstructure occurred because the tube was improperly heat treated. The cold worked structure indicated that specified annealing 3 i temperatures were not reached for this end of the tube, while the fine j grained structure indicates that recrystallization temperatures were reached, but not high enough to allow proper grain growth. The presence 1 of carbides in the prior-history grain boundaries is also an indication that proper temperatures were not reached to solubilize the carbides. Sensitization of the cold worked structure reveals that the lower section of the tube was exposed to temperatures between 800 F and 1450 F, which is the temperature range which carbides will precipitate in grain boundaries without recrystallization taking place.

4 i }- j i Table 11-1 X-RAY FLUORESCENCE RESULTS !n Assumed Oxides Tube Oxide Present, Percent 4 Elements Transition Area Cold Leg i Al 0

• I9
• II 23

{ SiO .21 .17 2 ] P0 .05 .07 25 TiO .12 .17 2 J Cr 0 .88 1.2 23 Mn0 3.2 4.8 Fe 0 66.6* 69.5* 34 N10 1.0 1.0 Cu 20.3+ 18.0+ Zn0 7.0 6.0 I I i i I

• MajorPhase(301orgreater)

)

• Minor Phase (10t - 30t)

I } t i l 20-j ,..--e-.,. ,e -e,.,,g...-_, ~.,.,_,-,,,e::,-c._n,,,.p.e,, ~,,, ,,,ew ,,-,,e-,,-

___m.-- 0 I TubeR89 L151 was annealed in a muffle-tube furnace. This furnace con-sists of - five. tubes, side-by-side, which are surrounded by a box that j ' contains the heat source. The steam generator tubes are pulled through this furnace with hydrogen flowing in the direction opposite the movement of the tubes. The most likely cause of the improper heat treatment was r f an unscheduled furnace shutdown. When this occurs, all the tubes in the } furnace must be removed and run through the furnace again. This did not occur for the tube in question, resulting in the improper microstructure. ~ Though the exact nature of the problem has not been defined, it-is obvious f that the tube did not receive the mill processing required to produce a 3 . roper microstructure consisting of equiaxed, medium-size grains. p l l

13.0 CONCLUSION

S i l The visual and destructive examinations of the steam generator tube-l removed from SONGS Unit 2 during the 1984/1985 outage resulted in the following findings / conclusions:- 1 I 1 1. The tube-section from R89 L151 examined by Combustion Engineering i had a region where the ECT background changed (a transition region), as measured on.the absolute channel. I 2. No defects. were observed in the tube section during ECT, visual, radiographic and d,estructive examinations. 4 3. Dimensional measurements ' indicated that the tube possessed nominal i ~ imensions. d i _4. Microstructural analysis revealed a change in microstructure from a i cold worked structure to a fine grained structure, indicating a . manuf acturing anomaly. 5. Microhardness measurements also indicated a change in microstructure; the readings going from high to-low as the structure changed from i cold worked to annealed. l i

• l

6. Modified Huey testing revealed a substantial amount of sensitization in the cold worked structure and essentially no sensitization in the annealed structure. 7. Bulk chemical analysis results confirmed that the composition met i the specification requirements for Alloy 600. 8. X-ray diffraction /X-ray fluorescence analyses of the exterior oxides indicate a build-up of normal deposits for the environment to which the tube was exposed. 14.0 RECOMMENDATIONS The non-destructive and destructive examinations described herein charac-terized the cause of the ECT reading as a varying microstructure. The specific cause of the microstructure was identified as a temperature gradient along the length of the tube. Additional investigations into the tube processing history are necessary in order to determine the reason for this temperature gradient. These investigations should include a review of all records pertaining to tube R89.L151, as well as conversa-tions with personnel involved in the manuf acturing process. f f i t . i i

APPENDIX C Destructive Examination of Tube R96-L118 from San Onofre Nuclear Generating Station - Unit 2 Steam Generator 88 (Combustion Engineering Report) 4

l I e 2 4 Destructive Examination, of Tube R96 1.118 from. San Onofre Nuclear Generat'ng Station - Unit 2 Steam Generator 88 i n I 4 E. P. Kurdziel C. M. Owens 4 2 1 I t March 1985 i i a 1 Combustion Engineering, Inc. Nuclear Power Systems. 1000 Prospect Hill Road Windsor, Connecticut < 4

1 3 LEGAL NOTICE THIS REPORT WAS PREPARED AS AN ACCOUNT OF WORK SPONSORED BY COMBUSTION ENGINEERING, INC. NEITHER COMBUSTION ENGINEERING NOR ANY PERSON ACTING ON ITS BEHALF: A. MAKES ANY WARRANTY OR REPRESENTATION, EXPRESS OR IMPLIED INCLUDING THE WARRANTIE3 0F FITNESS FOR A PARTICULAR PURPOSE OR MERCHANTABILITY, WITH RESPECT TO THE ACCURACY, _ COMPLETENESS, OR USEFULNESS OF THE INFORMATION CON-TAINED IN THIS REPORT, OR THAT THE USE OF ANY INFORMATION, APPARATUE, METHOD, OR PROCESS DISCLOSED IN THIS REPORT MAY NOT INFRINGE PRIVATELY 0WNED RIGHTS: OR B. ASSUMES ANY LIABILITIES WITH RESPECT TO THE USE OF, OR FOR DAMAGES RESULTING FROM THE USE 0F, ANY INFORMATION,- APPARATUS, METHOD OR PROCESS DIS- ^ CLOSED IN THIS REPORT. e

l t TABLE OF CONTENTS Section Page No. List of Tables lii List of Figures iv 1.0 Summary 1 2.0 Introduction 1 i 3.0 Eddy-Current Testing 2 1 4.0 Visual Examination 3 l 5.0 Dimensional Measurements 3 6.0 Light Microscopy Examinations 3 7.0 Modified Huey Tests '6 ) 8.0 Chemical Analysis 14 e i 9.0 X-Ray Diffraction /X-Ray Fluorescence 14 l 10.0 Conclusions 17 t l' .6 -i-e ,.--_-n_-,

TABLE OF CONTENTS (Con't) Section Page No. 11.0 Recommendations 17 12.0 References 17 -ii-

LIST 0F TABLES Table No. Title Page'No. 5-1 Dimensions of ECT Transition Region 5 Dual Etch Procedure for NiCrFe Alloy 600 7 I 6-1 8-1 Chemical Analysis of Tube Sections 15 l i 9-1 X-Ray Fluorescence Results 16 1 I 4 1 4 .1 i J 4 t i i i-1 .s p 3'-.,, w-y,,-g-y --.-p-y --y mu. ,,wy ,. w,3phy g7g- .y. ,-g qp-w-g -.,,g-p g g r.

• +-s g

w

_ _.. ~. LIST OF FIGURES Figure No. Title Page No. 3-1 Eddy Current Test Indication 2 i i 5-1 Sectioning Diagram 4 i 6-1 Microstructure of Upper Reference Sample 8 6-2 . Microstructure of Upper Transition Region 9 i 6-3 Microstructure of Midpoint of Transition Region 10 6-4 Microstructure of Lower Transition Region .11 6-5 Microstructure of Peak Area 12 1 6-6 Microstructure of Lower Reference Sample 13 L 1 ) i l I I j -1V- .~

1.0

SUMMARY

Steam generator tube R96 Lil8 from the San Onofre Nuclear Generating Station (SONGS) Unit 2 was removed from steam generator #88 during the late 1984/1985 outage. The tube was removed due to an Eddy Current Test (ECT) reading that indicated a deviation from the absolute baseline read-A destructive examination was performed in order to determine the ing. cause of the ECT indications. This examination revealed a tube that possessed a microstructure that was typical of mill-annealed tubing used in Combustion Engineering supplied steam generators. Additionally, the dimensions of the tube conformed to as-ordered specifications.

2.0 INTRODUCTION

In late 1984, a tube (R89 L151) that had been plugged in one of the SONGS Unit 2 steam generators was removed for examination. This tube had failed earlier while in service. ECT on this tube revealed an area where the absolute test data deviated from the normal signal _ baseline (ECT transi-tion region). As described in Reference (1), Combustion Engineering performed a destructive examination on the section of this tube containing the ECT indication, and determined that the microstructure of the tube varied within the ECT transition region. Based upon this information, another tube (R96 Lil8) that displayed an ECT reading containing a devia-tion from baseline was removed for examination. The ECT indication of this tube did not display as severe a deviation as the previous tube, but was representative of numerous tubes in the generator. Combustion Engineering developed a program to destructively examine tube R96 Lil8 in order to determine the cause of the ECT indication. The pro-gram included the following: 1. Laboratory eddy-current testing 2. Visual examination and documentation of the condition of the as-received tube

n .:. : a.. 3. Dimensional measurements of the tube

4..

Light microscopy examination 5. Modified Huey tests for sensitization 6. Chemical Analysis 7. X-Ray diffraction /X-Ray fluorescence of deposit samples 3.0 EDDY-CURRENT TESTING Seven sections from tube R96 Lil8 were shipped to Combustion Engineering from SONGS Unit 2. ECT was performed on each of the sections until the section that initiated the anomalous reading was identified. The labora-tory ECT signal duplicated the field signal. Figure 3-1 shows the laboratory ECT readings at both 100 and 300 KHz. This signal exhibits a shift from the ECT baseline reading which showed a bulk material property change indicative of a conductivity or permeability change. Section 5 l 7 .m T ^ Baseline = I -l i 1 T e-_; _EFJ } } l 1' Transition I l

--.. Peak 1

1 1 I i l ---l T.. P ^

.

~.:r - ^ - ~ 3 l 100 KHz 300 KHz i Figure 3-1 Eddy-Current Test Indication at 100 and 300 KHz -w-y

Section 5 of the tube, located between the 3rd and 4th eggcrates, con-tained the deviation from the ECT baseline, and Section 4, located between the 2nd and 3rd eggcrates, contained that portion of the ECT reading where the signal slowly returned to the baseline. The tube sections were marked to record the positions where the signal began to deviate from the base-line, where the peak reading was reached, where the return to baseline be-gan, and where the return to baseline was completed. 4.0 VISUAL EXAMINATION Visual examination revealed no defects in the tube sections. The outside surface of the tube was covered with a discontinuous black oxide and ap-peared to be in a normal condition, with several minor scratches running along the length due to the tube pulling operation. The inside surface was smooth and showed no evidence of mistreatment from rotary straighten-ing. 5.0 DIMENSIONAL MEASUP.EMENTS Upon completion of the visual examination, the tube sections were cut to produce samples for further analysis, as shown in Figure 5-1. The piece of tube containing the ECT reading showing the deviation from baseline to peak reading (ECT transition region) was measured to determine 10, 00 and wall thickness. These readings are given in Table 5-1. The inside diameter was 0.652+.001"; the outside diameter was 0.745"+.001"; and the wall thickness was 0.045"+.002". These measurments are within the ordered size specifications of 0.750"+.000".0075" 0.D. and 0.048"+10% (average wall thickness). 6.0 LIGHT MICROSCOPY EXAMINATIONS Longitudinal sections of tube R96 Lll8 were mounted, ground on sic papers, polished and etched using a dual etch procedure to characterize the tube microstructure. Single etch techniques used on Alloy 600 may give mislead-

i f 7 !h. 5 4 no n i o t i c t v e c S a l e e a S n n i i l dt e un s t u a h i o B gM n o o t L nr u t d e e R i y /f e u ,I 1 1 i l d f o M h e l t p nm ua oS b M a y l e el a r uac A H ni i m k g dd e a euh e i t C e P f i n i gk i d nl l oou e MLB s a 8 B II i i 8 i / n l 1 4 3 o a i n 8 t i 2 i d t s d un n Jt u a i o r gM .*l T n z I i g ie o L 4 / 1 8 2 8 / 3 en 6 i 1 l l a e ce s il 2 a 4 k mp / B / l em 1 h , uh a o 3 BCS 0 t 4 1 1 l n a r n u i t d t e un R t u O i o gM no L d e c 4 O 4 < li yt f cs dIlue i T o M g m.OCx1P m' mn O EO o~2Ox g = ,a* G

TABLE 5-1 DIMENSIONS OF-ECT TRANSITION REGION -Reading No. (every 1/4 in.) ID (in) OD (in) Wall Thickness (in) Upper 1 .6528 .7461 .0465 2 .6530 .7460 .0460 3 .6530 .7466 .0460 4 .6529 .7457 .0460 5 .6528 .7449 .0460 6 .6538- .7454 .0460 7 .6524 .7450 .0460 -8 .6534 .7441 .0465 9 .6524 .7447 .0462 10 .6516 .7442 .0440 11 .6524 .7450 .0470 12 .6520 .7461- .0450 13 .6538 .7457 .0450 1 Lower End w

........c.n. e ing information on carbide presence and distribution. As a result, a dual electrolytic etch procedure using nital to etch grain boundaries and orthophosphoric acid to etch carbides was used. This procedure is des-cribed in Table 6-1. Samples for microstructural examinations were removed from four areas of the tube sections (see Figure 5-1). One pair of photomicrographs was taken of the Section 5 (upper) reference sample, one pair from the Section 4 (lower) reference sample, one pair from the peak area, and three pairs from the transition region. The photomicrographs for each of these samples are shown in Figures 6-1 thru 6-6. The microstructures from each of these areas were identical. The nital etch revealed a medium-grain sized (ASTM Grain Size 6.5), annealed structure. The orthophosphoric -acid etch revealed a discontinuous carbide network in the grain boundaries. This type of structure is characteristic of the mill-annealed Alloy-600 tubing used in Combustion Engineering supplied steam generators. During the preparation of the photomicrographs, microhardness measurements were performed on the samples. All possessed a microhardness of 160-171 (OPH), which is equivalent to a hardness reading of HRB 84-86. These hard-ness readings are consistent with the microstructures of the samples. 7.0 MODIFIED HUEY TESTS The Modified Huey test was performed to determine the degree of sensitiza-tion of the tubing. Sensitization is defined as grain boundary-chromium depletion caused by carbide precipitation, resulting in a chromium-defi-f cient area which is susceptible to -corrosive environments. The test consists of placing a pre-weighed sample into a solution of boiling 25% nitric acid solution for 48 hours. Typical mill-annealed material has a weight loss of up to 0.5% with 0.2 to 0.3 % typical. i

Table 6-1 DUAL ETCH PROCEDURE FOR NiCrFe-ALLOY 600 1. Etch electrolytically in 5% nital 2 1/4 - 3/14 volts approximately 10 seconds 2. Locate tic particle or other surf ace feature which can be used to relocate the same area 3. Photograph - the nital etch reveals grain boundaries 4. Lightly polish the specimen to remove the nital etch 5. Etch ' electrolytically in orthophosphoric acid / water, 8:1 2 1/2 - 3 1/2 volts approximately 10 seconds

l 6.

Photograph - orthophosphoric etch reveals carbides i i a i l' t. 4 e

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l '. ' '. = " 2.s 5w- .. #,* ',,n.... y e. ?.-

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......a.. s w,. . l g. h i ' 3,.:. l Orthophosphoric Etch FIGURE 6-1 Microstructure of Upper Reference Sample, 500X (NegativeNos. 71136,71137)

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y n.

. w.A. m + n 4. v1 Et 5 ~. n ...~. ;..., (. ?- ( 4 d. j...g s . w:g "" K 4 t n u, y:a .5 W. S ,V- .: h ..m :. r v k,. l :. . h. . a .c - n;;~;. $.-_.'. ty )t l}6, X::. - ly.{ Y. 4 W *1 1 i_.y^m; N- %.. v. x 1 . y%;My.g?x% ~.. - -', [g,., % t *

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.m B Orthophosphoric Etch FIGURE 6-2 Microstructure of Upper Transition Region, 500X (Negative Nos. 71125,71126) _9

i

. s

' - ? '.Y ~. ?!- m

j i

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2. s::,.

y . ; 1.-w . y' ! :.*. ~.< *,. y..:.. ]. ~. e- . f %.. *: 7..;.. c ,s . ~. qh. _., _ _- \\ ....~,. ,. u,. .A ;:~.'. ' "h,.l...: s.e....., m ./ .)'. ki: '.'p'd:f%. { i (. -. ;: p w;,.. ... ? ?'. h e- .s,.. ..li. g. ;.g)".. s.. .-' M . ~- ^ \\ ..) s.,.. l .' V.f.,*;;;. i \\ y' / ~ .s x.' s ?. e Orthophosphoric Etch FIGURE 6-3 Microstructure of Midpoint of Transition Region, 500X (Negative Nos. 71127,71128) .,,-----.--.,--n..,,._

G

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...,... W. . ~.. .5$$?N4f '.r,, a.*c.y... f. gg c 9lG5'd.) 6

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.. - W n N.y... N, j i .y.. r. m ..g; .; :.s n, ..,,..: ~g.. - }'.. 'T w -g.n .a g .g3 .g.- Nital Etch E.Ti '-}i' 42 '? h;k a ? T+M,:v ..s .h ^5

e..?/.l,.egMU?t

.s Y M)h.M.?k?kY';.-O d b' Y-(. ' ~ I ...d

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---

,-w.----

.. - -- - -, - -- - - -. -,.~. - - -.. - -,, - -, - - - - - - - -

... _ = _... _. Three samples were prepared for these tests. As shown in Figure 5-1, one piece was taken from the lower end of Section 4 (reference piece), one from the upper end of Section 5 (reference piece) and one from the peak area reading of Section 5. The results of these tests are shown below: Specimen Location T wt. Loss Upper Reference 0.12 Lower Reference 0.16 Peak Area 0.12 These values indicate the material was not sensitized. 8.0 CHEMICAL ANALYSIS Samples from two areas of Section 5 were sectioned and analyzed to deter-mine the chemical composition of the base metal. Figure 5-1 illustrates the areas from which the sections were removed. One piece was removed from the bas.eline region and the other from the peak area. The oxide layer was removed by scraping and grinding, and the specimens decontam-inated by immersion in a nitric / hydrofluoric acid solution. The chemical composition of the tubing, Table 8-1, is consistent with the ASME 58-163 (Alloy 600). 9.0 X-RAY DIFFRACTION /X-RAY FLUORESCENCE 0xide deposits were removed from the outside surface of the ECT transition region in order to determine the composition of these deposits. X-ray diffraction showed no major phase (30% or greater) to be present and the minor phases (10% to 30 %) to be Fe 0 and elemental Cu. The results 34 of the X-ray fluorescence analysis are given in Table 9-1.

Table 8-1 CHEMICAL ANALYSIS OF TUBE SECTIONS

• Element ASME**

Baseline Area Peak Area Fe 6.0-10.0 7.23 7.06 Si 0.5 Max. .134 .136 Cu 0.5 Max. .061 .063 Mn 1.0 Max. .200 .198 Ni 72.0 Min. 76.69 76.75 Cr 14.0-17.0 15.25 15.28 Co .030 .030 S 0.015 Max. .003 .002 C 0.15 Max. .023 .026

• Analysis performed by J. Dirats and Co., Inc., Westfield, MA.
  • ASME Boiler and Pressure Vessel Code, Section II, Part B, 58-163, July 1, 1983
    • Cobalt counts as Nickel

) o

. _ =. Table 9-1 X-RAY FLUORESCENCE RESULTS Assumed Element Concentration (Pct) Oxide Concentration (Pct) Al 1.5 Al 0 .8 23 Si 5.4 SiO 11.6 2 P .2 P0 .5 25 S .1 S0 .3 3 C1 .3 Cl .3 K .2 K0 .2 2 Ca .3 Ca0 .4 Ti .2 TiO .3 2 Cr 13.8 Cr 0 20.2 23 Mn 1.3 Mn0 1.7 Fe 19.9 Fe3 4 27.5 0 Ni 17.4 Ni0 22.2 Cu 10.8 Cu 10.8 Zn 1.4 Zn0 1.7 l l 1

.a.: a= ...: w_.

= n.:. a a -

10.0 CONCLUSION

S 1. Tube R96 Lil8 contained a region in which the ECT reading showed a gradual shift from the baseline reading. 2. _ No defects were observed in the tube section during ECT, visual and destructive examinations. 3. The dimensional measurements of the tube-(ID, 00, wall thickness) conform to specifications. 4. Microstructural analysis revealed a " normal" tube, with grain struc-ture typical of mill-annealed tube. 1 5. Modified Huey tests indicated no sensitization of the tube material. 6. Chemical composition of the tubing was consistent with the require-ments for ASME SB-163 (Alloy 600). 11.0 RECOMMENDATIONS The destructive examination of tube R96 L118 revealed a typical mill-annealed microstructure. Based upon this, C-E recommended that tubes that possessed a similar ECT indication in SONGS Units 2 and 3 not be plugged. However, further analysis must be performed to determine the material characteristics that caused a shift from the baseline reading.

12.0 REFERENCES

1. Kurdziel, E. P., Owens, C. M., Powell, D. E., " Destructive Examina-tion of Tube R89 L151 from San Onofre Nuclear Generating Station - Unit 2 Steam Generator 88," January 1985. j

APPENDIX 0 Fabrication and Testing of an As-Drawn / Mill Annealed Eddy Current Standard (Westinghouse Report) D

-... ~. - SG-85-02-046 Page 1 of 10 i E R.b c.A_c_,m %.1.c3o a O c(,,_T se as t. i. o.g of an A s -D r a w n / b11 1 1 Annealed _ EE d d v C L.Ar"r" e n t StEMdar"d Prepared by J. M. Gilkison and A. Sagar Westinghouse Electric Corporation Steam Generator Technology Division P. D. Box 955 Pittsburgh, Pennsylvania 15230

. ~,. i l SG-85-92-946 ^ t Page 2 of la l Fabrication and Testing of an As-Drawn / Mill Annealed Eddy Current Standard Introduction Westinghouse has prepared an eddy current standard using Inchnel.6BO steam generator tubing in which a portion of the tube material is in the as-drawn (cold worked) condition and the remainder of the tube is in the mill annealed condition. 'l The tubing was then eddy current tested to determine the effect of the metallurical condition of the tube material on the eddy current signals generated during inspection of the tube. Specifically, the testing was performed to determine if an eddy current technique can be used to differentimte between the as-drawn and mill annealed materials in the tube. The present report discusses the fabrication an eddy current inspection of the tubing. This effort was funded by the Southern California Edison Company (SCE). Material Inconel 600 tubing nominally 3/4-inch OD by B.843-inch wall f thickness was used in this investigation. The tubing was manufactured by the Westinghouse Specialty Metals Plant, Blairsville, PA, using standard procedures for manufacturing steam generator tubing except that the tubing was in the as-drawn condition, that is, the tubing was not mill annealed l or otherwise heat treated after the final drawing operation. The chemical analysis of the Inconel given in Table I. 600 in the tubing is Eddy 14!rrent Eauloment Zetec MIZ-12 eddy current test equipment was used for this during this investigation. The calibration dates for the signal generator was January 24, 1985 and for the CRT was November 38, 1984. I The eddy current signals were recorded on a Zetec Model i HP3968AZ tape recorder and could also be displayed on a Gould I Brush 220 two channel strip chart recorder. A S.631 inch diameter bobbin wound probe was used for all inspections. The eddy current probe was manually translated axially in the tube for all tests. Thus, some distortion exists in the tube length axis of the strip chart recordings. However, the voltage variations (y-axis) were calibrated to a known standard and are consistent for all tests. . e.

l 1 ~ SG-85-02-046 Page 3 of 10 Fabrication and Inspection Procedures The following procedures were employed during the f abrication and eddy current inspection of the standards 1.0 The tube was cut to length,'24-inches, and was rinsed with water. 2.0 The tube was addy current inspected in the as-drawn I condition. 3.0 Rockwell hardness tests were conduc?id on each end of the tube. 4.0 Thermocouples were attached within one inch of each end of the tube. 5.0 A tube furnace with a flowing hydrogen atmosphere was preheated to 1800 F. Approximately, 10-inches of the tube was inserted into the hot zone of the furnace and was held 1 minute at temperature. The heated end of the tube was moved to a cool zone in the furnace (area of the furnace where the furnace Jacket was water cooled) and was cooled to 600 F. The tube was then removed f rom the f urnace and allowed to cool in air to room temperature. The temperature of each end of the tube was monitored during the heat treatment by potentiometers attached to the thermocouples. 6.0 The thermocouples were removed from the tube and pieces of tubing approximately 3/8-inch in length were removed from each end of the tube for metallographic examination. 7.0 The tube was re-inspected by eddy currents using the same equipment and procedures as used previously. 8.0 The ID surface of the tube was honed using silicon carbide rotary flex hones, and the oxides on the tube i DD were removed using aluminum oxide 320 grit paper. i 9.0 The tube section was eddy current inspected again. Results and Discussion During the heat treatment, the thermocouple on the unheated end of the tube indicated a maximum temperature of 340 F while the heated and indicated the required 1800 F. ) The results of the hardness tests before the heat treatment showed the tube to have a nominal hardness of 100 Rockwell B on both ends of the tube, Table II. After the heat treatment, the hardness on the heated end of the tube was

. ~..... -.. SG-85-02-046 Page 4 of 10 reduced to nominally 70 Rockwell B. The hardness of the unheated tube end was unchanged. The microstructure of the heated and unheated ends of the tubing are shown in Figure 1 and confirm that the tubing materials was recrystallized by the heat treatment. The microstructure of the two tube ends are typical of as-drawn and mill annealed, as applicable, 3/4-inch OD Inconal 600 steam generator tubing. Strip chart recor dings of the eddy current inspections of the tube in the completely as-drawn condition and af ter the heat treatment are shown in Figures 2 and 3. These strip charts show a drift in the eddy current signal as the probe was moved from the as-drawn to heat treated portion of the tube. The cause of the drift could not be determined from the testing performed during the present investigation. However, eddy current drif ts of the type recorded during this investigation can result from changes in the electrical resistance or magnetic permeability of the tube material and may be the cause of the drif t in the laboratory sample. Eddy current drifts of this type can also be caused by the i presence of deposits on the tube wall or by variations in the tube wall thickness. The two latter conditions were not encountered during this laboratory investigation but may be of some concern during eddy current inspection of steam generators. Conclusions The inspection of a laboratory prepared eddy current standard showed a drift in the signal as the eddy current probe was translated along the tube from an as-drawn section of the tube to an annealed portion of the tube. These results indicate that an eddy current inspection procedure could be developed to dif f erentiate between as-drawn (cold worked) and annealed sections within the same tube. However, the testing program conducted within this investigation were insufficient to provide adequate data to form a conclusion as to the l sensitivity of the procedure to detect different percentages i of cold work. Further, the procedure detects changes in the cold-work condition of a tube and may not be practical for identifying tubes that are uniformly worked or uniformly annealed. Recomenations for Future Work The following minimal ef fort is recommended f or the development of the eddy current' procedure: 1.0 Additional eddy current inspection of the tube specimen using additional frequencies and probe designs such as an BX1 probe. 9

.~._m._.....___ . __ _ -.___r .__-_m.., SG-85-02-046 Page 5 of 10 2.0 Destructive examination of the specimen to correlate the microstructure with the drift in the eddy' current signal. This effort should include measurements of the electrical resistance locally along the tube and may require sophisticated microstructural inspection techniques such as transmission electron microscopy or x-ray diffraction. 3.0 The present effort should be repeated using additional tubes with varying degrees of cold reduction and with tubes that have deposits on the OD surface. The effects of other heating cycles should also be studied to determine the sensitivity to the amount of cold reduction in the tube material and to effects of partial recrystallization. This effort would also establish the reproducibility of the procedure. I r i j 1 2 1 i I 4 I j i l 1

l SG-85-32-346 Page 6 of 10 i Table,I Chemical Analysis of Inconel 600 Heat No. NX1118 Weight E_1_.ement Percent C B.033 Mn 74.65 Cr 14.91 Fe 9.76 Mn 0.22 Si 0.15 Cu 9.28 A1 0.24 Ti 2.22 Co 0.05 S 0.001 P 0.008 B 2.003 l l l O \\

~.. _ _ ~...... _ _ _.. SG-85-02-046 Pcga 7 of 10 Table II Hardness Test Results Hardness Tests on Tube, Rockwell B Heat No. Nx 1118 Heated End Unheated End Before+ After+ Before+ After+

• m **

103 72 105 107 103 72 102 104 102 71 104 106 Before or after the heat treatment. l e

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b. Heat Treated End (Mill Annealed)

Figure 1 - Microstructures of heated and unheated ends of tube (100X, Electrolytic Oxalic Acid (10%) Etchant) i

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__..m._ .o...~.~.; i 1 APPENDIX E Laboratory Induced Eddy Current Transition in a Steam Generator Tube (Combustion Engineering Report) i e L.

\\ i ll }< 4 LABORATORY INDUCED EDDY CURRENT 3 TRANSITION IN A STEAM GENERATOR TUBE E. P. KURDZIEL FEBRUARY 1985 COMBUSTION ENGINEERING, INC. 1000 PROSPECT HILL ROAD WINDSOR, CONNECTICUT O 9 ~. y

+ ...2.... ,m. 4 TABLE OF CONTENTS i Section No. Title Page No. List of Figures 11 1.0

SUMMARY

1-1 4

2.0 INTRODUCTION

2-l ' 3.0 EDDY CURRENT TESTING 3-1 i ) 4.0 LIGHT MICROSCOPY EXAMINATION 4-1 4 5.0 DISCUSSION 5-1 l

6.0 CONCLUSION

S 6-1 i

7.0 REFERENCES

7--I 4 4 1 i 6

a..~ . m x ;s l l LIST OF FIGURES Figure No. Title Page No. 3-1 Tube R89 L151 ECT Results 3-2 3-2 ECT Results for Laboratory Sample 3-2 Pulled from Annealed to Cold Worked End 3-3 Conductivity Loci 0 500 KHZ 3-3 4-1 Sectioning Diagram 4-3 4-2 Microstructure of Cold Worked End of the Tube 4-4 4-3 Microstructure of ECT Transition - Cold Worked 4-5 Side 4-4 Microstructure of ECT Transition - Midpoint 4-6 4-5 Microstructure of ECT Transition - Annealed 4-7 Side 4-6 Microstructure of Actual-Transition Area - 4-8 4-7 Microstructure of Annealed End of the Tube 4-9 11 j I

w., --..m_ _ _. - - - - - i I Section 1.0

SUMMARY

A sample piece of steam generator tubing in the cold worked condition was procured by Combustion Engineering to replicate the microstructural condition and the accompanying Eddy Current Testing (ECT) indication found in tube R89 L151 from the San Onofre Nuclear Generating Station (SONGS) Unit 2. The microstructure of the SONGS tube consisted of a cold worked structure that transformed to a fully annealed structure over a short distance (Ref. 1). This type of structure generated an ECT indication in which the absolute test data deviated from the normal signal baseline (ECT transition region). The sample tube was heat treated to produce a microstructure similar to that observed in tube R89 L151 and was then eddy current tested. The ECT results were similar to those found in the SONGS tube. There was a deviation from baseline, though the transition was opposite in phase from that found in tube R89 L151. 1-1

v....

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u ~n -~ c. Section

2.0 INTRODUCTION

C-E procured a piece of cold worked steam generator tubing with the intent of replicating the microstructure of tube R89 L151 from SONGS Unit 2. The tube used was from Heat No. NK 0471, with a carbon content of 0.02%. Additionally, the tube had received a final area reduction of 60% off the tube reducer. The tube was placed in a furnace, with half of the tube projecting out of the furnace through a hole in the furnace door. The tube section that was outside of the furnace was wrapped in wet rags, and argon gas was pumped through the inside of the tube to prevent oxidation. The furnace required 3-5 minutes to reach the annealing temperature of 1780*F. The tube remained in the furnace at this temperature for 6-8 minutes af ter which it was removed from the furnace and air cooled. This treatment was sufficient to transform the cold worked structure in the furnace to an annealed structure. 2-1 t

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w.-_ _., Section 3.0 EDDY CURRENT TESTING After the tube section was removed from the furnace, an eddy current examination was conducted. Examination frequencies and test sensitivities were adjusted to compensate for the difference in wall thickness between the sample tube (.042") and the SONGS steam generator tubing (.048"). The sample was tested with a conventional I.D. bobbin coil and an absolute signal transition was evidenced approximately mid-way through the tube. Although the transition itself was opposite in phase from those in the SONGS steam generators, the end result, increased net conductivity, was the same (see Figures 3-1 and 3-2). The conductivity loci test, performed with a mid-range surface coil, also indicated an increased conductivity in the cold worked section, as compared to the mill annealed ASME standard (Figure 3-3). The annealed section of the sample tube displayed wide variations in the conductivity loci but these variations are believed to be attributable to magnetic permeability ef fects rather than conductivity. 3-1 l

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._ n. -..n .._.n 6 -1 -_];5 i~ kdi_ W :~ iiig normal Q&%& E :--% . nd _4 __. f" (annealed) w __._ ~ (, x_ ._~_ 55Ms.E#55%fdPN l_ = - .;== abnormal _. -qu--. (cold Worked) m 4 = T M -21. ,l_ ~ i. g p 200 in </deE2,b Flaws Figure 3-1. Tube R89 LISI ECT Results L ._A a _ =. - 9.wam he cold worked ~ 3, . _ e.a : ,w,- . ~g annealed =w_I g t O 200 W/l*. Figure 3-2. ECT Results for Laboratory Sample Pulled from Annealed to Cold Worked End l l l 3-2

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i l I Figure 3-3. Conductivity Loci @ 500KHz l 1 ) ) l l l l 1 I l 1 l I 3-3 1 I 1 -,-,,.n

+,. .- - ~... I Section 4.0 l l LIGHT MICROSCOPY EXAMINATIONS Upon completion of ECT, the tube was marked to identify the transition region-of the tube. -Metallographic sections were taken at each end of the tube and in the transition region. Longitudinal sections were mounted, ground-on sic j papers, polished and etched using a dual etch procedure to characterize the The electro 1 tic oxalic etchant generally used on Alloy j tube' microstructure. 3 600 may give misleading information on carbide presence and distribution. As a result, a dual electrolytic etch procedure using nital etch to reveal grain 1 boundaries and orthophosphoric acid to reveal carbides was used. The microstructure of the tube in the ECT transition region did not reveal a cold worked structure which changed to an annealed structure, so additional samples for metallographic evaluation were taken from - the tube. Figure 4-1 is a i sectioning diagram of the tube. As was expected.-the microstructure of the cold worked end, as seen in Figure j 4-2, showed elongated grains (typical of a cold worked material) with grain f boundary carbides. Figures 4-3, 4-4 and 4-5 are photomicrographs of the l transition region, moving from the cold worked end to the annealed end. Based i on the ECT indications, these microstructures were expected to show a transition from a cold worked structure to an annealed structure consisting of i equiaxed, recrystallized grains. However, the photomicrographs revealed the f whole ECT transition region to contain a cold worked structure. Only Figure 4-5, which was the annealed side of the transition region, showed the presence of a small number of recrystallized grains. The actual microstructure transition occurred approximately 3" from the ECT transition area 'in the annealed portion of'the tube. The microstructure at this location is shown in ] Figure 4-6. .4-1 4 .f;* n,- ,c-m,,, ..n,,.--, v-e-- -,n._

~m.a.. The nital etch at this location revealed a structure containing a mixture of cold worked, elongated grains and equiaxed grains, while the orthophosphoric etch revealed carbides in the remaining and in prior cold worked grain boundaries. This structure was essentially identical to that found in the SONGS tube containing the ECT transition. (Tube R89 L151). Figure 4-7, which is a photomicrograph of the annealed end, had a microstructure of equiaxed grains, as expected. 4-2 l I r - --r-,

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.-..--.-.__...a . ~.... - 3 Section 5.0 DISCUSSION C-E was able to simulate the structure found in SONGS Unit 2 tube R89 L151 by taking a cold worked tube and annealing one end. The ECT response was similar to that present in the SONGS tube although it was out of phase. The microstructure transition from a cold worked structure to an annealed structure did not coincide with the ECT transition. The section of the tube that contained the microstructural change was approximately 1.5" inside the furnace, while the section with the ECT was at the outside surface of the furnace door, about 3" from the area with the structure change. The difference in location for the two transitions suggests that microstructure alone was not responsible for the change in ECT response in the absolute mode. However, the results do indicate that variations in heat treatment can produce changes in the material that will be reflected in ECT signals. l 5-1 1 1

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2

(.,, 4 's Section 6.0 CONCLUSIONS 1. .The microstructure, including the transition from cold-worked to annealed structures, of tube R89 L151 from SONGS Unit 2 was duplicated in the laboratory by heat-treating a cold worked tube. 2. ECT generated ' signals showed a transition similar to that found in the SONGS tube, though out of phase. 3. The ECT transition area was offset 3" from the section of the tube showing a microstructural transition. l i 6-1 l J ) 4 i

-~ - d f ~ Section 7.0 i REFERENCES l. Kurdziel, E. P., Owens, C. M., Powell, D. E., " Destructive Examination of Tube R89 L151 from San Onofre Nuclear Generating Station - Unit 2 Steam Generator 88" January 1985. 0 7-1* 4 L}}