ML20084Q633
| ML20084Q633 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 03/05/1970 |
| From: | Staehle R PARAMETER, INC. |
| To: | US ATOMIC ENERGY COMMISSION (AEC) |
| Shared Package | |
| ML20084P985 | List: |
| References | |
| DC-65, NUDOCS 8306130514 | |
| Download: ML20084Q633 (55) | |
Text
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Dcamination ard Evaluation of Safe End From Nozzle N6B on Nine Mile Point Reactor i
DC-65 5 March,1970 Preparci for: Division of Cmpliance United States Atcmic Energy Cmmission AT Contract No. AT(ll-1)-1658 Parameter No. DC-65 Subcontract No. 5 Prepared by: R. W. Staehle Columbus, Ohio Prepared through: Parameter, Inc.
Consulting Engineers Elm Grove, Wisconsin m:
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TABLS T CONTENTS Page 1.0 Sumary and Conclusions . . . . . . . . . . . . . . . . . . . 1 2.0 Ihckground and Introduction . . . . . . . . . . . . . . . . . 2 3.0 Experimental and Specimens. . . . . . . . . . . . . . . . . . 2 4.0 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1 Optical arxl Scannirg Metallography. . . . . . . . . . . . 4 4.1.1 General Examination of the Inside Surface . . . . . . . 4 4.1.2 Cross Sectiom1 Examimtion of Cracks Starting at Inside Surface . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1.3 Cross Sectional Examimtion of Cracks Starting at Outside Surface . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1.4 Examim tion of the Crack Face . . . . . . . . . . . . . 5 4.1.5 Examination of the Field Weld and Weld Affected Zone. . 6 4.2 Electron Probe Results . . . . . . . . . . . . . . . . . . 7 4.2.1 'fraces Across Crack Cross Section . . . . . . . . . . . 7 4.2.2 Amlysis of Oxide Fourri in Cross Sectioml Examination. 8
) 4.2.3 Amlysis of Oxide on the Crack Surface. . . . . . . . . 8 5.0 Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.1 Mode of Cracking fran Inside surface. . . . . . . . . . . 11 5.2 Causative Elenents. . . . . . . . . . . . . . . . . . . . 11 5.3 Segaerx:e of Events. . . . . . . . . . . . . . . . . . . . 14 5.4 Significance of Transgramlar Cracking. . . . . . . . . . 16 5.5 Significance of Intergramlar Penetrations on Material Not Furnace Sensitized. . . . . . . . . . . . . . . . . . . . 16 5.6 Confidence Level for Safe Ends If Not Repaired. . . . . . 16 l
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s't k 11 Table of Contents (Continued)
Page 5.7 Inplications fcr Repair Procedure . . . . . . . . . . 17 l
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- 5.8 liechanistic Processes . . . . . . . . . . . . . . . . 17 6.0 Acknowledgenents . . . . . . . . . . . . . . . . . . . . . 17 1
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- 7.0 References. . . . . . . . . . . . . . . . . . . . . . . . 18 1
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'a h lii List of Figures
- 1. Schematic diagram showing safe erd configuration.
2 Schenatic description of radiographic irdications frm safe end showirg location of cracks. The 12:00 position was the top of the tube.
Radiographs were taken through the double thickness of the tube before specimens were removed. Location of the E specimen is noted.
Approximate location of leaks roted.
3 Schmatic location of cuts made in the core spray line for renovirg the E-GE specimen. Cut numbers refer to same numbers of figure 1.
4 Schenatic arrargement slowing orientation ard description of E specimen. (a) Orientation of specimen with respect to tube ard coordinate axes. (b) Letter designations describirg specimens taken frun E specime , together with coordinate axes. (c) Optical macrographs showing views of inside surface, edges, ard erd of E specimen. Only that portion shown in the -x direction frm the field weld.
- 5. Ootical macrograph showing view of inside surface of the E specimen.
Coordimtes relate to figure 4. Circled region shows site where scannirs electron micrograph obtainal (figure 6).
- 6. Scanning electron micrograph frcrn inside surface of E specimen.
Note site of micrcgraph in figure 5.
- 7. Optical micrograph of crack starting at inside surface ard extending in the +z direction. Specimen taken from specimen B. Face being examined is in the x-z plane. Left figure is initial portion of crack and right figure is bottm. The bottcm of the left figure ard top of right are exactly contiguous.
- 8. Optical micrograph of crack starting at inside surface ard exterdirg in the +z direction. Specimen taken frm specimen B. Face being examined is in the x-z plane. Left figure is initial portion of crack and right figure is bottcm. The bottcm of the left figure and top of -
right are exactly contiguous.
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- 9. Optical micrograph of crack startig at inside surface aM extending in the +z direction. Specimen taken frm specimen E. Face being examined is in the x-z plane. Left figure is initial portion of crack and right figure is bottom. The bott e of the left figure and top of right are exactly contiguous.
- 10. Schmatic arrangment showing location of specimens taken frm outside surface of specimen B. Surface viewed is in the x-z plane.
- 11. Photmicrographs of cracks on outside surface of specimen B. Cracks 4 are propagatig in the -z direction. Surface viewed in the x-z plane.
, 12. View of face of crack. Crack face is in the y-z plane. Specimen broken apart for viewig. (a) Colar print (b) Schmatic diagram shwing various regions of crack. Distances frm inside surfaces noted locate sites where scanning micrographs taken. Specimen broken open to show surface of stress carrosian crack. Shiny bright region results frm this treaking process.
- 13. Scanning electron micrographs of stress corrosion crack surface corresponding to O m position noted in figure 12.
- 14. Scanning electron micrographs of stress corrosion crack surface carrespoMing to 1 m position noted in figure 12.
- 15. Scanning electron micrographs of stress corrosion crack surface corresponding to 3 m position noted in figure 12.
- 16. Scanning electron micrographs of stress carrosion crack surface carrespoMing to 91/2 m position noted in figure 12.
- 17. Scanning electron micrographs of stress corrosion crack surface carresponding to 8 m position noted in figure 12.
- 18. Optical micrograph from specimen H along inside surface along d 2 3*
Plane viewed is x-2.
19 Optical micrograph frm specimen H alog the outside surface along d
2 3 Plane viewed is x-z.
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- 20. optical micrograph frm specimen K shcwig inside surface along e 2-*3*
Plane viewed is x-z,
- 21. optical micrographs frm specimen K showing outside surface. Penetrations are ectending in the -z direction frm the surface. The plane viewed here is alog the e -f 2 2 line.
- 22. (a) Schenatic location showing where micrvsube trace across crack ms performed. (b) Photographs showig where traces performed. (c)-(d) Results frm separate determination at different sites alog crack.
23 Typical aside in crack showig where the x-ray spLun was analyzed.
This is not the region examined but the oxide is typical.
- 24. Intensity of x-ray emissions fra spectral scan of one piece of oxide in crack by microprobe. Spectral regions where primary peaks should have occurred for particularly sought elsnents are noted with asterisk.
- 25. Montage showig scanning micrograph fra face of stress carrosion crack.
Special notations locate regions where micrurube determinations were made. The location of each micrograph is the same as that in figure 12.
Most of the scanning micrographs are frm the series shown in figures 13-17.
For reference the Cr/Te ratio of scale-free type 304 stainless steel is 0.4. These ratios are frm direct scale readings and are not corrected.
- 26. Schenatic relationship showing possible pattern of interaction of stress, sensitization, dissolved arygen, tenperature at pH in the stress corrosion crackig of sensitized stainless steels.
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.. L O O 1.0 Stamary and Corrlusions A safe eM fran a core spray line as renoved and examined from the Nine Mile Point Reactor; this was fran nozzle N6B at the west core spray safe end. This specimen was examined because snall leaks were observed during a reactor shutdown. A portion of this specimen was examined at Battelle Northwest Iaboratories at the request of the AEC; the renainder was examined by General Electric personnel. The BNWL-AEC investigation was performed accordirg to a procedure outlined by R. W. Staehle and W. J. Collins.
The investigation at BNWL was supervised by L. A. Hartoorn. This report describes and amlyzes only the examination at BNNL.
Impartant conclusions fran the examimtion at BNWL are as follows:
- 1. The failure as most likely causal by stress corrosion cracking of heavily sensitized stainless steel as opposed to the possibility of fatigue or overload.
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2 There is a high probability that the failure initiated and propagated while the reactar was at power.
3 The crackirg was intergramlar ani started fran the inside surface.
A snall amount of transgranular crackirs started fran the outside surface, but the intergramlar crackirg caused the cbserved leaks.
- 4. The primary causative elenents in this failure are most reasonably heavy sensitization, high stress, arx1 dissolved oxygen. Minor causative elenents may be slightly loweral pH (relative to PWR's) and elevated tenperature (relative to roan tarperature) . If any one of the primary causative elenents had been negligible, crackiry would probably not have occurred. The minimum critical canbinations of individual values of the primary causative elenents are not clear. The form of stress carrosion cracking described herein has been observed to occur at roon tenperature (Oyster Creek) .
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' 's - (a) 2.0 Backgraturl ard Introduction This report describes the observations and their analysis made on a specimen reoved frm nozzle N6B at the west core spray nozzle safe end in the Nine Mile Point Reactor. This reactor is operated by Niagara Mohawk Power Corp.
The specimen described in this report was renoved during the weekend of 14-15 March 1970. It was carried by W. J. Collins of AIE to Battelle Northwest Iabaratories in Richlard 'Ashington. Significant portions of this examination were cmpleted by 24 March,1970.
The specimen was retoved frm the safe end of the core spray nozzle at a point outboard of the weld between the carbon steel and the safe end.
Renoval of this particular specimen was basal on a leak observed whN the reactor ms shut down. The leak took the form of a fine sgay ard penetratal the pipe at two points.
This report describes specifically the operations involved in renoving the specimen, the procedures ard results of examination, and an analysis of the possible causes of failure.
3.0 Experimental and Specimens The region frm which the specimen was renoved is shown in figure 1.
The region of cracking ms determined frm radiography performed tefore the specimen was renoved. The arrargenent of cracks as determined in radiography is shown schenatically in figure 2. This radiograph a s taken by GE personnel ard the determination was made through a double thickness of metal (i.e.
through both walls).
The specimen was renoved frm its installed location using a power hacksaw. No coolant was ussi duriry the operation. A schenatic arrangement of the location frm which the specimen was renoved together with the cuts is shown in figure 3. The first cut was made approximately at position #1.
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.When this cut was empleted, the portion of the pipe connected to the vert ical riser shifted to the left about 3/4" relative to the portion of the pipe connectd to the vessel. This resulted frm the lockM in deformation (cold spring) in the piping; this ms thus relieved where the cut was made.
Following the first cut a secod one was made at position #2. This cut form d the outer boundary of the specimen. Following this cut a shieldirg plug was inserted to, prevent radiation streaming. Special care was taken durirg this operation to avoid damaging the inside surface mechanically or introducing chemical contamination. The fim1 cut was made at position #3 aM this formed the other boundary of the specimen. The specimen as cut contained both material frm the furmce sensitizM safe end as well as non-sensitized pipe. The portion of this pipe next to the weld can be presumed to be slightly sensitized.
The entire " specimen" for both the E ard GE constituted the region between cuts #2 ard #3 of figure 3. The portion examind at Battelle Northwest on behalf of the E was the length of the specimen tube and one irch wide; it es taken approximately frm the site shown on the radiographic layout of figure 2 ard shown also in figure 4. This location was between the 10:45 aM 11:00 positions with the 12:00 being straight up. This particular location as selected to give GE the vau Linity to examine both leaks.
For convenierce herein the specimen describd in this report will be called the "E specimen" ard the renairder will be called the "GE specimen."
The entire specimen taken between the cuts #2 ard #3 will be called the "E-GE specimen."
The E specimen was cut by Niagara Mohawk personnel using a dry milling procedure. (i.e. no coolant was used) . This cutting operation, as well as the one involving taking the entire specimen, was monitored by Staehle ard Collins. Special care as taken to avoid spurious contamination.
The E specimen was taken by Collins to BNWL on 15 March,1970.
The GE-E specimen is shown in figure 4 with respect to orientations and specimen designations. X-y-z ocordinates are applied for convenience a describing the specimen. In addition to these directions a series of y 2 etc.) outline specific interfaces.
letter designations (a --b l
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The AEC specimen was cut by manual hacksaw into the specimens l
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- shown in figure 4b. Figure 4c shows the part of the '
- .T specimen before i it ws cut into snaller specimens for subsequent evaluation.
The specimen was examined by BNL people using the followirg techniques:
optical metallography, scanniIg electron metallography (SEM), electron microprobe amlysis. No umsual or novel techniques were anployal. These techniques were applied to this amlysis in ways that are relatively straight forward in failure analysis.
i 4.0 Results This section describes the results frm examinations using optical arrl electron metallography and micro-probe amlysis.
4.1 Optical and ' scanning Metallography t
4.1.1 General Examination'of the Inside Surface i
Figure 5 shows an inside view of the ABC specimen surface where the j crackirg was most extensive. This cracking was most extensive next to
- cut number 3, i.e. the end of the specimen toward the reactor vessel.
Figure 6 is a scannirg electron micrograph frcm the region circled in figure 5. This shows the inside surface of the pipe and looks into the crack in the +z direction. The crackirg is clearly intergranular. There l appears to be little evidence of any significant plastic deformation. A further significant observations here is that there is no pattern of intergranular pene-tration on the surface. This suggests that the crackirg process is not a stress assistal intergranular corrosion but is a type of stress corrosion crackirg.
l 4.1.2 Cross Sectional Examimtion of Cracks Starting at Inside Surface I
Figures 7 and 8 are optical micrographs from specimen B on the a2-b2 face
!' lookirg in the 4y direction. The cracks start frm the inside surface and progress in the +z direction. On these figures it should be particularly noted that there is little evidence of significant~ plastic behavior of the material. There are no strain markings nor do the grains at the fracture edge have a " taffy-like" appearance indicative of gross local clastic deformation.
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Figure 9 is an optical micrograph showing a crack frm specimen E along the b2-*3 face and looking in the +y direction. This cracking node is again intergranular but the extent drops off with the distance away
(+x direction) frm ay-a4 interface. This suggests that residual stresses frm the weld ard also the adjacent thickness discontinuity (See Fig.1) interacted with the mment in the pipe due to differential expansion of the vessel ard vertical pipe. This mcment was presumably constant over the safe end. This interaction suggests that the observed cracking is strorgly stress deperdant. It further suggests that the crack may have initiated and propagatsi only when the reactor was at power.
4.1.3 Cross Sectional Examination of Cracks Starting At Outside Surface The outside surface of the AE specimen was also examined metallographically.
Figure 10 shows schenatically where a specific set of specimens was taken frm spe:Imen B. These specimens were taken frcm the a -b face of specimen B 2 2 ard lookirg in the 47 direction. The capital letters (A,B,C ...) irdicate approximate locations where micrographs were taken. These micrographs are shown in figure 11 ard have letter designations correspordirg to tirse of figure 10. Figure 11 shows that transgranular cracks had initiated frcm the outside surface. Sme of the specimens show extensive evidence of strain markings near the surface. These should be cmpared with the edge of the crack in figures 8 ard 9 where no such strain markings are evident.
This cmparison is again significant with respect to establishing whether the intergrarnle cracking frcm the inside surface was due to stress corrosion or to a purely mechanical type of failure. The strain markirgs of figure 11 are no doubt the result of rough hardling of the piping during construction.
This amount of cold work, in itself, at this location is probably not significant. The point to be made here is simply that the evidence of strain markings on the surface is not associated with the intergranular cracks.
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Another factor of significame is the relatively limited extent of these cracks into the metal. Since relatively high stresses are known to be required for the intergranular cracks and sime the chloride induced transgranular cracks propagate e sily at 20-50% of the yield, it would se m that the transaranular cracks muld have progressed further if the chlorido were present at the site of cracking over the entire duration of the plant operation. While there are obvious alternatives to this interpretation (i.e. lack of moisture, too low a chloride comentration), this limited extent of cracking will be discussed later with respect to fixing the time of cracking.
4.1.4 Examination of the Crack Face The surfaces of the cracks were examined optically and a color photograph of the cracked surface is shown in figure 12a. The cracked surface appears to have three zones. The first (next to the inside surface) is generally dark brown; the next is a dull gray; and the third (next to crack root) is a bright gray. Beyond this the shiny bright region is the result of breakiry the specimen open to expose the surface.
These different regions on the crack face were examined using the scanning electron microscope. A series of pictures at various mgnifications was taken at each of these discrete regions. A schaatic figure showing these regions is given in figure 12b. The scanning electron micrographs are shown in figures 13-17. These figures are designated as O m,1 m, 3 m, 5-1/2 m, and 8 m- these being the respective distances away frm the inner surface in the +z direction. The plane of the crack surface is approximately parallel to the y-z plane.
The micrographs of figures 13-17 show clearly that the crack is intergranular and that very little gross plastic deformation occured.
l Of particular interest, especially at the higher e ngifications is the norphology of the corrosion product. The corrosion product on the 5-1/2 m point (figure 16) has a shape often fourd on surface exposed to high taperature water at high pH . These shapes are typical of those fonned by precipitation frm the solution. The precipitates at the 3 m site (figure 15) appear to have sme structure but to be less well formed. At 1 m (figure 14) the precipitates seen to have little structure at all. At the O m (figure 13) site, just inside the inside surface of the specimen, the precipitates
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4.1.5 Examination of the Field Weld and Weld Affectd Zone The region of the field weld as examined to provide a cmparison between the behavior of furnace sensitized material and the weld sensitized material.
Figure 18 shows a weld defect on the ID surface frcm specimen H looking in the y direction on the c 2-d2 fa e. The apparent defect runs along the d -d line. There is no clear eviderce for this defect beim caused by SCC 2 3 but it may have been. The general appearance suggests that it nay be due to a weld defect.
Figure 19 shows the weld region on the outside diameter of specimen H alog the line d -d 2 3. No gross attack is evident here.
Figure 20 rJ10ws the inside diameter frm specimen K along the line e2 -83*
same integranular penetration is evident here although it appears to have extended inward only a relatively small fraction of a grain diameter.
Figure 21 shows a' set of miu@aphs fra the outside surface . .
of specimen K along the interface e2~f2. A small amount of intergranular.
per=L.ation is noted in all cases. These pictures were taken to ascertain the behavior of the weld sensitizal region; however, it is not possible to detemine whether these intergranular penetrations are related to pickling of the tubing prior to installation or are related to a stress corrosion process which occurred durig the operation of the reactor.
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4.2 Electron Probe Results Three regions were examined to determine local chenical cmposition usirg the microprobe. Traces were made across the cross section of cracks; individual axide particles found in cross sections were e::amined; arri the camposition of the precipitates on the crack faces was determined. Deperrling on the site, various chenical elements were sought.
4.2.1 Traces Across Crack Cross Section Figure 22a is a schenatic diagram showirg the region examined by a microprobe scan; figure 22b shows scannirg micrographs of the regions '
examined; figure 22c and d are results fran scans across the crack. These scans were conducted step-wise across the cracks. The corr:entrations of i these elenents were calculated using an analyzed Type 316 stainless steel '
standard. Steps were taken at 0.5 micron intervals. These scans were conducted on the ay -g surface of specimen A. The zero point of the coordinates was taken as the center of the crack. The trace was taken to be approximately pw.gilicular to the plane of the crack face. No particularly unusual distribution of iron, chromium, or nickel was noted. The cxxnposition of all the elenents naturally decreased in the vicinity of the crack. The slight lunps and irregularities are associated with grayish precipitates.
4.2.2 Analysis of exide Found in Cross Sectional Examination The axide netterial in a crack similar to the one shown in figure 23 was analyzed for the entire spectrum of elenents startirg with the element magnesium.
Only elenents Fe, Cr, Ni, Mn and a trace of Si and Mn were detected. Special attention was directed toward firrling elements Cu, F, Cl, Br, Pb, S, As, but none was found in significant quantities above the mininun sensitivity of 0.03 to 0.1 weight percent. There appears to be a slight indication of copper enrichnent. The enrichnent of copper on iron base surfaces is ccmnon because high purity water is often exposed to heat excha1gers constructed This material was used because the iron, chronium, and nickel had been chenically analyzed. Tlus the x-ray snissions fran the subject experiments could be canpared with the peak heights fran the 316 and the approoriate experimental corrections made. The Type 316 was used because of the press of time and because its use is considered to involve virtually no error, t
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of copper base alloys. Figure 24 shows the result of the scan of the spectrum.
On figure 24 the various sps: tral paks are designated for the respective alloy elenents which were found. Also the locations of primary peaks for other elenents, sought but not fourd, are noted.
4.2.3 Analysis of Oxide on the Crack Surface Figures 13-17 showed distributions and morphologies of precipitates on the crack surface. The chenical canposition of these precipitates was determined approximately using the microprobe. Figure 25 shows the canplete set of results irrludirg the site from which the analyses were taken and the Cr to Fe ratio as determined directly fran the x-ray peaks for the two elenents. In addition, for cunparison, the morphology ard chenistry of surface scale is slown in figure 25. There are three very important observations here:
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- 1. The Cr/Fe ratios are generally higher than that in the alloy.
The theoretical ratio in the alloy is Cr:Fe = 18:74 = 0.24. However, the uncorrected ratio fran x-ray enission fran examining a clean surface is 0.4. Thus, the ratios in figure 25 should be conpared with the latter number.
- 2. The Cr/Fe ratio at the crack mouth is very high. ,
- 3. There are wide local ranges as noted at the 3 mn position.
The basis for these enrichnents in chranium appear to be related to the relative solubility products of the iron hydraxides and chroniun hydroxide.
The latter are less soluble and therefore would tend to precipitate earlier.
The enrichnent at the crack mouth is probably relavent to a change in pH l
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, thereat. Wark on chenistries in restricted geonetries has shown that inside cracks of iron base alloys the pH at roan tenperature lies in a range of about pH = 3.0. Presumably the basis for the very great enrichnent of Cr at the o m position is related to the great driviry force for precipitation when the pH changes fran that inside the crack (low pH) to that in the bulk enviroment (neutral pH) .
The wide range of Cr/Fe ratios at the 3 m position suggests that the crystals formed at different times when the solution contained widely varyiry ratios of Fe arri Cr soluble ions. For example, the Cr/Fe = 0.8 value was fran a gererally broad area which certainly includes sme of the substrate.
Most of the dissolution contributing tc the enriched chromian may have come fran the dissolution of the carbides which are rich in chrmium. The iron rich crystals may have cane at a time when the chronium free zone was dissolvirg at the tip of the propagatirg crack.
The octahairal morphology of the crystals at the 5-1/2 m position of figure 25 has been observal before by Fabele and Daniel who examined the corrosion products on stainless steel surfaces in static autoclaves. This octahedral marpholcgy is that usuallly preferred by magnetite.
The difference in Cr/Fe ratio of the " pebbles" and " needles" at the o m position suggest again that the cmposition of the solution inside the crack cha.ges with time. This, it is reasonable that the first precipitate .
to fann, i.e. the inside or pebble part, is higher in chronium because of its lower solubility product; the needles are still high in chronium but liave a relatively lower Cr/Fe ratio.
The relatively high Cr/Fe ratio found in the surface scale of figure 25 l suggests that the dissolviry alloy precipitates so that the chronian is enriched accordirg to the previous cmment concerniry the effect of pH
! on solubility products.
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. O O 5.0 Discussion The essential technical objectives for this investigation were to supply the kind of infannation necessary to answer or assess the following questions:
- 1. ms the cracking mode related to stress corrosion cracking, high strain range low cycle fatigue, or mechanical overload?
- 2. If the failure mode is stress corrosion cracking, what were the causative elsnents and their relative importarce (tsnperature, degree of sensitization, oxygen, stress)?
- 3. With respect to a possible stress corrosion failure what is the most likely sequerce of events?
- 4. What is the significance of the transgranular crackiry observed on the outside surface?
- 5. What is the significance of the intergranular penetrations in the region next to the weld opposite the safe end?
- 6. To what degree of confidence is information available that would permit leaving sane of the safe ends in the vessel without repair?
- 7. To what degree of confidence can a repair proce: lure be specified based on the information herein?
The following discussion is addressed to these questions. In addition, there is a brief discussion of the mechanistic processes involved in the stress corrosion cracking of sensitized material.
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. . _12-5.1 Mode of Crackiry fram Inside surface Figures 5-9 ard 13-17 suggest, witlnut nuch question, that the failure mode us stress corrosion crackig. There is very little evidence for local plastic deformation which would support a view of fatigue or overload failures. If either of these latter two nodes were operatirg, the grains, for example, in figures 7-9 wuld have a taffy-like pullig apart appearance.
Also, the etch would have shown the kird strain nakings which were observed near the surface of figure 11.
There is no evidence that this failure is simply an intergranular corrosion phenanenon which was accelerated by stress. The SEM micrograph of figure 6 shows no evidence of general intergranular corrosion. The only attack apparent is associated with the crack itself.
- 5.2 Causative Elc.ents The causative elenents to be considered here are:
- 1. Degree of sensitization
- 2. Applied stress
- 3. Dissolved aKygen
- 4. Other environrental contaminants
- 5. Tenperature
- 6. Solution pH There is virtually no information presently available to permit assessing very specifically the relative magnitudes and canbination of the above factors which are critical with respect to causing stress corrosion l cracking. Available infonnation makes possible the following qualitative comnents:
This appears to be a minor distinction but it is not. For example, sensitized stainless steel would dissolve intergrarularly ard rapidly in boiling concentrated nitric acid; with applied stress the process would be scmewhat accelerated. However, for stress corrosion crackirg, no significant attack occurs unless stress is applied.
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- 1. The sensitization required for this type of crack must be fairly extensive. The paper of Mrd, Mathis and Staehle shows that light sensitization is not sufficient to cause this intergranular stress corrosion crackig in fluoride contaminated solutions at roan tenperature. Conversa.ly, the stainless steel fuel elenents which failed intergranularly in the high power density program conductal by GE in the early 1960's (See GEAP-4400 for example) were not sensitized.
2 'Ihe stress requirement for SCC of sensitized material is not well definal but the general pattern of events suggest that it must be above the yield strength. This is generally well above the range of stress required for the transgranular cracking of stainless steel induced by chloride. For the latter case stresses as low as 10-30%
of the yield strength are sufficient to cause SCC.
The failures which occurred in the stub tubes at Oyster Cred were generally associated with the regions of highest stress; in that case the stresses were producal by welding and were shown to be very high.
In the case of the failure described herein the magnitude of stress at t?e joint in question is clearly high b2t definitive values are not yet available. The fact that the major cracks were corcentratal near the weld and the section discontinuity suggests that on the stresses causing crackiry were related to a sunination of the applied plus the residual weldirg and the discontinuity stresses.
The stresses appear to have been highest during operation since the maximum therrial differential between the vessel and vertical portion of the core spray line occurs at this time. It should be noted that the usual argtunent about the inside surface in compression and outside in tension do not apply here since the neutral axis is j the pipe centerline.
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.4 . V U 3 The anmnt of dissolved oxygen required for _this intergranular TC of sensitized material is not clear. The intergranular cracking observed to date for all causes suggest that axvuen is required hit how much is not clear. The lack of a similar occurrence in PWR type systens suggests that the minimtzn range may be in the range of 1 ppn.
The general pattern of SCC observed in various reactars together with data in the literature obtained on higher nickel alloys suggests that axygen is certainly an accelerant.
4 In general it appears that intergranular SCC of sensitized material can occur in tin absence of contaminants such as chla-ide, fluaride, etc. Ibwever, it also appears that these can accelerate the process as irdicated by the data of mrd et' al. ' discussed earlier.
- 5. The critical tenperature far EC of the sensitizal material is not clear. It certainly must be interrelatsi in scrne way with the other factors. Ibwever, the Oyster Creek failure suggests that crackirg can occur readily at rocxn tenperature.
- 6. The pH of the solution is an important variable with respect to SCC ard this may be part of the basis for observed differences in the behavior of BhR and PWR systens. However, this is presently a speculative matter ard care should be exercised in reaching conclusions here.
The general pattern suggested by the above is that sensitivity to TC 'of sensitized material can be interrelated according to the followirq qualitative relationship for cracking to occur:
[ stress]* x [ degree of sensitization]# x [ oxygen]P x [tenperature]9 x [ contaminants]* x 1-Y 7 K (1)
L.,PHg i.e. crackirg will occur when the product exceeds K.
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one can conclude fran this equation that SCC will not occur when any of these quantities is 0 (except for sane obvious difficulties of interpretation associated with too low pH) . Further, an approach to el*unisel.irg the TC problen is to minimize all the factors. Graphically this TC problen might be considered as having the depeMancies shown in figure 26. This figure suggests that the tw most critical parameters are stress and degree of sensitization. Below the hyperblic lines SCC will not occur Mt will above; the incrase of oxygen ard tenperature, and decreasing pH may cause these lines to shift to a broader range of cracking by going progressively fran lines 3 to 2 aM 1. Increasirg the concentration of contamimnts (like fluoride) would presumably act to move fran line 3 to 1.
The exponents in equation 1 should be indicative of the daninance of their respective quantity. While this expression is only qualitative, it should serve as a reasonable basis for considering the general aspects of material-envircrrnental control.
5.3. Sequence of Events In assessing the sequence of events questions naturally raised include:
- 1. Did the failure start prior to operation? -
- 2. Did it occur at elevated tenperature?
- 3. Did it occur at roon tenperature?
- 4. Did it occur during heat-up or cool-down cycles?
While abolute eviderce is lackirg in order to be specifically definite with respect to the above questions, the evidence esoecially of. figures 12, 13-17 suggests the following:
l 1. Significant propagation occurred during the last 3 to 5 cycles ad r each band of figure 12 correspords to a different time when the crack was propagating.
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.. 2. Crackirg prnMhly did not occur prior to operation but this cannot be proved.
- 3. The excessive brown coloration of the first stage of cracking in figure 12 suggests that this region nny have existed for a fairly long time. This may be the result of a large number of snall incrcments of propagation or a simple propagation which existed very early.
There is also the possibility that this propagation occurred at high temperature ard the others occurred at low temperature.
4 The change in morpholcny of the precipitates suggests that the well formed ones at 5-1/2 nm of figure 16 were the result of cooling slowly. This implies that the last stage of cracking occurred durirg the last cycle ard that it occurred at high temperature. This is bascd on the assumption that such crystals were formed while cooling down.
The different shapes of precipitates at the 3-1/2 and 1 nm position suggest that at one time the well formed crystals existed tut with successive thermal cycles they dissolved and reprecipitated.
- 5. The crack propagation appears to have involved sane amcunt of dissolution during propagation of Mth the carbides ard the region depleted of chramium. This helps justify the higher Cr/Fe ratios.
- 6. The fact that the stresses presumably are raised in the safe erd considerably during hat operation provides the essential basis for an argument that crack propagation occurs while the reactor is at power.
In sunmary, it appears that the cracks propagated at the high temperature ard did so durirg reactor operation.
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(b 5.4 Significance of Transgranular Crackirg The transgranular cracks observed on the outside surface as shown in figure 11 were alnest certainly induced by chloride. In view of the stresses it is surprisig that they have exterded only such short distances. If the chloride ard high stress had existed for a lorg time, it is likely that the transgranular cracking would have empletely penetrated .the wall.
This lack of extent probably results frm the fact that they may not have occurred until after the leak induced by the internally generated cracks.
It is possible that the water deposited briefly on the outside surface was evaporated, and left (after enough evaporation) a chloride residuum.
It is also possible that these cracks could have occurred earlier Mt the chloride concentration was not sufficient to cause significant propagation.
5.5 ' Significarce of Intergranular Penetrations on Material Not Furnace Sensitized Figure 21 shows intergrarular penetration on the outside diameter of the pipe in the +x direction frm the field weld. Their general pattern suggests that this mode of attack may have been produced during pickling. Neither possibility (pickling or in service) can be excluded by the available evidence.
However, if pickling were the case, it would seen that similar penetration should have occurred on the inner surface.
5.6 Confidence Level for Safe Ends If Not Repaired This question is not easily answered by available data. The only readily manipilable variable is stress ard the question relates to a stress level below which ECC of sensitized material will not occur. It is reasonable
'that such a level may in fact exist and that, by taking additional specimens of equivalent gemetries but at lower stresses, a reasonable estimate for a safe stress level could be obtained. Ibwever, any conclusion based on even this evidence would be questionable for the following reasons:
- 1. TMre is no clear evidence that the applied plus residual stress patterns presently existing and during reactor life can be specified with the necessary confidence.
- 2. The canplexity of interaction suggested by equation 1 and figure 26 produce an additional degree of uncertainty that makes clear conclusions difficult.
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,s - J 5.7 rnplications for Repair Procedure Based on the evidence fran this work it can only be said that sensitization ard high stresses should be avoidcd. More specific reccmnerdations are too speculative ard argumentative ard are not considered a c giate o
for this report. Ibwever, it is also appropriate to point out here that there is virtually no useful information on the mechanical properties of heavily sensitized material: 1.e. fatigue, rapid crack propagation, creep rupture.
5.8 Mechanistic Processes Wilile this is not the appropriate place to discuss at length the mechanisn for cracking, several ocmnents are a m giate.
e Sensitized stainless steel appear to crack according to an electrochenically controlled process. Oxygen serves the function of a reducible species ard therefore prcmotes the anodic process at the crack tip. The same function would be provided by hydrogen ions if the pH is lowered. The actual propagation process appears to be related to a transient dissolution of the chrcmium-depleted grain boundary material; the capacity of this material to repassivate appear to be substantially reduced relative to the bulk grain containing higher chronium. Applied stress breaks the protective film; the chranium containing grains repassivate rapidly ard no crack penetration occurs; the depleted region does not repassivate so rapidly ard cracks propagate.
6.0 Acknowledgenents The metallographic work at INE was supervised by L. A. Hartcorn; microprobe analysis was performed by W. Clark; ard scanning electron metallography was performed by J. L. Daniel j
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.O 7.0 References
- 1. T. J. Kabele and J. L. Daniel, " Corrosion Product Study by Scanning Electron Microscope", BN. ell 84, November,1969.
- 2. C. T. Hard, D. L. Mathis, ard R. W. Staehle, Corrosion, Vol. 25. , No. 9, Septenber, (1969) .
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(a) Ektachrcme color print and (b) Schenatic diagram showirg various regions of crack.
Distarces frcm inside surfaces noted locate sites where scanning micrographs taken. Specimen Iroken open to show surface of stress corrosion crack. Shiny bright region results fran this breakirg process.
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C b g = uy. b d51mm D Figure 16. Scanning electron micrographs of stress corrosion crack surface corresponding to 5-1/2 nm position noted in figure 12.
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10 5 0 5 10 10 5 0 5 10 Distance (microns)
(c) (d)
Figure 22. (a) Schanatic location showirg where microprobe trace across crack was performed. (b) Photographs showing where traces performed. '-
(c) - (d) Results fran separate detennimtion at different sites along crack.
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4 Typical Oxide Analyzed in i
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Pigure 23 Typical mide in crack showing where the x-ray spectrtan was analyzed.
1 This is not the region examined but the mide is typical.
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O o Figure 25. Montage showing scanning micrograph from face of stress corrosion crack. Special notations locate regions where microprobe detennination were made. The location of each micrograph is the same as that in figure 12. Most of the scanning micrographs are fran the series shown in figures 13-17. For reference the Cr/Fe ratio of scale-free type 304 stainless steel is 0.4. These ratios are fran direct scale readings and are not corrected.
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' Scanning Electron Micrograph O o POSITION Cr/Fe RATIO l
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C-Broken Grain C=1.7 Near Crack Mouth
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Higher Magnification
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Scarinind Electron Mkgraph POSITION O Cr/Fe RATIO G G = 0.7 9
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(' ) I I = 0.025 4 3mm (higher magnification)
.*? aws ide 9 H = 0.8
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a Cracking No Cracking
///
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- 3 e
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I Sensitization Figure 26. Schenatic relationship showirg possible pattern of interaction of stress, sensitization, dissolved cmcygen, tenperature anl pH in the stress carrosion crackirg of sensitized stainless steels.
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