ML20072E434
| ML20072E434 | |
| Person / Time | |
|---|---|
| Site: | Indian Point  |
| Issue date: | 06/30/1983 |
| From: | Czajkowski C BROOKHAVEN NATIONAL LABORATORY |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| CON-FIN-A-3400 BNL-NUREG-51670, NUREG-CR-3281, NUDOCS 8306240413 | |
| Download: ML20072E434 (51) | |
Text
{{#Wiki_filter:. . - ._- . o NUREG/CR-3281
- BNL-NUREG-51670 Investigation of Shell Cracking on the Steam Generators at Indian Point Unit No. 3 Prepared by C. J. Czajkowski Brookhaven National Laboratory b huclear Regulatory hDR DD K O O O 86 p PDR
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l NUREG/CR-3281 BNL-NUREG-51670 Investigation of : Shell Cracking on the Steam Generators at Indian Point Unit No. 3 eta Pu hed J no C ajkowski Brookhaven National Laboratory rt nt f uclear Energy l Pr: pared for Divi:lon of Engineering l Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission W chington, D.C. 20666 NRC FIN A3400 l l
ABSTRACT A metallurgical investigation was performed on specimens from the shell , of steam generators #31 and 32 of the Indian Point 3 Power Plant. The exam-inntion consisted of optical microscopy,' SEM/EDS, hardness measurements, and two different heat treatments. The shell material exhibited high values in hardness prior to the heat treatments, which was indicative that relatively high residual stresses may have been present in the areas of the welds.- All observed cracks were transgranular in appearance and were associated with pits on the vessel's inside surfaces. Beach marks were observed on a fracture face from steam generator #32, as well as possible fatigue striations. 1 The report concludes that the cracking was caused by a low cycle corro-i sion fatigue phenomenon with cracks initiating at areas of localized corrosion and propagating by fatigue. The cause of the pitting / cracking is considered l to be related to the unit's relatively high operating 02 levels and copper species in solution. The report also concludes that stress corrosion cracking cannot be entirely discounted as a possible failure mechanism. 1 i
TABLE OF CONTENTS Page ABSTRACT . . . . . . . .. ... ..... . ... . . . . . . . . i LIST OF TABLES . . . .. ....... . .... . . . . . . . . . iv LIST OF GRAPHS . . . . . ....... .... . . . . . . . . . . iv LIST OF FIGURES. . . . .. ........ . ... . . . . . . . . v
- 1. INTRODUCTION . . . . . . . . .. ... . .. ... . . . . . . . . 1 II. VISUAL EXAMINATION . . . .......... .. . . . . . . . . . 2 III. HARDNESS MEASUREMENTS / HEAT. TREATMENT . ..... . . . . . . . . . 2 IV. OPTICAL MICROSCOPY . . . . . . . . . . . . . . ... . . . . . . . . 3 V. SEM/EDS. . . . . . . .. ...... . ... ... . . . . . . . . 4 VI. DISCUSSIONS / CONCLUSIONS. . .. . .. . ..... . . . . . . . . . 6 VII. ACKNOWLEDGEMENTS . . .. ... ... ... . . .. . . . . . . . . 8 VIII. REFERENCES . . . . . . . . . . ... .... ... . . . . . . . . 9 i
iii i
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LIST OF TABLES Table No. Pajgt 1 Chemical and Tensile Properties of A302 Grade B Steel . . . . . 10 2 Chemical and Mechanical Properties of E8018 C-3 Welding Electrode. . .......... . . . . .. . ... 11 3 Hardness Readings of Plug Sample. . . . . . . . . . . . . . . . 12 4 Hardness Readings of Boat Samples . . . . . . . . . . . . .. . 14 LIST OF CRAPHS Graph No. Page 1 Hardness Values of the Base Metal Af ter Heat Treatment. . .. . 15 2 Hardness Values of the Weld Metal After Heat Treatment. . . . . 16 3 Hardness Values of the Heat Affected Zone After Heat Treatment. . . . . . .......... .. . . . . . . . . 17 ( iv
LIST OF FIGURES Figure No. Page 1 Schematic of Steam Generator . . . . . . . . . . . . . . . . 18 2 Photograph of 6" Plug 0.D. . . . . . . . . . . . . . . . . . 19 3 Photograph of 6" Plug I.D. . . . . . . . . . . . . . . . . . 20 4 Photograph of 6" Plug Cross Section. . . . . . . . . . . . . 21 5 Photograph of 6" Plug Showing Through Wall Crack . . . . . . 22 6 Photograph of a Specimen from the 6" Plug. . . . . . . . . . 23 7 Top View of Boat Sample MT #44 . . . . . . . . . . . . . . . 24 8 Top View of Boat Sample MT #46 . . . . . . . . . . . . . . . 24 9 Top View of Boat Samples MT #37, 38. . . . . . . . . . . . . 24 10 Photomicrograph of Pit - No Crack. . . . . . . . . . . . . . 25 11 Photomicrograph of Pit - With Crack. . . . . . . . . . . . . 25 12 Photomicrograph of Transgranular Crack . . . . . . . . . . . 25 13 Photomicrograph of Open-Mouthed Crack. . . . . . . . . . . . 26 14 Photomicrograph of a Crack at the Bottom of a Pit. . . . . . 26
- 15 Photomicrograph of Area of Lack of Fusion. . . . . . . . . . 27
( i 16 Higher Magnification Photo of Figure 15. . . . . . . . . . . 27 17 Photomicrograph of Base Metal Structure. . . . . . . . . . . 28 ! 18 Photomicrograph of Weld Metal Structure. . . . . . . . . . . 28 19 . Photomicrograph of Heat Affected Zone Structure. . . . . . . 28 20 . Comparison Photomicrograph /Fractograph . . . . . . . . . . . 29 21 Low Magnification Photomicrograph of MT 37/38 1 ! a) Highe r Magnifica tion - Base Metal . . - . . . . . . . . . . 30 b) Higher Magnification - Weld Metal. . . . . . . . . . . . 30 c) Higher Magnification - Heat Affected Zone. . . . . . . . 30 V i i
l l l l LIST OF FIGURES , (Cont'd) l Figure No. Page 22 Low Magnification Photomicrograph of MT 44 a) Higher Magnification - Base Metal. . . . . . . . . . . . 31 b) Higher Magnification - Weld Metal. . . . . . . . . . . . 31 c) Higher Magnification - Heat Affected Zone. . . . . . . . 31 23 Low Magnification Photomicrograph of HT 46 a) Higher Magnification - Base Metal. . . . . . . . . . . . 32 b) . Higher Magnification - Weld Metal. . . . . . . . . . . . 32 c) Higher Magnification - Heat Affected Zone. . . . . . . . 32 24 SEM Photo of "Through Wall Leaker" . . . . . . . . . . . .. 33 25 a) Higher Magnification Photo of Copper Deposits. . . . . . 33 b) EDS Scan for Constituents. . . . . . . . . . . . . . . . 33 26 Low Magnification Fractograph of _6" Plug Fracture Face . . . 34 27 a) 100x Fractograph of Possible Fatigue Area. . . . . . . . 35 b) 50x Fractograph of Possible Fatigue Area. . . . . . . . 35 c) 200x Fractograph of Possible Fatigue Area. . . . . . . . 35 d) 500x Fractograph of Possible Fatigue Area. . . . . . . . 35 28 Low Magnification SEM Photo of Another Fracture Surface from the 6" Plug . . .'. . . . . . . . . . . . . . 36 29 SEM Fractograph of " Beach Marks" . . . . . . . . . . . . . . 37 30 EDS Scan of Typical Constituents of MT 37/38 . . . . . . . . 37 31 EDS Scan of Typical Constituents of MT 44. . . . . . . . . . 37 32 EDS Scan of Typical Constituents of MT 46. . . . . . . . . . 37 33 Low Magnification Fractograph of MT 44 . . . . . . . . . . . 38 34 Low Magnification Fract'o graph of MT 46 . . . . . . . . . . . 39 35 Low Magnification Fractograph of MT 37/38. . . . . . . . . . 40
-36 SEM Photo of MT 37/38 Initiation Site. . . . . . . . . . . . 41 vi
LIST OF FIGURES l (Cont'd) l 1 Figure No. Page 37 a) Higher Magnification Photo of MT 37/38 Pit ....... 41 b) EDS Scan of MT 37/38 Pit for Constituents ..... .. 41 38 a) SEM Photo of Start of a Pit on MT 44 . . . . . . . . . . 42 b) EDS Scan of 38a Area . ... ......... ..... 42 39 a) SEM Photo of MT 46 Pit .. . ..... ......... 42
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- b) EDS Scan of MT 46 Pit.
.. . ............ .. 42 ,
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'40 a) SEM Photo of MT 37/38 Pit. .......... ..... 43 b) EDS Scan of MT 37/38 Pit - Rim . . ..... . ..... 43 c) EDS Scan of MT 37/38 Pit - Interior. . . ... ... . . 43 T ,
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Q /-# - I. s INTRODUCTION i/ N , _, " 4 On March 27, 1982, during a refueling outage (with the reactor in a cold ).) s hut _down condition), a small leak was detected on the shell side of steam gen-
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,; f erator #32 at the Indian Point-3 Nuclear Power Plant. Further examination of Glj : steam generator #32 disclosed that the detected leak originated in the circum-hhO ferential weld. joining the transition cone to clie upper shell (closure weld) of-g the' steam generator (Figure 1). W' I IndianPoint-3isa925MWepress'urizedwaterjreactor(PWR)with,four Westingho'use Model 44 steam generators (vertical 7U tube design). The unit has
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p % Nd'approximately three years of' effectiv'e full power operation since its com-
. g, nj N TEarcial
; starting date in 1976.
g , M J- The steam ' generator shell is constructed of SA302 Grade B material (Table f 1) ofL4"~ approximate-thickness. The closure weid had a nominal 45* included angle weld preparation and was welded from the odtside surface of the vessel by f" the submerged are process with backing. The' spacer strip was then back gouged s-and tlpe weld completed by welding from the inside7 3urface with the shielded 4- metal' arc (SMAW) process using E8018-C3 electrode (Table 2). The weld was then continuously stress relieved at 1000*F minimum for three hours / inch of thick-
, ness-(12 hours total soak time). .
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Closer examination of the. area of the leak d closed a hole on the outer j- surface of steam generator #32 approximately 16 mm lopg by 5 mm wide (Figure j, (2). .-:The owner of Indian Point-3, Power Authority of' the State of New York j PASNY), then instituted a nondestructive, testing progran consisting of visual !. W iinspection, ultrasonic examination, and magnetic particle examination of the p, sffected welds. These examinations disclos'ed that the closure weld on each of G< , 'thesteam generators had in excess of J0O~ cracks associated with them, although
' ^
Ithe only through-crack was the " leaker" lu# steam generator #32. A significant amount of pitting.(Figure 3) was also as9cciated with the closure welds of the 6
' steam generator inside surfaces.
g 1. ,.- qr f C In ordept'o characterize the cracking, PASNk' had a 4" x 1" elliptically Q shaped boat. 'rample (containing circumferentially oriented cracks) removed from j
/ the-in'sids s.;rface of steam generator.#32 in the area of the closure weld.
This-sample' was first sent to Lucitiiditkin, Inc. and then to General Electric f Co. for metallurgi' cal evaluation and fatiti,re analyses. These analyses yielded s'omewhat dif fering conclusions as to the 'catise of the cracking. Another larger sample (6", plug) was removed from steam generator #32 encompassing the through j ~ wall leake'r (Figures 2 and t 3), and sent to' Lucius Pitkin for further analysis. u Owing to th'.possible e " generic" nature of'the problem, the Westinghouse Corpor-
.ation reco. amended that certain pisnts initiate inspections of their PWR steam ger.erators during their scheduled [ shutdowns. Additionally, the Materials
; Engineering Branch (MTEB) of th@ United States Nuclear Regulatory Commission H' (USNRC) commissioned Brookhaven' National Laboratory (BNL) to perform an inde-f penddnt failure snalysis on the 6" plug (after completion of the evaluation by 7 Lucius Pitkin), and on three additional boat samples containing cracks cist from steam generator (31.- _
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The BNL evaluation was to encompass the following tasks in order to de-termine the possible cause(s) of the cracking:
.1) Visual examination / photography
- 2) Hardness measurements / heat treatment
- 3) Optical microscopy
- 4) Scanning electron microscopy (SEM)/ energy dispersive spec-troscopy (EDS)
These examinations were performed on both the plug specimens and the three boat samples.
- 11. VISUAL EXAMINATION Figure 4 is a section of the 6" plug removed from steam generator #32.
It is cut vertically, relative to the steam generator weld. The section was etched for one minute using a 10% ammonium persulfate + water etch. The multi-ple weld passes of the crown weld are clearly seen, as well as a large area of repair weld at the inside surface. A crack approximately 20 mm in length is visible on the upper shell side of the weld 6 mm away from the repair weld heat affected zone-(HAZ). Circled section A in Figure 4 is an area of lack of fu-sion (higher magnification micrographs are in optical microscopy section) which did not appear to propagate. Area B is an area of porosity which is related to the original-repair welding of the steam generator vessel. The actual path of the through wall " leaker" seemingly followed the weld heat affected zone and is clearly defined in Figure 5. The broadening of the crack near the outside surface of the steam generator is most probably the re-sult of a steam channel erosion effect after the crack had breached the outer surface. Figure 6 is a macrophotograph of another specimen received from Lucius Pitkin depicting the relative location of another crack to the repair weld area. - All three of the boat samples received from PASNY (Figures 7, 8 and 9) showed evidence of pitting on the inside surface of the steam generator, es-pecially in the case of MT indications 44 and 46, whose cracks appeared as an array of pits following the crack outline. III. HARDNESS MEASUREMENTS / HEAT TREATMENT d The section of plug weld (Figure 4) was polished and etched and then microhardness measurements were performed on the specimen as received. Table 3 is a tabulation of these measurements. Additionally, Rockwell measurements 1 were performed for comparison of the weldments' bulk properties. The plug sec-tion was then subjected to two differing heat treatments: a) 1000*F for 3 hours / inch of thickness with a cooling rate not ex-ceeding 100*F/ hour until 600*F, then furnace-cooled. 2
b) 1125'F for 1 hour / inch of thickness with a cooling rate not ex-ceeding 100'F/ hour until 600*F, then furnace-cooled. Hardness measurements (both Knoop and Rockwell) were taken after each heat treatment and are also listed in Table 3. All values recorded for the base metal, weld metal and heat affected areas were then plotted on Graphs 1-3. The solid line in each graph represents the maximum hardness values recorded, while the dashed line is the simple arithmetical average of the recorded values. It is evident from the three tables that each of the heat treatments reduced not only the maximum hardness values, but also the average hardness values recorded. These results indicate that this particular section of the plug weld had received a heat treatment of less than 1000*F in the location of the hardness measurements. This reduction of maximum and average hardness values af ter heat treatment were also substantiated by the Rockwell measurements. The only ex-ception to this was that the maximum value of the base metal hardness measure-ments appeared to increase slightly af ter the 1000*F heat treatment. This increase was well within the sensitivity range of the equipment. The highest value obtained during these tests was KHN 367, or the equivalent of RC 36.6, which is not inordinately high for this material in the as-welded condition. Table 4 is a tabulation of microhardness readings for the three ' oat samples removed from steam generator #31. A maximum value of KHN 396 was re-corded, translating to RC 39.4. This peak value is an indication that relatively high residual welding stresses may be present in this weldment.
'IV. OPTICAL MICROSCOPY Various pits on the inside surface of the plug sample were examined by optical microscopy after etching with a 10% Nital solution. The specimens were prepared by making a cut perpendicular to the inside surface of the plug and then grinding and polishing back until the pits were reached. Some pits ex-l amined (Figure 10)-had.no cracks associated with them. The structure of the l material in the area adjacent to the pits was that of a tempered martensite, which would be normal for this type of steel.
Some shallow pits had cracks associated with them (Figures 11 and 12). !- The cracks were tight and transgranular in nature, emanating at the bottom of the shallow pits. These cracks were continuous with virtually no branching evi-dent. Other pits had shallow, opened-mouthed cracks associated with them (Figures 13 and 14). These cracks had the appearance of hasing a more active corrosion process associated with them than the predominantly transgranular j ones. The microstructure surrounding these cracks was also a tempered marten-l site. The area of lack of fusion in Figure 4 (area A) was also investigated. Figure 15 is a photomicrograph showing the area of lack of fusion (boxed area) in relation to the weld and heat affected zone of the repair weld. Figure 16 is a higher magnification photomicrograph showing that no subcracks are asso-ciated with the area of lack of fusion. 3 l
The microstructure of the various base metals (Figure 17), weld metals (Figure 18) and heat af fected zones (Figure 19) from the 6" plug were examined. The base metal had a tempered martensitic structure conaistent with A302 Grade , B material. The weld metal had a' dendritic structure which would be normal for both manually deposited shielded metal arc weld of E8018-C3 material and sub- . merged arc welding. The heat.affected zone of the plug weld had a more acicu- l lar structure normally associated with untempered martensite. j A comparison (Figure 20) photomicrograph /fractograph was made of the j crack shown in the specimen in Figure 6. It is quite evident that the crack is transgranular with virtually no branching associated with it. Although'there l is no definitive heat affected zone visible in the immediate vicinity of this crack, there was a grain size difference between areas closer to the inner sur- i face and'those about mid-depth of the crack. This grain size difference is
- similar to that which would normally be associated with weld heat affected
- zones or those encountered with mechanically worked thick sections in materi-als. The fractograph clearly shows a non-continuous crack growth ini.tiating from a point 'on the fracture surface not included on the fractograph due to the limited thickness of this particular -specimen. The radiating lines in the upper section of the fractograph would normally point to the initiation site of
- the fracture.
Optical cross sections were cut, mounted and etched from the three boat samples cut from steam generator #31. Figure 21 shows the crack identified as MT indication #37. The crack.is extremely straight, transgranular, and ex-4 hibits virtually no crack branching. - The crack appears to initiate at a pit on the inside surface of the specimen. The higher magnification photomicrographs , of the weld metal (Figure 21a) shows a normal dendritic structure for E8018-C3 material. The base metal had a tempered martensitic structure (Figure 21b) which is also normal for the material condition. Figure 21c is a photomicro- ' i- graph 1of the heat affected zone which has a more acicular structure not nor- I I' mally associated with the " tempered martensitic" c'ondition, but more normally associated with untempered martensite. The cracks associated with MT indications 44 (Figure 22) and 46 (Figure
- 23) were both associatd with surface pits and were transgranular with very little branching evident. Both cracks exhibited arrests along their length where it -appears that the corrosion process was dominant for some period of time. The higher magnification photomicrographs (Figures 22a,b,c, and 23a,b,c) of the' base metal, weld metal and heat affected zone structure of these two cracks correspond to the same observations noted previously on MT indication
#37.
V. SEM/EDS i Various fracture faces from both the plug and the three boat samples were examined, in the hope of characterizing the failure mode. Additionally, the fracture faces and 'various pits were examined by Energy Dispersive Spectroscopy
~
(EDS) in an attempt to determine if corrosive ..anstituents were present. 4
The first specimens examined were those from the 6" plug. Initial ,ex-amination of the fracture faces' disclosed that the surfaces were covered by a tight and adherent oxide film. In order to more fully characterize the fracto-graphic features of the cracks, an electrolytic cleaning procedure was insti-tuted on the plug weld specimens. This cleaning procedure is described below. A working solution of Endox-214 was prepared by adding 8 ounces of Endox-214 powder to 1000 ml of cold water and stirring until it was completely dissolved. A small amount of Photoflow was added to the solution to aid the wetting of the specimen and eliminate some of the featuring during the electrochemical clean-ing step. A glass beaker with 500 ml of ~ the Endox-214 solution was placed in an ultrasonic cleaner. The specimen was made the cathode, and a platinum wire loop used as an anode. A current density of N 250 mA/cm2 was applied for 15 seconds. The specimen was removed from the electrolyte and ultrasonically washed in a detergent solution consisting of Alconox and Photoflow for one minute, then rinsed in clean water, dipped in methanol and' dried in hot air. The above procedure comprises one cycle. It may be necessary to repeat'the above cycle several times before removing all the corrosion products. It was not possible to predetermine the exact number of cleaning cycles for any given specimen, since it depended upon the severity of the oxidation, roughness of surface, and the physical size of the sample. The specimen was observed op-tically af ter each cycle so that the process could be discontinued after the oxide or the corrosion product was removed and the specimen surface looked clean. Af ter the specimen was thoroughly dried, it was either examined imme-diately, since it was subject to reoxidation at ambient atmosphere, or it was stored in a good desiccant awaiting examination. The 'first plug specimen examined was from the ' area of the through hole
" leaker" (Figure 24). Salient features of the surface were virtually non-existent due to the erosive environment of the leaking secondary fluid. This particular specimen had visible copper colored deposits in evidence af ter elec-trolytic cleaning. These deposits were confirmed by EDS as copper, with zinc
- also in evidence (Figures 25a and 25b).
l l Due second specimen examined (Figure 26) exhibited a transgranular frac-ture face with the " wood-like" appearance normally associated with. progressive or fatigue-type fractures. Higher magnification scrutiny of the fracture face l disclosed an area of possible fatigue interaction (Figures 27a-27d). The struc- [ ture of low alloy steels makes a positive identification of fatigue striations l extremely dif ficult. Area B of this fracture face (Figures 26) appears to be an area where the crack started to change its direction of growth, due to the effect of a dominant stress field, perhaps the vessel's hoop stress. 7 The third fracture face examined from the plug coupon (Figure 28) also j displayed a fracture face characteristic of fatigue fractures. This particular fracture initiated at pits which were quite evident in the fractograph. Figure 29 is a fractograph of the same fracture surface, which clearly shows " beach marks" in evidence. These again are indicative of " fatigue-type" fractures. The fracture faces associated with the three boat samples were also ex-amined prior to electrolytic cleaning in order to ascertain any information re-garding possible corrosive species which may have contributed to the failures. 5
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]
- All scans were performed near the leading edge of the cracks. The fracture faces exhibited normal iron and manganese, with traces of silicon and copper being the most prevalent additional constituents (Figures 30-32). With the exception of some scans having zine or nickel present, these scans were typical
- of all scans on the fracture faces.
Af ter electrolytic cleaning, the fracture faces for the three boat sam-ples were examined.. . All three fractures ' exhibited transgranular features with , the ~ wood-like characteristics of fatigue fractures (Figures 33, 34 and 35). Additionally, the fractures had. varying degrees of pitting associated with the initiation points of the fracture. _ An EDS scan of the area of initiation on fracture face NT indication 37/38 indicated Fe, Cr, Ni, Si, S, Cu and Zn (Fig-ures 36, 37, and 37a).
. Other pits were examined at the mouth of the cracks (Figures 38-40).
1 Only the pit in Figure 38a exhibited" additional elements present other than the typical ones of Fe, Mn, Ni, Cu and Zn. This pit also had present Al, Si, Cl i and K. It is also interesting to note that.the EDS analysis-of the pit in ! -Figure 40a had a difference in composition between the rim of the pit and the interior portion of the pit.. The difference was evidenced by the element nickel not being,present on the pit's rim but is present on the pit's interior surface. f' VI. DISCUSSIONS / CONCLUSIONS Indian Point-3 has operated on an all-volatile treatment from initial startup. This is one of only two units which has developed moderate to severe denting without a prior history of the phosphate treatment. The Hudson River estuary at the Indian _ Point site ranges from fairly fresh water to a brackish environment, depending on the season. Copper alloys l in the condenser tubes at the site have been corroded by this in-leaking l brackish water so that the sludge analysis in Indian Point-3 shows concentra-tions of copper as high as 45% and iron 40%. A significant amount of Cl- is present as well. Analysis of this sludge has shown significant quantities of copper as cuprous. oxide (Cu2 0 ) and is also the only plant that contains sig-nificant amounts of alpha hematite (alpha-Fe2 O 3) in the sludge pile.. The presence of both of these constituents indicates that oxygen control in the-Indian Point-3 steam generators has been poer for a considerable period of time. The aforementioned poor oxygen control, plus the fact that hydrazine levels have been kept low in the steam generators (due to environmental con-cerns), have definitely contributed to the pitting of the shell material. f Additionally, -in January 1981, the unit suffered a turbine blade failure which. damaged approximately 50 condenser tubes and allowed chloride into the g steam generators with recorded levels of up to 325 parts- per million (ppm). l This chloride intrusion may have had a decided influence on initiating pits at the inside surface of the steam generator shell. 6
1 1
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l Steam generators.have had thermal fatigue indicated as the causative factor in other failures, such as cracking of feedwater nozzles [1]. These nozzle cracks showed large numbers of crack arrests and beach marks, suggesting that their propagation was also discontinuous in nature and caused by a stress mode that was cyclic in nature. The conclusions on the failure mechanism of these nozzles was low cycle corrosion fatigue. It is interesting to note that in the case of the feedwater nozzle cracking, machining marks played a role in + the initiation of the cracks since they formed sites for localized corrosion (pits). The cracks observed on this investigation were quite similar in ap-pearance to the. feedwater cracks (by optical and electron microscopy). Constant Extension Rate Tests (CERT) performed on A508 C12 steel [2] (another pressure vessel steel) in oxygenated water at higher temperatures have
. demonstrated that this particular steel is susceptible to transgranular stress corrosion cracking in pure water between 100*C to 288'C if the water contains.
1 or 8 ppm oxygen. This work also described pitting corrosion occurring in pure water (of the same oxygen content) at 100*C and 150*C. Transgranular cracks nucleating from the corrosion pits were also noted. A recent EPRI report [3] on environment concerns for carbon steel piping concluded that carbon steel is susceptible to environmental assisted cracking in high temperature oxygenated water environments, especially under high ampli- ! tude, low frequency cyclic loading.. Low frequency cyclic stresses would be the type encountered during steam generator operation. 1-Oxygen has been considered to be a strong influencing factor in the gen- , eral corrosion of carbon steels in nuclear reactor environments [4]. Oxygen content has also been shown to exert considerable control on the incubation l time of crack growth in A302 Grade B material [5,6] subjected to corrosion fatigue testing. These results stemmed from a testing program initiated af ter
. the Japanese Power Demonstration Reactor developed cracks in its stainless steel overlay which propagated into the A302 Grade B base material.
The nucleation of the pits on the shell walls was probably the combined effect of oxygen with the copper species promoting a more oxidizing potential in solution. The effect of chlorides [7,8] in promoting pits in iron has been investigated and probably aided in pit nucleation. Carbon steel has also been known to be susceptible to cracking by chloride solutions at 316*C [9]. i Although residual stresses and their relative importance in fatigue life have been argued extensively [10,11,12], it is generally accepted that fatigue strength increases when specimens tested have compressive residual stresses. If a weld has received an insufficient or inadequate stress relief, the stresses present would more than likely be of the tensile rather than com-pressive variety in the weld. . Another characteristic of fatigue is that many i fatigue failures are the result of material stress raisers on a free surface. i Pits on the inside surface of a pressure vessel are effective stress raisers. The aforementioned discussion and previous observations hav' led to the following conclusions regarding the cracking of the Indian Poine 3 steam generators: 7
r
$4
- I
-1) All'of the cracks examined were transgranular in' nature and'were as-- '
sociated-with pits on the inside surface of the steam generators.
- 2) The-hardness' measurements after heat treatment and'on the "as're-
"ceived" material indicate the possibility of high' residual stresses
~
associated:with the welds. These hardness values were not: excessive, however. 1
~
3); The'relatively high 02 levels- and the copper species in solution , contributed to'the pitting on the inside' surface of the vessels. The previous chloride intrusion may also have complemented this effect.
=4) The~ observation of " beach marks". and possible fatigue striations,
+ coupled with~the transgranularity of the cracks, is characteristic of l' ta low cycle corrosion fatigue phenomenon.
- 5) Although corrosion fatigue is considered the primary cause of degra-dation, the relative importance of corrosion in the crack propagation rate is currently undetermined. The possibility of stress corrosion cracking being a contributing cause of the failure cannot be entirely i~ ' discounted due to the lack of literature concerning this alloy's re-sponse in copper containing oxygenated water.
1
'VII. ACKNOWLEDGEMENTS r-
< c The author wishes to thank R. Sabatini for the SEM/EDS work, L. Gerlach for his able assistance in testing, A. Cendrowski and D. Horn for the hardness measurements, D.m Thompson for her typing skills, and Dr. J. R. Weeks for his
' continued support.
4 s i e e 1 4 d r I 8-
. - -- _ . ..~. . . _ . . . , _ _ . _ _ ___ _ _ , _ . . , _ _ , _ _ _ _ .
VIII. REFERENCES l
- 1. _Vyas, B. , Czajkowski, C. J. and Weeks, J. R. , Nuclear Technology, j!5, (November 1981).
- 2. Choi, H. , Beck, F. H. , Szklarska-Smialowska, Z. and MacDonald , D. D. ,
Corrosion, 38, No. 3, (March 1982).
- 3. Weinstein, D., EPRI NP-2406, Project 1248-1, Final Report, (May 1982).
~4. Pearl, W. L. and Wazadlo, G. P. , Corrosion, 2],, (August 1965).
- 5. Kondo, T., Kikuyama, T., Nakajima, H., Shindo, M. and Nagasaki, R.,
Proceedings: Corrosion Fatigue: Chemistry, Mechanics and Micro-structure, (June 1971) (NACE)
- 6. Kondo, T. , Nakajima, H. and Nagasaki, R. , Nuclear Engineering & Design, 16, (1971).
- 7. Janik-Czachor, M. , Wood , G. C. and Thompson, G. E. , Br. Corros. J. , 15, No. 4, (1980).
- 8. Szauer, T. and Jakobs, J. , Corrosion Sc. , 16, (1976).
- 9. Strauss, M. B. and Bloom, M. C., Corrosion, 553t-556t (May 1961).
- 10. Welding Handbook, Volume 1, 7th Edition.
- 11. Mechanical Metallurgy, 2nd Edition, G. E. Dieter, ed. , McGraw Hill (1976).
- 12. Physical Metallurgy Principles, R. E. Reed-Hill, ed., D. Van Nostrand Co., (1964).
t 9
TABLE 1 Chemical and Tensile Properties of A302 Grade' B Steel Chemical Requirements Elements Composition, % Carbon,-max *: Up to 1 in. (25 mm) incl. in thickness 0.20 Over 1 to 2 in. (50 mm) incl. 0.23 Over 2 in. in thickness 0.25 Manganese: Heat analysis 1.15-1.50 Product analysis 1.07-1.62 Phosphorus, max
- 0.035 Sulfur, max
- 0.040 Silicon:
Heat analysis 0.15-0.40 Product analysis 0.13-0.45 Molybdenum: Heat analysis 0.45-0.60 Product analysis 0.41-0.64 Nickel: Heat analysis Product analysis l
- Applies to both heat and product analyses i
Tensile Requirements j [ Tensile Strength, ksi (MPa) 80-100 (550-690) Yield Strength, min, kai (MPa) 50 (345) Elongation in 8 in, or 200 mm. min. % 15* Elongation in 2 in. or 50 mm. min. % 18* I~ h
*See Specification ASTM A20 t l l
l 10 l i
TABLE 2 Chemical. and Mechanical Properties of E8018 C-3 Welding Electrode 4 Chemical Requirements Percent
- Carbon- 0.12 Manganese 0.40-1.25 Phosphorus 0.030 Sulfur 0.030 Silicon 0.80 Nickel. 0.80-1.10 Chromium 0.15 Molybdenum 0.35 Vanadium 0.05 i
- Single values are maximum percentages Mechanical Requirements l - Tensile Strength, min, psi 80,000 f
Yield Strength @ 0.2% offset, psi 68,000-80,000 Elongation in 2 in., min, percent 24 r t l t 1 11
1 i i TABLE 3 j Hardness Readings of Plug Sample As Received (KHN Values /500 gm) Base Metal HAZ Weld 248 296 240 323 267 287 311 259, 289 243 342 280 280 28 0 251 282 246 332 281 289 278 248 307 247 367 280 295 28 0 1 247 278 246 356 284 292 245 269 244 298 303 366 365 Af ter 1000*F Heat- Treatment (KHN Values /500 gm) Base Metal HAZ Weld 231 264 280 314 295 282 247
-244 263 264 346 296 316 232 226- 267 274 328 311 311 243 238 263 270 314 295 322 237 228 258 288 284 291 321 235 Af ter 1125*F Heat Treatment (KHN Values /500 gm)
Base Metal HAZ Weld 229 210 279 280 269 262 231 220- 213 274 29 1 246 280 221 l 224 221 282 288 279 267 237 221 220 234 277 267 265 234- ( l 217 215 280 295 262 248 221 4 i 12
d TABLE 3 (Cont'd) Rockwell Hardness Values (RB unless noted) As Received Base Metal HAZ Weld 90.5 94 97 43.5(Rp) 96.5 99.5 92.5 92.5 43.5(Rp) 96 96.5 92.5 92.5 93 46.5(RD) 44.5(RD ) 95.5 92 92.5 95 99.5 50(Rp) 97 91.5 93 _41(RO ) 98 After 1000*F Heat Treatment (RB unless noted) Base Metal HAZ Weld 90 95.5 98 42(RO) 97 98
- 92 91 43.5(RD ) 41(RD ) 95 95 93 93 48(RD ) 47(RD) 98 92 94.5 93 97 43(RD ) 97 95 95 40(RD) 96.5 "
After 1125'F Heat Treatment (RB unless noted) { Base Metal HAZ Weld 93 92 100 41(Rp) 97 97 93 92 43(Rp) 100 96 96 k 96 93 42(RD ) 41(Rp) 97 96 I. 92 '92 97 40(Rp) 96 95 92 40(RD ) 95 13
\
\
TABLE 4 Hardness Readings of Boat Samples (As Received) Sample 37/38 Base Metal HAZ Weld 227 220 324 312 300 255 228 240 307 307 281 264 229 303 373 396 357 Sample 44 Base Metal HAZ Weld 274 272 252 320 267- 261 274 261 338 324 264 255 255 329 246 Sample 46 Only one microstructure noted 227 229 240 235 219 233 233 233 237 240 14
GRAPH #1 A Graphical Comparison of the Hardness Values of the BASE METAL After Heat Treatment 310 BASE METAL 300 A 290 a - 4 O J 280 - 2 o . . o 270 . o - m - V 260 g t- - - - -_ ______ ~ uJ s '
$ 250 -
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= Maximum Hardness Values
--- = Average Hardness Values 15 I
I l . _ _ _ .___..,_ ,,. -- . .__ _ _.
l r GRAPH #2 A Graphical Comparison of the Hardness Values of the WELD METAL After Heat Treatment I l WELD METAL i l 310 - l 300 0 o
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= Maximum Hardness Values
--- = Average Hardness Values 16
GRAPH #3 A Graphical Comparison of the Hardness Values of the HEAT AFFECTED ZONE After Heat Treatment
)
l 370 HEAT AFFECTED ZONE i 360 340 - N. A 330 - o - 0 320 : E :. O 310 .. o H - - - - - - - - - - - o .4s W '
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= Maximum Hardness Values
--- = Average Hardness Values 17
, , STEAM OUTLET N0ZZLE
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l O-Figure 6. Photograph of another crack in the 6" plug specimen and its relative position to the repair wel d . 23
Y l' IlllilllillIllililllIlilililll'lilililljililililllililililIlllflililtjilililigijijilil ligi[ijijij O 1 2 3 4* 1 j O 1 2 ~3 4 5 6 7 8 9 10 11 12 lilllH ullilllllilllilllllll h lilllllllii n !i gilg lilg illt i g h g Illgllg g h g ]lg glg glg g] g g li j g lg glji nlg g g' Figure 7. Photograph of the top side l (inside surface) of the boat sample itT #44 . 1 i tlIltlilllIlllIltill!!Ililllfl'lilIlilll'lil 2 l'l'l'I'l'I'I'l'I'l'I'I'l'I'I'I'I'I'I'I'I'I'I'l'I'I 3 4 O 1 j 6 7 8 9 10 11 12 O 1 2 3 4 5 in:icohmindunhmlun!nMiw d4&ilMm'onkinh;ulauluuloianninnlH , Figure 8. Photograph of the top side of i boat sample !1T #46. J i p p lIli j i j s plilllIlllililll f illiit illIlllilli flIlil'Ill'l'lllilil'l'l'l'lil' \'l'I'l'l'l'lil'I'I'l i 0 -1 2 3 4 S O 1 2 3 4 5 6 7 8 9 10 11 12 i Hillinilullhuilillilitillll!!!hHill!!hil!IIHlhillhillhlHIHilllill!Illl!HilhlllIUU!UUbul!UU!Ihl HU U 2 Figure 9. Photograph of the top side of ! boat sample tit #37,38. Note the extensive pitting .
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250X 400X r '; kigura^13i Photomicrograph of one of the Figure 14. Another smaller crack found at
, - - shah os , oprn-em thed'cr6cks the bottom of a pit.
,, at the bottom.of a pit.
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" 62.5X Figure 15. Photomicrograph depicting the area of " lack of fusion" shown in Figure 4 ( Area - A ).
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f i4' - . (&Dl?ge. (:h; + k, : . { L 400X Figure 16. Higher magnification photomicrograph of the area of " lack of fusion" showing blunted ends and no additional cracking evident. l 27
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" Figure 17. Photomicrograph depicting the Figure 18. Photomicrograph of the weld base metal's structure as a metal , showing a characteristic tempered martensite. dendritic structure.
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cfa*u'h&e S tes INh e) n { 400X Figure 19. Photomicrograph of the 6" plug's heat affected zone -hard marten-site.
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3.y' tAbfig,&*Tf? ~h 3 a Figure 20. A comparison between the optical photomicrograph and the fractograph =- of the crack shown in Figure 6.There is virtually no crack branching = evident. Note the discontinuous nature of the fractograph. M A 29 =
=
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c ~, -; a Figure 21. Photomicrographs of the MT 37/38 crack. Higher mag. photos of the areas of interest are also shown. 400X 30 21c. Heat affected zone.
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,s 22a. Base metal.
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'.ia;2 + s 4 QwQ Figure 22. Photomicrographs of the MT 44 crack. Higher mag. photos b.
El " I
.[.m b w
of the areas of interest V
.4 '
Q ,
'*^ %' N N 4' are also shown. i
';.; iM ! i 6 b'? ,, . _ a. .:
. waar ru 31 22c. Heat affected zone.
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bg3%fg.%m.49lg , :,.pgr 1M, 3 t;mwplM- r W7 %2 - Q! , : $ 7, xt y (fM {lk. t; f&!
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fVA'Ci%Q:19 h .f \? $ ? f' ?g. % 8,k N N[.ke ! IN. t . 3d.$ 400X 23a. Base metal.
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. W: , 400X n
Q [ ~ 23b. Weld metal . x ' Ti7
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4..
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.s 4 Figure 23. Photomicrographs of the MT ' ". ~ , , . . . ~'-' r' Q, #:;;~; ,..
46 crack. Higher mag. photos # 4., g, :
$.' c.g . / $, f g., r.,# .-'[f*..
of the areas of interest G . ,$' C, ' , , '
.c ,
are also shown. 6 1 '- *
- 32 23c. Heat affected zone.
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+. y 3 ,, . . , - ,. .u ,
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' 4' y 3 cW % i :
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'9gkf).. d $%. .. : ey jg,g.
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.,4 + ..?
e y p,w Q' . K
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& /, 4 Figure 24. SEM fractograph of the fracture face of an area of the "through wall leaker". The boxed area is one of the de-posited copper locations.
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s.- - Y: ;.fd. (. , .a ' "/4,.7 y;. g ;, i TASX: I., [ g.. *t,f ' . hHA12 ADD.820 KEV /CH ite. SECS 9- A ' y G l : a ,- ?*^ ).q
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10.228 1000X Figure 25a. Higher magnification photo of Figure 25b. EDS scan of the deposit in 25a the copper deposit. showing a definite copper and zinc peak in addition to. iron. 33
.+
.e.~, 3..
.-;c.-d T.g; i @ i'd : 'r . ':+ <; .
, ~3r$;G%
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'lQ $s/ Wfi42f m:a kWit wl}Q J' iox
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, ~a.m' u: p{,n,-
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,_s t 'A 7 c.
f g"- L 9 s Figure 26. Low magnification SEM fractograph of one of the fracture faces associated with the 6" plug sample. Area "A" is an area of possible fatigue interaction while location"B" is an area of crack redirection. 34
WEP 3{[l}R$ 50XF
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< ; 27b. Low magnification photo of
} ll area in 27a.
f, ,> 01 J, 7 > e .1;p 200X 'E-jp:q: 3 M Aqh% ' j '9:;&fe
~
cAwG.c .gg:.;- M 8 1 A, p.a [5j;bh'Y 4 27c. Higher magnification photo
. M ;a D- of same area showing def-inite regular spacing of I k4 4 [*f. .h; striations.
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g _jj , {,, song a-gy$n,r;e e 'z nr r .. y , - $i
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e a !%:s? - # f. = > p 8 Mh.$gQ }+QMrg.9pf.e 3 b # ' ] 3 gw s., % n ,&i ayf r m i ,, .R , dMib 91NEsh 1 E 27d. High magnification photo Figure 27a. Fractograph of the area"A" in of the same area. Fig.26 showing possible fatigue striations. Note the intimation of progressive fatigue characteristics. 35 _ _ _ _ _ _ _ _ _ _ i
l .. y ,: 2 Figure 28. Low magnification SEf1
'f' ,g photo of another fracture
; a4 surface from the 6" lug.
yi The photo clearly d picts [Hf D*%j ,.i+p c. the type of surface char-
. .L . s-dy_k acteristic of progressive h ;h i M5d y fatigue.
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d1 K^ kQi//!'-3 - :/ ( r .,. l5[ '[p AREA OF
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CRACK g k<l % M - INITIATION u gy; &
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INSIDE SURFACE 36
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10X Figure 29. An SEM fractograph showing def- Figure 30. EDS scan of the fracture surface inite indications of " beach of specimen MT 37/38, depicting O marks" on the fracture surface. the typical elements found;on the surface of the fracture.
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100. Figure 31. EDS scan of the fracture surface Figure 32. EDS scan for chemical constituents of specimen MT 44 for typical on specimen MT 46 fracture face. elements present. j l
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10X Figure 33. Low magnification fractograph of the fracture face on specimen MT 44, after electrolytic cleaning. The initiation site of the fracture originated on the inside surface of the steam generator, where a considerable amount of pitting is visible.
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# 10x Figure 34. SEM fractograph depicting the fracture face of specimen MT 46. The photgraph clearly shows the crack initiated at pits on the inside surface of the steam generator and has the general characteristics of a progressive or fatigue-like fracture.
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10X Figure 35. Fractograph of the fracture face on specimen MT 37/38. The fractograph clearly has the " wood-like" structure characteristic of progressive or fatigue cracking. The - photo also shows the initiation site to be on the steam generator's inside surface and apparently emanating from surface pitting. _ r
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1 I
" . OE R MsseWM DDC/
U.S. NUCLEAR f.ECULATORY CcMMISSION
" " ' NUREG/CR 3281 DIBLIOGRAPHIC DATA SHEET BNL/NUREG 51670
- 2. T'IT LE AND r.UaT ITLE (Add Volume No.. # appespressel 2.ILee-e o'*aAA INVESTIGATION OF THE SHELL CRACKING ON THE STEAM CENERATORS
- 3. REciPIENM ACCESSION NO.
OF INDIAN POINT #3
- 5. DATE REPORT COMPLETED h.AUTHnRMt Mrw Y H l vEa C:rl J. Czajkowski April 1983
- 9. PE nf onulNG ORGANIZATION A E AND MAILING ADDRESS (include les Codel. DATE REPORT ISSUFO
" "'" *r Brookhaven National a oratory June l"1983 Dzpartment of Nuclear Energy s.(tene os,aes Upton, New York 11973 8.14eore blaaki
- 12. SPONSORING ORGAN ZATION NAME AND MAILING ADDRESS flaciude Esa Codel ,
Division of Engineering Office of Nuclear Reactor Regulation si. CONTRACT NO. I U.S. Nuclear Regulatory Commission FIN A-3400 WIchington, D.C. 20555
- 13. TYPE OF REPORT PE RioQ Cove REo flactus.'ve deresJ T2chnical
- 15. SUPPLEMENTARY NOTES 14. (Leave clea4/
- 16. ABSTA ACT C00 words or sessl A metallurgical investigation was performed on specimens. from the shell of steam generacors
#31 and 32 of the Indian Point 3 Power Plant. The examination consisted of optical micro-scopy,SEM/EDS, hardness measurements, and two different heat treatments. The shell material sxhibited high values in hardness prior to the heat treatments, which was indicative that ralatively high residual stresses may have been present in the areas of the welds. All observed cracks were transgranular in appearance and were associated with pits on the vasel's inside surfaces. The report concludes that the cracking was caused by a low cycle.
corrosion fatigue phenomenon with cracks initiating at areas of localized corrosion and propagating by fatigue.
- 17. KCY viORDS AND DCcVMENT AN ALYSIS ITa. DCScatti Tons corrosion fatigue shell cracking transgranular cracking pits
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