ML20148G830
| ML20148G830 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 12/14/1987 |
| From: | Dupont E, Hellner R PUBLIC SERVICE CO. OF COLORADO |
| To: | |
| Shared Package | |
| ML20148G775 | List: |
| References | |
| 138, TAC-65992, NUDOCS 8801270045 | |
| Download: ML20148G830 (71) | |
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METALLURGICAL' ANALYSIS OF COMPONENTS FROM HELIUM-CIRCULATOR C-2101 Lab Report No. 138-Prepared By: . R. L. Hellner Materials Engineering Coordinator Fort St. Vrain Nuclear Generating Station December 14 1987 Prepared By /M E/ /._Oupont / ~ QK Engineer Fort St. Vrain Nuclear Generating Station December 14, 1987 m
Page 2 'of 20 ABSTRACT On July 22, 1987, Helium Circulator S/N C-2101 tripped on PPS overspeed. Shaft wobble outside allowable limits and an interspace leak in excess of technical specification limits resulted in the circulator being declared inoperable. On July 31, 1987, the decision to remove circulator S/N C-2101 was made. During removal failed circulator hardware pieces were found in the steam outlet plenum. During inspection at GA Technologies in San Diego, California, it was revealed that the labyrinth spacer, labyrinth seal and insulation cover dropped and came in contact with the steam turbine. This resulted in significant damage to the steam ducting and steam turbine rotor. As a result of these findings several components were metallurgically examined. A detailed study was performed on twenty-four different components including fasteners, expansion joint and turbine wheel. This included visual examination, scanning electron microscopy, energy-dispersive spectrometry, metallographic examination, x-ray diffraction and chemical analysis. Failure or damage has occurred in several different materials: 410 stainless steel, A-286 stainless steel, carbon steel, 4140 low alloy steel, 5% chromium steel, Monel 400 and Inconel 600. Stress corrosion cracking (SCC) was identified in several components including 410 SS bolts, A-286 SS bolts, carbon steel spring plunger, and Monel 400 lockwire. In addition, caustic SCC was found in the Inconel 600 bellows and high concentrations of sodium were identified on crack surfaces. Sodium was also found in the corrosion products from the C2101-460-10, 11 and NG5250-A-59 cap screws. Caustic will cause corrosion and SCC in 410 SS, A-286 SS, carbon steel, 5% chromium steel, 4140 low alloy steel, Inconel 600 and Monel 400 materials. A common environment to all the failed and damaged components is steam utilized for the turbine drive suggesting this was the source of caustic contamination. From review of water chemistry records, during the period of 12/83 - 4/85 significant levels of sodium hydroxide and sodium phosphate were carried over into the circulators from auxiliary boiler steam and is considered to be the major cause of the corrosion and SCC observed. Other causes of corrosion and SCC were considered such as hydrogen sulfide which could result from the degradation of molydisulfide fastener lubricant. However, since SCC was found in nonlubricated components, such as the expansion bellows, Monel lockwire and fractured surface of pressure tap bolt, molydisulfide degradation is not considered to be the main cause of the SCC observed in C-2101.
Page 3 of 20
1.0 INTRODUCTION
The purpose of thjs report is to provide results of_ metallurgical analysis performed on components from helium circulator S/N C-2101.= On July 22, 1987, the circulator tripped on PPS overspeed. Shaft wobble outside allowable limits and an interspace leak in excess of technical specification limits resulted in the circulator being declared inoperable. On July 31, 1987, the decision to remove circulator S/N C-2101 was made. Several failed circulator hardware pieces were found in the steam outlet plenum during removal. The machine was then shipped to GA Technologies in San Diego, California for disassembly and inspection. Otsassembly and inspection revealed the labyrinth spacer, labyrinth seal and insulation cover had dropped and come in contact with the steam turbine rotor. This resulted in significant rubbing damage to these components, to the steam ducting and the steam turbine rotor as previously reported (Ref. 1). As a result of these findings several components which are listed below, were metallurgically examined. The components are listed by relative locations shown in Figures 1, 2 and 3. PART NAME PART NO MATERIAL REPORT LOCATION Labyrinth Seal C2101-300-46 410 SS Figure 1 Mounting Bolts Spring Plunger C2101-300-53 Carbon Steel Figure 1 Steam Ducting C2101-300-52 A-286 SS Figure 1 Pressure Tap Bolt Steam Ducting C2101-300-45 A-286 SS Figure 1 Bolting Steam Ducting C2101-300-23 Inconel 718 Figure 1 Stator C2101-460-11 410 SS Figure 1 Alignment Sleeve Cap Screws Stator Water C2101-460-10 410 SS Figure 1 Deflector Retaining Screws Lockwire for Monel 400* Figure 1 460-10 Cap Screws Stator C2101-300-44 410 SS Figure 1 Installation Cap Screws re
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Page 4 of 20 i Labyrinth C2101-300-27 430 SS- Figure 1 Spacer
-Labyrinth C2101-300-28 -430 SS Figure 1 Seal Insulation C2101-300-11 430 SS Figure 1 Cover Steam Turbine C2101-300-24 422 SS Figure 1 Rotor Pelton Wheel C2101-300-38 Inconel 718 Figure 1 Tie Bolt Pelton Wheel C2101-300-14 Inconel 600 Figure 1 Lockwasher Pelton Wheel C2101-300-13 Inconel 718 Figure 1 Nut Nozzle Support NG5250-A-59 4140 Figure 2 Socket Cap Screw Nozzle Bolt NG5250-A-57 501(5%Cr) Figure 2 Assembly Lockwire for 304 SS* Figure 2 NG5250-A-57 Bolts Special Stud NG5250-A-42 501 (5% Cr) Figure 2 Bellows Support NG5250-A-68 4140 Figure 2 Ring Cap Screws Lockwire for Inconel 600 Figure 2 NG5250-A-68 Cap Screws Expansion Joint NG5250-A-27 Inconel 600 Figure 2, 3 Deflector Plate NG5250-A-50 Carbon Steel Figure 2, 3 *Specified to be Inconel 600 nr
Page 5 of ?.0 2.0 ANALYSIS LABYRINTH SEAL MOUNTING BOLTS Item 46 C2101-300 Originally there were twelve 1/4" -200NC-2A 5/8" long hex head bolts attaching the labyrinth seal to the steam ducting weldment. Inspection after disassembly found that hole locations 12, 13 and 1E (Figure 4) were empty with no apparent damage to the threads. The remaining holes had remnants of fractured bolts. The majority of the fractures occurred approximately one thread below the surface with the remainder flush with the surface. The bolt material specification is ASTM A193 Grade B6 which is a 410 martensitic stainless steel. A tempering operation at 1150' F 25* F for one ' hour was performed to increase resistance to stress corrosion cracking (SCC). Six of the fractured bolts were removed for examination. The bolts were removed by drilling, tapping and utilization of an extraction tool. A typical fractured bolt removed is shown in Figure 5. Two additional fractured bolt sections of unknown location were also examined. Chemical analysis indicated the bolts meet the ASTM specification requirements. The fractured surfaces had a brittle appearance indicating a progressive type of failure typical of stress corrosion cracking (SCC) (Figure 6). Corrosion products were observed on the fractured surface and analyzed utilizing scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). This analysis indicated that the corrosian product consisted of iron oxides and no evidence of contaminants such as C1, Na, etc. were found. The six bolts and two bolt sections all exhibited cracking in the root of the threads (Figure 7). The cracking was generally in the first thread below the fractured face with cracking in more than one thread observed in several of the bolts. Crack propagation was transgranular and perpendicular to the applied load which is typical of a SCC failure mechanism. The largest crack observed was .0424" in length and is shown in Figure 8. Some cracks were observed initiating at pits as shown in Figure 9. The microstre n.re was spheroidized which is not typical of a quenched and tempertd 410 SS. This structure may have occurred as a result of an extremely long tempering operation. The material specification ASTM A193 does not specify microstructure requirements. The hardness of the bolts, Rc22-26, is not normally a condition susceptible to SCC (Ref. 2). However,' SCC in 410 SS has been reported in 10*4 caustic soda solutions at 600 F-630 F at hardness levels of Rc22-28 (Ref. 3).
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Page 6 of 20 SB ING PLUNGER Item 53 C2101-300 There is one spring plunger located in hole No.11 (Figure 4). The spring plunger was utilized for locking the insulation cover into place once it was mated and rotated into the cam locks of the labyrinth seal. Only a portion of the plunger body was remaining and all internals were missing. The plunger was manufactured by Vlier Engineering (Part No. 554B-104-
- 02) and is a carbon steel material. The plunger body was found to be heavily corroded and pitted as shown in Figures 10, 11 and 12.
SEM/EDS analysis of the pit shown in Figure 11 indicated the t corrosion product consisted of iron oxide and no evidence of contaminants were found. Transgranular SCC was observed initiating at the roots of threads as shown in Figure 13. STEAM DUCTING PRESSURE TAP BOLT Item 52 C2101-300 The 3/4" dia X 3.52" long pressure tap bolt which was located in hole No. 9 (Figure 4) had fractured in the shank above the first thread (Figure 14). The fractured surface shown in Figure 15 had a brittle appearance and was covered with corrosion products. The Item 52 bolt is an AMS 5737 Type A-286 precipitation hardening austenitic stainless steel manufactured by Standard Pressed Steel Co. (SPS). The minimum ultimate tensile strength specified by SPS was 200,000 PSI. In order to achieve this strength level cold forming was utilized for fabrication whict. would generate high residual stress levels. Chemical analysis indicated that the Item 52 bolt met the chemical requirements of AMS 5737. SEM analysis revealed extensive secondary cracking on the fractured surface and heavy corrosion products as shown in Figure 16. SEM/EDS analysis indicated the corrosion product consisted of iron oxides with no evidence of contaminants found. Numerous secondary cracks initiating at the fractured surface and propagating along the longitudinal axis were observed. The cracks were transgranular and exhibited branching typical of SCC (Figures 17 and 18). Cracking initiating from the fractured surface indicates an environmental assisted cracking mechanism typical of a SCC failure. Stress corrosion cracking propagating parallel to the applied load indicates that the cracking occurred after fracture or failure of the bolt. The stresses leading to crack initiation and propagation in this direction would be residual stresses from cold forming. Similar cracks were also observed initiating at the roots of the first three threads. The cracks were transgranular and propagating perpendicular to the applied load (Figures 19 and 20). These observations indicate that the pressure tap bolt failure initiating mechanism was SCC. The hardness of the bolt was found to be Rc40.
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Page 7 of 20 STEAM DUCTING BOLTING Item 45 C2101-300 There are fourteen 3/4" dia X 3.52" long Item 45 bolts located in hole Nos. 1-8 & 10-15, (Figure 4). The heads of the bolts had been worn down by as much as 1/4" (Figure 21). This damage was a result of metal components being rubbed between the steam turbine wheel and the steam ducting. Part of a bolt head was jammed between the head of bolt no. 1 and the hole counterbore (Figure 21). The Item 45 bolts are a precipitation hardening austenitic (A-286) stainless steel manufactured by Standard Pressed Steel Co. (SPS) 'n accordance with' AMS 5737. The minimum ultimate tensile strength specified by SPS was 200,000 PSI and were cold formed to obtain high strength levels . Bolt No. 10 had fractured in the shank .6)" above the first thread (Figure 22). SEM analysis of the fractured surface was not useful since the surface had been heavily rubbed. Secondary cracking on the fracture surface similar to that found on the Item 52 bolt was observed. As found in the Item 52 bolt, cracks initiating from the roots of the first three threads below the shank were observed. The cracks were transgranular and propagating perpendicular to the applied load typical of a SCC mechanism. The largest crack was 0.113" in length. Bolt No. 8 was found to be cracked in the shank approximately 1" below the bottom of the head as shown in Figure 23. The crack was opened for examination and 1/2 of the fracture is shown in Figure 24. Evident are two distinct areas on the fractured surface. The crescent-shaped region (located on the bottom of Figure 24) is a precracked area and the remaining surface is a result of overload. The crescent-shaped region was covered with corrosion product and was examined utlilizing SEM/EDS. The EDS analysis indicated the corrosion product consisted of iron oxides with no evidence of contaminants found. The area was then cleaned for SEM fractographic examination. Several distinct arrest marks were observed as shown in Figure 25 . The extreme edge of the fracture origin in Figure 25 was damaged and this could not be examined. The fracture was transgranular with some evidence of post-fracture corrosion or oxidation damage and extensive secondary cracking was noted in some regions as shown in Figure 26. No fatigue striations could be resolved in the origin area. Fractograph, Figure 27, shows the tip of the crescent-shaped region near the side of the bolt. Some faint arrest marks were visually evident in the transition from the crescent-shaped region to a flat overload area. Poorly defined fatigue striations could be resolved in this region as shown in Figure 28. rs
pago 8 of 20 These fatigue striations are typical of situations where fatigue crack growth rates accelerate just prior to final overload fracture. Metallographic examination revealed cracks initiating from the roots of the first two threads below the shank. The cracks were transgranular and propagating perpendicular to the applied load typical of SCC. This cracking is identical to that observed in the roots of threads of bolt No. 10 and the pressure tap bolt. Therefore, the precracked area on the fractured surface was most likely .due to SCC and the overload area occurred as a result of the loads imposed on the bolts by the debris-rubbing described earlier. The fatigue striations observed were a result of fatigue loading from mechanical rubbing just prior to overload. In addition the twelve other Item 45 bolts were examined utilizing liquid dye penetrant and metallographic examination. Cracking at the roots of the first three threads similar to that found in the pressure tap bolt and bolt No.10 was observed in five out of twelve bolts examined. STEAM DUCTING Item 23 C2101-300 The steam ducting, location shown in Figure 1, is an Inconel 718 forging per AMS 5662. During inspection two cracks on the bottom side (turbine side) 180' apart around bolt hole No. 9 were observed. The steam ducting was then inspected from both the bottom (turoine side) and top (bearing side) by liquid dye penetrant examination. The crack in hole No. 9 at the radially inward side extended 0.400" from the edge of the 13/16" dia, hole on the top side and through the 1-15/16" dia. counterbore towards the center of the steam ducting as shown in Figure 30. At 180' on the top side the opposite crack extended from the edge of the hole 0.350" to the wall and up the wall 1.40". This crack then extended through the bolt hole and counterbore and radially outward 7/16" from the counterbore on the bottom side as shown in Figure 29. A crack in bolt hole No. 10 was found extending radially from the top side outward approximately 0.25" from the 13/16" dia. hole as shown in Figure 30. A small i sample 0.18" wide X 0.60" long was removed from hole No. 9 for examination as shown in Figures 31 and 32. , Visual examination of the fracture surface of the crack sample from the steam scroll bolt hole revealed the presence of a significant corrosion deposit. Also evident are faint indications of fatigue beachmarks (shown in Figure 33). No distinct crack initiation site was observed. cc
,. s Page 9 of 20 1 X-ray diffractions analysis was performed on the fracture surface to identify the type of oxide present. The analysis revealed predominantly magnetite (Fe:0.) with some hematite (Fe:0 ) also 3 present. Magnetite is a high temperature oxide, and its presence could be attributed to either a pre-exposed crack surface during heat treatment, or more likely from carryover and deposition from. the steam. SEM analysis of the fracture surface was performed and initially, the oxide layer was too thick to observe any features. The deposit was removed by ultrasonic cleaning in inhibited hydrochloric acid. Prior to removal, SEM/EOS analysis of the deposit was performed and no contaminants were identified. Figure 34 shows a low magnification view of the fracture surface from area 1 of Figure 33. Evident is the predominantly transgranular fracture surface, with some secondary cracking and areas of corrosion damage observed. Figure 35 is a higher magnification view of area 2, where evident are faint fatigue striations. Random areas exhibiting these striations were observed, indicating at least partial fatigue crack propagation. Another area of extensive secondary cracking observed near that of Figure 34 is shown in Figure 36. Metallographic examination was performed, and the extensive secondary cracking was observed, as shown in Figure 37. The crack propagation mode was transgranular, and is better illustrated in Figure 38, a high magnification view of area 1. The microstructure consists of coarse Ni:Cb precipitates (dark) in a gamma-nickel matrix. While there was extensive secondary cracking observed, the cracks were all predominantly transgranular. Microhardness testing of the sample was performed (the sample was not large enough for macrohardness), and the hardness ranged from Rc43-51 (HB 392-496). This wide range of hardness values is most likely the result of microstructural effects inherent in this precipitation hardening material. These values meet the requirements of AMS 5662 of HB 331 (Rc36) minimum. , The primary cause of the cracking could not be determined since only a portion of one crack was examined. Since SCC in both caustic and acidic environments is known to be intergranular in Inconel 718 (Ref. 4, 5), SCC is not considered to be a likely cause of this cracking. It is theorized that the cracking occurred from mechanical loading when 1/2 of the failed bolts from bolt hole No. 9 and 10 were pulled out as a result of debris rubbing. This is supported by the fact that cracking was only observed at these two locations. m
Page 10 of 20 I i STATOR ALIGNMENT SLEEVE cap SCREWS Item 11 C2101-460 There are four 1/4"-28 UNF X 1" long Item 11 socket head cap screws. The location of the cap screws are shown in Figure 39. The cap screws are a 410 martensitic stainless steel and were tempered at 1150*F 25*F for one hour. One of the cap screws fractured during removal at the root of a thread approximately 9 threads below the head. The fractured end is shown in Figure 40; also evident is cracking at the root of adjacent threads. Heavy corrosion products were observed as shown in Figure 40 and were analyzed utilizing SEM/EDS. This analysis indicated the corrosion product consisted of iron oxides with no evidence of contaminants found. Metallographic examination revealed multiple SCC initiating at roots of threads as shown in Figure 41. Cracking at the roots of threads was also observed in two other Item 11 cap screws examined as shown in Figure
- 42. Heavy corrosion products were observed and analyzed by semi-quantitative spectrographic analysis. The results are listed in Table 1 and notable is ti.e presence of 0.3% by weight sodium in the corrosion product. Also, x-ray diffraction analysis indicated the presence of Fe 0 3 (50%) which would be expected, and two other compounds MOS: (1-10%) and SiO: (1-10%). Meta 11ographic examination revealed intergranular cracking along prior austenitic grain boundaries typical of SCC (Figure 43). The hardness in the vicinity of the intergranular crack was Rc29 and the hardness in the center was Rc27.5. Cracks were also observed initiating from the face of threads as shown in Figure 44. These cracks may have initiated at original manufacturing defects. Some slight branchi.ng typical of SCC was also observed as shown in Figure 45.
STATOR WATER DEFLECTOR RETAINING SCREWS Item 10 C2101-460 There are sixteen 1/4"-28-UNF X 1" long Item 10 socket head cap screws. The location of the Item 10 cap screws are shown in Figure
- 46. The cap screws are a 410 martensitic stainless steel and were tempered at 1150 F 25' F for 1 hour. The Item 10 cap screws were covered with corrosion products which were removed for semi-quantitative spectrographic analysis. Results of the analysis are listed in Table 1 and notable is the presence of .3% sodium by weight in the corrosion product. Metallographic examination revealed minor general corrosion and some pitting corrosion. Cracks initiating from the faces of threads were also observed. These cracks were limited to one side of the thread face on each thread and were identical to cracks observed on the face of Item 11 cap screws (Figures 44 and 45). These cracks likely initiated at original manufacturing defects. The hardness of the cap screws was determined to be Rc26-30.
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Page 11 of 20 The lockwire for the Item 10 cap screws was also examined. The specified material for the lockwire was Inconel 600 but chemical analysis determined it to be Monel 400. Several areas were found to have surface corrosion and cracking as shown in Figure 47. SEM/EDS analysis .of the corrosion product revealed no evidence of contaminants. Cracking was predominate in crevice areas where two
-wire strands overlapped as shown in Figure 48. The cracking was intergranular as shown in Figures 49 and 50. Intergranular crackino in a Monel 400 material indicates a SCC mechanism.
STATOR INSTALLATION CAP SCREWS Item 44 C2101-300 There are four 1/4" - 20 UNC-2A x 1 1/2" long Item 44 socket nead cap screws. The cap screws are a 410 martensitic stainless steel and were tempered at 1150*F 25'F for one hour. The cap screws were covered with corrosion product and were analyzed utilizing SEM/EOS. The EDS analysis revealed no evidence of contarainants. The surface deposits were removed and the cleaned surfaces examined utilizing a stereo microscope and SEM. Cracking at the apex of the last thread were observed on one cap screw. Metallographic examination revealed several stress corrosion cracks propagating from the apex of the thread as shown in Figure 51. It is unusual for cracking to occur at this location in a fastener since this is not the highest stressed area. It is possible that original manufacturing defects contributed to crack initiation at this location. Previous examination of other new bolts has shown cracking at the apex of the threads. The hardness was found to be Rc22-24. LABYRINTH SPACER Item 27 C2101-300 Examination of two samples from the labyrinth spacer was performed. The spacer met the chemical requirements of 430 ferritic stainless steel, with a hardness of Rb81. While all of the samples examined exhibited significant general ductility, fractographic examination revealed a brittle fracture surface, with fracture initiating solely at the machined nctches. Oxidation was present, and no secondary cracking was observed. The fractographic observations are considered typical of the extremely notch sensitive 430 ferritic stainless steel (overall general ductility with brittle fracture occurring at notches). The microstructure of the spacer was typical of a uniform forging structure, with a uniform grain size distribution and a fine distribution of matrix and grait, boundary carbides. Failure of the spacer was a result of the irpact and subsequent mechanical rubbing when the spacer, insulation cover, and labyrinth seal dropped and came into contact with the steam wheel.
Page 12 of 20 INSULATION COVER Item 11 C2101-300 Examination of seven samples from the insulation cover wm performed. Significant post-serv'ce rubbing damage was observed. The cover met the chemical requirerrents of 430 ferritic stainless steel, with a hardness of Rb83.5. SEM fractor.raphic examination revealed brittle, transgranular cleavage fracture, again initiating at the machined notches. Significant oxidation of tt.e surface was observed. Metallographic analysis revealed extensive transgrant.;ar secondary cracking with branching observed. SEM/EOS analysis revealed no evidence of contamination within the oxides of the fracture surface and secondary cracks. While some of the cracks were typical of stress corrosion cracking (SCC), SCC has very rarely been observed in ferritic stainless steel. In the literature when any reference to SCC has been made it has always been intergranular. The microstructure observed consisted of coarse grain boundary chromium carbides (intensifying the inherent brittleness of the material), partially lamellar carbides, and equiaxed recrystallized ferrite. grains. The possibility of frictional heating producing che structure is considered unlikely, because the initial solutionizing temperature would have to be above 1600 F for at least 30 minutes. Transformation would then occur as a function of exposure time and f temperature, such as at *.100 F (15 hours) to 1300 F (2 hours). If this had occurred, microstructural transformation of other circulator components (e.g. spheroidization of the carbon steel plunger) would have been observed. Failure of the cover was also a result of the impact and subsequent mechanical rubbing with the steam wheel. LABYRINTH SEAL Item 28 C2101-300 Examination of three fragments from the 430 SS labyrinth seal was performed. Significant rubbing damage was observed. The seal met the chemical requirements of 430 SS with a hardness of Rb88.5. Fractographic examination also revealed brittle, transgranular cleavage fracture, again initiating at the machined notches. Oxidation of the fracture surface was observed, and SEM/EDS analysis revealed no evidence of contaminants. Some secondary cracking was observed, however no branching was observed. The microstructure observed was similar to that of the insulation cover. Failure of the seal was also a result of the impact and subsequent mechanical rubbing with the steam wheel. l l l w
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Page 13 of 20 b STEAM TURBINE ROTOR Item 24 C2101-300 Visual examination of the 422 martensitic stainless steel steam turbine rotor revealed significant damage throughout the disc, with sections from the insulation ~ cover embedded n it. This was attributed to rubbing between the insulation cover and the disk (Ref. 1). Minor mechanical . damage to the turbine rator blades (Greek Ascoloy alloy) was also observed. Metallographic examination revealed minor pitting (.003") of the disk, along the dovetail notch. No evidence of SCC of the rotor was observed. The hardness was determined to be Rc35 which is consistent with ASTM A565, type 422 martensitic stainless steel in the HT heat treatment condition. PELTON WHEEL TIE BOLT ltem 38 C2101-300 The pelton wheel bolt is 20" long with varying diameters between 1.125" - 1.687". The material of the bolt is a Inconel 718 per AMS 5662. The bolt was visually inspected and no evidence of pitting or cracking was found. A fluorescent dye liquid penetrant examination was performed and no indications were found. PELTON WHEEL LOCKWASHER Item 14 C2101-300 i Visual examination of the .lockwasher (Inconel 600 material) revealed very minor mechanical damage (i.e., slight gouging). No cracking or corrosion damage was observed. Metallographic examination revealed no indications of corrosion damaga or SCC. PELTON WHEEL NUT Item 15 C2101-300 Visual examination of the nut (Inconel 718) revealed no indications of damage. Metallographic examination also revealed no damage from corrosion or SCC. The hardness was determined ;o be Rb91.5. [ re
Page 14 of 20 N0ZZLE SUPPORT SOCKET CAP SCREW Item 59 NG5250-A There are twenty-four Item 59 10-32 X 3/4" long socket head cap screws. The cap screws material specification is ASTM A193 Grade 87(4140). Three of these cap screws were analyzed. The threads were covered with corrosion product as shown in Figure 52. The _ corrosion product was removed and semi quantitative spectrographic analysis was performed. The results of the analysis are listed -in Table 1 and notable is the presence of 0.2% sodium. The cleaned surfaces were examined for cracking utilizing a SEM. Cracking was observed at the head to shank junction and at the roots of several threads as shown in Figures 53 and 54. X-ray diffraction analysis indicated the corrosion product consisted of Fe30,, (20-50%), Fe 0 (20-50%) and 3 SiO2 (1-10%). Metallographic examination revealed that the crack like indications observed from SEM examination could have resulted from corrosion attack of an original manufacturing defect. This is illustrated in Figure 55 which show the heavy corrosive attack initiating at the root of a thread. Other possible mechanisms are SCC or corrosion fatigue cracking. The hardness was determined to be Rc28.5. N0ZZLE BOLT ASSEMBLY Item 57 NG5250-A There are six Item 57 3/8" x 7/8" long hex head bolts. The Item 57 material specification is ASTM A193 Grade B5 which is equivalent to an AISI 501 (5% Cr) material. Tnree out of six bolts were analyzed. Heavy corrosion products were observed and analyzed utilizing SEM/EDS and no evidence of contaminants were found. Metallographic examination revealed pitting and general corrosion had occurred. Stress corrosion cracks were found initiating at the head to shank junction in two of three bolts examined. The lockwire for the Ite, 57 bolts were also examined. The lockwire specified to be Inconel 600 but was found to be a 304 SS material by chemical analysis. No corrosion or stress corrosion cracking was observed. SPECIAL STUD Item 42 NG5250-A There are sixteen Item 42 5/8" x 17'3 1/2" long special studs. The Item 42 studs material specification is ASTM A193 Grade 85 (5% Cr). A 1 3/4" long section, at the pelton nozzle end, was cut off one stud for examination. SEM examination revealed pitting along the thread face and pitting in the root of threads as shown in Figure 56. Metallographic examination revealed shallow pitting but no evidence of cracking e , shown in Figure 57. f _
Page.15 of 20 BELLOWS SUPPORT RING CAP SCREW
- Item 68 NG5250-A There are 40 Item 68 1/2" - 13 UNC-2A x 2 1/2" long socket head cap screws. The cap screws are a ASTM A193 Grade 87 material. Four of the ' Item 68 cap screws were examined. Heavy surface corrosion products wera observed and analyzed utilizing SEM/EOS and no evidence of contaminants were found. Transgranular SCC was observed in one of the four cap screws examined as shown in Figures 58 & 59. The crack initiated at the root of a thread and had propagated through the entire cross section over 3/4" of the circumference. General and pitting corrosion was observed in the other three cap screws. The hardness was-found to be Rc27.5. The lockwire (Inconel 600) was examined and no corrosion or cracking was observed.
EXPANSION JOINT (BELLOWS) Item (7 NG5250-A The bellows assembly is located in the inlet piping as shown in Figures 2 and 3. The failed bellows was located in the six-inch pipe separating the steam turbine inlet steam passage from the pelton wheel water drain passage. Steam did not directly impinge the bellows since a deflector plate was located above the bellows. The chamber around the bellows, however, is filled with steam during operation. The bellows is a six-inch, six-convolution three-ply component fabricated from Inconel 600 per ASME SB-168 by Pathway Bellows, Inc., El Cajon, CA. Pathway Bellows Inc. reported tha't the bellows was cold formed from cold-rolled-annealed sheet with no subsequent heat treatment. Several cracks in the convolution area of the bellows were observed (Figures 60 & 61). Cracking of all three plies in several areas, and complete separation of one area was observed. Scanning electron microscope examination of several crack surfaces revealed that the cracking was intergranular as shown in Figures 62 and 63. Secondary intergranular cracks were also observed as shown in Figure 63. The cracking initiated from the outside surface or s:eam side of the bellows. Many of the cracks initiated and propagated along the forming marks. Forming marks associated with the cracking are areas of stress intensification. Metallographic examination was performed on several cross sections removed from all three plies. Numerous cracks were observed adjacent and in the vicinity of the main fracture. Photomicrograph, Figure 64 , shows a typical crack adjacent to a main crack, also note the intergranular and delta branching nature of the cracking propagation. Other intergranular cracks in the vicinity of the main fracture, which have not propagated completely through the section thickness are shown in Figure 65. l L e rg
Page 16 of 20 Auger Microprobe Analysis was performed on a crack surface to determine the compost?. ion of surface deposits. The analysis was performed at the tip of a crack from ply no.1 (outerply) (Figure 62) that was opened just prior to performing the analysis to minimize any contamination of the surfaces. The analysis showed high concentrations of sodium even after sputter etching to remove 600 angstroms (A.U.), 4000 A.U. and 6000 '..U. of surface deposit. For comparison an analysis was also performed on the fresh fracture area and no sodium was found. The results of the Auger Microprobe analysis is listed in Table 2. The high concentration of sodium at the crack tip and intergranular cracking in an Inconel 600 material indicates a cau'stic SCC failure mechanism. These are the same conclusions drawn from a 1983 failure analysis (Ref. 6) of an identical Item 27 bellows in piping S/N 2000 which had been installed with Circulator C-2105. The failed bellows from C-2105 examined in 1983 is shown in Figure 66, cracking is less severe than that found in the bellows from C2101 (Figures 60 and 61). DEFLECTOR PLATE Item 50 NG5250-A The location of the Item 50 deflector plate is shown in Figures 2 and
- 3. A photograph of the 16" tee inlet nozzle, 6" pelton wheel water drain line and deflector plate is shown in Figure 67. The attachment weld of the deflector plate to the 6" drain line was found to be cracked over much of its length as shown in Figure 68. Visual inspection revealed that the cracking was located in the weld metal
- close to the fusion line in the plate. A section of the deflector plate was removed for metallographic examination. A cross section of the deflector plate is shown in Figure 69. The weld penetration was required to be 1/4" and was found only to be approximately 1/8". The crack had occurred in the weld metal near the fusion line as shown in Figure 69. The hardness of weld metal was DPH 239, OPH 273 for the heat affected zone and OPH 168 for the base metal. Secondary cracking was also observed near the main crack and is shown in Figure
- 70. The cracks were filled with oxide and exhibited slight branching. A cross section through this area revealed what appeared to be shallow (0.00075") corrosion fatigue cracks (Figure 71). The cracking in the delfector plate was most likely due to simple everload because of reduced cross sectional area of the attachment weld.
Page 17 of 20 3.0 OISCUSSKON AND CONCLUSIONS The results of metallurgical analysis performed on components from Circulator C-2101 are summarized in Table 3. Failure or !amage has occurred in several different materials including 410 stainless steel, A-286 stainless steel, carbon steel, 4140 low alloy steel, 5% chromium steel, Monel 400 and Inconel 600. Stress corrosion cracking was identified in several components including 410 SS bolts, A-286 SS bolts, carbon steel spring plunger, and Monel 400 lockwire. In addition caustic SCC was identified in the Inconel 600 bellows where high concentrations of sodium were identified on crack surfaces. Caustic will cause corrosion and SCC in most metallic materials including those which showed evidence of SCC or corrosion from C-2101. A common environment to all the failed and damaged components is steam utilized for the turbine drive suggesting this was the source of caustic contamination. The need for proper water chemistry control has long been recognized to prevent corrosion in steam systems. When the concentration of caustic in steam exceeeds its solubility, droplets of concentrated caustic solution will precipitate and deposit on metal surfaces. Caustic contamination can result frora condenser tube leaks, misoperation of the condensate demineralizers, or carry over from.the auxiliary boilers. Depending upon operational mode, the steam operating conditions for the circulators are 150-800 PSIA at 560-700 F. The solubility of caustic in steam for various conditions can be estimated from Figure 72 (Ref. 15). From this diagram the solubility of caustic in steam for . operating conditions of C-2101 could be as low as 1-10 PPB and the concentration of the precipitate can be as high as 80%. Limited review of Water Chemistry records indicated auxiliary boiler feedwater sodium levels during steam operation has been as high as 40 PPB for C-2101 and 200 PPB for other circulators (Ref.16). The potentially greatest damaging time period occurred from 12/83 - 4/85 when significant levels of caustic were injected into auxiliary boiler steam (Ref. 16). As a result of boiler tube corrosion problems in December, 1983, the chemical feedwater treatment for the auxiliary boiler was changed to a coordinated phosphate program utilizing sodium hydroxide and trisodium phosphate. During maintenance activities in April 1985 isolation valve V-5280 between the 150 lb. steam header and the auxilary boilers would not close. Upon disassembly 6" of deposits were found in the 9" supply line for V-5280. Laboratory analysis found the deposits consisted of sodium phosphate, sodium sul f ate, sodium carbonate and iron phosphate. Sampling of steam from the 150 lb header found sodium concentrations l of 9-820 PPB and phosphate as high as 100 PPB. Review of steam drum l level charts indicate that during boiler startups water levels in the l drum were erratic resulting in sodium hydroxide and trisodium phosphate being carried into the supply steam for the circulators, i 1 l l l 1
- - e
Page 18 o'f 20 1 The possibility of molydisulfide lubricant degradation was also considered as a cause of SCC in the fasteners although limited information is available on exact conditions that will cause degradation.. It has been reported that moisture at high temperatures in contact with molydisulfide may cause formation of hydrogen sulfide (Ref. 17). Hydrogen sulfide can lead to corrosion and SCC of high strength fasteners. Since SCC was found in nonlubricated components such as the expansion bellows, Monel lockwire and fractured surface of the pressure tap bolt, it is unlikely that molydisulfide breakdown was the primary cause of corrosion and SCC described in this report. Stress corrosion cracking and corrosion have been observed in a wide range of materials from C-2101, many of which are not normally susceptible to SCC in a steam environment without caustic in high-concentrations. Therefore, it can be concluded that the cause of the damage and SCC observed in components from C-2101 was sodium hydroxide contamination from the steam supply for the turbine drive. Other observations supporting this conclusion are: Caustic SCC was identified as the failure mechanism of the NG5250-A-27 Inconel 600 bellows.
- High levels of sodium were detected on the crack surfaces of the Inconel 600 bellows.
- Sodium was detected in the corrosion products from C2101-460-10, C2101-460-11 and NG5250-A-59 cap screws.
- SCC was observed in nonlubricated components including the NG5250-A-27 bellows, Monel lockwire and fractured surfaces of the C2101-300-52 pressure tap bolt.
- Significant sodium hydroxide contamination was present in the steam from the auxiliary boilers during the period of 12/83 - 4/85.
~
i Page 19 of 20 4.0 RdFERENCES
- 1. Preliminary Report of Helium Circulator S/N C-2101 Damage
~ Including Licensing Assessment, Sept. 11, 1987, Public Service Co. of Colorado.
- 2. NACE Standard MR-01-75 (1980 Revision), Sulfide Stress Cracking Resistant Metallic Material for Oil Field Equipment, in H2 S Corrosion in Oil and Gas Production - A Compilation of Papers, R. N. Tuttle and R. D. Kane(Eds.),
National Assoc. of Corrosion Engineers, Houston, Texas, 1981.
- 3. I. L. Wilson, R. G. Aspden, "The Influence of Specimen Type and Heat Treatment on the Caustic Stress Corrosion Cracking of Some Stainless- Alloys". Corrosion Vol 35, No.5, Max 1976.
- 4. GA Topical Report, GA-A13175, "Investigation Into the Cause and Consequence of the Fort St. Vrain Pelton Wheel Incipient Fractures". October 16, 1974.
- 5. I. L. Wilson and T. R. Mager. "Stress Corrosion of Age Hardenable NiFeCr Alloys", Corrosion 1985, pp. 352-361.
- 6. R. L. Hellner, "Failure Analysis of Expansio1 Bellows From Helium Circulator 1B Inlet Piping", Laboratory Report No.
43, November 23, 1983. 7, A. W. Loginow, "Failure Analysis of Expansion Bellows in High Pressure Steam", Corrosion, Vol. 35, October (1979).
- 8. L. O. Kramer, S. T. Michael, F. W. Pement, "Service Experience and Stress Corrosion of Inconel 600 Bellows Expansion Joints in Turbine Steam Environments,"
Corrosion /75, Paper No. 105,(1975).
- 9. Funk, G. W. , Baxton, "Caustic Stress Corrosion Cracking",
Paper 54, Corrosion /77, San Francisco, California, March 14-18, 1977.
- 10. Wilson, I. L. W. , and R. G. Aspen, "Caustic SCC of Fe-Ni-Cr Alloys," Conference June 12-16, 1973. Unieux-Firminy, France, NACE-5, pp. 1189-1199.
- 11. D. Singbeil, D. Tromans, "Caustic Stress Corrosion Cracking of Mild Steels", Metallurgical Transactions, Vol.
13A, June 1982.
- 12. J. A. Boards, "Stress Corrosion In The Power Industry",
Journal of the Institute of Metal, Vol. 101, 1973. rg
Page 20 of 20 l 1 l i l
- 13. P. Borge, et al., "Caustic SCC of Fe-Ni-Cr Austenitic Alloys Corrosion", September, 1976.
- 14. El-Sayad, H. A. et al., "SCC Behavior of B6-410 Stainless Steel in Boiling 70% Na0H Solution", Surface Technology 14, pp. 245-256 (1981),
- 15. R. J. Lindinger, R. M. Curran, "Corrosion Experience In large Steam Turbines", Power Engineering, Oct., 1981.
- 16. L. Zoller, "Water Chemistry Treatment", PSC Memos NFG 0547, NFG-87-0548, December 4, 1987,
- 17. C. J. Czajkowski, "Testing of Nuclear Grade Lubricants and Their Effects On A 'b40 824 AND A 193 87 Bolting Materials", Paper 112 Corrosion /85, March 25-29, 1985.
. cv
SEMI-QUANTITATIVE ANALYSIS OF_ CORROSION PRODUCTS _ TABLE 1 WEICHT 1 NG5250-59/ 'f Licment 1f1 4 0 -10 84 60-11 10-32 Calcium 7. 1. 1. Ch romi um .1 8. 1 Iron 25. 50. 66. Lead --- --- .3 Magnesium .2 .1- .2 Hanganese .06 .3 .4 Molybdenum .6 1 .04
- Nickel .8 4. .1 Silicon 18. 4. .6 Sodium .3 .3 .2 Sulfur .4 1.
Titanium .2 .4 .1 Zinc .06 --- .06 AUGLR MICROP! LOBE ANALYSIS Of OPENEO CRACK fROM NG5250-A-27 DELLOWS TABLE 2 poncentration Expressed as Atomic Percent - Ca C CI Cr le Ni N O K Na S Sample Designation 0.2 14.4 4.2 714 6 .09 3.3 --- -- . O.3
#1 I resh f racture ---
2.1 3.8 5.0 2.3 1.3 14.5 2.6 11.5 0.6
#1 as rec'd (crack tip) 0.5 $7.9 ---
58.5 2.1 --- 4.6 1.1 1.3 15.0 is . 3 12.0 1.1
#1 600'A remove <d 55.0 1.1 ---
4.7 1.0 0.9 16.4 4.9 14.6 1.4.
#1 4000' A removed ---
4.2 2.6 1.2 17.4 5.2 13.5 1.6
#1 6000'A removed 0.4 53.2 0.7 ---
Page-1 or 2
SUMMARY
OF METAILURCICAL EXAMINATION
- IABLE 3 l l 1 l YS KSI l UIS KSI i 10lAL l NO. H 1 l
OBSLRVA[ IONS 1 MATERIAL 1 MINIMUM _1 MINIMUM l NUMH[R1 EXAMIN[D Pitting and SCC inthreads in i i PARI NO. [ MAME l l Labyrinth i ! l l l 110 12 8 lait 8 bolts examined l I C2101-300-46 ISeal Mounting l 410SS l 85 l l 1 Bolts i I I I I I I l 1 l l 1 l l l Spring l Carbon l l lPicting and SCC in threads ! l C2101-300-53 i Plunger l Steel l- --- l --- 1 1 1 1 I l I 1 l l 1 l I l l l 1 l Steam Oucting l l l l l ISCC in shank and threads ! l Pressure Tap l A-286SS I 183 200 l 1 1 l C2101-300-52 I 1 I I I i 1 l l Bolt 1 l l Steam Ducting l l l l l l SCC in shanks and threads of l i Bolting i A-286SS I .83 l 200 1 14 l 14 18 of 14 bolts i l C2101-300-45 l 1 I I I I i 1 I l [ l ICracking bolt hole 9 & 10 l l l l Inconel l 1- liikely from mecahnical rubbingl I C2101-300-23 ISteam Ductis- l 718 l 125 l 140 1 1 1 i l I ! l l l l damage. _l l~ IPitting and SCC in threads or l l l Steam lurbine l l 110
'l 4
l 3 13 cap screws l l C2101-460-11 l Stator Cap l 410S$ l 85 l l 1 l l l l 'l l l l Sc rews l l l SCC along race or threads l l l Steam Iurbine l l l 110 16 l 16 linitiating at manufacturing i l C2101-460-10 l Stator Cap i 410SS l 85 l l l I I Iderects in 10 cap sc rews l i l_ Screws i I I i i 1 -l l I I Itockwire for i ISCC l l --- 1460-10 Bolts IMONEL 400 1 --- l --- l 1 1 1 l l 8 I I i 1 l l l l l l l SCC at the apex of a thread l l l Socket Head l 110 4 4 lini t ia ting a t a manufacturing l l C2101-300-44 i Cap Screws 1 410SS I 85 l 1 1 l l l l Idefect_in one cap _sgrew l l l l l '. l l l Damage due to mechanical l l l Labyrinth l i Spacer I 430SS I 45 75 l 1 l 1 trubbing . 1 C2101-300-27 I 1 I I I I l l i I I l Labyrinth ! l l l l 10amage due to mechanical l l Irubbing l l C2101-300-28 l Seal l 430SS I 45 1 75 l 1 1 1 I I l ! l l I 'l 1 I insulation l l l l l l Damage due to mechanical l 1 430SS l 45 75 i 1 l 1 Irubbing i l C2101-300-11 l Cover 1 1 l i i l I I i ! l l l Slight pitting or disk along l l l Steam Iurbine l l 111 l 140 l l Idovetail notch l C2101-442-2 l Rotor Disk l 422SS l 1 l 1 1 l I I I I I I I I I lNo appa rent damage - l l l Pelton l INCONEL I l l l l l C2'41-300-38 l Wheel Tie Bolt! 718 1 150 1 180 l 1 1 1 l I I I l I i i l 1
Page 2 of 2.
SUMMARY
or METAttORGICAL EXAMINATioq IABLE 3 l j j i YS KSI l U1S KSI i 10iAL l NO. l l PART No. I NAME I MATIRIAL_j MINIMUM i MINIMUM NUMBERI EXAMINEDj OBSIRVATIONS l l lPelton Wheel l INCONEL l l l lNo appa rent damage l l 'l l C2101-300-14 l tock Wa she r 1 600 1 35 1 80 1 1 1 1 1 I I I I I I I I i I I I I I I I I I l Pelton l INCONEL l l 1 l lNo appa rent damage l l l C2101-300-15 l Wheel Nut i 718 1 150 1 185 l 1 1 1 1 I 3 I I I I I i _I I l l l l l l l Crack like indications l I 1(possible SCC) at threads l J INozzle Supportl ! I I l l NG5250-A-59 l Socket Cap l 4140 1 105 l 125 1 24 1 3 1 & head or shank . I t Sc rew I l l l l ljunction or 2 out_nr 3 screws l .. l SCC at head to shank junction l j j Nozzle Bolt l 5% 1 1 I l l NG5250-A-57 l Assembly l Chrome 1 80 1 100 1 6 1 3 lin 2 or 3 bolts. Gene ra l and l l l l l l l l pitting Corr. in all 3 bolts. I l l tockwire for I i i i l INO appa rent damage 1 I I NC5250-A-57 304SS I --- 1 l 1 1 1 1 I l 1 I I polis I _ l l __ l l l 1 I I Special I 5% i i i i 1 I Pitting and general co r ro s i on.1 1 NG5250-A-42 i Stud l Chrome 1 80 1 100 l 16 l (1 3/4" l I J l i I I I I Sectionll i I i 15e l lows i I I I I I I NG5250-A-68 l Support Ring i i l l l IPitting in all rour and SCC I l I through enti re cross sect ion i I l Cap Screw 1 4140 1 105 1 125 1 40 1 4 I I I I I I lin one cap scryw I I l tockwi re f or i INCONEL I l l l lNo appa rent damage l I l NG5250-A-68 600 --- --- l 1 1 1 l l l --- 1 1 1 l 1 l__Can_ Screws __l l l l 1 I l l Expansion l INCONEL l l l l ISCC l i 600 l 132 144 1 l l l NC5250-A-27 l Joint 1 1 1 1 i
~
l l __f ile l lowa l i I I I I I l l Der lec to r l Carbon l l l l l Cracking due to inadequate 1 1 NG5250-A-50 1 Plate l Steel l 38 1 70 1 1 1 1 tweld penetration aiso 1 2 I I I I ishallow corrosion ratigue I I I I i 1 I i l l I le rackino I
///h 23 STEAM DUCTING
' '" SPRING PLUNGER ROTATED IN PLANE
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I i STEAM INLET DEFLECTOR PLATE DETAll FIGURE 3
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- =
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-27 (6 IN. NOM.)
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. i Figure 4. Photograph of bottom of steam ducting showing location of labyrinth seal mounting bolts (outer numbers),
spring plunger (No. 11 sp), pressure tap bolting (NO. 9 pt) and steam ducting tolting.
-- rmymm
- lh 1
1 Figure 5. Macrograph of typical fractured labyrinth seal mounting bolt, Item 46. Magnification: 7.5X. n.
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i Figure 14. Macrograph of pressure tap bolt (Item 52) located in hole No.9 (Figure 4) which had fractured in the shank above the first thread. Magnification: 3.8X. l .. - - _. -. - - . . . . . . - . . - . - . . - . . .- - -. . l ! i l- l 1 i t i h i l t Figure 15. Macrograph of fractured surface of pressure tap bolt (Item 52) shown in Figure 14. Note brittle fracture appearance and heavy corrosion products. Magnification: 4.75X. n.
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j 'I// Figure 21. Photograph of steam ducting bolting showing damage resulting from components being rubbed between the steam turbine wheel and the steam ducting. Note part of a bolt head jammed in Hole No. 1 upper left hand corner. I u;. g ~- ; 14!?s$4{)h0d)IilD. .
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Figure 22. Macrograph of steam ducting bolt (Item 45) located in hole No.10 (Figure 4) which had fractured in the shank .60" above the first thread.
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i i. ! Figure 23. Macrogrcph of steam ducting bolt located in hole No. 8 which had a crack approximately 1" below the head. Magnification: 1.15X. , I ;f g: d' .
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& l Figure 30. Photograph of cracks observed in Item 52 Bolt Hole No. 9 and Item 45 Bolt Hole No. 10. View is from top or bearing side of steam ducting.
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n ) - Pigure 39. Photograpn snowing location of stator alignment sleeve cap screws, 460-11.
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Figure 41. Photomicrograph of crack at root of thread shown in Figure 40. Magnification: 266X, Vile 11a's etch. , + .. c.w ... .-
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Figure 46. Photograph showing location of stator water deflector retaining screws, 460-10.
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gyj . gM suu e5 e "u 's "uu" "u u" ' agure 47. SEM macrograph of 460-10 cap screw lockwire showing ge-eral corrosion and SCC. Vagnification. 100X.
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4g re :s. s ? a . reg ra en o < 4e0-10 ca e sc re, ,c .w're snc.i g SCC COse <e: <r : e.,ce areas ere two w, e stra-as 0,er'a: pea. Va'r "*' Cat Cr i 351 R. . . _
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k it / Figure 49. Photomiccograon of cross section of 460-10 lockwire s t' o n n in Figure 47. Note cracking propgating from tne outsice l Sur# ace. Magnification: 266X, unetChed, i 3 s u, I
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Cigure 50. P1;ure 49 sno n; <ntergranular ratere o crac=4rg ;r:Dagat'or f Vag"i'icatiar: 266X, Nitr ic, acetic ( glac'a') etcn. rt_.
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Figure 53. SEM nacrograph of NG5250-A Item 59 cap screw showing -{ cracking at the root of threads and head to shank junction. g- . Magnification: 10X. g_ __y.-_. - _e
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_ Figure 55. Photomicrograph of crack observed in root of thread g of a NG5250-A Item 59 Nozzle Support Cap Screws, Magnification: 100X, unetched. m. E. m b =
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c_ % }. \ l000 15 0050 00 Figure 56. SEM macrograph of NG5250-A Item 42 special stud showing pitting at root threads and along thread faces, j Magnification: 50X. 1 Figura 57. Photomicrograph of pitting shown in Figure 56. Magnification: 133X, unetched.
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C'gere 62. SEM Cractograph of NG5250-A Item 27 expansior cel'ons cracked surface from oly no. 1 showing intergrarua'ar a-d secordary cracas. Auger Microprobe analysis was also
;erfor-ec at this area. Vagnification: 500x.
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sm gwre 65. botom'crograph of a cross section of ply no. 1 from
? Item 27 be'iows srow ing intergra"ular cracting from C.D.
( stea side) . Magr'fication: 266X, IC, oxalic electolytic etch. [
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Cigare 69 P"OtOSiCr0 graph Of Cr0SS Sect 4 0n of ceflector plate Sr0ning C r a C s. i r g through ne'd metal (ind1Cated by arrow). Magrif1 Cat'00; 34, 3o nital etch.
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