ML14253A318

Enclosure 3: Babcock & Wilcox Report, S-1473-002, Revision 0, Examination of Clevis Bolts Removed from D. C. Cook Nuclear Plant. Part 1 of 3

ML14253A318
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Site:	Cook
Issue date:	12/31/2013
From:	Hyres J W B & W Technical Services
To:	Office of Nuclear Reactor Regulation
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ML14253A310	List: AEP-NRC-2014-59, Enclosure 3: Babcock & Wilcox Report, S-1473-002, Revision 0, Examination of Clevis Bolts Removed from D. C. Cook Nuclear Plant. Part 3 of 3 ML14253A316 ML14253A317 ML14253A318 ML14253A319
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AEP-NRC-2014-59 S-1473-002
Download: ML14253A318 (61)

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ENCLOSURE 3 TO AEP-NRC-2014-59 Babcock & Wilcox Report, S-1473-002, Revision 0, "Examination of Clevis Bolts Removed fromD. C. Cook Nuclear Plant" technical services groupFINAL REPORT:EXAMINATION OF CLEVIS BOLTS REMOVEDFROM D. C. COOK NUCLEAR PLANTPrepared by:Babcock & Wilcox Technical Services Group2016 Mount Athos RoadLynchburg, Virginia 24504-5447 (434) 522-6000PREPARED BY:A-VrsPE A&Picipal EngineerNuclar Materials Engineering Nuclear Material

& Inspection ServicesS-1473-002 DECEMBER 2013s-I 473-002 DECEMBER 2013 SUMMARYThis report covers laboratory examinations performed by Babcock & Wilcox Technical ServicesGroup (B&W TSG) on failed clevis bolts removed from the Lower Radial Support System(LRSS) at D. C. Cook Unit 1. Sixteen (16) broken bolts and thirteen (13) intact bolts (basedon the post-removal visual inspections) were provided for laboratory analysis to evaluatethe degradation, identify the failure mechanism(s),

characterize the bolt material, and toevaluate the integrity of the intact bolts. The laboratory work scope included visual andstereovisual examinations of all bolts. Based on the results of these examinations, fourbolts (two broken, two intact) were selected for more detailed analysis/testing, including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), opticalmetallography, microhardness, chemical analysis by inductively coupled plasma-mass spectroscopy (ICP-MS),

Rockwell hardness

testing, and tensile testing.All of the submitted bolts, including those considered to be intact, contained cracking in thehead-to-shank transition.

No cracking was identified in the threaded region of any of thebolts. There was a generally uniform open crack fracture pattern consisting of crackinitiation at two diametrically opposing sides of the bolt in the head-to-shank transition andcrack growth that extended upward into the bolt head at a -350 angle relative to the boltseating surface.

The head separated from the shank when the two opposing cracks linkedup near the center of the bolt cross section.For each bolt, crack growth occurred along an axis of symmetry created by the opposingdirections of crack growth. These crack growth axes indicated the direction of prevailing stresses in each bolt. No correlation between the orientations of the crack growth axes andthe in-service orientations of the bolts within each clevis was observed.

Minor differences in crack morphology around the circumference of the head-to-shank transition suggested that the magnitude of the prevailing stresses varied from bolt to bolt.Fractographic SEM analysis and cross section metallographic examinations determined thefracture mode was essentially 100% intergranular with crack branching for all of the bolts.The chemical analysis results for all four bolts were consistent with Alloy X-750 material.

The mechanical properties and microstructure of the bolts were consistent with thosepublished for Alloy X-750. No unexpected characteristics in the material properties, microstructures, or form of the bolts were identified.

The laboratory data indicated the bolts failed by intergranular stress corrosion cracking(IGSCC).

The reported heat treatment for the bolts included a low solution annealing temperature and two-step aging treatment.

Alloy X-750 material heat treated in thismanner is known to have poor SCC cracking resistance in both high and low temperature water. There was no evidence that the bolts failed due to fatigue cracking or mechanical overload.

ii TABLE OF CONTENTSSECTION PAGELIST O F TABLES ................................................................................................................

ivLIST O F FIG URES ...............................................................................................................

vLIST O F ACRO NYM S ....................................................................................................

x1.0 INTRO DUCTIO N ........................................................................................................

12.0 BACKG RO UND .........................................................................................................

13.0 RECEIPT VISUAL EXAM INATIO NS .......................................................................

34.0 BO LT SELECTIO N ................................................................................................

65.0 VISUAL/STEREOVISUAL INSPECTIO NS .............................................................

76.0 SECTIO NING .............................................................................................................

87.0 SEM/EDS EXAM INATIO NS .................................................................................

108.0 M ETALLO G RAPHIC EXAM INATIO NS ................................................................

129.0 VICKERS M ICRO HARDNESS

..............................................................................

1510.0 INDUCTIVELY COUPLED PLASMA-MASS SPECTROSCOPY (ICP-MS)

...........

1711.0 ROCKW ELL HARDNESS TESTING .....................................................................

1812.0 TENSILE TESTING ..............................................................................................

1813.0 ANALYSIS OF HEAD-TO-SHANK TRANSITION

...............................................

2014.0 DISCUSSIO N ...........................................................................................................

2115.0 CO NCLUSIO NS .......................................................................................................

24

16.0 REFERENCES

.........................................................................................................

25iii LIST OF TABLESTABLE PAGE1 S um m ary of subm itted bolts .......................................................................................

12 Typical mechanical properties for Alloy X-750 .....................................................

23 Visual examination summary of the bolts ...............................................................

34 Summary of bolts selected for destructive examinations

.......................................

65 Bolt 300°-1 head thickness measurements

..........................................................

96 Summary of Vickers microhardness (HV) results for bolt 120'-2 cross section ....... 157 Summary of Vickers microhardness (HV) results for bolt 120'-6 cross section ....... 158 Summary of Vickers microhardness (HV) results for bolt 2400-7 cross section ....... 169 Summary of Vickers microhardness (HV) results for bolt 300°-1 cross section ....... 1610 Sum m ary of IC P-M S data ...................................................................................

1711 Rockwell C hardness measurements

.................................................................

1812 Sum m ary of tensile test data ...............................................................................

19iv LIST OF FIGURESFIGURE PAGE1 Schematic diagram showing the six clevis locations around the vesselcircum fe re nce .................................................................................................

..262 Schematic showing the typical bolt configuration for each clevis insert ............

263 Bolt 120°-2, annotated with laboratory rotational orientations

..........................

274 Bolt 120'-6, annotated with laboratory rotational orientations

..........................

275 Bolt 240'-7, annotated with laboratory rotational orientations

..........................

286 Bolt 3000-1, annotated with laboratory rotational orientations

..........................

287 Receipt macro photograph for bolt 0°-1 ............................................................

298 Receipt macro photograph for bolt 0°-3 .............................................................

299 Receipt macro photographs for bolt 0Q-5 ...........................................................

3010 Receipt macro photograph for bolt 0°-7 .............................................................

3111 Receipt macro photograph for bolt 60°-1 ..........................................................

3112 Receipt macro photograph for bolt 60°-3 ..........................................................

3213 Receipt macro photograph for bolt 600-5 ..........................................................

3214 Receipt macro photograph for bolt 60'-7 ..........................................................

3315 Receipt macro photographs for bolt 1200-1 ......................................................

3416 Receipt macro photographs for bolt 120'-2 ......................................................

3517 Receipt macro photographs for bolt 120'-3 .......................................................

3618 Receipt macro photographs for bolt 120°-4 .......................................................

3719 Receipt macro photographs for bolt 1200-5 ......................................................

3820 Receipt macro photographs for bolt 120'-6 ......................................................

3921 Receipt macro photograph for bolt 1200-7 .......................................................

4022 Receipt macro photographs for bolt 120'-8 ......................................................

4123 Receipt macro photographs for bolt 180°-1 .....................................................

4224 Receipt macro photograph for bolt 180°-3 .........................................................

4325 Receipt macro photographs for bolt 180°-7 ......................................................

4426 Receipt macro photographs for bolt 1800-8 ......................................................

4527 Receipt macro photograph for bolt 2400-1 .........................................................

4628 Receipt macro photograph for bolt 2400-3 ........................................................

4629 Receipt macro photograph for bolt 2400-5 ........................................................

47V LIST OF FIGURES (CONTINUED)

FIGURE PAGE30 Receipt macro photograph for bolt 240°-7 .........................................................

4731 Receipt macro photograph for bolt 300°-1 .........................................................

4832 Receipt macro photographs for bolt 3000-3 .....................................................

4933 Receipt macro photographs for bolt 300'-5 .......................................................

5034 Receipt macro photographs for bolt 3000-6 ......................................................

5135 Receipt macro photographs for bolt 3000-7 .....................................................

5236 Receipt macro photographs for bolts from the 1200 clevis ..............................

5337 Receipt macro photographs for bolts 7 and 8 from the 180' clevis ...................

5538 Receipt macro photographs for bolts 5, 6, and 7 from the 300° clevis ..............

5639 Macro and stereo photographs of bolt 1200-2 taken at 450 increments

............

5740 Macro and stereo photographs of bolt 120°-6 taken at 450 increments

............

6141 Macro and stereo photographs of bolt 2400-7 taken at 450 increments

............

6542 Macro and stereo photographs of bolt 3000-1 taken at 450 increments

............

6943 Photograph showing wire EDM used to section each bolt for destructive e xa m inatio ns ...................................................................................................

..7344 Section photograph for bolt 2400-7 showing the typical locations chosen forsubsequent analysis

........................................................................................

..7345 Typical location of tensile specimens machined from each bolt ........................

7446 Miniature tensile specimen design showing dimensions

..................................

7447 Bolt 2400-7 after breaking open the crack for SEM/EDS ...................................

7548 Bolt 3000-1 showing plunge cut EDM surface, which removed much of thein-service cracking

..........................................................................................

..7549 Macro photograph showing the open crack surface for bolt 1200-2 .................

7650 O D of 120°-2 near 900, 50X ..............................................................................

7751 C enter of Figure 50, 50O X .................................................................................

7752 M id-diam eter of 120°-2, 50X .............................................................................

7853 C enter of Figure 52, 50O X .................................................................................

7854 Center of 120'-2 fracture, 50X ..........................................................................

7955 C enter of Figure 54, 50O X .................................................................................

7956 Macro photograph showing the open crack surface for bolt 1200-6 ...................

80vi LIST OF FIGURES (CONTINUED)

FIGURE PAGE57 OD of bolt 120°-6 near 450, 5OX ......................................................................

8158 Center of Figure 57, 50OX .................................................................................

8159 Mid-diameter of bolt 1200-6, 5OX ......................................................................

8260 Center of Figure 59, 50OX .................................................................................

8261 Center of 1200-6 fracture, 50X ...........................................................................

8362 Center of Figure 61,500X .................................................................................

8363 Stereo microscope photograph showing the open crack surface forbo lt 24 0 °-7 .........................................................................................................

8464 OD of bolt 240°-7 near 3150, 50X ......................................................................

8565 Center of Figure 64, 50OX .................................................................................

8566 Mid-diameter of 240°-7, 50X .............................................................................

8667 Center of Figure 66, 50OX .................................................................................

8668 Center of 240'-7 fracture, 50X ...........................................................................

8769 Center of Figure 68,500X .................................................................................

8770 BSE image of titanium nitride -15 pm in size, 1,500X ......................................

8871 EDS spectrum collected from precipitate shown in Figure 70 ..........................

8872 BSE image of niobium-titanium intermetallic

-20 pm long, 1,500X ..................

8973 EDS spectrum collected from precipitate shown in Figure 72 ..........................

8974 Typical area of polished cross section, 1,500X .................................................

9075 EDS spectrum collected from entire area shown in Figure 74 ...........................

9076 EDS dot maps collected from area shown in Figure 74 ....................................

9177 Tensile fracture surface for 2400-7L4, 80X ......................................................

9278 Same as Figure 77 with annotated reduction in area measurements, 80X ..... 9279 As-polished overview of bolt 120'-2 cross section.

6X ....................................

9380 Same area as Figure 79 after phosphoric

+ nital etch. Structure is banded. 6X ...9381 As-polished micrograph montage of crack. -10X .............................................

9482 Higher magnification detail of Figure 81, as-polished.

140X ............................

9583 Detail of initiation near 90'. 180X ...................................................................

9684 Same area as Figure 83 above after phosphoric

+ nital etch. 180X .................

9685 Detail of initiation near 270°. 180X .................................................................

97vii LIST OF FIGURES (CONTINUED)

FIGURE PAGE86 Same area as Figure 85 after phosphoric

+ nital etch. 180X ...........................

9787 Typical microstructure after phosphoric etch, DIC, 660X ..................................

9888 Same area as Figure 87 after phosphoric

+ nital etch, DIC, 660X ....................

9889 Bolt 120°-6 cross section as-polished overview.

6X ..........................................

9990 Same area as Figure 89 after phosphoric

+ nital etch. 6X ...............................

9991 As-polished micrograph montage of crack. -1OX ...............................................

10092 Higher magnification detail of Figure 91, as-polished.

140X ...............................

10193 D etail of initiation near 450. 180X .......................................................................

10294 Same area as Figure 93 after phosphoric

+ nital etch. 180X ..............................

10295 D etail of initiation near 2250. 180X .....................................................................

10396 Same area as Figure 95 after phosphoric

+ nital etch. 180X ..............................

10397 Typical microstructure after phosphoric etch, DIC, 660X .....................................

10498 Same area as Figure 97 after phosphoric

+ nital etch, DIC, 660X .......................

10499 As-polished overview of 240°-7 cross section.

6X ..............................................

105100 Same area as Figure 99 after phosphoric

+ nital etch. 6X ..................................

105101 As-polished micrograph montage of crack. 1OX .................................................

106102 Higher magnification detail of Figure 101, as-polished.

120X .............................

107103 Detail of initiation near 3150 after etching.

180X .................................................

108104 Second crack near 3150after etching.

180X .......................................................

108105 Detail of initiation near 1350 after etching.

180X .................................................

109106 Typical appearance of intermittent duplex grain structure, DIC. 90X ..................

109107 Typical microstructure after phosphoric etch, DIC, 660X .....................................

110108 Same area as Figure 107 after phosphoric

+ nital etch, DIC, 660X .....................

110109 As-polished overview of 300°-1 cross section.

6X ..............................................

111110 Same area as Figure 109 after phosphoric

+ nital etch. 6X ................................

111111 Detail of initiation near 900. 180X .......................................................................

112112 Same area as Figure 111 after etching.

180X ....................................................

113113 Detail of initiation near 2700. 180X .....................................................................

114114 Same area as Figure 113 after etching.

180X ....................................................

114115 Typical microstructure after phosphoric etch, DIC, 660X .....................................

115viii LIST OF FIGURES (CONTINUED)

FIGURE PAGE116 Same area as Figure 115 after phosphoric

+ nital etch, DIC, 660X .....................

115117 Photograph showing the microhardness locations for 120'-2 ..............................

116118 Photograph showing the microhardness locations for 120'-6 ..............................

116119 Photograph showing the microhardness locations for 240'-7 ..............................

117120 Photograph showing the microhardness locations for 300°-1 ..............................

117121 Bolt 2400-7 at 135°. Circle radius is 0.065". 20X ...............................................

118122 Bolt 2400-7 at 315'. Circle radius is 0.065". 20X ...............................................

118123 Bolt 3000-1 at 900. Circle radius is 0.069". 20X .................................................

119124 Bolt 3000-1 at 270'. Circle radius is 0.069". 20X ...............................................

119125 Higher magnification montage of bolt 240'-7 at 1350. As-polished,

-70X ....... 120126 Higher magnification montage of bolt 240'-7 at 315'. As-polished,

-65X ....... 121127 Higher magnification montage of bolt 300°-1 at 90'. As-polished,

-80X ............

122128 Higher magnification montage of bolt 3000-1 at 2700. As-polished,

-75X ..........

123ix LIST OF ACRONYMSASME ................

AMERICAN SOCIETY OF MECHANICAL ENGINEERS ASTM ................

AMERICAN SOCIETY FOR TESTING AND MATERIALS B&W TSG .........

BABCOCK & WILCOX TECHNICAL SERVICES GROUPBSE ...................

BACKSCATTERED ELECTRON (SEM IMAGING)CW ....................

CLOCKW ISECCW .................

COUNTERCLOCKWISE DIC ....................

DIFFERENTIAL INTERFERENCE CONTRAST (OPTICAL IMAGING)EDM ..................

ELECTRICAL DISCHARGE MACHINING EDS ..................

ENERGY DISPERSIVE SPECTROSCOPY HRC ..................

ROCKWELL C HARDNESS (MACRO HARDNESS)

HTSCC .............

HIGH TEMPERATURE STRESS CORROSION CRACKINGHV .....................

VICKERS HARDNESS (MICROHARDNESS)

ICP-MS .............

INDUCTIVELY COUPLED PLASMA-MASS SPECTROSCOPY IGSCC ..............

INTERGRANULAR STRESS CORROSION CRACKINGLRSS ................

LOWER RADIAL SUPPORT SYSTEMLTCP .................

LOW TEMPERATURE CRACK PROPAGATION OD ....................

OUTER DIAMETEROES ..................

OPTICAL EMISSION SPECTROSCOPY PWR .................

PRESSURIZED WATER REACTORRA .....................

REDUCTION IN AREASCC ..................

STRESS CORROSION CRACKINGSE .....................

SECONDARY ELECTRON (SEM IMAGING)SEM ..................

SCANNING ELECTRON MICROSCOPE/MICROSCOPY X

B&W Technical Services GroupS-1473-002 I

1.0 INTRODUCTION

This report covers laboratory examinations performed by Babcock & Wilcox Technical ServicesGroup (B&W TSG) on failed clevis bolts removed from the Lower Radial Support System(LRSS) at D. C. Cook Unit 1.Sixteen (16) broken bolts and thirteen (13) intact bolts were shipped to the B&W Lynchburg Technology Center for laboratory analysis to evaluate the degradation, identify the failuremechanism(s),

characterize the bolt material, and to evaluate the integrity of the intactbolts. A summary of the submitted bolt samples (locations per Figure 1 and Figure 2) isprovided in the table below:Table 1: Summary of submitted bolts.Broken Bolt Intact BoltLocation Locations Locations 00 #5 #1, #3, #7600 #1,#3,#5,#7 1200 #1 through #8 ---1800 #1, #7, #8 #32400 --- #1, #3, #5, #73000 #3, #5, #6, #7 #1STotals 16 13The laboratory work scope included visual and stereovisual examinations, scanningelectron microscopy (SEM), energy dispersive spectroscopy (EDS), optical metallography, Vickers microhardness, chemical analysis by inductively coupled plasma-mass spectroscopy (ICP-MS),

Rockwell hardness testing and tensile testing.

The goal of theseexaminations was to determine the most likely cause of the bolt failures.

2.0 BACKGROUND

Seven clevis bolts and one dowel pin in the LRSS had visual indications during the March2010 refueling outage at D. C. Cook Unit 1. AEP replaced a minimum bolt pattern thatencompassed all bolts with visual indications during the March/April 2013 refueling outageand it was determined that a total of 16 bolts had failed. In each case, the failure locationwas below the head in the head-to-shank transition.

The lower radial support system consists of six (6) support clevises spaced evenly aroundthe reactor vessel circumference as shown schematically in Figure 1. Each clevis iscomprised of a wear plate (insert) attached to the lugs by eight (8) bolts and two (2) pressfit dowel pins (Figure 2). In all, there are 48 clevis bolts and 12 dowel pins in the lowerradial support system.

B&W Technical Services GroupS-1473-002 2The clevis bolts are approximately 3" long by 0.75" in diameter and are manufactured fromAlloy X-750. The clevis bolt head is approximately 1.5" in diameter with a slotted internalhex socket to accommodate a locking bar. The locking bar is welded in place afterinstallation to prevent backing out during operation.

The on-site inspections indicated some of the locking bars were worn due to contact with the clevis bolt head. Inspection photos of the four bolts selected for detailed destructive analyses are presented in Figure 3through Figure 6.The clevis bolts were heat treated using a two-step aging treatment that consisted of:* Hot roll* Equalize (solution anneal) for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at 1625°F (885C) and air cool* Solution anneal at 17750F (968C) for one hour and air cool* Age at 1,350°F (732C) for 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />s* Furnace cool to 1,150°F (621C) and age for 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />s* Air coolThis exact heat treatment was not found in any of the Alloy X-750 material specifications.

Elements of this heat treatment are common to condition AH and condition BH, except thatthese heat treatments specify a single aging treatment at 1,300°F (704C) for 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> inlieu of the two-step aging treatment.

Other two-step aging processes, such as ASME CodeCase N-60-5, SB-637, Grade 688, Type 2, employ a higher solution annealing temperature, 1,800°F (982C) along with the same two-step process described above (Ref. 1).Mechanical properties for each of these heat treatments are summarized in the tablebelow:Table 2: Typical mechanical properties for Alloy X-750.Condition UTS, ksi I YS, ksi % Elong. I RA, %AH 173 typ. 119 typ. 26 typ. 44 typ.BH 198 typ. 145 typ. 22 typ. 41 typ.SB-637, Type 2 170 min. 115 min. 18 min. 18 min.It can be seen that the ultimate strength and yield strength values are significantly higherfor condition BH compared to condition AH. The higher strength of condition BH is offsetby lower elongation and reduction in area (RA) values compared to condition AH. Thecondition AH and SB-637, Type 2 strength levels are comparable.

B&W Technical Services GroupS-1473-002 33.0 RECEIPT VISUAL EXAMINATIONS Macro photographs were taken of the intact bolts and bolt fragments to document their as-received condition.

The bolt fragments (i.e. head and shank) were photographed separately; the intact bolts were photographed in the head-to-shank transition region.Detailed visual inspections were then performed on the as-received bolts and fragments under the stereomicroscope at magnifications up to 50X. These inspections were used toassess the overall condition of the bolts/fragments and help select candidate bolts for thedestructive examinations.

The results of these inspections are detailed in the following table. The four bolts selected for detailed examinations are shown in bold.Table 3: Visual examination summary of the bolts.In-Service Shipping Report lBroken!

Rubbed Typical ALocation*

ID Figure Intact Fracture?

Pattern?

Additional Comments001 AYes Most angled cracking atthis clevis location.

-3 AaYes Less angled cracking0°3A-8 8 Intact N/A thnes-than 00-1Fracture surface lostdue to EDM plunge cut.0-7 AYes Least angled crackingat this clevis location.

60-1 B-1 11 Intact N/A Yes Most angled cracking atthis clevis location.

600-3 B-13 12 Intact N/A Yes Less angled crackingcompared to 600-1.600-5 B-4 13 Intact N/A Yes Cracking tighter and not3600 around.600-7 B-14 14 Intact N/A Yes Less angled crackingcompared to 600-1.1200-1 B-15 15 Broken Yes Yes Angled crackingpresent around OD.Less angled cracking1200-2 B-12 16 Broken No Yes compared to 1200-1 or1200-4.Less angled cracking1200-3 B-5 17 Broken Yes Yes compared to 1200-1 or1200-4.1200-4 B-3 18 Broken Yes Yes Angled crackingI_ _ III_ IIpresent around OD.*In-service location consists of the angular orientation of the clevis (00, 600, 1200, etc.) along withthe bolt position (1, 2, 3, etc.). Refer to Figure 1 and Figure 2.

B&W Technical Services GroupS-1473-002 4Table 3 (continued):

Visual examination summary of the bolts.In-Service Shipping Report Broken! Rubbed Typical 1Location*

ID Figure Intact Fracture?

Pattern?

Additional Comments120°-5 B-2 19 Broken Yes Yes Angled crackingpresent around OD.1200-6 B-16 20 Broken No No Very minor angledcracking around OD.Fracture surface lost120 °- 7 B- 6 21 Broken N/A N/A det D lnectdue to EDM plunge cut.120°-8 B-11 22 Broken Yes Yes Angled crackingpresent around OD.Fracture surface lost180 o_1 A- 12 23 Broken N/A N/A det D lnectdue to EDM plunge cut.1800-3 A-3 24 Intact N/A Yes Minor angled crackingaround OD.Less angled cracking180°-7 A-14 25 Broken Yes Yes compared to 180°-8;asymmetric center.Most angled cracking at180°-8 A-13 26 Broken Yes Yes this clevis location; asymmetric center.Less angled cracking240-1 A-16 27 Intact N/A Yes247.240°-3 A-15 28 Intact N/A Yes Less angled crackingcompared to 2400-5.Less angled cracking240°-5 A-4 29 Intact N/A Yes compared toackingcompared to 240°-1.2400-7 A-6 30 Intact NIA Yes Most angled crackingat this clevis location.

300°-1 A-7 31 Intact N/A No Very minor angledcracking around OD.Fracture surface lost3000-3 A-2 32 Broken NIA NIA Frcuesfaelt due to EDM plunge cut.3000-5 B-9 33 Broken Yes Yes Less angled crackingcompared to 3000-7.3000-6 B-10 34 Broken Yes Yes Most angled cracking atthis clevis location.

300'-7 B-8 35 Broken Yes Yes Less angled crackingI I compared to 300-6*In-service location consists of the angular orientation of the clevis (00, 600, 1200, etc.) along withthe bolt position (1, 2, 3, etc.). Refer to Figure 1 and Figure 2.

B&W Technical Services GroupS-1473-002 5It was determined that the fracture surfaces on four of the broken bolts (0°-5, 120'-7, 180°-1, and 300'-3) were destroyed during removal due to EDM plunge cutting.

These four bolts(shown in italics in Table 3) were not examined further.Macro photographs were taken of the twelve (12) open fracture surfaces using side lightingto highlight the fracture surface texture.

The bolt photographs are presented in Figure 36(1200 clevis),

Figure 37 (1800 clevis),

and Figure 38 (300' clevis).

In these figures, thebolts are arranged by their position within the clevis. Also, the orientation of each bolt wasmatched to the in-service orientation, i.e. the 12:00 position in the photograph matches the12:00 in-service position.

Many of the open fractures sustained considerable rubbing damage and were notconsidered good candidates for the higher magnification examinations.

However, it wasevident that all of the open fracture surfaces followed a similar pattern.

Crack growthprogressed inward from the head/shank transition at a -35° angle relative to horizontal toward the center of the bolt from two diametrically opposed sides. The axis of symmetrycreated by these two opposing sides is annotated on each fracture surface in Figure 36through Figure 38. The orientations of the axes of symmetry were random in nature, whichindicated the directions of prevailing stresses were variable within each clevis and betweendifferent clevises.

Final fracture occurred when the two opposing cracks linked togethernear the center of the bolt.All of the thirteen (13) intact bolts contained cracking in the head/shank transition.

Nocracking was identified in the threaded region of any bolt. Most of the intact bolts exhibited a common cracking pattern consisting of a straight, unbranched crack for approximately half of the circumference, while the other half had many angular cracks that may or maynot have linked up. Broken bolts exhibited a similar pattern when observing crack elevation variations around the bolt OD in the head to shank transition.

The amount of angled cracking varied somewhat among the bolts, from many angledcracks (e.g. 240'-7) to very few angled cracks (e.g. 300°-1).

The amount of angledcracking is expected to decrease as the stress increases, but this variation in crackmorphology could also indicate a failure mode change.

B&W Technical Services GroupS-1473-002 64.0 BOLT SELECTION It was decided to select two intact and two broken bolts for the destructive examinations.

Itwas also important to select bolts that exhibited a higher degree of angled cracking andbolts with little or no angled cracking.

Of secondary concern was capturing bolts from thedifferent clevis orientations and from different positions within a particular clevis (i.e.high/low, left/right).

It was determined that the following bolts would be subjected to the destructive examinations:

120'-2, 120'-6, 240'-7, and 300°-1. The goal of the selection process wasto capture as many variables as possible within the limits of the authorized work scope.A summary of the selected bolts is provided in the table below:Table 4: Summary of bolts selected for destructive examinations.

Bolt ID Broken/intact Fits Typical Left/Right High/LowBotID rknItc Pattern?120-2 Broken Yes Left High120-6 Broken No Right High240-7 Intact Yes Right Low300-1 Intact No Left HighThe selected bolts were located in three of the six clevis locations and included two boltsfrom the 120' clevis. The 1200 clevis location experienced the greatest population of in-service bolt failures (all 8 bolts failed).

B&W Technical Services GroupS-1473-002 75.0 VISUAL/STEREOVISUAL INSPECTIONS Detailed visual and stereovisual inspections were performed on the four selected bolts.Photographs were taken at 450 increments to document the extent and nature of thecracking.

The angular orientations were established by assigning the 0' position to the12:00 in-service position and increasing degrees in the clockwise direction when viewingthe bolt head.Bolt 120°-2The macro and stereo photographs for bolt 120'-2 are presented in Figure 39. The fracturesurface was dark brown in color and exhibited some branching around the circumference.

The fracture surface axis of symmetry was in the 900-2700 direction.

Bolt 120°-6The macro and stereo photographs for bolt 1200-6 are presented in Figure 40. The fracturesurface was lighter in color than bolt 1200-2, which suggested less surface deposits werepresent and this fracture likely occurred more recently than the 120'-2 bolt failure.Cracking was generally straight and exhibited very minor branching around thecircumference.

The fracture surface axis of symmetry was in the 45o-2250 direction.

Bolt 240°-7The macro and stereo photographs for bolt 2400-7 are presented in Figure 41. Crackbranching is evident around nearly the entire circumference with the exception of the 3150orientation (i.e. 10:30 in-service).

Bolt 3000-1The macro and stereo photographs for bolt 300°-1 are presented in Figure 42. Crackingwas generally straight and exhibited very minor branching around the circumference.

B&W Technical Services GroupS-1473-002 86.0 SECTIONING Sectioning was required to permit the higher magnification metallographic and SEMexaminations of the bolt fractures, as well as chemical analysis and mechanical testing ofthe bolt material.

The sectioning was accomplished using wire electrical discharge machining (EDM).Hardness,

Tensile, and Chemical Analysis Specimens The hardness and tensile specimens were machined from the threaded portion of eachbolt. To produce these specimens, a rectangular bar measuring 0.5" x 0.5" x 2.2" wasmachined from the threaded region as shown in Figure 43. One surface of each bar wasthen ground for Rockwell C hardness measurements.

The location of these measurements for bolt 2400-7 is shown in Figure 44. The hardness measurement locations were typicalfor all four bolts. Also shown in Figure 44 is the typical location of the disk-shaped chemical analysis

specimen, which was machined from the unthreaded portion of each boltshank.After the hardness testing was complete, the tensile specimens were profiled out of therectangular bar as shown in Figure 45. At least 0.030" of material was machined from thehardness testing surface to ensure the tensile specimens were not influenced by localized cold working introduced during hardness testing.

In all, 20 tensile specimens weremachined from each bolt, 10 "upper" specimens from near the head (identified as Ulthrough U10) and 10 "lower" specimens away from the head (identified as Li through L10).The miniature tensile specimen dimensions are provided in Figure 46. This design wasselected because it is consistent with ASTM E 8 (Ref. 2), and owing to its smaller size,permits testing of several specimens from a relatively small amount of material.

Open Crack SEM and Cross Section Metallographic Specimens The open crack SEM examinations were performed on the head side of each fracturesurface.

For the broken bolts, 120'-2 and 120'-6, the entire fracture surface wasexamined.

For the intact bolt, 2400-7, the bolt head was first split through the 1350-315' orientation to produce specimens for open crack SEM and cross section metallography.

Figure 47 shows the mating halves of the fracture surface of bolt 2400-7 after breakingopen the crack. It was estimated that the crack opened with just a few pounds of force, i.e.there was a very small amount of remaining ligament as evidenced by the lack of shinylaboratory fracture on the open crack surfaces.

It was necessary to reduce the thickness of the three open crack specimens to facilitate theSEM examinations.

Care was taken to ensure the bottom of the hex and the fracturesurface were not damaged during cutting.

The location of this cut for bolt 240'-7 is shownin Figure 44. The broken bolt heads from bolt 1200-2 and bolt 1200-6 were cut in a similarmanner.

B&W Technical Services GroupS-1473-002 9After the SEM examinations were completed on the two broken bolt heads, the fracturesurfaces were sectioned through their axis of symmetry for the cross section metallography examinations.

The 300°-1 bolt was plunge EDM cut during removal from service, which eliminated most ofthe in-service cracking (Figure 48). Measurements of the EDM plunge cut depth indicated only -0.020" of the cracking remained for examination.

In order to capture the maximumextent of the remaining crack, thickness measurements were taken around the headcircumference as shown in the table below:Table 5: Bolt 300°-1 head thickness measurements.

Orientation Thickness, in.00 0.242900 0.2531350 0.2501800 0.2442700 0.2313150 0.236These measurements indicated the EDM plunge cutter was located slightly toward the 2700orientation (i.e. minimum thickness value). The 900-2700 orientation was selected for thecross section metallographic examinations to provide the highest probability of capturing the longest remaining crack length, which was expected to be at the 900 orientation.

Since relatively little cracking was present in bolt 3000-1, it was decided to perform theSEM examinations on a polished cross section prepared from the threaded region ratherthan an open crack specimen.

The cross section analysis would provide an opportunity toexamine the microstructure at higher magnifications and perform EDS chemical analysis tocharacterize the bulk material and precipitates.

B&W Technical Services GroupS-1473-002 107.0 SEM/EDS EXAMINATIONS Fracture surfaces and polished metallographic cross sections of the four bolts wereexamined by SEM equipped with EDS for elemental analysis.

Both secondary electron(SE) and backscattered electron (BSE) imaging modes were utilized to characterize thebolts. SE imaging was used to document the fracture surface morphology, while BSEimaging (identified as AUX1 in Figures 70, 72, and 74) was used to characterize thematerial microstructure on the cross sections.

The fractographic examinations wereperformed on bolts 120'-2, 120'-6, and 240o-7. The microstructural characterizations wereperformed on an as-polished cross section prepared from the threaded region of bolt 3000-1.Bolt 120°-2 Open CrackA low magnification photograph showing the open crack from bolt 120'-2 is shown inFigure 49. Annotated on this figure are three areas selected for higher magnification examinations, which included the OD surface near 90' (Figure 50 and Figure 51), the mid-diameter (Figure 52 and Figure 53), and near the center of the fracture surface (Figure 54and Figure 55). Cracking was essentially 100% intergranular

fracture, with the exception ofa small amount of mixed mode fracture near the center of the bolt fracture surface.

Thiscentral area contained a mixture of intergranular

fracture, transgranular

cleavage, and asmall amount of ductile fracture (Figure 55). Surface deposits were noted on the fracturesurface.

EDS analysis indicated these deposits were base metal oxides. The presence ofthese deposits was consistent with the relatively dark macro appearance of the fracturesurface.Bolt 120*-6 Open CrackA low magnification photograph showing the open crack from bolt 1200-6 is shown inFigure 56. Annotated on this figure are three areas selected for higher magnification examinations, which included the OD surface near 450 (Figure 57 and Figure 58), the mid-diameter (Figure 59 and Figure 60), and near the center of the fracture surface (Figure 61and Figure 62). Cracking was essentially 100% intergranular fracture.

There was noevidence of mixed mode near the center as was the case for the 120'-2 bolt. Less surfaceoxides/deposits were present on this fracture surface.

This is consistent with the macroappearance of the fractures, i.e. bolt 120'-6 appeared "cleaner" than bolt 1200-2.Bolt 240°-7 Open CrackA low magnification photograph showing the open crack from bolt 240'-7 is shown inFigure 63. Annotated on this figure are three areas selected for higher magnification examinations, which included the OD surface near 315' (Figure 64 and Figure 65), the mid-diameter (Figure 66 and Figure 67), and near the center of the fracture surface (Figure 68and Figure 69). The fracture mode was essentially 100% intergranular.

B&W Technical Services GroupS-1473-002 11Bolt 300- 1 Mounted Cross SectionA polished cross section prepared through the threaded portion of bolt 300°-1 wasanalyzed by SEM/EDS.

Typical precipitates identified during these examinations includedtitanium nitride measuring

-15 pm in size (Figure 70 and Figure 71) and niobium-titanium intermetallic measuring

-20 pm long by -6 pm wide (Figure 72 and Figure 73). Carbonwas not detected in any of the examined areas; therefore, specific carbide types (M23C6 vs.MC) could not be differentiated.

Subsequent optical examinations described in Section 5.0indicated the carbides were very fine (<1 pm) and smaller than the EDS analysis volume.The typical microstructure for this material is shown in Figure 74. The standardless quantitation performed on this area is shown in Figure 75. The chemical analysis resultswere generally consistent with Alloy X-750 material.

High resolution EDS dot maps were also collected from this area and are presented inFigure 76. When interpreting these maps, note that the concentration of an elementincreases with increasing color density within that element's window, i.e. darker colorindicates more of that element is present.

The maps indicated the primary elementspresent were nickel, chromium, and iron; trace amounts of titanium and niobium weredetected as well. The maps also slow that the composition of the alloy base metal wasuniform (no significant, widespread contrast in the maps) and that there was a higherconcentration of niobium and titanium in the precipitates, as indicated by the discretedarker regions within the niobium and titanium element windows.Tensile Fracture SurfacesLow magnification (80X) SE imaging was used to document each tensile specimenfracture.

Low magnification micrographs were taken of each surface in order to accurately measure the reduced section area, since standard techniques such as calipers can beproblematic when measuring miniature specimens.

A typical example of a tensile fractureis shown in Figure 77. The fracture surface was mixed mode that consisted ofintergranular facets and ductile microvoid coalescence.

The potential implications of thisfinding are discussed further in Section 13.0.Thickness and width measurements were then made to determine the reduction in area foreach specimen.

Typical measurements are annotated on the micrograph as shown inFigure 78.

B&W Technical Services GroupS-1473-002 128.0 METALLOGRAPHIC EXAMINATIONS Metallographic examinations were performed on the mounted cross section specimens prepared through cracking in the four bolts. The mounting material used was a long curetwo-part epoxy compound.

The mounts were analyzed first in the as-polished condition andafter chemical etching to reveal the material microstructure.

The dual etch procedure was used on the bolt material.

This procedure involves etching thepolished cross section in concentrated phosphoric acid to reveal the carbides, then etchingin 5% nital to reveal the material grain boundaries.

Electrolytic etching (3V for 15 seconds)was used for both steps. The dual etch technique is frequently used to determine thecarbide distribution (i.e. intergranular vs. intragranular) in Alloy 600. Differential interference contrast (DIC) lighting was used to evaluate the carbide distribution.

Bolt 1200-2 Cross SectionLow magnification stereo microscope photographs were taken of the bolt 1200-2 crosssection in the as-polished (Figure 79) and etched (Figure 80) conditions.

The bandednature of the microstructure was evident at low magnification in the etched condition.

Crack propagation traversed the banded regions, an indication that cracking was notdirectly influenced by the presence of a banded microstructure.

A multi-frame montage of the entire crack is presented in Figure 81. A higher magnification montage showing the extent of cracking toward the bottom of the hex (top of montage) ispresented in Figure 82. Cracking exhibited a branched intergranular morphology in allareas examined.

Higher magnification detail micrographs were also taken of each initiation region. The 90"micrographs are shown in Figure 83 and Figure 84. The 2700 micrographs are shown inFigure 85 and Figure 86. Evidence of a second, shallower crack was noted at bothlocations, as was the banded microstructure.

There was no obvious evidence of surfacecold work at either initiation region.Typical DIC micrographs showing the microstructure after phosphoric acid etching and nitaletching are presented in Figure 87 and Figure 88, respectively.

The carbides (fine blackdots) were present in discrete bands, as evidenced by a vertical band on the left and rightof the micrographs.

The center region contained relatively few carbides.

The carbideswere typically intragranular; a slight amount of grain boundary carbide coverage was noted(<25%).

B&W Technical Services GroupS-1473-002 13Bolt 1200-6 Cross SectionLow magnification stereo microscope photographs were taken of the bolt 1200-6 crosssection in the as-polished (Figure 89) and etched (Figure 90) conditions.

The bandednature of the microstructure was evident at low magnification.

Crack propagation traversed the banded regions, an indication that cracking was not directly influenced by the presenceof a banded microstructure.

A multi-frame montage of the entire crack is presented in Figure 91. A higher magnification montage showing the extent of cracking at the crack apex is presented in Figure 92.Cracking exhibited a branched intergranular morphology in all areas examined.

Higher magnification detail micrographs were also taken of each initiation region. The 450micrographs are shown in Figure 93 and Figure 94. The 2250 micrographs are shown inFigure 95 and Figure 96. Very little, if any, crack branching was noted at each initiation, which is consistent with the relative lack of angled cracking noted around the bolt OD.There was no obvious evidence of surface cold work at either initiation region.Typical DIC micrographs showing the microstructure after phosphoric acid etching and nitaletching are presented in Figure 97 and Figure 98, respectively.

A discrete band ofcarbides (fine black dots) is visible toward the left side of the micrograph.

Relatively fewcarbides were noted elsewhere.

The carbides were typically intragranular; a slight amountof grain boundary carbide coverage was noted (<25%).Bolt 240°-7 Cross SectionLow magnification stereo microscope photographs were taken of the bolt 2400-7 crosssection in the as-polished (Figure 99) and etched (Figure 100) conditions.

The bandednature of the microstructure was evident at low magnification.

Crack propagation traversed the banded regions, an indication that cracking was not directly influenced by the presenceof a banded microstructure.

A multi-frame montage of the entire crack is presented in Figure 101. A highermagnification montage showing the nature of the crack branching is presented in Figure102. Two cracks were located at the 3150 orientation, i.e. the cross section cut through twoadjacent, overlapping cracks at this orientation.

Cracking exhibited a branchedintergranular morphology in all areas examined.

B&W Technical Services GroupS-1473-002 14Higher magnification detail micrographs were also taken of each initiation region. The 3150micrographs are shown in Figure 103 and Figure 104. Two separate cracks were presentat this location.

The 135' micrograph is shown in Figure 105. There was no obviousevidence of surface cold work at either initiation region. A low magnification DICmicrograph showing the typical appearance of the duplex microstructure is presented inFigure 106. The microstructure was primarily comprised of fine, equiaxed grains (ASTM-7-8) with some large, abnormal grains (ASTM 2).Typical DIC micrographs showing the microstructure after phosphoric acid etching and nitaletching are presented in Figure 107 and Figure 108, respectively.

A discrete carbide band(fine black dots) is visible toward the right side of the micrograph.

A few carbides werenoted on the left side of the micrograph.

The carbides were typically intragranular; a slightamount of grain boundary carbide coverage was noted (<25%).Bolt 300 -1 Cross SectionLow magnification stereo microscope photographs were taken of the bolt 300°-1 crosssection in the as-polished (Figure 109) and etched (Figure 110) conditions.

The bandednature of the microstructure was evident at low magnification.

Crack propagation traversed the banded regions, an indication that cracking was not directly influenced by the presenceof a banded microstructure.

The EDM plunge cut depth is also indicated in Figure 109.Higher magnification detail micrographs were taken of each initiation region. The 90'micrographs are shown in Figure 111 (as-polished) and Figure 112 (etched).

The 2700micrographs are shown in Figure 113 (as-polished) and Figure 114 (etched).

The available crack length was approximately 0.025" at 900 and 0.0200 at 2700. Cracking was primarily intergranular, except for one instance where cracking was transgranular through a largegrain at the 90' orientation (denoted by arrows in Figure 112). Minor secondary crackingwas noted in this cross section and was mainly located adjacent to the plunge EDM surface(along the top edge of the micrographs).

Crack branching was also minor and extended 1-2grains deep from the main crack.Typical DIC micrographs showing the microstructure after phosphoric acid etching and nitaletching are presented in Figure 115 and Figure 116, respectively.

A discrete carbide band(fine black dots) is visible toward the right side of the micrograph.

Relatively few carbideswere noted elsewhere.

The carbides were typically intragranular; a slight amount of grainboundary carbide coverage was noted (-<25%).

B&W Technical Services GroupS-1473-002 159.0 VICKERS MICROHARDNESS Several Vickers microhardness (HV) readings (500 gram load) were taken on the mountedspecimens to characterize the hardness in various areas of interest, including near the ODin the initiation

regions, mid-thickness, and bulk material away from the cracking.

Theareas selected are shown in Figure 117 through Figure 120. For the intact bolts, hardnessmeasurements were taken both above and below the cracking.

The Vickers microhardness (HV) results are summarized in the tables below:Table 6: Summary of Vickers microhardness (HV) results for bolt 1200-2 cross section.Reading Initiation 900 1 Initiation 2700 Mid-Crack Bulk 900 Bulk 1800#1 433 397 408 408 388#2 338* 374 410 398 407#3 413 377 445 413 412#4 416 399 421 405 413#5 398 416 429 400 407*Reading disregarded due to edge effect (i.e. too close to edge of specimen).

Table 7: Summary of Vickers microhardness (HV) results for bolt 1200-6 cross section.Reading Initiation 3150 Initiation 1350 Mid-Crack J Bulk 3150 Bulk 1350#1 396 397 400 399 405#2 392 394 395 422 409#3 380 358 365 400 396#4 380 394 407 387 390#6 392 393 387 405 395 B&W Technical Services GroupS-1473-002 16Table 8: Summary of Vickers microhardness (HV) results for bolt 2400-7 cross section.[Reading

]Initiation 135 [Initiation 315° [ Mid-Crack Bulk 1350 ] Bulk 3150#1 389 408 407 412 426#2 390 403 390 411 426#3 371 394 396 413 404#4 375 408 402 409 420#5 401 403 392 393 414#6 400 388 .........

1. 7 391 384 ---....#8 392 440 ---......

1. 9 386 390 ...#10 393 389 ...NoteInitiation readings

1. 1-#5 were taken above the crack; readings

1. 6-#10 were taken below thecrack. Five readings were taken at the mid-crack, bulk 1350, and bulk 3150 locations.

Table 9: Summary of Vickers microhardness (HV) results for bolt 300°-1 cross section.Reading Initiation 900 Initiation 270 [ Bulk 900 Bulk 1800#1 394 405 404 410#2 412 402 434 415#3 409 395 420 395#4 402 415 432 406#5 390 402 373 415The results for the initiation regions and bulk material were generally consistent and asexpected for Alloy X-750 material

(-400 HV). The results were also consistent with theRockwell C hardness values, refer to Section 11.0. There was no evidence of significant surface cold work near the initiation regions.

B&W Technical Services GroupS-1473-002 1710.0 CHEMICAL ANALYSISSmall pieces (-100mg each) were removed from each of the four candidate bolts andanalyzed by Inductively Coupled Plasma -Mass Spectroscopy (ICP-MS) for base materialconstituents, including:

nickel, chromium, iron, cobalt, niobium,

titanium, aluminum, manganese,

silicon, copper, sulfur, phosphorus, boron, zirconium, and vanadium.

Notethat this technique cannot determine carbon content.

The results are summarized in thetable below:Table 10: Summary of ICP-MS Data.Element Alloy X-750 120°-2 120°-6 J 240°-7 300--1Nickel 70.0 min 71.8 73.0 72.8 72.4Chromium 14.0- 17.0 15.7 15.6 15.7 15.5Iron 5.0 -9.0 7.5 7.3 7.2 7.3Titanium 2.25 -2.75 2.32 2.10 2.13 2.46Aluminum 0.4- 1.0 0.87 0.82 0.81 0.80Niobium + Tantalum 0.7 -1.2 0.85 0.62 0.82 0.92Manganese 1.0 max 0.20 0.12 0.12 0.12Silicon 0.5 max 0.02 0.02 0.03 0.02Sulfur 0.01 max <0.003 <0.003 <0.003 <0.003Copper 0.5 max 0.03 0.01 0.01 0.01Carbon* 0.08 max -- -- -- --Cobalt 1.0 max 0.31 0.06 0.06 0.06Phosphorus

-- 0.006 0.003 0.006 0.005Boron -- 0.004 0.004 0.004 0.004Zirconium

-- 0.05 0.07 0.08 0.08Vanadium

-- 0.26 0.26 0.24 0.27*Not determined.

The niobium + tantalum and titanium concentrations were marginally low for the 120'-6 boltand the titanium was marginally low for the 240'-7 bolt (bolded values in Table 10).However, the concentrations were considered to be consistent with Alloy X-750, whenaccounting for the uncertainty of the ICP-MS technique and the additional tolerance allowed by the material specification when performing product (also called "check")analysis (Ref. 3). This additional tolerance is provided to account for variations that mayoccur between components made from the same heat lot. For titanium, the producttolerance is 0.07% below the minimum, or 2.18%. For niobium + tantalum, the producttolerance is 0.05% below the minimum, or 0.65%.It was noted that the trace element levels (manganese, cobalt, and copper) were higher inthe 120'-2 bolt compared to the other three bolts. This indicated the 120'-2 bolt likelyoriginated from a different heat lot.

B&W Technical Services GroupS-1473-002 18Carbon concentration cannot be reliably determined using ICP-MS due to low ionefficiency.

Techniques typically used to determine the carbon concentration in bulk metalsinclude combustion and optical emission spectroscopy (OES). OES is also preferred overICP-MS for determining bulk chemical analysis of metals. Unfortunately, no laboratories capable of performing OES on irradiated specimens were identified during the course ofthis examination.

11.0 ROCKWELL HARDNESS MEASUREMENTS Rockwell C hardness measurements (diamond

indenter, 150 kg load) were taken todetermine the bolt material bulk hardness.

These measurements were taken on the tensilespecimen blank removed from each of the four candidate bolts. Figure 44 shows thetypical location of the five (5) hardness measurements taken on each tensile blank. TheEDM recast layer was ground off prior to performing the hardness measurements.

Theresults are summarized below:Table 11: Rockwell C hardness measurements.

Reading 120°-2 120o-6 240--7 300--1#1 39.4 38.8 38.7 39.0#2 39.0 38.9 38.7 39.0#3 39.1 38.9 39.0 38.8#4 38.9 39.1 39.1 39.0#5 39.1 39.2 39.0 39.1Average 39.1 39.0 [ 38.9 j 39.0The results were very consistent for each bolt and between the four bolts and were asexpected for Alloy X-750 material.

The ASTM A 370 (Ref. 4) conversion for HRC 39 is 177ksi UTS, which is generally consistent with the tensile testing results, refer to Section 12.0.12.0 TENSILE TESTINGTensile testing was performed in accordance with B&W Technical Procedure TP-78,"Tension Testing of Metallic Materials" (Ref. 5). This specification is consistent with ASTME8 (Ref. 2). These tests were used to determine the bolt material yield strength, ultimatetensile strength, percent elongation, and reduction in area. Figure 3.5 shows the miniature tensile specimen

geometry, which was consistent with ASTM E8, except on a miniature scale to increase the number of samples tested per bolt.

B&W Technical Services GroupS-1473-002 19Eight tensile specimens were tested from each bolt, including the #1, #4, #7, and #10"upper" specimens located closer to the bolt head and the #1, #4, #7, and #10 "lower"specimens located away from the head. The tensile test data was very consistent for the#1, #4, and #7 samples.

All of these specimens broke in the center of the gage length.However, the #10 samples were not as consistent and tended to break toward the top ofthe gage section.

This appeared to be an artifact of the fixturing/machining

process, as the#10 specimens were slightly thicker and wedge-shaped from end to end. Because of theuncertainty and data scatter among the #10 specimens, only the data for the #1, #4, and #7specimens are reported here. The test results are summarized in Table 12.Table 12: Summary of tensile test data.Specimen ID UTS, ksi YS, ksi % Elongation RA, %120-2U1 164.3 120.6 29.0 46.5120-2U4 167.9 122.3 29.2 48.5120-2U7 168.6 119.6 29.0 38.4120-2L1 168.1 121.8 29.0 43.7120-2L4 167.6 122.3 29.4 45.0120-2L7 168.3 122.6 29.1 39.8120o-2 Average 167.5 121.5 29.1 43.7120-6U 1 160.7 116.4 29.9 42.7120-6U4 163.9 117.0 30.9 44.1120-6U7 162.5 117.1 30.6 39.7120-6L1 162.0 117.3 30.2 43.0120-6L4 163.4 118.3 30.1 43.7120-6L7 162.0 116.1 29.9 41.6120V-6 Average 162.4 117.0 30.3 42.5240-7U1 161.2 115.6 30.3 45.8240-7U4 160.3 114.6 30.4 43.4240-7U7 167.4 120.6 30.2 44.5240-711 164.8 117.7 31.2 40.0240-71L4 162.8 117.8 29.6 40.5240-7L7 160.6 115.1 28.9 38.9240-7 Average 162.9 116.9 30.1 42.2300-1 U1 162.2 116.5 30.7 39.0300-1 U4 162.3 116.5 30.6 40.4300-1 U7 160.2 111.8 31.0 42.6300-1 Ll 159.3 114.5 29.4 44.8300-1 L4 164.9 119.1 30.4 36.9300-1L7 160.6 115.3 31.0 39.930V- 1 Average 161.6 115.6 30.5 40.6 B&W Technical Services GroupS-1473-002 20The data was generally consistent among all of the bolts, with the yield strength rangingfrom -115 ksi to 122 ksi and the ultimate strength ranging from the -160 ksi to 167 ksi.The average elongation was -30% and the average reduction in area was -42%. Thesevalues were consistent with published mechanical properties for Alloy X-750 material.

13.0 ANALYSIS OF HEAD-TO-SHANK TRANSITION Additional examinations were performed to further characterize the head-to-shank transition in the bolts to determine if anomalies existed that may have contributed to thecracking.

These examinations included:

estimating the surface finish, determining theradius of curvature between the head and shank, measuring the angle between theunderside of the head and the shank, and performing higher magnification metallographic examinations of the radius between the head and shank. All examinations were performed on bolt 2400-7 and bolt 300°-1 (mounted specimens and loose pieces, as appropriate) unless otherwise noted.The surface finish was estimated to be 32 RMS or better for both bolts based on a lowmagnification visual examination.

It should be noted that the surface finish was difficult todetermine accurately due to oxide buildup and relatively small amount of available

material, especially for bolt 2400-7.An optical comparator was used to determine the radius of curvature on three bolts: the twonoted above plus bolt 1800-3 selected at random. None of the radii were perfectly formedor smooth; however, they did not contain unusually sharp transitions or features that wouldhave played a significant role in the crack initiation.

Consequently, the radii were estimated based on the best curvature fit. The results were as follows:

bolt 2400-7 was -0.065";

bolt3000-1 was -0.069";

and bolt 1800-3 was -0.080".

Higher magnification stereo microscope photographs taken of the mounted cross sections are provided in Figure 121 throughFigure 124. Each of these figures includes a semi-transparent circle that represents thebest curvature fit.Efforts were made to measure the angle between the underside of the head and the shank.It was determined that none of the bolts were conducive to this measurement due to thecracking; therefore, no valid data was obtained.

Higher magnification metallographic examinations were performed on the radius betweenthe head and shank of the two mounted specimens.

A three-frame montage was taken ofeach radius as shown in Figure 125 through Figure 128. The radii were not perfectly formed or smooth as noted earlier.

In addition, there were a few wavy areas observed, e.g.toward the shank side of bolt 3000-1, but there did not appear to be any sharp transitions, machining

grooves, or notches associated with the cracking.

There was also no obviousevidence of surface cold work at any initiation region as previously noted in Section 8.0.

B&W Technical Services GroupS-1473-002 2114.0 DISCUSSION The visual examinations performed on the twelve (12) open cracks indicated a generalcrack pattern consisting of crack initiation at two diametrically opposing sides of the bolt inthe head-to-shank transition region. The cracking extended upward into the bolt head at a-350 angle relative to horizontal.

Final fracture occurred when the two opposing crackslinked together near the center of the bolt. An analysis of the axis of symmetry (i.e. the linedrawn between the two opposing initiations) indicated no clear pattern in the direction ofthe prevailing

stresses, either within bolts removed from a particular clevis or betweendifferent clevises.

This finding indicated the stresses were variable from bolt to bolt. It alsocould suggest that stresses redistribute and/or change direction as multiple bolts failed at aparticular clevis location.

All of the thirteen (13) intact bolts contained cracking in the head/shank transition.

Nocracking was identified in the threaded region of any bolt. The amount of angled crackingvaried among the bolts, from many angled cracks (e.g. 240'-7) to very minor angledcracking (e.g. 3000-1).Four bolts were selected for destructive examinations.

The bolts were chosen to captureseveral variables, including intact and broken bolts, bolts with highly angled cracks andbolts with minor angled cracks. The selected bolts originated from three separate clevislocations, 1200, 2400, and 300'. Two bolts were chosen from the 120' clevis, since thislocation experienced the greatest number of bolt failures (all 8 failed at this location).

High magnification SEM examinations performed on the open cracks indicated the fracturesurface was essentially 100% intergranular fracture.

One small area near the center of bolt1200-2 was mixed mode fracture consisting of transgranular cleavage and ductile microvoid coalescence.

Cross section metallographic examinations confirmed the branchedintergranular nature of the cracking in all four bolts examined.

SEM/EDS analysis of a polished cross section prepared through the 300°-1 bolt identified titanium nitride precipitates (typically

-15 pm in size) and niobium titanium intermetallic compounds (typically

-20 pm in size). Carbon was not positively detected in any location, presumably because the carbides were very fine, i.e. <1 pm in size. Subsequent opticalmetallography using the dual etch technique confirmed the carbide size was <1 pm, whichis smaller than the EDS excitation volume. Consequently, no conclusions could be drawnregarding the carbide type(s),

i.e. M23C6or MC, present in the material microstructure.

B&W Technical Services GroupS-1473-002 22The optical metallographic examinations also revealed the discrete, banded nature of themicrostructure.

The banded regions contained both carbides and second phaseprecipitates such as titanium nitrides.

The carbides were generally intragranular within thebanded regions.

Carbide coverage at the grain boundaries was estimated to be 25% orless. The microstructure contained a duplex grain structure consisting primarily of finegrains (ASTM 7-8), with some very large grains (ASTM 1-2). There was no evidence ofsurface cold work or other surface anomaly noted at any of the initiation regions.The lack of cold work was confirmed during the Vickers microhardness

testing, whichindicated uniform microhardness values at the initiation

regions, mid-crack, and in the bulkmaterial away from the cracking.

The average microhardness was -400 HV at alllocations.

These results match the bulk Rockwell C hardness values, which averaged

-40HRC and were consistent with those expected for Alloy X-750 material.

Mechanical testing was performed on several miniature tensile specimens machined fromthe threaded region of the bolts. The data was generally uniform for all of the bolts, withthe yield strength ranging from -115 ksi to -122 ksi and the ultimate tensile strengthranging from -160 ksi to -167 ksi. The average elongation was -30% and the averagereduction in area was -40%; all of these results are as expected for Alloy X-750 material.

Low magnification SEM examination of the tensile fracture surfaces indicated the fracturemode was mixed and consisted of intergranular (IG) facets and ductile microvoid coalescence.

The presence of IG facets typically suggests that the material was embrittled by some mechanism.

Ref. 1 provides a possible explanation:

"Two-step age-hardening treatments after solution annealing tend to precipitate MC rather than M23C6 carbides.

TheMC-type carbides are principally titanium

carbides, TiC, which are likely trapping sites forhydrogen and increase the stay time of hydrogen at the grain boundaries, thus increasing the likelihood of SCC. The presence of residual hydrogen in the bulk material could alsobe responsible for the IG fracture on the tensile specimens.

However, since themechanical properties (especially elongation and reduction in area values) were asexpected for Alloy X-750 material, it is believed that the presence of IG facets on thefracture surface is not significant.

Chemical analysis results by ICP-MS were within the expected ranges for Alloy X-750material.

The results for niobium + tantalum and titanium were slightly below the specified values in one or two cases, but were considered to be within the uncertainty of the ICP-MStechnique, especially when accounting for additional product tolerances permitted by thematerial specification (Ref. 3).The laboratory

results, including tensile testing, hardness and microhardness testing,metallographic and SEM examinations, and chemical analysis were all consistent with AlloyX-750 material heat treated in accordance with the reported two-step aging heat treatment.

The literature indicates that Alloy X-750 in this heat treatment will have poor IGSCCresistance in a PWR environment (Ref. 1, 6, 7).

B&W Technical Services GroupS-1473-002 23For SCC to occur, three conditions must be present:

1) a susceptible

material,

2) tensilestresses, and 3) an environment capable of causing stress corrosion cracking in thatmaterial.

Each of these conditions is discussed briefly below:MaterialThe bolt microstructure consisted of a duplex grain structure and also contained discretebands of carbides and other precipitates.

The duplex structure and banding are bothinfluenced by prior work in the material and indicate a lower solution annealing temperature was used (1,625°F or 885C), i.e. it was not sufficiently high enough to dissolve the carbidesand precipitates.

For maximum SCC resistance, a 2,000°F (1,093C) solution annealing temperature is recommended, followed by a single-step aging treatment.

At the highersolution annealing temperature, all of the carbides are dissolved and can re-precipitate asM23C6 type carbides at the grain boundaries during subsequent aging. An added benefit ofthe higher temperature solution anneal is grain growth, which reduces the grain boundaryvolume, thereby increasing resistance to IGSCC.Tensile StressThe prevailing stresses on the bolts were applied tensile stresses.

An analysis of thefracture surfaces indicated the direction of the stresses were variable within each clevisand between different clevises.

It also appears possible that stresses could redistribute and/or change direction as bolt failures progressed at a particular clevis location.

The amount of angled cracking around the bolt OD varied between the bolts from manyangled cracks to very minor angled cracking.

The amount of angled cracking typically decreases as stresses increases.

This suggests that bolts with minor amounts of angledcracking (60'-3, 1200-6, 1800-3, 2400-3, and 300°-1) failed after other bolts at theirrespective clevis location.

Varying amounts of angled cracking could also indicate thepresence of a stress concentrator or other surface anomaly, although none were identified in the examined bolts. In all cases, though, fracture was essentially 100% intergranular.

Environment Alloy X-750 material heat treated using a low temperature solution anneal and two-stepaging treatment is known to have poor SCC cracking resistance in both high temperature and low temperature water (Ref. 1). The low temperature crack propagation (LTCP) ratesare orders of magnitudes greater than crack growth rates at high temperatures.

It shouldbe noted that LTCP will initiate from a crack-like defect, such as a high temperature SCC(HTSCC) defect. LTCP will not initiate from a smooth surface or notch (Ref. 7). Thisindicates the bolt cracking most likely initiated at high temperatures.

Crack propagation may have occurred at high or low temperatures;

however, the laboratory testing performed was not unable to conclusively determine the temperature(s) at which crack propagation occurred.

In addition, the laboratory data could not determine the age of the cracks, whenthe cracks initiated, or how fast they propagated.

B&W Technical Services GroupS-1473-002 24

15.0 CONCLUSION

S

• All of the submitted bolts, including those considered to be intact, contained cracking inthe head-to-shank transition.

No cracking was identified in the threaded region of anyof the bolts.* There was a generally uniform open crack fracture pattern consisting of crack initiation at two diametrically opposing sides of the bolt in the head-to-shank transition and crackgrowth that extended upward into the bolt head at a -35' angle relative to the boltseating surface.

The head separated from the shank when the two opposing crackslinked up near the center of the bolt cross section.* For each bolt, crack growth occurred along an axis of symmetry created by theopposing directions of crack growth. These crack growth axes indicated the direction ofprevailing stresses in each bolt. No correlation between the orientations of the crackgrowth axes and the in-service orientations of the bolts within each clevis wasobserved.

• Minor differences in crack morphology around the circumference of the head-to-shank transition suggested that the magnitude of the prevailing stresses varied from bolt tobolt.* Fractographic SEM analysis and cross section metallographic examinations determined the fracture mode was essentially 100% intergranular with crack branching for all of thebolts." The chemical analysis results for all four bolts were consistent with Alloy X-750material.

• The mechanical properties and microstructure of the bolts were consistent with thosepublished for Alloy X-750 material.

• No unexpected characteristics in the material properties, microstructures, or form of thebolts were identified.

• The laboratory data indicated the bolts failed by intergranular stress corrosion cracking(IGSCC).

The reported heat treatment for the bolts included a low solution annealing temperature and two-step aging treatment.

Alloy X-750 material heat treated in thismanner is known to have poor SCC cracking resistance in both high and lowtemperature water.* There was no evidence that the bolts failed due to fatigue cracking or mechanical overload.

B&W Technical Services GroupS-1473-002 25

16.0 REFERENCES

1. EPRI Materials

Handbook, Chapter 2, "Alloy X-750 -Precipitation Hardening Nickel-Base Alloy".2. ASTM E 8, "Test Methods for Tension Testing of Metallic Materials."

3. ASTM B 637, "Specification for Precipitation-Hardening Nickel Alloy Bars, Forgings, andForging Stock for High Temperature Service."

4. ASTM A 370, "Test Methods and Definitions for Mechanical Testing of Steel Products."

5. B&W TSG Technical Procedure TP-78, "Tension Testing of Metallic Materials."

6. "Microstructure and Stress Corrosion Resistance of Alloys X-750, 718, and A-286 inLWR Environments, EPRI NP-6392-SD, June 1989.7. Mills, W.J., et al, "Hydrogen Embrittlement, Grain Boundary Segregation, and StressCorrosion Cracking of Alloy X-750 in Low- and High-temperature Water", Metallurgical Transactions A, Volume 30A, June 1999.

B&W Technical Services GroupS-1473-002 26NOTE: BUILDING O0AND REACTORVESSEL V, ARE DIFFERENT.

60" -120"INISO*0*300"' .-" 240"---,EOUIPMENT HATCH___ _ IS LOCATEDSOUTHCLEVIS IDENTIFICATION REACTOR VESSEL ORIENTATION Figure 1: Schematic diagram showing the six clevis locations around the vesselcircumference.

BOLT I DOWEL PINLOCATIONS Figure 2: Schematic showing the typical bolt configuration for each clevis insert.

B&W Technical Services GroupS-1473-002 27I B&W Technical Services GroupS-1473-002 28B&W Technical Services Group S-I 473-002 28laooratory rotationai orentations.

IB&W Technical Services GroupS-1473-002 29B&W Technical Services Group S-1473-002 29eipt macro pnotograpn Tor origure o: meceipt macro pnotograpn Tor DOlt B&W Technical Services GroupS-1473-002 30B&W Technical Services Group S-I 473-002 30Ftol p-h5otoanK Figure 9: Receipt macro photographs for bolt 00-5.d B&W Technical Services GroupS-1473-002 31B&W Technical Services Group S-I 473-002 31.ceipt macro photograph for bolt11: Receipt macro photograpl tor B&W Technical Services GroupS-1473-002 32B&W Technical Services Group S-I 473-002 32eipt macro photograph for bolt B&W Technical Services GroupS-1473-002 33B&W Technical Services Group S-I 473-002 33macro photograph B&W Technical Services GroupS-1473-002 34FR t mao- p h fnanKFigure 15: Receipt macro photographs for bolt 120°-1.

B&W Technical Services GroupS-1473-002 35B&W Technical Services Group S-I 473-002 35Figure 16: Receipt macro photographs for bolt 1200-2.

B&W Technical Services GroupS-1473-002 36B&W Technical Services Group S-I 473-002 36Boit macr-3 5hanKFigure 17: Receipt macro photographs for bolt 120"-3.

B&W Technical Services GroupS-1473-002 37B&W Technical Services Group S-1473-002 37Fuore t1z--4 m p nanKFigure 18: Receipt macro photographs for bolt 1200-4.

B&W Technical Services GroupS-1473-002 38B&W Technical Services Group S-I 473-0 02 38bont 120l-5 bnanKFigure 19: Receipt macro photographs for bolt 1200-5.

B&W Technical Services GroupS-1473-002 39B&W Technical Services Group S-I 473-002 39DUEL 14W -0 011411KFigure 20: Receipt macro photographs for bolt 1200-6.

B&W Technical Services GroupS-1473-002 40B&W Technical Services Group S-I 473-002 40Figure 21: Receipt macro photograph for bolt 120laboratory.

to the B&W Technical Services GroupS-1473-002 41F e R t mazuc-r p nanKFigure 22: Receipt macro photographs for bolt 1200-8.

B&W Technical Services GroupS-1473-002 42B&W Technical Services Group S-I 473-002 42Figure 23: Receipt macro photographs for bolt 1800-1.

B&W Technical Services GroupS-1473-002 43 B&W Technical Services GroupS-1473-002 44B&W Technical Services Group S-1473-002 44IOlt 1UU-fi fnanKFigure 25: Receipt macro photographs for bolt 180-7.

B&W Technical Services GroupS-1473-002 45B&W Technical Services Group S-I 473-002 45Figure 26: Receipt macro photographs for bolt 180°-8.

B&W Technical Services GroupS-1473-002 46 B&W Technical Services GroupS-1473-002 47 B&W Technical Services GroupS-1473-002 48 B&W Technical Services GroupS-1473-002 49Figure 32: Receipt macro photographs for bolt 3000-3.

B&W Technical Services GroupS-1473-002 50B&W Technical Services Group s-I 473-002 50bolt .1UU-5 bfanKFigure 33: Receipt macro photographs for bolt 300-5.