ML14253A318
| ML14253A318 | |
| Person / Time | |
|---|---|
| Site: | Cook  |
| Issue date: | 12/31/2013 |
| From: | Hyres J W B & W Technical Services |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML14253A310 | List: |
| References | |
| AEP-NRC-2014-59 S-1473-002 | |
| Download: ML14253A318 (61) | |
Text
ENCLOSURE 3 TO AEP-NRC-2014-59 Babcock & Wilcox Report, S-1473-002, Revision 0, "Examination of Clevis Bolts Removed from D. C. Cook Nuclear Plant" technical services group FINAL REPORT: EXAMINATION OF CLEVIS BOLTS REMOVED FROM D. C. COOK NUCLEAR PLANT Prepared by: Babcock & Wilcox Technical Services Group 2016 Mount Athos Road Lynchburg, Virginia 24504-5447 (434) 522-6000 PREPARED BY: A-VrsPE A&Picipal Engineer Nuclar Materials Engineering Nuclear Material & Inspection Services S-1473-002 DECEMBER 2013 s-I 473-002 DECEMBER 2013
SUMMARY
This report covers laboratory examinations performed by Babcock & Wilcox Technical Services Group (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 (based on the post-removal visual inspections) were provided for laboratory analysis to evaluate the degradation, identify the failure mechanism(s), characterize the bolt material, and to evaluate the integrity of the intact bolts. The laboratory work scope included visual and stereovisual examinations of all bolts. Based on the results of these examinations, four bolts (two broken, two intact) were selected for more detailed analysis/testing, including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), optical metallography, 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 the head-to-shank transition.
No cracking was identified in the threaded region of any of 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 crack growth that extended upward into the bolt head at a -350 angle relative to the bolt seating surface. The head separated from the shank when the two opposing cracks linked up near the center of the bolt cross section.For each bolt, crack growth occurred along an axis of symmetry created by the opposing directions 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 and the 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 the fracture 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 those published 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 this manner 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 CONTENTS SECTION PAGE LIST O F TABLES ................................................................................................................
iv LIST O F FIG URES ...............................................................................................................
v LIST O F ACRO NYM S ....................................................................................................
x 1.0 INTRO DUCTIO N ........................................................................................................
1 2.0 BACKG RO UND .........................................................................................................
1 3.0 RECEIPT VISUAL EXAM INATIO NS .......................................................................
3 4.0 BO LT SELECTIO N ................................................................................................
6 5.0 VISUAL/STEREOVISUAL INSPECTIO NS .............................................................
7 6.0 SECTIO NING .............................................................................................................
8 7.0 SEM/EDS EXAM INATIO NS .................................................................................
10 8.0 M ETALLO G RAPHIC EXAM INATIO NS ................................................................
12 9.0 VICKERS M ICRO HARDNESS ..............................................................................
15 10.0 INDUCTIVELY COUPLED PLASMA-MASS SPECTROSCOPY (ICP-MS) ...........
17 11.0 ROCKW ELL HARDNESS TESTING .....................................................................
18 12.0 TENSILE TESTING ..............................................................................................
18 13.0 ANALYSIS OF HEAD-TO-SHANK TRANSITION
...............................................
20 14.0 DISCUSSIO N ...........................................................................................................
21 15.0 CO NCLUSIO NS .......................................................................................................
24
16.0 REFERENCES
.........................................................................................................
25 iii LIST OF TABLES TABLE PAGE 1 S um m ary of subm itted bolts .......................................................................................
1 2 Typical mechanical properties for Alloy X-750 .....................................................
2 3 Visual examination summary of the bolts ...............................................................
3 4 Summary of bolts selected for destructive examinations
.......................................
6 5 Bolt 300°-1 head thickness measurements
..........................................................
9 6 Summary of Vickers microhardness (HV) results for bolt 120'-2 cross section ....... 15 7 Summary of Vickers microhardness (HV) results for bolt 120'-6 cross section ....... 15 8 Summary of Vickers microhardness (HV) results for bolt 2400-7 cross section ....... 16 9 Summary of Vickers microhardness (HV) results for bolt 300°-1 cross section ....... 16 10 Sum m ary of IC P-M S data ...................................................................................
17 11 Rockwell C hardness measurements
.................................................................
18 12 Sum m ary of tensile test data ...............................................................................
19 iv LIST OF FIGURES FIGURE PAGE 1 Schematic diagram showing the six clevis locations around the vessel circum fe re nce .................................................................................................
..26 2 Schematic showing the typical bolt configuration for each clevis insert ............
26 3 Bolt 120°-2, annotated with laboratory rotational orientations
..........................
27 4 Bolt 120'-6, annotated with laboratory rotational orientations
..........................
27 5 Bolt 240'-7, annotated with laboratory rotational orientations
..........................
28 6 Bolt 3000-1, annotated with laboratory rotational orientations
..........................
28 7 Receipt macro photograph for bolt 0°-1 ............................................................
29 8 Receipt macro photograph for bolt 0°-3 .............................................................
29 9 Receipt macro photographs for bolt 0Q-5 ...........................................................
30 10 Receipt macro photograph for bolt 0°-7 .............................................................
31 11 Receipt macro photograph for bolt 60°-1 ..........................................................
31 12 Receipt macro photograph for bolt 60°-3 ..........................................................
32 13 Receipt macro photograph for bolt 600-5 ..........................................................
32 14 Receipt macro photograph for bolt 60'-7 ..........................................................
33 15 Receipt macro photographs for bolt 1200-1 ......................................................
34 16 Receipt macro photographs for bolt 120'-2 ......................................................
35 17 Receipt macro photographs for bolt 120'-3 .......................................................
36 18 Receipt macro photographs for bolt 120°-4 .......................................................
37 19 Receipt macro photographs for bolt 1200-5 ......................................................
38 20 Receipt macro photographs for bolt 120'-6 ......................................................
39 21 Receipt macro photograph for bolt 1200-7 .......................................................
40 22 Receipt macro photographs for bolt 120'-8 ......................................................
41 23 Receipt macro photographs for bolt 180°-1 .....................................................
42 24 Receipt macro photograph for bolt 180°-3 .........................................................
43 25 Receipt macro photographs for bolt 180°-7 ......................................................
44 26 Receipt macro photographs for bolt 1800-8 ......................................................
45 27 Receipt macro photograph for bolt 2400-1 .........................................................
46 28 Receipt macro photograph for bolt 2400-3 ........................................................
46 29 Receipt macro photograph for bolt 2400-5 ........................................................
47 V LIST OF FIGURES (CONTINUED)
FIGURE PAGE 30 Receipt macro photograph for bolt 240°-7 .........................................................
47 31 Receipt macro photograph for bolt 300°-1 .........................................................
48 32 Receipt macro photographs for bolt 3000-3 .....................................................
49 33 Receipt macro photographs for bolt 300'-5 .......................................................
50 34 Receipt macro photographs for bolt 3000-6 ......................................................
51 35 Receipt macro photographs for bolt 3000-7 .....................................................
52 36 Receipt macro photographs for bolts from the 1200 clevis ..............................
53 37 Receipt macro photographs for bolts 7 and 8 from the 180' clevis ...................
55 38 Receipt macro photographs for bolts 5, 6, and 7 from the 300° clevis ..............
56 39 Macro and stereo photographs of bolt 1200-2 taken at 450 increments
............
57 40 Macro and stereo photographs of bolt 120°-6 taken at 450 increments
............
61 41 Macro and stereo photographs of bolt 2400-7 taken at 450 increments
............
65 42 Macro and stereo photographs of bolt 3000-1 taken at 450 increments
............
69 43 Photograph showing wire EDM used to section each bolt for destructive e xa m inatio ns ...................................................................................................
..73 44 Section photograph for bolt 2400-7 showing the typical locations chosen for subsequent analysis ........................................................................................
..73 45 Typical location of tensile specimens machined from each bolt ........................
74 46 Miniature tensile specimen design showing dimensions
..................................
74 47 Bolt 2400-7 after breaking open the crack for SEM/EDS ...................................
75 48 Bolt 3000-1 showing plunge cut EDM surface, which removed much of the in-service cracking ..........................................................................................
..75 49 Macro photograph showing the open crack surface for bolt 1200-2 .................
76 50 O D of 120°-2 near 900, 50X ..............................................................................
77 51 C enter of Figure 50, 50O X .................................................................................
77 52 M id-diam eter of 120°-2, 50X .............................................................................
78 53 C enter of Figure 52, 50O X .................................................................................
78 54 Center of 120'-2 fracture, 50X ..........................................................................
79 55 C enter of Figure 54, 50O X .................................................................................
79 56 Macro photograph showing the open crack surface for bolt 1200-6 ...................
80 vi LIST OF FIGURES (CONTINUED)
FIGURE PAGE 57 OD of bolt 120°-6 near 450, 5OX ......................................................................
81 58 Center of Figure 57, 50OX .................................................................................
81 59 Mid-diameter of bolt 1200-6, 5OX ......................................................................
82 60 Center of Figure 59, 50OX .................................................................................
82 61 Center of 1200-6 fracture, 50X ...........................................................................
83 62 Center of Figure 61,500X .................................................................................
83 63 Stereo microscope photograph showing the open crack surface for bo lt 24 0 °-7 .........................................................................................................
84 64 OD of bolt 240°-7 near 3150, 50X ......................................................................
85 65 Center of Figure 64, 50OX .................................................................................
85 66 Mid-diameter of 240°-7, 50X .............................................................................
86 67 Center of Figure 66, 50OX .................................................................................
86 68 Center of 240'-7 fracture, 50X ...........................................................................
87 69 Center of Figure 68,500X .................................................................................
87 70 BSE image of titanium nitride -15 pm in size, 1,500X ......................................
88 71 EDS spectrum collected from precipitate shown in Figure 70 ..........................
88 72 BSE image of niobium-titanium intermetallic
-20 pm long, 1,500X ..................
89 73 EDS spectrum collected from precipitate shown in Figure 72 ..........................
89 74 Typical area of polished cross section, 1,500X .................................................
90 75 EDS spectrum collected from entire area shown in Figure 74 ...........................
90 76 EDS dot maps collected from area shown in Figure 74 ....................................
91 77 Tensile fracture surface for 240 0-7L4, 80X ......................................................
92 78 Same as Figure 77 with annotated reduction in area measurements, 80X ..... 92 79 As-polished overview of bolt 120'-2 cross section. 6X ....................................
93 80 Same area as Figure 79 after phosphoric
+ nital etch. Structure is banded. 6X ...93 81 As-polished micrograph montage of crack. -10X .............................................
94 82 Higher magnification detail of Figure 81, as-polished.
140X ............................
95 83 Detail of initiation near 90'. 180X ...................................................................
96 84 Same area as Figure 83 above after phosphoric
+ nital etch. 180X .................
96 85 Detail of initiation near 270°. 180X .................................................................
97 vii LIST OF FIGURES (CONTINUED)
FIGURE PAGE 86 Same area as Figure 85 after phosphoric
+ nital etch. 180X ...........................
97 87 Typical microstructure after phosphoric etch, DIC, 660X ..................................
98 88 Same area as Figure 87 after phosphoric
+ nital etch, DIC, 660X ....................
98 89 Bolt 120°-6 cross section as-polished overview.
6X ..........................................
99 90 Same area as Figure 89 after phosphoric
+ nital etch. 6X ...............................
99 91 As-polished micrograph montage of crack. -1OX ...............................................
100 92 Higher magnification detail of Figure 91, as-polished.
140X ...............................
101 93 D etail of initiation near 450. 180X .......................................................................
102 94 Same area as Figure 93 after phosphoric
+ nital etch. 180X ..............................
102 95 D etail of initiation near 2250. 180X .....................................................................
103 96 Same area as Figure 95 after phosphoric
+ nital etch. 180X ..............................
103 97 Typical microstructure after phosphoric etch, DIC, 660X .....................................
104 98 Same area as Figure 97 after phosphoric
+ nital etch, DIC, 660X .......................
104 99 As-polished overview of 240°-7 cross section. 6X ..............................................
105 100 Same area as Figure 99 after phosphoric
+ nital etch. 6X ..................................
105 101 As-polished micrograph montage of crack. 1OX .................................................
106 102 Higher magnification detail of Figure 101, as-polished.
120X .............................
107 103 Detail of initiation near 3150 after etching. 180X .................................................
108 104 Second crack near 315 0 after etching. 180X .......................................................
108 105 Detail of initiation near 1350 after etching. 180X .................................................
109 106 Typical appearance of intermittent duplex grain structure, DIC. 90X ..................
109 107 Typical microstructure after phosphoric etch, DIC, 660X .....................................
110 108 Same area as Figure 107 after phosphoric
+ nital etch, DIC, 660X .....................
110 109 As-polished overview of 300°-1 cross section. 6X ..............................................
111 110 Same area as Figure 109 after phosphoric
+ nital etch. 6X ................................
111 111 Detail of initiation near 900. 180X .......................................................................
112 112 Same area as Figure 111 after etching. 180X ....................................................
113 113 Detail of initiation near 2700. 180X .....................................................................
114 114 Same area as Figure 113 after etching. 180X ....................................................
114 115 Typical microstructure after phosphoric etch, DIC, 660X .....................................
115 viii LIST OF FIGURES (CONTINUED)
FIGURE PAGE 116 Same area as Figure 115 after phosphoric
+ nital etch, DIC, 660X .....................
115 117 Photograph showing the microhardness locations for 120'-2 ..............................
116 118 Photograph showing the microhardness locations for 120'-6 ..............................
116 119 Photograph showing the microhardness locations for 240'-7 ..............................
117 120 Photograph showing the microhardness locations for 300°-1 ..............................
117 121 Bolt 2400-7 at 135°. Circle radius is 0.065". 20X ...............................................
118 122 Bolt 2400-7 at 315'. Circle radius is 0.065". 20X ...............................................
118 123 Bolt 3000-1 at 900. Circle radius is 0.069". 20X .................................................
119 124 Bolt 3000-1 at 270'. Circle radius is 0.069". 20X ...............................................
119 125 Higher magnification montage of bolt 240'-7 at 1350. As-polished, -70X ....... 120 126 Higher magnification montage of bolt 240'-7 at 315'. As-polished, -65X ....... 121 127 Higher magnification montage of bolt 300°-1 at 90'. As-polished, -80X ............
122 128 Higher magnification montage of bolt 3000-1 at 2700. As-polished, -75X ..........
123 ix LIST OF ACRONYMS ASME ................
AMERICAN SOCIETY OF MECHANICAL ENGINEERS ASTM ................
AMERICAN SOCIETY FOR TESTING AND MATERIALS B&W TSG .........
BABCOCK & WILCOX TECHNICAL SERVICES GROUP BSE ...................
BACKSCATTERED ELECTRON (SEM IMAGING)CW ....................
CLOCKW ISE CCW .................
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 CRACKING HV .....................
VICKERS HARDNESS (MICROHARDNESS)
ICP-MS .............
INDUCTIVELY COUPLED PLASMA-MASS SPECTROSCOPY IGSCC ..............
INTERGRANULAR STRESS CORROSION CRACKING LRSS ................
LOWER RADIAL SUPPORT SYSTEM LTCP .................
LOW TEMPERATURE CRACK PROPAGATION OD ....................
OUTER DIAMETER OES ..................
OPTICAL EMISSION SPECTROSCOPY PWR .................
PRESSURIZED WATER REACTOR RA .....................
REDUCTION IN AREA SCC ..................
STRESS CORROSION CRACKING SE .....................
SECONDARY ELECTRON (SEM IMAGING)SEM ..................
SCANNING ELECTRON MICROSCOPE/MICROSCOPY X
B&W Technical Services Group S-1473-002 I
1.0 INTRODUCTION
This report covers laboratory examinations performed by Babcock & Wilcox Technical Services Group (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 failure mechanism(s), characterize the bolt material, and to evaluate the integrity of the intact bolts. A summary of the submitted bolt samples (locations per Figure 1 and Figure 2) is provided in the table below: Table 1: Summary of submitted bolts.Broken Bolt Intact Bolt Location Locations Locations 00 #5 #1, #3, #7 600 #1,#3,#5,#7 1200 #1 through #8 ---1800 #1, #7, #8 #3 2400 --- #1, #3, #5, #7 3000 #3, #5, #6, #7 #1 STotals 16 13 The laboratory work scope included visual and stereovisual examinations, scanning electron 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 these examinations 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 March 2010 refueling outage at D. C. Cook Unit 1. AEP replaced a minimum bolt pattern that encompassed all bolts with visual indications during the March/April 2013 refueling outage and it was determined that a total of 16 bolts had failed. In each case, the failure location was below the head in the head-to-shank transition.
The lower radial support system consists of six (6) support clevises spaced evenly around the reactor vessel circumference as shown schematically in Figure 1. Each clevis is comprised of a wear plate (insert) attached to the lugs by eight (8) bolts and two (2) press fit dowel pins (Figure 2). In all, there are 48 clevis bolts and 12 dowel pins in the lower radial support system.
B&W Technical Services Group S-1473-002 2 The clevis bolts are approximately 3" long by 0.75" in diameter and are manufactured from Alloy X-750. The clevis bolt head is approximately 1.5" in diameter with a slotted internal hex socket to accommodate a locking bar. The locking bar is welded in place after installation 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 3 through 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 1775 0 F (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 cool This 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 that these 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 /> in lieu of the two-step aging treatment.
Other two-step aging processes, such as ASME Code Case 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 table below: 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 higher for condition BH compared to condition AH. The higher strength of condition BH is offset by lower elongation and reduction in area (RA) values compared to condition AH. The condition AH and SB-637, Type 2 strength levels are comparable.
B&W Technical Services Group S-1473-002 3 3.0 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 to assess the overall condition of the bolts/fragments and help select candidate bolts for the destructive 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 A Location*
ID Figure Intact Fracture?
Pattern? Additional Comments 001 AYes Most angled cracking at this clevis location.-3 AaYes Less angled cracking 0°3A-8 8 Intact N/A thnes-than 00-1 Fracture surface lost due to EDM plunge cut.0-7 AYes Least angled cracking at this clevis location.60-1 B-1 11 Intact N/A Yes Most angled cracking at this clevis location.600-3 B-13 12 Intact N/A Yes Less angled cracking compared to 600-1.600-5 B-4 13 Intact N/A Yes Cracking tighter and not 3600 around.600-7 B-14 14 Intact N/A Yes Less angled cracking compared to 600-1.1200-1 B-15 15 Broken Yes Yes Angled cracking present around OD.Less angled cracking 1200-2 B-12 16 Broken No Yes compared to 1200-1 or 1200-4.Less angled cracking 1200-3 B-5 17 Broken Yes Yes compared to 1200-1 or 1200-4.1200-4 B-3 18 Broken Yes Yes Angled cracking I_ _ III_ IIpresent around OD.*In-service location consists of the angular orientation of the clevis (00, 600, 1200, etc.) along with the bolt position (1, 2, 3, etc.). Refer to Figure 1 and Figure 2.
B&W Technical Services Group S-1473-002 4 Table 3 (continued):
Visual examination summary of the bolts.In-Service Shipping Report Broken! Rubbed Typical 1 Location*
ID Figure Intact Fracture?
Pattern? Additional Comments 120°-5 B-2 19 Broken Yes Yes Angled cracking present around OD.1200-6 B-16 20 Broken No No Very minor angled cracking around OD.Fracture surface lost 120 °- 7 B- 6 21 Broken N/A N/A det D lnect due to EDM plunge cut.120°-8 B-11 22 Broken Yes Yes Angled cracking present around OD.Fracture surface lost 180 o_1 A- 12 23 Broken N/A N/A det D lnect due to EDM plunge cut.1800-3 A-3 24 Intact N/A Yes Minor angled cracking around OD.Less angled cracking 180°-7 A-14 25 Broken Yes Yes compared to 180°-8;asymmetric center.Most angled cracking at 180°-8 A-13 26 Broken Yes Yes this clevis location;asymmetric center.Less angled cracking 240-1 A-16 27 Intact N/A Yes247.240°-3 A-15 28 Intact N/A Yes Less angled cracking compared to 2400-5.Less angled cracking 240°-5 A-4 29 Intact N/A Yes compared toacking compared to 240°-1.2400-7 A-6 30 Intact NIA Yes Most angled cracking at this clevis location.300°-1 A-7 31 Intact N/A No Very minor angled cracking around OD.Fracture surface lost 3000-3 A-2 32 Broken NIA NIA Frcuesfaelt due to EDM plunge cut.3000-5 B-9 33 Broken Yes Yes Less angled cracking compared to 3000-7.3000-6 B-10 34 Broken Yes Yes Most angled cracking at this clevis location.300'-7 B-8 35 Broken Yes Yes Less angled cracking I I compared to 300-6*In-service location consists of the angular orientation of the clevis (00, 600, 1200, etc.) along with the bolt position (1, 2, 3, etc.). Refer to Figure 1 and Figure 2.
B&W Technical Services Group S-1473-002 5 It 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 lighting to 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, the bolts are arranged by their position within the clevis. Also, the orientation of each bolt was matched to the in-service orientation, i.e. the 12:00 position in the photograph matches the 12:00 in-service position.Many of the open fractures sustained considerable rubbing damage and were not considered good candidates for the higher magnification examinations.
However, it was evident that all of the open fracture surfaces followed a similar pattern. Crack growth progressed 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 symmetry created by these two opposing sides is annotated on each fracture surface in Figure 36 through Figure 38. The orientations of the axes of symmetry were random in nature, which indicated the directions of prevailing stresses were variable within each clevis and between different clevises.
Final fracture occurred when the two opposing cracks linked together near the center of the bolt.All of the thirteen (13) intact bolts contained cracking in the head/shank transition.
No cracking 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 may not 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 angled cracks (e.g. 240'-7) to very few angled cracks (e.g. 300°-1). The amount of angled cracking is expected to decrease as the stress increases, but this variation in crack morphology could also indicate a failure mode change.
B&W Technical Services Group S-1473-002 6 4.0 BOLT SELECTION It was decided to select two intact and two broken bolts for the destructive examinations.
It was also important to select bolts that exhibited a higher degree of angled cracking and bolts with little or no angled cracking.
Of secondary concern was capturing bolts from the different 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 was to 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/Low BotID rknItc Pattern?120-2 Broken Yes Left High 120-6 Broken No Right High 240-7 Intact Yes Right Low 300-1 Intact No Left High The selected bolts were located in three of the six clevis locations and included two bolts from the 120' clevis. The 1200 clevis location experienced the greatest population of in-service bolt failures (all 8 bolts failed).
B&W Technical Services Group S-1473-002 7 5.0 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 the cracking.
The angular orientations were established by assigning the 0' position to the 12:00 in-service position and increasing degrees in the clockwise direction when viewing the bolt head.Bolt 120°-2 The macro and stereo photographs for bolt 120'-2 are presented in Figure 39. The fracture surface 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°-6 The macro and stereo photographs for bolt 1200-6 are presented in Figure 40. The fracture surface was lighter in color than bolt 1200-2, which suggested less surface deposits were present and this fracture likely occurred more recently than the 120'-2 bolt failure.Cracking was generally straight and exhibited very minor branching around the circumference.
The fracture surface axis of symmetry was in the 45o-2250 direction.
Bolt 240°-7 The macro and stereo photographs for bolt 2400-7 are presented in Figure 41. Crack branching is evident around nearly the entire circumference with the exception of the 3150 orientation (i.e. 10:30 in-service).
Bolt 3000-1 The macro and stereo photographs for bolt 300°-1 are presented in Figure 42. Cracking was generally straight and exhibited very minor branching around the circumference.
B&W Technical Services Group S-1473-002 8 6.0 SECTIONING Sectioning was required to permit the higher magnification metallographic and SEM examinations of the bolt fractures, as well as chemical analysis and mechanical testing of the 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 each bolt. To produce these specimens, a rectangular bar measuring 0.5" x 0.5" x 2.2" was machined from the threaded region as shown in Figure 43. One surface of each bar was then 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 typical for 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 bolt shank.After the hardness testing was complete, the tensile specimens were profiled out of the rectangular bar as shown in Figure 45. At least 0.030" of material was machined from the hardness testing surface to ensure the tensile specimens were not influenced by localized cold working introduced during hardness testing. In all, 20 tensile specimens were machined from each bolt, 10 "upper" specimens from near the head (identified as Ul through 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 was selected 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 fracture surface. For the broken bolts, 120'-2 and 120'-6, the entire fracture surface was examined.
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 breaking open 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 shiny laboratory fracture on the open crack surfaces.It was necessary to reduce the thickness of the three open crack specimens to facilitate the SEM examinations.
Care was taken to ensure the bottom of the hex and the fracture surface were not damaged during cutting. The location of this cut for bolt 240'-7 is shown in Figure 44. The broken bolt heads from bolt 1200-2 and bolt 1200-6 were cut in a similar manner.
B&W Technical Services Group S-1473-002 9 After the SEM examinations were completed on the two broken bolt heads, the fracture surfaces 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 of the 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 maximum extent of the remaining crack, thickness measurements were taken around the head circumference as shown in the table below: Table 5: Bolt 300°-1 head thickness measurements.
Orientation Thickness, in.00 0.242 900 0.253 1350 0.250 1800 0.244 2700 0.231 3150 0.236 These measurements indicated the EDM plunge cutter was located slightly toward the 2700 orientation (i.e. minimum thickness value). The 900-2700 orientation was selected for the cross 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 300 0-1, it was decided to perform the SEM examinations on a polished cross section prepared from the threaded region rather than an open crack specimen.
The cross section analysis would provide an opportunity to examine the microstructure at higher magnifications and perform EDS chemical analysis to characterize the bulk material and precipitates.
B&W Technical Services Group S-1473-002 10 7.0 SEM/EDS EXAMINATIONS Fracture surfaces and polished metallographic cross sections of the four bolts were examined by SEM equipped with EDS for elemental analysis.
Both secondary electron (SE) and backscattered electron (BSE) imaging modes were utilized to characterize the bolts. SE imaging was used to document the fracture surface morphology, while BSE imaging (identified as AUX1 in Figures 70, 72, and 74) was used to characterize the material microstructure on the cross sections.
The fractographic examinations were performed on bolts 120'-2, 120'-6, and 240o-7. The microstructural characterizations were performed on an as-polished cross section prepared from the threaded region of bolt 3000-1.Bolt 120°-2 Open Crack A low magnification photograph showing the open crack from bolt 120'-2 is shown in Figure 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 54 and Figure 55). Cracking was essentially 100% intergranular fracture, with the exception of a small amount of mixed mode fracture near the center of the bolt fracture surface. This central area contained a mixture of intergranular fracture, transgranular cleavage, and a small amount of ductile fracture (Figure 55). Surface deposits were noted on the fracture surface. EDS analysis indicated these deposits were base metal oxides. The presence of these deposits was consistent with the relatively dark macro appearance of the fracture surface.Bolt 120*-6 Open Crack A low magnification photograph showing the open crack from bolt 1200-6 is shown in Figure 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 61 and Figure 62). Cracking was essentially 100% intergranular fracture.
There was no evidence of mixed mode near the center as was the case for the 120'-2 bolt. Less surface oxides/deposits were present on this fracture surface. This is consistent with the macro appearance of the fractures, i.e. bolt 120'-6 appeared "cleaner" than bolt 1200-2.Bolt 240°-7 Open Crack A low magnification photograph showing the open crack from bolt 240'-7 is shown in Figure 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 68 and Figure 69). The fracture mode was essentially 100% intergranular.
B&W Technical Services Group S-1473-002 11 Bolt 300- 1 Mounted Cross Section A polished cross section prepared through the threaded portion of bolt 300°-1 was analyzed by SEM/EDS. Typical precipitates identified during these examinations included titanium 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). Carbon was not detected in any of the examined areas; therefore, specific carbide types (M 2 3 C 6 vs.MC) could not be differentiated.
Subsequent optical examinations described in Section 5.0 indicated 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 results were generally consistent with Alloy X-750 material.High resolution EDS dot maps were also collected from this area and are presented in Figure 76. When interpreting these maps, note that the concentration of an element increases with increasing color density within that element's window, i.e. darker color indicates more of that element is present. The maps indicated the primary elements present were nickel, chromium, and iron; trace amounts of titanium and niobium were detected as well. The maps also slow that the composition of the alloy base metal was uniform (no significant, widespread contrast in the maps) and that there was a higher concentration of niobium and titanium in the precipitates, as indicated by the discrete darker regions within the niobium and titanium element windows.Tensile Fracture Surfaces Low magnification (80X) SE imaging was used to document each tensile specimen fracture.
Low magnification micrographs were taken of each surface in order to accurately measure the reduced section area, since standard techniques such as calipers can be problematic when measuring miniature specimens.
A typical example of a tensile fracture is shown in Figure 77. The fracture surface was mixed mode that consisted of intergranular facets and ductile microvoid coalescence.
The potential implications of this finding are discussed further in Section 13.0.Thickness and width measurements were then made to determine the reduction in area for each specimen.
Typical measurements are annotated on the micrograph as shown in Figure 78.
B&W Technical Services Group S-1473-002 12 8.0 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 cure two-part epoxy compound.
The mounts were analyzed first in the as-polished condition and after chemical etching to reveal the material microstructure.
The dual etch procedure was used on the bolt material.
This procedure involves etching the polished cross section in concentrated phosphoric acid to reveal the carbides, then etching in 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 the carbide 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 Section Low magnification stereo microscope photographs were taken of the bolt 1200-2 cross section in the as-polished (Figure 79) and etched (Figure 80) conditions.
The banded nature of the microstructure was evident at low magnification in the etched condition.
Crack propagation traversed the banded regions, an indication that cracking was not directly 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) is presented in Figure 82. Cracking exhibited a branched intergranular morphology in all areas 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 in Figure 85 and Figure 86. Evidence of a second, shallower crack was noted at both locations, as was the banded microstructure.
There was no obvious evidence of surface cold work at either initiation region.Typical DIC micrographs showing the microstructure after phosphoric acid etching and nital etching are presented in Figure 87 and Figure 88, respectively.
The carbides (fine black dots) were present in discrete bands, as evidenced by a vertical band on the left and right of the micrographs.
The center region contained relatively few carbides.
The carbides were typically intragranular; a slight amount of grain boundary carbide coverage was noted (<25%).
B&W Technical Services Group S-1473-002 13 Bolt 1200-6 Cross Section Low magnification stereo microscope photographs were taken of the bolt 1200-6 cross section in the as-polished (Figure 89) and etched (Figure 90) conditions.
The banded nature of the microstructure was evident at low magnification.
Crack propagation traversed the banded regions, an indication that cracking was not directly influenced by the presence of 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 450 micrographs are shown in Figure 93 and Figure 94. The 2250 micrographs are shown in Figure 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 nital etching are presented in Figure 97 and Figure 98, respectively.
A discrete band of carbides (fine black dots) is visible toward the left side of the micrograph.
Relatively few carbides were noted elsewhere.
The carbides were typically intragranular; a slight amount of grain boundary carbide coverage was noted (<25%).Bolt 240°-7 Cross Section Low magnification stereo microscope photographs were taken of the bolt 2400-7 cross section in the as-polished (Figure 99) and etched (Figure 100) conditions.
The banded nature of the microstructure was evident at low magnification.
Crack propagation traversed the banded regions, an indication that cracking was not directly influenced by the presence of a banded microstructure.
A multi-frame montage of the entire crack is presented in Figure 101. A higher magnification montage showing the nature of the crack branching is presented in Figure 102. Two cracks were located at the 3150 orientation, i.e. the cross section cut through two adjacent, overlapping cracks at this orientation.
Cracking exhibited a branched intergranular morphology in all areas examined.
B&W Technical Services Group S-1473-002 14 Higher magnification detail micrographs were also taken of each initiation region. The 3150 micrographs are shown in Figure 103 and Figure 104. Two separate cracks were present at this location.
The 135' micrograph is shown in Figure 105. There was no obvious evidence of surface cold work at either initiation region. A low magnification DIC micrograph showing the typical appearance of the duplex microstructure is presented in Figure 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 nital etching 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 were noted on the left side of the micrograph.
The carbides were typically intragranular; a slight amount of grain boundary carbide coverage was noted (<25%).Bolt 300 -1 Cross Section Low magnification stereo microscope photographs were taken of the bolt 300°-1 cross section in the as-polished (Figure 109) and etched (Figure 110) conditions.
The banded nature of the microstructure was evident at low magnification.
Crack propagation traversed the banded regions, an indication that cracking was not directly influenced by the presence of 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 2700 micrographs 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 large grain at the 90' orientation (denoted by arrows in Figure 112). Minor secondary cracking was 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 1-2 grains deep from the main crack.Typical DIC micrographs showing the microstructure after phosphoric acid etching and nital etching 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 carbides were noted elsewhere.
The carbides were typically intragranular; a slight amount of grain boundary carbide coverage was noted (-<25%).
B&W Technical Services Group S-1473-002 15 9.0 VICKERS MICROHARDNESS Several Vickers microhardness (HV) readings (500 gram load) were taken on the mounted specimens to characterize the hardness in various areas of interest, including near the OD in the initiation regions, mid-thickness, and bulk material away from the cracking.
The areas selected are shown in Figure 117 through Figure 120. For the intact bolts, hardness measurements 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 Group S-1473-002 16 Table 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 .........#7 391 384 ---....#8 392 440 ---......#9 386 390 ...#10 393 389 ...Note Initiation readings #1-#5 were taken above the crack; readings #6-#10 were taken below the crack. 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#5 390 402 373 415 The results for the initiation regions and bulk material were generally consistent and as expected for Alloy X-750 material (-400 HV). The results were also consistent with the Rockwell 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 Group S-1473-002 17 10.0 CHEMICAL ANALYSIS Small pieces (-100mg each) were removed from each of the four candidate bolts and analyzed by Inductively Coupled Plasma -Mass Spectroscopy (ICP-MS) for base material constituents, including:
nickel, chromium, iron, cobalt, niobium, titanium, aluminum, manganese, silicon, copper, sulfur, phosphorus, boron, zirconium, and vanadium.
Note that this technique cannot determine carbon content. The results are summarized in the table below: Table 10: Summary of ICP-MS Data.Element Alloy X-750 120°-2 120°-6 J 240°-7 300--1 Nickel 70.0 min 71.8 73.0 72.8 72.4 Chromium 14.0- 17.0 15.7 15.6 15.7 15.5 Iron 5.0 -9.0 7.5 7.3 7.2 7.3 Titanium 2.25 -2.75 2.32 2.10 2.13 2.46 Aluminum 0.4- 1.0 0.87 0.82 0.81 0.80 Niobium + Tantalum 0.7 -1.2 0.85 0.62 0.82 0.92 Manganese 1.0 max 0.20 0.12 0.12 0.12 Silicon 0.5 max 0.02 0.02 0.03 0.02 Sulfur 0.01 max <0.003 <0.003 <0.003 <0.003 Copper 0.5 max 0.03 0.01 0.01 0.01 Carbon* 0.08 max -- -- -- --Cobalt 1.0 max 0.31 0.06 0.06 0.06 Phosphorus
-- 0.006 0.003 0.006 0.005 Boron -- 0.004 0.004 0.004 0.004 Zirconium
-- 0.05 0.07 0.08 0.08 Vanadium -- 0.26 0.26 0.24 0.27*Not determined.
The niobium + tantalum and titanium concentrations were marginally low for the 120'-6 bolt and 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, when accounting 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 may occur between components made from the same heat lot. For titanium, the product tolerance is 0.07% below the minimum, or 2.18%. For niobium + tantalum, the product tolerance is 0.05% below the minimum, or 0.65%.It was noted that the trace element levels (manganese, cobalt, and copper) were higher in the 120'-2 bolt compared to the other three bolts. This indicated the 120'-2 bolt likely originated from a different heat lot.
B&W Technical Services Group S-1473-002 18 Carbon concentration cannot be reliably determined using ICP-MS due to low ion efficiency.
Techniques typically used to determine the carbon concentration in bulk metals include combustion and optical emission spectroscopy (OES). OES is also preferred over ICP-MS for determining bulk chemical analysis of metals. Unfortunately, no laboratories capable of performing OES on irradiated specimens were identified during the course of this examination.
11.0 ROCKWELL HARDNESS MEASUREMENTS Rockwell C hardness measurements (diamond indenter, 150 kg load) were taken to determine the bolt material bulk hardness.
These measurements were taken on the tensile specimen blank removed from each of the four candidate bolts. Figure 44 shows the typical location of the five (5) hardness measurements taken on each tensile blank. The EDM recast layer was ground off prior to performing the hardness measurements.
The results 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 39.1 Average 39.1 39.0 [ 38.9 j 39.0 The results were very consistent for each bolt and between the four bolts and were as expected for Alloy X-750 material.
The ASTM A 370 (Ref. 4) conversion for HRC 39 is 177 ksi UTS, which is generally consistent with the tensile testing results, refer to Section 12.0.12.0 TENSILE TESTING Tensile testing was performed in accordance with B&W Technical Procedure TP-78,"Tension Testing of Metallic Materials" (Ref. 5). This specification is consistent with ASTM E8 (Ref. 2). These tests were used to determine the bolt material yield strength, ultimate tensile 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 Group S-1473-002 19 Eight 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 of the 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 the uncertainty and data scatter among the #10 specimens, only the data for the #1, #4, and #7 specimens 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 46.5 120-2U4 167.9 122.3 29.2 48.5 120-2U7 168.6 119.6 29.0 38.4 120-2L1 168.1 121.8 29.0 43.7 120-2L4 167.6 122.3 29.4 45.0 120-2L7 168.3 122.6 29.1 39.8 120o-2 Average 167.5 121.5 29.1 43.7 120-6U 1 160.7 116.4 29.9 42.7 120-6U4 163.9 117.0 30.9 44.1 120-6U7 162.5 117.1 30.6 39.7 120-6L1 162.0 117.3 30.2 43.0 120-6L4 163.4 118.3 30.1 43.7 120-6L7 162.0 116.1 29.9 41.6 120V-6 Average 162.4 117.0 30.3 42.5 240-7U1 161.2 115.6 30.3 45.8 240-7U4 160.3 114.6 30.4 43.4 240-7U7 167.4 120.6 30.2 44.5 240-711 164.8 117.7 31.2 40.0 240-71L4 162.8 117.8 29.6 40.5 240-7L7 160.6 115.1 28.9 38.9 240-7 Average 162.9 116.9 30.1 42.2 300-1 U1 162.2 116.5 30.7 39.0 300-1 U4 162.3 116.5 30.6 40.4 300-1 U7 160.2 111.8 31.0 42.6 300-1 Ll 159.3 114.5 29.4 44.8 300-1 L4 164.9 119.1 30.4 36.9 300-1L7 160.6 115.3 31.0 39.9 30V- 1 Average 161.6 115.6 30.5 40.6 B&W Technical Services Group S-1473-002 20 The data was generally consistent among all of the bolts, with the yield strength ranging from -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%. These values 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 the cracking.
These examinations included:
estimating the surface finish, determining the radius of curvature between the head and shank, measuring the angle between the underside 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 low magnification visual examination.
It should be noted that the surface finish was difficult to determine 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 two noted above plus bolt 1800-3 selected at random. None of the radii were perfectly formed or smooth; however, they did not contain unusually sharp transitions or features that would have 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"; bolt 3000-1 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 through Figure 124. Each of these figures includes a semi-transparent circle that represents the best 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 the cracking; therefore, no valid data was obtained.Higher magnification metallographic examinations were performed on the radius between the head and shank of the two mounted specimens.
A three-frame montage was taken of each 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 obvious evidence of surface cold work at any initiation region as previously noted in Section 8.0.
B&W Technical Services Group S-1473-002 21 14.0 DISCUSSION The visual examinations performed on the twelve (12) open cracks indicated a general crack pattern consisting of crack initiation at two diametrically opposing sides of the bolt in the 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 cracks linked together near the center of the bolt. An analysis of the axis of symmetry (i.e. the line drawn between the two opposing initiations) indicated no clear pattern in the direction of the prevailing stresses, either within bolts removed from a particular clevis or between different clevises.
This finding indicated the stresses were variable from bolt to bolt. It also could suggest that stresses redistribute and/or change direction as multiple bolts failed at a particular clevis location.All of the thirteen (13) intact bolts contained cracking in the head/shank transition.
No cracking was identified in the threaded region of any bolt. The amount of angled cracking varied among the bolts, from many angled cracks (e.g. 240'-7) to very minor angled cracking (e.g. 3000-1).Four bolts were selected for destructive examinations.
The bolts were chosen to capture several variables, including intact and broken bolts, bolts with highly angled cracks and bolts with minor angled cracks. The selected bolts originated from three separate clevis locations, 1200, 2400, and 300'. Two bolts were chosen from the 120' clevis, since this location experienced the greatest number of bolt failures (all 8 failed at this location).
High magnification SEM examinations performed on the open cracks indicated the fracture surface was essentially 100% intergranular fracture.
One small area near the center of bolt 1200-2 was mixed mode fracture consisting of transgranular cleavage and ductile microvoid coalescence.
Cross section metallographic examinations confirmed the branched intergranular 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 optical metallography using the dual etch technique confirmed the carbide size was <1 pm, which is smaller than the EDS excitation volume. Consequently, no conclusions could be drawn regarding the carbide type(s), i.e. M 2 3 C 6 or MC, present in the material microstructure.
B&W Technical Services Group S-1473-002 22 The optical metallographic examinations also revealed the discrete, banded nature of the microstructure.
The banded regions contained both carbides and second phase precipitates such as titanium nitrides.
The carbides were generally intragranular within the banded regions. Carbide coverage at the grain boundaries was estimated to be 25% or less. The microstructure contained a duplex grain structure consisting primarily of fine grains (ASTM 7-8), with some very large grains (ASTM 1-2). There was no evidence of surface 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, which indicated uniform microhardness values at the initiation regions, mid-crack, and in the bulk material away from the cracking.
The average microhardness was -400 HV at all locations.
These results match the bulk Rockwell C hardness values, which averaged -40 HRC and were consistent with those expected for Alloy X-750 material.Mechanical testing was performed on several miniature tensile specimens machined from the threaded region of the bolts. The data was generally uniform for all of the bolts, with the yield strength ranging from -115 ksi to -122 ksi and the ultimate tensile strength ranging from -160 ksi to -167 ksi. The average elongation was -30% and the average reduction 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 fracture mode 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 M 2 3 C 6 carbides.
The MC-type carbides are principally titanium carbides, TiC, which are likely trapping sites for hydrogen 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 also be responsible for the IG fracture on the tensile specimens.
However, since the mechanical properties (especially elongation and reduction in area values) were as expected for Alloy X-750 material, it is believed that the presence of IG facets on the fracture surface is not significant.
Chemical analysis results by ICP-MS were within the expected ranges for Alloy X-750 material.
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-MS technique, especially when accounting for additional product tolerances permitted by the material specification (Ref. 3).The laboratory results, including tensile testing, hardness and microhardness testing, metallographic and SEM examinations, and chemical analysis were all consistent with Alloy X-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 IGSCC resistance in a PWR environment (Ref. 1, 6, 7).
B&W Technical Services Group S-1473-002 23 For SCC to occur, three conditions must be present: 1) a susceptible material, 2) tensile stresses, and 3) an environment capable of causing stress corrosion cracking in that material.
Each of these conditions is discussed briefly below: Material The bolt microstructure consisted of a duplex grain structure and also contained discrete bands of carbides and other precipitates.
The duplex structure and banding are both influenced 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 carbides and 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 higher solution annealing temperature, all of the carbides are dissolved and can re-precipitate as M 2 3 C 6 type carbides at the grain boundaries during subsequent aging. An added benefit of the higher temperature solution anneal is grain growth, which reduces the grain boundary volume, thereby increasing resistance to IGSCC.Tensile Stress The prevailing stresses on the bolts were applied tensile stresses.
An analysis of the fracture surfaces indicated the direction of the stresses were variable within each clevis and 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 many angled cracks to very minor angled cracking.
The amount of angled cracking typically decreases as stresses increases.
This suggests that bolts with minor amounts of angled cracking (60'-3, 1200-6, 1800-3, 2400-3, and 300°-1) failed after other bolts at their respective clevis location.
Varying amounts of angled cracking could also indicate the presence 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-step aging 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) rates are orders of magnitudes greater than crack growth rates at high temperatures.
It should be 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). This indicates 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, when the cracks initiated, or how fast they propagated.
B&W Technical Services Group S-1473-002 24
15.0 CONCLUSION
S
- All of the submitted bolts, including those considered to be intact, contained cracking in the head-to-shank transition.
No cracking was identified in the threaded region of any of 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 crack growth that extended upward into the bolt head at a -35' angle relative to the bolt seating surface. The head separated from the shank when the two opposing cracks linked up near the center of the bolt cross section.* For each bolt, crack growth occurred along an axis of symmetry created by the opposing directions 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 and the 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 the fracture 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 those published for Alloy X-750 material.* 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 this manner 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.
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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, and Forging 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 in LWR Environments, EPRI NP-6392-SD, June 1989.7. Mills, W.J., et al, "Hydrogen Embrittlement, Grain Boundary Segregation, and Stress Corrosion Cracking of Alloy X-750 in Low- and High-temperature Water", Metallurgical Transactions A, Volume 30A, June 1999.
B&W Technical Services Group S-1473-002 26 NOTE: BUILDING O0AND REACTOR VESSEL V, ARE DIFFERENT.
60" -120" IN ISO*0*300"' .-" 240"---,EOUIPMENT HATCH___ _ IS LOCATED SOUTH CLEVIS IDENTIFICATION REACTOR VESSEL ORIENTATION Figure 1: Schematic diagram showing the six clevis locations around the vessel circumference.
BOLT I DOWEL PIN LOCATIONS Figure 2: Schematic showing the typical bolt configuration for each clevis insert.
B&W Technical Services Group S-1473-002 27 I B&W Technical Services Group S-1473-002 28 B&W Technical Services Group S-I 473-002 28 laooratory rotationai orentations.
I B&W Technical Services Group S-1473-002 29 B&W Technical Services Group S-1473-002 29 eipt macro pnotograpn Tor o rigure o: meceipt macro pnotograpn Tor DOlt B&W Technical Services Group S-1473-002 30 B&W Technical Services Group S-I 473-002 30 Ftol p-h5otoanK Figure 9: Receipt macro photographs for bolt 00-5.d B&W Technical Services Group S-1473-002 31 B&W Technical Services Group S-I 473-002 31.ceipt macro photograph for bolt 11: Receipt macro photograpl tor B&W Technical Services Group S-1473-002 32 B&W Technical Services Group S-I 473-002 32 eipt macro photograph for bolt B&W Technical Services Group S-1473-002 33 B&W Technical Services Group S-I 473-002 33 macro photograph B&W Technical Services Group S-1473-002 34 FR t mao- p h fnanK Figure 15: Receipt macro photographs for bolt 120°-1.
B&W Technical Services Group S-1473-002 35 B&W Technical Services Group S-I 473-002 35 Figure 16: Receipt macro photographs for bolt 1200-2.
B&W Technical Services Group S-1473-002 36 B&W Technical Services Group S-I 473-002 36 Boit macr-3 5hanK Figure 17: Receipt macro photographs for bolt 120"-3.
B&W Technical Services Group S-1473-002 37 B&W Technical Services Group S-1473-002 37 Fuore t1z--4 m p nanK Figure 18: Receipt macro photographs for bolt 1200-4.
B&W Technical Services Group S-1473-002 38 B&W Technical Services Group S-I 473-0 02 38 bont 120l-5 bnanK Figure 19: Receipt macro photographs for bolt 1200-5.
B&W Technical Services Group S-1473-002 39 B&W Technical Services Group S-I 473-002 39 DUEL 14W -0 011411K Figure 20: Receipt macro photographs for bolt 1200-6.
B&W Technical Services Group S-1473-002 40 B&W Technical Services Group S-I 473-002 40 Figure 21: Receipt macro photograph for bolt 120 laboratory.
to the B&W Technical Services Group S-1473-002 41 F e R t mazuc-r p nanK Figure 22: Receipt macro photographs for bolt 1200-8.
B&W Technical Services Group S-1473-002 42 B&W Technical Services Group S-I 473-002 42 Figure 23: Receipt macro photographs for bolt 1800-1.
B&W Technical Services Group S-1473-002 43 B&W Technical Services Group S-1473-002 44 B&W Technical Services Group S-1473-002 44 IOlt 1UU-fi fnanK Figure 25: Receipt macro photographs for bolt 180-7.
B&W Technical Services Group S-1473-002 45 B&W Technical Services Group S-I 473-002 45 Figure 26: Receipt macro photographs for bolt 180°-8.
B&W Technical Services Group S-1473-002 46 B&W Technical Services Group S-1473-002 47 B&W Technical Services Group S-1473-002 48 B&W Technical Services Group S-1473-002 49 Figure 32: Receipt macro photographs for bolt 3000-3.
B&W Technical Services Group S-1473-002 50 B&W Technical Services Group s-I 473-002 50 bolt .1UU-5 bfanK Figure 33: Receipt macro photographs for bolt 300-5.