ML20071N542
| ML20071N542 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 03/31/1983 |
| From: | Bingham W, Gonzales J, Schmidt G BECHTEL GROUP, INC. |
| To: | |
| Shared Package | |
| ML20071N533 | List: |
| References | |
| 0383-03-FA, 383-3-FA, NUDOCS 8306070107 | |
| Download: ML20071N542 (20) | |
Text
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FAILURE ANALYSIS OF AN ASTM A-354 BOLT
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Report for
- Palo Verde Nuclear Generating Station i
Prepared for W. G. Bingham j l
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! By .
M. C. Gonzales (/ j l
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! Approved G. R. Schmidt l l
Metallurgical Engineering and i
Laboratory Services Group Manager
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Approved ff f Z!I B. D. Hackney "
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Assistant Manager l
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l Materials and Quality Services Department Research and Engineering BECHTEL GROUP, INC.
SAN FRANCISCO Job No. 10407-002 Tech Report No. 0383-03 FA Log No. D131177 BLN No. 283-13
( March 1983 8306070107 830524 PDR 6
ADOCK 05000529 pyg
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ABSTRACT
( This report summarizes Materials & Quality Services Failure Analysis of a fractured bolt sample from the Palo Verde Nuclear Generating Station. The bolt reportedly broke during torquing. The report concludes that the bolt had been manufactured to specification and that the failure was due to overtorquing.
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FAILURE ANALYSIS OF AN ASTM A-354 BOLT CO NTENTS h
ABSTRACT ............................................................... i CONTENTS............................................................... ii LI ST O F I LLU ST RAT IO N S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
- 1. 0 I NT RO D U CT I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 2.0 CO NCLUSIO NS AND RECOMME ND ATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I
! 3.0 MATERIAIS ...................................................... 1 4.0 EVALUATION ..................................................... 1 i
5.0 D ISCUS S ION OF EV ALU ATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
6.0 REFERENCES
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LIST OF ILLUSTRATIONS Tables g I Chemical Analysis of Fractured Bolt ............................. 4 II Hardness Data.................................................... 5 III Knoop Hardness Test Data Close to Thread Surfaces ............... 6 Figures 1 The Fractured Bolt as-received................................... 7 2 The Bolt Showing Pitch Measurements ............................. 8 3 The Bolt as-cut for Chemical Analysis and Examination ........... 9 4 Longitudinal Section of Bolt Showing Stripped Threads ........... 10 5 Microstructure of the Bolt ...................................... 11 6 Typical Inclusion on Bolt Longitudinal Section .................. 12
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7 Mac r ogra ph o f the Frac t ure Face . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 8 SEM Micrograph From One Periphery of Fracture Area Face (A) ..... 14 1
9 Elongated Dimples at Area (C) ................................... 15 i 10 SEM Micrograph of an Axial Secondary Crack or Split Near I the Center of the Fracture at Area D ............................ 16 ,
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1.0 INTRODUCTION
The fractured bolt, ASTM A-354, Grade BD, was received from Palo Verde
( Nuclear Generating Station. It is one of the bolts used inside the containment area to secure a column base to the top of a concrete wall. This bolt was being torqued when it failed. Materials & Quality Services was requested to investigate the cause of the failure.
2.0 CONCLUSION
S AND RECOMMENDATIONS 2.1 Conclusion 2.1.1 The bolt chemistry met the product analysis requirement of ASTM A-354, Grade BD and SAE 4140.
2.1.2 Bolt hardness was within the hardness range of 33 to 38 HRC (Rockwell C) as specified for ASTM A-354, Grade BD.
2.1.3 The bolt had an acceptable quenched and tempered microstructure.
2.1.4 Optical examination at low magnification revealed no signs of prior cracking or metal fatigue.
2.1.5 Scanning Electron Microscope (SEM) examination showed the predominant fracture mode to be ductile shear overload as seen on the fracture periphery, and elongated dimples at various areas of the fracture face.
2.1.6 Torsional loading is evidenced by the smearing of separation faces and the generally clockwise direction of the elongated dimples observed in the SEM.
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2.1.7 The examination concluded that the primary cause of fracture of the bolt was overtorquing.
2.2 RecomLendation Materials & Quality Services recommends investigation of the application procedures of applying torque on the bolts. This may include pressure settings on the torque wrenches being used in the field. It may involve more regular checking of torque settings than previously practiced routines.
3.0 MATERIALS The bolt was said to be Specification ASTM A-354, Grade BD. In this specification, the only elements controlled are the carbon, phosphorus and sulfur. Any alloying elements added to meet the mechanical properties requirements of the specification is left to the manufacturer. The results of the analysis, given in Table I, are in accordance with ASTM A-354, Grade BD.
4.0 EVALUATION The bolt was subjected to the following examination and tests:
(1) Visual examination k
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(2) Bolt extension calculations
( (3) Hardness testing using Rockwell (with 150 Kg load, and "C" Brale -
Rockwell C) and Vickers (Vickers Indenter with 10 Kg and Knoop Indenter with 1 Kg load.)
(4) Meta 11ographic examination using both optical as well as Scanning Electron Microscope (SEM).
(1) Visual Examination. The fractured bolt was 1"9 x 2.4" long as shown in Figure 1. The fracture face was very irregular apparently caused by longitudinal secondary cracks or splits in the center of the bolt section, which happened when the bolt broke. Denting on the nut was observed on only one side of the corners. This is a sign it was not a re-used nut, or that there had been no opposing nut removal force applied. Very obvious permanent elongation of the bolt can be seen in Figure 2 by looking at the thread pitch on each side of the nut.
(2) Bolt Extension. Figure 2 shows where measurements were made and the elongation calculated. Elongation in the fracture area was calculated to be 12%.
(3) Hardne resting. Figure 3 shows the different sections of the bolt taken fo examination or evaluation. Hardness tests revealed the bolt to be within the hardness range of 33 to 38 Rockwell C required by Specification ASTM A-354, Grade BD. Table II presents the hardness data obtained on both longitudinal and transverse sections of the bolt.
To determine the cold working or work hardening effect of torquing the
( bolt, microhardness tests were made close to the surface of the stripped threads. Results revealed an increase in hardness in the cold worked areas due to overtorquing of the nut (Table III).
(4) Meta 11ographic Examination Optical Examinations. Examination of the threads of the bolt af ter removal of the nut revealed the total absence of heat treating scale on the newly exposed surface due to stripping. Figure 4 shows the stripped threads of the bolt. Visual and low magnification examination of the fracture surface revealed no evidence of prior cracking or metal fatigue.
Examination of a mounted and etched section the bolt revealed a typical, 0.4% carbon quenched and tempered martensite microstructure (Ref. 2), Figure 5.
Inclusion content appears to be normal for this type of bolt as can be seen in Figure 6.
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(5) Scanning Electron Microscope (SEM) Examination
( Figure 7 shows the fracture face of the bolt and the letters A, C and D the areas where photographs of significant features were taken. SEM examination revealed that the fracture mode at two opposite peripheries of the fracture face (areas A and C) were by ductile shear overload.
Photographs of these two locations are shown in Figures 8 and 9. The elongated dimples fracture feature is predominant in the entire fracture face, a characteristic feature of ductile shear overload.
Torsional load is evidenced by the orientation of the dimples and smearings at these two locations A and C (Figure 7). The torsion direction was observed to be clockwise and consistent with the load applied during tightening of the bolt. Figure 10 shows an SEM micrograph of the axial secondary crack or split near the center of the bolt. The " woody" texture of the fracture face is due to the ductile rupture around elongated inclusions. This " woody" textured fracture was also found in other secondary cracks or splits in other areas of the fracture face.
5.0 RESULTS AND DISCUSSION OF EXAMINATION The chemical analysis, general hardness data and metallurgical structure of the bolt indicates that the bolt had been manufactured to the requirements of
, ASTM A-354, Grade BD.
The elongation of the bolt and dents on the hexagonal nut on only one side of each of the six corners are two visual signs of overtorquing. The 12%
elongation which is only 2% short of the 14% elongation required for the tensile
( test in ASTM A-354 is an indication that the bolt was overtorqued. The absence of evidence of prior cracking or metal f atigue on the fracture surface is another indirect indication of failure due to overtorquing. Stripping of the threads and an increase in hardness near the stripped threads surface (Table III) are other indications of overtorquing. The total absence of scale (due to heat treating) on the stripped thread surface suggests that the stripping was due to overtorquing and not due to threading system problems.
Elongated dimples characteristic of a ductile shear overload fracture were observed by SEM (Figures 8 and 9) examination all around the periphery of the fracture. This characteristic indicates that the overall bolt fracture was by torsional overload. The orientation of the elongated dimples in the direction of torquing is clear evidence that the failure was by overtorquing rather than by non-torsional overloading. Since torsional overload initiates at the outer 1 surface, the effect of non-metallic inclusions would be negligible, as non-metallic inclusions are mostly found near the center of the bolt.
6.0 REFERENCES
- 1. Failure analysis of an ASTM A-354 Bolt Palo Verde (GRS-061-23/SMY),
July 15, 1081.
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THE I CHEMICAL ANALYSIS OF FRACTURED BOLT
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ASTM A3542 REQUIRED FOR GRADE BD SAE 41402 ELEMENT ,
FRACTURED BOLTI MIN. MAX. MIN. MAX.
Aluminum 0.30%
Carbon 0.40 0. 28 0.55 0. 38 % 0.43%
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Chromium 0.89 0.80 1.10 Cobalt <0.005 Columbium <0.005 Copper 0.10 Iron Rem.
Lead <0.005 Manganese 0.80 0. 75% 1.00%
( Molybdenum 0.20 0. 15 0.25 Nickel 0.06 Phosphorus 0.013 0.040% 0.035 Selenium <0. 005 0.15 0.30 Silicon 0.21 0.20 0.35 Sulfur 0. 0 28 0.045 0.040 Tin 0.006 Titanium <0.005 Vanadium 0.005 Zirconium <0.005 The bolt conforms to ASTM A354, Grade BD and to SAE 4140 composition liuits.
- 1. By Emission Spectrographic Analysis, carbon by Leco combustion process.
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TABLE II HARDNESS DATA
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A. LONGITUDINAL SECTION
- 1. Unaffected End 2. Near Fractured End 3.
Indent EV10 BRCI Indent HV10 BRCI Indent BRC2 No. .(DPH) No. (DPH) No.
1 339 34.5 13 373 38 A 33.0 2 357 35.5 14 394 40 B 35.5
. 3 330 33.5 15 405 41 C 35.0 4 333 34 16 363 37 D 33.5 5 342 35 17 325 33 E 34.5 i 6 319 32 18 360 37 F 33.0 l 7 336 34 19 366 37 C 33.0 33.5 8 325 33 20 354 36 H 9 330 33.5 I 34.5 10 342 35 11 336 34 12 333 34 2
I 13 3 14 g g j 3 H 15 E
' 6 C F Longitudinal Section G
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9 19 10 E 20 11 12 B. TRANSVERSE SECTION Indent HV10 HRCI No. (DPH) 1 37.0 2 1 363 2 366 37.0 3 3 357 36.5 4 4 339 34.5 10 12 325 33.0 5 % ,y,,,, ,,
5 6 333 34.0 11 13 7 351 36.0 6 8 357 36.5 7 36.5 8 9 360 10 348 35.5 9 11 3 54 36.0 12 351 36.0 13 363 37.0 1 By conversion from HV10 to Rockwell C valves based on ASTM -340-77 2 Using Rockwell C indenter and 150 Kg load
TABLE III KNOOP HARDNESS TEST DATA f CLOSE TO THREAD SURFACES 1 2 3 4 5 6 7 0
f H
ti Threads Nos. 5,6,7, J 13,14, and 15 were partially stripped.
B D, G 9 10 11 12 13 14 15 16 17 Indent HK11 RRC2 Remarks A 339 34.5 C.W.3 B 356 35.5 C.W.
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C 345 34.5 C.W. .
I D 317 31.0 N.C.W.4 I
E 305 30.5 a.C.W. l l
F 323 33.0 N.C.W.
I G 317 31.0 N.C.W.
N 332 34.0 C.W.
I 306 30.0 N.C.W.
J 339 34.5 C.W.
1 E 370 37.0 C.W.
L 305 30.5 N.C.W.
1 Knoop Hardness - 1 Eg load 2 Rockwell C by conversion based on ASTN E-140 3 Cold worked 4 Not cold worked
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(a) The fracture face showing longitudinal cracks or splits at the center.
(b) Side view of the bolt.
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Figure 2: The bolt, showing locations of vernier caliper measurements between pitches of the threads of the stretched and unstretched sections.
(a) Thread tips 1 to 3 - fracture (stretched) end (b) Thread tips 4 to 6 - other end untouched by the torquing process Lf = distance between thread tips 1 to 3 Li - distance between thread tips 4 to 5 Measured values Lf = 0.29" Li = 0.26" l
% Elongation = 0.29"-0.26" x 100 = 12%
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Figure 3: The bolt cut to pieces for chemical analysis (1), hardness test, microstructure and inclusion content (2 & 3) and scanning electron microscope (SEM) examination (4).
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Figure 7: A macrograph of the fracture face of the bolt. Note the very irregular fracture plane, following several thread roots and the radial secondary fractures in the center. 4.5x l
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Figure 8: Dual magnification SEM micrograph from the periphery of the fracture in the area (A) in Figure 7. The elongated dimples are characteristic of ductile overload by shear.
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,f Figure 10: Area D (Figure 7) an SEM micrograph of the axial secondary crack or split near the center of the fracture. Arrow points to a non-metallic inclusion: The " woody" texture of the fracture face is due to ductile rupture around elongated inclusions. 180x 1800x l
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