ML15331A168

From kanterella
Revision as of 18:10, 30 June 2018 by StriderTol (talk | contribs) (Created page by program invented by StriderTol)
Jump to navigation Jump to search
Official Exhibit - NYS00488A-00-BD01 - NUREG/CR-7184 (ANL-12/56), Chen, Et Al., Crack Growth Rate and Fracture Toughness Tests on Irradiated Cast Stainless Steels (Revised December 2014) (ML14356A136)
ML15331A168
Person / Time
Site: Indian Point  Entergy icon.png
Issue date: 12/31/2014
From:
State of NY, Office of the Attorney General
To:
Atomic Safety and Licensing Board Panel
SECY RAS
References
RAS 27898, ASLBP 07-858-03-LR-BD01, 50-247-LR, 50-286-LR
Download: ML15331A168 (86)
v • d • e


Text

NUREG/CR-7184 ANL-12/56

Crack Growth Rate and Fracture Toughness Tests on Irradiated Cast Stainless Steels

Y. Chen, B. Alexandreanu, and K. Natesan Nuclear Engineering Division Argonne National Laboratory

A. S. Rao, NRC Project Manager

Draft Manuscript: March 2014 Revised Manuscript: December 2014

iii ABSTRACT Cast austenitic stainless steel, which has a ferrite-austenite duplex microstructure, is used in the cooling system of light water reactors for components with complex shapes, such as pump casings, valve bodies, and coolant piping. In the present study, crack grow th rate and fracture toughness J-R curve tests were performed on irradi ated cast stainless st eels and unirradiated control samples in low-corrosion-potential environments (high-purity water with low dissolved oxygen or simulated pressurized water reactor) at 320°C. Both as-received and thermally aged materials were included to investigate the combined effect of thermal aging and irradiation embrittlement on the fracture behavior of cast stainless steels. The samples were irradiated to approximately 0.08 dpa at the Halden reactor. Good resistance to corrosion fatigue and stress corrosion cracking was observed for all cast stainless steel specimens. Thermal aging had little effect on the crack growth be havior at 0.08 dpa. Cleavage-like fracture was the dominant cracking morphology during the crack growth rate tests, and the ferrite phase was deformed to a lesser extent compared with the surrounding au stenite phase. The fracture toughness results showed a dominant effect of neutron irradiati on, and the fracture resistances were decreased considerably for all cast specimens regardless of their thermal aging. The reduction in fracture toughness was more significant in the unaged than thermally aged specimens. Nonetheless, the fracture toughness values of thermally aged specimens were 20-30% lower than their unaged counterparts, suggesting a combined effect of thermal aging and neutr on irradiation in cast stainless steel.

v FOREWORD Cast austenitic stainless steel (C ASS) is used in the cooling system of light water reactors for components with complex shapes, such as pump casings, valve bodies, coolant piping, control rod guide tubes and housing, etc.

The ferrite phase is vulnerable to thermal aging embrittlement after long-term exposure to reactor coolant. In addition, neutron irradiation can decrease the fracture resistance of CASS significantly. It is suspected that a combined effect of thermal aging and irradiation embrittlement could reduce the fracture resistance even further to a level neither of these degradation mechanisms can impart alone.

In the early 1990s, NRC has conducted research on investigating the long term embrittlement of cast duplex stainless steels under LWR systems. Under those programs, experimental data was collected on the microstructure and the mechani cal properties of cast steels were subjected to commercial and experimental heats as well as reactor aged cast st eels (such as CF-3, CF-8 and CF-8M). The tests concluded that the chemical composition of the st eels and the ferrite morphology strongly affect the kinetics of the embrittlement of steels. For example, while the low carbon cast steels (CF-3) are most resistant to embrittlement, the high carbon cast steels with molybdenum (CF-8M) are the most susceptible to embrittlement. The delta ferrite content of these steels ranged between 3 to 30%

, and the steels containing

> 5% ferrite in the steels have either a lacy or acicular vermicular ferrite morphology. The embrittlement in these steels was found to occur when the failure is dominated by brit tle fracture that is associated with either cleavage of ferrite and or the separation of ferrite/austenite phase boundaries.

While the thermal aging embrittlement of CASS has been studied extensively, there are no data available at present with regard to the synergistic combined effect of thermal aging and irradiation embrittlement. In response to a 2009 request from the Office of Nuclear Regulatory Research, a test program has been initiated to investigate the joint effect of thermal aging and irradiation damage on the cracking suscepti bility and fracture resistance of CASS.

Crack growth rate (CGR) and fr acture toughness J-R curve tests we re conducted on three grades of CASS (CF-3, CF-8 and CF-8M) containing high levels of delta ferrite (>23%). These samples were irradiated at the Halden reactor to a low dose of 0.08 displacements per atom (dpa). Both as-received (labeled as un-aged) and thermally aged specimens were investigated to determine the combined effect of thermal aging and irradiation embrittlement. The CGR tests were conducted in a low dissolved oxygen (DO) water environment or a pressurized water reactor (PWR) environment at 320°C. Following the CGR tests, the fracture toughness - J-R curve tests were performed on the same samples in the test environments.

The CASS materials showed good resistance to both corrosion fatigue and stress corrosion cracking (SCC) at the 0.08 dpa dose level. Tr ans-granular cleavage-like cracking was the dominant fracture mode during the CGR tests, and it was found that the extent of deformation of ferrite phase was significantly lesser than the surrounding austenite phase of the steel. The neutron irradiation even at low dose of 0.08 dpa significantly affected on the fracture toughness of both aged and un-aged CASS. The extent of irradiation-induced embrittlement is more evident in the un-aged than aged CASS alloys.

vi Brian Thomas, Director (Acting)

Division of Engineering Office of Nuclear Re gulatory Research U.S. Nuclear Regulatory Commission

vii TABLE OF CONTENTS

Abstract .................................................................................................................................... iii Foreword .................................................................................................................................. v Figures...................................................................................................................................... ix Tables ....................................................................................................................................... xiii Executive Summary ................................................................................................................. xv Acknowledgments.................................................................................................................... xvii Acronyms and Abbreviations ..................................................................................................

xix 1 Introduction .......................................................................................................................... 1 2 Experimental ........................................................................................................................ 5 2.1 Materials and Specimens .......................................................................................... 5 2.2 Irradiation...............................................................................................................

... 7 2.3 Test Facility ............................................................................................................

.. 9 2.4 Crack Growth Rate and Fracture Toughness J-R curve Tests .................................. 10 2.4.1 Crack Growth Rate Test ............................................................................... 10 2.4.2 Fracture Toughness J-R curve Test .............................................................. 12 2.4.3 Fractographic Examination ........................................................................... 14 3 Results .................................................................................................................................. 15 3.1 CF-3 Cast Stainless Steel .......................................................................................... 15 3.1.1 Unaged CF-3 CASS ...................................................................................... 15 3.1.2 Thermally Aged CF-3 CASS ........................................................................ 44 3.2 CF-8 Cast Stainless Steel .......................................................................................... 67 3.2.1 Unaged CF-8 CASS ...................................................................................... 67 3.2.2 Thermally Aged CF-8 CASS ........................................................................ 89 3.3 CF-8M Stainless Steel ............................................................................................... 111 3.3.1 Unaged CF-8M CASS .................................................................................. 111 3.3.2 Thermally Aged CF-8M CASS .................................................................... 123 4 Discussion ....................................................................................................................... 135 4.1 Cyclic Crack Growth Rates ...................................................................................... 136 4.2 Constant-load Crack Growth Rates .......................................................................... 139 4.3 Fracture Toughness ................................................................................................... 141 5 Summary ......................................................................................................................... 145 References ................................................................................................................................ 147

ix LIST OF FIGURES 1. Metallurgical images of the unaged and thermally aged CASS materials. ........................... 6

2. Schematic of 1/4T-CT specimen used in this study (red lines represent electrical leads). ... 7
3. Irradiation capsule of low-dose Halden irradiation. .............................................................. 8
4. Recirculation water loop for Cell 1 of the IML (items in red are safety significant components) ...................................................................................................................

..... 10 5. Schematic for inducing environmentally assisted cracking in test environment. ................. 12

6. Crack-length-vs.-time plot for specimen A-N1 (unirradiated and unaged CF-3 with 24%

ferrite): test periods (a) a-c, (b) d-j, (c) k-p, (d) q-u, (e) v-aa, and (f) 1. ............................. 17

7. Cyclic CGRs of specimen A-N1. .......................................................................................... 19
8. The J-R curve for specimen A-N1. ....................................................................................... 20
9. Fracture surface of specimen A-N1 tested in low-DO high-purity water. ........................... 21
10. Fracture surface of specimen A-N1 along the sample central line. ..................................... 22
11. Transgranular fracture in the precracking region of specimen A-N1. ............................... 23
12. Ductile dimple fracture in JR test region of specimen A-N1. ............................................ 24
13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% ferrite):

test periods (a) a-e, (b) f-j, (c) k-m, and (d) 1. .................................................................... 26

14. Cyclic CGRs of specimen A-1. ........................................................................................... 2 8 15. Fracture surface of specimen A-1 tested in PWR water..................................................... 29
16. Fracture surface of specimen A-1 along the sample central line. ........................................ 30
17. Cleavage-like fracture in specimen A-1: (a

) pre-cracking, and (b) end of the CGR test. Crack propagation from bottom to top............................................................................... 31

18. Fracture surface of specimen A-1 showing that delta ferrite deformed to a lesser extent than austenite. Crack propagation from bottom to top. .................................................... 33
19. Crack-length-vs.-time plot for specimen A-2 (0.08-dpa unaged CF-3 with 24% ferrite) tested in low-DO high-purity water environment: test periods (a) a-e, (b) f-j, (c) k-o, (d) p-s, and (e) 1-2. .............................................................................................................

...... 35

20. Cyclic CGRs of specimen A-2. ........................................................................................... 3 7 21. The J-R curve for specimen A-2. ........................................................................................ 3 8 22. Fracture surface of specimen A-2 tested in low-DO high-purity water. ............................ 39
23. Fracture surface of specimen A-2 along the sample central line. ........................................ 40
24. Cleavage-like steps at the beginning of CGR test of specimen A-2. Crack propagation from bottom to top. ............................................................................................................ 41 25. Fracture surface of specimen A-2 showing that ferrite deformed to a lesser extent than austenite. Crack propagation from bottom to top. ............................................................ 42
26. Ductile dimple morphology in the JR test region of specimen A-2. Crack propagation from bottom to top. ............................................................................................................ 43 27. Crack-length-vs.-time plot for specimen B-N1 (unirradiated, aged CF-3 with 24% ferrite) in PWR water: test pe riods (a) a-d, (b) e-h, (c) i-n, (d

) o-s, (e) t-w, (f) x-aa, and (g) 1a-1b. ....................................................................................................................

......... 45

28. Cyclic CGRs of specimen B-N1. ........................................................................................ 48
29. The J-R curve for specimen B-N1. ...................................................................................... 48 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

x 30. Fracture surface of specimen B-N1 tested in PWR water. ................................................. 50

31. Fracture surface of specimen B-N1 along the sample central line. ..................................... 51
32. Transgranular fracture in specimen B-N1 at the beginning of the precracking. Crack propagation from bottom to top. ........................................................................................ 52
33. Transgranular fracture in specimen B-N1 at the end of the precracking. Crack propagation from bottom to top. ........................................................................................ 53
34. Ductile dimple fracture in the JR test region of specimen B-N1. Crack propagation from bottom to top. ............................................................................................................ 54 35. Crack-length-vs.-time plot for specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water: test periods (a) a-e, (b) f-k, (c) l-u, (d) v-y, (e) z-ac, (f) ad-ag, (g) ah-al, and (h) am-1. .................................................................................................................

...... 57

36. Cyclic CGRs of specimen B-1. ........................................................................................... 6 0 37. SCC CGRs of unaged and thermally aged CF-3 with 24% ferrite. ..................................... 60
38. The J-R curve for specimen B-1. .........................................................................................

61 39. Fracture surface of specimen B-1 tested in PWR water. .................................................... 62

40. Fracture surface of specimen B-1 along the sample central line. ........................................ 63
41. Deformation steps in the pre-cracking region of specimen B-1. Crack propagation from bottom to top. ..................................................................................................................... 64
42. Delta ferrite at dendrite cores in specimen B-1. Crack propagation from bottom to top. . 65
43. Fracture surface of specimen B-1 showing that delta ferrite is surrounded by heavily deformed austenite phase. Crack propagation from bottom to top. .................................. 66
44. Crack-length-vs.-time plot for specimen E-N1 (unirradiated, unaged CF-8 with 23%

ferrite): test periods (a) a-c, (b) d-f, (c) g-i, (d) j-m, (e) n-q, (f) r-u, and (g) 1a-1b. ........... 68

45. Cyclic CGRs of specimen E-N1. ......................................................................................... 71
46. J-R curve data of specimen E-N1. ....................................................................................... 72
47. Fracture surface of specimen E-N1 tested in low-DO high-purity water. .......................... 73
48. Transgranular fracture at the beginning of the precracking for specimen E-N1. Crack propagation from bottom to top. ........................................................................................ 74
49. Transgranular fracture at the end of the CGR test for specimen E-N1. Crack propagation from bottom to top. ........................................................................................ 75
50. Ductile dimple fracture in the JR test region of specimen E-N1. Crack propagation from bottom to top. ............................................................................................................ 77 51. Crack-length-vs.-time plot for specimen E-1 (0.08-dpa unaged CF-8 with 23% ferrite):

test periods (a) a-d, (b) e-f, (c) g-j, (d) k-o, and (e) p-2. ..................................................... 79

52. Cyclic CGRs of specimen E-1. ............................................................................................

81 53. The J-R curve of specimen E-1. ..........................................................................................

82 54. Fracture surface of specimen E-1 tested in low-DO high-purity water. ............................ 83

55. Fracture surface of specimen E-1 along the sample central line. ........................................ 84
56. Cleavage-like cracking at the beginning of the CGR test of specimen E-1. Crack propagation from bottom to top. ........................................................................................ 85
57. Cyclic CGR test region of specimen E-1. Crack propagation from bottom to top. .......... 86 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xi 58. Smooth fracture surface at the end of the CGR test in specimen E-1. Crack propagation from bottom to top. ............................................................................................................ 87 59. Ductile dimple fracture in the J-R test region of specimen E-1. Crack propagation from bottom to top. ..................................................................................................................... 88

60. Crack-length-vs.-time plot for specimen F-N1 (unirradiated, aged CF-8 with 23%

ferrite): test periods (a) a-d, (b) e-h, (c) i-m, (d) n-q, and (e) 1a-1b. .................................. 90

61. Cyclic CGRs of specimen F-N1. ......................................................................................... 92
62. The J-R curve of specimen F-N1. ....................................................................................... 93
63. Fracture surface of specimen F-N1 tested in low-DO high-purity water. .......................... 94
64. Fracture surface of specimen F-N1 along the sample central line. ..................................... 95
65. Transgranular fracture in the CGR test of specimen F-N1: (a) in the precracking region and (b) at the end of CGR test. Crack advance direction from bottom to top. ................ 96
66. Transition region from CGR to J-R curve tests of specimen F-N1. Crack advance direction from bottom to top. ............................................................................................. 98
67. Ductile dimple fracture in the JR test region of specimen F-N1. Crack advance direction from bottom to top. ............................................................................................. 10 0 68. Crack-length-vs.-time plot for specimen F-1 (0.08-dpa aged CF-8 with 23% ferrite): test periods (a) a-f, (b) g-j, (c) k-m, and (d) 1. .................................................................... 102
69. Cyclic CGRs of specimen F-1. ............................................................................................

103 70. SCC CGRs of unaged and aged CF-8 CASS with 23% ferrite. .......................................... 104

71. The J-R curve of specimen F-1. ..........................................................................................

105 72. Fracture surface of specimen F-1 tested in low-DO high-purity water. ............................. 106

73. Fracture surface of specimen F-1 along the sample central line. ........................................ 107
74. Fracture surface of the CGR region in specimen F-1. Crack propagation from bottom to top. .......................................................................................................................

........... 108

75. Deformation steps in austenite grain ar ound ferrite phase in the CGR test region of specimen F-1. Crack propagation from bottom to top. ..................................................... 109
76. Dimple fracture in the JR test region of specimen F-1. Crack propagation from bottom to top. ................................................................................................................................. 110
77. Crack-length-vs.-time plot for specime n I-1 (0.08-dpa unaged CF-8M with 28% ferrite): test periods (a) a-f, (b) g-k, (c) l-o, (d) p-r, (e) s-v, (f) w-z, (g) aa-ac, and (h) 1. .. 113
78. Cyclic CGRs of specimen I-1. .............................................................................................

116 79. The J-R curve of specimen I-1. ...........................................................................................

116 80. Fracture surface of specimen I-1 tested in low-DO high-purity water. .............................. 118

81. Fracture surface of specimen I-1 along the sample central line. ......................................... 119
82. Precracking region in the CGR test of specimen I-1. Crack propagation from bottom to top. ..................................................................................................................................... 120
83. Fracture surface at the end of CGR test of specimen I-1. Crack propagation from bottom to top. ..................................................................................................................... 121
84. Heavily deformed micros tructure in the JR test region of specimen I-1. ........................... 122
85. Crack-length-vs.-time plot for specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite):

test periods (a) a-g, (b) h-n, (c) o-r, (d) s-u, (e) 1a-1b, and (f) 2a-2c. ................................. 124 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xii 86. Cyclic CGRs of specimen J-1. ............................................................................................

127 87. SCC CGRs of unaged and aged CF-8M CASS, irradiated to 0.08 dpa. ............................. 127

88. The J-R curve of specimen J-1. ...........................................................................................

128 89. Fracture surface of specimen J-1 tested in low-DO high-purity water. ............................. 129

90. Fracture surface of specimen J-1 along the sample central line. ......................................... 130
91. Precracking region of specimen J-1. Crack propagation from bottom to top. .................. 131
92. Ferrite microstructure at the end of CGR test of specimen J-1. Crack propagation from bottom to top. ..................................................................................................................... 132
93. Cleavage-like fracture at the end of CGR test of specimen J-1. Crack propagation from bottom to top. ..................................................................................................................... 133
94. Fracture along ferrite at dendrite core in the JR test region of specimen J-1. Crack propagation from bottom to top. ........................................................................................ 134
95. Best-fit curves of cyclic CGRs at 0.08-dpa dose: (a) unage d and aged CF-3, (b) unaged and aged CF-8, and (c) unaged and aged CF-8M. .............................................................. 137
96. Fitting coefficient A for the corrosion fatigue superposition model. .................................. 139
97. Constant-load CGRs of the low-dose CASS with more than 23% ferrite in low-DO high-purity and PWR water environments. ........................................................................ 140
98. Fracture toughness values of unirradiated and irradiated CASS in unaged and aged conditions. Note that most of the results are from 1/4T-CT specimens tested at ~320°C in water environments. The unirradiated results for CF-8M CASS are from 1T-CT specimens tested at ~290°C in an air atmosphere. .............................................................. 142 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xiii LIST OF TABLES 1. Chemical compositions of the cast stainless steels examined in this study. ....................... 5

2. Thermal aging conditions for the cast stainless steels in this study. ................................... 5
3. Test matrix of unirradiated and irradiated low-dose CASS specimens. ............................. 15
4. Crack growth rates of specimen A-N1 (unirradiated and unaged CF-3 with 24% ferrite) in low-DO high-purity water. ............................................................................. 16
5. Crack growth rates of specimen A-1 (0.08-dpa unaged CF-3 sample with 24% ferrite) in PWR water. ..................................................................................................... 2 5 6. Crack growth rates of specimen A-2 (0.08-dpa unaged CF-3 with 24% ferrite) in low-DO high-purity water environment. ........................................................................... 34
7. CGR test of specimen B-N1 (unirradiated, thermally aged CF-3 with 24% ferrite) in PWR water. .................................................................................................................... 44 8. CGR test of specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water. ........ 56
9. CGR test of specimen E-N1 (unirradiated, unaged CF-8 with 23% ferrite) in low-DO high-purity water. ........................................................................................................

67 10. CGR test of specimen E-1 (0.08-dpa unaged CF-8 with 23% ferrite) in low-DO high-purity water. ............................................................................................................... 78 11. Crack growth rates of specimen F-N1 (unirradiated, aged CF-8 with 23% ferrite) in a low-DO high-purity water environment. .................................................................... 89

12. Crack growth rates of specimen F-1 (0.08-dpa aged CF-8 with 23% ferrite) in a low-DO high-purity water environment. ........................................................................... 101
13. Crack growth rates of specimen I-1 (0.08-dpa unaged CF-8M with 28% ferrite) in a low-DO high-purity water environment.......................................................................... 112
14. Crack growth rates of specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite) in a low-DO high-purity water environment. ........................................................................... 123
15. CGR test results at ~320°C for CASS specimens with high ferrite contents. ................... 135
16. Fracture toughness JR test results for CASS with high ferrite contents. ........................... 136 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xv EXECUTIVE SUMMARY Cast austenitic stainless steel (C ASS) is used in the cooling system of light water reactors for components with complex shapes, such as pum p casings, valve bodies, coolant piping, and control rod guide tube spacers. In contrast to a fully austenitic microstructure of wrought stainless steel (SS), CASS consists of a ferrite-austenite duplex microstructure. A certain amount of delta ferrite phase is intentionally designed into CASS and SS weld metals to engineer against hot cracking. However, the ferrite phase is vulnerable to thermal aging embrittlement after long-term exposure to reactor coolant. In addition, neutron irradi ation can decrease the fracture resistance of CASS significantly. It is suspected that a combined effect of thermal aging and irradiation embrittlement could reduce the fracture resistance even further to a level neither of these degradation mechanisms can impart alone. While the thermal aging embrittlement of CASS has been studied extensively, no data are available at present with regard to the combined effect of thermal aging and irradiation embrittlement. A test program has been initiated to investigate the joint effect of thermal aging and irradiation damage on the cracking susceptibility and fracture resistance of CASS.

Crack growth rate (CGR) and fracture toughness J-integral resi stance (J-R) curve tests were conducted on three grades of CASS (CF-3, CF-8, and CF-8M) containing high levels of delta ferrite (>23%). The samples were irradiated at the Halden reactor to a low dose of 0.08 displacements per atom (dpa) or 5.56 x 10 19 n/cm2 (E > 1 MeV). Both as-received and thermally aged specimens were included to determine the combined effect of thermal aging and irradiation embrittlement. The CGR tests were conducted at ~320°C on the irradiated and unirradiated control samples in a high purity water envir onment with low dissolved oxygen (DO) or a pressurized water reactor (PWR) environment. Following the CGR tests, the fracture toughness J-R curve tests were performed on the same samples in the test environments.

Cyclic CGRs and constant-load CGRs were measured to evaluate the corrosion fatigue and stress corrosion cracking (SCC) of the CASS specimens. In the cyclic CGR tests, environmental enhanced cracking was more difficu lt to establish in the CASS specimens than in wrought SSs. In SCC CGR tests, only moderate CGRs, in the range of 10

-11 m/s, were recorded in most of the CASS specimens regardless of their thermal agin g history. In general, the CASS materials showed good resistance to both co rrosion fatigue and SCC without irradiation and at 0.08 dpa. Transgranular cleavage-like cracking was the dominant fracture mode during the CGR tests, and the ferrite phase was often deformed to a less extent compared with its surrounding austenite phase. This observation supports the hypothesis that the beneficial effect of ferrite for SCC resistance arises, in part, from the low plastic deformation of ferrite phase compared to austenite.

A previous study showed that the SCC CGRs of thermally aged CASS were one order of magnitude higher than those of unaged materials at high DO concentrations (>1 ppm). In contrast, a similar cracking behavior between thermally aged and unaged materials was observed for irradiated and unirradiated control specimens in the low-DO high-purity water and simulated PWR water. The lack of sensitivity to thermal aging history in these tests might be a result of the low-corrosion-potential environments employed. The effect of neutron irradiation on the SCC behaviors of CASS materials was not clear in the pr esent study.

xvi Neutron irradiation did significan tly affect the fracture toughn ess of CASS. At 0.08 dpa, the fracture toughness values of unaged specimens were much lower than thei r initial unirradiated values. Fracture toughness was also reduced by 20-30% for thermally aged specimens after irradiation. This observation s uggests an interaction between thermal aging and irradiation damage. When both conditions of thermal aging and irradiation damage are present, the combined effect is not a simple addition of bot h degradations. The interaction between thermal aging and irradiation damage can lead to different microstructural evolutions in CASS materials (viz. by prompting G-phase formation through radiation-induced segregations of minor elements), reducing the fracture resistance to a higher extent than either one of them can achieve alone. Neutron irradiation appears to affect not only the kinetics of thermal aging embrittlement, but also the lower bound values of fracture toughness (i.e., the satu ration state). For this reason, the effects of neutron irradiation should be considered when the degree of thermal aging embrittlement is being evaluated for CA SS components.

xvii ACKNOWLEDGEMENTS The authors would like to thank Drs. O. K. Chopra and W. J. Shack for their invaluable contribution to this project. Our special tha nks go out to Ms. T. M. Karlsen, OECD Halden Reactor Project, Halden, Norway, for reactor irradiation experiments and sample transfer. L. A.

Knoblich, E. E. Gruber, R. Clark, and E. J. Listwan of Argonne, Y. Yang of University of Florida, and J. Pakarinen and Y. Huang of University of Wi sconsin-Madison are acknowledged for their contributions to the experimental effort. We are also gr ateful to Drs. R. Tregoning and W. J. Shack for their careful reviews and comments on the manuscript. This work is sponsored by the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, under Job Code N6519 and V6454; Program Manager: A. S. Rao.

xix ACRONYMS AND ABBREVATIONS ASTM American Society for Testing and Materials BWR Boiling Water Reactor CASS Cast Austenitic Stainless Steels CGR Crack Growth Rate CT Compact Tension DCPD Direct Current Potential Drop DO Dissolved Oxygen ECP Electrochemical Potential HWC Hydrogen Water Chemistry IASCC Irradiation-Assisted Stress Corrosion Cracking IGSCC Intergranular Stress Corrosion Cracking IML Irradiated materials Laboratory J-R J-Integral Resistance LVDT Linear Voltage Displacement Transducer LWR Light Water Reactor NWC Normal Water Chemistry PPU Periodical Partial Unloading PWR Pressurized Water Reactor SCC Stress Corrosion Cracking SEM Scanning Electron Microscopy SS Stainless Steel

TG Transgranular

xx Units of Measure

°C degrees centigrade cm centimeter dpa displacement per atom hr hour J joule m meter mm millimeter MPa megapascal ppb part per billion ppm part per million psig pound per square inch gauge s second micron 1 1 Introduction Stainless steel is an important class of engineering materials used in light water reactors (LWRs). While the main structure of reactor core internals is constructed from wr ought stainless steels (SSs), components with complex shapes, such as pump casings, valve bodies, coolant piping, elbows, and control rod guide tube spacers, are often made of cast austenitic stainless steels (CASS).11 The CF grades of CASS whose compositions are similar to those of 300-series austenitic SSs are the most widely used corrosion-resistant CASS. The CF-3 and CF-8 grades contain nominal 19% Cr and 9% Ni and are th e cast equivalents of Types 304L and 304 SSs, respectively. In addition, the CF-3M and CF-8M grades are the molybdenum-containing cast versions of 316L and 316 SSs, respectively. Like their wrought equivalents, the CF grades of CASS possess excellent corrosion resistance and mechanical properties, 22 and, thus, are ideal for LWR applications in aqueous environments. The strength and ductility of CF grades are comparable to those of wrought SSs.

At room temperature, the yiel d and tensile strengths of CF-3 and CF-8 grades are greater than 200 MPa and 480 MPa, 33 respectively, similar to those of solution-annealed SSs. A good combination of strength and ductility also gives rise to excellent fracture toughness. Stable tearing occurs in CASS samples well above thei r yield strength before final fracture. Chopra and Sather 44 showed that the crack initia tion toughness of CF-3 and CF-8 CASS varies from ~200 kJ/m 2 to over 1000 kJ/m 2 at room temperature; these values are comparable to those of wrought SSs reported by Mills.

55 In contrast to the fully austenitic microstructu re of wrought SSs, CASS consists of a ferrite-austenite duplex microstructure. It is common that a certain amount of ferrite is present along with austenite () in the solidification microstructure such as CASS or weld metals. The precise fractions of ferrite and austenite phases depend on the chemical composition and casting thermal history. Experimental methods, such as quantitative metallography and ferromagnetic measurement, can be used to determine the ferrite content in CASS or weld metals.

66,77 Empirical models have also been developed to predict and control ph ase content with alloy composition. Based on the phase-stabilizing effects of Cr and Ni in Fe-Cr-Ni systems, several constitution diagrams have been established to estimate phase content.

88,99,1010 The contributions of minor alloying elements are inco rporated with Cr and Ni equivalent numbers and computed with empirical equations. The applicable range of composition, incorporated alloying elements, and weighing factors vary in these models, and thus the predicted phase contents differ among them to some extent.1111 For steel castings of CF grades, use of the Scheofer diagram, which is a modified version of the Schaeffler diagram 88, is recommended by the American Society for Testing and Materials (ASTM) for estimating ferrite content.1212 The ferrite phase is critical for the mechanical properties and corrosion resistance of CASS and weld metals. Since hardening mechanisms by thermal-mechanical treatments cannot be easily implemented in castings, the strength of CASS mainly relies on the ferrite-austenite duplex microstructure. Beck et al. 1313 showed that the tensile and yiel d strengths of CASS increase with ferrite content up to 40% at both room and elevated temperatures. The strengthening effect of ferrite phase is attri buted to ferrite-austenite boundaries and can be explained with the Hall-Petch model.

1414 Ferrite phase is also crucial fo r the soundness and weld ability of steel castings. A minimum ferrite content is often specified for SS welds to reduce the tendency of hot cracking. In addition, the presence of ferrite phase can improve the resistance to Formatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormattegrammar 2 sensitization and stress corrosion cracking (SCC).1515 In susceptible environments, CASS tends to be more resistant to SCC than the same grade of wrought SS. This benefi cial effect of ferrite was clearly demonstrated by Hughes et al.1616 in boiling water reactor (BWR) environments.

Using slow strain rate tests, they showed that CF-3, CF-3A, and CF-8 have an exceptional resistance to intergranular SCC (IGSCC) in hi gh-purity (HP) water containing 6-8 ppm dissolved oxygen (DO).

While the presence of ferrite in CASS is mostly benefici al, ferrite phase can also exert a detrimental effect on the fracture resistance of CASS under certain conditions. Exposed to temperatures of 300-500°C, ferrite phase is vulnerable to a low-temperature thermal aging phenomenon known as "475°C embrittlement." 1717,1818 The consequences of the thermal aging embrittlement are an increased te nsile strength and reduced ductility.1919,2020 The upper-shelf impact energy is also reduced, and the ductile-to-brittle transition temperature shifts higher.2020 Because of thermal aging embrittlement, the long-term performance of CASS materials at elevated service temperatures is of concern. The degradation of CASS components resulting from thermal aging embrittlement has been recognized as a potential issue for aging reactors.2121 Several research programs have been conducted to assess Charpy impact properties and JR resistance curves of thermally aged CASS.

2222,2323 It was found that thermal aging at 290-450°C up to 30,000 hr leads to a significant deterior ation in the fracture properties of CASS. The lower bound of impact energy and fracture toughness (J IC) can be as low as 20 J/cm 2 and 25 kJ/m2, respectively, at room temperature. The duct ile-to-brittle transition temperature of CASS is also shifted to around 0°C for the thermally aged CASS. The extent of the thermal aging embrittlement increases with ferrite content and is sensitive to ferrite morphology.2222 The mechanism of thermal aging of dupl ex SSs has been studied extensively.1717-2020, 2222-2828 It is widely accepted that embrittlement is caused by the instability of the ferrite phase under thermal aging. The main reasons of the hardening and loss of toughness are (1) the formation of Cr-rich ' phase through spinodal decomposition, and (2) the precipitation and growth of carbides and G-phase at ferrite-a ustenite phase boundaries. Obviously, these microstructural changes are thermally activated and are fundame ntally controlled by solid-state diffusion. Therefore, the thermal aging time for a given exte nt of degradation (e.g., an increase in hardness or decrease in toughness) follows an Arrhenius-type relationship.2424 Besides thermal aging, neutron irradiation can also affect the micr ostructural evolution of CASS profoundly. Under fast neutron bombardment, lattice atoms in a crystalline material are displaced from their original sites by cascade damage. An avalanche of lattice displacements gives rise to point defect supersaturation, which does not exist under thermal equilibrium.2929,3030 These point defects evolve at irradiation temperatures to form irra diation defects, giving rise to irradiation hardening and embrittlement. The irradiation embrittlement can generate further degradation in the ferrite phase, leading to an addition al loss of fracture toughness. Furthermore, the presence of nonequilibrium point defects in irradiated microstructures can enhance the transportation of solutes in materials by radiation-enhanced diffusion.3030 The elevated diffusivity under neutron irradiation could certainly affect the kinetics of thermal aging.

Formatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gram 3 Thus, a combined effect of irradiation embrittlement and thermal aging could not only produce a higher degree of embrittlement, but also affect the rate of degradation development.

While the thermal aging embrittlement of CASS has been studied extensively, very limited data exist in the open literature for neutron-irradiated CASS.

55,3131,3232 Two tests conducted on thermally aged and irradiated CF-8M showed a higher degree of embrittlement in the CASS material than wrought SSs.3131 It is not clear, however, if the simultaneous exposure to irradiation and thermal aging would reduce the fracture resistance to a lower level than either of the degradation mechanisms can impart alone. If so, the combined effect is not only important for internal components made of CASS, but also for SS weld metals that possess a similar austenite-ferrite duplex microstructure. While weld metals may contain less ferrite phase than that in CASS materials, a minimum ferrite content is usually specified for weld metals to engineer against hot cracking. Thus, weld metals may be subjected to the same type of degradation as CASS. A better understanding of the combined effect of thermal aging and irradiation would also be helpful to address issues c oncerning SS weld metals.

In the current study, several CF grades of CASS were irradiated to approximately 0.08 displacements per atom (dpa). Both as-received and thermally aged specimens are included.

Fracture toughness J-integral resistance (J-R) curve tests were conducted on the irradiated and unirradiated control samples to assess the extent of embrittlement resulting from neutron irradiation and thermal aging. Additionally, crack growth rate (CGR) tests were carried out prior to the fracture toughness tests in low electrochemical potential (ECP) environments to evaluate their SCC performance. At 0.08 dpa, irradiation embrittlement of ferrite phase is anticipated to some extent, but little effect either on embrittlement or stress corrosion susceptibility would be expected for austenite phase. Thus, the current work is focused on the effects of thermal aging and irradiation on the ferrite phase. At higher irradiation levels, additional effect would occur due to embrittlement and increased SCC susceptibility of austenite phase. Since elevated susceptibility to irradiation-assisted stress corrosion cracking (IASCC) is unlikely at this low dose, the SCC CGR test durations were kept relatively short in th e present study. Still, the CGR tests could provide corrosion fa tigue starter cracks for the su bsequent fracture toughness J-R curve tests, so that any environmental contribut ion to the fracture behavior of CASS could be detected.

Formatteand gramFormatteand gramFormatteand gramFormatteand gram

5 2 Experimental 2.1 Materials and Specimens Three experimental heats of corrosion-resistant CASS (CF-3, CF-8, and CF-8M) were obtained from a previous ANL research program.

44 The CASS heats are static cast slabs with dimensions of 610 x 610 x 76 mm. A ferrite scope measurement on these heats showed that the CF-3 and CF-8 heats contained approxima tely 24% and 23% delta ferrite, respectively, and the CF-8M heat had 28% delta ferrite. Table 1 gives the compositions of these materials. Both as-received (or unaged) and thermally aged specimens were prepared from these heats. The thermal aging conditions are given in Table 2. Figure 1 shows metallurgical images of the CASS materials used in this study. The samples were polished wi th SiC papers up to 1200 grit. After a final finish with 1 m diamond slurry, the polished surfaces were etched with ferric chloride. Both the unaged and thermally aged microstructures are similar, as shown in Fig. 1. Note that a slight decline in ferrite content was reported for all grades of CASS after thermal aging.

44 For the aging conditions used in the current study (10,000 hr at 400°C), the decrease in ferrite content is insignificant. Thus, the average ferrite contents were considered unchanged after thermal aging.

Table 1. Chemical compositions of the cast stainless steels examined in this study.

Cast Grade FerriteContent Heat ID Composition (wt. %) Measured a Calculated b Mn Si P S Mo Cr Ni N C CF-3 24% 21% 69 0.631.130.0150.0050.3420.18 8.59 0.028 0.023CF-8 23% 14% 68 0.641.070.0210.0140.3120.46 8.08 0.062 0.063CF-8M 28% 25% 75 0.530.670.0220.0122.5820.86 9.12 0.052 0.065

a. Measured with a ferrite scope,Ref. [

44]. b. Calculated with Hull's equations, Ref. [4]

Table 2. Thermal aging conditions for th e cast stainless steels in this study. Cast Grade Ferrite a Spec. Code Heat ID Thermal Aging Condition CF-3 24% A 69 Unaged B 69 10,000 hr at 400°C CF-8 23% E 68 Unaged F 68 10,000 hr at 400°C CF-8M 28% I 75 Unaged J 75 10,000 hr at 400°C

a. Measured with a ferrite scope,Ref. [

44].

Formatteand gramFormatteand gramField CoFormatteand gramField CoFormatteand gram 6 CF-3, 24% unaged CF-3, 24% , aged CF-8, 23% unaged CF-8, 23% aged CF-8M, 28% unaged (from ref. [

44] ) CF-8M, 28% aged Figure 1. Metallurgical images of the unaged and thermally aged CASS materials.

FormatteField Co 7 Constrained by irradiation space, sub-sized compact tension (CT) specimens were used in this study. The sample was about 6.5-mm thick (i.e., 1/4T-CT) and 14-mm high. The starter notch size was about 6 mm. To ensure an in-plane crack growth, side grooves approximately 5% of the thickness were machined on both sides of the sample. Figure 2 is a schematic of the sample used in this study. The red lines in the figure are electrical leads spot-welded on the specimen.

Figure 2. Schematic of 1/4T-CT specimen used in this study (red lines represent electrical leads).

2.2 Irradiation The CASS specimens were irradiated in a helium-filled capsule in the Halden reactor, a boiling heavy water reactor in Norway. Figure 3 shows the assembly of the irradiation capsule. Some disk-shaped samples for transmission electron microscopy analyses were also included in a cylindrical container in the irradiation capsule (item 7 in Fig. 3). The irradiation temperature was ~315°C, and two sets of melting alloy temperature monitors (item 5) were installed in the irradiation assembly.

During the irradiation experiment

, three fluence monitor wires (F e, Ni, and Al/Co alloy) were placed outside the irradiation capsule. After the irradiation, dosimetry was performed by the Halden researchers on these wires. Using the activation cross sections determined previously, they estimated the accumulated neutron fluence fo r the irradiation capsule. The obtained fast neutron fluence (E > 1MeV) was about 5.56 x 10 19 n/cm2, which corresponds to a displacement damage of 0.08 dpa for the samples.

Field Co 8

Figure 3. Irradiation capsule of low-dose Halden irradiation.

3333 4. End Guide Sleeve

8. Spacer 3. Heat Transfer Body
7. TEM Specimens Containe r 2. Outer Capsule
6. CT Specimens
1. End Plug
5. Melting Alloy Assembl y

Field CoFormattept, Checkby 3 pt 9 2.3 Test Facility Two servo-hydraulic mechanical test systems located in the Irradiated Materials Laboratory (IML) at Argonne National Laboratory were used in this study. The IML is a radiological facility equipped with four air-atmosphere beta-gamma hot cells. The hot cells are maintained at a negative pressure with respec t to the surroundings to maintain a proper radiological barrier. The two test systems are installed in separate hot cells. Each of th e hot cells is equipped with its own loading frame, autoclave, load cell, linear voltage displaceme nt transducer (LVDT), Instron control console, and data acquisition system.

In the current study, the tests were performed either in a simulated pressurized water reactor (PWR) environment or in a low-DO high-purity water environment. Both environments have low corrosion potentials, which are known to reduce the sensitivity of SSs to IASCC.3434 The simulated PWR water contained ~2 ppm lithium, ~1000 ppm boron, and ~2 ppm hydrogen. The conductivity was about 20

µS/cm. The low-DO high-purity wa ter contained less than 10 ppb dissolved oxygen, and the conductivity was kept below 0.07

µS/cm during the tests. The test environments were provided by two water recirculation loops, and a schematic diagram of loop #1 is shown in Fig. 4. Each loop consists of a storage tank, a high pressure pump, a regenerative heat exchanger, an autoclave, an ECP cell, a back-pressure regulator, two ion-exchange

cartridges, and several heaters.

The high-pressure section of the loop extends from the high-pressure pump (item 13 in Fig.

4) through the back-pressure regulator (item 10 in Fig. 4). The rest of the loop is kept at low pressure. Over-pressurization of the high-pressure portion of the system is prevented by two rupture disks (items 3 and 15 in Fig. 4) located at the downstream of the high-pressure pump. The autoclaves are one-liter Type 316 stainless steel autoclaves and rated to 2900 psig for 350°C. The loop for hot cell #1 is a PWR water system, and hydrogen is used as cover gas. A hydrogen leak detection/alarm unit (items 36 - 38 in Fig. 4) is installed.

For the hot cell #2 loop, a low-corrosion-potential environment is simulated in high-purity water using a mixture of nitrogen with 4% hydrogen as cover gas. For both systems, water is circulated at a rate of 20-30 mL/min through th e autoclaves. The water conductivity and pH can be monitored with inline sensors. During the tests, the temperature and pressure of the autoclaves were kept at ~320°C and ~1800 psig, respectively.

Formatteand gram 10 1. Autoclave 15. Rupture disk 29. Solenoid valve 2. Autoclave preheater 16. Pressure transducer 30. Excess flow valve 3. Rupture disk 17. High pressure gauge 31. Low-pressure regulator 4. ECP preheater

18. Conductiv ity sensor 32. Check valve 5. ECP cell 19. pH sensor 33. Vacuum & pressure gauge 6. Check valve 20. Solenoid valve 34. Pressure relief valve 7. Air Cooled Coil 21. Feedwater storage tank 35. Flash arrestor 8. Heat exchanger 22. Sparge tube 36. Hydrogen alarm control panel 9. Chilled block 23. Tank recirculation pump 37. Hydrogen leak sensors 10. Back-pressure regulator
24. Check valve 38. Hydrogen leak sensors 11. Ion exchange cartridge 25. Ion exchange cartridge V = Valve 12. Pressure relief valve 26. Cover gas supply cylinder T = Thermocouples 13. High-pressure pump
27. High-pressure regulator
14. Accumulator 28. Flash arrestor Figure 4. Recirculation water loop for Cell 1 of the IML (items in red are safety significant components) 2.4 Crack Growth Rate and Fr acture Toughness J-R curve Tests 2.4.1 Crack Growth Rate Test All CGR tests in this study were performed on the 1/4T-CT samples described above. Crack extensions were monitored with the direct current potential drop (DCPD) method during the tests. With this method, four electrical leads were spot welded on the CT sample (red lines shown in Fig. 2). A constant current was passed through the sample

, and the potential drop across the crack mouth was measured and related to the crack extens ion of the CT sample with a calibrated correlation curve. Field Co 11 The CGR tests were carried out in either a simulated PWR water environment or a low-DO high-purity water environment.

A typical CGR test is started with cyclic loading to pre-crack the sample in the test environment.

A load ratio around 0.2-0.3, frequency of 1-2 Hz, and maximum stress intensity factor (Kmax) between 10 and 16 MPa m 1/2 are used for pre-cracking. The object ive of this step is to generate a sharp fatigue crack and to advance the crack tip passing beyond the area close to the machine notch. The properties at this area may have b een altered by machining and do not represent the material's intrin sic behavior.

The stress intensity factor K for a CT specimen is calculated by:

1/23/2(2/)()()(1/)NPaWaKfBBWaWW+ (1) where P is applied load; B is the specimen thickness; BN is the net specimen thickness (or distance between the root s of the side grooves);

a is crack length; and W is specimen width (measured from the load line to the back edge of the specimen). The geometry factor [f(a/W)] for a CT specimen is:

234()0.8864.6413.3214.725.60aaaaafWWWWW=++ (2) Once a fatigue crack is initiated, a series of test steps is carried out with gradually increased rise times and load ratios. The measured CGRs in these test steps include the contributions from both mechanical fatigue and corrosion fatigue. With the change in loading conditions, the contribution of mechanical fatigue is gradually reduced while the environmental effect is enhanced. Figure 5 illustrates the principle of introducing environmentally assisted cracking in a test. The changes in test conditions should produce cyclic CG Rs along the green line. The cyclic CGR test is transitioned to a SCC CGR test when a significant environmental enhancement is observed.

Because of the beneficial effects of ferrite and the reduced sensitivity to IASCC in low-corrosion-potential environments, relatively low CGRs are expected for the CASS specimens even after irradiation to 0.08 dpa. To measure SCC CGRs at such low growth rates, a considerable amount of time is needed to collect data for adequate measurements. Since the focus of this study was on embrittlement (i.e., loss of facture toughness), a limited amount of time was spent on collecting SCC CGR data.

Field CoField Co 12 Crack Growth Rate in Water, Log ScaleCrack Growth Rate in Air, Log ScaleEnvironmental EffectMechanical FatigueIncrease Load Ratio and Rise Time Figure 5. Schematic for inducing environmentally assisted cracking in test environment.

2.4.2 Fracture Toughness J-R curve Test After each SCC CGR test, a fracture toughness J-R curve test was conducted on the same sample in the test environment. This test was performed with a consta nt stain rate of 0.43

µm/s, and the load and sample extension were recorded cont inuously outside the auto clave. The load-line displacement at load points was determined by s ubtracting the extension of the load train, which had been measured prior to the test. During th e test, the loading was interrupted periodically, and the specimen was held at a constant extension to measure crack length with the DCPD method. Before each DCPD measurement, the sample stress was allowed to relax at a constant displacement for 30 s.

The J-integral was calculated from the load (

P) vs. load-line displacement (

v) curve according to ASTM Specification E 1820-8a.3535 The J is the sum of the elastic and plastic components, elplJJJ=+ (3) At a hold point i corresponding to a crack length ai, as well as vi and Pi on the load vs. load-line displacement curve, the elastic component Jel(i) is given by:

22()()()(1)ieliKJE= (4) Field CoFormatteand gramField CoField Co 13 where is Poisson's ratio, and the stress intensity K(i) is calculated from Eqs.

11 and 22. The plastic component Jpl(i) is given by: (1)()(1)()(1)()(1)(1)(1)(1)1iplipliiiplipliiiNiAAaaJJbBb=+ (5) where b(i-1) is the remaining ligament at point i-1; Apl(i) is the area under the load vs. load-line displacement curve; and BN is the net specimen thickness. In addition, (i) and (i) are factors that account for the crack growth effects on J during the test a nd are expressed as: (1)(1)2.00.522iibW+= (6) (1)(1)1.00.76iibW=+ (7) The quantity Apl(i) - Apl(i-1) is the increment of the plastic area under the load vs. load-line displacement curve between lines of constant plastic displacement at points i - 1 and i. The plastic area under the load vs. load-line displacement curve is given by

[]1()(1)()(1)2iiplipliplipliPPvvAA+=+ (8) where the plastic components of the load-line displacement, vpl(i), are: ()()()pliiiLLivvPC= (9) In the above, v(i) is the total load-line displacement, and CLL(i) is the compliance required to give the current crack length ai and can be determined as follows:

23()'21.6217.80(/)4.88(/)1.27(/)[1(/)]iiiLLieiaWaWaWCEBaW++= (10) where Be is the specimen effective thickness given by B - (B - B N)2/B, and E=E/(1 - 2). A J-R curve is constructed by fitting the calculated J values and corresponding crack lengths to a

power law relationship. The J value at the in tersection of the power law curve and the 0.2-mm offset blunting line are reported. Note that a blunting line of four times flow stress (4f) is recommended by Mills 55 for materials with high strain hardening coefficients, and has been used in the previous thermal aging studies on unirradiated CASS.

44 To be consistent with the previous analyses, the same blunting line (i.e., J/4f) was also used in the current work.

In this study, the estimated flow stresses are approximately 280-340 MPa and 420-520 MPa for unirradiated and irradiated CASS materials, respectively. Th is relatively low strength allows a maximum J value of 280-360 kJ/m 2 for a typical 1/4T-CT specimen. The maximum crack extension is limited below ~1.3 mm in most cases. Because of the low strength and high fracture toughness of CASS materials, the re quired crack tip constraint and upper limit of J integral are often invalid with the 1/4T-CT specimen. This is especially true for the unirradiated CASS samples since the strength is ev en lower without irra diation hardening. For this reason, the J values determined from this study normally cannot be validated for J IC per ASTM E182-8a.

Field CoField CoField CoField CoField CoField CoFormatteand gramFormatteand gram 14 2.4.3 Fractographic Examination After each CGR/JR test, the final crack size was marked by fatigue cycling in an air atmosphere at room temperature. The specimen was then fractured, and the fracture surface was examined by scanning electron microscopy (SEM). Since the irradiated sample was highly radioactive, a

two-step replication technique was developed for the fractographic examination. After cleaning the tested sample remotely using manipulators, we used a two-part synthetic compound to produce a negative replica of the fracture surface inside the hot cell. The negative replica was then removed from the hot cell and transferred to a radiological fume hood for decontamination.

The second step of casting was to apply a low-viscosity epoxy on the surface of the negative replica. The cast was kept at approximately 70°C for several days to hard en the epoxy replica. The negative replica was then removed. The obtai ned epoxy replica was co ated with a layer of gold before it was transferred for SEM examination.

The CGR test and JR test regions were identified on the SEM images, and their fracture

morphologies were analyzed. The physical crack length was also measured on the images, and a

9/8 averaging technique was used to account for the uneven extensions at the crack front. With this technique, nine measurements were taken along the crack front sp aced across the width of the sample at equal intervals. The two near-surface measurements were averaged, and the resultant value was averaged with the remaining seven measurements to obtain the average crack length. All crack extensions determined from th e DCPD method were scal ed proportionately to match the final SEM-measured crack length.

15 3 Results Seven irradiated and four unirradiated control specimens from three heats of CASS were tested in the current study. The test matrix is shown in Table 3. The specimens were unaged and thermally aged pairs. The irradiated samples received a displacement dose of ~0.08 dpa. The CF-3 specimens were tested in either low-DO high-purity water or PWR water, and all CF-8 and CF-8M specimens were tested in low-DO high-purity water.

Table 3. Test matrix of unirradiated and irradiated low-dose CASS specimens.

Sample ID Dose (dpa) Heat ID Materials Test Environment Facility CGR JR SEM A-N1 - 69 CF-3, 24% , unaged Low-DO high-purity Cell 2 A-1 0.08 69 CF-3, 24% , unaged PWR Cell 1 - A-2 0.08 69 CF-3, 24% , unaged Low-DO high-purity Cell 2 B-N1 - 69 CF-3, 24% , aged PWR Cell 1 B-1 0.08 69 CF-3, 24% , aged PWR Cell 1 E-N1 - 68 CF-8, 23% , unaged Low-DO high-purity Cell 2 E-1 0.08 68 CF-8, 23% , unaged Low-DO high-purity Cell 2 F-N1 - 68 CF-8, 23% , aged Low-DO high-purity Cell 2 F-1 0.08 68 CF-8, 23% , aged Low-DO high-purity Cell 2 I-1 0.08 75 CF-8M, 28% , unaged Low-DO high-purity Cell 2 J-1 0.08 75 CF-8M, 28% , aged Low-DO high-purity Cell 2 3.1 CF-3 Cast Stainless Steel 3.1.1 Unaged CF-3 CASS 3.1.1.1 Unirradiated specimen A-N1 tested in low-DO high-purity water Crack growth rate test

Specimen A-N1 was an unirradiated control sample tested in low-DO hi gh-purity water. The material was an unaged CF-3 CASS with ~24% ferri te (Heat 69). The objective of this test was to provide a baseline for the irradi ated tests. The test conditions and results are summarized in Table 4, and a crack-length histor y plot is shown in Fig. 6.

Fatigue pre-crack was initiated at ~15.4 MPa m 1/2 with a triangular waveform of 1 Hz and a load ratio of ~0.16. A cyclic CGR slightly below the fatigue growth rate in air was readily obtained in this sample. After ~220-

µm crack extension, the load ratio and rise time were increased slowly to induce environmenta lly assisted cracking. The environmental effect on CGR was difficult to establish, and the crack stalled several times in the following test periods.

Nonetheless, elevated crack growth rates were observed for this CASS sample under cyclic Field Co 16 loading in the low-DO high-purity water. The cyclic CGR data obtained from this test are plotted against the fatigue CGRs in air in Fig. 7. The fatigue CGRs are estimated based on James and Jones3737 with the load ratios and rise times given in Table 4. A corrosion fatigue curve proposed by Shack and Kassner3838 for unirradiated SSs in 0.2-ppm DO water at 290°C is also included in the figure as a reference. Note that the data point above the reference line (from test period z) was due to a temperature in crease in the autocl ave during that test period. After the temperature was stabilized, a much lower CGR was measured in the next test period. It is clear that the corrosion fatigue response of this CASS sample is better than that of typical SSs in low-DO high-purity water.

After a total of ~900-

µm crack extension, the test was set to a constant load with periodical partial unloading (PPU) every 2 hr. The SCC CGR was negligibly low and was below the detection limit of the DCPD measurement. The SCC behavior of this sample will be discussed at the end of this section along with the other CGR data obtained from the same heat.

Table 4. Crack growth rates of specimen A-N1 (unirradiated and unaged CF-3 with 24% ferrite) in low-DO high-purity water.

Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2m/s m/s mm Start 0.3 5.978 A 3.2 317 0.16 0.39 0.39 0.11 15.4 12.9 5.94E-08 5.68E-08 6.204 B 5.1 317 0.29 0.36 0.36 0.14 14.5 10.3 1.98E-08 3.36E-08 6.257 C 9.1 317 0.40 0.69 0.69 0.31 14.3 8.6 1.01E-09 1.11E-08 6.265 D 21.8 317 0.21 0.38 0.38 0.12 16.0 12.6 7.14E-08 5.72E-08 6.419 E 24.5 317 0.30 0.74 0.74 0.26 16.5 11.6 3.02E-08 2.49E-08 6.528 F 29.4 317 0.45 0.68 0.68 0.32 16.8 9.3 1.35E-08 1.50E-08 6.611 G 43.2 317 0.60 3.04 3.04 1.96 16.6 6.7 3.02E-10 1.30E-09 6.618 H 50.7 317 0.50 1.65 1.65 0.85 16.9 8.5 5.37E-09 4.87E-09 6.664 I 66.6 317 0.50 6.57 1.31 3.43 16.8 8.4 6.07E-10 1.18E-09 6.685 J 80.2 317 0.49 9.91 3.30 5.09 16.9 8.5 6.25E-10 8.24E-10 6.702 K 97.4 317 0.49 19.9 3.31 10.1 17.0 8.6 3.28E-10 4.25E-10 6.713 L 118.2 317 0.49 39.7 7.94 20.3 17.1 8.7 2.68E-10 2.16E-10 6.728 M 145 317 0.49 99.7 7.98 50.3 17.3 8.8 1.57E-10 9.04E-11 6.737 N 170.5 317 0.49 199.8 7.99 100.2 17.3 8.9 7.96E-11 4.59E-11 6.742 O 196.2 317 0.48 333.6 8.01 166.4 17.3 8.9 1.51E-10 2.80E-11 6.751 P 236.1 318 0.48 667.8 8.01 332.2 17.3 8.9 negligible 1.42E-11 6.750 Q 285.6 318 0.49 133.3 8.00 66.7 17.4 8.9 6.58E-11 7.06E-11 6.758 R 308.1 318 0.49 66.6 7.99 33.4 17.5 8.9 8.93E-11 1.40E-10 6.763 S 316.7 318 0.44 34.2 8.21 15.8 17.4 9.7 3.90E-10 3.46E-10 6.769 t a 332.2 318 0.44 68.5 8.22 31.5 17.7 9.8 5.06E-10 1.81E-10 6.795 U 355.8 319 0.49 133.1 7.99 66.9 17.6 9.0 6.71E-11 7.25E-11 6.799 V 363.7 319 0.45 13.6 3.41 6.4 17.6 9.8 1.36E-09 8.90E-10 6.814 W 379.4 319 0.47 33.7 8.09 16.3 17.8 9.5 6.68E-10 3.33E-10 6.837 X 403.6 318 0.48 66.4 7.97 33.6 17.4 9.1 1.62E-10 1.48E-10 6.846 Y 430.8 318 0.49 133.2 8.00 66.8 17.8 9.2 8.67E-11 7.69E-11 6.851 z b 452.8 317 0.49 333.8 8.01 166.2 18.1 9.3 3.74E-10 3.19E-11 6.872 aa 500.3 319 0.48 669.5 8.03 330.5 18.1 9.4 3.65E-11 1.64E-11 6.876 1 811.3 318 0.50 12 12 7200 18.0 9.0 negligible 1.37E-12 6.880 a The CGR value was obtained from the later part of the test period.

b The autoclave temperature was unstable in the test period.

Formatteand gramFormatteand gramField Co 17 (a) 6.006.106.206.306.404812 16 2024283205101520Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCa, R=0.11 Hzb, R=0.21 Hzc, R=0.40.5 HzHold at low K (b) 6.206.30 6.406.506.60 6.706.804812 16 2024283220304050607080Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCd, R=0.21 Hze, R=0.31 Hzf, R=0.450.5 Hzg, R=0.60.1 Hzh, R=0.50.2 Hzi, R=0.510s/up, 2s/downj, R=0.515s/up, 5s/down (c) 6.656.70 6.756.8048 121620242832100150200Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCk, R=0.530s/up, 5s/downl, R=0.560s/up, 12s/downm, R=0.5150s/up, 12s/downn, R=0.5300s/up, 12s/downo, R=0.5500s/up, 12s/downp, R=0.51000s/up, 12s/down Figure 6. Crack-length-vs.-time plot for specime n A-N1 (unirradiated and unaged CF-3 with 24% ferrite): test periods (a) a-c, (b) d-j, (c) k-p, (d) q-u, (e) v-aa, and (f) 1. Field Co 18 (d) 6.706.756.80 6.854812 16 20242832240260280300320340Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCq, R=0.5200s/up, 12s/downr, R=0.5100s/up, 12s/downs, R=0.4550s/up, 12s/downt, R=0.45100s/up, 12s/downPressure unstable caused by cooling water problem.u, R=0.5200s/up, 12s/down5.1E-10 m/s (e) 6.756.806.856.906.954812 16 20242832360380400420440460480500Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCv, R=0.4520s/up, 5s/downw, R=0.4550s/up, 12s/downx, R=0.5100s/up, 12s/downy, R=0.5200s/up, 12s/downz, R=0.5500s/up, 12s/downaa, R=0.51000s/up, 12s/down (f) 6.8006.8206.8406.8606.8806.900 6.92048 121620242832500550600650700750800850Crack Length (mm)K (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oC1, PPU, 2 hr hold Figure 6. (Contd.)

19 CF curve for 0.2 ppm DO by Shack & Kassner 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-cTest periods: d-pTest periods: q-uTest periods: v-aaCGRenv (m/s)CGRair (m/s)Specimen A-N1 CF-3, unaged, unirradiated Low-DO high-purity water~318oC Figure 7. Cyclic CGRs of specimen A-N1.

Fracture toughness J-R curve test

A fracture toughness J-R curve test was conducte d on the sample in the low-DO high-purity water environment. The sample was load ed at a constant ex tension rate of 0.43

µm/s while the load and load-line displacement were recorded. During the test, loading was interrupted periodically to measure the crack extension by DC PD. The obtained J an d crack extension data are plotted in Fig. 8. Note that the measurement capacity of 1/4T-CT samples is significantly lower than the fracture toughness an ticipated for the CASS material s, and thus a validated J-R curve test is not possible for this sample. Nonetheless, the available data points fit to a power-law relationship, and the estimated J value at the 0.2-mm offset line is about 320 kJ/m

2. The straightness of the crack extension and the Jmax requirement are not met in this analysis due to lack of constraint at the crack tip.

Field Co 20 0160320480 6400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen A-N1CF-3, unaged, unirradiatedLow-DO high-purity water, ~318 oCJ= 536.5*a0.68JQ=320 kJ/m 2Jmaxamax Figure 8. The J-R curve for specimen A-N1.

Fractographic examination

After the J-R curve test, the sample was fatigued and broken open at room temperature in an air atmosphere. The fracture surface was examined with SEM (Fig. 9). While the CGR test region shows a transgranular (TG) morphology, the entir e JR test region is covered with ductile dimples. A major secondary crack that covers n early a half of the width of the sample can be seen at the machined notch. This large se condary crack may affect the crack propagation, leading to a curved crack front in the CGR test. Consequently, the final crack size of the JR test is also affected with a significantly higher cr ack extension on the right

-hand side of the SEM image. Figure 10 shows an enlarged view of the fracture surface along the sample central line.

More details of the CGR and JR test regions can be seen in Figs. 11 and 12, respectively.

Field Co 21 Figure 9. Fracture surface of specimen A-N1 tested in low-DO high-purity water.

Field Co 22 Figure 10. Fracture surface of specimen A-N1 along the sample central line.

Crack advance Dimple fractureMachined

notchSecondary

crackTG Field Co 23 Figure 11. Transgranular fracture in the precracking region of specimen A-N1. Field Co 24 Figure 12. Ductile dimple fracture in JR test region of specimen A-N1. Field Co 25 3.1.1.2 Irradiated specimen A-1 tested in simulated PWR water Crack growth rate test

Specimen A-1 was an irradiated CF-3 CASS sample (Heat 69) tested in the simulated PWR water environment. The specimen had ~24% fe rrite and was in the as-cast condition. The objective of this test was to compare the results with those from an identical test performed in low-DO high-purity water. Also, the CGR results of this sample were compared with those obtained from its thermally aged equivalent. Th e test conditions and results are summarized in Table 5, and a crack-length history plot is shown in Fig. 13. Note that the starting crack length for this sample was about 1 mm longer than that of a typical 1/4T-CT specimen. This larger-than-normal initial crack length was due to a rest art of the pre-cracking to correct a loose DCPD lead. The initial crack size reported in Table 5 was determined with SEM images on the fracture surface after the test.

Table 5. Crack growth rates of specimen A-1 (0.08-dpa unaged CF-3 sample with 24% ferrite) in PWR water.

a Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s S s MPa m1/2 MPa m1/2 m/s m/s mm Start 63.9 6.802 b a 64.8 319 0.21 0.43 0.43 0.07 24.3 19.2 1.58E-07 2.04E-07 7.009 b 66 319 0.31 0.41 0.41 0.09 23.8 16.4 1.17E-07 1.43E-07 7.218 c 68.6 319 0.41 0.78 0.78 0.22 23.1 13.6 3.21E-08 4.47E-08 7.340 d 72.3 319 0.41 1.54 1.54 0.46 21.9 12.8 9.77E-09 1.89E-08 7.395 e 87.6 319 0.46 2.96 1.48 1.04 20.3 11.0 2.89E-10 6.18E-09 7.403 f 91 319 0.41 3.08 1.54 0.92 22.2 13.1 9.46E-09 1.01E-08 7.462 g 94.1 320 0.41 7.69 1.54 2.31 22.4 13.2 5.67E-09 4.17E-09 7.503 h 103 320 0.46 22.6 3.76 7.42 22.6 12.2 1.51E-09 1.16E-09 7.535 i 114.3 320 0.46 45.1 3.76 14.9 22.7 12.3 7.63E-10 5.86E-10 7.558 j 118.7 319 0.41 15.3 3.83 4.66 23.0 13.6 4.61E-09 2.27E-09 7.602 k 127.9 319 0.41 46.0 3.83 14.0 23.0 13.7 1.25E-09 7.72E-10 7.632 l 150.3 318 0.40 114.9 9.19 35.1 23.0 13.7 3.90E-10 3.13E-10 7.655 m 168.7 319 0.40 385.6 9.25 114.4 23.9 14.3 3.56E-10 1.05E-10 7.673 1 209 318 0.40 12 12 7200 23.9 14.4 4.83E-11 5.75E-12 7.684 a Simulated PWR water with 2 ppm Li and 1000 ppm B. DO<10 ppb. Conductivity ~20

µS/cm. b Determined from the SEM image after the test.

The specimen was pre-cracked in the PWR environment with a triangular waveform at 1 Hz, a maximum stress intensity factor of ~24 MPa m1/2, and load ratio of 0.2. After about 200

µm extension, the load ratio was incr eased while the stress intensity f actor and cyclic frequency were reduced gradually. The measured CGRs in the initial stage of the pre-cracking (test periods a-d) followed closely with the fatigue growth rates in ai

r. Next, several test periods were carried out with saw-tooth waveforms and increased rise times to stimulate environmentally assisted cracking. The environmental enhancement of CGRs started to appear and became stabilized after some additional crack extension. The cyclic CGR data obtained from this test are plotted against the estimated fatigue CGRs in air as shown in Fig. 14. The corrosion fatigue curve proposed by Shack and Kassner is also included in the figure as a reference. Obviously, the Field Co 26 corrosion fatigue response of this sample is lo wer than that of typi cal SSs, suggesting good IASCC resistance of CASS in PWR water.

After the cyclic CGR test, the specimen was subj ected to a constant stress intensity factor of

~24 MPa m1/2 with PPU every 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.

The measured SCC CGR was about 4.8x10

-11 m/s, a factor of four lower than that of the NUREG-0313 curve,3636 a SCC disposition curve based on unirradiated SSs tested in a high-DO environment. The SCC response of this sample will be discussed at the end of this section along with the other CGR data obtained from the same heat.

(a) 6.806.907.007.107.207.307.407.507.604812162024286570758085Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCabcdeKmaxCrack length (b) 7.357.40 7.45 7.50 7.55 7.607.6548 12 16202428859095100105110115120Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCfeghijKmaxCrack length Figure 13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% ferrite): test periods (a) a-e, (b) f-j, (c) k-m, and (d) 1.

Formatteand gramField Co 27 (c) 7.557.60 7.65 7.707.7548 12 16202428120130140150160170Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCklmKmaxCrack length (d) 7.667.677.687.697.707.71 7.72481216202428170180190200210220230240250Crack Length (mm)K (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oC1KCrack length Figure 13. (Contd.)

28 10-1110-1010-910-810-710-1110-1010-910-810-7CGRenv (m/s)CGRair (m/s)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCTest period with <10

µm extension are excluded.CF curve for 0.2 ppm DO by Shack & Kassner Figure 14. Cyclic CGRs of specimen A-1.

Fractographic examination

After the test, the specimen was pulled apart in an air atmosphere at room temperature, and the fracture surface was examined with SEM. Figure 15 shows the entire crack front of specimen A-1. The straight line in the middl e of the picture corresponds to th e restart of the pre-cracking.

Figure 16 is an enlarged view along the central section of the specimen. Transgranular cleavage-like fracture is the dominant morphology thr oughout the CGR region. Figure 17 shows the

typical river pattern of cleavage cracking at the beginning an d the end of the CGR test region. Vermicular ferrite at dendrite cores can also be seen in a few places on the fracture surface. As

shown in Fig. 18, fewer slip ledges can be seen with in ferrite, suggesting th at the ferrite dendrite core might be deformed to a lesser extent th an the surrounding austenite. Beyond the CGR test, the dominant fracture mode is ductile dimples resulting from microvoid coalescence. Field Co 29 Figure 15. Fracture surface of specimen A-1 tested in PWR water.

Field Co 30 Figure 16. Fracture surface of specimen A-1 along the sample central line.

Machined notchRestart of pre-cracking Vermicular ferrite at

dendrite coreDimple fractureCrack advance Field Co 31 Figure 17. Cleavage-like fracture in specimen A-1: (a) pre-cracking, and (b) e nd of the CGR test.

Crack propagation from bottom to top.

a Field Co 32 Figure 17. (Contd.)

b 33 Figure 18. Fracture surface of specimen A-1 showing that delta ferrite deformed to a lesser extent than austenite. Crack propagation from bottom to top.

Field Co 34 3.1.1.3 Irradiated specimen A-2 te sted in low-DO high-purity water Crack growth rate test

Specimen A-2 was an irradiated unaged CF-3 CASS with ~24% ferrite (Heat 69) tested in high-purity water with low DO. The objective of this test was to compare the irradiated specimen A-1 tested in PWR water. Also, the fracture toughness of this specimen was compared with that of thermally aged CF-3 CASS. The CGR test conditions and results are summarized in Table 6, and a crack-length history pl ot is shown in Fig. 19.

Table 6. Crack growth rates of specim en A-2 (0.08-dpa unaged CF-3 with 24% ferrite) in low-DO high-purity water environment.

Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm Start 14.9 5.899 aa 18.1 319 0.34 0.45 0.45 0.05 17.3 11.4 4.09E-08 4.04E-08 6.041 b 20 319 0.41 0.45 0.45 0.05 17.1 10.1 3.21E-08 2.96E-08 6.141 c 21.6 319 0.50 0.43 0.43 0.07 15.7 7.8 1.94E-09 1.43E-08 6.151 da 24.3 319 0.45 0.44 0.44 0.06 16.5 9.1 2.27E-08 2.16E-08 6.219 e 30.4 319 0.45 0.88 0.88 0.12 16.0 8.8 5.26E-09 9.85E-09 6.274 f 39.3 319 0.50 1.74 1.74 0.26 15.9 8.0 1.80E-09 3.82E-09 6.304 g 40.4 319 0.40 0.45 0.45 0.05 17.2 10.4 5.87E-08 3.21E-08 6.410 h 42.4 319 0.45 0.44 0.44 0.06 16.3 9.0 1.25E-08 2.13E-08 6.453 i 44.6 319 0.45 0.88 0.88 0.12 16.6 9.2 1.29E-08 1.14E-08 6.497 j 49.0 319 0.45 1.75 1.75 0.25 16.8 9.2 9.20E-09 5.84E-09 6.564 k 62.2 319 0.45 4.38 4.38 0.62 17.0 9.4 3.07E-09 2.45E-09 6.627 l 74.2 319 0.46 8.73 4.37 1.27 17.2 9.4 2.37E-09 1.23E-09 6.686 m 88.4 319 0.45 26.2 4.37 3.79 17.2 9.5 7.77E-10 4.25E-10 6.719 n 110.6 319 0.44 52.5 10.5 7.52 17.2 9.7 6.45E-10 2.24E-10 6.756 o 135.7 319 0.43 105.0 10.5 15.0 17.0 9.7 3.77E-10 1.13E-10 6.780 p 184.3 319 0.43 262.6 10.5 37.4 17.3 9.8 8.72E-11 4.69E-11 6.795 q 232.7 319 0.43 525.9 10.5 74.1 17.6 10.0 1.29E-10 2.49E-11 6.815 r 278.6 320 0.43 876.3 10.5 123.7 17.6 10.1 1.73E-10 1.52E-11 6.840 s 326.7 319 0.48 865.9 10.4 134.1 17.6 9.2 1.27E-10 1.21E-11 6.860 1 423.9 319 0.50 12 12 7200 17.6 8.8 2.33E-11 1.27E-12 6.875 2-a 575.3 320 0.50 12 12 7200 19.6 9.8 4.89E-11 1.80E-12 6.907 2-b 687.3 321 1 - - - 19.8 - 4.94E 6.912 2-c 784 321 0.50 12 12 3600 19.8 9.9 4.26E-11 3.73E-12 6.924 a The CGR value was obtained from the later part of the test period.

Pre-cracking in this sample was started with a triangle waveform at 1 Hz. The applied maximum stress intensity factor and load ratio were ~17 MPa m 1/2 and ~0.35, respectively. The initial cyclic CGRs were comparable to fatigue growth rates but were quickly diminished as the load ratio was increased to 0.5. After lowering the lo ad ratio of 0.4 and increasing the frequency to 1 Hz, the high growth rate was re-established in test period g over a 100-

µm crack extension. Next, the rise time was increased gradually while the maximum stress intensity factor and load ratio were kept approximately the same. In the following test periods, environmentally enhanced cracking started to appear and became evident with further increases in rise time and load ratio.

By the end of test period s, the measured CGR was more than one order of magnitude higher than the fatigue growth rate in air. All cyclic CGRs obtained from this sample are plotted in Field Co 35 Fig. 20 along with the corrosion fatigue curve for unirradiated SSs in water with 0.2 ppm DO. A comparison between Figs. 14 and 20 shows that the cyclic CGRs of specimen A-1 is slightly lower than that of specimen A-2.

(a) 5.906.00 6.106.206.304812 1620242832141618202224262830Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCabc4.1E-8 m/s de2.3E-8 m/s KmaxCrack length (b) 6.206.25 6.306.356.40 6.45 6.50 6.556.60481216202428323035404550Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCfghijKmaxCrack length Figure 19. Crack-length-vs.-time plot for specimen A-2 (0.08-dpa unaged CF-3 with 24%

ferrite) tested in low-DO high-purity water environment: test periods (a) a-e, (b) f-j, (c) k-o, (d) p-s, and (e) 1-2. Field Co 36 (c) 6.506.55 6.60 6.65 6.70 6.75 6.806.856.904812 16202428326080100120140Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCklmnoKmaxCrack length (d) 6.706.756.806.85 6.90 6.957.0048121620242832160200240280320Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCpqrsKmaxCrack length (e) 6.866.886.906.926.944812 16 20242832400500600700800Crack Length (mm)K (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oC12-a2-b2-cKCrack length Figure 19. (Contd.)

37 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-fTest periods: g-sCGRenv (m/s)CGRair (m/s)Specimen A-2CF-3, unaged, 0.08 dpa.

Low-DO high-purity water320oCCF curve for 0.2 ppm DO by Shack & Kassner Figure 20. Cyclic CGRs of specimen A-2.

The first constant-load test period was conducted at 17.6 MPa m 1/2 with PPU every 2 hr (test period 1). After nearly 100 hr, a CGR of 2.3 x 10

-11 m/s was measured over ~15-

µm crack extension. The load was increased to 19.6 MPa m 1/2 for the second constant-load test period.

Again, PPU was applied every 2 hr. A CGR of 4.9 x 10

-11 m/s was obtained over 150 hr. After the PPU was removed in test period 2-b, the measured CGR decreased one order of magnitude. Constrained by test time, only 5-

µm crack extension was accumulated for this test period. In test period 2-c, the PPU was re-introduced but with 1-hour interval. The measured CGR was nearly identical to that obtai ned in test period 2-a. It appears that the hold time between PPU does not affect the SCC response of CF-3 at this stress intensity level.

Fracture toughness J-R curve test

After the crack growth test, a fracture toughness J-R curve test was conducted on this sample in the low-DO high-purity water environment. The samp le was loaded at a c onstant extension rate of 0.43 µm/s while the load and load-line displacement were recorded. During the test, loading was interrupted periodically to measure the crac k extension by DCPD. The obtained J and crack extension data are plotted in Fig. 21. A power-law curve fitting of the data gives a relationship of J = 430a0.64. The estimated J value at the 0.2-mm offset line is about 204 kJ/m

2. Note that the J-R curve data cannot be validated for this sample because one of the nine measurements of the final crack size was above the limit, and the Jmax requirement was omitted in this analysis.

Field Co 38 01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCJ= 430*a0.64JQ=204 kJ/m 2Jmaxamax Figure 21. The J-R curve for specimen A-2.

Fractographic examination

After the J-R curve test, the sample was cyclically loaded at room temperature in an air atmosphere to break the ligament. The fracture surface was then examined with the replication technique using SEM. Figure 22 shows the entire fracture surface of specimen A-2. Note that the round smooth areas on the SEM image are ai r bubbles trapped during replication, not the original morphology of the fracture surface. Both the CGR and post-JR fatigue regions are relatively flat, clearly contrasting with the heavily deformed JR region. The crack front of the CGR test is straight, indicati ng a well-controlled loading c ondition during the CGR test. A curved crack front due to a non-constant constrai nt can be seen for the JR test region.

Figure 23 shows an enlarged view of specimen A-2 along its central line. Similar to specimen A-1, which was tested in PWR water, transgranular cleavage-like fracture was also the dominant morphology during the CGR test. Figure 2424 shows the cleavage-like cracking at the initial part of the CGR test, and deformation steps resulting from brittle fracture can be seen in some places. An area of delta ferrite at the dendrite cores can be seen close to the end of the CGR test (see Fig.

2525). Similar to that found in the specimen A-1, less plastic deformation activities were observed in the ferrite phase. Beyond the CGR test, ductile dimples were the only fracture

morphology on the fracture surface, as shown in Fig.

2626. Field Co 39 Crack advanceDelta ferrite in dendritesAir bubbles CGRJRDimplesCrack advanceDelta ferrite in dendritesAir bubbles CGRJRDimples Figure 22. Fracture surface of specimen A-2 tested in low-DO high-purity water.

Field Co 40 Figure 23. Fracture surface of specimen A-2 along the sample central line.

CGR test TG Vermicular ferrite in

dendrite cores Dimple fractureCrack advance JR test Post-JR fatigue Machined

notchField Co 41 Figure 24. Cleavage-like steps at the be ginning of CGR test of specimen A-2.

Crack propagation from bottom to top.

Field Co 42 Figure 25. Fracture surface of specimen A-2 sh owing that ferrite deformed to a lesser extent than austenite. Crack propagatio n from bottom to top.

Field Co 43 Figure 26. Ductile dimple morphology in the JR test region of specimen A-2. Crack propagation from bottom to top.

Field Co 44 3.1.2 Thermally Aged CF-3 CASS 3.1.2.1 Unirradiated specimen B-N1 tested in PWR water Crack growth rate test Specimen B-N1 was an unirradiated control sample tested in PWR water. The material was a thermally aged CF-3 with ~24% ferrite. The obj ective of the test was to provide a baseline for irradiated tests on thermally aged specimens. The CGR test conditions and results of this sample are summarized in Table 7, and a crack-le ngth history plot is shown in Fig. 27.

Table 7. CGR test of specimen B-N1 (unirradiated, thermally aged CF-3 with 24% ferrite) in PWR water.

a Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm Start 0.3 5.997 a b 3.0 319 0.20 0.43 0.43 0.07 14.8 11.9 5.57E-08 4.04E-08 6.182 b b 7.2 319 0.35 0.42 0.42 0.08 15.0 9.8 3.25E-08 2.67E-08 6.353 c b 13.3 319 0.49 0.40 0.40 0.10 15.1 7.7 2.19E-08 1.45E-08 6.474 d 23 319 0.54 0.77 0.77 0.23 15.0 6.9 4.85E-09 5.50E-09 6.538 e 47 319 0.54 1.92 1.92 0.58 15.0 6.9 1.63E-09 2.24E-09 6.597 f 71.2 319 0.54 3.82 1.53 1.18 15.0 6.8 1.53E-10 1.07E-09 6.608 g b 76.9 319 0.54 0.77 0.77 0.23 15.3 7.0 4.78E-09 5.77E-09 6.641 h 101.8 319 0.55 2.29 0.76 0.71 15.1 6.8 1.64E-10 1.79E-09 6.649 i 123.9 319 0.54 3.83 3.83 1.17 15.3 7.0 7.97E-11 1.14E-09 6.652 j 143.8 319 0.52 1.93 1.93 0.57 15.3 7.3 1.76E-09 2.64E-09 6.690 k c 152.8 320 0.53 3.85 1.54 1.15 15.2 7.2 5.86E-10 1.25E-09 6.701 l 192.1 320 0.52 7.72 1.54 2.28 15.3 7.3 1.77E-10 6.60E-10 6.714 m 200.2 319 0.49 1.96 1.96 0.54 15.6 7.9 2.31E-09 3.22E-09 6.744 n 215 320 0.49 3.91 3.91 1.09 15.7 7.9 2.54E-09 1.66E-09 6.794 o 224 320 0.49 7.81 1.56 2.19 15.7 7.9 8.81E-10 8.30E-10 6.813 p 241.5 319 0.49 15.6 3.91 4.37 15.8 8.0 7.40E-10 4.27E-10 6.841 q 270.9 319 0.49 23.4 3.91 6.56 15.8 8.0 1.21E-10 2.87E-10 6.850 r 289.5 319 0.49 46.9 3.91 13.1 15.7 8.1 3.27E-11 1.45E-10 6.852 s 311.3 319 0.49 11.7 3.90 3.30 15.7 8.0 7.10E-11 5.65E-10 6.855 t 316.3 320 0.32 12.4 4.14 2.59 16.5 11.1 9.66E-09 1.33E-09 6.957 u 320 319 0.35 24.6 4.10 5.37 16.5 10.7 2.99E-09 6.14E-10 6.983 v 334.4 319 0.37 48.9 4.08 11.1 16.8 10.5 1.35E-09 2.98E-10 7.037 w 347.5 319 0.42 96.3 9.63 23.7 16.8 9.7 4.65E-10 1.23E-10 7.054 x 368.1 320 0.42 241.0 9.64 59.0 16.9 9.8 2.09E-10 5.02E-11 7.065 y 383.7 319 0.41 401.9 9.65 98.1 16.8 9.9 1.26E-10 3.05E-11 7.071 z 408.5 319 0.42 804.3 9.65 195.7 17.1 9.9 1.43E-10 1.57E-11 7.083 aa 436.2 319 0.42 1206.2 9.65 293.8 17.2 10.0 7.73E-11 1.05E-11 7.090 1a 512.5 319 0.50 12 12 7200 17.2 8.6 2.72E-11 1.17E-12 7.106 1b 647.5 319 1.00 - - - 17.1 - 2.29E 7.107 a Simulated PWR water with 2 ppm Li and 1000 ppm B. Conductivity ~20

µS/cm. b The CGR of the later part of the test period is reported.

c The CGR of the initial part of the test period is reported.

Field Co 45 Fatigue pre-cracking was started at ~15 MPa m1/2 with a triangle waveform of 1 Hz and a load ratio of 0.2. A CGR close to the fatigue growth rate in air was quickly established in this sample after about 1-hr cyc ling. After about 180-

µm crack extension, the load ratio was increased in the next two test periods. The observed CGRs decreased as expected with the reduced contribution of mechanical fatigue (as shown in Fig. 28).

However, the crack stalled in test period f after the triangle waveform was replaced with a saw-tooth waveform. After the crack was re-activated, several attempts were made to induce environmentally assisted cracking. The environmental effect became evident after a CGR of ~1.4 x 10

-9 m/s was established in test period

v. The load ratio and rise time were then increased simultaneously to stabilize the environmentally enhanced cracking. By the end of the cyclic loading test, the observed CGR in water was about a factor of seven higher than the fatigue growth rate in air. All cyclic CGRs obtained from this sample are plotted in Fig. 28 along with the corrosion fatigue curve for SSs in high-purity water with 0.2 ppm DO.

(a) 6.006.08 6.166.246.32 6.406.486.564812 16 2024283205101520Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCa, R=0.21 Hzb, R=0.351 Hzc, R=0.51 Hzd, R=0.550.5 Hz (b) 6.456.506.556.60 6.65 6.706.756.8048 12162024283230405060708090100Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCe, R=0.550.2 Hzf, R=0.555s/up, 2s/downg, R=0.550.5 Hzh, R=0.553s/up, 1s/down Figure 27. Crack-length-vs.-time plot for specimen B-N1 (unirradiated, aged CF-3 with 24% ferrite) in PWR water: test periods (a) a-d, (b) e-h, (c) i-n, (d) o-s, (e) t-w, (f) x-aa, and (g) 1a-1b. Field Co 46 (c) 6.606.656.706.75 6.806.854812 16 20242832120140160180200Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCi, R=0.520.1 Hzj, R=0.530.2 Hzk, R=0.525s up, 2s downl, R=0.5210s up, 2s downm, R=0.50.2 Hzn, R=0.50.1 Hz (d) 6.706.75 6.806.856.90 6.957.004812 16 20242832220240260280300Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCo, R=0.510s up, 2s downp, R=0.520s up, 5s downq, R=0.530s up, 5s downr, R=0.560s up, 5s downs, R=0.515s up, 5s down (e) 6.806.856.906.95 7.00 7.057.107.1548 121620242832315320325330335340345Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCt, R=0.3215s up, 5s downu, R=0.3530s up, 5s downv, R=0.3760s up, 12s downw, R=0.42120s up, 12s down Checkcompliance Figure 27. (Contd.)

47 (f) 7.007.057.10 7.154812 16 20242832350360370380390400410420430Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCx, R=0.4300s up, 12s downy, R=0.4500s up, 12s down z, R=0.41000s up, 12s downaa, R=0.41500s up, 12s down (g) 7.067.087.107.127.144812 16 20242832450500550600650Crack Length (mm)K (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oC1a,UUP, 2 hr hold 1b,Constant Figure 27. (Contd.)

After the environmentally assisted cracking was stabilized, the test was set to a constant stress intensity factor (~17 MPa m 1/2) with PPU every 2 hr. Following a short period of rapid growth, a CGR about 2.7 x 10-11 m/s was measured over 16-

µm crack extension. Without PPU, the observed CGR dropped more than two orders of magnitude under a near constant-K condition. Limited by the test time, the CGR test was terminated after 135 hr with just 1-

µm crack extension.

Fracture toughness J-R curve test

The fracture toughness J-R curve test was performed in the same PWR water environment. The test was conducted at a cons tant strain rate of 0.43

µm/s, and the crack extension was measured with the DCPD method. Before each DCPD measurement, the st ress was allowed to relax for 30 s at a constant displacement. The obtained J-R curve is shown in Fig. 29. The estimated J 48 value at 0.2 mm offset line is about 170 kJ/m

2. Note that the J-R curve data cannot be validated for this test because the requirements of the crack straightness and Jmax were not met.

10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-fTest periods: g-iTest periods: j-lTest periods: m-sTest periods: t-aaCGRenv (m/s)CGRair (m/s)Specimen B-N1 CF-3, aged, unirradiated PWR water~319oCCF curve for 0.2 ppm DO by Shack & Kassner Figure 28. Cyclic CGRs of specimen B-N1.

0160320480 6400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen B-N1CF-3, aged, unirradiatedPWR water, ~319 oCJ= 352.6*a0.66JQ=170 kJ/m 2Jmaxamax Figure 29. The J-R curve for specimen B-N1. Field CoField Co 49 Fractographic examination

After the JR test, the sample was broken open at room temperature in air. The fracture surface of the tested sample was examined with SEM. Figure 30 is a global view of the entire fracture surface of the CGR and JR test regions. The crack front of the CGR test is quite straight, indicating a well-controlled loading condition. Figure 31 shows more details of the fracture along the central line of the specimen. The CGR test region shows a transganular morphology, and the JR test region is a ductile dimple fract ure. A significant crack extension can be seen during the JR test, which is consistent with the low fracture resistance observed in this sample.

In the CGR test region, the initial fracture close to the machined notch is characterized by coarse ledges resulting from fatigue fracture (Fig. 32), while at the later stage of the CGR test, the fracture surface is smoother (Fig. 33). Details of ductile dimple fracture in the JR test region can be seen in Fig. 34.

50 Figure 30. Fracture surface of specim en B-N1 tested in PWR water.

Field Co 51 Figure 31. Fracture surface of specimen B-N1 along the sample central line.

CGR test Fatigue fracture TG Dimple fractureCrack advance JR test Post-JR fatigue Machined

notchField Co 52 Figure 32. Transgranular fracture in specim en B-N1 at the beginning of the precracking. Crack pr opagation from bottom to top.

Field Co 53 Figure 33. Transgranular fracture in speci men B-N1 at the end of the precracki ng. Crack propagation from bottom to top.

Field Co 54 Figure 34. Ductile dimple fracture in the JR test region of specimen B-N1. Crack propagation from bottom to top.

Field Co 55 3.1.2.2 Irradiated specimen B-1 tested in PWR water Crack growth rate test Specimen B-1 was a 0.08-dpa thermally aged CF-3 CASS with ~24% ferrite. This sample was obtained from the same heat (Heat 69) as specimens A-1 and A-2. The thermal aging condition was 400°C and 10,000 hr. The objective was to compare the results with those of the unaged CF-3 at the same dose level. The CGR test conditions and results ar e summarized in Table 8, and a crack-length history plot is shown in Fig. 35.

The test was started with fatigue pre-cracking at a maximum stress intensity factor of

~18 MPa m1/2 and a load ratio of 0.3. A triangular waveform was used with a frequency of 1 Hz. Upon successful crack initiation, the maximum stress intensity f actor was lowered to prepare for transitioning. In the following te st periods, the load ratio and rise time were increased slowly to stimulate environmentally enhanced cracking. Despite the effort, the measured CGRs fell well below the fatigue growth rate line. A more aggressive loading condi tion was applied to re-activate the fatigue crack. Following that, repeated attempts (test periods e-j, k-q, and r-af) were made to induce environmental enhanced cracki ng. However, no elevated CGRs could be maintained. It appears that cracking cannot be sustained with a load ratio higher than 0.4 and a stress intensity factor less than 19 MPa m 1/2 in this sample. Environmentally enhanced cracking was only observed after increasing the stre ss intensity factor to about 22 MPa m 1/2. At this stress intensity, the measured CGR by the end of the cyclic test (period ao) was a factor of three higher than the fatigue crack growth rate in air. Subs equently, the test was set at constant load with PPU every 2 hr in test period

1. A CGR of 2.8 x 10

-11 m/s was obtained at a stress intensity factor of 22 MPa m 1/2.

56 Table 8. CGR test of specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water.

a Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm Start 0.52 6.019 a 3.0 319 0.27 0.38 0.38 0.12 17.6 12.8 7.88E-08 6.55E-08 6.250 b 6.7 320 0.39 0.35 0.35 0.15 17.0 10.4 4.11E-08 4.06E-08 6.442 c 10.5 320 0.50 0.64 0.64 0.36 15.8 7.9 2.75E-09 9.97E-09 6.460 d 20.1 320 0.45 1.32 1.32 0.68 15.9 8.8 3.63E-10 6.50E-09 6.466 e 23.3 320 0.27 0.37 0.37 0.13 17.9 13.1 5.89E-08 7.29E-08 6.698 f 25.4 320 0.27 0.36 0.36 0.14 17.1 12.4 4.86E-08 6.25E-08 6.822 g 27.8 320 0.38 0.34 0.34 0.16 16.8 10.4 2.77E-08 4.22E-08 6.905 h 32.2 320 0.38 0.66 0.66 0.34 16.3 10.1 8.33E-09 1.93E-08 6.950 i 44.2 319 0.44 3.20 3.20 1.80 16.2 9.2 2.63E-10 3.07E-09 6.955 j 47.5 320 0.38 1.32 1.32 0.68 16.2 10.0 7.00E-10 9.49E-09 6.958 k b 52.3 320 0.37 0.67 0.67 0.33 17.1 10.8 1.01E-08 2.32E-08 7.000 l 59.1 320 0.37 3.36 1.34 1.64 17.2 10.9 4.05E-09 4.80E-09 7.048 m 69 320 0.37 6.71 1.34 3.29 17.4 11.0 2.24E-09 2.48E-09 7.092 n 82.8 320 0.42 13.0 3.25 6.99 17.3 10.1 3.18E-10 1.01E-09 7.102 o 95.2 320 0.36 13.4 3.35 6.60 17.3 11.0 4.09E-11 1.23E-09 7.104 p 105 320 0.36 6.73 3.37 3.27 17.5 11.2 1.01E-10 2.59E-09 7.107 q 116.8 320 0.36 13.6 3.40 6.41 17.8 11.5 9.16E-11 1.40E-09 7.107 r 124.2 320 0.36 1.36 1.36 0.64 18.0 11.5 3.85E-09 1.43E-08 7.142 s 130.6 320 0.36 3.39 1.36 1.61 18.1 11.6 2.66E-09 5.86E-09 7.171 t 142.5 320 0.36 6.77 1.35 3.23 18.2 11.7 1.21E-09 3.00E-09 7.201 u 145.9 319 0.35 13.5 3.38 6.46 18.1 11.7 9.45E-11 1.49E-09 7.202 v 164.7 319 0.35 13.6 3.40 6.40 18.3 11.9 1.23E-10 1.56E-09 7.207 w 170 319 0.35 10.2 3.40 4.80 18.4 11.9 1.53E-10 2.09E-09 7.208 x 188.7 319 0.35 10.2 3.42 4.75 18.9 12.3 1.39E-09 2.28E-09 7.251 y 199 319 0.35 20.5 3.42 9.51 18.9 12.3 7.57E-10 1.15E-09 7.265 z 238 319 0.35 41.0 3.41 19.0 19.1 12.3 2.14E-10 5.88E-10 7.287 aa 284.5 319 0.36 81.8 8.18 38.2 19.3 12.4 2.61E-10 3.00E-10 7.318 ab 356.3 319 0.35 204.4 8.18 95.6 19.2 12.4 4.36E-11 1.20E-10 7.325 ac 442.2 318 0.35 413.0 8.26 187.0 19.9 12.9 2.88E-11 6.65E-11 7.331 ad 501.5 319 0.35 693.0 8.32 307.0 20.2 13.2 1.41E-11 4.26E-11 7.334 ae 527.8 318 0.46 78.0 7.80 42.0 19.9 10.8 5.94E-12 2.23E-10 7.336 af 574.5 318 0.45 39.4 7.88 20.6 20.6 11.3 2.11E-10 4.98E-10 7.356 ag 644.4 318 0.45 78.8 7.88 41.2 20.6 11.2 negligible 2.48E-10 7.355 ah 648 318 0.29 3.97 3.97 1.03 22.5 15.9 2.93E-08 1.30E-08 7.477 ai 650 318 0.34 11.7 3.89 3.32 22.3 14.7 7.43E-09 3.61E-09 7.509 aj 653.2 318 0.41 22.6 3.76 7.43 21.5 12.8 2.97E-09 1.27E-09 7.532 ak 671.3 318 0.39 45.8 9.16 14.2 22.6 13.8 1.02E-09 7.91E-10 7.577 al 693.8 318 0.39 91.5 9.15 28.5 22.8 14.0 7.29E-10 4.09E-10 7.618 am 717 318 0.46 194.0 7.76 106.0 22.1 11.9 4.90E-10 1.22E-10 7.644 an 747.7 319 0.46 387.7 7.75 212.3 22.0 11.9 1.66E-10 6.16E-11 7.655 ao 788.7 319 0.46 645.4 7.74 354.6 22.1 11.9 1.20E-10 3.71E-11 7.667 1 837.5 318 0.40 12 12 7200 22.1 13.3 2.81E-11 4.42E-12 7.675 a Simulated PWR water with 2 ppm Li and 1000 ppm B. Conductivity ~20

µS/cm. b The CGR value was obtained from the later part of the test period. Field Co 57 (a) 6.006.106.206.306.406.506.606.7048 12 1620242804812162024Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCabcdeCompliancetest Crack length Kmax (b) 6.706.75 6.806.856.90 6.95 7.00 7.057.104812162024282428323640444852Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCfhgijkCrack length Kmax (c) 6.957.00 7.05 7.10 7.15 7.207.2548 12 162024286080100120140Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oClmnopqrstuCrack length Kmax Figure 35. Crack-length-vs.-time plot for specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water: test periods (a) a-e, (b) f-k, (c) l-u, (d) v-y, (e) z-ac, (f) ad-ag, (g) ah-al, and (h) am-1. Field Co 58 (d) 7.157.20 7.257.3048 12 16202428150160170180190200Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCvwxyCrack length Kmax (e) 7.267.28 7.307.327.347.367.38481216202428200250300350400450Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCzaaabacCrack length Kmax (f) 7.307.327.347.367.387.4048 12 16202428480520560600640Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCafagaeadCrack length Kmax Figure 35. (Contd.)

59 (g) 7.357.40 7.45 7.50 7.55 7.607.6548 12 16202428650660670680690Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCahaiajakalCrack length Kmax (h) 7.607.627.647.66 7.68 7.707.72481216202428700750800850Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCamanao1Crack length Kmax Figure 35. (Contd.)

The cyclic CGRs with a significant crack extension (defined as >10

µm) from this sample are plotted in Fig. 36 along with the corrosion fatigue curve for unirradiated SSs. No elevated corrosion fatigue response can be seen for this material despite its thermal aging condition. Comparing Figs. 14 and 36, we foun d the corrosion fatigue behavi ors of unaged and aged CF-3 to be similar in PWR water. It seems that a combination of irradiation damage and thermal aging does not increase cracking suscep tibility of CF-3 at 0.08 dpa, as would be expected.

All SCC CGRs obtained from specimens A-N1, A-1, A-2, B-N1, and B-1 are shown in Fig. 37.

The open and closed symbols are for the unaged and thermally aged CF-3, and the blue and red symbols are for the unirradiated and irradiated specimens, respectively. The data points are all well below the NUREG-0313 curve, as expected in such low-corrosion-potential environments. No obvious difference can be seen between the unirradiated and irradiated specimens. The SCC CGRs of the unaged CF-3 (specimens A-1 and A-2) are also similar for the low-DO high-purity water and PWR water environments. The thermally aged specimen (B-1) has a slightly lower 60 CGR than that of unaged CF-3. However, given the large scatter of the SCC CGR data, this difference is considered insignificant.

10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-dTest periods: e-jTest periods: k-qTest periods: r-afTest periods: ah-aoCGRenv (m/s)CGRair (m/s)Specimen B-1CF-3, aged, 0.08 dpaPWR water, 320 oCCF curve for 0.2 ppm DO by Shack & KassnerTest periods with <10

µm extension are excluded.

Figure 36. Cyclic CGRs of specimen B-1.

10-1310-1210-1110-1010-910-8101520253035Spec. A-N1, unirr., PPU 2 hr, Low-DO water Spec. A-1, 0.08 dpa, PPU 2 hr, PWR waterSpec. A-2, 0.08 dpa, PPU 2 hr, Low-DO waterSpec. A-2, 0.08 dpa, PPU 1 hr, Low-DO waterSpec. A-2, 0.08 dpa, w/o PPU, Low-DO water Spec. B-N1, unirr., PPU 2 hr, PWR waterSpec. B-N1, unirr., w/o PPU, PWR waterSpec. B-1, 0.08 dpa, PPU 2 hr, PWR waterCGR (m/s)K (MPa m1/2)NUREG-0313CurveCASS CF-3 with 24% ferrite low-DO high-purity or PWR water318 - 320 oCOpen = UnagedClosed = AgedBlue = UnirradiatedRed = 0.08 dpa Figure 37. SCC CGRs of unaged and thermally aged CF-3 with 24% ferrite. Field CoField Co 61 Fracture toughness J-R curve test After the CGR test, a fracture toughness J-R curve test was conducted on the same sample at 320°C in PWR water. The obtained J-R curve is shown in Fig. 38. A power-law fitting of the JR data gives a relationship of J = 362a0.85. The estimated J value is about 116kJ/m 2 at the 0.2-mm offset line. The J-R curve data cannot be validated with the ASTM standard because both measurements of the initial and final crack size did not meet the requirements. The data points above the Jmax limit were also used in the analysis.

01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen B-1CF-3, aged, 0.08 dpaPWR water, 320 oCJ= 362*a0.85JQ=116 kJ/m 2Jmaxamax Figure 38. The J-R curve for specimen B-1.

Fractographic examination

Fractographic analysis of specimen B-1 was carri ed out with replicas, and Fig. 39 shows the entire fracture surface. Transgranular cleavage-like cracking is the dominant fracture morphology in the CGR test region.

The overall crack extension for the CGR test is a little more on one side of the sample than the other, leading to a slightly skewed crack front. Figure 40 shows an enlarged view of the fracture surface along the sample central line. The CGR and JR test regions can be clearly dis tinguished by their appearance. The CGR region is relatively flat, and the JR region indicates heavily deformed ductil e tearing. Note that ai r bubbles trapped in the replica are more excessive in the JR test region, a rough and dimpled fracture surface, than the flat CGR test region. Casting dendrite morphology with ferrite core s was seen at the end of the CGR test.

Cleavage-like morphology dominated the fatigue pre-cracking region, as shown in Fig. 41. Deformation steps are clearly visible on the fracture surface. With the advance of the crack, an area with delta ferrite at dendrite cores starte d to appear (Fig. 42). Compared with the surrounding austenitic phase, fewer deformation step s can be seen within the ferrite phase, as shown in Fig. 43. Field Co 62 Figure 39. Fracture surface of specimen B-1 tested in PWR water.

Field Co 63 Figure 40. Fracture surface of specimen B-1 along the sample central line. Machine notch CGR test JR test Fatigue crackingVermicular

ferrite at dendrite cores Dimple fractureCrack advance Post-JR fatigue Field Co 64 Figure 41. Deformation steps in the pr e-cracking region of speci men B-1. Crack propag ation from bo ttom to top.

Field Co 65 Figure 42. Delta ferrite at dendrite cores in specimen B-1. Crack propagation from bottom to top.

Field Co 66 Figure 43. Fracture surface of specimen B-1 showing that delta ferrite is surrounded by heavily deformed austenite phase. Cra ck propagation from bottom to top.

Field Co NUREG/CR-7184 ANL-12/56

Crack Growth Rate and Fracture Toughness Tests on Irradiated Cast Stainless Steels

Y. Chen, B. Alexandreanu, and K. Natesan Nuclear Engineering Division Argonne National Laboratory

A. S. Rao, NRC Project Manager

Draft Manuscript: March 2014 Revised Manuscript: December 2014

iii ABSTRACT Cast austenitic stainless steel, which has a ferrite-austenite duplex microstructure, is used in the cooling system of light water reactors for components with complex shapes, such as pump casings, valve bodies, and coolant piping. In the present study, crack grow th rate and fracture toughness J-R curve tests were performed on irradi ated cast stainless st eels and unirradiated control samples in low-corrosion-potential environments (high-purity water with low dissolved oxygen or simulated pressurized water reactor) at 320°C. Both as-received and thermally aged materials were included to investigate the combined effect of thermal aging and irradiation embrittlement on the fracture behavior of cast stainless steels. The samples were irradiated to approximately 0.08 dpa at the Halden reactor. Good resistance to corrosion fatigue and stress corrosion cracking was observed for all cast stainless steel specimens. Thermal aging had little effect on the crack growth be havior at 0.08 dpa. Cleavage-like fracture was the dominant cracking morphology during the crack growth rate tests, and the ferrite phase was deformed to a lesser extent compared with the surrounding au stenite phase. The fracture toughness results showed a dominant effect of neutron irradiati on, and the fracture resistances were decreased considerably for all cast specimens regardless of their thermal aging. The reduction in fracture toughness was more significant in the unaged than thermally aged specimens. Nonetheless, the fracture toughness values of thermally aged specimens were 20-30% lower than their unaged counterparts, suggesting a combined effect of thermal aging and neutr on irradiation in cast stainless steel.

v FOREWORD Cast austenitic stainless steel (C ASS) is used in the cooling system of light water reactors for components with complex shapes, such as pump casings, valve bodies, coolant piping, control rod guide tubes and housing, etc.

The ferrite phase is vulnerable to thermal aging embrittlement after long-term exposure to reactor coolant. In addition, neutron irradiation can decrease the fracture resistance of CASS significantly. It is suspected that a combined effect of thermal aging and irradiation embrittlement could reduce the fracture resistance even further to a level neither of these degradation mechanisms can impart alone.

In the early 1990s, NRC has conducted research on investigating the long term embrittlement of cast duplex stainless steels under LWR systems. Under those programs, experimental data was collected on the microstructure and the mechani cal properties of cast steels were subjected to commercial and experimental heats as well as reactor aged cast st eels (such as CF-3, CF-8 and CF-8M). The tests concluded that the chemical composition of the st eels and the ferrite morphology strongly affect the kinetics of the embrittlement of steels. For example, while the low carbon cast steels (CF-3) are most resistant to embrittlement, the high carbon cast steels with molybdenum (CF-8M) are the most susceptible to embrittlement. The delta ferrite content of these steels ranged between 3 to 30%

, and the steels containing

> 5% ferrite in the steels have either a lacy or acicular vermicular ferrite morphology. The embrittlement in these steels was found to occur when the failure is dominated by brit tle fracture that is associated with either cleavage of ferrite and or the separation of ferrite/austenite phase boundaries.

While the thermal aging embrittlement of CASS has been studied extensively, there are no data available at present with regard to the synergistic combined effect of thermal aging and irradiation embrittlement. In response to a 2009 request from the Office of Nuclear Regulatory Research, a test program has been initiated to investigate the joint effect of thermal aging and irradiation damage on the cracking suscepti bility and fracture resistance of CASS.

Crack growth rate (CGR) and fr acture toughness J-R curve tests we re conducted on three grades of CASS (CF-3, CF-8 and CF-8M) containing high levels of delta ferrite (>23%). These samples were irradiated at the Halden reactor to a low dose of 0.08 displacements per atom (dpa). Both as-received (labeled as un-aged) and thermally aged specimens were investigated to determine the combined effect of thermal aging and irradiation embrittlement. The CGR tests were conducted in a low dissolved oxygen (DO) water environment or a pressurized water reactor (PWR) environment at 320°C. Following the CGR tests, the fracture toughness - J-R curve tests were performed on the same samples in the test environments.

The CASS materials showed good resistance to both corrosion fatigue and stress corrosion cracking (SCC) at the 0.08 dpa dose level. Tr ans-granular cleavage-like cracking was the dominant fracture mode during the CGR tests, and it was found that the extent of deformation of ferrite phase was significantly lesser than the surrounding austenite phase of the steel. The neutron irradiation even at low dose of 0.08 dpa significantly affected on the fracture toughness of both aged and un-aged CASS. The extent of irradiation-induced embrittlement is more evident in the un-aged than aged CASS alloys.

vi Brian Thomas, Director (Acting)

Division of Engineering Office of Nuclear Re gulatory Research U.S. Nuclear Regulatory Commission

vii TABLE OF CONTENTS

Abstract .................................................................................................................................... iii Foreword .................................................................................................................................. v Figures...................................................................................................................................... ix Tables ....................................................................................................................................... xiii Executive Summary ................................................................................................................. xv Acknowledgments.................................................................................................................... xvii Acronyms and Abbreviations ..................................................................................................

xix 1 Introduction .......................................................................................................................... 1 2 Experimental ........................................................................................................................ 5 2.1 Materials and Specimens .......................................................................................... 5 2.2 Irradiation...............................................................................................................

... 7 2.3 Test Facility ............................................................................................................

.. 9 2.4 Crack Growth Rate and Fracture Toughness J-R curve Tests .................................. 10 2.4.1 Crack Growth Rate Test ............................................................................... 10 2.4.2 Fracture Toughness J-R curve Test .............................................................. 12 2.4.3 Fractographic Examination ........................................................................... 14 3 Results .................................................................................................................................. 15 3.1 CF-3 Cast Stainless Steel .......................................................................................... 15 3.1.1 Unaged CF-3 CASS ...................................................................................... 15 3.1.2 Thermally Aged CF-3 CASS ........................................................................ 44 3.2 CF-8 Cast Stainless Steel .......................................................................................... 67 3.2.1 Unaged CF-8 CASS ...................................................................................... 67 3.2.2 Thermally Aged CF-8 CASS ........................................................................ 89 3.3 CF-8M Stainless Steel ............................................................................................... 111 3.3.1 Unaged CF-8M CASS .................................................................................. 111 3.3.2 Thermally Aged CF-8M CASS .................................................................... 123 4 Discussion ....................................................................................................................... 135 4.1 Cyclic Crack Growth Rates ...................................................................................... 136 4.2 Constant-load Crack Growth Rates .......................................................................... 139 4.3 Fracture Toughness ................................................................................................... 141 5 Summary ......................................................................................................................... 145 References ................................................................................................................................ 147

ix LIST OF FIGURES 1. Metallurgical images of the unaged and thermally aged CASS materials. ........................... 6

2. Schematic of 1/4T-CT specimen used in this study (red lines represent electrical leads). ... 7
3. Irradiation capsule of low-dose Halden irradiation. .............................................................. 8
4. Recirculation water loop for Cell 1 of the IML (items in red are safety significant components) ...................................................................................................................

..... 10 5. Schematic for inducing environmentally assisted cracking in test environment. ................. 12

6. Crack-length-vs.-time plot for specimen A-N1 (unirradiated and unaged CF-3 with 24%

ferrite): test periods (a) a-c, (b) d-j, (c) k-p, (d) q-u, (e) v-aa, and (f) 1. ............................. 17

7. Cyclic CGRs of specimen A-N1. .......................................................................................... 19
8. The J-R curve for specimen A-N1. ....................................................................................... 20
9. Fracture surface of specimen A-N1 tested in low-DO high-purity water. ........................... 21
10. Fracture surface of specimen A-N1 along the sample central line. ..................................... 22
11. Transgranular fracture in the precracking region of specimen A-N1. ............................... 23
12. Ductile dimple fracture in JR test region of specimen A-N1. ............................................ 24
13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% ferrite):

test periods (a) a-e, (b) f-j, (c) k-m, and (d) 1. .................................................................... 26

14. Cyclic CGRs of specimen A-1. ........................................................................................... 2 8 15. Fracture surface of specimen A-1 tested in PWR water..................................................... 29
16. Fracture surface of specimen A-1 along the sample central line. ........................................ 30
17. Cleavage-like fracture in specimen A-1: (a

) pre-cracking, and (b) end of the CGR test. Crack propagation from bottom to top............................................................................... 31

18. Fracture surface of specimen A-1 showing that delta ferrite deformed to a lesser extent than austenite. Crack propagation from bottom to top. .................................................... 33
19. Crack-length-vs.-time plot for specimen A-2 (0.08-dpa unaged CF-3 with 24% ferrite) tested in low-DO high-purity water environment: test periods (a) a-e, (b) f-j, (c) k-o, (d) p-s, and (e) 1-2. .............................................................................................................

...... 35

20. Cyclic CGRs of specimen A-2. ........................................................................................... 3 7 21. The J-R curve for specimen A-2. ........................................................................................ 3 8 22. Fracture surface of specimen A-2 tested in low-DO high-purity water. ............................ 39
23. Fracture surface of specimen A-2 along the sample central line. ........................................ 40
24. Cleavage-like steps at the beginning of CGR test of specimen A-2. Crack propagation from bottom to top. ............................................................................................................ 41 25. Fracture surface of specimen A-2 showing that ferrite deformed to a lesser extent than austenite. Crack propagation from bottom to top. ............................................................ 42
26. Ductile dimple morphology in the JR test region of specimen A-2. Crack propagation from bottom to top. ............................................................................................................ 43 27. Crack-length-vs.-time plot for specimen B-N1 (unirradiated, aged CF-3 with 24% ferrite) in PWR water: test pe riods (a) a-d, (b) e-h, (c) i-n, (d

) o-s, (e) t-w, (f) x-aa, and (g) 1a-1b. ....................................................................................................................

......... 45

28. Cyclic CGRs of specimen B-N1. ........................................................................................ 48
29. The J-R curve for specimen B-N1. ...................................................................................... 48 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

x 30. Fracture surface of specimen B-N1 tested in PWR water. ................................................. 50

31. Fracture surface of specimen B-N1 along the sample central line. ..................................... 51
32. Transgranular fracture in specimen B-N1 at the beginning of the precracking. Crack propagation from bottom to top. ........................................................................................ 52
33. Transgranular fracture in specimen B-N1 at the end of the precracking. Crack propagation from bottom to top. ........................................................................................ 53
34. Ductile dimple fracture in the JR test region of specimen B-N1. Crack propagation from bottom to top. ............................................................................................................ 54 35. Crack-length-vs.-time plot for specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water: test periods (a) a-e, (b) f-k, (c) l-u, (d) v-y, (e) z-ac, (f) ad-ag, (g) ah-al, and (h) am-1. .................................................................................................................

...... 57

36. Cyclic CGRs of specimen B-1. ........................................................................................... 6 0 37. SCC CGRs of unaged and thermally aged CF-3 with 24% ferrite. ..................................... 60
38. The J-R curve for specimen B-1. .........................................................................................

61 39. Fracture surface of specimen B-1 tested in PWR water. .................................................... 62

40. Fracture surface of specimen B-1 along the sample central line. ........................................ 63
41. Deformation steps in the pre-cracking region of specimen B-1. Crack propagation from bottom to top. ..................................................................................................................... 64
42. Delta ferrite at dendrite cores in specimen B-1. Crack propagation from bottom to top. . 65
43. Fracture surface of specimen B-1 showing that delta ferrite is surrounded by heavily deformed austenite phase. Crack propagation from bottom to top. .................................. 66
44. Crack-length-vs.-time plot for specimen E-N1 (unirradiated, unaged CF-8 with 23%

ferrite): test periods (a) a-c, (b) d-f, (c) g-i, (d) j-m, (e) n-q, (f) r-u, and (g) 1a-1b. ........... 68

45. Cyclic CGRs of specimen E-N1. ......................................................................................... 71
46. J-R curve data of specimen E-N1. ....................................................................................... 72
47. Fracture surface of specimen E-N1 tested in low-DO high-purity water. .......................... 73
48. Transgranular fracture at the beginning of the precracking for specimen E-N1. Crack propagation from bottom to top. ........................................................................................ 74
49. Transgranular fracture at the end of the CGR test for specimen E-N1. Crack propagation from bottom to top. ........................................................................................ 75
50. Ductile dimple fracture in the JR test region of specimen E-N1. Crack propagation from bottom to top. ............................................................................................................ 77 51. Crack-length-vs.-time plot for specimen E-1 (0.08-dpa unaged CF-8 with 23% ferrite):

test periods (a) a-d, (b) e-f, (c) g-j, (d) k-o, and (e) p-2. ..................................................... 79

52. Cyclic CGRs of specimen E-1. ............................................................................................

81 53. The J-R curve of specimen E-1. ..........................................................................................

82 54. Fracture surface of specimen E-1 tested in low-DO high-purity water. ............................ 83

55. Fracture surface of specimen E-1 along the sample central line. ........................................ 84
56. Cleavage-like cracking at the beginning of the CGR test of specimen E-1. Crack propagation from bottom to top. ........................................................................................ 85
57. Cyclic CGR test region of specimen E-1. Crack propagation from bottom to top. .......... 86 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xi 58. Smooth fracture surface at the end of the CGR test in specimen E-1. Crack propagation from bottom to top. ............................................................................................................ 87 59. Ductile dimple fracture in the J-R test region of specimen E-1. Crack propagation from bottom to top. ..................................................................................................................... 88

60. Crack-length-vs.-time plot for specimen F-N1 (unirradiated, aged CF-8 with 23%

ferrite): test periods (a) a-d, (b) e-h, (c) i-m, (d) n-q, and (e) 1a-1b. .................................. 90

61. Cyclic CGRs of specimen F-N1. ......................................................................................... 92
62. The J-R curve of specimen F-N1. ....................................................................................... 93
63. Fracture surface of specimen F-N1 tested in low-DO high-purity water. .......................... 94
64. Fracture surface of specimen F-N1 along the sample central line. ..................................... 95
65. Transgranular fracture in the CGR test of specimen F-N1: (a) in the precracking region and (b) at the end of CGR test. Crack advance direction from bottom to top. ................ 96
66. Transition region from CGR to J-R curve tests of specimen F-N1. Crack advance direction from bottom to top. ............................................................................................. 98
67. Ductile dimple fracture in the JR test region of specimen F-N1. Crack advance direction from bottom to top. ............................................................................................. 10 0 68. Crack-length-vs.-time plot for specimen F-1 (0.08-dpa aged CF-8 with 23% ferrite): test periods (a) a-f, (b) g-j, (c) k-m, and (d) 1. .................................................................... 102
69. Cyclic CGRs of specimen F-1. ............................................................................................

103 70. SCC CGRs of unaged and aged CF-8 CASS with 23% ferrite. .......................................... 104

71. The J-R curve of specimen F-1. ..........................................................................................

105 72. Fracture surface of specimen F-1 tested in low-DO high-purity water. ............................. 106

73. Fracture surface of specimen F-1 along the sample central line. ........................................ 107
74. Fracture surface of the CGR region in specimen F-1. Crack propagation from bottom to top. .......................................................................................................................

........... 108

75. Deformation steps in austenite grain ar ound ferrite phase in the CGR test region of specimen F-1. Crack propagation from bottom to top. ..................................................... 109
76. Dimple fracture in the JR test region of specimen F-1. Crack propagation from bottom to top. ................................................................................................................................. 110
77. Crack-length-vs.-time plot for specime n I-1 (0.08-dpa unaged CF-8M with 28% ferrite): test periods (a) a-f, (b) g-k, (c) l-o, (d) p-r, (e) s-v, (f) w-z, (g) aa-ac, and (h) 1. .. 113
78. Cyclic CGRs of specimen I-1. .............................................................................................

116 79. The J-R curve of specimen I-1. ...........................................................................................

116 80. Fracture surface of specimen I-1 tested in low-DO high-purity water. .............................. 118

81. Fracture surface of specimen I-1 along the sample central line. ......................................... 119
82. Precracking region in the CGR test of specimen I-1. Crack propagation from bottom to top. ..................................................................................................................................... 120
83. Fracture surface at the end of CGR test of specimen I-1. Crack propagation from bottom to top. ..................................................................................................................... 121
84. Heavily deformed micros tructure in the JR test region of specimen I-1. ........................... 122
85. Crack-length-vs.-time plot for specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite):

test periods (a) a-g, (b) h-n, (c) o-r, (d) s-u, (e) 1a-1b, and (f) 2a-2c. ................................. 124 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xii 86. Cyclic CGRs of specimen J-1. ............................................................................................

127 87. SCC CGRs of unaged and aged CF-8M CASS, irradiated to 0.08 dpa. ............................. 127

88. The J-R curve of specimen J-1. ...........................................................................................

128 89. Fracture surface of specimen J-1 tested in low-DO high-purity water. ............................. 129

90. Fracture surface of specimen J-1 along the sample central line. ......................................... 130
91. Precracking region of specimen J-1. Crack propagation from bottom to top. .................. 131
92. Ferrite microstructure at the end of CGR test of specimen J-1. Crack propagation from bottom to top. ..................................................................................................................... 132
93. Cleavage-like fracture at the end of CGR test of specimen J-1. Crack propagation from bottom to top. ..................................................................................................................... 133
94. Fracture along ferrite at dendrite core in the JR test region of specimen J-1. Crack propagation from bottom to top. ........................................................................................ 134
95. Best-fit curves of cyclic CGRs at 0.08-dpa dose: (a) unage d and aged CF-3, (b) unaged and aged CF-8, and (c) unaged and aged CF-8M. .............................................................. 137
96. Fitting coefficient A for the corrosion fatigue superposition model. .................................. 139
97. Constant-load CGRs of the low-dose CASS with more than 23% ferrite in low-DO high-purity and PWR water environments. ........................................................................ 140
98. Fracture toughness values of unirradiated and irradiated CASS in unaged and aged conditions. Note that most of the results are from 1/4T-CT specimens tested at ~320°C in water environments. The unirradiated results for CF-8M CASS are from 1T-CT specimens tested at ~290°C in an air atmosphere. .............................................................. 142 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xiii LIST OF TABLES 1. Chemical compositions of the cast stainless steels examined in this study. ....................... 5

2. Thermal aging conditions for the cast stainless steels in this study. ................................... 5
3. Test matrix of unirradiated and irradiated low-dose CASS specimens. ............................. 15
4. Crack growth rates of specimen A-N1 (unirradiated and unaged CF-3 with 24% ferrite) in low-DO high-purity water. ............................................................................. 16
5. Crack growth rates of specimen A-1 (0.08-dpa unaged CF-3 sample with 24% ferrite) in PWR water. ..................................................................................................... 2 5 6. Crack growth rates of specimen A-2 (0.08-dpa unaged CF-3 with 24% ferrite) in low-DO high-purity water environment. ........................................................................... 34
7. CGR test of specimen B-N1 (unirradiated, thermally aged CF-3 with 24% ferrite) in PWR water. .................................................................................................................... 44 8. CGR test of specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water. ........ 56
9. CGR test of specimen E-N1 (unirradiated, unaged CF-8 with 23% ferrite) in low-DO high-purity water. ........................................................................................................

67 10. CGR test of specimen E-1 (0.08-dpa unaged CF-8 with 23% ferrite) in low-DO high-purity water. ............................................................................................................... 78 11. Crack growth rates of specimen F-N1 (unirradiated, aged CF-8 with 23% ferrite) in a low-DO high-purity water environment. .................................................................... 89

12. Crack growth rates of specimen F-1 (0.08-dpa aged CF-8 with 23% ferrite) in a low-DO high-purity water environment. ........................................................................... 101
13. Crack growth rates of specimen I-1 (0.08-dpa unaged CF-8M with 28% ferrite) in a low-DO high-purity water environment.......................................................................... 112
14. Crack growth rates of specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite) in a low-DO high-purity water environment. ........................................................................... 123
15. CGR test results at ~320°C for CASS specimens with high ferrite contents. ................... 135
16. Fracture toughness JR test results for CASS with high ferrite contents. ........................... 136 Field CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField CoField Co

xv EXECUTIVE SUMMARY Cast austenitic stainless steel (C ASS) is used in the cooling system of light water reactors for components with complex shapes, such as pum p casings, valve bodies, coolant piping, and control rod guide tube spacers. In contrast to a fully austenitic microstructure of wrought stainless steel (SS), CASS consists of a ferrite-austenite duplex microstructure. A certain amount of delta ferrite phase is intentionally designed into CASS and SS weld metals to engineer against hot cracking. However, the ferrite phase is vulnerable to thermal aging embrittlement after long-term exposure to reactor coolant. In addition, neutron irradi ation can decrease the fracture resistance of CASS significantly. It is suspected that a combined effect of thermal aging and irradiation embrittlement could reduce the fracture resistance even further to a level neither of these degradation mechanisms can impart alone. While the thermal aging embrittlement of CASS has been studied extensively, no data are available at present with regard to the combined effect of thermal aging and irradiation embrittlement. A test program has been initiated to investigate the joint effect of thermal aging and irradiation damage on the cracking susceptibility and fracture resistance of CASS.

Crack growth rate (CGR) and fracture toughness J-integral resi stance (J-R) curve tests were conducted on three grades of CASS (CF-3, CF-8, and CF-8M) containing high levels of delta ferrite (>23%). The samples were irradiated at the Halden reactor to a low dose of 0.08 displacements per atom (dpa) or 5.56 x 10 19 n/cm2 (E > 1 MeV). Both as-received and thermally aged specimens were included to determine the combined effect of thermal aging and irradiation embrittlement. The CGR tests were conducted at ~320°C on the irradiated and unirradiated control samples in a high purity water envir onment with low dissolved oxygen (DO) or a pressurized water reactor (PWR) environment. Following the CGR tests, the fracture toughness J-R curve tests were performed on the same samples in the test environments.

Cyclic CGRs and constant-load CGRs were measured to evaluate the corrosion fatigue and stress corrosion cracking (SCC) of the CASS specimens. In the cyclic CGR tests, environmental enhanced cracking was more difficu lt to establish in the CASS specimens than in wrought SSs. In SCC CGR tests, only moderate CGRs, in the range of 10

-11 m/s, were recorded in most of the CASS specimens regardless of their thermal agin g history. In general, the CASS materials showed good resistance to both co rrosion fatigue and SCC without irradiation and at 0.08 dpa. Transgranular cleavage-like cracking was the dominant fracture mode during the CGR tests, and the ferrite phase was often deformed to a less extent compared with its surrounding austenite phase. This observation supports the hypothesis that the beneficial effect of ferrite for SCC resistance arises, in part, from the low plastic deformation of ferrite phase compared to austenite.

A previous study showed that the SCC CGRs of thermally aged CASS were one order of magnitude higher than those of unaged materials at high DO concentrations (>1 ppm). In contrast, a similar cracking behavior between thermally aged and unaged materials was observed for irradiated and unirradiated control specimens in the low-DO high-purity water and simulated PWR water. The lack of sensitivity to thermal aging history in these tests might be a result of the low-corrosion-potential environments employed. The effect of neutron irradiation on the SCC behaviors of CASS materials was not clear in the pr esent study.

xvi Neutron irradiation did significan tly affect the fracture toughn ess of CASS. At 0.08 dpa, the fracture toughness values of unaged specimens were much lower than thei r initial unirradiated values. Fracture toughness was also reduced by 20-30% for thermally aged specimens after irradiation. This observation s uggests an interaction between thermal aging and irradiation damage. When both conditions of thermal aging and irradiation damage are present, the combined effect is not a simple addition of bot h degradations. The interaction between thermal aging and irradiation damage can lead to different microstructural evolutions in CASS materials (viz. by prompting G-phase formation through radiation-induced segregations of minor elements), reducing the fracture resistance to a higher extent than either one of them can achieve alone. Neutron irradiation appears to affect not only the kinetics of thermal aging embrittlement, but also the lower bound values of fracture toughness (i.e., the satu ration state). For this reason, the effects of neutron irradiation should be considered when the degree of thermal aging embrittlement is being evaluated for CA SS components.

xvii ACKNOWLEDGEMENTS The authors would like to thank Drs. O. K. Chopra and W. J. Shack for their invaluable contribution to this project. Our special tha nks go out to Ms. T. M. Karlsen, OECD Halden Reactor Project, Halden, Norway, for reactor irradiation experiments and sample transfer. L. A.

Knoblich, E. E. Gruber, R. Clark, and E. J. Listwan of Argonne, Y. Yang of University of Florida, and J. Pakarinen and Y. Huang of University of Wi sconsin-Madison are acknowledged for their contributions to the experimental effort. We are also gr ateful to Drs. R. Tregoning and W. J. Shack for their careful reviews and comments on the manuscript. This work is sponsored by the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, under Job Code N6519 and V6454; Program Manager: A. S. Rao.

xix ACRONYMS AND ABBREVATIONS ASTM American Society for Testing and Materials BWR Boiling Water Reactor CASS Cast Austenitic Stainless Steels CGR Crack Growth Rate CT Compact Tension DCPD Direct Current Potential Drop DO Dissolved Oxygen ECP Electrochemical Potential HWC Hydrogen Water Chemistry IASCC Irradiation-Assisted Stress Corrosion Cracking IGSCC Intergranular Stress Corrosion Cracking IML Irradiated materials Laboratory J-R J-Integral Resistance LVDT Linear Voltage Displacement Transducer LWR Light Water Reactor NWC Normal Water Chemistry PPU Periodical Partial Unloading PWR Pressurized Water Reactor SCC Stress Corrosion Cracking SEM Scanning Electron Microscopy SS Stainless Steel

TG Transgranular

xx Units of Measure

°C degrees centigrade cm centimeter dpa displacement per atom hr hour J joule m meter mm millimeter MPa megapascal ppb part per billion ppm part per million psig pound per square inch gauge s second micron 1 1 Introduction Stainless steel is an important class of engineering materials used in light water reactors (LWRs). While the main structure of reactor core internals is constructed from wr ought stainless steels (SSs), components with complex shapes, such as pump casings, valve bodies, coolant piping, elbows, and control rod guide tube spacers, are often made of cast austenitic stainless steels (CASS).11 The CF grades of CASS whose compositions are similar to those of 300-series austenitic SSs are the most widely used corrosion-resistant CASS. The CF-3 and CF-8 grades contain nominal 19% Cr and 9% Ni and are th e cast equivalents of Types 304L and 304 SSs, respectively. In addition, the CF-3M and CF-8M grades are the molybdenum-containing cast versions of 316L and 316 SSs, respectively. Like their wrought equivalents, the CF grades of CASS possess excellent corrosion resistance and mechanical properties, 22 and, thus, are ideal for LWR applications in aqueous environments. The strength and ductility of CF grades are comparable to those of wrought SSs.

At room temperature, the yiel d and tensile strengths of CF-3 and CF-8 grades are greater than 200 MPa and 480 MPa, 33 respectively, similar to those of solution-annealed SSs. A good combination of strength and ductility also gives rise to excellent fracture toughness. Stable tearing occurs in CASS samples well above thei r yield strength before final fracture. Chopra and Sather 44 showed that the crack initia tion toughness of CF-3 and CF-8 CASS varies from ~200 kJ/m 2 to over 1000 kJ/m 2 at room temperature; these values are comparable to those of wrought SSs reported by Mills.

55 In contrast to the fully austenitic microstructu re of wrought SSs, CASS consists of a ferrite-austenite duplex microstructure. It is common that a certain amount of ferrite is present along with austenite () in the solidification microstructure such as CASS or weld metals. The precise fractions of ferrite and austenite phases depend on the chemical composition and casting thermal history. Experimental methods, such as quantitative metallography and ferromagnetic measurement, can be used to determine the ferrite content in CASS or weld metals.

66,77 Empirical models have also been developed to predict and control ph ase content with alloy composition. Based on the phase-stabilizing effects of Cr and Ni in Fe-Cr-Ni systems, several constitution diagrams have been established to estimate phase content.

88,99,1010 The contributions of minor alloying elements are inco rporated with Cr and Ni equivalent numbers and computed with empirical equations. The applicable range of composition, incorporated alloying elements, and weighing factors vary in these models, and thus the predicted phase contents differ among them to some extent.1111 For steel castings of CF grades, use of the Scheofer diagram, which is a modified version of the Schaeffler diagram 88, is recommended by the American Society for Testing and Materials (ASTM) for estimating ferrite content.1212 The ferrite phase is critical for the mechanical properties and corrosion resistance of CASS and weld metals. Since hardening mechanisms by thermal-mechanical treatments cannot be easily implemented in castings, the strength of CASS mainly relies on the ferrite-austenite duplex microstructure. Beck et al. 1313 showed that the tensile and yiel d strengths of CASS increase with ferrite content up to 40% at both room and elevated temperatures. The strengthening effect of ferrite phase is attri buted to ferrite-austenite boundaries and can be explained with the Hall-Petch model.

1414 Ferrite phase is also crucial fo r the soundness and weld ability of steel castings. A minimum ferrite content is often specified for SS welds to reduce the tendency of hot cracking. In addition, the presence of ferrite phase can improve the resistance to Formatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormattegrammar 2 sensitization and stress corrosion cracking (SCC).1515 In susceptible environments, CASS tends to be more resistant to SCC than the same grade of wrought SS. This benefi cial effect of ferrite was clearly demonstrated by Hughes et al.1616 in boiling water reactor (BWR) environments.

Using slow strain rate tests, they showed that CF-3, CF-3A, and CF-8 have an exceptional resistance to intergranular SCC (IGSCC) in hi gh-purity (HP) water containing 6-8 ppm dissolved oxygen (DO).

While the presence of ferrite in CASS is mostly benefici al, ferrite phase can also exert a detrimental effect on the fracture resistance of CASS under certain conditions. Exposed to temperatures of 300-500°C, ferrite phase is vulnerable to a low-temperature thermal aging phenomenon known as "475°C embrittlement." 1717,1818 The consequences of the thermal aging embrittlement are an increased te nsile strength and reduced ductility.1919,2020 The upper-shelf impact energy is also reduced, and the ductile-to-brittle transition temperature shifts higher.2020 Because of thermal aging embrittlement, the long-term performance of CASS materials at elevated service temperatures is of concern. The degradation of CASS components resulting from thermal aging embrittlement has been recognized as a potential issue for aging reactors.2121 Several research programs have been conducted to assess Charpy impact properties and JR resistance curves of thermally aged CASS.

2222,2323 It was found that thermal aging at 290-450°C up to 30,000 hr leads to a significant deterior ation in the fracture properties of CASS. The lower bound of impact energy and fracture toughness (J IC) can be as low as 20 J/cm 2 and 25 kJ/m2, respectively, at room temperature. The duct ile-to-brittle transition temperature of CASS is also shifted to around 0°C for the thermally aged CASS. The extent of the thermal aging embrittlement increases with ferrite content and is sensitive to ferrite morphology.2222 The mechanism of thermal aging of dupl ex SSs has been studied extensively.1717-2020, 2222-2828 It is widely accepted that embrittlement is caused by the instability of the ferrite phase under thermal aging. The main reasons of the hardening and loss of toughness are (1) the formation of Cr-rich ' phase through spinodal decomposition, and (2) the precipitation and growth of carbides and G-phase at ferrite-a ustenite phase boundaries. Obviously, these microstructural changes are thermally activated and are fundame ntally controlled by solid-state diffusion. Therefore, the thermal aging time for a given exte nt of degradation (e.g., an increase in hardness or decrease in toughness) follows an Arrhenius-type relationship.2424 Besides thermal aging, neutron irradiation can also affect the micr ostructural evolution of CASS profoundly. Under fast neutron bombardment, lattice atoms in a crystalline material are displaced from their original sites by cascade damage. An avalanche of lattice displacements gives rise to point defect supersaturation, which does not exist under thermal equilibrium.2929,3030 These point defects evolve at irradiation temperatures to form irra diation defects, giving rise to irradiation hardening and embrittlement. The irradiation embrittlement can generate further degradation in the ferrite phase, leading to an addition al loss of fracture toughness. Furthermore, the presence of nonequilibrium point defects in irradiated microstructures can enhance the transportation of solutes in materials by radiation-enhanced diffusion.3030 The elevated diffusivity under neutron irradiation could certainly affect the kinetics of thermal aging.

Formatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gramFormatteand gram 3 Thus, a combined effect of irradiation embrittlement and thermal aging could not only produce a higher degree of embrittlement, but also affect the rate of degradation development.

While the thermal aging embrittlement of CASS has been studied extensively, very limited data exist in the open literature for neutron-irradiated CASS.

55,3131,3232 Two tests conducted on thermally aged and irradiated CF-8M showed a higher degree of embrittlement in the CASS material than wrought SSs.3131 It is not clear, however, if the simultaneous exposure to irradiation and thermal aging would reduce the fracture resistance to a lower level than either of the degradation mechanisms can impart alone. If so, the combined effect is not only important for internal components made of CASS, but also for SS weld metals that possess a similar austenite-ferrite duplex microstructure. While weld metals may contain less ferrite phase than that in CASS materials, a minimum ferrite content is usually specified for weld metals to engineer against hot cracking. Thus, weld metals may be subjected to the same type of degradation as CASS. A better understanding of the combined effect of thermal aging and irradiation would also be helpful to address issues c oncerning SS weld metals.

In the current study, several CF grades of CASS were irradiated to approximately 0.08 displacements per atom (dpa). Both as-received and thermally aged specimens are included.

Fracture toughness J-integral resistance (J-R) curve tests were conducted on the irradiated and unirradiated control samples to assess the extent of embrittlement resulting from neutron irradiation and thermal aging. Additionally, crack growth rate (CGR) tests were carried out prior to the fracture toughness tests in low electrochemical potential (ECP) environments to evaluate their SCC performance. At 0.08 dpa, irradiation embrittlement of ferrite phase is anticipated to some extent, but little effect either on embrittlement or stress corrosion susceptibility would be expected for austenite phase. Thus, the current work is focused on the effects of thermal aging and irradiation on the ferrite phase. At higher irradiation levels, additional effect would occur due to embrittlement and increased SCC susceptibility of austenite phase. Since elevated susceptibility to irradiation-assisted stress corrosion cracking (IASCC) is unlikely at this low dose, the SCC CGR test durations were kept relatively short in th e present study. Still, the CGR tests could provide corrosion fa tigue starter cracks for the su bsequent fracture toughness J-R curve tests, so that any environmental contribut ion to the fracture behavior of CASS could be detected.

Formatteand gramFormatteand gramFormatteand gramFormatteand gram

5 2 Experimental 2.1 Materials and Specimens Three experimental heats of corrosion-resistant CASS (CF-3, CF-8, and CF-8M) were obtained from a previous ANL research program.

44 The CASS heats are static cast slabs with dimensions of 610 x 610 x 76 mm. A ferrite scope measurement on these heats showed that the CF-3 and CF-8 heats contained approxima tely 24% and 23% delta ferrite, respectively, and the CF-8M heat had 28% delta ferrite. Table 1 gives the compositions of these materials. Both as-received (or unaged) and thermally aged specimens were prepared from these heats. The thermal aging conditions are given in Table 2. Figure 1 shows metallurgical images of the CASS materials used in this study. The samples were polished wi th SiC papers up to 1200 grit. After a final finish with 1 m diamond slurry, the polished surfaces were etched with ferric chloride. Both the unaged and thermally aged microstructures are similar, as shown in Fig. 1. Note that a slight decline in ferrite content was reported for all grades of CASS after thermal aging.

44 For the aging conditions used in the current study (10,000 hr at 400°C), the decrease in ferrite content is insignificant. Thus, the average ferrite contents were considered unchanged after thermal aging.

Table 1. Chemical compositions of the cast stainless steels examined in this study.

Cast Grade FerriteContent Heat ID Composition (wt. %) Measured a Calculated b Mn Si P S Mo Cr Ni N C CF-3 24% 21% 69 0.631.130.0150.0050.3420.18 8.59 0.028 0.023CF-8 23% 14% 68 0.641.070.0210.0140.3120.46 8.08 0.062 0.063CF-8M 28% 25% 75 0.530.670.0220.0122.5820.86 9.12 0.052 0.065

a. Measured with a ferrite scope,Ref. [

44]. b. Calculated with Hull's equations, Ref. [4]

Table 2. Thermal aging conditions for th e cast stainless steels in this study. Cast Grade Ferrite a Spec. Code Heat ID Thermal Aging Condition CF-3 24% A 69 Unaged B 69 10,000 hr at 400°C CF-8 23% E 68 Unaged F 68 10,000 hr at 400°C CF-8M 28% I 75 Unaged J 75 10,000 hr at 400°C

a. Measured with a ferrite scope,Ref. [

44].

Formatteand gramFormatteand gramField CoFormatteand gramField CoFormatteand gram 6 CF-3, 24% unaged CF-3, 24% , aged CF-8, 23% unaged CF-8, 23% aged CF-8M, 28% unaged (from ref. [

44] ) CF-8M, 28% aged Figure 1. Metallurgical images of the unaged and thermally aged CASS materials.

FormatteField Co 7 Constrained by irradiation space, sub-sized compact tension (CT) specimens were used in this study. The sample was about 6.5-mm thick (i.e., 1/4T-CT) and 14-mm high. The starter notch size was about 6 mm. To ensure an in-plane crack growth, side grooves approximately 5% of the thickness were machined on both sides of the sample. Figure 2 is a schematic of the sample used in this study. The red lines in the figure are electrical leads spot-welded on the specimen.

Figure 2. Schematic of 1/4T-CT specimen used in this study (red lines represent electrical leads).

2.2 Irradiation The CASS specimens were irradiated in a helium-filled capsule in the Halden reactor, a boiling heavy water reactor in Norway. Figure 3 shows the assembly of the irradiation capsule. Some disk-shaped samples for transmission electron microscopy analyses were also included in a cylindrical container in the irradiation capsule (item 7 in Fig. 3). The irradiation temperature was ~315°C, and two sets of melting alloy temperature monitors (item 5) were installed in the irradiation assembly.

During the irradiation experiment

, three fluence monitor wires (F e, Ni, and Al/Co alloy) were placed outside the irradiation capsule. After the irradiation, dosimetry was performed by the Halden researchers on these wires. Using the activation cross sections determined previously, they estimated the accumulated neutron fluence fo r the irradiation capsule. The obtained fast neutron fluence (E > 1MeV) was about 5.56 x 10 19 n/cm2, which corresponds to a displacement damage of 0.08 dpa for the samples.

Field Co 8

Figure 3. Irradiation capsule of low-dose Halden irradiation.

3333 4. End Guide Sleeve

8. Spacer 3. Heat Transfer Body
7. TEM Specimens Containe r 2. Outer Capsule
6. CT Specimens
1. End Plug
5. Melting Alloy Assembl y

Field CoFormattept, Checkby 3 pt 9 2.3 Test Facility Two servo-hydraulic mechanical test systems located in the Irradiated Materials Laboratory (IML) at Argonne National Laboratory were used in this study. The IML is a radiological facility equipped with four air-atmosphere beta-gamma hot cells. The hot cells are maintained at a negative pressure with respec t to the surroundings to maintain a proper radiological barrier. The two test systems are installed in separate hot cells. Each of th e hot cells is equipped with its own loading frame, autoclave, load cell, linear voltage displaceme nt transducer (LVDT), Instron control console, and data acquisition system.

In the current study, the tests were performed either in a simulated pressurized water reactor (PWR) environment or in a low-DO high-purity water environment. Both environments have low corrosion potentials, which are known to reduce the sensitivity of SSs to IASCC.3434 The simulated PWR water contained ~2 ppm lithium, ~1000 ppm boron, and ~2 ppm hydrogen. The conductivity was about 20

µS/cm. The low-DO high-purity wa ter contained less than 10 ppb dissolved oxygen, and the conductivity was kept below 0.07

µS/cm during the tests. The test environments were provided by two water recirculation loops, and a schematic diagram of loop #1 is shown in Fig. 4. Each loop consists of a storage tank, a high pressure pump, a regenerative heat exchanger, an autoclave, an ECP cell, a back-pressure regulator, two ion-exchange

cartridges, and several heaters.

The high-pressure section of the loop extends from the high-pressure pump (item 13 in Fig.

4) through the back-pressure regulator (item 10 in Fig. 4). The rest of the loop is kept at low pressure. Over-pressurization of the high-pressure portion of the system is prevented by two rupture disks (items 3 and 15 in Fig. 4) located at the downstream of the high-pressure pump. The autoclaves are one-liter Type 316 stainless steel autoclaves and rated to 2900 psig for 350°C. The loop for hot cell #1 is a PWR water system, and hydrogen is used as cover gas. A hydrogen leak detection/alarm unit (items 36 - 38 in Fig. 4) is installed.

For the hot cell #2 loop, a low-corrosion-potential environment is simulated in high-purity water using a mixture of nitrogen with 4% hydrogen as cover gas. For both systems, water is circulated at a rate of 20-30 mL/min through th e autoclaves. The water conductivity and pH can be monitored with inline sensors. During the tests, the temperature and pressure of the autoclaves were kept at ~320°C and ~1800 psig, respectively.

Formatteand gram 10 1. Autoclave 15. Rupture disk 29. Solenoid valve 2. Autoclave preheater 16. Pressure transducer 30. Excess flow valve 3. Rupture disk 17. High pressure gauge 31. Low-pressure regulator 4. ECP preheater

18. Conductiv ity sensor 32. Check valve 5. ECP cell 19. pH sensor 33. Vacuum & pressure gauge 6. Check valve 20. Solenoid valve 34. Pressure relief valve 7. Air Cooled Coil 21. Feedwater storage tank 35. Flash arrestor 8. Heat exchanger 22. Sparge tube 36. Hydrogen alarm control panel 9. Chilled block 23. Tank recirculation pump 37. Hydrogen leak sensors 10. Back-pressure regulator
24. Check valve 38. Hydrogen leak sensors 11. Ion exchange cartridge 25. Ion exchange cartridge V = Valve 12. Pressure relief valve 26. Cover gas supply cylinder T = Thermocouples 13. High-pressure pump
27. High-pressure regulator
14. Accumulator 28. Flash arrestor Figure 4. Recirculation water loop for Cell 1 of the IML (items in red are safety significant components) 2.4 Crack Growth Rate and Fr acture Toughness J-R curve Tests 2.4.1 Crack Growth Rate Test All CGR tests in this study were performed on the 1/4T-CT samples described above. Crack extensions were monitored with the direct current potential drop (DCPD) method during the tests. With this method, four electrical leads were spot welded on the CT sample (red lines shown in Fig. 2). A constant current was passed through the sample

, and the potential drop across the crack mouth was measured and related to the crack extens ion of the CT sample with a calibrated correlation curve. Field Co 11 The CGR tests were carried out in either a simulated PWR water environment or a low-DO high-purity water environment.

A typical CGR test is started with cyclic loading to pre-crack the sample in the test environment.

A load ratio around 0.2-0.3, frequency of 1-2 Hz, and maximum stress intensity factor (Kmax) between 10 and 16 MPa m 1/2 are used for pre-cracking. The object ive of this step is to generate a sharp fatigue crack and to advance the crack tip passing beyond the area close to the machine notch. The properties at this area may have b een altered by machining and do not represent the material's intrin sic behavior.

The stress intensity factor K for a CT specimen is calculated by:

1/23/2(2/)()()(1/)NPaWaKfBBWaWW+ (1) where P is applied load; B is the specimen thickness; BN is the net specimen thickness (or distance between the root s of the side grooves);

a is crack length; and W is specimen width (measured from the load line to the back edge of the specimen). The geometry factor [f(a/W)] for a CT specimen is:

234()0.8864.6413.3214.725.60aaaaafWWWWW=++ (2) Once a fatigue crack is initiated, a series of test steps is carried out with gradually increased rise times and load ratios. The measured CGRs in these test steps include the contributions from both mechanical fatigue and corrosion fatigue. With the change in loading conditions, the contribution of mechanical fatigue is gradually reduced while the environmental effect is enhanced. Figure 5 illustrates the principle of introducing environmentally assisted cracking in a test. The changes in test conditions should produce cyclic CG Rs along the green line. The cyclic CGR test is transitioned to a SCC CGR test when a significant environmental enhancement is observed.

Because of the beneficial effects of ferrite and the reduced sensitivity to IASCC in low-corrosion-potential environments, relatively low CGRs are expected for the CASS specimens even after irradiation to 0.08 dpa. To measure SCC CGRs at such low growth rates, a considerable amount of time is needed to collect data for adequate measurements. Since the focus of this study was on embrittlement (i.e., loss of facture toughness), a limited amount of time was spent on collecting SCC CGR data.

Field CoField Co 12 Crack Growth Rate in Water, Log ScaleCrack Growth Rate in Air, Log ScaleEnvironmental EffectMechanical FatigueIncrease Load Ratio and Rise Time Figure 5. Schematic for inducing environmentally assisted cracking in test environment.

2.4.2 Fracture Toughness J-R curve Test After each SCC CGR test, a fracture toughness J-R curve test was conducted on the same sample in the test environment. This test was performed with a consta nt stain rate of 0.43

µm/s, and the load and sample extension were recorded cont inuously outside the auto clave. The load-line displacement at load points was determined by s ubtracting the extension of the load train, which had been measured prior to the test. During th e test, the loading was interrupted periodically, and the specimen was held at a constant extension to measure crack length with the DCPD method. Before each DCPD measurement, the sample stress was allowed to relax at a constant displacement for 30 s.

The J-integral was calculated from the load (

P) vs. load-line displacement (

v) curve according to ASTM Specification E 1820-8a.3535 The J is the sum of the elastic and plastic components, elplJJJ=+ (3) At a hold point i corresponding to a crack length ai, as well as vi and Pi on the load vs. load-line displacement curve, the elastic component Jel(i) is given by:

22()()()(1)ieliKJE= (4) Field CoFormatteand gramField CoField Co 13 where is Poisson's ratio, and the stress intensity K(i) is calculated from Eqs.

11 and 22. The plastic component Jpl(i) is given by: (1)()(1)()(1)()(1)(1)(1)(1)1iplipliiiplipliiiNiAAaaJJbBb=+ (5) where b(i-1) is the remaining ligament at point i-1; Apl(i) is the area under the load vs. load-line displacement curve; and BN is the net specimen thickness. In addition, (i) and (i) are factors that account for the crack growth effects on J during the test a nd are expressed as: (1)(1)2.00.522iibW+= (6) (1)(1)1.00.76iibW=+ (7) The quantity Apl(i) - Apl(i-1) is the increment of the plastic area under the load vs. load-line displacement curve between lines of constant plastic displacement at points i - 1 and i. The plastic area under the load vs. load-line displacement curve is given by

[]1()(1)()(1)2iiplipliplipliPPvvAA+=+ (8) where the plastic components of the load-line displacement, vpl(i), are: ()()()pliiiLLivvPC= (9) In the above, v(i) is the total load-line displacement, and CLL(i) is the compliance required to give the current crack length ai and can be determined as follows:

23()'21.6217.80(/)4.88(/)1.27(/)[1(/)]iiiLLieiaWaWaWCEBaW++= (10) where Be is the specimen effective thickness given by B - (B - B N)2/B, and E=E/(1 - 2). A J-R curve is constructed by fitting the calculated J values and corresponding crack lengths to a

power law relationship. The J value at the in tersection of the power law curve and the 0.2-mm offset blunting line are reported. Note that a blunting line of four times flow stress (4f) is recommended by Mills 55 for materials with high strain hardening coefficients, and has been used in the previous thermal aging studies on unirradiated CASS.

44 To be consistent with the previous analyses, the same blunting line (i.e., J/4f) was also used in the current work.

In this study, the estimated flow stresses are approximately 280-340 MPa and 420-520 MPa for unirradiated and irradiated CASS materials, respectively. Th is relatively low strength allows a maximum J value of 280-360 kJ/m 2 for a typical 1/4T-CT specimen. The maximum crack extension is limited below ~1.3 mm in most cases. Because of the low strength and high fracture toughness of CASS materials, the re quired crack tip constraint and upper limit of J integral are often invalid with the 1/4T-CT specimen. This is especially true for the unirradiated CASS samples since the strength is ev en lower without irra diation hardening. For this reason, the J values determined from this study normally cannot be validated for J IC per ASTM E182-8a.

Field CoField CoField CoField CoField CoField CoFormatteand gramFormatteand gram 14 2.4.3 Fractographic Examination After each CGR/JR test, the final crack size was marked by fatigue cycling in an air atmosphere at room temperature. The specimen was then fractured, and the fracture surface was examined by scanning electron microscopy (SEM). Since the irradiated sample was highly radioactive, a

two-step replication technique was developed for the fractographic examination. After cleaning the tested sample remotely using manipulators, we used a two-part synthetic compound to produce a negative replica of the fracture surface inside the hot cell. The negative replica was then removed from the hot cell and transferred to a radiological fume hood for decontamination.

The second step of casting was to apply a low-viscosity epoxy on the surface of the negative replica. The cast was kept at approximately 70°C for several days to hard en the epoxy replica. The negative replica was then removed. The obtai ned epoxy replica was co ated with a layer of gold before it was transferred for SEM examination.

The CGR test and JR test regions were identified on the SEM images, and their fracture

morphologies were analyzed. The physical crack length was also measured on the images, and a

9/8 averaging technique was used to account for the uneven extensions at the crack front. With this technique, nine measurements were taken along the crack front sp aced across the width of the sample at equal intervals. The two near-surface measurements were averaged, and the resultant value was averaged with the remaining seven measurements to obtain the average crack length. All crack extensions determined from th e DCPD method were scal ed proportionately to match the final SEM-measured crack length.

15 3 Results Seven irradiated and four unirradiated control specimens from three heats of CASS were tested in the current study. The test matrix is shown in Table 3. The specimens were unaged and thermally aged pairs. The irradiated samples received a displacement dose of ~0.08 dpa. The CF-3 specimens were tested in either low-DO high-purity water or PWR water, and all CF-8 and CF-8M specimens were tested in low-DO high-purity water.

Table 3. Test matrix of unirradiated and irradiated low-dose CASS specimens.

Sample ID Dose (dpa) Heat ID Materials Test Environment Facility CGR JR SEM A-N1 - 69 CF-3, 24% , unaged Low-DO high-purity Cell 2 A-1 0.08 69 CF-3, 24% , unaged PWR Cell 1 - A-2 0.08 69 CF-3, 24% , unaged Low-DO high-purity Cell 2 B-N1 - 69 CF-3, 24% , aged PWR Cell 1 B-1 0.08 69 CF-3, 24% , aged PWR Cell 1 E-N1 - 68 CF-8, 23% , unaged Low-DO high-purity Cell 2 E-1 0.08 68 CF-8, 23% , unaged Low-DO high-purity Cell 2 F-N1 - 68 CF-8, 23% , aged Low-DO high-purity Cell 2 F-1 0.08 68 CF-8, 23% , aged Low-DO high-purity Cell 2 I-1 0.08 75 CF-8M, 28% , unaged Low-DO high-purity Cell 2 J-1 0.08 75 CF-8M, 28% , aged Low-DO high-purity Cell 2 3.1 CF-3 Cast Stainless Steel 3.1.1 Unaged CF-3 CASS 3.1.1.1 Unirradiated specimen A-N1 tested in low-DO high-purity water Crack growth rate test

Specimen A-N1 was an unirradiated control sample tested in low-DO hi gh-purity water. The material was an unaged CF-3 CASS with ~24% ferri te (Heat 69). The objective of this test was to provide a baseline for the irradi ated tests. The test conditions and results are summarized in Table 4, and a crack-length histor y plot is shown in Fig. 6.

Fatigue pre-crack was initiated at ~15.4 MPa m 1/2 with a triangular waveform of 1 Hz and a load ratio of ~0.16. A cyclic CGR slightly below the fatigue growth rate in air was readily obtained in this sample. After ~220-

µm crack extension, the load ratio and rise time were increased slowly to induce environmenta lly assisted cracking. The environmental effect on CGR was difficult to establish, and the crack stalled several times in the following test periods.

Nonetheless, elevated crack growth rates were observed for this CASS sample under cyclic Field Co 16 loading in the low-DO high-purity water. The cyclic CGR data obtained from this test are plotted against the fatigue CGRs in air in Fig. 7. The fatigue CGRs are estimated based on James and Jones3737 with the load ratios and rise times given in Table 4. A corrosion fatigue curve proposed by Shack and Kassner3838 for unirradiated SSs in 0.2-ppm DO water at 290°C is also included in the figure as a reference. Note that the data point above the reference line (from test period z) was due to a temperature in crease in the autocl ave during that test period. After the temperature was stabilized, a much lower CGR was measured in the next test period. It is clear that the corrosion fatigue response of this CASS sample is better than that of typical SSs in low-DO high-purity water.

After a total of ~900-

µm crack extension, the test was set to a constant load with periodical partial unloading (PPU) every 2 hr. The SCC CGR was negligibly low and was below the detection limit of the DCPD measurement. The SCC behavior of this sample will be discussed at the end of this section along with the other CGR data obtained from the same heat.

Table 4. Crack growth rates of specimen A-N1 (unirradiated and unaged CF-3 with 24% ferrite) in low-DO high-purity water.

Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2m/s m/s mm Start 0.3 5.978 A 3.2 317 0.16 0.39 0.39 0.11 15.4 12.9 5.94E-08 5.68E-08 6.204 B 5.1 317 0.29 0.36 0.36 0.14 14.5 10.3 1.98E-08 3.36E-08 6.257 C 9.1 317 0.40 0.69 0.69 0.31 14.3 8.6 1.01E-09 1.11E-08 6.265 D 21.8 317 0.21 0.38 0.38 0.12 16.0 12.6 7.14E-08 5.72E-08 6.419 E 24.5 317 0.30 0.74 0.74 0.26 16.5 11.6 3.02E-08 2.49E-08 6.528 F 29.4 317 0.45 0.68 0.68 0.32 16.8 9.3 1.35E-08 1.50E-08 6.611 G 43.2 317 0.60 3.04 3.04 1.96 16.6 6.7 3.02E-10 1.30E-09 6.618 H 50.7 317 0.50 1.65 1.65 0.85 16.9 8.5 5.37E-09 4.87E-09 6.664 I 66.6 317 0.50 6.57 1.31 3.43 16.8 8.4 6.07E-10 1.18E-09 6.685 J 80.2 317 0.49 9.91 3.30 5.09 16.9 8.5 6.25E-10 8.24E-10 6.702 K 97.4 317 0.49 19.9 3.31 10.1 17.0 8.6 3.28E-10 4.25E-10 6.713 L 118.2 317 0.49 39.7 7.94 20.3 17.1 8.7 2.68E-10 2.16E-10 6.728 M 145 317 0.49 99.7 7.98 50.3 17.3 8.8 1.57E-10 9.04E-11 6.737 N 170.5 317 0.49 199.8 7.99 100.2 17.3 8.9 7.96E-11 4.59E-11 6.742 O 196.2 317 0.48 333.6 8.01 166.4 17.3 8.9 1.51E-10 2.80E-11 6.751 P 236.1 318 0.48 667.8 8.01 332.2 17.3 8.9 negligible 1.42E-11 6.750 Q 285.6 318 0.49 133.3 8.00 66.7 17.4 8.9 6.58E-11 7.06E-11 6.758 R 308.1 318 0.49 66.6 7.99 33.4 17.5 8.9 8.93E-11 1.40E-10 6.763 S 316.7 318 0.44 34.2 8.21 15.8 17.4 9.7 3.90E-10 3.46E-10 6.769 t a 332.2 318 0.44 68.5 8.22 31.5 17.7 9.8 5.06E-10 1.81E-10 6.795 U 355.8 319 0.49 133.1 7.99 66.9 17.6 9.0 6.71E-11 7.25E-11 6.799 V 363.7 319 0.45 13.6 3.41 6.4 17.6 9.8 1.36E-09 8.90E-10 6.814 W 379.4 319 0.47 33.7 8.09 16.3 17.8 9.5 6.68E-10 3.33E-10 6.837 X 403.6 318 0.48 66.4 7.97 33.6 17.4 9.1 1.62E-10 1.48E-10 6.846 Y 430.8 318 0.49 133.2 8.00 66.8 17.8 9.2 8.67E-11 7.69E-11 6.851 z b 452.8 317 0.49 333.8 8.01 166.2 18.1 9.3 3.74E-10 3.19E-11 6.872 aa 500.3 319 0.48 669.5 8.03 330.5 18.1 9.4 3.65E-11 1.64E-11 6.876 1 811.3 318 0.50 12 12 7200 18.0 9.0 negligible 1.37E-12 6.880 a The CGR value was obtained from the later part of the test period.

b The autoclave temperature was unstable in the test period.

Formatteand gramFormatteand gramField Co 17 (a) 6.006.106.206.306.404812 16 2024283205101520Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCa, R=0.11 Hzb, R=0.21 Hzc, R=0.40.5 HzHold at low K (b) 6.206.30 6.406.506.60 6.706.804812 16 2024283220304050607080Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCd, R=0.21 Hze, R=0.31 Hzf, R=0.450.5 Hzg, R=0.60.1 Hzh, R=0.50.2 Hzi, R=0.510s/up, 2s/downj, R=0.515s/up, 5s/down (c) 6.656.70 6.756.8048 121620242832100150200Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCk, R=0.530s/up, 5s/downl, R=0.560s/up, 12s/downm, R=0.5150s/up, 12s/downn, R=0.5300s/up, 12s/downo, R=0.5500s/up, 12s/downp, R=0.51000s/up, 12s/down Figure 6. Crack-length-vs.-time plot for specime n A-N1 (unirradiated and unaged CF-3 with 24% ferrite): test periods (a) a-c, (b) d-j, (c) k-p, (d) q-u, (e) v-aa, and (f) 1. Field Co 18 (d) 6.706.756.80 6.854812 16 20242832240260280300320340Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCq, R=0.5200s/up, 12s/downr, R=0.5100s/up, 12s/downs, R=0.4550s/up, 12s/downt, R=0.45100s/up, 12s/downPressure unstable caused by cooling water problem.u, R=0.5200s/up, 12s/down5.1E-10 m/s (e) 6.756.806.856.906.954812 16 20242832360380400420440460480500Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oCv, R=0.4520s/up, 5s/downw, R=0.4550s/up, 12s/downx, R=0.5100s/up, 12s/downy, R=0.5200s/up, 12s/downz, R=0.5500s/up, 12s/downaa, R=0.51000s/up, 12s/down (f) 6.8006.8206.8406.8606.8806.900 6.92048 121620242832500550600650700750800850Crack Length (mm)K (MPa m0.5)Time (h)Spec. A-N1, CF-3, 24% unagedLow-DO high-purity water, ~318 oC1, PPU, 2 hr hold Figure 6. (Contd.)

19 CF curve for 0.2 ppm DO by Shack & Kassner 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-cTest periods: d-pTest periods: q-uTest periods: v-aaCGRenv (m/s)CGRair (m/s)Specimen A-N1 CF-3, unaged, unirradiated Low-DO high-purity water~318oC Figure 7. Cyclic CGRs of specimen A-N1.

Fracture toughness J-R curve test

A fracture toughness J-R curve test was conducte d on the sample in the low-DO high-purity water environment. The sample was load ed at a constant ex tension rate of 0.43

µm/s while the load and load-line displacement were recorded. During the test, loading was interrupted periodically to measure the crack extension by DC PD. The obtained J an d crack extension data are plotted in Fig. 8. Note that the measurement capacity of 1/4T-CT samples is significantly lower than the fracture toughness an ticipated for the CASS material s, and thus a validated J-R curve test is not possible for this sample. Nonetheless, the available data points fit to a power-law relationship, and the estimated J value at the 0.2-mm offset line is about 320 kJ/m

2. The straightness of the crack extension and the Jmax requirement are not met in this analysis due to lack of constraint at the crack tip.

Field Co 20 0160320480 6400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen A-N1CF-3, unaged, unirradiatedLow-DO high-purity water, ~318 oCJ= 536.5*a0.68JQ=320 kJ/m 2Jmaxamax Figure 8. The J-R curve for specimen A-N1.

Fractographic examination

After the J-R curve test, the sample was fatigued and broken open at room temperature in an air atmosphere. The fracture surface was examined with SEM (Fig. 9). While the CGR test region shows a transgranular (TG) morphology, the entir e JR test region is covered with ductile dimples. A major secondary crack that covers n early a half of the width of the sample can be seen at the machined notch. This large se condary crack may affect the crack propagation, leading to a curved crack front in the CGR test. Consequently, the final crack size of the JR test is also affected with a significantly higher cr ack extension on the right

-hand side of the SEM image. Figure 10 shows an enlarged view of the fracture surface along the sample central line.

More details of the CGR and JR test regions can be seen in Figs. 11 and 12, respectively.

Field Co 21 Figure 9. Fracture surface of specimen A-N1 tested in low-DO high-purity water.

Field Co 22 Figure 10. Fracture surface of specimen A-N1 along the sample central line.

Crack advance Dimple fractureMachined

notchSecondary

crackTG Field Co 23 Figure 11. Transgranular fracture in the precracking region of specimen A-N1. Field Co 24 Figure 12. Ductile dimple fracture in JR test region of specimen A-N1. Field Co 25 3.1.1.2 Irradiated specimen A-1 tested in simulated PWR water Crack growth rate test

Specimen A-1 was an irradiated CF-3 CASS sample (Heat 69) tested in the simulated PWR water environment. The specimen had ~24% fe rrite and was in the as-cast condition. The objective of this test was to compare the results with those from an identical test performed in low-DO high-purity water. Also, the CGR results of this sample were compared with those obtained from its thermally aged equivalent. Th e test conditions and results are summarized in Table 5, and a crack-length history plot is shown in Fig. 13. Note that the starting crack length for this sample was about 1 mm longer than that of a typical 1/4T-CT specimen. This larger-than-normal initial crack length was due to a rest art of the pre-cracking to correct a loose DCPD lead. The initial crack size reported in Table 5 was determined with SEM images on the fracture surface after the test.

Table 5. Crack growth rates of specimen A-1 (0.08-dpa unaged CF-3 sample with 24% ferrite) in PWR water.

a Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s S s MPa m1/2 MPa m1/2 m/s m/s mm Start 63.9 6.802 b a 64.8 319 0.21 0.43 0.43 0.07 24.3 19.2 1.58E-07 2.04E-07 7.009 b 66 319 0.31 0.41 0.41 0.09 23.8 16.4 1.17E-07 1.43E-07 7.218 c 68.6 319 0.41 0.78 0.78 0.22 23.1 13.6 3.21E-08 4.47E-08 7.340 d 72.3 319 0.41 1.54 1.54 0.46 21.9 12.8 9.77E-09 1.89E-08 7.395 e 87.6 319 0.46 2.96 1.48 1.04 20.3 11.0 2.89E-10 6.18E-09 7.403 f 91 319 0.41 3.08 1.54 0.92 22.2 13.1 9.46E-09 1.01E-08 7.462 g 94.1 320 0.41 7.69 1.54 2.31 22.4 13.2 5.67E-09 4.17E-09 7.503 h 103 320 0.46 22.6 3.76 7.42 22.6 12.2 1.51E-09 1.16E-09 7.535 i 114.3 320 0.46 45.1 3.76 14.9 22.7 12.3 7.63E-10 5.86E-10 7.558 j 118.7 319 0.41 15.3 3.83 4.66 23.0 13.6 4.61E-09 2.27E-09 7.602 k 127.9 319 0.41 46.0 3.83 14.0 23.0 13.7 1.25E-09 7.72E-10 7.632 l 150.3 318 0.40 114.9 9.19 35.1 23.0 13.7 3.90E-10 3.13E-10 7.655 m 168.7 319 0.40 385.6 9.25 114.4 23.9 14.3 3.56E-10 1.05E-10 7.673 1 209 318 0.40 12 12 7200 23.9 14.4 4.83E-11 5.75E-12 7.684 a Simulated PWR water with 2 ppm Li and 1000 ppm B. DO<10 ppb. Conductivity ~20

µS/cm. b Determined from the SEM image after the test.

The specimen was pre-cracked in the PWR environment with a triangular waveform at 1 Hz, a maximum stress intensity factor of ~24 MPa m1/2, and load ratio of 0.2. After about 200

µm extension, the load ratio was incr eased while the stress intensity f actor and cyclic frequency were reduced gradually. The measured CGRs in the initial stage of the pre-cracking (test periods a-d) followed closely with the fatigue growth rates in ai

r. Next, several test periods were carried out with saw-tooth waveforms and increased rise times to stimulate environmentally assisted cracking. The environmental enhancement of CGRs started to appear and became stabilized after some additional crack extension. The cyclic CGR data obtained from this test are plotted against the estimated fatigue CGRs in air as shown in Fig. 14. The corrosion fatigue curve proposed by Shack and Kassner is also included in the figure as a reference. Obviously, the Field Co 26 corrosion fatigue response of this sample is lo wer than that of typi cal SSs, suggesting good IASCC resistance of CASS in PWR water.

After the cyclic CGR test, the specimen was subj ected to a constant stress intensity factor of

~24 MPa m1/2 with PPU every 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.

The measured SCC CGR was about 4.8x10

-11 m/s, a factor of four lower than that of the NUREG-0313 curve,3636 a SCC disposition curve based on unirradiated SSs tested in a high-DO environment. The SCC response of this sample will be discussed at the end of this section along with the other CGR data obtained from the same heat.

(a) 6.806.907.007.107.207.307.407.507.604812162024286570758085Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCabcdeKmaxCrack length (b) 7.357.40 7.45 7.50 7.55 7.607.6548 12 16202428859095100105110115120Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCfeghijKmaxCrack length Figure 13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% ferrite): test periods (a) a-e, (b) f-j, (c) k-m, and (d) 1.

Formatteand gramField Co 27 (c) 7.557.60 7.65 7.707.7548 12 16202428120130140150160170Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCklmKmaxCrack length (d) 7.667.677.687.697.707.71 7.72481216202428170180190200210220230240250Crack Length (mm)K (MPa m0.5)Time (h)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oC1KCrack length Figure 13. (Contd.)

28 10-1110-1010-910-810-710-1110-1010-910-810-7CGRenv (m/s)CGRair (m/s)Specimen A-1CF-3, unaged, 0.08 dpaPWR water, 320 oCTest period with <10

µm extension are excluded.CF curve for 0.2 ppm DO by Shack & Kassner Figure 14. Cyclic CGRs of specimen A-1.

Fractographic examination

After the test, the specimen was pulled apart in an air atmosphere at room temperature, and the fracture surface was examined with SEM. Figure 15 shows the entire crack front of specimen A-1. The straight line in the middl e of the picture corresponds to th e restart of the pre-cracking.

Figure 16 is an enlarged view along the central section of the specimen. Transgranular cleavage-like fracture is the dominant morphology thr oughout the CGR region. Figure 17 shows the

typical river pattern of cleavage cracking at the beginning an d the end of the CGR test region. Vermicular ferrite at dendrite cores can also be seen in a few places on the fracture surface. As

shown in Fig. 18, fewer slip ledges can be seen with in ferrite, suggesting th at the ferrite dendrite core might be deformed to a lesser extent th an the surrounding austenite. Beyond the CGR test, the dominant fracture mode is ductile dimples resulting from microvoid coalescence. Field Co 29 Figure 15. Fracture surface of specimen A-1 tested in PWR water.

Field Co 30 Figure 16. Fracture surface of specimen A-1 along the sample central line.

Machined notchRestart of pre-cracking Vermicular ferrite at

dendrite coreDimple fractureCrack advance Field Co 31 Figure 17. Cleavage-like fracture in specimen A-1: (a) pre-cracking, and (b) e nd of the CGR test.

Crack propagation from bottom to top.

a Field Co 32 Figure 17. (Contd.)

b 33 Figure 18. Fracture surface of specimen A-1 showing that delta ferrite deformed to a lesser extent than austenite. Crack propagation from bottom to top.

Field Co 34 3.1.1.3 Irradiated specimen A-2 te sted in low-DO high-purity water Crack growth rate test

Specimen A-2 was an irradiated unaged CF-3 CASS with ~24% ferrite (Heat 69) tested in high-purity water with low DO. The objective of this test was to compare the irradiated specimen A-1 tested in PWR water. Also, the fracture toughness of this specimen was compared with that of thermally aged CF-3 CASS. The CGR test conditions and results are summarized in Table 6, and a crack-length history pl ot is shown in Fig. 19.

Table 6. Crack growth rates of specim en A-2 (0.08-dpa unaged CF-3 with 24% ferrite) in low-DO high-purity water environment.

Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm Start 14.9 5.899 aa 18.1 319 0.34 0.45 0.45 0.05 17.3 11.4 4.09E-08 4.04E-08 6.041 b 20 319 0.41 0.45 0.45 0.05 17.1 10.1 3.21E-08 2.96E-08 6.141 c 21.6 319 0.50 0.43 0.43 0.07 15.7 7.8 1.94E-09 1.43E-08 6.151 da 24.3 319 0.45 0.44 0.44 0.06 16.5 9.1 2.27E-08 2.16E-08 6.219 e 30.4 319 0.45 0.88 0.88 0.12 16.0 8.8 5.26E-09 9.85E-09 6.274 f 39.3 319 0.50 1.74 1.74 0.26 15.9 8.0 1.80E-09 3.82E-09 6.304 g 40.4 319 0.40 0.45 0.45 0.05 17.2 10.4 5.87E-08 3.21E-08 6.410 h 42.4 319 0.45 0.44 0.44 0.06 16.3 9.0 1.25E-08 2.13E-08 6.453 i 44.6 319 0.45 0.88 0.88 0.12 16.6 9.2 1.29E-08 1.14E-08 6.497 j 49.0 319 0.45 1.75 1.75 0.25 16.8 9.2 9.20E-09 5.84E-09 6.564 k 62.2 319 0.45 4.38 4.38 0.62 17.0 9.4 3.07E-09 2.45E-09 6.627 l 74.2 319 0.46 8.73 4.37 1.27 17.2 9.4 2.37E-09 1.23E-09 6.686 m 88.4 319 0.45 26.2 4.37 3.79 17.2 9.5 7.77E-10 4.25E-10 6.719 n 110.6 319 0.44 52.5 10.5 7.52 17.2 9.7 6.45E-10 2.24E-10 6.756 o 135.7 319 0.43 105.0 10.5 15.0 17.0 9.7 3.77E-10 1.13E-10 6.780 p 184.3 319 0.43 262.6 10.5 37.4 17.3 9.8 8.72E-11 4.69E-11 6.795 q 232.7 319 0.43 525.9 10.5 74.1 17.6 10.0 1.29E-10 2.49E-11 6.815 r 278.6 320 0.43 876.3 10.5 123.7 17.6 10.1 1.73E-10 1.52E-11 6.840 s 326.7 319 0.48 865.9 10.4 134.1 17.6 9.2 1.27E-10 1.21E-11 6.860 1 423.9 319 0.50 12 12 7200 17.6 8.8 2.33E-11 1.27E-12 6.875 2-a 575.3 320 0.50 12 12 7200 19.6 9.8 4.89E-11 1.80E-12 6.907 2-b 687.3 321 1 - - - 19.8 - 4.94E 6.912 2-c 784 321 0.50 12 12 3600 19.8 9.9 4.26E-11 3.73E-12 6.924 a The CGR value was obtained from the later part of the test period.

Pre-cracking in this sample was started with a triangle waveform at 1 Hz. The applied maximum stress intensity factor and load ratio were ~17 MPa m 1/2 and ~0.35, respectively. The initial cyclic CGRs were comparable to fatigue growth rates but were quickly diminished as the load ratio was increased to 0.5. After lowering the lo ad ratio of 0.4 and increasing the frequency to 1 Hz, the high growth rate was re-established in test period g over a 100-

µm crack extension. Next, the rise time was increased gradually while the maximum stress intensity factor and load ratio were kept approximately the same. In the following test periods, environmentally enhanced cracking started to appear and became evident with further increases in rise time and load ratio.

By the end of test period s, the measured CGR was more than one order of magnitude higher than the fatigue growth rate in air. All cyclic CGRs obtained from this sample are plotted in Field Co 35 Fig. 20 along with the corrosion fatigue curve for unirradiated SSs in water with 0.2 ppm DO. A comparison between Figs. 14 and 20 shows that the cyclic CGRs of specimen A-1 is slightly lower than that of specimen A-2.

(a) 5.906.00 6.106.206.304812 1620242832141618202224262830Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCabc4.1E-8 m/s de2.3E-8 m/s KmaxCrack length (b) 6.206.25 6.306.356.40 6.45 6.50 6.556.60481216202428323035404550Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCfghijKmaxCrack length Figure 19. Crack-length-vs.-time plot for specimen A-2 (0.08-dpa unaged CF-3 with 24%

ferrite) tested in low-DO high-purity water environment: test periods (a) a-e, (b) f-j, (c) k-o, (d) p-s, and (e) 1-2. Field Co 36 (c) 6.506.55 6.60 6.65 6.70 6.75 6.806.856.904812 16202428326080100120140Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCklmnoKmaxCrack length (d) 6.706.756.806.85 6.90 6.957.0048121620242832160200240280320Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCpqrsKmaxCrack length (e) 6.866.886.906.926.944812 16 20242832400500600700800Crack Length (mm)K (MPa m0.5)Time (h)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oC12-a2-b2-cKCrack length Figure 19. (Contd.)

37 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-fTest periods: g-sCGRenv (m/s)CGRair (m/s)Specimen A-2CF-3, unaged, 0.08 dpa.

Low-DO high-purity water320oCCF curve for 0.2 ppm DO by Shack & Kassner Figure 20. Cyclic CGRs of specimen A-2.

The first constant-load test period was conducted at 17.6 MPa m 1/2 with PPU every 2 hr (test period 1). After nearly 100 hr, a CGR of 2.3 x 10

-11 m/s was measured over ~15-

µm crack extension. The load was increased to 19.6 MPa m 1/2 for the second constant-load test period.

Again, PPU was applied every 2 hr. A CGR of 4.9 x 10

-11 m/s was obtained over 150 hr. After the PPU was removed in test period 2-b, the measured CGR decreased one order of magnitude. Constrained by test time, only 5-

µm crack extension was accumulated for this test period. In test period 2-c, the PPU was re-introduced but with 1-hour interval. The measured CGR was nearly identical to that obtai ned in test period 2-a. It appears that the hold time between PPU does not affect the SCC response of CF-3 at this stress intensity level.

Fracture toughness J-R curve test

After the crack growth test, a fracture toughness J-R curve test was conducted on this sample in the low-DO high-purity water environment. The samp le was loaded at a c onstant extension rate of 0.43 µm/s while the load and load-line displacement were recorded. During the test, loading was interrupted periodically to measure the crac k extension by DCPD. The obtained J and crack extension data are plotted in Fig. 21. A power-law curve fitting of the data gives a relationship of J = 430a0.64. The estimated J value at the 0.2-mm offset line is about 204 kJ/m

2. Note that the J-R curve data cannot be validated for this sample because one of the nine measurements of the final crack size was above the limit, and the Jmax requirement was omitted in this analysis.

Field Co 38 01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen A-2CF-3, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCJ= 430*a0.64JQ=204 kJ/m 2Jmaxamax Figure 21. The J-R curve for specimen A-2.

Fractographic examination

After the J-R curve test, the sample was cyclically loaded at room temperature in an air atmosphere to break the ligament. The fracture surface was then examined with the replication technique using SEM. Figure 22 shows the entire fracture surface of specimen A-2. Note that the round smooth areas on the SEM image are ai r bubbles trapped during replication, not the original morphology of the fracture surface. Both the CGR and post-JR fatigue regions are relatively flat, clearly contrasting with the heavily deformed JR region. The crack front of the CGR test is straight, indicati ng a well-controlled loading c ondition during the CGR test. A curved crack front due to a non-constant constrai nt can be seen for the JR test region.

Figure 23 shows an enlarged view of specimen A-2 along its central line. Similar to specimen A-1, which was tested in PWR water, transgranular cleavage-like fracture was also the dominant morphology during the CGR test. Figure 2424 shows the cleavage-like cracking at the initial part of the CGR test, and deformation steps resulting from brittle fracture can be seen in some places. An area of delta ferrite at the dendrite cores can be seen close to the end of the CGR test (see Fig.

2525). Similar to that found in the specimen A-1, less plastic deformation activities were observed in the ferrite phase. Beyond the CGR test, ductile dimples were the only fracture

morphology on the fracture surface, as shown in Fig.

2626. Field Co 39 Crack advanceDelta ferrite in dendritesAir bubbles CGRJRDimplesCrack advanceDelta ferrite in dendritesAir bubbles CGRJRDimples Figure 22. Fracture surface of specimen A-2 tested in low-DO high-purity water.

Field Co 40 Figure 23. Fracture surface of specimen A-2 along the sample central line.

CGR test TG Vermicular ferrite in

dendrite cores Dimple fractureCrack advance JR test Post-JR fatigue Machined

notchField Co 41 Figure 24. Cleavage-like steps at the be ginning of CGR test of specimen A-2.

Crack propagation from bottom to top.

Field Co 42 Figure 25. Fracture surface of specimen A-2 sh owing that ferrite deformed to a lesser extent than austenite. Crack propagatio n from bottom to top.

Field Co 43 Figure 26. Ductile dimple morphology in the JR test region of specimen A-2. Crack propagation from bottom to top.

Field Co 44 3.1.2 Thermally Aged CF-3 CASS 3.1.2.1 Unirradiated specimen B-N1 tested in PWR water Crack growth rate test Specimen B-N1 was an unirradiated control sample tested in PWR water. The material was a thermally aged CF-3 with ~24% ferrite. The obj ective of the test was to provide a baseline for irradiated tests on thermally aged specimens. The CGR test conditions and results of this sample are summarized in Table 7, and a crack-le ngth history plot is shown in Fig. 27.

Table 7. CGR test of specimen B-N1 (unirradiated, thermally aged CF-3 with 24% ferrite) in PWR water.

a Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm Start 0.3 5.997 a b 3.0 319 0.20 0.43 0.43 0.07 14.8 11.9 5.57E-08 4.04E-08 6.182 b b 7.2 319 0.35 0.42 0.42 0.08 15.0 9.8 3.25E-08 2.67E-08 6.353 c b 13.3 319 0.49 0.40 0.40 0.10 15.1 7.7 2.19E-08 1.45E-08 6.474 d 23 319 0.54 0.77 0.77 0.23 15.0 6.9 4.85E-09 5.50E-09 6.538 e 47 319 0.54 1.92 1.92 0.58 15.0 6.9 1.63E-09 2.24E-09 6.597 f 71.2 319 0.54 3.82 1.53 1.18 15.0 6.8 1.53E-10 1.07E-09 6.608 g b 76.9 319 0.54 0.77 0.77 0.23 15.3 7.0 4.78E-09 5.77E-09 6.641 h 101.8 319 0.55 2.29 0.76 0.71 15.1 6.8 1.64E-10 1.79E-09 6.649 i 123.9 319 0.54 3.83 3.83 1.17 15.3 7.0 7.97E-11 1.14E-09 6.652 j 143.8 319 0.52 1.93 1.93 0.57 15.3 7.3 1.76E-09 2.64E-09 6.690 k c 152.8 320 0.53 3.85 1.54 1.15 15.2 7.2 5.86E-10 1.25E-09 6.701 l 192.1 320 0.52 7.72 1.54 2.28 15.3 7.3 1.77E-10 6.60E-10 6.714 m 200.2 319 0.49 1.96 1.96 0.54 15.6 7.9 2.31E-09 3.22E-09 6.744 n 215 320 0.49 3.91 3.91 1.09 15.7 7.9 2.54E-09 1.66E-09 6.794 o 224 320 0.49 7.81 1.56 2.19 15.7 7.9 8.81E-10 8.30E-10 6.813 p 241.5 319 0.49 15.6 3.91 4.37 15.8 8.0 7.40E-10 4.27E-10 6.841 q 270.9 319 0.49 23.4 3.91 6.56 15.8 8.0 1.21E-10 2.87E-10 6.850 r 289.5 319 0.49 46.9 3.91 13.1 15.7 8.1 3.27E-11 1.45E-10 6.852 s 311.3 319 0.49 11.7 3.90 3.30 15.7 8.0 7.10E-11 5.65E-10 6.855 t 316.3 320 0.32 12.4 4.14 2.59 16.5 11.1 9.66E-09 1.33E-09 6.957 u 320 319 0.35 24.6 4.10 5.37 16.5 10.7 2.99E-09 6.14E-10 6.983 v 334.4 319 0.37 48.9 4.08 11.1 16.8 10.5 1.35E-09 2.98E-10 7.037 w 347.5 319 0.42 96.3 9.63 23.7 16.8 9.7 4.65E-10 1.23E-10 7.054 x 368.1 320 0.42 241.0 9.64 59.0 16.9 9.8 2.09E-10 5.02E-11 7.065 y 383.7 319 0.41 401.9 9.65 98.1 16.8 9.9 1.26E-10 3.05E-11 7.071 z 408.5 319 0.42 804.3 9.65 195.7 17.1 9.9 1.43E-10 1.57E-11 7.083 aa 436.2 319 0.42 1206.2 9.65 293.8 17.2 10.0 7.73E-11 1.05E-11 7.090 1a 512.5 319 0.50 12 12 7200 17.2 8.6 2.72E-11 1.17E-12 7.106 1b 647.5 319 1.00 - - - 17.1 - 2.29E 7.107 a Simulated PWR water with 2 ppm Li and 1000 ppm B. Conductivity ~20

µS/cm. b The CGR of the later part of the test period is reported.

c The CGR of the initial part of the test period is reported.

Field Co 45 Fatigue pre-cracking was started at ~15 MPa m1/2 with a triangle waveform of 1 Hz and a load ratio of 0.2. A CGR close to the fatigue growth rate in air was quickly established in this sample after about 1-hr cyc ling. After about 180-

µm crack extension, the load ratio was increased in the next two test periods. The observed CGRs decreased as expected with the reduced contribution of mechanical fatigue (as shown in Fig. 28).

However, the crack stalled in test period f after the triangle waveform was replaced with a saw-tooth waveform. After the crack was re-activated, several attempts were made to induce environmentally assisted cracking. The environmental effect became evident after a CGR of ~1.4 x 10

-9 m/s was established in test period

v. The load ratio and rise time were then increased simultaneously to stabilize the environmentally enhanced cracking. By the end of the cyclic loading test, the observed CGR in water was about a factor of seven higher than the fatigue growth rate in air. All cyclic CGRs obtained from this sample are plotted in Fig. 28 along with the corrosion fatigue curve for SSs in high-purity water with 0.2 ppm DO.

(a) 6.006.08 6.166.246.32 6.406.486.564812 16 2024283205101520Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCa, R=0.21 Hzb, R=0.351 Hzc, R=0.51 Hzd, R=0.550.5 Hz (b) 6.456.506.556.60 6.65 6.706.756.8048 12162024283230405060708090100Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCe, R=0.550.2 Hzf, R=0.555s/up, 2s/downg, R=0.550.5 Hzh, R=0.553s/up, 1s/down Figure 27. Crack-length-vs.-time plot for specimen B-N1 (unirradiated, aged CF-3 with 24% ferrite) in PWR water: test periods (a) a-d, (b) e-h, (c) i-n, (d) o-s, (e) t-w, (f) x-aa, and (g) 1a-1b. Field Co 46 (c) 6.606.656.706.75 6.806.854812 16 20242832120140160180200Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCi, R=0.520.1 Hzj, R=0.530.2 Hzk, R=0.525s up, 2s downl, R=0.5210s up, 2s downm, R=0.50.2 Hzn, R=0.50.1 Hz (d) 6.706.75 6.806.856.90 6.957.004812 16 20242832220240260280300Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCo, R=0.510s up, 2s downp, R=0.520s up, 5s downq, R=0.530s up, 5s downr, R=0.560s up, 5s downs, R=0.515s up, 5s down (e) 6.806.856.906.95 7.00 7.057.107.1548 121620242832315320325330335340345Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCt, R=0.3215s up, 5s downu, R=0.3530s up, 5s downv, R=0.3760s up, 12s downw, R=0.42120s up, 12s down Checkcompliance Figure 27. (Contd.)

47 (f) 7.007.057.10 7.154812 16 20242832350360370380390400410420430Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oCx, R=0.4300s up, 12s downy, R=0.4500s up, 12s down z, R=0.41000s up, 12s downaa, R=0.41500s up, 12s down (g) 7.067.087.107.127.144812 16 20242832450500550600650Crack Length (mm)K (MPa m0.5)Time (h)Spec. B-N1, CF-3, 24% agedLow-DO high-purity water, ~319 oC1a,UUP, 2 hr hold 1b,Constant Figure 27. (Contd.)

After the environmentally assisted cracking was stabilized, the test was set to a constant stress intensity factor (~17 MPa m 1/2) with PPU every 2 hr. Following a short period of rapid growth, a CGR about 2.7 x 10-11 m/s was measured over 16-

µm crack extension. Without PPU, the observed CGR dropped more than two orders of magnitude under a near constant-K condition. Limited by the test time, the CGR test was terminated after 135 hr with just 1-

µm crack extension.

Fracture toughness J-R curve test

The fracture toughness J-R curve test was performed in the same PWR water environment. The test was conducted at a cons tant strain rate of 0.43

µm/s, and the crack extension was measured with the DCPD method. Before each DCPD measurement, the st ress was allowed to relax for 30 s at a constant displacement. The obtained J-R curve is shown in Fig. 29. The estimated J 48 value at 0.2 mm offset line is about 170 kJ/m

2. Note that the J-R curve data cannot be validated for this test because the requirements of the crack straightness and Jmax were not met.

10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-fTest periods: g-iTest periods: j-lTest periods: m-sTest periods: t-aaCGRenv (m/s)CGRair (m/s)Specimen B-N1 CF-3, aged, unirradiated PWR water~319oCCF curve for 0.2 ppm DO by Shack & Kassner Figure 28. Cyclic CGRs of specimen B-N1.

0160320480 6400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen B-N1CF-3, aged, unirradiatedPWR water, ~319 oCJ= 352.6*a0.66JQ=170 kJ/m 2Jmaxamax Figure 29. The J-R curve for specimen B-N1. Field CoField Co 49 Fractographic examination

After the JR test, the sample was broken open at room temperature in air. The fracture surface of the tested sample was examined with SEM. Figure 30 is a global view of the entire fracture surface of the CGR and JR test regions. The crack front of the CGR test is quite straight, indicating a well-controlled loading condition. Figure 31 shows more details of the fracture along the central line of the specimen. The CGR test region shows a transganular morphology, and the JR test region is a ductile dimple fract ure. A significant crack extension can be seen during the JR test, which is consistent with the low fracture resistance observed in this sample.

In the CGR test region, the initial fracture close to the machined notch is characterized by coarse ledges resulting from fatigue fracture (Fig. 32), while at the later stage of the CGR test, the fracture surface is smoother (Fig. 33). Details of ductile dimple fracture in the JR test region can be seen in Fig. 34.

50 Figure 30. Fracture surface of specim en B-N1 tested in PWR water.

Field Co 51 Figure 31. Fracture surface of specimen B-N1 along the sample central line.

CGR test Fatigue fracture TG Dimple fractureCrack advance JR test Post-JR fatigue Machined

notchField Co 52 Figure 32. Transgranular fracture in specim en B-N1 at the beginning of the precracking. Crack pr opagation from bottom to top.

Field Co 53 Figure 33. Transgranular fracture in speci men B-N1 at the end of the precracki ng. Crack propagation from bottom to top.

Field Co 54 Figure 34. Ductile dimple fracture in the JR test region of specimen B-N1. Crack propagation from bottom to top.

Field Co 55 3.1.2.2 Irradiated specimen B-1 tested in PWR water Crack growth rate test Specimen B-1 was a 0.08-dpa thermally aged CF-3 CASS with ~24% ferrite. This sample was obtained from the same heat (Heat 69) as specimens A-1 and A-2. The thermal aging condition was 400°C and 10,000 hr. The objective was to compare the results with those of the unaged CF-3 at the same dose level. The CGR test conditions and results ar e summarized in Table 8, and a crack-length history plot is shown in Fig. 35.

The test was started with fatigue pre-cracking at a maximum stress intensity factor of

~18 MPa m1/2 and a load ratio of 0.3. A triangular waveform was used with a frequency of 1 Hz. Upon successful crack initiation, the maximum stress intensity f actor was lowered to prepare for transitioning. In the following te st periods, the load ratio and rise time were increased slowly to stimulate environmentally enhanced cracking. Despite the effort, the measured CGRs fell well below the fatigue growth rate line. A more aggressive loading condi tion was applied to re-activate the fatigue crack. Following that, repeated attempts (test periods e-j, k-q, and r-af) were made to induce environmental enhanced cracki ng. However, no elevated CGRs could be maintained. It appears that cracking cannot be sustained with a load ratio higher than 0.4 and a stress intensity factor less than 19 MPa m 1/2 in this sample. Environmentally enhanced cracking was only observed after increasing the stre ss intensity factor to about 22 MPa m 1/2. At this stress intensity, the measured CGR by the end of the cyclic test (period ao) was a factor of three higher than the fatigue crack growth rate in air. Subs equently, the test was set at constant load with PPU every 2 hr in test period

1. A CGR of 2.8 x 10

-11 m/s was obtained at a stress intensity factor of 22 MPa m 1/2.

56 Table 8. CGR test of specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water.

a Test Test Time, Test Temp., Load Ratio Rise Time, Return Time, Hold Time, Kmax, K, CGR in Env., CGR in Air, Crack Length, Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm Start 0.52 6.019 a 3.0 319 0.27 0.38 0.38 0.12 17.6 12.8 7.88E-08 6.55E-08 6.250 b 6.7 320 0.39 0.35 0.35 0.15 17.0 10.4 4.11E-08 4.06E-08 6.442 c 10.5 320 0.50 0.64 0.64 0.36 15.8 7.9 2.75E-09 9.97E-09 6.460 d 20.1 320 0.45 1.32 1.32 0.68 15.9 8.8 3.63E-10 6.50E-09 6.466 e 23.3 320 0.27 0.37 0.37 0.13 17.9 13.1 5.89E-08 7.29E-08 6.698 f 25.4 320 0.27 0.36 0.36 0.14 17.1 12.4 4.86E-08 6.25E-08 6.822 g 27.8 320 0.38 0.34 0.34 0.16 16.8 10.4 2.77E-08 4.22E-08 6.905 h 32.2 320 0.38 0.66 0.66 0.34 16.3 10.1 8.33E-09 1.93E-08 6.950 i 44.2 319 0.44 3.20 3.20 1.80 16.2 9.2 2.63E-10 3.07E-09 6.955 j 47.5 320 0.38 1.32 1.32 0.68 16.2 10.0 7.00E-10 9.49E-09 6.958 k b 52.3 320 0.37 0.67 0.67 0.33 17.1 10.8 1.01E-08 2.32E-08 7.000 l 59.1 320 0.37 3.36 1.34 1.64 17.2 10.9 4.05E-09 4.80E-09 7.048 m 69 320 0.37 6.71 1.34 3.29 17.4 11.0 2.24E-09 2.48E-09 7.092 n 82.8 320 0.42 13.0 3.25 6.99 17.3 10.1 3.18E-10 1.01E-09 7.102 o 95.2 320 0.36 13.4 3.35 6.60 17.3 11.0 4.09E-11 1.23E-09 7.104 p 105 320 0.36 6.73 3.37 3.27 17.5 11.2 1.01E-10 2.59E-09 7.107 q 116.8 320 0.36 13.6 3.40 6.41 17.8 11.5 9.16E-11 1.40E-09 7.107 r 124.2 320 0.36 1.36 1.36 0.64 18.0 11.5 3.85E-09 1.43E-08 7.142 s 130.6 320 0.36 3.39 1.36 1.61 18.1 11.6 2.66E-09 5.86E-09 7.171 t 142.5 320 0.36 6.77 1.35 3.23 18.2 11.7 1.21E-09 3.00E-09 7.201 u 145.9 319 0.35 13.5 3.38 6.46 18.1 11.7 9.45E-11 1.49E-09 7.202 v 164.7 319 0.35 13.6 3.40 6.40 18.3 11.9 1.23E-10 1.56E-09 7.207 w 170 319 0.35 10.2 3.40 4.80 18.4 11.9 1.53E-10 2.09E-09 7.208 x 188.7 319 0.35 10.2 3.42 4.75 18.9 12.3 1.39E-09 2.28E-09 7.251 y 199 319 0.35 20.5 3.42 9.51 18.9 12.3 7.57E-10 1.15E-09 7.265 z 238 319 0.35 41.0 3.41 19.0 19.1 12.3 2.14E-10 5.88E-10 7.287 aa 284.5 319 0.36 81.8 8.18 38.2 19.3 12.4 2.61E-10 3.00E-10 7.318 ab 356.3 319 0.35 204.4 8.18 95.6 19.2 12.4 4.36E-11 1.20E-10 7.325 ac 442.2 318 0.35 413.0 8.26 187.0 19.9 12.9 2.88E-11 6.65E-11 7.331 ad 501.5 319 0.35 693.0 8.32 307.0 20.2 13.2 1.41E-11 4.26E-11 7.334 ae 527.8 318 0.46 78.0 7.80 42.0 19.9 10.8 5.94E-12 2.23E-10 7.336 af 574.5 318 0.45 39.4 7.88 20.6 20.6 11.3 2.11E-10 4.98E-10 7.356 ag 644.4 318 0.45 78.8 7.88 41.2 20.6 11.2 negligible 2.48E-10 7.355 ah 648 318 0.29 3.97 3.97 1.03 22.5 15.9 2.93E-08 1.30E-08 7.477 ai 650 318 0.34 11.7 3.89 3.32 22.3 14.7 7.43E-09 3.61E-09 7.509 aj 653.2 318 0.41 22.6 3.76 7.43 21.5 12.8 2.97E-09 1.27E-09 7.532 ak 671.3 318 0.39 45.8 9.16 14.2 22.6 13.8 1.02E-09 7.91E-10 7.577 al 693.8 318 0.39 91.5 9.15 28.5 22.8 14.0 7.29E-10 4.09E-10 7.618 am 717 318 0.46 194.0 7.76 106.0 22.1 11.9 4.90E-10 1.22E-10 7.644 an 747.7 319 0.46 387.7 7.75 212.3 22.0 11.9 1.66E-10 6.16E-11 7.655 ao 788.7 319 0.46 645.4 7.74 354.6 22.1 11.9 1.20E-10 3.71E-11 7.667 1 837.5 318 0.40 12 12 7200 22.1 13.3 2.81E-11 4.42E-12 7.675 a Simulated PWR water with 2 ppm Li and 1000 ppm B. Conductivity ~20

µS/cm. b The CGR value was obtained from the later part of the test period. Field Co 57 (a) 6.006.106.206.306.406.506.606.7048 12 1620242804812162024Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCabcdeCompliancetest Crack length Kmax (b) 6.706.75 6.806.856.90 6.95 7.00 7.057.104812162024282428323640444852Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCfhgijkCrack length Kmax (c) 6.957.00 7.05 7.10 7.15 7.207.2548 12 162024286080100120140Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oClmnopqrstuCrack length Kmax Figure 35. Crack-length-vs.-time plot for specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water: test periods (a) a-e, (b) f-k, (c) l-u, (d) v-y, (e) z-ac, (f) ad-ag, (g) ah-al, and (h) am-1. Field Co 58 (d) 7.157.20 7.257.3048 12 16202428150160170180190200Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCvwxyCrack length Kmax (e) 7.267.28 7.307.327.347.367.38481216202428200250300350400450Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCzaaabacCrack length Kmax (f) 7.307.327.347.367.387.4048 12 16202428480520560600640Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCafagaeadCrack length Kmax Figure 35. (Contd.)

59 (g) 7.357.40 7.45 7.50 7.55 7.607.6548 12 16202428650660670680690Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCahaiajakalCrack length Kmax (h) 7.607.627.647.66 7.68 7.707.72481216202428700750800850Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen B-1CF-3, aged 10,000 hrs @ 400°C, 0.08 dpaPWR water, 320 oCamanao1Crack length Kmax Figure 35. (Contd.)

The cyclic CGRs with a significant crack extension (defined as >10

µm) from this sample are plotted in Fig. 36 along with the corrosion fatigue curve for unirradiated SSs. No elevated corrosion fatigue response can be seen for this material despite its thermal aging condition. Comparing Figs. 14 and 36, we foun d the corrosion fatigue behavi ors of unaged and aged CF-3 to be similar in PWR water. It seems that a combination of irradiation damage and thermal aging does not increase cracking suscep tibility of CF-3 at 0.08 dpa, as would be expected.

All SCC CGRs obtained from specimens A-N1, A-1, A-2, B-N1, and B-1 are shown in Fig. 37.

The open and closed symbols are for the unaged and thermally aged CF-3, and the blue and red symbols are for the unirradiated and irradiated specimens, respectively. The data points are all well below the NUREG-0313 curve, as expected in such low-corrosion-potential environments. No obvious difference can be seen between the unirradiated and irradiated specimens. The SCC CGRs of the unaged CF-3 (specimens A-1 and A-2) are also similar for the low-DO high-purity water and PWR water environments. The thermally aged specimen (B-1) has a slightly lower 60 CGR than that of unaged CF-3. However, given the large scatter of the SCC CGR data, this difference is considered insignificant.

10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-dTest periods: e-jTest periods: k-qTest periods: r-afTest periods: ah-aoCGRenv (m/s)CGRair (m/s)Specimen B-1CF-3, aged, 0.08 dpaPWR water, 320 oCCF curve for 0.2 ppm DO by Shack & KassnerTest periods with <10

µm extension are excluded.

Figure 36. Cyclic CGRs of specimen B-1.

10-1310-1210-1110-1010-910-8101520253035Spec. A-N1, unirr., PPU 2 hr, Low-DO water Spec. A-1, 0.08 dpa, PPU 2 hr, PWR waterSpec. A-2, 0.08 dpa, PPU 2 hr, Low-DO waterSpec. A-2, 0.08 dpa, PPU 1 hr, Low-DO waterSpec. A-2, 0.08 dpa, w/o PPU, Low-DO water Spec. B-N1, unirr., PPU 2 hr, PWR waterSpec. B-N1, unirr., w/o PPU, PWR waterSpec. B-1, 0.08 dpa, PPU 2 hr, PWR waterCGR (m/s)K (MPa m1/2)NUREG-0313CurveCASS CF-3 with 24% ferrite low-DO high-purity or PWR water318 - 320 oCOpen = UnagedClosed = AgedBlue = UnirradiatedRed = 0.08 dpa Figure 37. SCC CGRs of unaged and thermally aged CF-3 with 24% ferrite. Field CoField Co 61 Fracture toughness J-R curve test After the CGR test, a fracture toughness J-R curve test was conducted on the same sample at 320°C in PWR water. The obtained J-R curve is shown in Fig. 38. A power-law fitting of the JR data gives a relationship of J = 362a0.85. The estimated J value is about 116kJ/m 2 at the 0.2-mm offset line. The J-R curve data cannot be validated with the ASTM standard because both measurements of the initial and final crack size did not meet the requirements. The data points above the Jmax limit were also used in the analysis.

01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen B-1CF-3, aged, 0.08 dpaPWR water, 320 oCJ= 362*a0.85JQ=116 kJ/m 2Jmaxamax Figure 38. The J-R curve for specimen B-1.

Fractographic examination

Fractographic analysis of specimen B-1 was carri ed out with replicas, and Fig. 39 shows the entire fracture surface. Transgranular cleavage-like cracking is the dominant fracture morphology in the CGR test region.

The overall crack extension for the CGR test is a little more on one side of the sample than the other, leading to a slightly skewed crack front. Figure 40 shows an enlarged view of the fracture surface along the sample central line. The CGR and JR test regions can be clearly dis tinguished by their appearance. The CGR region is relatively flat, and the JR region indicates heavily deformed ductil e tearing. Note that ai r bubbles trapped in the replica are more excessive in the JR test region, a rough and dimpled fracture surface, than the flat CGR test region. Casting dendrite morphology with ferrite core s was seen at the end of the CGR test.

Cleavage-like morphology dominated the fatigue pre-cracking region, as shown in Fig. 41. Deformation steps are clearly visible on the fracture surface. With the advance of the crack, an area with delta ferrite at dendrite cores starte d to appear (Fig. 42). Compared with the surrounding austenitic phase, fewer deformation step s can be seen within the ferrite phase, as shown in Fig. 43. Field Co 62 Figure 39. Fracture surface of specimen B-1 tested in PWR water.

Field Co 63 Figure 40. Fracture surface of specimen B-1 along the sample central line. Machine notch CGR test JR test Fatigue crackingVermicular

ferrite at dendrite cores Dimple fractureCrack advance Post-JR fatigue Field Co 64 Figure 41. Deformation steps in the pr e-cracking region of speci men B-1. Crack propag ation from bo ttom to top.

Field Co 65 Figure 42. Delta ferrite at dendrite cores in specimen B-1. Crack propagation from bottom to top.

Field Co 66 Figure 43. Fracture surface of specimen B-1 showing that delta ferrite is surrounded by heavily deformed austenite phase. Cra ck propagation from bottom to top.

Field Co

Retrieved from "https://kanterella.com/w/index.php?title=ML15331A168&oldid=771632"