ML15160A156

NYS000488A - 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)

ML15160A156
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Site:	Indian Point
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
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NYS00488A Submitted: June 9, 2015 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

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 growth rate and fracture toughness J-R curve tests were performed on irradiated cast stainless steels 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 behavior 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 austenite phase. The fracture toughness results showed a dominant effect of neutron irradiation, 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 neutron irradiation in cast stainless steel.

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FOREWORD Cast austenitic stainless steel (CASS) 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 mechanical properties of cast steels were subjected to commercial and experimental heats as well as reactor aged cast steels (such as CF-3, CF-8 and CF-8M). The tests concluded that the chemical composition of the steels 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 brittle 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 susceptibility and fracture resistance of CASS.

Crack growth rate (CGR) and fracture toughness 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%). 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. Trans-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.

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Brian Thomas, Director (Acting)

Division of Engineering Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission vi

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 vii

LIST OF FIGURES

1. Metallurgical images of the unaged and thermally aged CASS materials. ........................... 6 Field Co

2. Schematic of 1/4T-CT specimen used in this study (red lines represent electrical leads). ... 7 Field Co

3. Irradiation capsule of low-dose Halden irradiation. .............................................................. 8 Field Co

4. Recirculation water loop for Cell 1 of the IML (items in red are safety significant Field Co components) ........................................................................................................................ 10

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

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

8. The J-R curve for specimen A-N1. ....................................................................................... 20 Field Co

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

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

11. Transgranular fracture in the precracking region of specimen A-N1. ............................... 23 Field Co

12. Ductile dimple fracture in JR test region of specimen A-N1. ............................................ 24 Field Co

13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% ferrite): Field Co test periods (a) a-e, (b) f-j, (c) k-m, and (d) 1. .................................................................... 26

14. Cyclic CGRs of specimen A-1. ........................................................................................... 28 Field Co

15. Fracture surface of specimen A-1 tested in PWR water..................................................... 29 Field Co

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

17. Cleavage-like fracture in specimen A-1: (a) pre-cracking, and (b) end of the CGR test. Field Co Crack propagation from bottom to top............................................................................... 31

18. Fracture surface of specimen A-1 showing that delta ferrite deformed to a lesser extent Field Co 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) Field Co 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. ........................................................................................... 37 Field Co

21. The J-R curve for specimen A-2. ........................................................................................ 38 Field Co

22. Fracture surface of specimen A-2 tested in low-DO high-purity water. ............................ 39 Field Co

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

24. Cleavage-like steps at the beginning of CGR test of specimen A-2. Crack propagation Field Co from bottom to top. ............................................................................................................ 41

25. Fracture surface of specimen A-2 showing that ferrite deformed to a lesser extent than Field Co austenite. Crack propagation from bottom to top. ............................................................ 42

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

27. Crack-length-vs.-time plot for specimen B-N1 (unirradiated, aged CF-3 with 24% Field Co 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. ............................................................................................................................. 45

28. Cyclic CGRs of specimen B-N1. ........................................................................................ 48 Field Co

29. The J-R curve for specimen B-N1. ...................................................................................... 48 Field Co ix

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

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

32. Transgranular fracture in specimen B-N1 at the beginning of the precracking. Crack Field Co propagation from bottom to top. ........................................................................................ 52

33. Transgranular fracture in specimen B-N1 at the end of the precracking. Crack Field Co propagation from bottom to top. ........................................................................................ 53

34. Ductile dimple fracture in the JR test region of specimen B-N1. Crack propagation Field Co 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 Field Co 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. ........................................................................................... 60 Field Co

37. SCC CGRs of unaged and thermally aged CF-3 with 24% ferrite. ..................................... 60 Field Co

38. The J-R curve for specimen B-1.......................................................................................... 61 Field Co

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

40. Fracture surface of specimen B-1 along the sample central line. ........................................ 63 Field Co

41. Deformation steps in the pre-cracking region of specimen B-1. Crack propagation from Field Co bottom to top. ..................................................................................................................... 64

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

43. Fracture surface of specimen B-1 showing that delta ferrite is surrounded by heavily Field Co 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% Field Co 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 Field Co

46. J-R curve data of specimen E-N1. ....................................................................................... 72 Field Co

47. Fracture surface of specimen E-N1 tested in low-DO high-purity water........................... 73 Field Co

48. Transgranular fracture at the beginning of the precracking for specimen E-N1. Crack Field Co propagation from bottom to top. ........................................................................................ 74

49. Transgranular fracture at the end of the CGR test for specimen E-N1. Crack Field Co propagation from bottom to top. ........................................................................................ 75

50. Ductile dimple fracture in the JR test region of specimen E-N1. Crack propagation Field Co from bottom to top. ............................................................................................................ 77

51. Crack-length-vs.-time plot for specimen E-1 (0.08-dpa unaged CF-8 with 23% ferrite): Field Co 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 Field Co

53. The J-R curve of specimen E-1. .......................................................................................... 82 Field Co

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

55. Fracture surface of specimen E-1 along the sample central line. ........................................ 84 Field Co

56. Cleavage-like cracking at the beginning of the CGR test of specimen E-1. Crack Field Co propagation from bottom to top. ........................................................................................ 85

57. Cyclic CGR test region of specimen E-1. Crack propagation from bottom to top. .......... 86 Field Co x

58. Smooth fracture surface at the end of the CGR test in specimen E-1. Crack propagation Field Co from bottom to top. ............................................................................................................ 87

59. Ductile dimple fracture in the J-R test region of specimen E-1. Crack propagation from Field Co bottom to top. ..................................................................................................................... 88

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

62. The J-R curve of specimen F-N1. ....................................................................................... 93 Field Co

63. Fracture surface of specimen F-N1 tested in low-DO high-purity water. .......................... 94 Field Co

64. Fracture surface of specimen F-N1 along the sample central line. ..................................... 95 Field Co

65. Transgranular fracture in the CGR test of specimen F-N1: (a) in the precracking region Field Co 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 Field Co direction from bottom to top. ............................................................................................. 98

67. Ductile dimple fracture in the JR test region of specimen F-N1. Crack advance Field Co direction from bottom to top. ............................................................................................. 100

68. Crack-length-vs.-time plot for specimen F-1 (0.08-dpa aged CF-8 with 23% ferrite): Field Co test periods (a) a-f, (b) g-j, (c) k-m, and (d) 1. .................................................................... 102

69. Cyclic CGRs of specimen F-1. ............................................................................................ 103 Field Co

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

71. The J-R curve of specimen F-1. .......................................................................................... 105 Field Co

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

73. Fracture surface of specimen F-1 along the sample central line. ........................................ 107 Field Co

74. Fracture surface of the CGR region in specimen F-1. Crack propagation from bottom Field Co to top. .................................................................................................................................. 108

75. Deformation steps in austenite grain around ferrite phase in the CGR test region of Field Co 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 Field Co to top. ................................................................................................................................. 110

77. Crack-length-vs.-time plot for specimen I-1 (0.08-dpa unaged CF-8M with 28% Field Co 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 Field Co

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

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

81. Fracture surface of specimen I-1 along the sample central line. ......................................... 119 Field Co

82. Precracking region in the CGR test of specimen I-1. Crack propagation from bottom to Field Co top. ..................................................................................................................................... 120

83. Fracture surface at the end of CGR test of specimen I-1. Crack propagation from Field Co bottom to top. ..................................................................................................................... 121

84. Heavily deformed microstructure in the JR test region of specimen I-1............................ 122 Field Co

85. Crack-length-vs.-time plot for specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite): Field Co test periods (a) a-g, (b) h-n, (c) o-r, (d) s-u, (e) 1a-1b, and (f) 2a-2c. ................................. 124 xi

86. Cyclic CGRs of specimen J-1. ............................................................................................ 127 Field Co

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

88. The J-R curve of specimen J-1. ........................................................................................... 128 Field Co

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

90. Fracture surface of specimen J-1 along the sample central line. ......................................... 130 Field Co

91. Precracking region of specimen J-1. Crack propagation from bottom to top. .................. 131 Field Co

92. Ferrite microstructure at the end of CGR test of specimen J-1. Crack propagation from Field Co bottom to top. ..................................................................................................................... 132

93. Cleavage-like fracture at the end of CGR test of specimen J-1. Crack propagation from Field Co bottom to top. ..................................................................................................................... 133

94. Fracture along ferrite at dendrite core in the JR test region of specimen J-1. Crack Field Co propagation from bottom to top. ........................................................................................ 134

95. Best-fit curves of cyclic CGRs at 0.08-dpa dose: (a) unaged and aged CF-3, (b) unaged Field Co and aged CF-8, and (c) unaged and aged CF-8M. .............................................................. 137

96. Fitting coefficient A for the corrosion fatigue superposition model. .................................. 139 Field Co

97. Constant-load CGRs of the low-dose CASS with more than 23% ferrite in low-DO Field Co high-purity and PWR water environments. ........................................................................ 140

98. Fracture toughness values of unirradiated and irradiated CASS in unaged and aged Field Co 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 xii

LIST OF TABLES

1. Chemical compositions of the cast stainless steels examined in this study. ....................... 5 Field Co

2. Thermal aging conditions for the cast stainless steels in this study. ................................... 5 Field Co

3. Test matrix of unirradiated and irradiated low-dose CASS specimens. ............................. 15 Field Co

4. Crack growth rates of specimen A-N1 (unirradiated and unaged CF-3 with 24% Field Co 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% Field Co ferrite) in PWR water. ..................................................................................................... 25

6. Crack growth rates of specimen A-2 (0.08-dpa unaged CF-3 with 24% ferrite) in Field Co low-DO high-purity water environment. ........................................................................... 34

7. CGR test of specimen B-N1 (unirradiated, thermally aged CF-3 with 24% ferrite) Field Co in PWR water. .................................................................................................................... 44

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

9. CGR test of specimen E-N1 (unirradiated, unaged CF-8 with 23% ferrite) in low- Field Co DO high-purity water. ........................................................................................................ 67

10. CGR test of specimen E-1 (0.08-dpa unaged CF-8 with 23% ferrite) in low-DO Field Co high-purity water................................................................................................................ 78

11. Crack growth rates of specimen F-N1 (unirradiated, aged CF-8 with 23% ferrite) Field Co 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 Field Co 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 Field Co 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 Field Co low-DO high-purity water environment. ........................................................................... 123

15. CGR test results at ~320°C for CASS specimens with high ferrite contents. ................... 135 Field Co

16. Fracture toughness JR test results for CASS with high ferrite contents. ........................... 136 Field Co xiii

EXECUTIVE

SUMMARY

Cast austenitic stainless steel (CASS) is used in the cooling system of light water reactors for components with complex shapes, such as pump 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 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. 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 resistance (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 1019 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 environment 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 difficult 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 aging history. In general, the CASS materials showed good resistance to both corrosion 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 present study.

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Neutron irradiation did significantly affect the fracture toughness of CASS. At 0.08 dpa, the fracture toughness values of unaged specimens were much lower than their initial unirradiated values. Fracture toughness was also reduced by 20-30% for thermally aged specimens after irradiation. This observation suggests 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 both 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 saturation state). For this reason, the effects of neutron irradiation should be considered when the degree of thermal aging embrittlement is being evaluated for CASS components.

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ACKNOWLEDGEMENTS The authors would like to thank Drs. O. K. Chopra and W. J. Shack for their invaluable contribution to this project. Our special thanks 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 Wisconsin-Madison are acknowledged for their contributions to the experimental effort. We are also grateful 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.

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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 xix

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 xx

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 wrought 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 Formatte and gram 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 the 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 Formatte and gram for LWR applications in aqueous environments. The strength and ductility of CF grades are comparable to those of wrought SSs. At room temperature, the yield and tensile strengths of CF-3 and CF-8 grades are greater than 200 MPa and 480 MPa,33 respectively, similar to those of Formatte and gram 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 their yield strength before final fracture. Chopra and Sather 44 showed that the crack initiation toughness of CF-3 and CF-8 Formatte and gram CASS varies from ~200 kJ/m2 to over 1000 kJ/m2 at room temperature; these values are comparable to those of wrought SSs reported by Mills.55 Formatte and gram In contrast to the fully austenitic microstructure 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 Formatte and gram Empirical models have also been developed to predict and control phase content with alloy composition. Based on the phase-stabilizing effects of Cr and Ni in Fe-Cr-Ni systems, several Formatte and gram constitution diagrams have been established to estimate phase content.88,99,1010 The Formatte contributions of minor alloying elements are incorporated with Cr and Ni equivalent numbers and gram and computed with empirical equations. The applicable range of composition, incorporated Formatte alloying elements, and weighing factors vary in these models, and thus the predicted phase and gram contents differ among them to some extent.1111 For steel castings of CF grades, use of the Formatte Scheofer diagram, which is a modified version of the Schaeffler diagram88, is recommended by and gram the American Society for Testing and Materials (ASTM) for estimating ferrite content.1212 Formatte and gram The ferrite phase is critical for the mechanical properties and corrosion resistance of CASS and Formatte and gram 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 Formatte and gram microstructure. Beck et al. 1313 showed that the tensile and yield strengths of CASS increase Formatte with ferrite content up to 40% at both room and elevated temperatures. The strengthening and gram effect of ferrite phase is attributed to ferrite-austenite boundaries and can be explained with the Hall-Petch model.1414 Ferrite phase is also crucial for the soundness and weldability of steel Formatte castings. A minimum ferrite content is often specified for SS welds to reduce the tendency of grammar hot cracking. In addition, the presence of ferrite phase can improve the resistance to 1

sensitization and stress corrosion cracking (SCC).1515 In susceptible environments, CASS tends Formatte and gram to be more resistant to SCC than the same grade of wrought SS. This beneficial effect of ferrite was clearly demonstrated by Hughes et al.1616 in boiling water reactor (BWR) environments. Formatte and gram Using slow strain rate tests, they showed that CF-3, CF-3A, and CF-8 have an exceptional resistance to intergranular SCC (IGSCC) in high-purity (HP) water containing 6-8 ppm dissolved oxygen (DO).

While the presence of ferrite in CASS is mostly beneficial, 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 Formatte and gram aging embrittlement are an increased tensile strength and reduced ductility.1919,2020 The upper-shelf impact energy is also reduced, and the ductile-to-brittle transition temperature shifts Formatte and gram higher.2020 Formatte and gram Because of thermal aging embrittlement, the long-term performance of CASS materials at Formatte elevated service temperatures is of concern. The degradation of CASS components resulting and gram from thermal aging embrittlement has been recognized as a potential issue for aging reactors.2121 Formatte Several research programs have been conducted to assess Charpy impact properties and JR and gram resistance curves of thermally aged CASS.2222,2323 It was found that thermal aging at 290- Formatte 450°C up to 30,000 hr leads to a significant deterioration in the fracture properties of CASS. and gram The lower bound of impact energy and fracture toughness (JIC) can be as low as 20 J/cm2 and 25 Formatte and gram kJ/m2, respectively, at room temperature. The ductile-to-brittle transition temperature of CASS is also shifted to around 0°C for the thermally aged CASS. The extent of the thermal aging Formatte and gram embrittlement increases with ferrite content and is sensitive to ferrite morphology.2222 Formatte and gram The mechanism of thermal aging of duplex SSs has been studied extensively.1717-2020, 2222-2828 Formatte It is widely accepted that embrittlement is caused by the instability of the ferrite phase under and gram thermal aging. The main reasons of the hardening and loss of toughness are (1) the formation of Formatte Cr-rich phase through spinodal decomposition, and (2) the precipitation and growth of and gram carbides and G-phase at ferrite-austenite phase boundaries. Obviously, these microstructural Formatte changes are thermally activated and are fundamentally controlled by solid-state diffusion. and gram Therefore, the thermal aging time for a given extent of degradation (e.g., an increase in hardness Formatte and gram or decrease in toughness) follows an Arrhenius-type relationship.2424 Formatte and gram Besides thermal aging, neutron irradiation can also affect the microstructural 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 Formatte

,3030 These point defects evolve at irradiation temperatures to form irradiation defects, giving and gram rise to irradiation hardening and embrittlement. The irradiation embrittlement can generate Formatte and gram further degradation in the ferrite phase, leading to an additional 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 Formatte and gram elevated diffusivity under neutron irradiation could certainly affect the kinetics of thermal aging.

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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 Formatte and gram 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 Formatte and gram irradiation and thermal aging would reduce the fracture resistance to a lower level than either of Formatte the degradation mechanisms can impart alone. If so, the combined effect is not only important and gram for internal components made of CASS, but also for SS weld metals that possess a similar Formatte austenite-ferrite duplex microstructure. While weld metals may contain less ferrite phase than and gram 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 concerning 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 the present study. Still, the CGR tests could provide corrosion fatigue starter cracks for the subsequent fracture toughness J-R curve tests, so that any environmental contribution to the fracture behavior of CASS could be detected.

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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 Formatte and gram of 610 x 610 x 76 mm. A ferrite scope measurement on these heats showed that the CF-3 and CF-8 heats contained approximately 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 with 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 Formatte and gram 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. Field Co Cast FerriteContent Heat Composition (wt. %)

Grade Measured a Calculated b ID Mn Si P S Mo Cr Ni N C CF-3 24% 21% 69 0.63 1.13 0.015 0.005 0.34 20.18 8.59 0.028 0.023 CF-8 23% 14% 68 0.64 1.07 0.021 0.014 0.31 20.46 8.08 0.062 0.063 CF-8M 28% 25% 75 0.53 0.67 0.022 0.012 2.58 20.86 9.12 0.052 0.065

a. Measured with a ferrite scope,Ref. [44]. Formatte

b. Calculated with Hulls equations, Ref. [4] and gram Table 2. Thermal aging conditions for the cast stainless steels in this study. Field Co Cast Grade Ferrite a Spec. Code Heat ID Thermal Aging Condition A 69 Unaged CF-3 24%

B 69 10,000 hr at 400°C E 68 Unaged CF-8 23%

F 68 10,000 hr at 400°C I 75 Unaged CF-8M 28%

J 75 10,000 hr at 400°C

a. Measured with a ferrite scope,Ref. [44]. Formatte and gram 5

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 Formatte Field Co Figure 1. Metallurgical images of the unaged and thermally aged CASS materials.

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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 Field Co 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 (Fe, 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 for the irradiation capsule. The obtained fast neutron fluence (E > 1MeV) was about 5.56 x 1019 n/cm2, which corresponds to a displacement damage of 0.08 dpa for the samples.

7

8

1. End Plug 2. Outer Capsule 3. Heat Transfer Body 4. End Guide Sleeve

5. Melting Alloy Assembly 6. CT Specimens 7. TEM Specimens Container 8. Spacer Figure 3. Irradiation capsule of low-dose Halden irradiation.3333 Formatte pt, Check Field Co by 3 pt

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 respect to the surroundings to maintain a proper radiological barrier.

The two test systems are installed in separate hot cells. Each of the hot cells is equipped with its own loading frame, autoclave, load cell, linear voltage displacement 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 Formatte and gram simulated PWR water contained ~2 ppm lithium, ~1000 ppm boron, and ~2 ppm hydrogen. The conductivity was about 20 S/cm. The low-DO high-purity water contained less than 10 ppb dissolved oxygen, and the conductivity was kept below 0.07 S/cm during the tests. The test environments were provided by two water recirculation loops, and a schematic diagram of loop

1. 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 the 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.

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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. Conductivity 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 Field Co components) 2.4 Crack Growth Rate and Fracture 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 extension of the CT sample with a calibrated correlation curve.

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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 m1/2 are used for pre-cracking. The objective 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 been altered by machining and do not represent the materials intrinsic behavior.

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

P (2 + a / W ) a Field Co K= 1/ 2 3/ 2 f( ) (1)

( B BN W ) (1 a / W ) W where P is applied load; B is the specimen thickness; BN is the net specimen thickness (or distance between the roots 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:

2 3 4 Field Co a a a a a f ( ) = 0.886 + 4.64 13.32 + 14.72 5.60 (2)

W W W W W 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 CGRs 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.

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Crack Growth Rate in Water, Log Scale Mechanical Fatigue Increase Load Ratio and Rise Time Environmental Effect Crack Growth Rate in Air, Log Scale Figure 5. Schematic for inducing environmentally assisted cracking in test environment. Field Co 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 constant stain rate of 0.43 m/s, and the load and sample extension were recorded continuously outside the autoclave. The load-line displacement at load points was determined by subtracting the extension of the load train, which had been measured prior to the test. During the 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, Formatte and gram J = J el + J pl (3) Field Co 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:

Field Co

( K ( i ) ) 2 (1 ) 2 J el ( i ) = (4)

E 12

where is Poissons ratio, and the stress intensity K(i) is calculated from Eqs. 11 and 22. The plastic component Jpl(i) is given by:

( i 1) Apl ( i ) Apl ( i 1) a( i ) a( i 1) Field Co J pl ( i ) = J pl ( i 1) + 1 ( i 1) (5) b( i 1) BN b( i 1) 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 and are expressed as:

b( i 1) Field Co

( i 1) = 2.0 + 0.522 (6)

W b( i 1) Field Co

( i 1) = 1.0 + 0.76 (7)

W 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

[ Pi + Pi 1 ] v pl (i ) v pl (i 1) Field Co Apl (i ) = Apl (i 1) + (8) 2 where the plastic components of the load-line displacement, vpl(i), are:

v pl (i ) = v(i ) PC i LL (i ) (9) Field Co 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:

1.62 + 17.80(ai / W ) 4.88(ai / W )2 + 1.27( ai / W )3 Field Co CLL (i ) = (10)

' 2 E Be [1 (ai / W ) ]

where Be is the specimen effective thickness given by B - (B - BN)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 intersection 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 Formatte and gram used in the previous thermal aging studies on unirradiated CASS.44 To be consistent with the Formatte previous analyses, the same blunting line (i.e., J/4f) was also used in the current work. and gram In this study, the estimated flow stresses are approximately 280-340 MPa and 420-520 MPa for unirradiated and irradiated CASS materials, respectively. This relatively low strength allows a maximum J value of 280-360 kJ/m2 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 required 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 even lower without irradiation hardening. For this reason, the J values determined from this study normally cannot be validated for JIC per ASTM E182-8a.

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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 harden the epoxy replica.

The negative replica was then removed. The obtained epoxy replica was coated 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 spaced 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 the DCPD method were scaled proportionately to match the final SEM-measured crack length.

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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. Field Co Sample Dose Heat Materials Test Environment Facility CGR JR SEM ID (dpa) ID 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 high-purity water. The material was an unaged CF-3 CASS with ~24% ferrite (Heat 69). The objective of this test was to provide a baseline for the irradiated tests. The test conditions and results are summarized in Table 4, and a crack-length history plot is shown in Fig. 6.

Fatigue pre-crack was initiated at ~15.4 MPa m1/2 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-m crack extension, the load ratio and rise time were increased slowly to induce environmentally 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 15

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 Formatte and gram 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 Formatte and gram test period z) was due to a temperature increase in the autoclave 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-m 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% Field Co ferrite) in low-DO high-purity water.

Test Test Load Rise Return Hold CGR in CGR Crack Test Time, Temp., Ratio Time, Time, Time, Kmax, K, Env., in Air, Length, Period h °C s s s MPa m1/2 MPa m1/2 m/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 ta 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 zb 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.

16

6.40 (a) Spec. A-N1, CF-3, 24% , unaged 32 Low-DO high-purity water, ~318 oC 6.30 28 24 Crack Length (mm)

Kmax (MPa m 0.5) c, Hold at low K R=0.4 6.20 b, 20 0.5 Hz R=0.2 1 Hz 16 6.10 12 a,

8 R=0.1 6.00 1 Hz 4

0 5 10 15 20 Time (h) 6.80 (b) Spec. A-N1, CF-3, 24% , unaged 32 Low-DO high-purity water, ~318 oC 6.70 f, j, 28 R=0.45 i, R=0.5 0.5 Hz R=0.5 15s/up, 5s/down 6.60 24 Crack Length (mm) h, 10s/up, 2s/down Kmax (MPa m 0.5) g, R=0.6 R=0.5 0.1 Hz 0.2 Hz 20 6.50 16 6.40 e, 12 R=0.3 1 Hz 8

6.30 d, R=0.2 4 1 Hz 6.20 20 30 40 50 60 70 80 Time (h) 6.80 (c) Spec. A-N1, CF-3, 24% , unaged p, 32 Low-DO high-purity water, ~318 oC o, R=0.5 n, R=0.5 1000s/up, m, R=0.5 500s/up, 28 12s/down l, R=0.5 300s/up, 12s/down 6.75 k, R=0.5 150s/up, 12s/down 24 Crack Length (mm)

Kmax (MPa m 0.5)

R=0.5 60s/up, 12s/down 30s/up, 12s/down 20 5s/down 16 6.70 12 8

4 6.65 100 150 200 Time (h)

Figure 6. Crack-length-vs.-time plot for specimen A-N1 (unirradiated and unaged CF-3 with Field Co 24% ferrite): test periods (a) a-c, (b) d-j, (c) k-p, (d) q-u, (e) v-aa, and (f) 1.

17

6.85 (d) Spec. A-N1, CF-3, 24% , unaged u, 32 Low-DO high-purity water, ~318 oC t, R=0.5 R=0.45 200s/up, 28 s, 100s/up, 12s/down q, r, R=0.45 12s/down 6.80 24 Crack Length (mm) 50s/up, Kmax (MPa m 0.5)

R=0.5 R=0.5 200s/up, 100s/up, 12s/down 12s/down 20 12s/down 5.1E-10 m/s 16 6.75 Pressure unstable 12 caused by cooling water problem.

8 4

6.70 240 260 280 300 320 340 Time (h) 6.95 (e) Spec. A-N1, CF-3, 24% , unaged 32 Low-DO high-purity water, ~318 oC aa, z, R=0.5 y, R=0.5 1000s/up, 28 6.90 500s/up, x, R=0.5 12s/down w, R=0.5 12s/down 24 200s/up, Crack Length (mm)

Kmax (MPa m 0.5)

R=0.45 100s/up, 12s/down 50s/up, 12s/down 12s/down 20 6.85 16 12 6.80 v, 8 R=0.45 20s/up, 4 5s/down 6.75 360 380 400 420 440 460 480 500 Time (h) 6.920 (f) Spec. A-N1, CF-3, 24% , unaged 32 Low-DO high-purity water, ~318 oC 6.900 1, 28 PPU, 2 hr hold 6.880 24 Crack Length (mm)

K (MPa m 0.5) 20 6.860 16 6.840 12 8

6.820 4

6.800 500 550 600 650 700 750 800 850 Time (h)

Figure 6. (Contd.)

18

Specimen A-N1 CF-3, unaged, unirradiated 10-7 Low-DO high-purity water

~318oC 10-8 CF curve for 0.2 ppm DO CGRenv (m/s) by Shack & Kassner 10-9 10-10 Test periods: a-c Test periods: d-p Test periods: q-u 10-11 Test periods: v-aa 10-11 10-10 10-9 10-8 10-7 CGRair (m/s)

Figure 7. Cyclic CGRs of specimen A-N1. Field Co Fracture toughness J-R curve test A fracture toughness J-R curve test was conducted on the sample in the low-DO high-purity water environment. The sample was loaded at a constant extension rate of 0.43 m/s while the load and load-line displacement were recorded. During the test, loading was interrupted periodically to measure the crack extension by DCPD. The obtained J and 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 anticipated for the CASS materials, 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/m2. 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.

19

640 Specimen A-N1 CF-3, unaged, unirradiated Low-DO high-purity water, ~318 oC 480 J (kJ/m2) 320 Jmax JQ=320 kJ/m2 160 J= 536.5*a0.68 amax 0

0.0 0.5 1.0 1.5 2.0 Crack Extension (mm)

Figure 8. The J-R curve for specimen A-N1. Field Co 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 entire JR test region is covered with ductile dimples. A major secondary crack that covers nearly a half of the width of the sample can be seen at the machined notch. This large secondary 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 crack 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.

20

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

Field Co

Dimple fracture Crack advance TG Secondary crack Machined notch Field Co Figure 10. Fracture surface of specimen A-N1 along the sample central line.

22

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

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% ferrite 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. The 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 restart 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) Field Co in PWR water.a Test Test Load Rise Return Hold CGR in CGR Crack Test Time, Temp., Ratio Time, Time, Time, Kmax, K, Env., in Air, 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 S/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 m extension, the load ratio was increased while the stress intensity factor 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 air. 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 25

corrosion fatigue response of this sample is lower than that of typical SSs, suggesting good IASCC resistance of CASS in PWR water.

After the cyclic CGR test, the specimen was subjected 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 Formatte and gram 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.

7.60 (a) Specimen A-1 28 7.50 CF-3, unaged, 0.08 dpa Crack length o

PWR water, 320 C e 7.40 d 24 c

Crack Length (mm) 7.30 20 Kmax (MPa m0.5)

Kmax 7.20 b 16 7.10 a

12 7.00 8

6.90 4

6.80 65 70 75 80 85 Time (h) 7.65 (b) Specimen A-1 28 CF-3, unaged, 0.08 dpa Kmax j 7.60 PWR water, 320oC i 24 7.55 Crack Length (mm) h 20 Kmax (MPa m0.5) g Crack length 16 7.50 f 12 7.45 e 8 7.40 4

7.35 85 90 95 100 105 110 115 120 Time (h)

Figure 13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% Field Co ferrite): test periods (a) a-e, (b) f-j, (c) k-m, and (d) 1.

26

7.75 (c) Specimen A-1 28 CF-3, unaged, 0.08 dpa Kmax PWR water, 320oC 24 7.70 m

Crack Length (mm) 20 Kmax (MPa m0.5) l 7.65 16 k

Crack length 12 7.60 8 4

7.55 120 130 140 150 160 170 Time (h) 7.72 (d) Specimen A-1 28 CF-3, unaged, 0.08 dpa 7.71 PWR water, 320oC 24 K

7.70 Crack Length (mm) 20 K (MPa m 0.5) 7.69 1 16 12 7.68 Crack length 8 7.67 4

7.66 170 180 190 200 210 220 230 240 250 Time (h)

Figure 13. (Contd.)

27

Specimen A-1 10-7 CF-3, unaged, 0.08 dpa PWR water, 320oC 10-8 CGRenv (m/s)

CF curve for 0.2 ppm DO by Shack & Kassner 10-9 10-10 Test period with <10 m extension are excluded.

10-11 10-11 10-10 10-9 10-8 10-7 CGRair (m/s)

Figure 14. Cyclic CGRs of specimen A-1. Field Co 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 middle of the picture corresponds to the 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 throughout the CGR region. Figure 17 shows the typical river pattern of cleavage cracking at the beginning and 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 within ferrite, suggesting that the ferrite dendrite core might be deformed to a lesser extent than the surrounding austenite. Beyond the CGR test, the dominant fracture mode is ductile dimples resulting from microvoid coalescence.

28

29 Figure 15. Fracture surface of specimen A-1 tested in PWR water.

Field Co

Dimple fracture Vermicular ferrite at dendrite core Crack advance Restart of pre-cracking Machined notch Field Co Figure 16. Fracture surface of specimen A-1 along the sample central line.

30

a 31 Figure 17. Cleavage-like fracture in specimen A-1: (a) pre-cracking, and (b) end of the CGR test. Crack propagation from bottom to top.

Field Co

b 32 Figure 17. (Contd.)

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

3.1.1.3 Irradiated specimen A-2 tested 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 plot is shown in Fig. 19.

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

Test Test Load Rise Return Hold CGR in CGR Crack Test Time, Temp., Ratio Time, Time, Time, Kmax, K, Env., in Air, 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 d a 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 m1/2 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 load 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-m 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 34

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) 6.30 Specimen A-2 CF-3, unaged, 0.08 dpa. 32 e

Low-DO high-purity water, 320oC d 28 c

6.20 Crack length 24 Crack Length (mm)

Kmax (MPa m0.5) b 2.3E-8 m/s 20 a

6.10 16 4.1E-8 m/s Kmax 12 6.00 8

5.90 4 14 16 18 20 22 24 26 28 30 Time (h) 6.60 (b) Specimen A-2 CF-3, unaged, 0.08 dpa. 32 6.55 Low-DO high-purity water, 320oC j 28 6.50 i h Crack length 24 Crack Length (mm)

Kmax (MPa m0.5) 6.45 g

20 6.40 f 16 6.35 Kmax 12 6.30 8

6.25 4

6.20 30 35 40 45 50 Time (h)

Figure 19. Crack-length-vs.-time plot for specimen A-2 (0.08-dpa unaged CF-3 with 24% Field Co 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

6.90 (c) Specimen A-2 CF-3, unaged, 0.08 dpa. 32 6.85 Low-DO high-purity water, 320oC o 28 6.80 24 Crack Length (mm) n Kmax (MPa m0.5) 6.75 m Crack length 20 6.70 l 16 6.65 k Kmax 12 6.60 8

6.55 4

6.50 60 80 100 120 140 Time (h) 7.00 (d) Specimen A-2 CF-3, unaged, 0.08 dpa. 32 6.95 Low-DO high-purity water, 320oC 28 6.90 s 24 Crack Length (mm)

Kmax (MPa m0.5)

Kmax q r 20 6.85 p 16 6.80 12 Crack length 8

6.75 4

6.70 160 200 240 280 320 Time (h)

(e) 6.94 Specimen A-2 CF-3, unaged, 0.08 dpa. 32 Low-DO high-purity water, 320oC 2-c 28 6.92 2-b 24 Crack Length (mm)

K 2-a K (MPa m 0.5) 20 6.90 16 1

6.88 12 Crack length 8

6.86 4 400 500 600 700 800 Time (h)

Figure 19. (Contd.)

36

Specimen A-2 10-7 CF-3, unaged, 0.08 dpa.

Low-DO high-purity water 320oC 10-8 CGRenv (m/s)

CF curve for 0.2 ppm DO by Shack & Kassner 10-9 10-10 Test periods: a-f 10-11 Test periods: g-s 10-11 10-10 10-9 10-8 10-7 CGRair (m/s)

Figure 20. Cyclic CGRs of specimen A-2. Field Co The first constant-load test period was conducted at 17.6 MPa m1/2 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-m crack extension. The load was increased to 19.6 MPa m1/2 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-m 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 obtained 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 sample was loaded at a constant extension rate of 0.43 m/s while the load and load-line displacement were recorded. During the test, loading was interrupted periodically to measure the crack 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/m2. 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.

37

640 Specimen A-2 CF-3, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC 480 J (kJ/m 2) 320 J= 430*a0.64 Jmax 160 amax JQ=204 kJ/m2 0

0.0 0.5 1.0 1.5 2.0 Crack Extension (mm)

Figure 21. The J-R curve for specimen A-2. Field Co 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 air 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, indicating a well-controlled loading condition during the CGR test. A curved crack front due to a non-constant constraint 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.

38

Air bubbles JR Dimples 39 Crack advance Delta ferrite in dendrites CGR Figure 22. Fracture surface of specimen A-2 tested in low-DO high-purity water.

Field Co

Post-JR fatigue JR test Dimple fracture Crack advance Vermicular ferrite in dendrite cores CGR test TG Machined notch Field Co Figure 23. Fracture surface of specimen A-2 along the sample central line.

40

41 Figure 24. Cleavage-like steps at the beginning of CGR test of specimen A-2. Crack propagation from bottom to top.

Field Co

42 Figure 25. Fracture surface of specimen A-2 showing that ferrite deformed to a lesser extent than austenite. Crack propagation 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

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 objective 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-length history plot is shown in Fig. 27.

Table 7. CGR test of specimen B-N1 (unirradiated, thermally aged CF-3 with 24% ferrite) in Field Co PWR water.a Test Test Load Rise Return Hold CGR in CGR Crack Test Time, Temp., Ratio Time, Time, Time, Kmax, K, Env., in Air, Length, Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm Start 0.3 5.997 ab 3.0 319 0.20 0.43 0.43 0.07 14.8 11.9 5.57E-08 4.04E-08 6.182 bb 7.2 319 0.35 0.42 0.42 0.08 15.0 9.8 3.25E-08 2.67E-08 6.353 cb 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 gb 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-13 - 7.107 a Simulated PWR water with 2 ppm Li and 1000 ppm B. Conductivity ~20 S/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.

44

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 cycling. After about 180-m 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) 6.56 Spec. B-N1, CF-3, 24% , aged 32 Low-DO high-purity water, ~319 oC 6.48 d,

28 6.40 R=0.55 b, 0.5 Hz 24 Crack Length (mm)

Kmax (MPa m 0.5)

R=0.35 c, 6.32 a, 1 Hz R=0.5 20 R=0.2 1 Hz 6.24 1 Hz 16 6.16 12 6.08 8 6.00 4 0 5 10 15 20 Time (h) 6.80 (b) Spec. B-N1, CF-3, 24% , aged 32 o

6.75 Low-DO high-purity water, ~319 C 28 h,

6.70 g, R=0.55 R=0.55 3s/up, 1s/down 24 Crack Length (mm)

Kmax (MPa m 0.5) f, 0.5 Hz 6.65 e, R=0.55 R=0.55 20 5s/up, 2s/down 0.2 Hz 6.60 16 12 6.55 8

6.50 4

6.45 30 40 50 60 70 80 90 100 Time (h)

Figure 27. Crack-length-vs.-time plot for specimen B-N1 (unirradiated, aged CF-3 with 24% Field Co 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.

45

6.85 (c) Spec. B-N1, CF-3, 24% , aged 32 Low-DO high-purity water, ~319 oC n, 6.80 R=0.5 28 m, 0.1 Hz R=0.5 24 Crack Length (mm)

Kmax (MPa m 0.5) l, 0.2 Hz 6.75 k, R=0.52 i, j, 20 R=0.52 10s up, 2s down R=0.52 R=0.53 5s up, 2s down 0.1 Hz 0.2 Hz 16 6.70 12 6.65 8 4

6.60 120 140 160 180 200 Time (h) 7.00 (d) Spec. B-N1, CF-3, 24% , aged 32 Low-DO high-purity water, ~319 oC 6.95 s, 28 r,

q, R=0.5 R=0.5 6.90 p, R=0.5 15s up, 24 Crack Length (mm) 60s up, Kmax (MPa m 0.5) o, R=0.5 30s up, 5s down 5s down R=0.5 20s up, 5s down 20 6.85 10s up, 5s down 2s down 16 6.80 12 8

6.75 4

6.70 220 240 260 280 300 Time (h) 7.15 (e) Spec. B-N1, CF-3, 24% , aged 32 o w, 7.10 Low-DO high-purity water, ~319 C R=0.42 v, 120s up, 12s down 28 7.05 u, R=0.37 R=0.35 60s up, 12s down 24 Crack Length (mm) t, Kmax (MPa m 0.5) 30s up, R=0.32 7.00 5s down 15s up, 20 5s down 6.95 16 12 6.90 Check 8 6.85 compliance 4

6.80 315 320 325 330 335 340 345 Time (h)

Figure 27. (Contd.)

46

7.15 (f) Spec. B-N1, CF-3, 24% , aged 32 Low-DO high-purity water, ~319 oC aa, 28 z, R=0.4 y, R=0.4 1500s up, 12s down 24 7.10 Crack Length (mm)

Kmax (MPa m 0.5) x, R=0.4 1000s up, 12s down R=0.4 500s up, 12s down 300s up, 12s down 20 16 7.05 12 8

4 7.00 350 360 370 380 390 400 410 420 430 Time (h)

(g) 7.14 Spec. B-N1, CF-3, 24% , aged 32 Low-DO high-purity water, ~319 oC 28 1b, 7.12 1a, Constant 24 Crack Length (mm)

UUP, 2 hr hold K (MPa m 0.5) 20 7.10 16 7.08 12 8

7.06 4 450 500 550 600 650 Time (h)

Figure 27. (Contd.)

After the environmentally assisted cracking was stabilized, the test was set to a constant stress intensity factor (~17 MPa m1/2) 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-m 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-m 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 constant strain rate of 0.43 m/s, and the crack extension was measured with the DCPD method. Before each DCPD measurement, the stress was allowed to relax for 30 s at a constant displacement. The obtained J-R curve is shown in Fig. 29. The estimated J 47

value at 0.2 mm offset line is about 170 kJ/m2. 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.

Specimen B-N1 CF-3, aged, unirradiated 10-7 PWR water

~319oC 10-8 CF curve for 0.2 ppm DO CGRenv (m/s) by Shack & Kassner 10-9 Test periods: a-f 10-10 Test periods: g-i Test periods: j-l Test periods: m-s Test periods: t-aa 10-11 10-11 10-10 10-9 10-8 10-7 CGRair (m/s)

Figure 28. Cyclic CGRs of specimen B-N1. Field Co 640 Specimen B-N1 CF-3, aged, unirradiated PWR water, ~319oC 480 J= 352.6*a0.66 J (kJ/m2) 320 160 Jmax amax JQ=170 kJ/m2 0

0.0 0.5 1.0 1.5 2.0 Crack Extension (mm)

Figure 29. The J-R curve for specimen B-N1. Field Co 48

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 fracture. 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.

49

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

Field Co

Post-JR fatigue Dimple fracture JR test Crack advance TG CGR test Fatigue fracture Machined notch Field Co Figure 31. Fracture surface of specimen B-N1 along the sample central line.

51

52 Figure 32. Transgranular fracture in specimen B-N1 at the beginning of the precracking. Crack propagation from bottom to top.

Field Co

53 Figure 33. Transgranular fracture in specimen B-N1 at the end of the precracking. 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

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 are 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 factor was lowered to prepare for transitioning. In the following test 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 condition 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 cracking. 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 m1/2 in this sample. Environmentally enhanced cracking was only observed after increasing the stress intensity factor to about 22 MPa m1/2. 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. Subsequently, 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 m1/2.

55

Table 8. CGR test of specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) in PWR water.a Field Co Test Test Load Rise Return Hold CGR in CGR Crack Test Time, Temp., Ratio Time, Time, Time, Kmax, K, Env., in Air, 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 kb 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 S/cm.

b The CGR value was obtained from the later part of the test period.

56

(a) 6.70 Specimen B-1 28 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa PWR water, 320oC e 6.60 Compliance 24 test Crack length 6.50 Crack Length (mm) 20 Kmax (MPa m0.5) c d 6.40 16 b

6.30 Kmax a 12 6.20 8

6.10 4

6.00 0 4 8 12 16 20 24 Time (h) 7.10 (b) Specimen B-1 28 7.05 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa PWR water, 320oC k 24 7.00 Crack length i j Crack Length (mm) 20 Kmax (MPa m0.5) 6.95 h 6.90 16 g

Kmax 6.85 f 12 6.80 8 6.75 4

6.70 24 28 32 36 40 44 48 52 Time (h) 7.25 (c) Specimen B-1 u 28 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa 7.20 PWR water, 320oC t 24 s

7.15 Crack Length (mm) q r 20 Kmax (MPa m0.5) p n o 7.10 16 m Kmax l Crack length 12 7.05 8

7.00 4

6.95 60 80 100 120 140 Time (h)

Figure 35. Crack-length-vs.-time plot for specimen B-1 (0.08-dpa aged CF-3 with 24% ferrite) Field Co 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

7.30 (d) Specimen B-1 28 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa y

PWR water, 320oC 24 Kmax x

7.25 Crack Length (mm) 20 Kmax (MPa m0.5) v w 16 Crack length 12 7.20 8

4 7.15 150 160 170 180 190 200 Time (h) 7.38 (e) Specimen B-1 28 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa 7.36 PWR water, 320oC Kmax 24 ac 7.34 ab Crack Length (mm) 20 Kmax (MPa m0.5) 7.32 aa 16 7.30 z Crack length 12 7.28 8 7.26 4 200 250 300 350 400 450 Time (h) 7.40 (f) Specimen B-1 28 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa PWR water, 320oC Kmax 7.38 24 ag Crack Length (mm) 20 Kmax (MPa m0.5) af 7.36 ad ae 16 Crack length 7.34 12 8

7.32 4

7.30 480 520 560 600 640 Time (h)

Figure 35. (Contd.)

58

(g) 7.65 Specimen B-1 28 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa Kmax al PWR water, 320oC 7.60 24 ak Crack Length (mm) 7.55 20 Kmax (MPa m0.5) aj ai Crack length 7.50 16 ah 7.45 12 7.40 8 7.35 4 650 660 670 680 690 Time (h) 7.72 (h) Specimen B-1 28 CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa 7.70 PWR water, 320oC Kmax 1 24 7.68 Crack Length (mm) 20 Kmax (MPa m0.5) ao an 16 7.66 am 12 7.64 Crack length 8

7.62 4

7.60 700 750 800 850 Time (h)

Figure 35. (Contd.)

The cyclic CGRs with a significant crack extension (defined as >10 m) 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 found the corrosion fatigue behaviors 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 susceptibility 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 59

CGR than that of unaged CF-3. However, given the large scatter of the SCC CGR data, this difference is considered insignificant.

Specimen B-1 10-7 CF-3, aged, 0.08 dpa PWR water, 320oC 10-8 CGRenv (m/s)

CF curve for 0.2 ppm DO by Shack & Kassner 10-9 Test periods with <10 m extension are excluded.

10-10 Test periods: a-d Test periods: e-j Test periods: k-q Test periods: r-af 10-11 Test periods: ah-ao 10-11 10-10 10-9 10-8 10-7 CGRair (m/s)

Figure 36. Cyclic CGRs of specimen B-1. Field Co CASS CF-3 with 24% ferrite 10-8 low-DO high-purity or PWR water Open = Unaged 318 - 320oC Closed = Aged Blue = Unirradiated 10-9 Red = 0.08 dpa NUREG-0313 CGR (m/s)

Curve 10-10 Spec. A-N1, unirr., PPU 2 hr, Low-DO water Spec. A-1, 0.08 dpa, PPU 2 hr, PWR water Spec. A-2, 0.08 dpa, PPU 2 hr, Low-DO water 10-11 Spec. A-2, 0.08 dpa, PPU 1 hr, Low-DO water Spec. A-2, 0.08 dpa, w/o PPU, Low-DO water 10-12 Spec. B-N1, unirr., PPU 2 hr, PWR water Spec. B-N1, unirr., w/o PPU, PWR water Spec. B-1, 0.08 dpa, PPU 2 hr, PWR water 10-13 10 15 20 25 30 35 K (MPa m 1/2)

Figure 37. SCC CGRs of unaged and thermally aged CF-3 with 24% ferrite. Field Co 60

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/m2 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.

640 Specimen B-1 CF-3, aged, 0.08 dpa PWR water, 320oC 480 J (kJ/m 2) 320 J= 362*a0.85 Jmax 160 JQ=116 kJ/m2 amax 0

0.0 0.5 1.0 1.5 2.0 Crack Extension (mm)

Figure 38. The J-R curve for specimen B-1. Field Co Fractographic examination Fractographic analysis of specimen B-1 was carried 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 distinguished by their appearance. The CGR region is relatively flat, and the JR region indicates heavily deformed ductile tearing. Note that air 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 cores 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 started to appear (Fig. 42). Compared with the surrounding austenitic phase, fewer deformation steps can be seen within the ferrite phase, as shown in Fig. 43.

61

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

Field Co

Post-JR fatigue JR test Dimple fracture Crack advance Vermicular ferrite at dendrite cores CGR test Fatigue cracking Machine notch Field Co Figure 40. Fracture surface of specimen B-1 along the sample central line.

63

64 Figure 41. Deformation steps in the pre-cracking region of specimen B-1. Crack propagation from bottom 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. Crack propagation from bottom to top.

Field Co