Crack Growth Rate and 

Fracture Toughness 

Tests on Irradiated 

Cast Stainless Steels

Office of Nuclear Regulatory Research

NUREG/CR-7184

ANL-12/56
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Crack Growth Rate and 

Fracture Toughness 

Tests on Irradiated 

Cast Stainless Steels

Manuscript Completed: March 2015

Date Published: July 2015

Prepared by: 

Y. Chen, B. Alexandreanu, and K. Natesan 

Nuclear Engineering Division

Argonne National Laboratory

A. S. Rao, NRC Project Manager

Office of Nuclear Regulatory Research

NUREG/CR-7184

ANL-12/56

 

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. 

iii

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 in this type of steel 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 has been postulated 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, the NRC conducted research on the long term embrittlement of cast duplex 

stainless steels used in LWR systems. Under those programs, experimental data were collected 

on the microstructure and the mechanical properties of commercial and experimental cast steels 

(such as CF-3, CF-8 and CF-8M) that were thermally aged in laboratories or in reactors. These

tests concluded that the chemical composition of the steels and the ferrite morphology strongly 

affect the kinetics of the embrittlement of steels. For example, the low carbon cast steels (CF-3) 

are most resistant to embrittlement, and 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 morphologies of ferrite are either lacy or vermicular in the steels 

containing > 5% ferrite . 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 had been studied extensively, the irradiationinduced embrittlement in CASS had not been investigated thoroughly. There was no data 

available on irradiated CASS alloys either with or without thermal aging. Thus, the present 

research was initiated to investigate the combined effects of thermal aging and irradiation 

damage on the 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 test 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 effects of thermal aging and irradiation embrittlement. The CGR tests 

were conducted at 320 °C in low-corrosion-potential environments. Following the CGR tests, 

the fracture toughness J-R curve tests were performed on the same samples in these 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 the fracture toughness of 

both aged and un-aged CASS. 

v
The report presents a focused research concentration for a better understanding of the 

embrittlement of CASS components. The results of this study enable the agency’s regulatory 

technical basis development efforts, such as the screening criteria of neutron fluence and upper 

bound value of delta ferrite in CASS components with acceptable fracture toughness properties.

 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

TABLE OF CONTENTS ........................................................................................................... vii

LIST OF FIGURES ..................................................................................................................... ix

LIST OF TABLES..................................................................................................................... xiii

EXECUTIVE SUMMARY .........................................................................................................xv

ACKNOWLEDGEMENTS ..................................................................................................... xvii

ACRONYMS AND ABBREVATIONS ................................................................................... xix

UNITS of MEASURE................................................................................................................ xxi

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

6. REFERENCES..................................................................................................................147

vii

LIST OF FIGURES

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

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

3. Irradiation capsule of low-dose Halden irradiation.33..............................................................8

4. Recirculation water loop for Cell 1 of the IML (items in red are safety significant 

components) ...........................................................................................................................10

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

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

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

7. Cyclic CGRs of specimen A-N1.............................................................................................19

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

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

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

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

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

13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% ferrite): 

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

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

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

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

17. Cleavage-like fracture in specimen A-1: (a) pre-cracking, and (b) end of the CGR test. 

Crack propagation from bottom to top...................................................................................31

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

than austenite. Crack propagation from bottom to top..........................................................33

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

tested in low-DO high-purity water environment: test periods (a) a-e, (b) f-j, (c) k-o, (d) 

p-s, and (e) 1-2........................................................................................................................35

20. Cyclic CGRs of specimen A-2................................................................................................37

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

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

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

24. Cleavage-like steps at the beginning of CGR test of specimen A-2. Crack propagation 

from bottom to top..................................................................................................................41

25. Fracture surface of specimen A-2 showing that ferrite deformed to a lesser extent than 

austenite. Crack propagation from bottom to top..................................................................42

26. Ductile dimple morphology in the JR test region of specimen A-2. Crack propagation 

from bottom to top..................................................................................................................43

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

ferrite) in PWR water: test 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

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

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

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

32. Transgranular fracture in specimen B-N1 at the beginning of the precracking. Crack 

propagation from bottom to top..............................................................................................52

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

propagation from bottom to top..............................................................................................53

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

from bottom to top..................................................................................................................54

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

PWR water: test periods (a) a-e, (b) f-k, (c) l-u, (d) v-y, (e) z-ac, (f) ad-ag, (g) ah-al, and 

(h) am-1. .................................................................................................................................57

36. Cyclic CGRs of specimen B-1................................................................................................60

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

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

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

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

41. Deformation steps in the pre-cracking region of specimen B-1. Crack propagation from 

bottom to top. .........................................................................................................................64

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

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

deformed austenite phase. Crack propagation from bottom to top. ......................................66

44. Crack-length-vs.-time plot for specimen E-N1 (unirradiated, unaged CF-8 with 23% 

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

45. Cyclic CGRs of specimen E-N1. ............................................................................................71

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

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

48. Transgranular fracture at the beginning of the precracking for specimen E-N1. Crack 

propagation from bottom to top..............................................................................................74

49. Transgranular fracture at the end of the CGR test for specimen E-N1. Crack 

propagation from bottom to top..............................................................................................75

50. Ductile dimple fracture in the JR test region of specimen E-N1. Crack propagation 

from bottom to top..................................................................................................................77

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

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

52. Cyclic CGRs of specimen E-1. ...............................................................................................81

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

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

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

56. Cleavage-like cracking at the beginning of the CGR test of specimen E-1. Crack 

propagation from bottom to top..............................................................................................85

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

x
58. Smooth fracture surface at the end of the CGR test in specimen E-1. Crack propagation 

from bottom to top..................................................................................................................87

59. Ductile dimple fracture in the J-R test region of specimen E-1. Crack propagation from 

bottom to top. .........................................................................................................................88

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

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

61. Cyclic CGRs of specimen F-N1. ............................................................................................92

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

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

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

65. Transgranular fracture in the CGR test of specimen F-N1: (a) in the precracking region 

and (b) at the end of CGR test. Crack advance direction from bottom to top......................96

66. Transition region from CGR to J-R curve tests of specimen F-N1. Crack advance 

direction from bottom to top. .................................................................................................98

67. Ductile dimple fracture in the JR test region of specimen F-N1. Crack advance direction 

from bottom to top................................................................................................................100

68. Crack-length-vs.-time plot for specimen F-1 (0.08-dpa aged CF-8 with 23% ferrite): test 

periods (a) a-f, (b) g-j, (c) k-m, and (d) 1.............................................................................102

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

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

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

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

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

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

top.........................................................................................................................................108

75. Deformation steps in austenite grain around ferrite phase in the CGR test region of 

specimen F-1. Crack propagation from bottom to top. .......................................................109

76. Dimple fracture in the JR test region of specimen F-1. Crack propagation from bottom 

to top.....................................................................................................................................110

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

test periods (a) a-f, (b) g-k, (c) l-o, (d) p-r, (e) s-v, (f) w-z, (g) aa-ac, and (h) 1..................113

78. Cyclic CGRs of specimen I-1. ..............................................................................................116

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

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

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

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

top.........................................................................................................................................120

83. Fracture surface at the end of CGR test of specimen I-1. Crack propagation from 

bottom to top. .......................................................................................................................121

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

85. Crack-length-vs.-time plot for specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite): 

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

xi
86. Cyclic CGRs of specimen J-1...............................................................................................127

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

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

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

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

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

92. Ferrite microstructure at the end of CGR test of specimen J-1. Crack propagation from 

bottom to top. .......................................................................................................................132

93. Cleavage-like fracture at the end of CGR test of specimen J-1. Crack propagation from 

bottom to top. .......................................................................................................................133

94. Fracture along ferrite at dendrite core in the JR test region of specimen J-1. Crack 

propagation from bottom to top............................................................................................134

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

and aged CF-8, and (c) unaged and aged CF-8M.................................................................137

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

97. Constant-load CGRs of the low-dose CASS with more than 23% ferrite in low-DO 

high-purity and PWR water environments...........................................................................140

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

conditions. Note that most of the results are from 1/4T-CT specimens tested at ~320°C 

in water environments. The unirradiated results for CF-8M CASS are from 1T-CT 

specimens tested at ~290°C in an air atmosphere. ...............................................................142

xii
LIST OF TABLES

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

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

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

4. Crack growth rates of specimen A-N1 (unirradiated and unaged CF-3 with 24% 

δ 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% 

δ ferrite) in PWR water. .........................................................................................................25

6. Crack growth rates of specimen A-2 (0.08-dpa unaged CF-3 with 24% δ ferrite) in 

low-DO high-purity water environment.................................................................................34

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

PWR water. ............................................................................................................................44

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

9. CGR test of specimen E-N1 (unirradiated, unaged CF-8 with 23% δ ferrite) in low-DO 

high-purity water. ...................................................................................................................67

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

11. Crack growth rates of specimen F-N1 (unirradiated, aged CF-8 with 23% δ ferrite) in a 

low-DO high-purity water environment.................................................................................89

12. Crack growth rates of specimen F-1 (0.08-dpa aged CF-8 with 23% δ ferrite) in a lowDO high-purity water environment. .....................................................................................101

13. Crack growth rates of specimen I-1 (0.08-dpa unaged CF-8M with 28% δ ferrite) in a 

low-DO high-purity water environment...............................................................................112

14. Crack growth rates of specimen J-1 (0.08-dpa aged CF-8M with 28% δ ferrite) in a 

low-DO high-purity water environment...............................................................................123

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

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

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 the 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 was 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 were available with regard to the 

combined effect of thermal aging and irradiation embrittlement. A test program was 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 either 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) response of the CASS specimens. In the cyclic CGR tests, 

environmental enhanced cracking was more difficult to establish in the CASS specimens when 

compared to 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.

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

xv
(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 embrittlement is 

evaluated for CASS components. 

xvi
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.

xvii

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

μ micron

xxi

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).

1 The CF grades of CASS whose compositions are similar to those of 300-series 

austenitic SSs are the most widely used corrosion-resistant CASS. The CF-3 and CF-8 grades 

contain nominal 19% Cr and 9% Ni and are 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,2 and, thus, are ideal for 

LWR applications in aqueous environments. The strength and ductility of CF grades are 

comparable to those of wrought SSs. At room temperature, the yield and tensile strengths of CF3 and CF-8 grades are greater than 200 MPa and 480 MPa,3 respectively, similar to those of 

solution-annealed SSs. A good combination of strength and ductility also gives rise to excellent

fracture toughness. Stable tearing occurs in CASS samples well above their yield strength before 

final fracture. Chopra and Sather 4 showed that the crack initiation toughness of CF-3 and CF-8

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.5

In contrast to the fully austenitic microstructure of wrought SSs, CASS consists of a ferriteaustenite duplex microstructure. It is common that a certain amount of δ ferrite is present along

with austenite (γ) 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.6,7 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 constitution 

diagrams have been established to estimate phase content.

8,9,10 The contributions of minor 

alloying elements are incorporated with Cr and Ni equivalent numbers and computed with 

empirical equations. The applicable range of composition, incorporated alloying elements, and 

weighing factors vary in these models, and thus the predicted phase contents differ among them 

to some extent.11 For steel castings of CF grades, use of the Scheofer diagram, which is a 

modified version of the Schaeffler diagram8, is recommended by the American Society for 

Testing and Materials (ASTM) for estimating ferrite content.12

The ferrite phase is critical for the mechanical properties and corrosion resistance of CASS and

weld metals. Since hardening mechanisms by thermal-mechanical treatments cannot be easily 

implemented in castings, the strength of CASS mainly relies on the ferrite-austenite duplex 

microstructure. Beck et al. 13 showed that the tensile and yield strengths of CASS increase with 

δ ferrite content up to 40% at both room and elevated temperatures. The strengthening effect of 

ferrite phase is attributed to ferrite-austenite boundaries and can be explained with the Hall-Petch 

model.14 Ferrite phase is also crucial for the soundness and weldability of steel castings. A 

minimum ferrite content is often specified for SS welds to reduce the tendency of hot cracking. 

In addition, the presence of ferrite phase can improve the resistance to sensitization and stress 

1
corrosion cracking (SCC).15 In susceptible environments, CASS tends 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.16 in boiling water reactor (BWR) environments. Using slow 

strain rate tests, they showed that CF-3, CF-3A, and CF-8 have an exceptional resistance to 

intergranular SCC (IGSCC) in high-purity (HP) water containing 6-8 ppm dissolved oxygen 

(DO). 

While the presence of δ 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.” 17,18 The consequences of the thermal aging

embrittlement are an increased tensile strength and reduced ductility.19,20 The upper-shelf 

impact energy is also reduced, and the ductile-to-brittle transition temperature shifts higher.20 

Because of thermal aging embrittlement, the long-term performance of CASS materials at 

elevated service temperatures is of concern. The degradation of CASS components resulting 

from thermal aging embrittlement has been recognized as a potential issue for aging reactors.21

Several research programs have been conducted to assess Charpy impact properties and JR 

resistance curves of thermally aged CASS.22,23 It was found that thermal aging at 290-450°C up 

to 30,000 hr leads to a significant deterioration in the fracture properties of CASS. The lower 

bound of impact energy and fracture toughness (JIC) can be as low as 20 J/cm2and 25 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 

embrittlement increases with ferrite content and is sensitive to ferrite morphology.22

The mechanism of thermal aging of duplex SSs has been studied extensively.17-20, 22-28 It is 

widely accepted that embrittlement is caused by the instability of the ferrite phase under thermal 

aging. The main reasons of the hardening and loss of toughness are (1) the formation of Cr-rich 

α’ phase through spinodal decomposition, and (2) the precipitation and growth of carbides and 

G-phase at ferrite-austenite phase boundaries. Obviously, these microstructural changes are 

thermally activated and are fundamentally controlled by solid-state diffusion. Therefore, the 

thermal aging time for a given extent of degradation (e.g., an increase in hardness or decrease in 

toughness) follows an Arrhenius-type relationship.24

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.29,30 

These point defects evolve at irradiation temperatures to form irradiation defects, giving rise to

irradiation hardening and embrittlement. The irradiation embrittlement can generate 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.30 The elevated diffusivity 

under neutron irradiation could certainly affect the kinetics of thermal aging. Thus, a combined 

2
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.5,31,32 Two tests conducted on thermally 

aged and irradiated CF-8M showed a higher degree of embrittlement in the CASS material than 

wrought SSs.31 It is not clear, however, if the simultaneous exposure to irradiation and thermal 

aging would reduce the fracture resistance to a lower level than either of the degradation 

mechanisms can impart alone. If so, the combined effect is not only important for internal 

components made of CASS, but also for SS weld metals that possess a similar austenite-ferrite 

duplex microstructure. While weld metals may contain less ferrite phase than that in CASS

materials, a minimum ferrite content is usually specified for weld metals to engineer against hot 

cracking. Thus, weld metals may be subjected to the same type of degradation as CASS. A 

better understanding of the combined effect of thermal aging and irradiation would also be 

helpful to address issues 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. 

3

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.4 The CASS heats are static cast slabs with dimensions

of 610 x 610 x 76 mm. A ferrite scope measurement on these heats showed that the CF-3 and 

CF-8 heats contained 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 μm 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.

4 For the aging 

conditions used in the current study (10,000 hr at 400°C), the decrease in ferrite content is

insignificant. Thus, the average ferrite contents were considered unchanged after thermal aging.

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

Cast 

Grade

Ferrite Content Heat 

ID

Composition (wt. %)

Measured a Calculated b 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. [4].

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

Table 2. Thermal aging conditions for the cast stainless steels in this study.

Cast Grade Ferrite a Spec. Code Heat ID Thermal Aging Condition

CF-3 24%

A 69 Unaged

B 69 10,000 hr at 400°C

CF-8 23%

E 68 Unaged

F 68 10,000 hr at 400°C

CF-8M 28%

I 75 Unaged

J 75 10,000 hr at 400°C

a. Measured with a ferrite scope, Ref. [4].

5
CF-3, 24% δ, unaged CF-3, 24% δ, aged

CF-8, 23% δ, unaged CF-8, 23% δ, aged

CF-8M, 28% δ, unaged (from ref. [4] ) CF-8M, 28% δ, aged

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

6
Constrained by irradiation space, sub-sized compact tension (CT) specimens were used in this 

study. The sample was about 6.5-mm thick (i.e., 1/4T-CT) and 14-mm high. The starter notch

size was about 6 mm. To ensure an in-plane crack growth, side grooves approximately 5% of 

the thickness were machined on both sides of the sample. Figure 2 is a schematic of the sample

used in this study. The red lines in the figure are electrical leads spot-welded on the specimen.

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

leads).

2.2 Irradiation

The CASS specimens were irradiated in a helium-filled capsule in the Halden reactor, a boiling 

heavy water reactor in Norway. Figure 3 shows the assembly of the irradiation capsule. Some

disk-shaped samples for transmission electron microscopy analyses were also included in a 

cylindrical container in the irradiation capsule (item 7 in Fig. 3). The irradiation temperature 

was ~315°C, and two sets of melting alloy temperature monitors (item 5) were installed in the 

irradiation assembly. 

During the irradiation experiment, three fluence monitor wires (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
 

Figure 3. Irradiation capsule of low-dose Halden irradiation.33

4. End Guide Sleeve8. Spacer

3. Heat Transfer Body7. TEM Specimens Container

2. Outer Capsule6. CT Specimens

1. End Plug5. Melting Alloy Assembly

8
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.

34 The 

simulated PWR water contained ~2 ppm lithium, ~1000 ppm boron, and ~2 ppm hydrogen. The 

conductivity was about 20 μS/cm. The low-DO high-purity water contained less than 10 ppb 

dissolved oxygen, and the conductivity was kept below 0.07 μS/cm during the tests. The test 

environments were provided by two water recirculation loops, and a schematic diagram of loop 

#1 is shown in Fig. 4. Each loop consists of a storage tank, a high pressure pump, a regenerative 

heat exchanger, an autoclave, an ECP cell, a back-pressure regulator, two ion-exchange 

cartridges, and several heaters. The high-pressure section of the loop extends from the highpressure 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. 

9
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 

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.

10
The CGR tests were carried out in either a simulated PWR water environment or a low-DO highpurity 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

material’s intrinsic behavior. 

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

1/2 3/2

(2 / ) ( ) ( ) (1 / ) NP aW a K f BB W aW W+ = ⋅⋅ ⋅⋅ − (1)

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 34

( ) 0.886 4.64 13.32 14.72 5.60 a aa aa f W WW WW

=+ − + −             (2)

Once a fatigue crack is initiated, a series of test steps is carried out with gradually increased rise 

times and load ratios. The measured CGRs in these test steps include the contributions from 

both mechanical fatigue and corrosion fatigue. With the change in loading conditions, the 

contribution of mechanical fatigue is gradually reduced while the environmental effect is 

enhanced. Figure 5 illustrates the principle of introducing environmentally assisted cracking in a 

test. The changes in test conditions should produce cyclic 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 lowcorrosion-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. 

11
Figure 5. Schematic for inducing environmentally assisted cracking in test environment.

2.4.2 Fracture Toughness J-R curve Test

After each SCC CGR test, a fracture toughness J-R curve test was conducted on the same sample 

in the test environment. This test was performed with a constant stain rate of 0.43 μm/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.

35 The J is the sum of the elastic and plastic components,

el pl JJ J = + (3)

At a hold point i corresponding to a crack length ai, as well as vi and Pi on the load vs. load-line 

displacement curve, the elastic component Jel(i) is given by: 

2 2

( )

( )

( ) (1 ) i

el i

K

J

E

−ν = (4) Crack Growth Rate in Water, Log Scale

Crack Growth Rate in Air, Log Scale

Environmental 

Effect

Mechanical 

Fatigue

Increase Load Ratio 

and Rise Time

12
where ν is Poisson’s ratio, and the stress intensity K(i) is calculated from Eqs. 1 and 2. The 

plastic component Jpl(i) is given by: 

( 1) ( ) ( 1) ( ) ( 1)

( ) ( 1) ( 1)

( 1) ( 1)

1 i pl i pl i i i

pl i pl i i

i N i

A A aa

J J

bB b

η

γ −− −

− −

− −

− − = + −

               

(5)

where b(i-1) is the remaining ligament at point i-1; Apl(i) is the area under the load vs. load-line 

displacement curve; and BN is the net specimen thickness. In addition, γ(i) and η(i) are factors that 

account for the crack growth effects on J during the test and are expressed as:

( 1)

( 1) 2.0 0.522 ii

b

W η −

− = + (6)

( 1)

( 1) 1.0 0.76 ii

b

W

γ −

− = + (7)

The quantity Apl(i) - Apl(i-1) is the increment of the plastic area under the load vs. load-line 

displacement curve between lines of constant plastic displacement at points i - 1 and i. The 

plastic area under the load vs. load-line displacement curve is given by 

[ 1 ( ) ( 1) ]

( ) ( 1) 2

i i pl i pl i

pl i pl i

PP v v

A A − −

−

+ −     = + (8)

where the plastic components of the load-line displacement, vpl(i), are:

pl i i i LL i () () () v v PC = − (9)

In the above, v(i) is the total load-line displacement, and CLL(i) is the compliance required to give 

the current crack length ai and can be determined as follows: 

2 3

( ) ' 2

1.62 17.80( / ) 4.88( / ) 1.27( / )

[1 ( / ) ]

ii i

LL i

e i

aW aW aW

C

EB a W

  +−+   = − (10)

where Be is the specimen effective thickness given by B - (B - BN)2/B, and =E/(1 - υ2). 

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 (4σf) is 

recommended by Mills 5 for materials with high strain hardening coefficients, and has been used 

in the previous thermal aging studies on unirradiated CASS.4 To be consistent with the previous 

analyses, the same blunting line (i.e., J/4σf) was also used in the current work. 

In this study, the estimated flow stresses are approximately 280-340 MPa and 420-520 MPa for 

unirradiated and irradiated CASS materials, respectively. 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. 

E′

13
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.

14
3. RESULTS

Seven irradiated and four unirradiated control specimens from three heats of CASS were tested 

in the current study. The test matrix is shown in Table 3. The specimens were unaged and 

thermally aged pairs. The irradiated samples received a displacement dose of ~0.08 dpa. The 

CF-3 specimens were tested in either low-DO high-purity water or PWR water, and all CF-8 and 

CF-8M specimens were tested in low-DO high-purity water. 

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

Sample 

ID

Dose 

(dpa)

Heat 

ID Materials Test Environment Facility CGR JR SEM

A-N1 - 69 CF-3, 24% δ, unaged Low-DO high-purity Cell 2 √ √ √

A-1 0.08 69 CF-3, 24% δ, unaged PWR Cell 1 √ - √

A-2 0.08 69 CF-3, 24% δ, unaged Low-DO high-purity Cell 2 √ √ √

B-N1 - 69 CF-3, 24% δ, aged PWR Cell 1 √ √ √

B-1 0.08 69 CF-3, 24% δ, aged PWR Cell 1 √ √ √

E-N1 - 68 CF-8, 23% δ, unaged Low-DO high-purity Cell 2 √ √ √

E-1 0.08 68 CF-8, 23% δ, unaged Low-DO high-purity Cell 2 √ √ √

F-N1 - 68 CF-8, 23% δ, aged Low-DO high-purity Cell 2 √ √ √

F-1 0.08 68 CF-8, 23% δ, aged Low-DO high-purity Cell 2 √ √ √

I-1 0.08 75 CF-8M, 28% δ, unaged Low-DO high-purity Cell 2 √ √ √

J-1 0.08 75 CF-8M, 28% δ, 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-μm 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 Jones37 with the load ratios and rise times given in Table 4. A corrosion fatigue curve 

proposed by Shack and Kassner38 for unirradiated SSs in 0.2-ppm DO water at 290°C is also 

included in the figure as a reference. Note that the data point above the reference line (from test 

period z) was due to a temperature 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 lowDO high-purity water. 

After a total of ~900-μm crack extension, the test was set to a constant load with periodical 

partial unloading (PPU) every 2 hr. The SCC CGR was negligibly low and was below the 

detection limit of the DCPD measurement. The SCC behavior of this sample will be discussed at 

the end of this section along with the other CGR data obtained from the same heat.

Table 4. Crack growth rates of specimen A-N1 (unirradiated and unaged CF-3 with 24% 

δ ferrite) in low-DO high-purity water. 

Test

Test 

Time,

Test

Temp.,

Load 

Ratio

Rise 

Time,

Return 

Time,

Hold 

Time, Kmax, ∆K,

CGR in 

Env.,

CGR 

in Air,

Crack 

Length,

Period h °C s s s MPa m1/2 MPa m1/2 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

t a 332.2 318 0.44 68.5 8.22 31.5 17.7 9.8 5.06E-10 1.81E-10 6.795

U 355.8 319 0.49 133.1 7.99 66.9 17.6 9.0 6.71E-11 7.25E-11 6.799

V 363.7 319 0.45 13.6 3.41 6.4 17.6 9.8 1.36E-09 8.90E-10 6.814

W 379.4 319 0.47 33.7 8.09 16.3 17.8 9.5 6.68E-10 3.33E-10 6.837

X 403.6 318 0.48 66.4 7.97 33.6 17.4 9.1 1.62E-10 1.48E-10 6.846

Y 430.8 318 0.49 133.2 8.00 66.8 17.8 9.2 8.67E-11 7.69E-11 6.851

z b 452.8 317 0.49 333.8 8.01 166.2 18.1 9.3 3.74E-10 3.19E-11 6.872

aa 500.3 319 0.48 669.5 8.03 330.5 18.1 9.4 3.65E-11 1.64E-11 6.876

1 811.3 318 0.50 12 12 7200 18.0 9.0 negligible 1.37E-12 6.880

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

b The autoclave temperature was unstable in the test period. 

16
(a) 

(b) 

(c) 

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

24% ferrite): test periods (a) a-c, (b) d-j, (c) k-p, (d) q-u, (e) v-aa, and (f) 1.

6.00

6.10

6.20

6.30

6.40

4

8

12

16

20

24

28

32

0 5 10 15 20

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. A-N1, CF-3, 24% δ, unaged

Low-DO high-purity water, ~318 oC

a, 

R=0.1

1 Hz

b, 

R=0.2

1 Hz

c, 

R=0.4

0.5 Hz

Hold at low K

6.20

6.30

6.40

6.50

6.60

6.70

6.80

4

8

12

16

20

24

28

32

20 30 40 50 60 70 80

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. A-N1, CF-3, 24% δ, unaged

Low-DO high-purity water, ~318 oC

d, 

R=0.2

1 Hz

e, 

R=0.3

1 Hz

f, 

R=0.45

0.5 Hz

g, 

R=0.6

0.1 Hz

h, 

R=0.5

0.2 Hz

i, 

R=0.5

10s/up, 2s/down

j, 

R=0.5

15s/up, 5s/down

6.65

6.70

6.75

6.80

4

8

12

16

20

24

28

32

100 150 200

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. A-N1, CF-3, 24% δ, unaged

Low-DO high-purity water, ~318 oC

k, 

R=0.5

30s/up, 

5s/down

l, 

R=0.5

60s/up, 

12s/down

m, 

R=0.5

150s/up, 

12s/down

n, 

R=0.5

300s/up, 

12s/down

o, 

R=0.5

500s/up, 

12s/down

p, 

R=0.5

1000s/up, 

12s/down

17
(d) 

(e) 

(f) 

Figure 6. (Contd.)

6.70

6.75

6.80

6.85

4

8

12

16

20

24

28

32

240 260 280 300 320 340

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. A-N1, CF-3, 24% δ, unaged

Low-DO high-purity water, ~318 oC

q, 

R=0.5

200s/up, 

12s/down

r, 

R=0.5

100s/up, 

12s/down

s, 

R=0.45

50s/up, 

12s/down

t, 

R=0.45

100s/up, 

12s/down

Pressure unstable 

caused by cooling 

water problem.

u, 

R=0.5

200s/up, 

12s/down

5.1E-10 m/s

6.75

6.80

6.85

6.90

6.95

4

8

12

16

20

24

28

32

360 380 400 420 440 460 480 500

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. A-N1, CF-3, 24% δ, unaged

Low-DO high-purity water, ~318 oC

v, 

R=0.45

20s/up, 

5s/down

w, 

R=0.45

50s/up, 

12s/down

x, 

R=0.5

100s/up, 

12s/down

y, 

R=0.5

200s/up, 

12s/down

z, 

R=0.5

500s/up, 

12s/down

aa, 

R=0.5

1000s/up, 

12s/down

6.800

6.820

6.840

6.860

6.880

6.900

6.920

4

8

12

16

20

24

28

32

500 550 600 650 700 750 800 850

Crack Length (mm)

K (MPa m0.5)

Time (h)

Spec. A-N1, CF-3, 24% δ, unaged

Low-DO high-purity water, ~318 oC

1, 

PPU, 2 hr hold

18
Figure 7. Cyclic CGRs of specimen A-N1.

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 μm/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 powerlaw 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. 

CF curve for 0.2 ppm DO 

by Shack & Kassner

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-c

Test periods: d-p

Test periods: q-u

Test periods: v-aa

CGRenv (m/s)

CGRair (m/s)

Specimen A-N1 

CF-3, unaged, unirradiated

Low-DO high-purity water

~318oC

19
Figure 8. The J-R curve for specimen A-N1.

Fractographic examination

After the J-R curve test, the sample was fatigued and broken open at room temperature in an air 

atmosphere. The fracture surface was examined with SEM (Fig. 9). While the CGR test region 

shows a transgranular (TG) morphology, the 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. 

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen A-N1

CF-3, unaged, unirradiated

Low-DO high-purity water, ~318oC

J= 536.5*∆a0.68

JQ=320 kJ/m2 Jmax

∆amax

20
Figure 9. Fracture surface of specimen A-N1 tested in low-DO high-purity water.

21
Figure 10. Fracture surface of specimen A-N1 along the sample central line.

Crack advance

Dimple 

fracture

Machined

notch

Secondary 

crack

TG 

22
Figure 11. Transgranular fracture in the precracking region of specimen A-N1.

23
Figure 12. Ductile dimple fracture in JR test region of specimen A-N1.

24
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 largerthan-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% δ ferrite)

in PWR water.a 

Test

Test 

Time,

Test

Temp.,

Load 

Ratio

Rise 

Time,

Return 

Time,

Hold 

Time, Kmax, ∆K,

CGR in 

Env.,

CGR 

in Air,

Crack 

Length,

Period h °C s S s MPa m1/2 MPa m1/2 m/s m/s mm

Start 63.9 6.802 b

a 64.8 319 0.21 0.43 0.43 0.07 24.3 19.2 1.58E-07 2.04E-07 7.009

b 66 319 0.31 0.41 0.41 0.09 23.8 16.4 1.17E-07 1.43E-07 7.218

c 68.6 319 0.41 0.78 0.78 0.22 23.1 13.6 3.21E-08 4.47E-08 7.340

d 72.3 319 0.41 1.54 1.54 0.46 21.9 12.8 9.77E-09 1.89E-08 7.395

e 87.6 319 0.46 2.96 1.48 1.04 20.3 11.0 2.89E-10 6.18E-09 7.403

f 91 319 0.41 3.08 1.54 0.92 22.2 13.1 9.46E-09 1.01E-08 7.462

g 94.1 320 0.41 7.69 1.54 2.31 22.4 13.2 5.67E-09 4.17E-09 7.503

h 103 320 0.46 22.6 3.76 7.42 22.6 12.2 1.51E-09 1.16E-09 7.535

i 114.3 320 0.46 45.1 3.76 14.9 22.7 12.3 7.63E-10 5.86E-10 7.558

j 118.7 319 0.41 15.3 3.83 4.66 23.0 13.6 4.61E-09 2.27E-09 7.602

k 127.9 319 0.41 46.0 3.83 14.0 23.0 13.7 1.25E-09 7.72E-10 7.632

l 150.3 318 0.40 114.9 9.19 35.1 23.0 13.7 3.90E-10 3.13E-10 7.655

m 168.7 319 0.40 385.6 9.25 114.4 23.9 14.3 3.56E-10 1.05E-10 7.673

1 209 318 0.40 12 12 7200 23.9 14.4 4.83E-11 5.75E-12 7.684

a Simulated PWR water with 2 ppm Li and 1000 ppm B. DO<10 ppb. Conductivity ~20 μS/cm. 

b Determined from the SEM image after the test.

The specimen was pre-cracked in the PWR environment with a triangular waveform at 1 Hz, a 

maximum stress intensity factor of ~24 MPa m1/2, and load ratio of 0.2. After about 200 μm 

extension, the load ratio was 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 hours. The measured SCC CGR was about 4.8x10-11 m/s, a 

factor of four lower than that of the NUREG-0313 curve,

36 a SCC disposition curve based on 

unirradiated SSs tested in a high-DO environment. The SCC response of this sample will be 

discussed at the end of this section along with the other CGR data obtained from the same heat. 

(a) 

(b) 

Figure 13. Crack-length-vs.-time plot for specimen A-1 (0.08-dpa unaged CF-3 with 24% 

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

6.80

6.90

7.00

7.10

7.20

7.30

7.40

7.50

7.60

4

8

12

16

20

24

28

65 70 75 80 85

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen A-1

CF-3, unaged, 0.08 dpa

PWR water, 320oC

a

b

c

d

e

Kmax

Crack length

7.35

7.40

7.45

7.50

7.55

7.60

7.65

4

8

12

16

20

24

28

85 90 95 100 105 110 115 120

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen A-1

CF-3, unaged, 0.08 dpa

PWR water, 320oC

f

e

g

h

i

K j max

Crack length

26
(c) 

(d) 

Figure 13. (Contd.)

7.55

7.60

7.65

7.70

7.75

4

8

12

16

20

24

28

120 130 140 150 160 170

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen A-1

CF-3, unaged, 0.08 dpa

PWR water, 320oC

k

l

m

Kmax

Crack length

7.66

7.67

7.68

7.69

7.70

7.71

7.72

4

8

12

16

20

24

28

170 180 190 200 210 220 230 240 250

Crack Length (mm)

K (MPa m0.5)

Time (h)

Specimen A-1

CF-3, unaged, 0.08 dpa

PWR water, 320oC

1

K

Crack length

27
Figure 14. Cyclic CGRs of specimen A-1.

Fractographic examination

After the test, the specimen was pulled apart in an air atmosphere at room temperature, and the 

fracture surface was examined with SEM. Figure 15 shows the entire crack front of specimen A1. 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 cleavagelike 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.

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CGRenv (m/s)

CGRair (m/s)

Specimen A-1

CF-3, unaged, 0.08 dpa

PWR water, 320oC

Test period with <10 μm 

extension are excluded.

CF curve for 0.2 ppm DO 

by Shack & Kassner

28
 

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

29
Figure 16. Fracture surface of specimen A-1 along the sample central line.

Machined

notch

Restart of 

pre-cracking

Vermicular 

ferrite at 

dendrite core

Dimple 

fracture

Crack advance

30
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.

a

31
Figure 17. (Contd.)

b

32
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.

33
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 highpurity 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% δ ferrite) in lowDO high-purity water environment.

Test

Test

Time,

Test

Temp.,

Load

Ratio

Rise

Time,

Return

Time,

Hold

Time, Kmax, ∆K,

CGR in

Env.,

CGR 

in Air,

Crack 

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 14.9 5.899

aa 18.1 319 0.34 0.45 0.45 0.05 17.3 11.4 4.09E-08 4.04E-08 6.041

b 20 319 0.41 0.45 0.45 0.05 17.1 10.1 3.21E-08 2.96E-08 6.141

c 21.6 319 0.50 0.43 0.43 0.07 15.7 7.8 1.94E-09 1.43E-08 6.151

da 24.3 319 0.45 0.44 0.44 0.06 16.5 9.1 2.27E-08 2.16E-08 6.219

e 30.4 319 0.45 0.88 0.88 0.12 16.0 8.8 5.26E-09 9.85E-09 6.274

f 39.3 319 0.50 1.74 1.74 0.26 15.9 8.0 1.80E-09 3.82E-09 6.304

g 40.4 319 0.40 0.45 0.45 0.05 17.2 10.4 5.87E-08 3.21E-08 6.410

h 42.4 319 0.45 0.44 0.44 0.06 16.3 9.0 1.25E-08 2.13E-08 6.453

i 44.6 319 0.45 0.88 0.88 0.12 16.6 9.2 1.29E-08 1.14E-08 6.497

j 49.0 319 0.45 1.75 1.75 0.25 16.8 9.2 9.20E-09 5.84E-09 6.564

k 62.2 319 0.45 4.38 4.38 0.62 17.0 9.4 3.07E-09 2.45E-09 6.627

l 74.2 319 0.46 8.73 4.37 1.27 17.2 9.4 2.37E-09 1.23E-09 6.686

m 88.4 319 0.45 26.2 4.37 3.79 17.2 9.5 7.77E-10 4.25E-10 6.719

n 110.6 319 0.44 52.5 10.5 7.52 17.2 9.7 6.45E-10 2.24E-10 6.756

o 135.7 319 0.43 105.0 10.5 15.0 17.0 9.7 3.77E-10 1.13E-10 6.780

p 184.3 319 0.43 262.6 10.5 37.4 17.3 9.8 8.72E-11 4.69E-11 6.795

q 232.7 319 0.43 525.9 10.5 74.1 17.6 10.0 1.29E-10 2.49E-11 6.815

r 278.6 320 0.43 876.3 10.5 123.7 17.6 10.1 1.73E-10 1.52E-11 6.840

s 326.7 319 0.48 865.9 10.4 134.1 17.6 9.2 1.27E-10 1.21E-11 6.860

1 423.9 319 0.50 12 12 7200 17.6 8.8 2.33E-11 1.27E-12 6.875

2-a 575.3 320 0.50 12 12 7200 19.6 9.8 4.89E-11 1.80E-12 6.907

2-b 687.3 321 1 - - - 19.8 - 4.94E-12 - 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-μm crack extension. 

Next, the rise time was increased gradually while the maximum stress intensity factor and load 

ratio were kept approximately the same. In the following test periods, environmentally enhanced 

cracking started to appear and became evident with further increases in rise time and load ratio. 

By the end of test period s, the measured CGR was more than one order of magnitude higher 

than the fatigue growth rate in air. All cyclic CGRs obtained from this sample are plotted in 

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) 

(b) 

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

ferrite) tested in low-DO high-purity water environment: test periods (a) a-e, (b) f-j, 

(c) k-o, (d) p-s, and (e) 1-2. 

5.90

6.00

6.10

6.20

6.30

4

8

12

16

20

24

28

32

14 16 18 20 22 24 26 28 30

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen A-2

CF-3, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

a

b

c

4.1E-8 m/s

d

e

2.3E-8 m/s

Kmax

Crack length

6.20

6.25

6.30

6.35

6.40

6.45

6.50

6.55

6.60

4

8

12

16

20

24

28

32

30 35 40 45 50

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen A-2

CF-3, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

f

g

h

i

j

Kmax

Crack length

35
(c) 

(d) 

(e) 

Figure 19. (Contd.)

6.50

6.55

6.60

6.65

6.70

6.75

6.80

6.85

6.90

4

8

12

16

20

24

28

32

60 80 100 120 140

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen A-2

CF-3, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

k

l

m

n

o

Kmax

Crack length

6.70

6.75

6.80

6.85

6.90

6.95

7.00

4

8

12

16

20

24

28

32

160 200 240 280 320

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen A-2

CF-3, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

p

q r

s

Kmax

Crack length

6.86

6.88

6.90

6.92

6.94

4

8

12

16

20

24

28

32

400 500 600 700 800

Crack Length (mm)

K (MPa m0.5)

Time (h)

Specimen A-2

CF-3, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

1

2-a

2-b

2-c

K

Crack length

36
Figure 20. Cyclic CGRs of specimen A-2.

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-μm 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-μm crack extension was accumulated for this test period. In test 

period 2-c, the PPU was re-introduced but with 1-hour interval. The measured CGR was nearly 

identical to that 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 μm/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 = 430∆a0.64. 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. 

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-f

Test periods: g-s

CGRenv (m/s)

CGRair (m/s)

Specimen A-2

CF-3, unaged, 0.08 dpa.

Low-DO high-purity water

320oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

37
Figure 21. The J-R curve for specimen A-2.

Fractographic examination

After the J-R curve test, the sample was cyclically loaded at room temperature in an air 

atmosphere to break the ligament. The fracture surface was then examined with the replication 

technique using SEM. Figure 22 shows the entire fracture surface of specimen A-2. Note that 

the round smooth areas on the SEM image are 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 24 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. 25). 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. 26.

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen A-2

CF-3, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

J= 430*∆a0.64

JQ=204 kJ/m2

Jmax

∆amax

38
Figure 22. Fracture surface of specimen A-2 tested in low-DO high-purity water.

39
Figure 23. Fracture surface of specimen A-2 along the sample central line.

CGR test

TG 

Vermicular 

ferrite in 

dendrite 

cores

Dimple 

fracture

Crack advance

JR test

Post-JR 

fatigue

Machined

notch

40
Figure 24. Cleavage-like steps at the beginning of CGR test of specimen A-2. Crack propagation from bottom to top.

41
Figure 25. Fracture surface of specimen A-2 showing that ferrite deformed to a lesser extent than austenite. Crack propagation 

from bottom to top.

42
Figure 26. Ductile dimple morphology in the JR test region of specimen A-2. Crack propagation from bottom to top.

43
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% δ ferrite) in 

PWR water.a

Test

Test 

Time,

Test

Temp.,

Load

Ratio

Rise 

Time,

Return 

Time,

Hold 

Time, Kmax, ∆K,

CGR in 

Env.,

CGR 

in Air,

Crack 

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 0.3 5.997

a b 3.0 319 0.20 0.43 0.43 0.07 14.8 11.9 5.57E-08 4.04E-08 6.182

b b 7.2 319 0.35 0.42 0.42 0.08 15.0 9.8 3.25E-08 2.67E-08 6.353

c b 13.3 319 0.49 0.40 0.40 0.10 15.1 7.7 2.19E-08 1.45E-08 6.474

d 23 319 0.54 0.77 0.77 0.23 15.0 6.9 4.85E-09 5.50E-09 6.538

e 47 319 0.54 1.92 1.92 0.58 15.0 6.9 1.63E-09 2.24E-09 6.597

f 71.2 319 0.54 3.82 1.53 1.18 15.0 6.8 1.53E-10 1.07E-09 6.608

g b 76.9 319 0.54 0.77 0.77 0.23 15.3 7.0 4.78E-09 5.77E-09 6.641

h 101.8 319 0.55 2.29 0.76 0.71 15.1 6.8 1.64E-10 1.79E-09 6.649

i 123.9 319 0.54 3.83 3.83 1.17 15.3 7.0 7.97E-11 1.14E-09 6.652

j 143.8 319 0.52 1.93 1.93 0.57 15.3 7.3 1.76E-09 2.64E-09 6.690

k c 152.8 320 0.53 3.85 1.54 1.15 15.2 7.2 5.86E-10 1.25E-09 6.701

l 192.1 320 0.52 7.72 1.54 2.28 15.3 7.3 1.77E-10 6.60E-10 6.714

m 200.2 319 0.49 1.96 1.96 0.54 15.6 7.9 2.31E-09 3.22E-09 6.744

n 215 320 0.49 3.91 3.91 1.09 15.7 7.9 2.54E-09 1.66E-09 6.794

o 224 320 0.49 7.81 1.56 2.19 15.7 7.9 8.81E-10 8.30E-10 6.813

p 241.5 319 0.49 15.6 3.91 4.37 15.8 8.0 7.40E-10 4.27E-10 6.841

q 270.9 319 0.49 23.4 3.91 6.56 15.8 8.0 1.21E-10 2.87E-10 6.850

r 289.5 319 0.49 46.9 3.91 13.1 15.7 8.1 3.27E-11 1.45E-10 6.852

s 311.3 319 0.49 11.7 3.90 3.30 15.7 8.0 7.10E-11 5.65E-10 6.855

t 316.3 320 0.32 12.4 4.14 2.59 16.5 11.1 9.66E-09 1.33E-09 6.957

u 320 319 0.35 24.6 4.10 5.37 16.5 10.7 2.99E-09 6.14E-10 6.983

v 334.4 319 0.37 48.9 4.08 11.1 16.8 10.5 1.35E-09 2.98E-10 7.037

w 347.5 319 0.42 96.3 9.63 23.7 16.8 9.7 4.65E-10 1.23E-10 7.054

x 368.1 320 0.42 241.0 9.64 59.0 16.9 9.8 2.09E-10 5.02E-11 7.065

y 383.7 319 0.41 401.9 9.65 98.1 16.8 9.9 1.26E-10 3.05E-11 7.071

z 408.5 319 0.42 804.3 9.65 195.7 17.1 9.9 1.43E-10 1.57E-11 7.083

aa 436.2 319 0.42 1206.2 9.65 293.8 17.2 10.0 7.73E-11 1.05E-11 7.090

1a 512.5 319 0.50 12 12 7200 17.2 8.6 2.72E-11 1.17E-12 7.106

1b 647.5 319 1.00 - - - 17.1 - 2.29E-13 - 7.107

a Simulated PWR water with 2 ppm Li and 1000 ppm B. Conductivity ~20 μS/cm. 

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

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

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-μm crack extension, the load ratio was increased in the 

next two test periods. The observed CGRs decreased as expected with the reduced contribution 

of mechanical fatigue (as shown in Fig. 28). However, the crack stalled in test period f after the 

triangle waveform was replaced with a saw-tooth waveform. After the crack was re-activated, 

several attempts were made to induce environmentally assisted cracking. The environmental

effect became evident after a CGR of ~1.4 x 10-9 m/s was established in test period v. The load 

ratio and rise time were then increased simultaneously to stabilize the environmentally enhanced 

cracking. By the end of the cyclic loading test, the observed CGR in water was about a factor of 

seven higher than the fatigue growth rate in air. All cyclic CGRs obtained from this sample are 

plotted in Fig. 28 along with the corrosion fatigue curve for SSs in high-purity water with 0.2 

ppm DO. 

(a) 

(b) 

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

ferrite) in PWR water: test periods (a) a-d, (b) e-h, (c) i-n, (d) o-s, (e) t-w, (f) x-aa, 

and (g) 1a-1b.

6.00

6.08

6.16

6.24

6.32

6.40

6.48

6.56

4

8

12

16

20

24

28

32

0 5 10 15 20

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. B-N1, CF-3, 24% δ, aged

Low-DO high-purity water, ~319 oC

a, 

R=0.2

1 Hz

b, 

R=0.35

1 Hz

c, 

R=0.5

1 Hz

d, 

R=0.55

0.5 Hz

6.45

6.50

6.55

6.60

6.65

6.70

6.75

6.80

4

8

12

16

20

24

28

32

30 40 50 60 70 80 90 100

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. B-N1, CF-3, 24% δ, aged

Low-DO high-purity water, ~319 oC

e, 

R=0.55

0.2 Hz

f, 

R=0.55

5s/up, 2s/down

g, 

R=0.55

0.5 Hz

h, 

R=0.55

3s/up, 1s/down

45
(c) 

(d) 

(e) 

Figure 27. (Contd.)

6.60

6.65

6.70

6.75

6.80

6.85

4

8

12

16

20

24

28

32

120 140 160 180 200

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. B-N1, CF-3, 24% δ, aged

Low-DO high-purity water, ~319 oC

i, 

R=0.52

0.1 Hz

j, 

R=0.53

0.2 Hz

k, 

R=0.52

5s up, 2s down

l, 

R=0.52

10s up, 2s down

m, 

R=0.5

0.2 Hz

n, 

R=0.5

0.1 Hz

6.70

6.75

6.80

6.85

6.90

6.95

7.00

4

8

12

16

20

24

28

32

220 240 260 280 300

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. B-N1, CF-3, 24% δ, aged

Low-DO high-purity water, ~319 oC

o, 

R=0.5

10s up, 

2s down

p, 

R=0.5

20s up, 

5s down

q, 

R=0.5

30s up, 

5s down

r, 

R=0.5

60s up, 

5s down

s, 

R=0.5

15s up, 

5s down

6.80

6.85

6.90

6.95

7.00

7.05

7.10

7.15

4

8

12

16

20

24

28

32

315 320 325 330 335 340 345

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. B-N1, CF-3, 24% δ, aged

Low-DO high-purity water, ~319 oC

t, 

R=0.32

15s up, 

5s down

u, 

R=0.35

30s up, 

5s down

v, 

R=0.37

60s up, 12s down

w, 

R=0.42

120s up, 12s down

Check

compliance

46
(f) 

(g) 

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-μm crack extension. Without PPU, the 

observed CGR dropped more than two orders of magnitude under a near constant-K condition. 

Limited by the test time, the CGR test was terminated after 135 hr with just 1-μm crack 

extension. 

Fracture toughness J-R curve test

The fracture toughness J-R curve test was performed in the same PWR water environment. The 

test was conducted at a constant strain rate of 0.43 μm/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 

7.00

7.05

7.10

7.15

4

8

12

16

20

24

28

32

350 360 370 380 390 400 410 420 430

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. B-N1, CF-3, 24% δ, aged

Low-DO high-purity water, ~319 oC

x, 

R=0.4

300s up, 12s down

y, 

R=0.4

500s up, 12s down

z, 

R=0.4

1000s up, 12s down

aa, 

R=0.4

1500s up, 12s down

7.06

7.08

7.10

7.12

7.14

4

8

12

16

20

24

28

32

450 500 550 600 650

Crack Length (mm)

K (MPa m0.5)

Time (h)

Spec. B-N1, CF-3, 24% δ, aged

Low-DO high-purity water, ~319 oC

1a,

UUP, 2 hr hold

1b,

Constant

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. 

Figure 28. Cyclic CGRs of specimen B-N1.

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

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-f

Test periods: g-i

Test periods: j-l

Test periods: m-s

Test periods: t-aa

CGRenv (m/s)

CGRair (m/s)

Specimen B-N1 

CF-3, aged, unirradiated

PWR water

~319oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen B-N1

CF-3, aged, unirradiated

PWR water, ~319oC

J= 352.6*∆a0.66

JQ=170 kJ/m2

Jmax ∆amax

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

50
Figure 31. Fracture surface of specimen B-N1 along the sample central line. CGR test

Fatigue 

fracture

TG

Dimple 

fracture

Crack advance

JR test

Post-JR 

fatigue

Machined

notch

51
Figure 32. Transgranular fracture in specimen B-N1 at the beginning of the precracking. Crack propagation from bottom to top.

52
Figure 33. Transgranular fracture in specimen B-N1 at the end of the precracking. Crack propagation from bottom to top.

δ 

53
Figure 34. Ductile dimple fracture in the JR test region of specimen B-N1. Crack propagation from bottom to top.

54
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 reactivate 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% δ ferrite) in PWR water.

a

Test

Test 

Time,

Test

Temp.,

Load

Ratio

Rise 

Time,

Return 

Time,

Hold 

Time, Kmax, ∆K,

CGR in 

Env.,

CGR 

in Air,

Crack 

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 0.52 6.019

a 3.0 319 0.27 0.38 0.38 0.12 17.6 12.8 7.88E-08 6.55E-08 6.250

b 6.7 320 0.39 0.35 0.35 0.15 17.0 10.4 4.11E-08 4.06E-08 6.442

c 10.5 320 0.50 0.64 0.64 0.36 15.8 7.9 2.75E-09 9.97E-09 6.460

d 20.1 320 0.45 1.32 1.32 0.68 15.9 8.8 3.63E-10 6.50E-09 6.466

e 23.3 320 0.27 0.37 0.37 0.13 17.9 13.1 5.89E-08 7.29E-08 6.698

f 25.4 320 0.27 0.36 0.36 0.14 17.1 12.4 4.86E-08 6.25E-08 6.822

g 27.8 320 0.38 0.34 0.34 0.16 16.8 10.4 2.77E-08 4.22E-08 6.905

h 32.2 320 0.38 0.66 0.66 0.34 16.3 10.1 8.33E-09 1.93E-08 6.950

i 44.2 319 0.44 3.20 3.20 1.80 16.2 9.2 2.63E-10 3.07E-09 6.955

j 47.5 320 0.38 1.32 1.32 0.68 16.2 10.0 7.00E-10 9.49E-09 6.958

k b 52.3 320 0.37 0.67 0.67 0.33 17.1 10.8 1.01E-08 2.32E-08 7.000

l 59.1 320 0.37 3.36 1.34 1.64 17.2 10.9 4.05E-09 4.80E-09 7.048

m 69 320 0.37 6.71 1.34 3.29 17.4 11.0 2.24E-09 2.48E-09 7.092

n 82.8 320 0.42 13.0 3.25 6.99 17.3 10.1 3.18E-10 1.01E-09 7.102

o 95.2 320 0.36 13.4 3.35 6.60 17.3 11.0 4.09E-11 1.23E-09 7.104

p 105 320 0.36 6.73 3.37 3.27 17.5 11.2 1.01E-10 2.59E-09 7.107

q 116.8 320 0.36 13.6 3.40 6.41 17.8 11.5 9.16E-11 1.40E-09 7.107

r 124.2 320 0.36 1.36 1.36 0.64 18.0 11.5 3.85E-09 1.43E-08 7.142

s 130.6 320 0.36 3.39 1.36 1.61 18.1 11.6 2.66E-09 5.86E-09 7.171

t 142.5 320 0.36 6.77 1.35 3.23 18.2 11.7 1.21E-09 3.00E-09 7.201

u 145.9 319 0.35 13.5 3.38 6.46 18.1 11.7 9.45E-11 1.49E-09 7.202

v 164.7 319 0.35 13.6 3.40 6.40 18.3 11.9 1.23E-10 1.56E-09 7.207

w 170 319 0.35 10.2 3.40 4.80 18.4 11.9 1.53E-10 2.09E-09 7.208

x 188.7 319 0.35 10.2 3.42 4.75 18.9 12.3 1.39E-09 2.28E-09 7.251

y 199 319 0.35 20.5 3.42 9.51 18.9 12.3 7.57E-10 1.15E-09 7.265

z 238 319 0.35 41.0 3.41 19.0 19.1 12.3 2.14E-10 5.88E-10 7.287

aa 284.5 319 0.36 81.8 8.18 38.2 19.3 12.4 2.61E-10 3.00E-10 7.318

ab 356.3 319 0.35 204.4 8.18 95.6 19.2 12.4 4.36E-11 1.20E-10 7.325

ac 442.2 318 0.35 413.0 8.26 187.0 19.9 12.9 2.88E-11 6.65E-11 7.331

ad 501.5 319 0.35 693.0 8.32 307.0 20.2 13.2 1.41E-11 4.26E-11 7.334

ae 527.8 318 0.46 78.0 7.80 42.0 19.9 10.8 5.94E-12 2.23E-10 7.336

af 574.5 318 0.45 39.4 7.88 20.6 20.6 11.3 2.11E-10 4.98E-10 7.356

ag 644.4 318 0.45 78.8 7.88 41.2 20.6 11.2 negligible 2.48E-10 7.355

ah 648 318 0.29 3.97 3.97 1.03 22.5 15.9 2.93E-08 1.30E-08 7.477

ai 650 318 0.34 11.7 3.89 3.32 22.3 14.7 7.43E-09 3.61E-09 7.509

aj 653.2 318 0.41 22.6 3.76 7.43 21.5 12.8 2.97E-09 1.27E-09 7.532

ak 671.3 318 0.39 45.8 9.16 14.2 22.6 13.8 1.02E-09 7.91E-10 7.577

al 693.8 318 0.39 91.5 9.15 28.5 22.8 14.0 7.29E-10 4.09E-10 7.618

am 717 318 0.46 194.0 7.76 106.0 22.1 11.9 4.90E-10 1.22E-10 7.644

an 747.7 319 0.46 387.7 7.75 212.3 22.0 11.9 1.66E-10 6.16E-11 7.655

ao 788.7 319 0.46 645.4 7.74 354.6 22.1 11.9 1.20E-10 3.71E-11 7.667

1 837.5 318 0.40 12 12 7200 22.1 13.3 2.81E-11 4.42E-12 7.675

a Simulated PWR water with 2 ppm Li and 1000 ppm B. Conductivity ~20 μS/cm. 

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

56
(a) 

(b) 

(c) 

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

in PWR water: test periods (a) a-e, (b) f-k, (c) l-u, (d) v-y, (e) z-ac, (f) ad-ag, (g) ahal, and (h) am-1.

6.00

6.10

6.20

6.30

6.40

6.50

6.60

6.70

4

8

12

16

20

24

28

0 4 8 12 16 20 24

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

a

b

c d

e

Compliance

test Crack length

Kmax

6.70

6.75

6.80

6.85

6.90

6.95

7.00

7.05

7.10

4

8

12

16

20

24

28

24 28 32 36 40 44 48 52

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

f

h

g

i j

k

Crack length

Kmax

6.95

7.00

7.05

7.10

7.15

7.20

7.25

4

8

12

16

20

24

28

60 80 100 120 140

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

l

m

n o p q r

s

t

u

Crack length

Kmax

57
(d) 

(e) 

(f) 

Figure 35. (Contd.)

7.15

7.20

7.25

7.30

4

8

12

16

20

24

28

150 160 170 180 190 200

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

v w

x

y

Crack length

Kmax

7.26

7.28

7.30

7.32

7.34

7.36

7.38

4

8

12

16

20

24

28

200 250 300 350 400 450

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

z

aa

ab ac

Crack length

Kmax

7.30

7.32

7.34

7.36

7.38

7.40

4

8

12

16

20

24

28

480 520 560 600 640

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

af

ag

ad ae

Crack length

Kmax

58
(g) 

(h) 

Figure 35. (Contd.)

The cyclic CGRs with a significant crack extension (defined as >10 μm) from this sample are 

plotted in Fig. 36 along with the corrosion fatigue curve for unirradiated SSs. No elevated 

corrosion fatigue response can be seen for this material despite its thermal aging condition. 

Comparing Figs. 14 and 36, we 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 

7.35

7.40

7.45

7.50

7.55

7.60

7.65

4

8

12

16

20

24

28

650 660 670 680 690

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

ah

ai

aj

ak

al

Crack length

Kmax

7.60

7.62

7.64

7.66

7.68

7.70

7.72

4

8

12

16

20

24

28

700 750 800 850

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen B-1

CF-3, aged 10,000 hrs @ 400°C, 0.08 dpa

PWR water, 320oC

am

an

ao

1

Crack length

Kmax

59
CGR than that of unaged CF-3. However, given the large scatter of the SCC CGR data, this

difference is considered insignificant. 

Figure 36. Cyclic CGRs of specimen B-1.

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

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-d

Test periods: e-j

Test periods: k-q

Test periods: r-af

Test periods: ah-ao

CGRenv (m/s)

CGRair (m/s)

Specimen B-1

CF-3, aged, 0.08 dpa

PWR water, 320oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

Test periods with <10 μm 

extension are excluded.

10-13

10-12

10-11

10-10

10-9

10-8

10 15 20 25 30 35

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

Spec. A-2, 0.08 dpa, PPU 1 hr, Low-DO water

Spec. A-2, 0.08 dpa, w/o PPU, Low-DO water

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

CGR (m/s)

K (MPa m1/2)

NUREG-0313

Curve

CASS CF-3 with 24% ferrite 

low-DO high-purity or PWR water

318 - 320oC Open = Unaged

Closed = Aged

Blue = Unirradiated

Red = 0.08 dpa

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 = 362∆a0.85. 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. 

Figure 38. The J-R curve for specimen B-1.

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. 

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen B-1

CF-3, aged, 0.08 dpa

PWR water, 320oC

J= 362*∆a0.85

JQ=116 kJ/m2

Jmax

∆amax

61
 

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

62
Figure 40. Fracture surface of specimen B-1 along the sample central line.

Machine notch

CGR test JR test

Fatigue 

cracking

Vermicular 

ferrite at 

dendrite cores

Dimple 

fracture

Crack advance

Post-JR fatigue 

63
Figure 41. Deformation steps in the pre-cracking region of specimen B-1. Crack propagation from bottom to top.

64
Figure 42. Delta ferrite at dendrite cores in specimen B-1. Crack propagation from bottom to top.

65
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.

γ

δ γ

66
3.2 CF-8 Cast Stainless Steel

3.2.1 Unaged CF-8 CASS

3.2.1.1 Unirradiated specimen E-N1 tested in low-DO high-purity water

Crack growth rate test

Specimen E-N1 was an unirradiated control sample tested in low-DO high-purity water. The 

material was an unaged CF-8 with ~23% ferrite. The objective of the test was to compare with 

the test of unaged CF-3 (specimen A-N1) and to provide a baseline for the irradiated tests on CF8 CASS. The CGR test conditions and results of this sample are summarized in Table 9, and a 

crack-length history plot is shown in Fig. 44.

Table 9. CGR test of specimen E-N1 (unirradiated, unaged CF-8 with 23% δ ferrite) in low-DO 

high-purity water.

Test

Test

Time,

Test

Temp.

,

Load

Ratio

Rise

Time,

Return

Time,

Hold

Time, Kmax, ∆K,

CGR in

Env.,

CGR

in Air

Crack

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 0.2 5.967

a a 2.7 319 0.21 0.36 0.36 0.14 15.2 12.0 8.06E-08 5.05E-08 6.172

b 3.9 319 0.21 0.35 0.35 0.15 14.1 11.1 4.98E-08 4.05E-08 6.248

c 9.2 319 0.32 0.33 0.33 0.17 13.3 9.1 1.67E-10 2.54E-08 6.252

d 92.3 -

94.2 319 0.24 0.35 0.35 0.15 13.6 10.4 5.84E-11 3.43E-08 6.250

e a 97.7 319 0.20 0.36 0.36 0.14 14.6 11.7 1.45E-08 4.68E-08 6.296

f 100.8 320 0.30 0.35 0.35 0.15 15.3 10.7 9.76E-09 4.10E-08 6.342

g

116.2 -

119.6 320 0.20 0.36 0.36 0.14 16.1 13.0 5.54E-08 6.45E-08 6.567

h 124.2 320 0.40 0.33 0.33 0.17 16.0 9.6 2.41E-09 3.36E-08 6.595

i 140.4 -

148.5 319 0.32 0.35 0.35 0.15 16.4 11.2 2.19E-10 4.84E-08 6.596

j 165.1 -

165.8 319 0.36 0.27 0.27 0.23 15.9 10.1 6.28E-08 4.61E-08 6.643

k1 167.1 -

170.1 319 0.43 0.52 0.52 0.48 16.4 9.3 2.36E-09 1.97E-08 6.693

k2 170.5 -

174.1 319 0.42 0.53 0.53 0.47 17.1 9.9 2.14E-08 2.35E-08 6.766

l a 191.6 319 0.50 1.22 1.22 1.28 17.2 8.6 9.12E-09 6.91E-09 6.823

m 213 319 0.56 2.27 2.27 2.73 17.0 7.6 1.59E-09 2.56E-09 6.854

n a 233.7 319 0.60 6.43 2.14 8.57 17.1 6.8 6.00E-10 6.77E-10 6.865

o 260.3 319 0.59 12.9 2.15 17.1 16.9 6.9 1.13E-11 3.43E-10 6.867

p 284.4 319 0.54 13.7 2.29 16.3 16.9 7.7 4.19E-10 4.49E-10 6.880

q 309.2 319 0.54 27.6 2.30 32.4 17.2 7.8 4.07E-10 2.35E-10 6.896

r 333 319 0.54 55.4 2.31 64.6 17.2 7.9 4.44E-10 1.22E-10 6.912

s 358.4 319 0.53 139.1 5.56 160.9 17.2 8.0 2.88E-10 5.00E-11 6.923

t 404.4 319 0.54 232.2 5.57 267.8 17.6 8.1 2.97E-10 3.11E-11 6.945

u 429.1 320 0.53 464.2 5.57 535.8 17.4 8.1 7.04E-11 1.55E-11 6.949

1a 553.3 319 0.55 12 12 7200 17.5 7.9 1.43E-11 9.27E-13 6.959

1b 718.7 320 1 - - - 17.5 0.3 8.09E-12 - 6.962

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

67
Fatigue precracking was started with a triangle waveform at 1 Hz and a load ratio of ~0.2. A 

CGR close to the fatigue growth rate in air was readily established at a Kmax of ~15.2 MPa m1/2. 

No stable crack growth could be maintained with a lower Kmax in the following test periods. 

After the machine compliance was confirmed, the crack was advanced for 500 μm at 16-17 MPa 

m1/2. Eventually, environmentally assisted cracking started to appear in test periods p and q. 

With further increases in load ratio and rise time, environmental enhancement was stabilized 

between test periods r and u. The cyclic CGRs obtained from this sample are shown in Fig. 45

along with the corrosion fatigue curve for SSs in low-DO water. It is clear that the corrosion 

fatigue response of the unaged CF-8 is comparable to that of the wrought SSs in low-DO water. 

After the cyclic CGR test, the test was set to a constant load with PPU every 2 hr. A SCC CGR 

of 1.4 x 10-11 m/s was obtained over 10-μm crack extension. This CGR was much higher than 

that observed in the unirradiated CF-3 CASS (specimen A-N1). After the PPU was removed, the 

CGR decreased to about 7.8 x 10-12 m/s, which was also much higher than that of the 

unirradiated CF-3 CASS.

(a) 

Figure 44. Crack-length-vs.-time plot for specimen E-N1 (unirradiated, unaged CF-8 with 23% 

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

6.00

6.10

6.20

6.30

6.40

4

8

12

16

20

24

28

32

0 5 10 15

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. E-N1, CF-8, 23% δ, unaged

Low-DO high-purity water, ~319 o

C

a, 

R=0.2

1 Hz

b, 

R=0.2

1 Hz

c, 

R=0.3

1 Hz Held at a low K

68
(b) 

(c) 

(d) 

Figure 44. (Contd.)

6.10

6.15

6.20

6.25

6.30

6.35

6.40

6.45

4

8

12

16

20

24

28

32

90 95 100 105

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. E-N1, CF-8, 23% δ, unaged

Low-DO high-purity water, ~319 oC

d, 

R=0.2

1 Hz

e, 

R=0.3

1 Hz

f, 

R=0.3

1 Hz Held at a low K

Held at 

a low K

Recheck compliance

6.30

6.40

6.50

6.60

6.70

4

8

12

16

20

24

28

32

120 130 140 150 160

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. E-N1, CF-8, 23% δ, unaged

Low-DO high-purity water, ~319 oC

g, 

R=0.2

1 Hz

h, 

R=0.4

1 Hz

Held at a low K

i, 

R=0.3

1 Hz

Held at a low K

6.50

6.55

6.60

6.65

6.70

6.75

6.80

6.85

6.90

4

8

12

16

20

24

28

32

170 180 190 200 210

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. E-N1, CF-8, 23% δ, unaged

Low-DO high-purity water, ~319 o

C

j, 

R=0.3

1 Hz

k1, 

R=0.4

0.5 Hz

k2, 

R=0.4

0.5 Hz

Check 

compliance

l, 

R=0.5

0.2 Hz m, 

R=0.55

0.1 Hz

69
(e) 

(f) 

(g) 

Figure 44. (Contd.)

6.800

6.850

6.900

4

8

12

16

20

24

28

32

220 240 260 280 300

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. E-N1, CF-8, 23% δ, unaged

Low-DO high-purity water, ~319 oC

n, 

R=0.6

15s/up, 5s/down

o, 

R=0.6

30s/up, 5s/down

p, 

R=0.55

30s/up, 5s/down

q, 

R=0.55

60s/up, 5s/down

6.860

6.880

6.900

6.920

6.940

6.960

6.980

4

8

12

16

20

24

28

32

320 340 360 380 400 420

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. E-N1, CF-8, 23% δ, unaged

Low-DO high-purity water, ~319 oC

r, 

R=0.55

120s/up, 5s/down

s, 

R=0.55

300s/up, 12s/down

t, 

R=0.55

500s/up, 12s/down

Computer 

crashed

u, 

R=0.55

1000s/up, 12s/down

6.900

6.920

6.940

6.960

6.980

7.000

4

8

12

16

20

24

28

32

440 480 520 560 600 640 680 720

Crack Length (mm)

K (MPa m0.5)

Time (h)

Spec. E-N1, CF-8, 23% δ, unaged

Low-DO high-purity water, ~319 oC

1a,

PPU, 2hr

1b,

Constant

70
Figure 45. Cyclic CGRs of specimen E-N1.

Fracture toughness J-R curve test

Following the CGR test, a fracture toughness J-R curve test was performed on this sample in the 

same low-DO high-purity water environment. The test was conducted at a constant strain rate of 

0.43 μm/s, and the crack extension was measured with the DCPD method. Before each DCPD 

measurement, the stress was allowed to relax at a constant displacement for 30 s. Due to the low 

flow stress and high ductility, significant plastic flow was observed in this sample during the J-R 

curve test. Very little crack extension was obtained before the maximum cross-head 

displacement was reached (limited by the load train inside the autoclave and the total range of 

LVDT). Consequently, no data point was available in the qualified range above the 0.2-mm 

offset line for a power-law curve fit (see Fig. 46). The J value measured at the end of the test 

was ~500 kJ/m2. A J value greater than 700 kJ/m2 was estimated by extrapolating the available 

data points to the 0.2-mm offset line. It is clear the fracture toughness of this sample is much 

higher than the measurement capacity of the 1/4T-CT specimen. 

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-f

Test periods: g-i

Test periods: j-o

Test periods: p-u

CGRenv (m/s)

CGRair (m/s)

Specimen E-N1 

CF-8, unaged, unirradiated

Low-DO high-purity water

~319oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

71
Figure 46. J-R curve data of specimen E-N1.

Fractographic examination

A fractographic analysis of the tested sample was carried out after the sample was broken open at 

room temperature in air. Figure 47 shows the entire fracture surface of the CGR and JR tests. 

The crack front of the CGR test is not very straight in this test, and the crack extension is smaller

on one side of the sample than the other. Transgranular cracking and ductile dimples are the 

dominant morphologies for the CGR and JR test regions, respectively. For the CGR test region, 

heavy deformation ledges resulting from fatigue loading can be seen close to the machined notch 

(Fig. 48). Fractured δ ferrites with little plastic deformation are more evident at the later stage of 

the CGR test (Fig. 49). The brittle fracture is not visible during the JR test, and ductile dimples 

are the main fracture morphology in the JR test region (Fig. 50). Only a narrow band of JR test 

region can be seen on the fracture surface, which is consistent with the high ductility observed in 

this sample. 

0

400

800

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen E-N1

CF-8, unaged, unirradiated

Low-DO high-purity water, ~318oC

> 700 kJ/m2

Jmax ∆amax

~500 kJ/m2

72
Figure 47. Fracture surface of specimen E-N1 tested in low-DO high-purity water.

73
Figure 48. Transgranular fracture at the beginning of the precracking for specimen E-N1. Crack propagation from bottom to top.

74
Figure 49. Transgranular fracture at the end of the CGR test for specimen E-N1. Crack propagation from bottom to top.

a

75
Figure 49. (Contd.)

δ 

γ 

b

76
Figure 50. Ductile dimple fracture in the JR test region of specimen E-N1. Crack propagation from bottom to top.

77
3.2.1.2 Irradiated specimen E-1 tested in low-DO high-purity water

Crack growth rate test

Specimen E-1, an unaged CF-8 CASS (Heat 68) irradiated to 0.08 dpa, was tested in low-DO

high-purity water at 320°C. This sample contained ~23% ferrite, similar to that of CF-3 in this 

study. The objective was to compare the results with those from thermally aged CF-8 at the 

same dose. The CGR test conditions and results are summarized in Table 10, and a crack-length 

history plot is shown in Fig. 51.

Table 10. CGR test of specimen E-1 (0.08-dpa unaged CF-8 with 23% δ ferrite) in low-DO 

high-purity water.

Test

Test

Time,

Test

Temp.

,

Load

Ratio

Rise

Time,

Return

Time,

Hold

Time, Kmax, ∆K,

CGR in

Env.,

CGR

in Air

Crack

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 1.3 6.001

a 3.0 320 0.20 0.44 0.44 0.06 17.4 13.9 6.98E-08 6.70E-08 6.153

b 6.6 319 0.30 0.43 0.43 0.07 16.4 11.5 2.26E-08 4.15E-08 6.294

c1 15.6 319 0.30 0.85 0.85 0.15 15.5 10.8 4.03E-09 1.73E-08 6.360

c2 18.3 319 0.30 0.85 0.85 0.15 15.7 11.0 1.57E-08 1.83E-08 6.421

c3 21.7 319 0.30 0.85 0.85 0.15 16.3 11.4 2.54E-08 2.04E-08 6.552

d 25.3 319 0.30 0.84 0.84 0.16 15.5 10.9 1.50E-08 1.76E-08 6.629

e1 36.5 319 0.40 0.81 0.81 0.19 14.5 8.7 9.26E-10 9.81E-09 6.650

e2 45.5 319 0.40 0.81 0.81 0.19 14.6 8.8 2.68E-09 1.01E-08 6.682

e3 50.5 319 0.40 0.81 0.81 0.19 14.5 8.8 6.48E-09 1.00E-08 6.733

e4 53.8 319 0.40 0.81 0.81 0.19 14.9 8.9 1.59E-08 1.08E-08 6.805

f 59.6 319 0.50 1.54 1.54 0.46 14.7 7.4 5.31E-09 3.28E-09 6.852

g 73 319 0.50 3.84 3.84 1.16 14.7 7.4 2.09E-09 1.32E-09 6.891

h 96.7 319 0.50 11.5 3.83 3.52 14.4 7.3 4.26E-10 4.20E-10 6.913

i 125.1 320 0.55 22.4 3.73 7.62 14.4 6.5 negligible 1.60E-10 6.911

j 144.6 319 0.44 23.5 3.91 6.54 14.4 8.0 2.41E-10 2.72E-10 6.925

k 152.2 320 0.45 11.8 3.92 3.25 14.8 8.2 1.68E-09 5.76E-10 6.951

l 167.4 319 0.49 23.0 3.83 7.01 14.8 7.5 6.61E-10 2.28E-10 6.973

m 181.7 320 0.49 46.0 9.19 14.0 14.7 7.5 3.64E-10 1.15E-10 6.988

n 217.5 320 0.49 92.0 9.20 28.0 14.7 7.5 1.97E-10 5.87E-11 7.004

o 262.6 320 0.49 229.8 9.19 70.2 14.7 7.5 1.01E-10 2.35E-11 7.018

p 320.5 320 0.49 459.7 9.19 140.3 14.8 7.6 1.04E-10 1.20E-11 7.032

q 360 321 0.49 765.3 9.18 234.7 14.9 7.6 8.40E-11 7.20E-12 7.041

1 431.8 321 0.45 12 12 7200 14.9 8.2 1.80E-11 9.53E-13 7.051

2 578.4 320 0.45 12 12 7200 16.8 9.3 2.71E-11 1.43E-12 7.073

Fatigue pre-cracking was started with a triangular waveform at a maximum stress intensity factor 

of ~17.5 MPa m1/2, load ratio of 0.2, and frequency of 1 Hz. After about 300-μm crack 

extension, the load ratio was increased to 0.3, and the maximum stress intensity factor was 

decreased to ~15.5 MPa m1/2. The measured CGR gradually increased in test period c after a 

short period of sluggish growth, and the final CGR was about 2.0 x 10-8 m/s. The rise time and 

load ratio were increased further in the subsequent test periods, and environmentally enhanced 

cracking started to appear at the end of test period e. In the following test periods, the maximum 

stress intensity factor was decreased to ~14-15 MPa m1/2. Environmental enhancement appears 

78
to have been readily established in this sample at a fairly low stress intensity level with a load 

ratio below 0.5. By the end of test period q, the measured CGR was more than one order of 

magnitude higher than the fatigue growth rate in air. All cyclic CGRs of this sample are plotted 

in Fig. 52. The corrosion fatigue curve for unirradiated SSs still bounds all data points from this 

sample. However, compared to the cyclic CGRs of CF-3 (specimens A-1, A-2, and B-1), the 

CF-8 sample shows a slightly lower sensitivity to corrosion fatigue.

(a) 

(b) 

Figure 51. Crack-length-vs.-time plot for specimen E-1 (0.08-dpa unaged CF-8 with 23% 

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

6.00

6.10

6.20

6.30

6.40

6.50

6.60

6.70

4

8

12

16

20

24

28

32

0 5 10 15 20 25

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen E-1

CF-8, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

a

b

c

d

2.54E-8 m/s

1.57E-8 m/s

4.03E-9 m/s

Kmax

Crack length

6.55

6.60

6.65

6.70

6.75

6.80

6.85

6.90

4

8

12

16

20

24

28

32

25 30 35 40 45 50 55 60

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen E-1

CF-8, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

e

1.59E-8 m/s

6.48E-9 m/s

f

Kmax

Crack length

79
(c) 

(d) 

(e) 

Figure 51. (Contd.)

6.75

6.80

6.85

6.90

6.95

7.00

4

8

12

16

20

24

28

32

60 70 80 90 100 110 120 130 140

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen E-1

CF-8, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

g

h

i

j

Kmax

Crack length

6.80

6.85

6.90

6.95

7.00

7.05

7.10

4

8

12

16

20

24

28

32

160 180 200 220 240 260

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen E-1

CF-8, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

k

l

m

n

o

Kmax

Crack length

6.85

6.90

6.95

7.00

7.05

7.10

7.15

7.20

4

8

12

16

20

24

28

32

280 320 360 400 440 480 520 560

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen E-1

CF-8, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

p

q

1 2

Kmax

Crack length

80
Figure 52. Cyclic CGRs of specimen E-1.

Following the pre-cracking, the test was set at constant load with PPU every 2 hr. The stress 

intensity factor was about 15 MPa m1/2. A SCC CGR of 1.8 x 10-11 m/s was measured over a 

10-μm crack extension. The stress intensity level was increased to ~17 MPa m1/2 with PPU 

every 2 hr for another SCC CGR measurement. A CGR of 2.7 x 10-11 m/s was recorded over a 

22-μm crack extension. 

Fracture toughness J-R curve test

A fracture toughness J-R curve test was performed on the sample after the CGR test. The J-R 

data are plotted in Fig. 53, and a power-law fitting gives rise to a resistance curve of J =

359∆a0.57. The J value at the 0.2-mm offset line is 183 kJ/m2 for this sample. The crack 

extension was heavily curved in this sample, and the J-R curve data could not be validated per 

the ASTM standard. Four of the nine measurements of the final crack size were above the limit, 

and the Jmax requirement was also ignored in the analysis.

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CGRenv (m/s)

CGRair (m/s)

Specimen E-1

CF-8, unaged, 0.08 dpa.

Low-DO high-purity water 

320oC

9 μm extension

CF curve for 0.2 ppm DO 

by Shack & Kassner

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

Fractographic examination

Figure 54 shows the entire fracture surface of specimen E-1. Different stages of the test can be 

clearly identified. The CGR crack front is not straight, and the crack extension on the right side 

of the sample is significantly less. Transgranular cleavage-like cracking is the dominant 

morphology close to the machine notch in the pre-cracking region. As the CGR test progressed, 

casting microstructure became more evident. Vermicular ferrites at the cores of casting dendrites 

were clearly visible. 

Figure 55 is an enlarged view of the fracture surface along the sample central line. Cleavage-like

morphology dominates the fatigue pre-cracking region. Large deformation steps can be seen in 

the early stage of the test (Fig. 56). With the advance of the crack, the fracture surface became

smoother, and deformation steps less pronounced. As shown in Fig. 57, deformation steps can 

still be seen in the austenite but are much less evident in the ferrite. At the end of the CGR test, 

the fracture surface became completely flat in both the ferrite and austenite (Fig. 58). Beyond 

the CGR test region, the fracture morphology changed to ductile dimples (Fig. 59), suggesting a 

heavy plastic deformation leading to a ductile fracture. 

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen E-1

CF-8, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

J= 359*∆a0.57

JQ=183 kJ/m2

Jmax

∆amax

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

83
Figure 55. Fracture surface of specimen E-1 along the sample central line.

Machined

notch

CGR test JR test

TG

Vermicular 

ferrite at 

dendrite cores

Dimple fracture

Post JR fatigue 

Crack advance

84
Figure 56. Cleavage-like cracking at the beginning of the CGR test of specimen E-1. Crack propagation from bottom to top.

85
Figure 57. Cyclic CGR test region of specimen E-1. Crack propagation from bottom to top.

γ

δ

γ

86
Figure 58. Smooth fracture surface at the end of the CGR test in specimen E-1. Crack propagation from bottom to top.

γ δ

JR

γ

87
Figure 59. Ductile dimple fracture in the J-R test region of specimen E-1. Crack propagation from bottom to top.

88
3.2.2 Thermally Aged CF-8 CASS

3.2.2.1 Unirradiated specimen F-N1 tested in low-DO high-purity water

Crack growth rate test

Specimen F-N1 was an unirradiated control sample tested in low-DO high-purity water. The 

material was a thermally aged CF-8 with ~23% ferrite. The objective of the test was to compare 

it with the test of the irradiated thermally aged CF-8 CASS. The CGR test conditions and results 

of this sample are summarized in Table 11, and a crack-length history plot is shown in Fig. 60.

Fatigue precracking was started with a triangle waveform of 1 Hz and a load ratio of ~0.2 at a 

maximum stress intensity factor of ~15.5 MPa m1/2. After an initial slow growth period, a CGR 

slightly below the fatigue growth rate in air was obtained. After about 200-μm crack extension, 

the load ratio and rise time were slowly increased to induce environmentally enhanced cracking. 

The environmental effect became evident in test period n with a load ratio of ~0.5. In the 

following test periods, the elevated CGR was stabilized with the further increases in rise time. 

Figure 61 shows all cyclic CGRs obtained from this sample along with the corrosion fatigue 

curve for SSs in high-purity water with 0.2 ppm DO. Similar to the unaged CF-8, the thermally 

aged CF-8 showed a good corrosion fatigue response in the low-DO high-purity water.

Table 11. Crack growth rates of specimen F-N1 (unirradiated, aged CF-8 with 23% δ ferrite) in 

a low-DO high-purity water environment.

Test

Test

time,

Test

Temp.,

Load

Ratio

Rise

Time,

Return

Time,

Hold

Time, Kmax, ∆K,

CGR in

Env.,

CGR 

in Air

Crack

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 1.3 5.983

a a 4.9 319 0.19 0.40 0.40 0.10 15.5 12.4 4.82E-08 5.13E-08 6.178

b 11.9 319 0.29 0.76 0.76 0.24 15.5 10.9 1.56E-08 1.97E-08 6.332

c 23.3 319 0.40 3.65 3.65 1.35 15.3 9.2 4.15E-10 2.63E-09 6.341

d 46.8 319 0.41 1.45 1.45 0.55 15.1 9.0 8.54E-11 6.10E-09 6.345

e a 51.4 319 0.35 0.75 0.75 0.25 15.5 10.1 1.16E-08 1.66E-08 6.385

f 54.5 319 0.40 1.46 1.46 0.54 15.5 9.3 3.19E-09 6.82E-09 6.398

g 73.8 319 0.40 3.63 1.45 1.37 15.3 9.2 3.66E-10 2.61E-09 6.415

h 98.5 319 0.40 7.27 1.45 2.73 15.4 9.2 1.72E-10 1.33E-09 6.423

i 119.2 318 0.40 3.64 1.45 1.36 15.9 9.5 2.45E-09 2.93E-09 6.514

j 142.6 319 0.40 7.25 1.45 2.75 16.1 9.7 1.68E-09 1.55E-09 6.604

k 171.8 319 0.45 14.2 3.54 5.85 16.3 9.0 5.53E-10 6.50E-10 6.645

l 195.3 319 0.50 41.3 8.27 18.7 16.3 8.2 2.41E-11 1.74E-10 6.647

ma 244.7 319 0.47 41.8 8.37 18.2 16.2 8.5 1.05E-10 1.93E-10 6.654

n 287 319 0.50 103.5 8.28 46.5 16.6 8.3 1.57E-10 7.27E-11 6.671

o 310.5 319 0.49 207.4 8.30 92.6 16.5 8.4 8.22E-11 3.70E-11 6.675

p 343.1 319 0.49 347.2 8.33 152.8 16.6 8.5 8.02E-11 2.33E-11 6.684

q 382.6 319 0.49 692.2 8.31 307.8 16.5 8.4 3.54E-11 1.13E-11 6.688

1a 478.4 318 0.50 12 12 7200 16.5 8.3 1.23E-11 1.03E-12 6.696

1b 621.5 319 1 - - - 16.5 - 1.17E-11 - 6.702

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

89
(a) 

(b) 

(c) 

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

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

6.00

6.10

6.20

6.30

6.40

4

8

12

16

20

24

28

32

0 10 20 30 40

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. F-N1, CF-8, 23% δ, aged

Low-DO high-purity water, ~319 oC

a, 

R=0.2

1 Hz

b, 

R=0.3

0.5 Hz c, 

R=0.4

0.1 Hz

d, 

R=0.4

0.25 Hz

6.30

6.35

6.40

6.45

6.50

4

8

12

16

20

24

28

32

50 60 70 80 90 100

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. F-N1, CF-8, 23% δ, aged

Low-DO high-purity water, ~319 oC

g, 

R=0.4

5s up, 2s down

h, 

R=0.4

f, 10s up, 2s down

R=0.4

0.25Hz

e, 

R=0.35

0.5Hz

6.40

6.45

6.50

6.55

6.60

6.65

6.70

6.75

4

8

12

16

20

24

28

32

100 120 140 160 180 200 220 240

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. F-N1, CF-8, 23% δ, aged

Low-DO high-purity water, ~319 oC

i, 

R=0.4

5s up, 

2s down

j, 

R=0.4

10s up, 

2s down

k, 

R=0.45

20s up, 

5s down

Unstable pressure

l, 

R=0.5

60s up, 

12s down

m, 

R=0.48

60s up, 

12s down

Unstable pressure

90
(d) 

(e) 

Figure 60. (Contd.)

After more than ~700-μm crack extension under cyclic loading, the test was transitioned to a 

constant load with PPU every 2 hr (test period 1a). Under this condition, a CGR of 1.2x10-11 m/s 

was obtained at ~16.5 MPa m1/2 after an initial short period of rapid growth. Next, the PPU was 

removed, and the test was held at a near constant-K condition (~16.5 MPa m1/2) for a total of 

~140 hr (test period 1b). Unlike the other tests conducted under low-corrosion-potential 

environments, the measured CGRs with and without PPU were almost identical in this test. This 

growth rate under a constant K was unexpectedly high, suggesting a dynamic loading condition 

during this test period. Note that the autoclave pressure was unstable during the test period 1b 

and several large pressure drops (>60 psig) were detected. Consequently, the applied stress 

intensity factor fluctuated in this test period (as shown in Fig. 60e). This dynamic loading 

condition may be responsible for the relatively high SCC CGR observed in this sample. 

6.60

6.65

6.70

6.75

4

8

12

16

20

24

28

32

260 280 300 320 340 360 380

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Spec. F-N1, CF-8, 23% δ, aged

Low-DO high-purity water, ~319 oC

n, 

R=0.5

150s up, 

12s down

o, 

R=0.5

300s up, 

12s down

p, 

R=0.5

500s up, 

12s down

q, 

R=0.5

1000s up, 

12s down

6.670

6.680

6.690

6.700

6.710

6.720

4

8

12

16

20

24

28

32

400 440 480 520 560 600

Crack Length (mm)

K (MPa m0.5)

Time (h)

Spec. F-N1, CF-8, 23% δ, aged

Low-DO high-purity water, ~319 oC

1a, 

PPU, 2 hr hold

1b, 

Constant-load

Unstable pressure

91
Figure 61. Cyclic CGRs of specimen F-N1.

Fracture toughness J-R curve test

After the CGR test, a fracture toughness J-R curve test was performed on the sample in the same 

low-DO high-purity water environment. The test was conducted with a constant extension rate 

of 0.43 μm/s. During the test, the loading was interrupted periodically to measure the crack 

extension by DCPD. The obtained J-R curve is shown in Fig. 62. The estimated J value at the 

0.2-mm offset line is about 220 kJ/m2. This fracture toughness value is significantly lower than 

that of the unaged CF-8, suggesting a strong thermal aging effect in this sample. Note that the JR curve data cannot be validated for this test since the requirements of the crack front 

straightness and Jmax were violated. 

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-d

Test periods: e-l

Test periods: m-q

CGRenv (m/s)

CGRair (m/s)

Specimen F-N1 

CF-8, aged, unirradiated

Low-DO high-purity water

~319oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

92
Figure 62. The J-R curve of specimen F-N1.

Fractographic examination

The tested sample was broken open under cyclic loading at room temperature in air. Figure 63

shows the entire fracture surface of the CGR and JR tests. The crack front of the CGR test was 

quite straight, indicating a well-controlled test condition. While the fracture surface of the CGR 

test region shows a TG morphology, the failure mode of the JR test region is ductile. More 

details of the fracture morphologies can be seen in Fig. 64 along the sample central line. At the 

beginning of the precracking, heavy deformation ledges resulting from fatigue loading can be 

seen on the fracture surface (Fig. 65a). As the crack advances deeper and environmental 

enhancement starts to appear, the fracture surface becomes flat. At the end of the CGR test, most 

δ ferrites appear to fracture in a brittle fashion with little plastic deformation (Fig. 65b). 

Figure 66 shows the details of the transition area from the CGR to JR tests. Some brittle 

morphology can be seen at the very beginning of the JR test. Beyond the initial 30-50 μm, the 

crack advances in a ductile tearing mode, and ductile dimples resulting from microvoid 

coalescence become the dominant morphology (Fig. 67). 

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen F-N1

CF-8, aged, unirradiated

Low-DO high-purity water, ~318oC

J= 395.3*∆a0.58

JQ=220 kJ/m2

Jmax

∆amax

93
Figure 63. Fracture surface of specimen F-N1 tested in low-DO high-purity water.

94
Figure 64. Fracture surface of specimen F-N1 along the sample central line.

CGR test JR test

TG

Dimple 

fracture

Post JR 

fatigue 

Crack advance

Machined

notch

95
Figure 65. Transgranular fracture in the CGR test of specimen F-N1: (a) in the precracking region and (b) at the end of CGR 

test. Crack advance direction from bottom to top.

a

96
Figure 65. (Contd.)

b

δ 

δ δ 

97
Figure 66. Transition region from CGR to J-R curve tests of specimen F-N1. Crack advance direction from bottom to top.

a

γ 

γ 

δ 

98
Figure 66. (Contd.)

b

γ γ 

δ 

99
Figure 67. Ductile dimple fracture in the JR test region of specimen F-N1. Crack advance direction from bottom to top.

100
3.2.2.2 Irradiated specimen F-1 tested in low-DO high-purity water

Crack growth rate test

Specimen F-1 was a CF-8 CASS with 23% ferrite cut from the same heat as specimen E-1 (Heat 

68). The specimen was thermally aged at 400°C for 10,000 hr prior to irradiation. This 

specimen was also tested in low-DO high-purity water at 320°C. The CGR test conditions and 

results are summarized in Table 12, and a crack-length history plot is shown in Fig. 68.

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

~17 MPa m1/2, load ratio of 0.2, and frequency of 1 Hz. Using a triangular waveform, the crack 

was advanced for about 500 μm with gradually increased load ratio. After a saw-tooth 

waveform was introduced in test period g, environmentally enhanced cracking started to appear. 

In the following test periods, the maximum stress intensity factor was maintained at

~16 MPa m1/2 while the load ratio and rise time were gradually increased. A significant degree 

of environmental enhancement was readily established in this sample, similar to that observed in 

unaged CF-8 (specimen E-1). By the end of the cyclic CGR test, the measured CGR in water 

was about a factor of seven higher than that of the fatigue growth rate. All cyclic CGRs obtained 

from this sample are plotted in Fig. 69. The corrosion fatigue curve for unirradiated SSs still 

bounds the data points of the aged CF-8 CASS.

After the cyclic CGR test, the test was set at constant load with PPU every 2 hr. A SCC CGR of 

2.69 x 10-11 m/s was measured at a stress intensity factor of 16 MPa m1/2 (Fig. 70). This growth 

rate is about a factor of three lower than the NUREG-0313 curve, and is very similar to that 

obtained from the unaged CF-8 CASS (specimen E-1). 

Table 12. Crack growth rates of specimen F-1 (0.08-dpa aged CF-8 with 23% δ ferrite) in a lowDO high-purity water environment.

Test

Test

time,

Test

Temp.,

Loa

d

Rati

o

Rise

Time,

Return

Time,

Hold

Time, Kmax, ∆K,

CGR in

Env.,

CGR 

in Air

Crack

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 0.4 6.038

a 2.0 319 0.20 0.45 0.45 0.05 17.3 13.8 6.84E-08 6.51E-08 6.196

b 3.9 319 0.30 0.44 0.44 0.06 16.4 11.4 4.28E-08 4.04E-08 6.323

c 5.7 319 0.40 0.42 0.42 0.08 15.4 9.2 1.11E-08 2.29E-08 6.358

d 8.2 319 0.40 0.84 0.84 0.16 15.4 9.3 6.40E-09 1.16E-08 6.382

e 11.7 319 0.35 0.85 0.85 0.15 15.6 10.1 1.13E-08 1.46E-08 6.440

f 24.2 319 0.35 4.25 4.25 0.75 15.6 10.1 3.20E-09 2.94E-09 6.505

g 36.7 319 0.35 10.2 4.25 1.81 15.7 10.2 2.06E-09 1.25E-09 6.561

h 49.4 319 0.40 16.8 4.19 3.25 15.6 9.4 1.36E-09 6.18E-10 6.601

i 76.8 319 0.39 33.5 10.05 6.5 15.8 9.6 7.62E-10 3.22E-10 6.650

j 120.9 319 0.39 83.6 10.04 16.4 15.7 9.6 3.05E-10 1.30E-10 6.685

k 168 319 0.44 247.6 9.91 52.4 15.8 8.9 1.69E-10 3.64E-11 6.708

l 224 319 0.44 495.0 9.90 105.0 16.0 8.9 8.94E-11 1.84E-11 6.723

m 290.3 320 0.44 824.9 9.90 175.1 16.0 9.0 7.17E-11 1.11E-11 6.735

1 359.3 318 0.45 12 12 7200 16.0 8.8 2.69E-11 1.20E-12 6.749

101
(a) 

(b) 

(c) 

Figure 68. Crack-length-vs.-time plot for specimen F-1 (0.08-dpa aged CF-8 with 23% ferrite): 

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

6.00

6.10

6.20

6.30

6.40

6.50

4

8

12

16

20

24

28

32

0 5 10 15 20 25

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen F-1

CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

a

b

c

d

e

f

Kmax

Crack length

6.45

6.50

6.55

6.60

6.65

6.70

6.75

4

8

12

16

20

24

28

32

40 60 80 100 120

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen F-1

CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

g

h

i

j

Kmax

Crack length

6.60

6.65

6.70

6.75

6.80

4

8

12

16

20

24

28

32

120 140 160 180 200 220 240 260 280

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen F-1

CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

k

l

m

Computer 

crashed.

Kmax

Crack length

102
(d) 

Figure 68. (Contd.)

Figure 69. Cyclic CGRs of specimen F-1.

Fracture toughness J-R curve test

Following the CGR test, a fracture toughness J-R curve test was performed on the same sample 

in the test environment. Figure 71 shows the obtained data, and a power-law fitting gives rise to 

a J-R curve of J = 372∆a0.62. The estimated J value at the 0.2-mm offset line is 171 kJ/m2. Note 

that the J-R curve data cannot be validated because one of the nine measurements of the final 

6.65

6.70

6.75

6.80

4

8

12

16

20

24

28

32

300 320 340 360 380

Crack Length (mm)

K (MPa m0.5)

Time (h)

Specimen F-1

CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

1

K

Crack length

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CGRenv (m/s)

CGRair (m/s)

Specimen F-1

CF-8, aged, 0.08 dpa.

Low-DO high-purity water 

320oC

9 μm extension

CF curve for 0.2 ppm DO 

by Shack & Kassner

103
crack size did not meet the requirements. Some J values used in the analysis were also above the 

limit for this sample. 

Fractographic examination

Replicas of the fracture surface of specimen F-1 were examined with SEM. As shown in Figs.

72 and 73, transgranular cleavage-like cracking is the main fracture mode during the precracking stage. Deformation steps are clearly visible next to the machine notch. As the crack 

advances, the fracture surface became increasingly smoother, suggesting the crack had 

propagated in a progressively more brittle fashion (Fig. 74). Also, as shown in Fig. 75, 

deformation steps seem to develop in the austenitic phase surrounding the ferritic phase at 

dendrite cores. Deformation ledges are seen less often within the ferrite. Finally, after the CGR 

test, the fracture surface became completely ductile. The sample was fractured by ductile tearing 

in the J-R curve test (Fig. 76). 

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

10-13

10-12

10-11

10-10

10-9

10-8

10 15 20 25

Spec. E-N1, unirr., PPU 2 hr, Low-DO water

Spec. E-N1, unirr., w/o PPU, Low-DO water

Spec. E-1, 0.08 dpa, PPU 2 hr, Low-DO water

Spec. F-N1, unirr., PPU 2 hr, Low-DO water

Spec. F-N1, unirr., w/o PPU, Low-DO water

Spec. F-1, 0.08 dpa, PPU 2 hr, Low-DO water

CGR (m/s)

K (MPa m1/2)

NUREG-0313

Curve

CASS CF-8 with 23% ferrite 

low-DO high-purity water

318 - 320oC Open = Unaged

Closed = Aged

Blue = Unirradiated

Red = 0.08 dpa

104
Figure 71. The J-R curve of specimen F-1. 

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen F-1

CF-8, aged, 0.08 dpa

Low-DO high-purity water, 320oC

J= 372*∆a0.62

JQ=171 kJ/m2

Jmax

∆amax

105
 

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

106
Figure 73. Fracture surface of specimen F-1 along the sample central line.

Machined

notch

CGR test JR test

Cleavage-like 

cracking

Vermicular 

ferrite

Dimple fracture

Post JR fatigue 

Crack advance

107
Figure 74. Fracture surface of the CGR region in specimen F-1. Crack propagation from bottom 

to top. 

γ δ

108
Figure 75. Deformation steps in austenite grain around ferrite phase in the CGR test region of specimen F-1. Crack propagation 

from bottom to top.

109
Figure 76. Dimple fracture in the JR test region of specimen F-1. Crack propagation from bottom to top.

110
3.3 CF-8M Stainless Steel

3.3.1 Unaged CF-8M CASS

Crack growth rate test

Specimen I-1 was an unaged CF-8M CASS with 28% ferrite (Heat 75) irradiated to 0.08 dpa. 

The specimen was tested in a low-DO high-purity water environment at 320°C. The objective 

was to compare the test results with those of its thermally aged equivalent. The CGR test 

conditions and results are summarized in Table 13, and a crack-length history plot is shown in 

Fig. 77.

Fatigue pre-cracking was started with a triangular waveform at a maximum stress intensity factor 

of ~14 MPa m1/2, a load ratio of 0.2, and frequency of 2 Hz. After the crack was initiated from 

the notch, several test periods with an increasing rise time and load ratio were carried out to 

stimulate environmentally assisted cracking. Two repeated attempts were made until 

environmental enhancement started to appear in test period n. In the following test periods, the 

enhancement was stabilized successfully at a maximum stress intensity factor of ~15.5 MPa 

m1/2. Before the test was set at constant load, a hydraulic pump tripped. Consequently, the 

actuator of the test system was switched off automatically. To eliminate any possible 

overloading effect, additional test periods (from s to ac) were added after the system was 

recovered to repeat the transition. Under a similar loading condition, a similar degree of

environmental enhancement was re-established in test period x and became stabilized in the 

following test periods. By the end of test period ac, the measured CGR was more than a factor 

of 10 higher than the fatigue crack growth rate curve in air. 

All cyclic CGRs obtained from this sample are plotted in Fig. 78. The data points are close to 

and sometime higher than the corrosion fatigue curve for unirradiated SSs. It appears that this

CF-8M CASS is more susceptible to cracking compared to CF-3 and CF-8. The test was then set 

at constant load with PPU every 2 hr. A CGR of 1.27 x 10-11 m/s was recorded at ~18 MPa m1/2

over 26-μm crack extension. This SCC CGR is still significantly lower than the NUREG-0313

curve. 

111
Table 13. Crack growth rates of specimen I-1 (0.08-dpa unaged CF-8M with 28% δ ferrite) in a 

low-DO high-purity water environment.

Test

Test 

Time,

Test

Temp.,

Load

Ratio

Rise 

Time,

Return 

Time,

Hold 

Time, Kmax, ∆K,

CGR in 

Env.,

CGR 

in Air,

Crack

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 0.9 5.977

aa 3.11 319 0.23 0.21 0.21 0.04 14.3 11.0 5.09E-08 6.72E-08 6.127

b 5.0 319 0.36 0.20 0.20 0.05 12.7 8.1 2.74E-09 3.02E-08 6.134

c 6.5 319 0.33 0.21 0.21 0.04 14.2 9.6 4.10E-08 4.87E-08 6.225

d 9.8 319 0.43 0.40 0.40 0.10 14.2 8.1 1.27E-08 1.61E-08 6.285

e 23.9 319 0.53 0.76 0.76 0.24 14.2 6.6 1.47E-09 4.79E-09 6.315

f 26.5 319 0.49 0.78 0.78 0.22 14.1 7.3 8.93E-10 6.14E-09 6.316

g 28.6 319 0.30 0.83 0.83 0.17 14.7 10.2 2.38E-08 1.47E-08 6.371

h 31.4 319 0.40 0.81 0.81 0.19 14.8 8.8 1.48E-08 1.04E-08 6.434

i 37 320 0.45 1.98 1.98 0.52 14.8 8.1 3.91E-09 3.33E-09 6.463

j 47.9 319 0.45 3.94 3.94 1.06 14.8 8.1 1.89E-09 1.67E-09 6.495

k 56.7 320 0.45 7.88 3.94 2.12 14.8 8.1 1.19E-09 8.46E-10 6.513

l 72.8 319 0.45 15.8 3.94 4.24 14.8 8.1 5.61E-10 4.27E-10 6.534

m 104.5 319 0.45 23.6 3.94 6.37 14.8 8.2 2.28E-10 2.87E-10 6.551

na 153.1 319 0.45 47.2 3.94 12.8 14.9 8.2 3.09E-10 1.46E-10 6.578

o 176.2 320 0.45 94.5 9.45 25.5 15.0 8.3 3.71E-10 7.47E-11 6.605

pa 240 319 0.50 231.1 9.24 68.9 15.1 7.5 3.17E-10 2.38E-11 6.634

q 335.4 320 0.50 461.4 9.23 138.6 15.0 7.6 1.49E-10 1.21E-11 6.676

r 363.8 320 0.51 768.0 9.22 232.0 15.3 7.6 1.95E-10 7.30E-12 6.690

Hydraulic pump trip

s1 394.3-

410.3 320 0.49 231.5 9.26 68.5 15.2 7.8 4.67E-10 2.63E-11 6.726

s2 433.6 320 0.49 231.5 9.26 68.5 15.2 7.8 1.35E-09 2.63E-11 6.726

t 440 320 0.48 463.4 9.27 136.6 15.1 7.9 3.16E-12 1.34E-11 6.731

u 505.6 319 0.48 464.4 9.29 135.6 15.2 7.9 3.76E-11 1.38E-11 6.740

v 530 319 0.49 116.5 9.32 33.5 15.7 8.1 1.44E-11 5.80E-11 6.740

w 532 319 0.29 8.32 4.16 1.68 16.5 11.7 1.12E-08 2.23E-09 6.787

x 538.7 319 0.39 24.2 4.04 5.76 16.5 10.0 1.52E-09 5.16E-10 6.813

y 559.3 320 0.50 46.6 9.33 13.4 16.5 8.3 2.81E-10 1.63E-10 6.831

z 601.7 319 0.48 93.5 9.35 26.5 16.3 8.5 8.88E-11 8.41E-11 6.845

aa 630.7 319 0.49 236.6 9.47 63.4 17.7 9.0 3.90E-10 4.17E-11 6.877

ab 672.7 319 0.49 473.0 9.46 127.0 17.7 9.1 2.37E-10 2.11E-11 6.902

ac 696.6 319 0.49 787.7 9.45 212.3 17.7 9.1 1.55E-10 1.26E-11 6.909

1 821.9 319 0.50 12 12 7200 17.9 8.9 1.89E-11 1.33E-12 6.925

a The CGR value was obtained from the later part of the test periods.

112
(a) 

(b) 

(c) 

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

ferrite): test periods (a) a-f, (b) g-k, (c) l-o, (d) p-r, (e) s-v, (f) w-z, (g) aa-ac, and (h) 

1.

5.90

6.00

6.10

6.20

6.30

6.40

4

8

12

16

20

24

28

32

0 5 10 15 20 25

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

a

b

c

d

5.09E-8 m/s

e

f

Kmax

Crack length

6.25

6.30

6.35

6.40

6.45

6.50

6.55

6.60

4

8

12

16

20

24

28

32

28 32 36 40 44 48 52

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

g

h

Check 

friction i

j

k

Kmax

Crack length

6.48

6.52

6.56

6.60

6.64

6.68

4

8

12

16

20

24

28

32

60 80 100 120 140 160 180

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

l m

n

3.1E-10 m/s o

Kmax

Crack length

113
(d) 

(e) 

(f) 

Figure 77. (Contd.)

6.50

6.55

6.60

6.65

6.70

6.75

4

8

12

16

20

24

28

32

200 250 300 350

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

q

p

r

Hydraulic 

pump 

tripped

Kmax

Crack length

6.65

6.70

6.75

6.80

6.85

4

8

12

16

20

24

28

32

400 420 440 460 480 500 520

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

s2

Hydraulic 

pump 

tripped t u v

s1

Kmax

Crack length

6.70

6.75

6.80

6.85

6.90

4

8

12

16

20

24

28

32

530 540 550 560 570 580 590 600

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

w

x

y z

Kmax

Crack length

114
(g) 

(h) 

Figure 77. (Contd.)

Fracture toughness J-R curve test

After the CGR test, a fracture toughness J-R curve test was performed on the same sample in the 

test environment. The obtained J and crack extension results are plotted in Fig. 79. A power-law 

fitting shows a J-R correlation of J = 336∆a0.66. The J value at the 0.2-mm offset line is about 

145 kJ/m2. Note that the J-R curve data cannot be validated because one of the nine 

measurements of the final crack size was above the limit. Some data points above the Jmax were 

also used in the analysis. 

6.75

6.80

6.85

6.90

6.95

7.00

4

8

12

16

20

24

28

32

600 620 640 660 680 700

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

aa

ab

ac

Kmax

Crack length

6.80

6.85

6.90

6.95

7.00

4

8

12

16

20

24

28

32

700 750 800 850

Crack Length (mm)

K (MPa m0.5)

Time (h)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water, 320oC

1

K

Crack length

115
Figure 78. Cyclic CGRs of specimen I-1.

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

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-f

Test periods: g-r

Test periods: s-v

Test periods: w-ac

CGRenv (m/s)

CGRair (m/s)

Specimen I-1

CF-8M, unaged, 0.08 dpa.

Low-DO high-purity water

320oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

Test periods with <10 μm 

extension are excluded.

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen I-1

CF-8M, unaged, 0.08 dpa

Low-DO high-purity water, 320oC

J= 336*∆a0.66

JQ=145 kJ/m2

Jmax

∆amax

116
Fractographic examination 

Following the J-R curve test, cyclic loading was applied at room temperature in an air

atmosphere to break the remaining ligament. Figure 80 shows the fracture surface of specimen I1. The crack front is relatively straight, indicating a well-controlled loading condition during the 

CGR test. The CGR region is flat, which shows a clear contrast from the heavily deformed 

plastic region in the JR test. Multiple secondary cracks perpendicular to the fracture surface can 

also be seen in the CGR test region. Figure 81 shows an enlarged view of the sample central 

line. Transgranular cleavage-like cracking can be seen at the beginning of the CGR test. With 

the advance of the crack, cleavage-like cracking became less pronounced and the vermicular 

ferrite that formed at the core of casting dendrites started to appear (Fig. 82). At the end of the 

CGR test, little deformation steps can be seen on the fracture surface (Fig. 83). In the JR test 

region, the fracture was a ductile dimple morphology, suggesting heavy plastic deformation prior 

to fracture (Fig. 84). 

117
 Figure 80. Fracture surface of specimen I-1 tested in low-DO high-purity water.

Crack advance Delta ferrite in dendrites

JR CGR

Dimples

Secondary cracking

Air bubbles

Crack advance Delta ferrite in dendrites

JR CGR

Dimples

Secondary cracking

Air bubbles

118
Figure 81. Fracture surface of specimen I-1 along the sample central line.

Machined

notch

CGR test JR test

Secondary 

cracking

Vermicular 

ferrite

Dimple 

fracture

Post-JR fatigue 

Crack advance

119
Figure 82. Precracking region in the CGR test of specimen I-1. Crack propagation from bottom to top.

120
Figure 83. Fracture surface at the end of CGR test of specimen I-1. Crack propagation from bottom to top.

121
Figure 84. Heavily deformed microstructure in the JR test region of specimen I-1.

122
3.3.2 Thermally Aged CF-8M CASS

Crack growth rate test

Specimen J-1 was the thermally aged version of specimen I-1, a CF-8M CASS with 28% ferrite. 

The sample was aged at 400°C for 10,000 hr and then irradiated to 0.08 dpa. The test was 

performed in the low-DO high-purity water at 320°C. The objective was to compare the results

with those of the unaged CF-8M at the same dose. The CGR test conditions and results are 

summarized in Table 14, and a crack-length history plot is shown in Fig. 85.

Table 14. Crack growth rates of specimen J-1 (0.08-dpa aged CF-8M with 28% δ ferrite) in a 

low-DO high-purity water environment.

Test

Test 

Time,

Test

Temp.,

Load

Ratio

Rise 

Time,

Return 

Time,

Hold 

Time, Kmax, ∆K,

CGR in 

Env.,

CGR in

Air,

Crack 

Length,

Period h °C s s s MPa m1/2 MPa m1/2 m/s m/s mm

Start 0.4 5.970

a 2.2 320 0.20 0.22 0.22 0.03 14.2 11.3 6.53E-09 6.82E-08 5.990

b 3.6 320 0.20 0.22 0.22 0.03 16.6 13.2 5.35E-08 1.13E-07 6.111

c 6.3 320 0.30 0.43 0.43 0.07 15.5 10.8 1.57E-08 3.37E-08 6.183

d 8.6 320 0.30 0.21 0.21 0.04 14.5 10.1 9.37E-09 5.43E-08 6.217

e 23.8 320 0.30 4.29 4.29 0.71 14.4 10.0 2.97E-10 2.67E-09 6.225

f 25.3 320 0.30 0.22 0.22 0.03 15.9 11.1 7.02E-08 7.34E-08 6.381

g a 28.7 320 0.30 0.43 0.43 0.07 15.0 10.4 2.98E-08 3.05E-08 6.494

h 33.5 320 0.40 0.83 0.83 0.17 15.1 9.0 1.24E-08 1.08E-08 6.587

i 37.1 320 0.45 1.64 1.64 0.36 15.1 8.2 6.15E-09 4.31E-09 6.620

j 48.5 320 0.52 3.98 3.98 1.02 15.0 7.2 6.21E-10 1.20E-09 6.632

k a 51.5 320 0.45 0.82 0.82 0.18 15.2 8.3 1.20E-08 8.81E-09 6.669

l 54.9 320 0.50 1.60 1.60 0.40 15.1 7.5 4.29E-09 3.41E-09 6.692

m 61.7 320 0.50 4.00 4.00 1.00 15.1 7.6 1.80E-09 1.39E-09 6.709

n 72.2 320 0.55 7.82 3.91 2.18 15.0 6.8 2.40E-10 5.15E-10 6.715

o 80.8 320 0.50 7.99 3.99 2.01 15.2 7.5 9.35E-10 6.91E-10 6.731

p 103.7 319 0.50 24.0 9.60 5.99 15.3 7.7 5.52E-10 2.41E-10 6.758

q 125.6 320 0.50 47.9 9.59 12.1 15.5 7.7 6.71E-10 1.23E-10 6.794

r 147.8 319 0.55 93.9 9.39 26.1 15.6 7.0 4.36E-10 4.81E-11 6.819

s 176.2 319 0.60 228.8 9.15 71.2 15.6 6.3 1.59E-10 1.44E-11 6.833

t 216.7 319 0.60 381.2 9.15 118.8 15.6 6.3 1.80E-10 8.77E-12 6.853

u 249.4 319 0.60 762.8 9.15 237.2 15.8 6.4 1.32E-10 4.47E-12 6.864

1-a 317.5 319 0.60 12 12 7200 15.5 6.2 1.79E-11 4.39E-13 6.874

1-b 365.2 319 0.60 12 12 3600 15.6 6.3 2.47E-11 8.97E-13 6.878

2-a a 416.7 319 0.60 12 12 7200 19.0 7.6 5.51E-11 8.49E-13 6.899

2-b 466 320 0.60 12 12 3600 18.9 7.6 6.42E-11 1.68E-12 6.910

2-c 503.9 320 1 - - - 18.9 - 2.02E-12 - 6.911

a The CGR value was obtained from the later part of the test periods.

Fatigue pre-cracking was carried out with a maximum stress intensity factor of 14-15 MPa m1/2, 

a load ratio of 0.2-0.3, and frequency of 2 Hz. After about 600-μm crack extension, a stable 

crack growth was obtained in test period h, and the measured CGRs were very close to the 

fatigue line. Next, both the rise time and load ratio were gradually increased to promote 

environmentally enhanced cracking, and an elevated CGR became evident in test period r. 

Additional increases in rise time and load ratio produced a further environmental enhancement. 

By the end of test period u, the measured CGR was a factor of 25 higher than the fatigue growth 

123
rate. Figure 86 shows the cyclic CGRs obtained from this sample. The corrosion fatigue 

behavior of this thermally aged specimen seems to be similar to that of its unaged counterpart. 

Both of the CF-8M specimens show a higher degree of sensitivity to environmentally enhanced 

cracking than the CF-3 and CF-8 CASS used in this study. 

After pre-cracking, the test was set at constant load with PPU every 2 hr in test period 1-a. A 

CGR of 1.8 x 10-11 m/s was recorded at a stress intensity factor of 15.5 MPa m1/2. With a shorter 

holding time (PPU every 1 hr), a slightly higher CGR (2.5 x 10-11 m/s) was obtained at the same 

stress intensity level. Next, the constant-load CGR (with PPUs) was measured at a higher stress 

intensity level (~19 MPa m1/2). A slightly higher CGR was once again observed with a shorter 

holding time (2-hr PPU in period 2-a and 1-hr PPU in period 2-b). When the PPU was removed

in test period 2-c, the CGR became much lower. Constrained by test time, the CGR test was 

concluded after 10-μm crack extension. 

Figure 87 shows the SCC CGRs obtained from the unaged and aged CF-8M CASS in this study. 

The CGR values are all well below the NUREG-0313 disposition curve, as expected at this dose

and ECP level. The unaged CF-8M may have performed slightly better than the aged sample. 

However, given the inherent uncertainty of CGR measurements, the difference in SCC CGRs of 

the aged and unaged CF-8M is insignificant. 

(a) 

Figure 85. Crack-length-vs.-time plot for specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite): 

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

5.90

6.00

6.10

6.20

6.30

6.40

6.50

6.60

4

8

12

16

20

24

28

32

0 5 10 15 20 25 30

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen J-1

CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

a b c d

e

f

g

3.0E-8 m/s Crack length

Kmax

124
(b) 

(c) 

(d) 

Figure 85. (Contd.)

6.45

6.50

6.55

6.60

6.65

6.70

6.75

6.80

4

8

12

16

20

24

28

32

30 40 50 60 70

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen J-1

CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

h

i

j

k

l

m

n

Crack length

Kmax

6.70

6.75

6.80

6.85

4

8

12

16

20

24

28

32

80 90 100 110 120 130 140 150

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen J-1

CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

o p

q

r

Crack length

Kmax

6.75

6.80

6.85

6.90

6.95

4

8

12

16

20

24

28

32

160 180 200 220 240

Crack Length (mm)

Kmax (MPa m0.5)

Time (h)

Specimen J-1

CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

s

t

Crack length u

Kmax

125
(e) 

(f) 

Figure 85. (Contd.)

Fracture toughness J-R curve test

After the CGR test, a fracture toughness J-R curve test was carried out on the same sample in the 

test environment. The J and crack extension results are shown in Fig. 88. A power-law fitting of 

the data shows a JR relationship of J = 259∆a0.64, which yields a J value of 106 kJ/m2 at the 0.2-

mm offset line. All J values obtained in this sample were below the Jmax limit. However, one of 

the nine measurements of the final crack size still exceeded the limit. Thus, the J-R curve cannot 

be validated. 

6.85

6.86

6.87

6.88

6.89

6.90

4

8

12

16

20

24

28

32

260 280 300 320 340 360

Crack Length (mm)

K (MPa m0.5)

Time (h)

Specimen J-1

CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

1-a

1-b Crack length

K

6.86

6.88

6.90

6.92

6.94

4

8

12

16

20

24

28

32

380 400 420 440 460 480 500

Crack Length (mm)

K (MPa m0.5)

Time (h)

Specimen J-1

CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.

Low-DO high-purity water, 320oC

2-a

2-b

5.51E-11 m/s

2-c Crack length

K

126
Figure 86. Cyclic CGRs of specimen J-1.

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

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

Test periods: a-e

Test periods: f-u

CGRenv (m/s)

CGRair (m/s)

Specimen J-1

CF-8M, aged, 0.08 dpa.

Low-DO high-purity water

320oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

Test periods with <10 μm 

extension are excluded.

10-11

10-10

10-9

10 15 20 25

Unaged CF-8M, Spec. I-1, PPU 2 hrs

Aged CF-8M, Spec. J-1, PPU 2 hrs

Aged CF-8M, Spec. J-1, PPU 1 hr

Aged CF-8M, Spec. J-1, w/o PPU

CGR (m/s)

K (MPa m1/2)

NUREG-0313

Curve

Low-DO high-purity water, ~320oC, ~0.08 dpa

w/o PPU

127
Figure 88. The J-R curve of specimen J-1.

Fractographic examination

The fracture surface of specimen J-1 was examined with replicas. Figure 89 shows the entire

fracture surface. Two distinct fracture regions, CGR and JR test areas, can be clearly identified. 

The crack front of the CGR test is straight, indicating a well-controlled loading condition during 

the test. The CGR region is relatively flat and shows a clear contrast from the heavily deformed

JR test region. Similar to the unaged CF-8M (specimen I-1), secondary cracks can be seen on 

the fracture surface.

Figure 90 shows an enlarged view of the fracture surface along the sample central line. Ferrite 

phase at the casting dendrite cores can be seen throughout the entire CGR region. Transgranular 

cleavage-like cracking is clearly visible in the pre-cracking region, as shown in Fig. 91. As the

crack advances, deformation steps became less pronounced in some areas, and little plastic 

deformation could be seen within the ferrites phase compared to the surrounding austenite phase

(Fig. 92). In some other areas, however, cleavage-like cracking remained the dominant fracture 

mode (Fig. 93). In the JR test region, the fracture morphology was mostly ductile dimples, 

suggesting heavy plastic flow during the JR test. In some areas, fracture occurred along the 

ferrite core of the columnar dendrites, as shown in Fig. 94. 

0

160

320

480

640

0.0 0.5 1.0 1.5 2.0

J (kJ/m

2)

Crack Extension (mm)

Specimen J-1

CF-8M, aged, 0.08 dpa

Low-DO high-purity water, 320oC

J= 259*∆a0.64

JQ=106 kJ/m2

Jmax

∆amax

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

129
Figure 90. Fracture surface of specimen J-1 along the sample central line.

Machined

notch

CGR test JR test

Fatigue 

cracking

Vermicular 

ferrite 

Dimple 

fracture

Post-JR fatigue 

Crack advance

Fractured 

ferrites

130
Figure 91. Precracking region of specimen J-1. Crack propagation from bottom to top.

131
Figure 92. Ferrite microstructure at the end of CGR test of specimen J-1. Crack propagation from bottom to top.

132
Figure 93. Cleavage-like fracture at the end of CGR test of specimen J-1. Crack propagation from bottom to top.

133
Figure 94. Fracture along ferrite at dendrite core in the JR test region of specimen J-1. Crack propagation from bottom to top.

134
4. DISCUSSION

Eleven unirradiated and irradiated 1/4T-CT specimens prepared from as-received and thermally 

aged CASS materials were tested in either low-DO high-purity water or simulated PWR water at 

~320°C. These specimens were fabricated from CF-3, CF-8, and CF-8M CASS with high ferrite 

contents (more than ~23%). Seven of the specimens were irradiated to 0.08 dpa in the Halden 

reactor. Thermal aging of the CASS samples was conducted at 400°C for 10,000 hr prior to the 

irradiation. This thermal aging treatment had been shown to yield a high degree of 

embrittlement in a previous study.4,39 Crack growth rate tests were performed on the specimens

in low-corrosion-potential environments. Cyclic and constant-load CGR tests were carried out at 

several stress intensity factors to assess the susceptibility of these materials to environmentally 

assisted cracking. The SCC CGRs obtained from the present study are summarized in Table 15. 

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

Material Ferrite

Content

Thermal

Aging

Dose

(dpa)

Sample

ID

Test

Environment

SCC CGR a

K (MPa m1/2) CGR (m/s)

CF-3 24%

Unaged

- A-N1 Low-DO high-purity 18.0 Negligible

0.08 A-1 PWR 23.9 4.8E-11

0.08 A-2 Low-DO high-purity

17.6 2.3E-11

19.6 4.9E-11

19.8 4.9E-12 (w/o PPU)

19.8 4.3E-11

Aged - B-N1 PWR

17.2 2.7E-11

17.1 2.3E-13 (w/o PPU)

0.08 B-1 PWR 22.1 2.8E-11

CF-8 23%

Unaged

- E-N1 Low-DO high-purity

17.5 1.4E-11

17.5 8.1E-12 (w/o PPU)

0.08 E-1 Low-DO high-purity

14.9 1.8E-11

16.8 2.7E-11

Aged - F-N1 Low-DO high-purity

16.5 1.2E-11

16.5 1.2E-11 (w/o PPU)

0.08 F-1 Low-DO high-purity 16.0 2.7E-11

CF-8M 28%

Unaged 0.08 I-1 Low-DO high-purity 17.9 1.9E-11

Aged 0.08 J-1 Low-DO high-purity

15.5 1.8E-11

15.6 2.5E-11

19.0 5.5E-11

18.9 6.4E-11

18.9 2.0E-12 (w/o PPU)

a Unless otherwise noted, SCC CGRs were measured under constant loads with PPU every 1 or 2 hours.

Fracture toughness J-R curve tests were also performed in the current study in the test

environments at ~320°C with environmentally enhanced starter cracks. Table 16 shows the J-R 

curve results along with some previous unirradiated results obtained in air at ~290°C. The 

parameters C and n in the table are the fitting coefficients of the power-law relationship of J = 

135
C∆an. The J value at the 0.2-mm offset line (JQ) is reported for each test. Note that the 

unirradiated specimens tested in air were 1T-CT samples, larger than the specimens used in the 

current study (1/4T-CT). No crack growth rate results in water were available for the 1T-CT 

specimens. Experimental details of the previous unirradiated tests in air can be found in 

references [22, 39].

Table 16. Fracture toughness JR test results for CASS with high ferrite contents.

Material a Ferrite

content

Thermal

aging

Sample

Size Test Env. b Test Temp. b

(°C)

Unirradiated Irradiated (0.08 dpa)

C n JQ (kJ/m2) C n JQ (kJ/m2)

CF-3 24%

Unaged

1/4T Water ~320 536 0.68 320 430 0.64 204

1T Air ~290 756 0.31 700 - - -

Aged

1/4T Water ~320 353 0.66 170 362 0.85 116

1T Air ~290 296 0.51 167 - - -

CF-8 23%

Unaged

1/4T Water ~320 - - > 500 c 359 0.57 183

1T Air ~290 783 0.27 753 - - -

Aged

1/4T Water ~320 395 0.58 220 372 0.62 171

1T Air ~290 396 0.51 242 - - -

CF-8M 28%

Unaged

1/4T Water ~320 - - - 336 0.66 145

1T Air ~290 583 0.45 437 - - -

Aged

1/4T Water ~320 - - - 259 0.64 106

1T Air ~290 274 0.46 156 - - -

a Irradiated unaged and aged materials were exposed to the irradiation temperature (~315°C) for approximately 4320 hr. The 

aging parameter P defined in reference [40] is 1.66, 1.82, and 2.07 for Material CF-3, CF-8, and CF-8M, respectively. Thus, 

the extent of embrittlement caused by the reactor temperature is negligible during the course of the irradiation. 

b All 1/4T-CT specimens were tested in low-corrosion-potential water environments at ~320°C in the current study. All 1T-CT 

specimens were tested in an air atmosphere at ~290°C in a previous study (NUREG/CR 4744, No.7).

c The last data point measured at the end of the test. A J value of ~700 kJ/m2 was estimated by extrapolating the available data to 

the 0.2-mm offset line. 

4.1 Cyclic Crack Growth Rates

Cyclic CGR data obtained from the unaged and aged CASS specimens were analyzed based on a 

superposition model previously developed by Shack and Kassner.38 By assuming that the 

environmental contribution to cyclic CGR is related to fatigue crack growth rate in air, Shack 

and Kassner determined the corrosion fatigue curves of unirradiated wrought and CASS SSs in 

high-purity water containing 0.2 ppm and 8 ppm DO. Using the corrosion fatigue curve of 0.2 

ppm DO as a reference, the best fit curves for each data set of the CASS specimens are 

compared. For the CF-3 specimens with 24% ferrite (Fig. 95a), the five fitting curves are all 

bounded by the line of 0.2-ppm DO, regardless of their irradiation, thermal aging, or test 

conditions. This observation suggests that irradiation does not increase the cracking 

susceptibility of CF-3 at this dose level. The relatively low environmental enhancement in the 

CF-3 can be attributed to the beneficial effect of ferrite in CASS. Several authors have reported 

a better SCC resistance for CASS than wrought SSs in aqueous environments.15,16 

136
Figure 95. Best-fit curves of cyclic CGRs at 0.08-dpa dose: (a) unaged and aged CF-3, (b) 

unaged and aged CF-8, and (c) unaged and aged CF-8M. 

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CGRenv (m/s)

CGRair (m/s)

(a) CF-3, PWR or 

low-DO high-purity water

320oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

Red: Best fit for A-1 data, 

unaged, irr. CF-3 in PWR water.

Blue: Best fit for A-2 data, 

unaged, irr. CF-3 in Low-DO water.

Black: Best fit for B-1 data, 

aged, irr. CF-3 in PWR water.

Purple: Best fit for B-N1 data, 

aged, unirr. CF-3 in PWR water.

Brick: Best fit for A-N1 data, 

unaged, unirr. CF-3 in Low-DO water.

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CGRenv (m/s)

CGRair (m/s)

(b) CF-8, low-DO high-purity water, 

320oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

Red: Best fit for E-1 data, 

unaged, irr. CF-8.

Blue: Best fit for F-1 data, 

aged, irr. CF-8.

Black: Best fit for E-N1 data, 

unaged, unirr. CF-8.

Purple: Best fit for F-N1 data, 

unaged, unirr. CF-8.

137
Figure 95. (Contd.)

As shown in Fig. 95b, the best fit curves of unaged and thermally aged CF-8 are also below the 

bounding line. The similar behaviors between CF-3 and CF-8 suggest that the difference in 

carbon content does not have a significant impact on corrosion fatigue behavior in low-DO highpurity or PWR water. For the CF-8M however, the fitting curves are slightly higher than the 

corrosion fatigue curve, as shown in Fig. 95c. Obviously, the CF-8M samples are more 

susceptible to environmentally assisted cracking under the current test conditions. Figure 96

shows the fitting coefficient “A” (in CGRenv = A·CGRair

0.5) for each data set obtained in the 

current study. While the fitting coefficients for CF-3 and CF-8 are similar, the values for CF-8M 

are much higher. Based on the current data, the corrosion fatigue growth rate of CF-8M is a 

factor of two to three higher than that of CF-3 and CF-8.

As shown in Fig. 96, the cyclic CGRs of thermally aged CASS are generally lower than those of 

unaged CASS, except for the unirradiated CF-3 where different test environments (PWR vs. lowDO water) were used in the different tests. The different cracking responses between the unaged 

and aged CASS suggest a better corrosion fatigue performance of the latter. However, given the 

large scatter in the CGR data, the observed differences between aged and unaged CASS may not 

be statistically significant. Nonetheless, the current study clearly shows that the corrosion 

fatigue behavior is similar between unaged and aged CASS in low-corrosion-potential 

environments. This observation contrasts with the results of unirradiated CASS tested in highDO water environments. The cyclic CGRs of thermally aged CASS were found to be one order 

of magnitude higher than those of unaged alloys in high-DO water (>1 ppm).38 The mechanism 

leading to similar cyclic CGRs between unaged and aged CASS in low-DO environments needs 

to be better understood.

10-11

10-10

10-9

10-8

10-7

10-11 10-10 10-9 10-8 10-7

CGRenv (m/s)

CGRair (m/s)

(c) CF-8M, low-DO high-purity water, 

320oC

CF curve for 0.2 ppm DO 

by Shack & Kassner

Red: Best fit for I-1 data, 

unaged, irr. CF-8M.

Blue: Best fit for J-1 data, 

aged, irr. CF-8M.

138
Figure 96. Fitting coefficient A for the corrosion fatigue superposition model.

4.2 Constant-load Crack Growth Rates

All constant-load CGRs obtained with or without PPU are plotted in Fig. 97. The open symbols 

represent the unaged CASS, and the closed symbols, their thermally aged counterparts. All data 

points are well below the NUREG-0313 line, and moderate CGRs in the range of 10-11 m/s are 

mainly obtained under loading conditions with PPU. Without PPU, the measured CGRs (the 

square symbols in Fig. 97) are much lower, except for the test on the unirradiated and thermally 

aged CF-8, where a dynamic loading condition resulting from pressure fluctuation was present. 

An accurate determination of the low growth rate exhibited by these CASS samples would 

require much longer test times than possible in the current study. In general, the tested CASS 

specimens show good SCC resistance, and neutron irradiation up to 0.08 dpa does not appear to 

elevate their cracking susceptibility significantly in the PWR and low-DO high-purity water 

environments. 

 

The unaged and aged data sets, regardless of their grades, irradiation, and test conditions, are 

fitted to a power-law expression with an exponent of 2.16 (same as the NUREG-0313 curve). As 

shown in Fig. 97, the fitting curve of the aged CASS is just slightly higher (<20%) than that of 

the unaged CASS. However, given the large scatter of the data sets and the inherent uncertainty 

in short-duration CGR tests like these, the difference is statistically insignificant. Thus, thermal 

aging does not appear to affect the cracking susceptibility of the CASS specimens in the low-DO 

high-purity and PWR water. This lack of sensitivity to thermal aging history is consistent with

that observed in cyclic CGR tests. 

139
Figure 97. Constant-load CGRs of the low-dose CASS with more than 23% ferrite in low-DO 

high-purity and PWR water environments. 

A low susceptibility to IASCC is expected for CASS owing to the beneficial effects of ferrite. It 

has been shown that unirradiated CASS samples are more resistant to SCC than wrought SSs in 

high-DO water.15,16 The superior SCC performance of the duplex microstructure may arise 

from the deformation behavior of the ferrite phase. Ferrite is more difficult to deform plastically 

compared with austenite under the same stress level. Using a nano-indentation measurement, 

Wang et al. 42 showed that the hardness of ferrite phase is higher than that of austenite phase in 

CF-8. Furthermore, the austenite is also more noble than the ferrite in corrosion potential 

measurements of single-phase alloys. By delaying the development of heavy plastic deformation 

in ferrite phase, a slip-dissolution mechanism could be hindered, to some extent, in a duplex 

microstructure. Our fractographic examinations support this hypothesis. As shown in the 

micrographs of the CGR test regions (e.g., Figs. 18, 25, 43, 49, 57, 74, 75, 83, and 91), little 

plastic deformation can be seen within the ferrite phase. In contrast, the surrounding austenite 

grains are often heavily deformed. If this mechanism is correct, the beneficial effect of ferrite 

could be diminished, in principle, by thermal aging or irradiation embrittlement. A deteriorated 

fracture resistance of the ferrite grains would accelerate the development of plastic strain in the 

surrounding austenite phase. In fact, elevated SCC CGRs have been observed in a thermally 

aged CF-8M at ~2.4 dpa.31 This observation suggests that the beneficial effect of a duplex 

10-12

10-11

10-10

10-9

10-8

5 10 15 20 25 30

Unaged, PPU 2hr

Unaged, PPU 1 hr

Unaged, Constant-load

Aged, PPU 2 hr

Aged, PPU 1 hr

Aged, Constant-load

CGR (m/s)

K (MPa m1/2)

NUREG-0313

Curve

Unirradiated and 0.08-dpa CF-3, CF-8, and CF-8M, 

tested in low-DO high-purity or PWR water, ~320 oC.

Unaged, Aged, ~K2.16

~K2.16

140
microstructure may be eliminated or greatly reduced by neutron exposure to a sufficiently high 

fluence level. 

4.3 Fracture Toughness 

Figure 98 shows all fracture toughness values (J at 0.2 mm offset) obtained from the current 

study. The blue and brick color bars are for the unirradiated and irradiated CASS specimens, 

respectively. Note that the J value for the unirradiated and unaged CF-8 is an estimated 

minimum (see Section 3.2.2.1 for details). Fracture toughness results of unirradiated CF-8M

tested in air from Ref. [39] are also included in Fig. 98 (green bars). Neutron irradiation, even at 

such a low dose (0.08 dpa), has a significant impact on the fracture toughness of CASS. The 

extent of irradiation embrittlement is much greater for unaged than aged specimens. After 

irradiation, the fracture toughness values of unaged CASS are significantly lower than the 

original unirradiated values. For aged CASS, fracture toughness is also reduced by 20-30% after 

irradiation. Since the comparison tests were performed in identical environments for CF-3 and 

CF-8, the differences between unirradiated and irradiated JR results can only be attributed to 

neutron irradiation. For the CF-8M, no unirradiated control tests were carried out in water at 

~320°C. Thus, we cannot rule out a potential effect of test environment on the fracture 

toughness. However, given the good SCC resistance observed in the CGR tests, it is unlikely 

that that test environment had a significant contribution to the loss of toughness in irradiated 

tests. In addition, the fractographic examinations showed that both irradiated and unirradiated 

specimens had similar fracture morphology (ductile dimples) in JR test regions, suggesting an 

insignificant role of the test environment in the irradiated J-R curve tests. Thus, the differences 

between unirradiated and irradiated JR results for CF-8M are also likely due to the neutron 

irradiation. 

Because the deterioration in fracture toughness developed more rapidly with neutron irradiation

in unaged CASS, the difference in fracture toughness between unaged and aged specimens was

reduced after irradiation. As shown in Fig. 98, the drastically different fracture toughness values 

between unaged and aged specimens (blue and green bars) are lessened after irradiation (brick

bars). This change suggests a dominant role of neutron irradiation (compared to thermal aging) 

in promoting embrittlement in CASS. The rapidly developed irradiation effect in unaged 

materials may also explain the inconsistent observations between the current study and the

previous work discussed in the last section. Shack and Kassner reported that thermal aging can 

considerably decrease the cracking resistance of unirradiated CASS in high-DO high-purity 

water.

38 However, in our study, both corrosion fatigue and SCC of irradiated CASS seem to be

insensitive to thermal aging history (e.g., Figs. 96 and 97). There is no doubt that neutron 

irradiation had introduced detrimental effects in both unaged and aged materials, but not 

necessarily at the same rate. It is possible that the unaged microstructure deteriorated more

quickly than did the aged microstructure at the current dose level. Consequently, the cracking 

behavior between the aged and unaged specimens became similar after irradiation. 

While the irradiation damage seems to be a dominant factor for embrittlement, a combined effect 

of thermal aging and irradiation damage does reduce the fracture resistance of CASS further. As 

shown in Fig. 98, the fracture toughness values of irradiated and aged specimens are

141
approximately 50 kJ/m2 lower than those of unirradiated and aged specimens. The decline of 

fracture resistance in thermally aged CASS samples at such low dose level is unexpected, and 

points toward an interaction between thermal aging and irradiation embrittlement. More 

important, these results show that the kinetics of thermal aging embrittlement could be altered by

irradiation, as could the saturation state (i.e., the lower bound of fracture toughness). This 

finding suggests that the conservatism assumed for thermal aging embrittlement needs to be 

examined closely under neutron irradiation. The current result does not show, however, how the 

ferrite content affects the extent of embrittlement. The samples tested in this study are all highferrite-content CASS materials. If only the changes in ferrite contribute to the embrittlement, the 

combined effect of thermal aging and irradiation damage should vary with the initial ferrite 

content. Additional tests on specimens with lower ferrite contents are needed to understand the

precise role of ferrite in the combined effect of thermal aging and neutron irradiation.

Figure 98. Fracture toughness values of unirradiated and irradiated CASS in unaged and aged 

conditions. Note that most of the results are from 1/4T-CT specimens tested at 

~320°C in water environments. The unirradiated results for CF-8M CASS are from 

1T-CT specimens tested at ~290°C in an air atmosphere.

142
The mechanisms of CASS thermal embrittlement are well understood. 17-20 A miscibility gap in 

the Fe-Cr phase diagram gives rise to a spinodal decomposition of two ferrite phases, a Fe-rich α

phase and a Cr-rich α’ phase. The α’ phase has a slightly different lattice parameter from the 

matrix and, thus, strengthens the ferrite grains and causes the embrittlement. The redistribution 

of Cr within ferrite phase is accompanied by the rearrangement of other alloying elements, which

can lead to additional nucleation and growth of precipitates within the ferrite phase or at the 

ferrite-austenite boundaries. Thus, carbides and Ni-rich G-phase are also found to be the main 

contributors to the thermal aging embrittlement of CASS. Under neutron irradiation, the kinetics 

of these embrittlement mechanisms may be affected.43 The natural miscibility gap could be 

widened, and new temperature-dependent wavelengths could be developed. While no irradiation 

microstructural work has been carried out in the current study, the mechanical test results suggest 

that an accelerated microstructural evolution occurs under neutron irradiation, and the initial 

microstructures of CASS may be a key factor for the evolution of irradiation microstructure. 

Detailed microstructural examinations of irradiation defects, precipitations, segregations, and 

phase stability in the ferrite phase and at austenite-ferrite boundaries would be helpful to explain

the combined effect of thermal aging and irradiation embrittlement. 

143

5. SUMMARY

Crack growth rate and fracture toughness J-R curve tests have been conducted on CF-3, CF-8,

and CF-8M CASS with high ferrite content (>23%). The samples were irradiated in the Halden 

test reactor to a low dose of 0.08 dpa. Both as-received and thermally aged specimens were 

included to show the combined effect of thermal aging and irradiation embrittlement. The CGR 

tests were conducted on irradiated and unirradiated control samples in low-DO high-purity water

or PWR water at 320°C. Following the CGR tests, fracture toughness J-R curve tests were 

performed on the same samples in the test environments. 

Cyclic CGRs and constant-load CGRs were obtained to evaluate the corrosion fatigue and SCC 

resistance of the CASS specimens. In cyclic CGR tests, environmentally 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 the CASS specimens, regardless of 

their thermal aging history or irradiation conditions. In general, the CASS materials showed

good resistance to both corrosion fatigue and SCC before 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 lesser extent than the surrounding austenite phase. This 

observation supports the hypothesis that the beneficial effect of ferrite arises, in part, from the 

high plastic deformation stress in ferrite phase.

All CASS specimens tested in this study failed in a ductile dimple mode during the fracture 

toughness J-R curve tests. Neutron irradiation had a significant impact on the fracture toughness 

of CASS. At 0.08 dpa, the fracture toughness values of unaged specimens were significantly 

lower than the initial unirradiated values. An additional 20-30% reduction in fracture toughness 

was also observed for thermally aged specimens after irradiation. The combined effect of 

thermal aging and irradiation damage can reduce the fracture resistance of CASS to a higher 

extent than any one of them can achieve alone. These results indicate that neutron irradiation 

can affect not only the kinetics of thermal aging embrittlement, but also the saturation state (i.e.,

lower bound values of fracture toughness). For this reason, the effects of neutron irradiation 

should be considered when the degree of thermal aging embrittlement is evaluated for CASS 

components. 

145

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NUREG/CR

-7184 Crack Growth Rate and Fracture Toughness Tests 

on Irradiated Cast Stainless Steels