ML15160A157

NYS000488B - NUREG/CR-7184 (ANL-12/56), Chen, Et Al., Crack Growth Rate and Fracture Toughness Tests on Irradiated Cast Stainless Steels (Revised December 2014) (ML14356A136)

ML15160A157
Person / Time
Site:	Indian Point
Issue date:	12/31/2014
From:	State of NY, Office of the Attorney General
To:	Atomic Safety and Licensing Board Panel
SECY RAS
References
RAS 27898, ASLBP 07-858-03-LR-BD01, 50-247-LR, 50-286-LR
Download: ML15160A157 (83)

Text

67 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 CF-8 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% 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-086.172 b 3.9 319 0.21 0.35 0.35 0.15 14.1 11.1 4.98E-08 4.05E-086.248 c 9.2 319 0.32 0.33 0.33 0.17 13.3 9.1 1.67E-10 2.54E-086.252 d 92.3 - 94.2 319 0.24 0.35 0.35 0.15 13.6 10.4 5.84E-11 3.43E-086.250 e a 97.7 319 0.20 0.36 0.36 0.14 14.6 11.7 1.45E-08 4.68E-086.296 f 100.8 320 0.30 0.35 0.35 0.15 15.3 10.7 9.76E-09 4.10E-086.342 g 116.2 - 119.6 320 0.20 0.36 0.36 0.14 16.1 13.0 5.54E-08 6.45E-086.567 h 124.2 320 0.40 0.33 0.33 0.17 16.0 9.6 2.41E-09 3.36E-086.595 i 140.4 - 148.5 319 0.32 0.35 0.35 0.15 16.4 11.2 2.19E-10 4.84E-086.596 j 165.1 - 165.8 319 0.36 0.27 0.27 0.23 15.9 10.1 6.28E-08 4.61E-086.643 k1 167.1 - 170.1 319 0.43 0.52 0.52 0.48 16.4 9.3 2.36E-09 1.97E-086.693 k2 170.5 - 174.1 319 0.42 0.53 0.53 0.47 17.1 9.9 2.14E-08 2.35E-086.766 l a 191.6 319 0.50 1.22 1.22 1.28 17.2 8.6 9.12E-09 6.91E-096.823 m 213 319 0.56 2.27 2.27 2.73 17.0 7.6 1.59E-09 2.56E-096.854 n a 233.7 319 0.60 6.43 2.14 8.57 17.1 6.8 6.00E-10 6.77E-106.865 o 260.3 319 0.59 12.9 2.15 17.1 16.9 6.9 1.13E-11 3.43E-106.867 p 284.4 319 0.54 13.7 2.29 16.3 16.9 7.7 4.19E-10 4.49E-106.880 q 309.2 319 0.54 27.6 2.30 32.4 17.2 7.8 4.07E-10 2.35E-106.896 r 333 319 0.54 55.4 2.31 64.6 17.2 7.9 4.44E-10 1.22E-106.912 s 358.4 319 0.53 139.1 5.56 160.9 17.2 8.0 2.88E-10 5.00E-116.923 t 404.4 319 0.54 232.2 5.57 267.8 17.6 8.1 2.97E-10 3.11E-116.945 u 429.1 320 0.53 464.2 5.57 535.8 17.4 8.1 7.04E-11 1.55E-116.949 1a 553.3 319 0.55 12 12 7200 17.5 7.9 1.43E-11 9.27E-136.959 1b 718.7 320 1 - - - 17.5 0.3 8.09E 6.962 a The CGR value was obtained from the later part of the test period.

Field Co 68 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 m 1/2. 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 cr acking 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) 6.006.106.206.306.4048 121620242832051015Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. E-N1, CF-8, 23% unagedLow-DO high-purity water, ~319 oCa, R=0.21 Hzb, R=0.21 Hzc, R=0.31 HzHeld at a low K 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.

Field Co 69 (b) 6.106.156.206.256.306.35 6.40 6.4548 1216202428329095100105Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. E-N1, CF-8, 23% unagedLow-DO high-purity water, ~319 oCd, R=0.21 Hze, R=0.31 Hzf, R=0.31 HzHeld at a low KHeld at a low KRecheck compliance (c) 6.306.406.506.606.7048 121620242832120130140150160Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. E-N1, CF-8, 23% unagedLow-DO high-purity water, ~319 oCg, R=0.21 Hzh, R=0.41 HzHeld at a low Ki, R=0.31 HzHeld at a low K (d) 6.506.55 6.606.656.70 6.756.806.85 6.9048 121620242832170180190200210Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. E-N1, CF-8, 23% unagedLow-DO high-purity water, ~319 oCj, R=0.31 Hzk1, R=0.40.5 Hzk2, R=0.40.5 HzCheck compliancel, R=0.50.2 Hzm, R=0.550.1 Hz Figure 44. (Contd.)

70 (e) 6.8006.8506.90048 121620242832220240260280300Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. E-N1, CF-8, 23% unagedLow-DO high-purity water, ~319 oCn, R=0.615s/up, 5s/downo, R=0.630s/up, 5s/downp, R=0.5530s/up, 5s/downq, R=0.5560s/up, 5s/down (f) 6.8606.880 6.9006.9206.9406.9606.98048 121620242832320340360380400420Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. E-N1, CF-8, 23% unagedLow-DO high-purity water, ~319 oCr, R=0.55120s/up, 5s/downs, R=0.55300s/up, 12s/downt, R=0.55500s/up, 12s/downComputer crashedu, R=0.551000s/up, 12s/down (g) 6.9006.920 6.9406.9606.980 7.00048 121620242832440480520560600640680720Crack Length (mm)K (MPa m0.5)Time (h)Spec. E-N1, CF-8, 23% unagedLow-DO high-purity water, ~319 oC1a,PPU, 2hr1b,Constant Figure 44. (Contd.)

71 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-fTest periods: g-iTest periods: j-oTest periods: p-uCGRenv (m/s)CGRair (m/s)Specimen E-N1 CF-8, unaged, unirradiated Low-DO high-purity water~319oCCF curve for 0.2 ppm DO by Shack & Kassner Figure 45. Cyclic CGRs of specimen E-N1.

Fracture toughness J-R curve test

Following the CGR test, a fracture toughness J-R curv e test was performed on this sample in the same low-DO high-purity water environment. The te st was conducted at a cons tant strain rate of 0.43 µm/s, and the crack extension was measured with the DCPD method. Before each DCPD measurement, the stress was allowed to relax at a constant displacement for 30 s. Due to the low flow stress and high ductility, sign ificant 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 avai lable 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/m

2. A J value greater than 700 kJ/m 2 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. Field Co 72 04008000.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen E-N1CF-8, unaged, unirradiatedLow-DO high-purity water, ~318 oC> 700 kJ/m 2Jmaxamax~500 kJ/m 2 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. Tr ansgranular cracking and ductile dimples are the dominant morphologies for the CG R and JR test regions, respectiv ely. For the CGR test region, heavy deformation ledges resulting from fatigue loading can be seen close to the machined notch (Fig. 48). Fractured ferrites with little plastic deformation are more evident at the later stage of the CGR test (Fig. 49). The br ittle fracture is not visible during the JR test, and ductile dimples are the main fracture morphology in the JR test re gion (Fig. 50). Only a na rrow band of JR test region can be seen on the fracture surface, which is consistent with the high ductility observed in this sample. Field Co 73 Figure 47. Fracture surface of specimen E-N1 tested in low-DO high-purity water.

Field Co 74 Figure 48. Transgranular fracture at the be ginning of the precracking for specimen E-N1. Crack pr opagation from bottom to top

. Field Co 75 Figure 49. Transgranular fracture at the e nd of the CGR test for specimen E-N1.

Crack propagation fro m bottom to top.

aField Co 76 Figure 49. (Contd.)

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

Field Co 78 3.2.1.2 Irradiated specimen E-1 te sted 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% 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 m 1/2, 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 m 1/2. The measured CGR gradually increased in test period c after a short period of sluggish growth, a nd the final CGR was about 2.0 x 10

-8 m/s. The rise time and load ratio were increased furthe r 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 wa s decreased to ~14-15 MPa m 1/2. Environmental enhancement appears Field Co 79 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) 6.006.106.206.306.406.506.606.704812 16202428320510152025Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen E-1CF-8, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCabcd2.54E-8 m/s1.57E-8 m/s4.03E-9 m/s KmaxCrack length (b) 6.556.606.656.706.756.806.856.90481216202428322530354045505560Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen E-1CF-8, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCe1.59E-8 m/s6.48E-9 m/s fKmaxCrack length 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.

Field Co 80 (c) 6.756.806.856.906.957.004812 162024283260708090100110120130140Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen E-1CF-8, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCghijKmaxCrack length (d) 6.806.856.906.95 7.00 7.057.1048121620242832160180200220240260Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen E-1CF-8, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCklmnoKmaxCrack length (e) 6.856.906.957.00 7.057.107.157.204812 1620242832280320360400440480520560Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen E-1CF-8, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCpq12KmaxCrack length Figure 51. (Contd.)

81 10-1110-1010-910-810-710-1110-1010-910-810-7CGRenv (m/s)CGRair (m/s)Specimen E-1CF-8, unaged, 0.08 dpa.

Low-DO high-purity water 320oC9 µm extensionCF curve for 0.2 ppm DO by Shack & Kassner Figure 52. Cyclic CGRs of specimen E-1.

Following the pre-cracking, the test was set at constant load w ith PPU every 2 hr. The stress intensity factor was about 15 MPa m 1/2. A SCC CGR of 1.8 x 10

-11 m/s was measured over a 10-µm crack extension. The stress intens ity level was increased to ~17 MPa m 1/2 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 =

359a0.57. The J value at the 0.2-mm offset line is 183 kJ/m 2 for this sample. The crack extension was heavily curved in this sample, a nd 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.

Field Co 82 01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen E-1CF-8, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCJ= 359*a0.57JQ=183 kJ/m 2Jmaxamax 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 pr e-cracking region. As th e 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 pr e-cracking region. Large deform ation 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 leadi ng to a ductile fracture.

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

Field Co 84 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 Field Co 85 Figure 56. Cleavage-like cracking at the beginning of the CGR test of specimen E-1. Crack propaga tion from botto m to top.

Field Co 86 Figure 57. Cyclic CGR test re gion of specimen E-1. Crack pr opagation from bottom to top.

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

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

Field Co 89 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% fe rrite. 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 pl ot 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 m 1/2. After an initial sl ow 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 we re 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 re sponse in the low-DO high-purity water.

Table 11. Crack growth rates of specimen F-N1 (unirradiated, aged CF-8 with 23% 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 m a 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 6.702 a The CGR value was obtained from the later part of the test period.

Field Co 90 (a) 6.006.106.206.306.404812 16 20242832010203040Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. F-N1, CF-8, 23% agedLow-DO high-purity water, ~319 oCa, R=0.21 Hzb, R=0.30.5 Hzc, R=0.40.1 Hzd, R=0.40.25 Hz (b) 6.306.35 6.406.456.504812 16 202428325060708090100Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. F-N1, CF-8, 23% agedLow-DO high-purity water, ~319 oCg, R=0.45s up, 2s downh, R=0.410s up, 2s downf, R=0.40.25Hze, R=0.350.5Hz (c) 6.406.456.506.55 6.60 6.656.706.7548 121620242832100120140160180200220240Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. F-N1, CF-8, 23% agedLow-DO high-purity water, ~319 oCi, R=0.45s up, 2s downj, R=0.410s up, 2s downk, R=0.4520s up, 5s downUnstable pressurel, R=0.560s up, 12s downm, R=0.4860s up, 12s downUnstable pressure 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. Field Co 91 (d) 6.606.656.70 6.754812 16 20242832260280300320340360380Crack Length (mm)

Kmax (MPa m0.5)Time (h)Spec. F-N1, CF-8, 23% agedLow-DO high-purity water, ~319 oCn, R=0.5150s up, 12s downo, R=0.5300s up, 12s downp, R=0.5500s up, 12s downq, R=0.51000s up, 12s down (e) 6.6706.6806.6906.700 6.7106.7204812 16 20242832400440480520560600Crack Length (mm)K (MPa m0.5)Time (h)Spec. F-N1, CF-8, 23% agedLow-DO high-purity water, ~319 oC1a, PPU, 2 hr hold1b, Constant-loadUnstable pressure Figure 60. (Contd.)

After more than ~700-

µm crack extension under cyclic loadi ng, the test was transitioned to a constant load with PPU every 2 hr (test period 1a). Under this condi tion, a CGR of 1.2x10-11 m/s was obtained at ~16.5 MPa m 1/2 after an initial shor t period of rapid growth. Next, the PPU was removed, and the test was held at a near constant-K c ondition (~16.5 MPa m 1/2) for a total of

~140 hr (test period 1b). Unlike the other tests conducte d 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 wa s 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.

92 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-dTest periods: e-lTest periods: m-qCGRenv (m/s)CGRair (m/s)Specimen F-N1 CF-8, aged, unirradiated Low-DO high-purity water~319oCCF curve for 0.2 ppm DO by Shack & Kassner 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 c onstant 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/m

2. 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 J-

R curve data cannot be validated for this test since the requirements of the crack front straightness and Jmax were violated.

Field Co 93 0160320480 6400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen F-N1CF-8, aged, unirradiatedLow-DO high-purity water, ~318 oCJ= 395.3*a0.58JQ=220 kJ/m 2Jmaxamax 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, th e 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 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 domina nt morphology (Fig. 67). Field Co 94 Figure 63. Fracture surface of specimen F-N1 tested in low-DO high-purity water.

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

CGR test JR test TG Dimple fracturePost JR fatigue Crack advance Machined notch Field Co 96 Figure 65. Transgranular fracture in the CG R test of specimen F-N1: (a) in the prec racking region and (b) at the end of CGR test. Crack advance direction from bottom to top.

aField Co 97 Figure 65. (Contd.)

b 98 Figure 66. Transition region from CGR to J-R curve tests of specimen F-N1. Crac k advance direction from bottom to top.

a Field Co 99 Figure 66. (Contd.)

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

Field Co 101 3.2.2.2 Irradiated specimen F-1 te sted 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 cr ack-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 m 1/2, 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 cr acking 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 gr adually 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 se t 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 m 1/2 (Fig. 70). This growth rate is about a factor of thr ee 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% 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 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 Field Co 102 (a) 6.006.10 6.20 6.306.406.504812 16202428320510152025Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen F-1CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oCabcdefKmaxCrack length (b) 6.456.506.556.60 6.65 6.706.7548121620242832406080100120Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen F-1CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oCghijKmaxCrack length (c) 6.606.65 6.70 6.756.804812 1620242832120140160180200220240260280Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen F-1CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oCklmComputer crashed.KmaxCrack length Figure 68. Crack-length-vs.-time plot for specimen F-1 (0.08-dpa ag ed CF-8 with 23% ferrite): test periods (a) a-f, (b) g-j, (c) k-m, and (d) 1.

Field Co 103 (d) 6.656.706.75 6.804812 16 20242832300320340360380Crack Length (mm)K (MPa m0.5)Time (h)Specimen F-1CF-8, aged 10,000 hr @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oC1KCrack length Figure 68. (Contd.)

10-1110-1010-910-810-710-1110-1010-910-810-7CGRenv (m/s)CGRair (m/s)Specimen F-1CF-8, aged, 0.08 dpa.

Low-DO high-purity water 320oC9 µm extensionCF curve for 0.2 ppm DO by Shack & Kassner 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 = 372a0.62. The estimated J value at the 0.2-mm offset line is 171 kJ/m

2. Note that the J-R curve data cannot be validated because one of the nine measurements of the final Field Co 104 crack size did not meet the requirements. Some J va lues 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 pre-cracking 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 au stenitic 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).

10-1310-1210-1110-1010-910-810152025Spec. E-N1, unirr., PPU 2 hr, Low-DO waterSpec. E-N1, unirr., w/o PPU, Low-DO waterSpec. E-1, 0.08 dpa, PPU 2 hr, Low-DO water Spec. F-N1, unirr., PPU 2 hr, Low-DO waterSpec. F-N1, unirr., w/o PPU, Low-DO waterSpec. F-1, 0.08 dpa, PPU 2 hr, Low-DO waterCGR (m/s)K (MPa m1/2)NUREG-0313CurveCASS CF-8 with 23% ferrite low-DO high-purity water318 - 320 oCOpen = UnagedClosed = AgedBlue = UnirradiatedRed = 0.08 dpa Figure 70. SCC CGRs of unaged and aged CF-8 CASS with 23% ferrite.

Field Co 105 01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen F-1CF-8, aged, 0.08 dpaLow-DO high-purity water, 320 oCJ= 372*a0.62JQ=171 kJ/m 2Jmaxamax Figure 71. The J-R curve of specimen F-1.

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

Field Co 107 Figure 73. Fracture surface of specimen F-1 along the sample central line. Machined notchCGR test JR test Cleavage-like crackingVermicular

ferriteDimple fracture Post JR fatigue Crack advance Field Co 108 Figure 74. Fracture surface of the CGR region in specimen F-1. Crack propagation from bottom to top. Field Co 109 Figure 75. Deformation steps in austenite grain around ferrite pha se in the CGR test region of specimen F-1. Crack propagatio n from bottom to top.

Field Co 110 Figure 76. Dimple fracture in the JR test region of specimen F-1. Cr ack propagation from bottom to top.

Field Co 111 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) irra diated 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 m 1/2, 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 starte d 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 loa d, a hydraulic pump tri pped. 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 da ta 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 compar ed 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 m 1/2 over 26-µm crack extension. This SCC CGR is stil l significantly lower than the NUREG-0313 curve.

112 Table 13. Crack growth rates of specimen I-1 (0.08-dpa unaged CF-8M with 28% ferrite) in a low-DO high-purity water environment.

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. Field Co 113 (a) 5.906.006.106.206.306.404812 16202428320510152025Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCabcd5.09E-8 m/s efKmaxCrack length (b) 6.256.306.356.406.456.506.556.604812162024283228323640444852Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCghCheck frictionijkKmaxCrack length (c) 6.486.526.566.606.64 6.684812 16202428326080100120140160180Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oClmno3.1E-10 m/s KmaxCrack length 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. Field Co 114 (d) 6.506.556.606.65 6.70 6.75481216 20 242832200250300350Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCqprHydraulic pump trippedKmaxCrack length (e) 6.656.706.756.806.8548121620242832400420440460480500520Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCs2Hydraulic pump trippedtuvs1KmaxCrack length (f) 6.706.756.806.856.9048121620242832530540550560570580590600Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCwxyzKmaxCrack length Figure 77. (Contd.)

115 (g) 6.756.806.856.906.957.004812 1620242832600620640660680700Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oCaaabacKmaxCrack length (h) 6.806.856.90 6.957.0048 121620242832700750800850Crack Length (mm)K (MPa m0.5)Time (h)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water, 320 oC1KCrack length 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 = 336a0.66. 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.

116 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-fTest periods: g-rTest periods: s-vTest periods: w-acCGRenv (m/s)CGRair (m/s)Specimen I-1CF-8M, unaged, 0.08 dpa.Low-DO high-purity water320oCCF curve for 0.2 ppm DO by Shack & KassnerTest periods with <10

µm extension are excluded.

Figure 78. Cyclic CGRs of specimen I-1.

01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen I-1CF-8M, unaged, 0.08 dpaLow-DO high-purity water, 320 oCJ= 336*a0.66JQ=145 kJ/m 2Jmaxamax Figure 79. The J-R curve of specimen I-1.

Field CoField Co 117 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 I-

1. The crack front is relatively straight, indica ting a well-controlled lo ading 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 seconda ry 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 morpho logy, suggesting heavy plastic deformation prior to fracture (Fig. 84).

118 Crack advanceDelta ferrite in dendritesCGRJRDimplesSecondary crackingAir bubbles Crack advanceDelta ferrite in dendritesCGRJRDimplesSecondary crackingAir bubbles Figure 80. Fracture surface of specimen I-1 tested in low-DO high-purity water.

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

Machined notchCGR test JR test Secondary crackingVermicular

ferriteDimple fracturePost-JR fatigue Crack advance Field Co 120 Figure 82. Precracking region in the CGR test of specimen I-1.

Crack propagation from bottom to top.

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

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

Field Co 123 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 dos

e. The CGR test conditions and results are summarized in Table 14, and a crack-le ngth history plot is shown in Fig. 85.

Table 14. Crack growth rates of specimen J-1 (0.08-dpa aged CF-8M with 28% ferrite) in a low-DO high-purity water environment.

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 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 m 1/2, a load ratio of 0.2-0.3, and freque ncy 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 Field Co 124 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 m 1/2. With a shorter holding time (PPU every 1 hr), a slightly high er 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 m 1/2). 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 di sposition 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) 5.906.006.106.20 6.306.406.506.604812 1620242832051015202530Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen J-1CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oCabcdefg3.0E-8 m/sCrack length Kmax 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.

Field Co 125 (b) 6.456.506.556.60 6.656.706.756.804812 16202428323040506070Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen J-1CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oChijklmnCrack length Kmax (c) 6.706.756.806.85481216202428328090100110120130140150Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen J-1CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oCopqrCrack length Kmax (d) 6.756.80 6.85 6.906.954812 1620242832160180200220240Crack Length (mm)

Kmax (MPa m0.5)Time (h)Specimen J-1CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oCstuCrack length Kmax Figure 85. (Contd.)

126 (e) 6.856.866.876.88 6.896.904812 16 20242832260280300320340360Crack Length (mm)K (MPa m0.5)Time (h)Specimen J-1CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oC1-a1-bCrack length K (f) 6.866.886.906.926.944812 16 20242832380400420440460480500Crack Length (mm)K (MPa m0.5)Time (h)Specimen J-1CF-8M, aged 10,000 hrs @ 400°C, 0.08 dpa.Low-DO high-purity water, 320 oC2-a2-b5.51E-11 m/s2-cCrack length K 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 = 259a0.64, which yields a J value of 106 kJ/m 2 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.

127 10-1110-1010-910-810-710-1110-1010-910-810-7Test periods: a-eTest periods: f-uCGRenv (m/s)CGRair (m/s)Specimen J-1CF-8M, aged, 0.08 dpa.Low-DO high-purity water320oCCF curve for 0.2 ppm DO by Shack & KassnerTest periods with <10

µm extension are excluded.

Figure 86. Cyclic CGRs of specimen J-1.

10-1110-1010-910152025Unaged CF-8M, Spec. I-1, PPU 2 hrsAged CF-8M, Spec. J-1, PPU 2 hrsAged CF-8M, Spec. J-1, PPU 1 hrAged CF-8M, Spec. J-1, w/o PPUCGR (m/s)K (MPa m1/2)NUREG-0313CurveLow-DO high-purity water, ~320 oC, ~0.08 dpaw/o PPU Figure 87. SCC CGRs of unaged and aged CF-8M CASS, irradiated to 0.08 dpa.

Field CoField Co 128 01603204806400.00.51.01.52.0J (kJ/m2)Crack Extension (mm)Specimen J-1CF-8M, aged, 0.08 dpaLow-DO high-purity water, 320 oCJ= 259*a0.64JQ=106 kJ/m 2Jmaxamax 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-controlle d 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 co res can be seen thr oughout the entire CGR region. Transgranular cleavage-like cracking is clearly visible in the pre-cracking regi on, 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 mos tly ductile dimples, suggesting heavy plastic flow during the JR test. In some ar eas, fracture occurred along the ferrite core of the columnar dendrites, as shown in Fig. 94. Field Co 129 Crack advanceDelta ferrite in dendritesCGRJRDimplesSecondary crackingCrack advanceDelta ferrite in dendritesCGRJRDimplesSecondary cracking Figure 89. Fracture surface of specimen J-1 tested in low-DO high-purity water.

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

Machined notchCGR test JR test Fatigue crackingVermicular

ferrite Dimple fracturePost-JR fatigue Crack advance Fractured ferrites Field Co 131 Figure 91. Precracking region of specimen J-1. Crack pr opagation from bottom to top.

Field Co 132 Figure 92. Ferrite microstructure at the end of CGR test of specimen J-1.

Crack propagation from bottom to top.

Field Co 133 Figure 93. Cleavage-like fracture at the end of CGR test of specimen J-

1. Crack propagati on from bottom to top.

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

p. Field Co 135 4 Discussion Eleven unirradiated and irradiated 1/4T-CT specimens prepared from as-received and thermally aged CASS materials were tested in either lo w-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.

44,3939 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 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.

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 unirradia ted results obtained in air at ~290°C. The Formatteand gramFormatteand gramField Co 136 parameters C and n in the table are the fitting coefficien ts of the power-law relationship of J =

Can. The J value at the 0.2-mm offset line (J Q) 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 prev ious unirradiated tests in air can be found in references [22, 39].

Table 16. Fracture toughness JR test resu lts for CASS with high ferrite contents.

Material a Ferrite content Thermal aging Sample Size Test Env.

bTest 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 0.68320 430 0.64 204 1T Air ~290 756 0.31700 - - -

Aged 1/4T Water ~320 353 0.66170 362 0.85 116 1T Air ~290 296 0.51167 - - -

CF-8 23% Unaged 1/4T Water ~320 - - > 500 c 359 0.57 183 1T Air ~290 783 0.27753 - - -

Aged 1/4T Water ~320 395 0.58220 372 0.62 171 1T Air ~290 396 0.51242 - - -

CF-8M 28% Unaged 1/4T Water ~320 - - - 336 0.66 145 1T Air ~290 583 0.45437 - - -

Aged 1/4T Water ~320 - - - 259 0.64 106 1T Air ~290 274 0.46156 - - -

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 [

4040] 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/m 2 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.

3838 By assuming that the environmental contribution to cyclic CGR is relate d to fatigue crack growth rate in air, Shack and Kassner determined the corrosion fatigue cu rves 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 irradia tion, 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.1515,1616 Field CoFormattegrammarFormattegrammarFormatteand gramFormatteand gram 137 10-1110-1010-910-810-710-1110-1010-910-810-7CGRenv (m/s)CGRair (m/s)(a) CF-3, PWR or low-DO high-purity water320oCCF curve for 0.2 ppm DO by Shack & KassnerRed: 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.

10-1110-1010-910-810-710-1110-1010-910-810-7CGRenv (m/s)CGRair (m/s)(b) CF-8, low-DO high-purity water, 320oCCF curve for 0.2 ppm DO by Shack & KassnerRed: 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.

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. Field Co 138 10-1110-1010-910-810-710-1110-1010-910-810-7CGRenv (m/s)CGRair (m/s)(c) CF-8M, low-DO high-purity water, 320oCCF curve for 0.2 ppm DO by Shack & KassnerRed: Best fit for I-1 data, unaged, irr. CF-8M.Blue: Best fit for J-1 data, aged, irr. CF-8M.

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 fa tigue behavior in low-DO high-purity or PWR water. For the CF-8M however, th e fitting curves are s lightly 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 CGR env = A*CGRair0.5) 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, th e corrosion fatigue growth rate of CF-8M is a factor of two to three higher th an 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. low-DO water) were used in the different tests. Th e different cracking resp onses 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, th e current study clearly s hows 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 high-DO 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).3838 The mechanism leading to similar cyclic CGRs between unaged and aged CASS in low-DO environments needs to be better understood.

Formatteand gram 139 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 thei r grades, irradiation, a nd test conditions, are fitted to a power-law expression with an expone nt of 2.16 (same as the NUREG-0313 curve). As shown in Fig. 97, the fitting curve of the aged CA SS is just slightly highe r (<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 Field Co 140 high-purity and PWR water. This lack of sensitivity to thermal ag ing history is consistent with that observed in cyclic CGR tests.

10-1210-1110-1010-910-851015202530Unaged, PPU 2hrUnaged, PPU 1 hrUnaged, Constant-loadAged, PPU 2 hrAged, PPU 1 hrAged, Constant-loadCGR (m/s)K (MPa m1/2)NUREG-0313CurveUnirradiated and 0.08-dpa CF-3, CF-8, and CF-8M, tested in low-DO high-purity or PWR water, ~320 oC.Unaged,

~K2.16Aged, ~K2.16 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.1515,1616 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.

4242 showed that the hardness of ferrite phase is higher than that of austenite phase in CF-8. Furthermore, the austenite is also mo re 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 ferr ite phase. In contrast

, the surrounding austenite grains are often heavily deformed. If this mechanism is correct, the benefi cial 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 Field CoFormatteand gramFormatteand gramFormattegrammar 141 surrounding austenite phase. In fact, elevated SCC CGRs have been observed in a thermally aged CF-8M at ~2.4 dpa.3131 This observation suggests that the beneficial effect of a duplex 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 th e 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). Fr acture 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 t oughness of CASS. The extent of irradiation embrittlement is much greater for unaged than aged specimens. After irradiation, the fracture toughness values of una ged 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 irra diated JR results can only be attributed to neutron irradiation. For the CF-8 M, 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 examinati ons 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 irradiat ed JR results for CF-8M are al so 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 t oughness between unaged and aged specimens was reduced after irradiation. As shown in Fig. 98, the drastically di fferent 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 th e current study and the previous work discussed in the last section. Shack and Kassner reported that thermal aging can considerably decrease the cracking resistan ce of unirradiated CASS in high-DO high-purity water.3838 However, in our study, both corrosion fatigue and SCC of irradiated CASS seem to be insensitive to thermal aging history (e.g., Fi gs. 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 micros tructure at the current dose le vel. Consequently, the cracking behavior between the aged and unaged specimens became similar after irradiation.

Formatteand gramFormatteand gram 142 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 approximately 50 kJ/m 2 lower than those of unirradiated and aged specimens. The decline of fracture resistance in thermally aged CASS sample s 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. Th e current result does not show, however, how the ferrite content affects the extent of embrittlement. The samples tested in this study are all high-ferrite-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 irradiat ed CASS in unaged and aged conditions. Note that most of the results are from 1/4T-CT specimens tested at Field Co 143 ~320°C in water environments. The unirrad iated results for CF-8M CASS are from 1T-CT specimens tested at ~290°C in an air atmosphere.

The mechanisms of CASS thermal embrittlement are well understood. 1717-2020 A miscibility gap in the Fe-Cr phase diagram gives rise to a spinodal decomposition of two ferrite phases, a Fe-rich phase and a Cr-rich ' phase. The ' 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.4343 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-fe rrite boundaries would be helpful to explain the combined effect of thermal aging and irradiation embrittlement.

Formatteand gramFormatteand gramFormatteand gram

145 5 Summary Crack growth rate and fracture toughness J-R cu rve tests have been conducted on CF-3, CF-8, and CF-8M CASS with high ferrite content (>23%). The samples we re 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 te sts, fracture toughness J-R curve tests were performed on the same samples in the test environments.

Cyclic CGRs and constant-load CGRs were obtai ned 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 ex tent than the surrounding austenite phase. This observation supports the hypothesis that the beneficial effect of ferrite arises, in part, from the high plastic deformation st ress 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 reduc e the fracture resistance of CASS to a higher extent than any one of them can achieve alone. These results indicate th at neutron irradiation can affect not only the kinetics of thermal aging embrittlement, but also the saturation state (i.e.,

lower bound values of fracture t oughness). For this reason, the e ffects of neutron irradiation should be considered when the degree of thermal aging embrittlement is evaluated for CASS components.

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