ISSN 2075-1133, Inorganic Materials: Applied Research, 2017, Vol. 8, No. 6, pp. 927–935. © Pleiades Publishing, Ltd., 2017. Original Russian Text © A.D. Erak, E.A. Kuleshova, S.A. Bubiakin, A.P. Bandura, D.A. Zhurko, 2016, published in Voprosy Materialovedeniya, 2016, No. 1(85), pp. 180–191.
RADIATION MATERIALS SCIENCE
Comparative Studies of Brittle Fracture Mechanism for Standard and Reconstructed CT Specimens Made of VVER-1000 RPV Materials A. D. Eraka, *, E. A. Kuleshovaa, b, S. A. Bubiakina, A. P. Banduraa, and D. A. Zhurkoa aNational
Research Center Kurchatov Institute, Kurchatov Centre for Nuclear Technologies, Institute for Reactor Materials and Technologies, Moscow, Russia b National Research Nuclear University MEPhI, Moscow, Russia *e-mail:
[email protected] Received November 30, 2015
Abstract⎯Before 2002, SE(B) specimens were included in surveillance specimen sets for operating VVER1000 reactors. The transition from SE(B) specimens to CT specimens in the fracture toughness tests is caused by the necessity to reduce the scatter and the conservativeness of results caused by the scale increase and the loading scheme. To make such a transition possible, SE(B) specimens were reconstructed into CT specimens. The comparative fractographic analysis of standard and reconstructed CT specimens after the fracture toughness tests shows that the crack initiation in both types starts from the “origin.” The types of “origins” and their ratios do not change, and the test results and the fractographic analysis data for standard and reconstructed CT specimens are described by one analytical dependence CTOD(CID) for each material and state. Keywords: VVER-1000 reactor vessel, brittle fracture, test results, comparative fractographic analysis, standard and reconstructed CT specimens. DOI: 10.1134/S207511331706003X
INTRODUCTION For the reliable evaluation of the current state and the determination of service life of a VVER-1000 reactor pressure vessel (RPV), direct data on the fracture toughness of materials are necessary for the calculation of the resistance of the RPV to brittle fracture. Reactors commissioned before 2002 were fracture toughness tested using SE(B) specimens of a small size (cross section of 10 × 10 mm2), which are included into the surveillance specimen programs for operating VVER-1000 reactor pressure vessels. This is related to the design of container assemblies of surveillance specimens and their locations in the reactor pressure vessel limiting the volume of metal for tests. For VVER-1000 reactor pressure vessels commissioned after 2002 and for all the new projects of reactor units, surveillance specimens are located on the inner surface of the reactor pressure vessel in flat containers; thus, full-scale CT-0.5 surveillance specimens can be used. However, to date, the experimental data on the fracture toughness have been collected using smallsize SE(B) specimens. In [1, 2], it was shown that the fracture toughness curve parameters determined on the basis of small-size specimens give underestimated results for the reference brittleness temperature T0 and show a wide dispersion of data compared to CT specimens.
Thus, with consideration of the necessity of broadening the database on the fracture toughness of pressure vessel steel, the technique for the reconstruction of CT specimens was developed [3, 4] using metal pieces of Charpy surveillance specimens with a V notch or with a sharp crack tested previously (Fig. 1). To provide the correct KJc values, the permissible yield point values of the holder material were determined depending on the yield point of material of the central insertion, under which the stress-strain state at the crack tip in a reconstructed and a standard specimens is almost identical. The sequence of welding in reconstructed CT specimens was chosen from the point of view of minimizing the residual weld stresses. The complex of three-dimensional finite element calculations and the experimental studies showed the optimality of the selected reconstruction technique [3]. On the basis of the performed calculations and studies, the reconstructed specimens were fabricated and the complex of experimental works on the determination of the welding mode parameters for the reconstruction of CT specimens was performed. This provided the original data on the fracture toughness of standard and reconstructed CT specimens of materials for VVER-1000 reactor pressure vessels. Also, the collected data on the fracture toughness for standard and reconstructed CT specimens were compared. The employed technique showed the ade-
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Holder
Insertion of studied material Fig. 1. Scheme of reconstruction of a CT specimen.
for the fabrication of a reconstructed CT specimen is shown in Fig. 2. At the preparation phase, to control the temperature of the central insertion, a thermocouple junction was welded to it and the temperature was measured after making each weld. The preliminary measurements showed that the temperature of the central insertion in the process of welding did not exceed the RPV operating temperature, and the phase composition did not differ from the initial material [4]. The sequence of welding is shown in Fig. 2. After making each weld, a welded blank was cooled for 5–10 min.
quacy of the fracture toughness data for standard and reconstructed specimens of materials for nuclear reactor pressure vessels [4]. The aim of this work is to confirm the applicability of the technique for the reconstruction of CT specimens from Charpy specimen halves on the basis of comparative studies of the brittle fracture mechanisms and the determination of brittle crack initiation sources in standard and reconstructed specimens. MATERIALS AND METHODS Specimens of low-alloy Cr–Ni–Mo pressure vessel steels with the bainite tempering structure were studied: ⎯OM-1 and MSh-1 base metal and weld metal of VVER-1000 RPV in the unirradiated (initial) state; ⎯OM-2 highly embrittled base metal of a prototype reactor cover operated for 20 years at the working temperature of ~275°C. The chemical composition of the studied materials is shown in Table 1.
Fracture Toughness Tests of Compact CT Specimens The mechanical tests for fracture toughness of standard and reconstructed specimens were performed using a Zwick/Roell Z100 installation in accordance with [5]. The temperature dependence of the fracture toughness KJc(T) of materials for reactor pressure vessels was developed by the master curve method [6]. The temperature dependences KJc(T) for standard and reconstructed specimens are shown in Fig. 3. According to the fracture toughness test results, the reference brittleness temperatures T0 of standard and reconstructed CT specimens were determined. The static tensile tests of cylindrical specimens [7] were also performed to determine the temperature dependence of the yield points of the studied materials. The data on the reference brittleness temperature T0 and the yield point values at room temperature Rp0.2(24°C) of the studied materials are shown in Table 2. As follows from the data in Table 2, the reference brittleness tem-
Technique for CT Specimen Reconstruction The fabrication of a reconstructed CT specimen consists in welding elements of a holder to ends and side surfaces of the central insertion by electron beam welding and the subsequent mechanical treatment of welded blanks to the size of a CT specimen (31.2 × 30 × 10 mm3). Specimens were fabricated using insertions with the cross section of 10 × 10 mm2 and the length of L = 20 mm. The scheme of location of holder elements and insertions Table 1. Chemical composition of studied materials
Content of chemical element, wt % Specimen OM-1 OM-2 MSh-1
C
Si
Mn
Cr
Ni
Mo
P
Cu
0.8 0.25 0.07
0.4 0.31 0.36
0.51 0.47 0.93
2.12 3.30 1.71
1.17 0.37 1.22
0.56 1.07 0.64
0.008 0.018 0.007
0.02 0.10 0.04
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the yield point values of specimens tested at different temperatures.
18 ± 0.1
weld 2
weld 4 weld 3
Technique for the Fractographic Studies of Specimens Tested for Fracture Toughness
31.2 ± 0.1
16.5 ± 0.1
∅6.3 ± 0.1
The fractographic studies of fractures were performed to evaluate the identity of the conditions of brittle crack initiation under the fracture toughness tests of standard and reconstructed CT specimens.
weld 1 10 ± 0.1
L 30 ± 0.1
Secondary electron images were obtained under the accelerating stress of 15 kV in the magnification range of 60–50000× using a Zeiss Supra 40VP scanning electron microscope. Hereinafter, the technique of the fractographic studies is described.
Fig. 2. Drawing of a reconstructed CT specimen with the locations of holder elements and insertions in a blank. The insertion size is 10 × 10 × 20 mm. The arrows show the sequence of welding.
At the first stage, the phased recording of the fracture surface of a specimen was performed under small (up to ~80×) magnifications along the front lines of a grown fatigue crack. In Fig. 4a, a typical SEM image of the fracture panorama of a steel CT specimen of a VVER-1000 reactor pressure vessel tested for fracture toughness is shown.
peratures of standard and reconstructed CT specimens showed good convergence. The analytical dependences relating the structural parameters of fractures with the fracture toughness parameters of tested specimens were developed on the basis of the crack tip open displacement (CTOD) parameter calculated using the following formula [6, 8]: CTOD =
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(
)
2 K Jc 1 − υ2 , 2E (T ) R p 0.2 (T )
In the panoramic image, according to the chevron relief of the fracture surface, which characterizes the direction of the crack propagation in a specimen, the focal zone of the brittle fracture was determined. In this panoramic image, one focal zone of the brittle fracture of a specimen is clearly seen. If there are two or more fracture initiation zones, the one from which a crack propagated to a larger distance is chosen. This indicates the earlier initiation of a brittle crack.
(1)
where KJc is the critical stress intensity factor, MPa m ; E(T) is the elastic modulus, GPa; Rp0.2 (T) is the yield point, MPa; and υ is the Poisson ratio. The CTOD value as the individual characteristic of each specimen was chosen to subsequently compare
K Jc , MPa m (a)
Then, under moderate magnifications (~500–7000×) (Fig. 4b) and higher magnifications (~10000–50000×) (Fig. 4c), according to the streamlet pattern of a transcrystallite fracture, the focal zone with an expressed
(b)
(c)
275 250 225 Pf = 0.95 Pf = 0.95 200 Pf = 0.5 175 150 Pf = 0.5 125 100 Pf = 0.05 Pf = 0.05 75 50 25 0 –140 –120 –100 –80 –60 –90 –70 –50 –80 –60 –40
Pf = 0.95
Pf = 0.5
Pf = 0.05
–80 –40 0 20 –60 –20 T, °C
Fig. 3. Temperature dependences of the fracture toughness for standard (⎯, j) and reconstructed (– – –, d) CT-0.5 specimens: OM-1 (a), OM-2 (b), and MSh-1 (c). INORGANIC MATERIALS: APPLIED RESEARCH
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Table 2. Test results for standard and reconstructed CT specimens Material
T0, °C
Type of specimen
OM-1 OM-2 MSh-1
Standard
–123
Reconstructed
–125
Standard
–84
Reconstructed
–94
Standard
–26
Reconstructed
–32
brittle fracture initiation source (so-called “origin”) was determined [9, 10]. For all the studied specimens, the cleavage initiation distance (CID), which is the minimum distance from the fracture initiation site (from a “origin”) to the tip of a grown fatigue crack [11], was measured (Fig. 4b).
Rp0.2(24°C), MPa 550 600 620
intercrystallite fracture from a “origin” (a structural boundary) is shown.
In [9, 10], it was shown that the main types of “origins” in pressure vessel steels are nonmetal inclusions (NI) and structural boundaries (SB) such as intergranular or subgranular boundaries (Fig. 5).
RESULTS AND DISCUSSION In Fig. 5, typical SEM images of the revealed brittle fracture initiation sources in standard and reconstructed CT specimens are shown. For standard and reconstructed specimens, in all the studied materials and states, the “origins” were nonmetal inclusions and structural boundaries of material.
Depending on the cohesive strength of the nonmetal inclusion/matrix boundary, the brittle fracture can propagate along the boundary (in this case, a whole inclusion or a hole made by an inclusion is observed, Fig. 5a) or across an inclusion (half of an inclusion is observed, Fig. 5b). In Fig. 5c, the brittle
Fractographic Studies of Standard and Reconstructed CT Specimens of OM-1 Material in the Initial State The results of the fractographic studies and the result of the fracture toughness tests of standard and
2 mm
(a)
CID
(b)
40 μm (c)
2 μm
Fig. 4. Typical SEM images of a fracture with different magnifications and scheme of fractographic studies with the determination of the brittle crack initiation source after the fracture toughness test of a CT specimen: (a) fracture panorama; (b) focal zone of fracture; (c) “origin.” INORGANIC MATERIALS: APPLIED RESEARCH
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2 μm (b)
(a)
(c)
2 μm
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2 μm
10 μm
(d)
Fig. 5. Typical images of brittle crack initiation sources in standard and reconstructed CT specimens of studied materials: (a, b) nonmetal inclusion; (c, d) structural boundary.
Fractographic Studies of Standard and Reconstructed OM-2 CT Specimens in the Embrittled State The main metal for a prototype reactor pressure vessel (OM-2) was chosen for the studies because, after 20 years of operation at the working temperature (~275°C), it showed the shift in the critical brittleness temperature of ~40°C, and the level of the grain INORGANIC MATERIALS: APPLIED RESEARCH
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boundary phosphorus concentration determined by Auger electron spectroscopy was 27%. Thus, the identity of the mechanical properties of standard and reconstructed specimens could be evaluated not only in the initial but also in the embrittled state. In Table 4, the results of the mechanical tests and the fractographic studies of standard and reconstructed OM-2 CT specimens after the fracture toughness tests in the embrittled state are summarized. For both standard and reconstructed CT specimens, the brittle crack initiation sources in the embrittled state are mainly grain boundaries (Figs. 5c and 5d). Thus, for reconstructed specimens, the types of “origins” are the same as for standard specimens. In Fig. 8, the histogram of the relative fraction of the brittle crack initiation from “origins” of diferent types in embrittled OM-2 specimens is shown. For standard OM-2 specimens in the embrittled state, the
Fraction of total number of initiations, %
reconstructed CT specimens made of steel OM-1 (the main metal for VVER-1000 RPV) tested in the initial state (Table 3) showed that the relation of different types of “origins” for standard and reconstructed specimens is identical (Fig. 6). In Figs. 7a and 7b, the established correlations between the crack tip open displacement (CTOD) and cleavage initiation distance (CID), which is the distance from a “origin” to the tip of a grown fatigue crack, for OM-1 CT specimens tested in the initial state are shown. The analysis of the experimental data (Table 3) using the Chow test [12] for standard and reconstructed CT specimens showed that the experimental points belong to the same CTOD(CID) dependence for each “origin” type. The obtained dependences have the following forms: ⎯for a NI “origin,” CTOD = 0.05 CID + 26.98; (2) ⎯for a SB “origin,” CTOD = 0.22 CID + 57.93. (3) The differences of the CTOD(CID) dependences for different types of “origins” are related to different strengths of such stress concentrators [13].
70 60 50 40 30 20 10 0
OM-1 MNI SB
Standard
Reconstructed
Fig. 6. Relative fraction of the brittle fracture initiation from “origins” of different types for standard and reconstructed OM-1 CT specimens in the initial state. No. 6
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Table 3. Results of mechanical tests and fractographic analysis data for standard and reconstructed OM-1 CT specimens in the initial state Specimen no.
Ttest, °C
KJc, MPa
m
CTOD, µm
Type of “origin”
Size of “origin,” µm
CID, µm
1
–100
144.8
65.35
NI
1.7
2
–100
277.8
240.54
SB
–
783.6
3
–100
79.3
19.60
NI
1.2
380.5
4
–110
94
26.66
NI
2.1
280
5
–110
81.5
20.04
SB
–
150
6
–100
117.4
42.96
NI
1.3
259.3
7
–100
105.4
34.63
NI
1.9
343.8
8
–100
130.6
53.16
NI
0.9
202.8
9
–100
192.3
115.26
SB
–
75
10
–100
140.9
61.88
NI
2.4
873.5
11
–100
208.9
136.02
SB
–
251.9
R1
–115
117.3
40.80
SB
–
39.4
R2
–115
150.7
67.35
SB
–
90.1
R3
–115
93.7
26.04
NI
1.7
139.6
R4
–115
63.2
11.84
NI
1.2
21.7
R5
–115
64.96
NI
1.3
215.8
R6
–115
69.1
14.16
NI
1
49.4
R7
–115
110.3
36.08
NI
0.7
68.7
R8
–115
184.2
100.62
SB
–
268.6
R9
–115
140.9
58.87
NI
2.3
355.5
R10
–115
205.2
124.87
SB
–
51.7
R11
–115
124.2
45.74
NI
1.8
127.3
148
brittle crack initiation sources are only structural boundaries; in reconstructed specimens, an individual case of brittle fracture initiation from a nonmetal inclusion was registered. Thus, the percentage of the fraction of brittle crack initiation sources hardly changed (Fig. 8). CTOD, μm 150
In Fig. 9, the established correlation between the crack tip open displacement (CTOD) and the cleavage initiation distance (CID) for standard and reconstructed OM-2 specimens tested in the embrittled state is shown.
(a)
(b) 300
100 = CTOD
6.98 ID + 2
200
100
300
600
+ CID
93 57.
.22
0.05C
50
0
700
900 0
OD CT
300
=0
600 CID, μm
Fig. 7. Dependence CTOD(CID) for different types of “origins” (NI (a) and SB (b)) registered in standard (s, n) and reconstructed (d, m) OM-1 CT specimens in the initial state. INORGANIC MATERIALS: APPLIED RESEARCH
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Table 4. Results of mechanical tests and fractographic analysis data for standard and reconstructed OM-2 CT specimens in the embrittled state Specimen no.
Ttest, °C
KJc, MPa m
CTOD, µm
Type of “origin”
Size of “origin,” µm
CID, µm
–63 –63 –63 –63 –63 –63 –63 –92 –40 –91 –90 –75 –75 –75 –75 –75 –75 –75 –75 –75 –75 –75
214.4 204.4 183.2 170.7 171.1 85.5 174 46.8 105.8 70.9 108.7 146.7 151.2 127.4 116.8 165.5 228.6 222.2 67.7 146.9 55.5 158.1
158.83 144.36 115.97 100.68 101.15 25.26 104.61 7.01 40.62 16.14 38.06 72.21 76.70 54.46 45.77 91.90 175.34 165.66 15.38 72.40 10.33 83.87
SB SB SB SB SB SB SB SB SB SB SB SB SB SB NI SB SB SB SB SB SB SB
– – – – – – – – – – – – – – 1.4 – – – – – – –
270 200 147 246 146 49.53 277 16.28 66 21 81 250.6 58.26 97.55 228 213.1 217 92.14 54.66 136 86.77 149
1 2 3 4 5 6 7 8 9 10 11 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11
The analysis of the experimental data using the Chow test [12] showed that CTOD(CID) within the dispersion range for standard and reconstructed specimens is described by one dependence: CTOD = 0.43 CID + 20.66.
(4)
Fraction of total number of initiations, %
Thus, it was shown that the tests of reconstructed CT specimens are also appropriate for steel in the embrittled OM-2 state. 100
Fractographic Studies of Standard and Reconstructed MSh-1 CT Specimens in the Initial State To confirm the adequacy of the results of the fracture toughness tests for standard and reconstructed specimens of weld metal in the VVER-1000 RPV with the cast structure and columnar grains, comparative studies of standard and reconstructed MSh-1 CT specimens were performed. CTOD, μm
OM-2 MNI SB
80
D+
200
D
CTO
60 40
3CI = 0.4
6
20.6
100
20 0 Standard
0
Reconstructed
300
Fig. 9. Dependence CTOD(CID) for standard and reconstructed OM-2 CT specimens in the embrittled state: (n, m) SB “origin,” standard and reconstructed specimens, respectively; (d) NI “origin,” reconstructed specimens.
Fig. 8. Relative fraction of the brittle fracture initiation from “origins” of different types for standard and reconstructed OM-2 CT specimens in the embrittled state. INORGANIC MATERIALS: APPLIED RESEARCH
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Table 5. Results of processing the mechanical tests and fractographic analysis data for standard and reconstructed MSh-1 CT specimens in the initial state Specimen no.
Size of “origin,” µm
Ttest, °C
KJc, MPa m
1
–60
87.5
24.06
NI
1.2
2
–60
106.7
35.77
SB
–
61.29
3
–60
85.8
23.13
SB
–
56.02
4
–60
58.9
10.90
NI
1.6
40.54
5
–60
71.7
16.15
NI
1.1
26.7
6
–45
66.3
14.22
NI
1.1
33.69
7
–45
89.9
26.15
SB
–
51.11
8
–45
88.9
25.57
NI
0.8
45
9
–45
106.5
36.69
SB
–
74.05
10
–45
79.4
20.40
NI
1.4
57.33
11
–35
84.8
23.68
SB
–
36.48
R1
–30
94.3
29.53
NI
0.8
R2
–30
125.7
52.46
NI
1.4
R3
–30
79.01
20.73
SB
–
64.17
R4
–30
56.81
10.72
SB
–
17.85
R5
–30
70.68
NI
0.9
R6
–30
99.5
32.87
SB
–
R7
–30
185.4
114.13
NI
1.5
509.3
R8
–30
128.3
54.65
NI
1.1
338.8
CTOD, µm
145.9
Fraction of total number of initiations, %
In Table 5, the results of the mechanical tests and the fractographic studies of standard and reconstructed MSh-1 CT specimens (weld metal in VVER1000 RPV) after the fracture toughness tests in the initial state are summarized. For both standard and reconstructed CT specimens of weld metal, the brittle crack initiation sources are grain boundaries and nonmetal inclusions (Figs. 5a–5d).
70 60 50 40 30 20 10 0
MSh-1 MNI SB
Standard
Reconstructed
Fig. 10. Relative fraction of the brittle fracture initiation from “origins” of different types for standard and reconstructed MSh-1 CT specimens of RPV in the unirradiated state.
Type of “origin”
CID, µm 49.25
99.97 207.3
241.3 60.41
In Fig. 10, it was shown that the percentage of brittle crack initiation sources for standard and reconstructed MSh-1 specimens hardly changed. In Fig. 11, the established correlation between the crack tip open displacement (CTOD) and the cleavage initiation distance (CID) for tested standard and reconstructed MSh-1 specimens in the initial state is shown. The analysis of the experimental data using the Chow test [12] showed that, within the dispersion of the data, the CTOD(CID) dependences for standard and reconstructed MSh-1 specimens in the initial state hardly differ. The resulting dependences have the following form: ⎯for NI “origins,” CTOD = 0.19 CID + 10.78; (5) ⎯for SB “origins,” CTOD = 0.39 CID + 5.61. (6) Hence, the comparative analysis of the structural parameters of fractures and the fracture toughness parameters showed that these dependences for standard and reconstructed CT specimens of weld metal, like those for base metal, are almost identical.
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CTOD, μm 150 100
(a)
D CTO
=
(b) + CID 0.19
8 10.7
ID +
.39C D=0
40
5.61
CTO
20
50
0
935
100
200
300
400
500 0
20
30
40
50
60 CID, μm
Fig. 11. Dependence CTOD(CID) for different types of “origins”: (NI (a) and SB (b)) registered in standard (s, n) and reconstructed (d, m) CT specimens of steel MSh-1 in the initial state.
CONCLUSIONS The comparative fractographic studies of standard and reconstructed CT surveillance specimens of base metal and weld metal of the VVER-1000 RPV in the initial and thermally embrittled states were performed. It was shown that the brittle fracture mechanisms in standard and reconstructed specimens are the same, which is confirmed by the following: —The processing of the test results on the fracture toughness of standard and reconstructed CT specimens by the master curve method gives approximately the same reference temperature T0 characterizing the temperature dependence KJc(T). —Brittle crack initiation sources in all the studied standard and reconstructed specimens are the same, namely, nonmetal inclusions and structural boundaries. —The relative fraction of the brittle fracture initiation from “origins” of different types in reconstructed specimens compared to standard specimens hardly changes for the same materials. —The analytical dependence between the crack tip open displacement (CTOD) and the cleavage initiation distance (CID) for standard and reconstructed specimens of the same composition in the same states are the same. ACKNOWLEDGMENTS This work was supported by OAO Rosenergoatom Concern and the Ministry of Education and Science of the Russian Federation, project no. 14.579.21.0060. REFERENCES 1. Heerens, J., Hellmann, D., and Ainsworth, R.A., Fracture toughness determination in the ductile-to-brittle transition regime pre-cracked Charpy specimens compared with standard compact specimens, in From Charpy to Present Impact Testing, Francois, D. and Pineau, A., Eds., Amsterdam: Elsevier, 2002, pp. 297–305. 2. Heerens, J., Ainsworth, R.A., Moskovic, R., and Wallin, K., Fracture toughness characterization in the ductile-to-brittle transition and upper shelf regimes using pre-cracked Charpy single-edge bend specimens, Int. J. Pressure Vessels Piping, 2005, vol. 82, pp. 649–667. INORGANIC MATERIALS: APPLIED RESEARCH
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Translated by E. Petrova No. 6
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