18th International Conference on Structural Mechanics in Reactor Technology (SMiRT 18) Beijing, China, August 7-12, 2005 SMiRT18-G11-10
HYDRIDE CRACK FORMATION IN Zr-Nb ALLOY UNDER CONSTANT
AND CYCLIC LOADING
Albertas Grybenas
Lithuanian Energy Institute, Breslaujos 3,
LT-44403 Kaunas, Lithuania
Phone: +37037401908, Fax:
+37037351271
E-mail: grybenas@mail.lei.lt
Vidas Makarevicius
Lithuanian Energy Institute, Breslaujos 3,
LT-44403 Kaunas, Lithuania
Phone: +37037401907, Fax:
+37037351271
E-mail:
makarev@mail.lei.lt
ABSTRACT
DHC rates in Zr-2.5Nb alloy were determined under constant and low cyclic loading conditions. Both CANDU and RBMK pressure tube materials (type TMT-1) were used for testing. Necessary amount of hydrogen ranging from 27 to 76 ppm was added to the specimens by depositing hydride layer and homogenizing it at elevated temperature to disperse the hydrogen at the required concentration. Testing was performed on compact tension specimens fabricated from hydrided material.
Delayed hydride cracking rates at constant loading were measured at 144, 250 and 283 °C, with initial KI
about 15 MPa·m½. It was found that DHC velocity increases with increase of the test temperature and varies from 9.3·10-10 to 4.7·10-8 m/s. Cycle loading tests were done at 250 °C with frequency of loading 1 cycle/min and different minimum/maximum load ratio R, changing from 0.13 to 1. At cyclic loading conditions DHC rate decreases with decrease of load ratio 6.7 times, from 3.2·10-8 m/s at constant loading to the value of 4.8·10-9 m/s at load ratio R=0.13.
It was observed that inter-striation spacing measured from the fracture surface photographs decreases with temperature. In case of cyclic loading testing, DHC rate and inter-striation spacing decreases with load ratio.
Keywords: zirconium, hydride, fuel channel, delayed hydride cracking, cyclic loading.
1. INTRODUCTION
condition is known as KIH, - the critical stress intensity factor in the presence of hydride, below which no crack
growth occurs. At KI values above KIH the rate of cracking, V, is essentially independent of KI.
The need to evaluate influence of cyclic loading to the formation of DHCis related with is uncertainty about KI values initiating DHC under cyclic loading conditions and influence of cyclic loading on DHC growth rate in
the pressure tube of the fuel channel. It is important to determine the life of the pressure tubes due to both crack initiation and the subsequent DHC propagation before the crack penetrates the pressure tubes and catastrophic failure occurs.
Temperature dependence of DHC velocity in axial direction was determined for both CANDU and RBMK Zr-2.5%Nb pressure tubes. Influence of cyclic loading was investigated for RBMK material at 250 °C.
2. EXPERIMENTAL
Test specimens were prepared using both CANDU cold worked and RBMK (TMT-1 heat-treated condition) Zr-2.5Nb pressure tube materials.
Sections of the tube pressure tubes were hydrided to a required hydrogen concentration using an electrolytic method and diffusion annealing treatment (Lepage, 1998). The hydrogen concentrations in the samples were chosen so that solubility limit at the test temperature was exceeded about 25 ppm.
After adding hydrogen curved compact toughness (CCT) specimens with the width of 17 mm were machined along the axis of the pressure tube, retaining the tube curvature. The CCT specimens have been fatigue pre-cracked at room temperature to produce an initial crack length about 1.7 mm
Table 1. Peak and test temperatures for DHC testing
Pressure tube material Peak temperature, °C Test temperature, °C Measured hydrogen concentration, ppm
CANDU 259 144 32
CANDU 315 250 62
CANDU 327 283 79
RBMK 275 144 27
RBMK 315 250 54
RBMK 327 283 76
DHC testing procedures are based on IAEA recommendations (Choubey, 1998).
R2 = 0.966
1.03 1.04 1.05 1.06 1.07 1.08
0.05 0.06 0.07 0.08 0.09 0.1
(a-af)/W
V/V
0
Fig. 1. Potential drop – crack length relationship
Potential drop output was measured using the wire (diameter 0.5 mm) made from the same pressure tube material to eliminate stray EMF. The wires have been attached to the specimen within 1 mm of either side of the notch by spot welding.
PD versus time changes has been continuously recorded using computer data acquisition system. DHC test has been completed after estimated crack growth reached 1.5 mm.
After completion of DHC test actual crack length was measured from fractographs.
Fractured specimens have been photographed using computerized image analysis system. On the digital image the DHC and fracture area boundaries have been outlined manually by drawing one pixel wide line, then image has been transformed to binary and corresponding areas have been measured.
The DHC velocity has been determined from the average DHC-length measured from fracture surface photograph for one half of the fractured specimen. Crack length was estimated as a ratio of measured crack area and specimen width.
3. DHC TEST RESULTS
It is known (Puls, 1990), that maximum DHC value is attained by cooling from a temperature higher than the solvus temperature of hydrogen in the specimen. Because DHC velocity is sensitive to the temperature history, a standardized procedure for measuring the cracking velocity was applied. Specimens have been heated at 5 °C per min to a peak temperature Tpk, soaked for 1h and cooled down at 1.5 °C per min to the test temperature Ttst with a
minimum 30 °C temperature drop without any under cooling. Peak and test temperatures for DHC velocity measurement are in Table 1. Temperature schedule for testing at 250 °C is illustrated in Fig. 2.
After holding the specimen at test temperature for 35 min, a constant static load sufficient to give an initial stress intensity factor of 15 MPa·m½ was applied on the specimen, then after 15 min cycling loading device was launched. DHC started after a short incubation time and was allowed to grow to an estimated crack length. Test data, including crack length, incubation and cracking time, DHC velocity and loading parameters are given in the Table 2. Figure 3 shows loading curves for cyclic loading test.
Table 2. Summary of test results for DHC velocity measurements at 250 °C
Specimen I.D. DHC crack length (mm) Incubation time (min) Cracking time (min) DHC velocity (m/s) Initial KImin / KImax(MPa·m1/2)
Final KImin / KImax
(MPa·m1/2)
Load ratio R Pmin/ Pmax
R4.0 1,003 30 3473 4,81E-09 2,1/15,29 2,54/18,56 0,13 R4.1 1,531 43 826 3,09E-08 15,68 21,25 1 R4.2 1,123 253 527 3,27E-08 15,1 18,7 1 R4.3 1,681 78 1220 2,16E-08 10,27/15,03 14,06/20,57 0,68 R4.4 1,157 28 2411 8,00E-09 4,75/14,88 5,95/18,62 0,32 R4.6 1,506 42 1583 1,59E-08 7,63/14,85 10,3/20,15 0,51 R4.7 1,528 55 1195 2,13E-08 9,9/15,27 10,3/20,15 0,65
T em p era tu re Time Heating
5 oC/min
Cooling
1,5 oC/min
Peak temperature
315oC , 60 min
Test temperature
250oC
35 min
Constant loading
~15MPa·m½ cycling
15 min
Fig. 2. Temperature-time schedule for DHC testing
Cyclic loading tests were done at 250 °C with specimens containing 55 ppm of hydrogen. Two specimens from the same section were tested under constant loading conditions. Other specimenswere tested under cycle loading at 1 cycle/min and a different load ratio R. Load ratio R was calculated as a ratio of minimum and maximum load in a cycle. Variation of loading during DHC test was recorded every 4 sec. to a computer data file. Loading charts are presented in Fig. 3; minimum and maximum KI values at start and at the end of test are recorded
0 100 200 300 400 500 600 700 800 900
0 60 120 180 240 300
time, s
load, N
R=0.13 R=0.32
R=0.51 R=0.68
Fig.
3. Specimen loading curves for DHC testing
Temperature dependence of DHC velocity is shown in Fig 5a. Figure 5b shows DHC axial velocity data at 250 °C depending on load ratio R.
Fatigue precrack DHC crack
Notch Crack boundary
a
b
1.0E-10 1.0E-09 1.0E-08 1.0E-07 1.0E-06
1.7 1.9 2.1 2.3 2.5
1000/T
VDHC
, m/s
RBMK CANDU
R2 = 0.9962
0.0E+00 1.0E-08 2.0E-08 3.0E-08 4.0E-08
0 0.5
R
VDHC, m/s
1
a
b
Fig. 5. DHC axial velocity data as a function of 1000/T (a) and load ratio R (b)
4. DISCUSSION
Tests performed show that DHC rate depends on test material, temperature and loading conditions.
Testing under different load ratio R (R=Pmin/Pmax) show that DHC rate decreases with R value 6.7 times, from
3.2·10-8 m/s at constant loading to the lowest value, which is nearly 4.8·10-9 m/s. During crack growth, ratio KImin
/KImax remained constant, while maximum KI valuesat the end of test increased at the average 4.5 MPa m1/2.
250 °C, RBMK 250 °C, CANDU
283 °C,RBMK 283 °C, CANDU
Fig. 6. Striations formed by DHC at different temperatures.
R=0.129, Specimen R4.0 R=0.319, Specimen R4.4
R=1, Specimen R4.1 R=1, Specimen R4.2
Fig. 7. Photographs of DHC fracture surface after cyclic loading test. Test temperature 250 °C
It can be observed a sharp increase in DHC rate (Fig. 8b) when loading mode changes from static to cyclic, which gradually decreases. Subsequent crack growth is even and a slope of PD versus crack length curve remains constant during the test, as KI values increases. Consequently, DHC rate remains nearly constant and depends on R.
DHC velocity, while R>0.1, and Kmax <21 MPa m1/2 follows relationship:
V·10-8 = -5.543·R3+9.827·R2-1.633·R+0.5324
Fabrication route influences microstructure, which as a result influences the strength of the alloy, terminal solid solubility and effective diffusivity of hydrogen. It was mentioned by Singh (2004) that RBMK pressure tube material has only marginally lower yield strength at 250 °C (542 MPa) compared to CANDU pressure tube (557-596.7 MPa). The differences that can affect DHC rate are grain structure, crystallographic texture and strength. Both microstructure and strength can independently affect crack velocity.
It was found that striation spacing increases with increase in test temperature and are larger in the specimens of the RBMK material than in the CANDU material. Inter-striation spacing measured from the fracture photographs (Fig. 6, 7) are shown in Fig. 8. The likely explanation is associated with decrease of matrix strength as temperature increases. DHC is an intermittent crack growth process, involving precipitation, growth and eventual of the hydride when it reaches critical length. Striation spacing is associated with the length of hydride (Shek et al, 1996).
0 10 20 30 40 50
1.7 1.9 2.1 2.3 2.5
1000/T Striation Spacing, CANDU RBMK 15 20 25 30 35 0 0.5 R
Striation spacing, mm
1
a
b
Fig. 8. Relationship between inter-striation spacing and temperature (a); inter-striation
spacing and load ratio R (b)
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
0 100 200 300
time, min
DHC crack length, mm
R = 1 R = 0.51 R = 0.13
0 0.05 0.1 0.15 0.2 0.25 0.3
0 2000 4000 6000
t, s
DHC crack length, mm static loading cyclic loading
a
b
Fig. 9. Crack length versus time at different load ratio R (a); sharp increase of DHC rate when
changing loading mode from static to cyclic (b)
The measured DHC velocity depends both on the temperature and on load ratio R and increases with increase in test temperature. Decreasing load ratio R from 1 to 0.13, DHC rate at 250 °C decreases 6.7 times from 3,2·10-8 m/s to 4,8·10-9 m/s.
DHC rate as well as inter-striation spacing, measured from the fracture surface photographs of specimens, subjected to constant loading test increases with test temperature. In case of cyclic loading testing at constant temperature, DHC rate and inter-striation spacing decreases with load ratio.
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