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Research Papers: Materials and Fabrication

Delayed Hydride Cracking Initiation at Notches in Zr-2.5Nb Alloys

[+] Author and Article Information
Jun Cui

 Kinectrics Inc., 800 Kipling Avenue, Toronto, ON M8Z 6C4, Canadajun.cui@kinectrics.com

Gordon K. Shek

 Kinectrics Inc., 800 Kipling Avenue, Toronto, ON M8Z 6C4, Canadagordon.shek@kinectrics.com

D. A. Scarth

 Kinectrics Inc., 800 Kipling Avenue, Toronto, ON M8Z 6C4, Canadadoug.scarth@kinectrics.com

Zhirui Wang

 University of Toronto, 184 College Street, Toronto, ON M5S 3E4, Canadazhirui.wang@utoronto.ca

J. Pressure Vessel Technol 131(4), 041407 (Jul 01, 2009) (12 pages) doi:10.1115/1.3141433 History: Received November 07, 2007; Revised September 01, 2008; Published July 01, 2009

Delayed hydride cracking (DHC) is an important crack initiation and growth mechanism in Zr-2.5Nb alloy pressure tubes of CANDU nuclear reactors. DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth, and fracture of a hydrided region at a flaw tip. In-service flaw evaluation requires analyses to demonstrate that DHC will not initiate from the flaw. The work presented in this paper examines DHC initiation behavior from V-notches with root radii of 15μm, 30μm, and 100μm, which simulate service-induced debris fretting flaws. Groups of notched cantilever beam specimens were prepared from two unirradiated pressure tubes hydrided to a nominal hydrogen concentration of 57wt.ppm. The specimens were loaded to different stress levels that straddled the threshold value predicted by an engineering process-zone (EPZ) model, and subjected to multiple thermal cycles representative of reactor operating conditions to form hydrides at the notch tip. Threshold conditions for DHC initiation were established for the notch geometries and thermal cycling conditions used in this program. Test results indicate that the resistance to DHC initiation is dependent on notch root radius, which is shown by optical metallography and scanning electron microscopy to have a significant effect on the distribution and morphology of the notch-tip reoriented hydrides. In addition, it is observed that one tube is less resistant to DHC initiation than the other tube, which may be attributed to the differences in their microstructure and texture. There is a reasonable agreement between the test results and the predictions from the EPZ model.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 2

Schematic diagram of the test setup

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Figure 4

Ratcheting cycle

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Figure 7

Failure probability in test groups in Tube A, as a function of applied KEFF

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Figure 18

Maximum notch-tip hydride length as a function of the applied KEFF in 15 μm radius samples in Tube B

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Figure 19

Effect of material variability on notch-tip hydrides (as opposed to Figs.  1717, Tube B): (a) optical micrograph and (b) SEM micrograph of the notch-tip hydrides in a 15 μm radius sample in test group U-2-1, KEFF=9 MPa√m, 85 cycles, Tube A

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Figure 20

Relatively coarse microstructure of Tube A

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Figure 21

Very fine and uniform microstructure of Tube B

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Figure 8

Failure probability in test groups in Tube B, as a function of applied KEFF

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Figure 9

Fracture surface of a 30 μm radius sample in test group U-10-4, KEFF=12 MPa√m, Tube B (initiated DHC in the 8th cycle and failed in the 11th cycle)

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Figure 10

Striations on the fracture surface of the specimen shown in Fig. 9

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Figure 11

SEM micrograph of the DHC zone near the notch-tip region in the specimen shown in Fig. 9

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Figure 12

SEM micrograph of a striation in the specimen shown in Fig. 9

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Figure 13

SEM micrograph of the ductile fracture zone 1 in Fig. 9

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Figure 14

SEM micrograph of the boxed area in Fig. 1 showing a typical layered morphology

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Figure 15

SEM micrograph of the solid boxed area in Fig. 1 showing ductile fracture

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Figure 16

SEM micrograph of the dashed boxed area in Fig. 1 showing local cleavage feature

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Figure 17

Effect of root radius on notch-tip hydrides: (a) optical micrograph and (b) SEM micrograph of the notch-tip hydrides in a 15 μm radius sample in test group U-8-3, KEFF=9 MPa√m, 96 cycles, Tube B; (c) optical micrograph and (d) SEM micrograph of the notch-tip hydrides in a 100 μm radius sample in test group U-12-2, KEFF=14 MPa√m, 198 cycles, Tube B.

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Figure 1

Schematic of a notched cantilever beam specimen

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Figure 5

Measured KTH under ratcheting conditions, as a function of notch root radius

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Figure 6

Measured σTH under ratcheting conditions, as a function of notch root radius

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