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

Abstract

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 15 μm, 30 μm, and 100 μm, 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 57 wt. 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.

Discrete Dislocation Plasticity Modeling of Hydrides in Zirconium under Thermal Cycling

“…The length scale of nucleation sites can be inferred from micrographs of typical hydride morphologies, which show the existence of large macroscopic hydride structures that can extend to be hundreds of microns in length [1,3]. On closer inspection, these macro-hydrides are actually assemblies of small microscopic hydride packets whose geometry resembles that of platelets or needles with lengths on the order of a micron [4,6].…”

“…The reduction in the fracture toughness ultimately leads to crack propagation in the hydride-rich zone. In the nuclear community, DHC universally serves as an umbrella term for the complexity associated with the repetition of the multiscale processes of atomic diffusion, microscale precipitation and fracture, as well as the coupled sub-processes therein, which can compromise the structural integrity of the zirconium alloy cladding [1][2][3][4].…”

An improved theoretical process-zone model for delayed hydride cracking initiation at a blunt V-notch

“…Reasonable agreement was observed between the experimental data [13][14][15] and model predictions on the threshold conditions for cracking in samples with different notch geometries.…”

“…It is noted that the emphasis of the previous analytical studies invovling the TPZ models [7][8][9] are on a blunt flaw in the shape of semi-ellipses or intrusions, whereas it is more common to use specimen containing a V-shaped notch with rounded root in the DHC initiation tests (e.g. [13][14][15]). The existing EPZ methods [10,11] for prediction of DHC initiation at such V-shaped flaws require the stress field directly obtained from a finite element model or access to parametric solutions for stress field [12].…”

Fracture Strength of Embedded Hydride Precipitates in Zirconium and its Alloys

“…Therefore, all effort was devoted to characterizing and understanding Fig. 9.1 Comparison of a optical and b SEM micrograph of the same hydrided region formed at a sharp (15 lm radius) flaw tip in Zr-2.5Nb pressure tube material under an effective K I = 9 MPaHm (from Cui et al [19]) the possibility of the rapid mode of fracture which could be caused by the reduction in overall fracture toughness of zirconium alloys as a result of an increase in volume fraction of hydride precipitates. Studies were carried out on the overall effect that various uniform distributions of hydride precipitates might have on reduction in fracture toughness of the material [15,22,26,27,33,34,52].…”