Effect of heat treatment on delayed hydride cracking in Zr-2.5 Wt Pct Nb
- PDF / 2,780,391 Bytes
- 8 Pages / 594 x 774 pts Page_size
- 5 Downloads / 191 Views
I.
INTRODUCTION
THE embrittling
effect of internal hydrogen in zirconium and other hydride-forming metals has been extensively studied in the past and the mechanisms of the embrittlement are fairly well understood. 1-4 The embrittlement in these exothermic absorbers is associated with the precipitation of brittle hydrides below the terminal solid solubility (TSS) temperatures. The presence of the hydrides results in a reduction in ductility 2 and fracture toughness 3'4 of the material. In addition to gross embrittlement, hydrides are also responsible for a delayed failure mode via slow, subcritical, crack growth. The first example of this in a zirconium alloy, namely Zr-2.5 wt pct Nb, was reported by Simpson and Ells, 5 in an experimental nuclear fuel sheath. Cracking occurred in the weld region after storage at room temperature over a two-year period. A second nuclear-related delayed cracking problem has emerged in some Zr-2.5 wt pct Nb pressure tubes in CANDU TM reactors. 6 Through-wall cracks developed in high-stress regions associated with the end fittings. Occurrence of the pressure tube failures led to extensive research efforts aimed at obtaining a better understanding of mechanisms responsible for such delayed failures. As a result, a good understanding of the process has been obtained. The mechanism has been studied both theoretically7 ~0 and experimentally. ~~ t~ Delayed hydride cracking (DHC) is believed to be produced by a diffusioncontrolled precipitation and growth of hydride at a crack tip (under the action of the stress gradient), followed by rapid crack propagation. The crack-tip advances through the embrittled region, then arrests in the dactile mamx, until a new hydrided zone is formed at the crack tip. The process repeats itself, in a step-wise manner, resulting in slow crack growth. K. E AMOUZOUVI, Materials Scientist. and L. J. CLEGG, Research Technologist. are with Atomic Energy of Canada Limited, Whiteshell Nuclear Research EstabLishment, Pinawa, MB, Canada ROE IL0. Manuscript submitted October 23, 1986.
METALLURGICAL TRANSACTIONS A
The theoretical model, 7-~~ which was developed to describe the process, assumes a relatively coarse distribution of hydrides, where the plastic zone at the crack tip is initially free of hydride precipitates. Hydrides present outside this highly stressed zone dissolve to provide a source of hydrogen for the hydride growing at the crack tip. The model was later improved ~~ to include the effect of internal stress (misfit stress associated with the precipitated hydrides) on the flux of hydrogen to the crack tip. The misfit stresses are generated in the vicinity of the hydride as a result of the hydride-matrix misfit (hydrides undergo a volume increase of about 17 pct upon precipitation). The misfit strains can be accommodated by elastic distortion or plastic deformation (generation of misfit dislocations) of the matrix. The associated strain energy has been shown to have a significant effect on the TSS. 16 The elastic part (reversible) of the accommodation en
Data Loading...
