NDLI: Effect of Hydrogen Concentration on the Threshold Stress Intensity Factor for Delayed Hydride Cracking in Zr-2.5Nb Pressure Tubes

Content Provider	The American Society of Mechanical Engineers (ASME) Digital Collection
Author	Gordon, K. Shek Don, R. Metzger
Copyright Year	2011
Abstract	The Zr-2.5Nb pressure tubes of CANDU reactors are susceptible to a crack initiation and growth mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, hydrided region formation and fracture at a flaw or crack tip. The threshold stress intensity for DHC initiation from a crack, KIH, is an important material parameter for assessing DHC initiation from flaws in pressure tubes. KIH is used to determine whether DHC initiation may occur from flaws which are postulated as crack-like. It is also an input parameter in the engineering process-zone methodology to assess DHC initiation from blunt flaws. Tests were performed to determine the effect of hydrogen concentration in solution on KIH in unirradiated Zr-2.5 Nb material, subjected to different thermo-mechanical treatments to obtain different yield strength or hardness. Hydrogen concentration in solution represents the diffusible hydrogen available for the DHC process, and is different than the total hydrogen concentration which includes the immobile hydrogen in the zirconium hydride phase. For all material conditions, the KIH values at 250°C are significantly higher when the hydrogen concentration in solution is low. Post test metallographic examination indicates that the crack-tip hydride is large and has a taper shape when the hydrogen concentration in solution is high. This suggests that KIH is reached due to insufficient stress to crack the hydrides. When the hydrogen concentration in solution is low, the crack-tip hydride is small and KIH is reached due to limited hydride growth. Finite element diffusion analysis was performed to determine the crack tip hydride accumulation as a function of KI and hydrogen in solution. For high hydrogen concentration in solution, the model predicts a taper hydride shape and hydride lengths which are consistent with the trend observed in the experiments. Another set of KIH tests was performed at 200°C on unirradiated pressure tube material hydrided to 60 and 100 ppm hydrogen. The test results indicated that KIH is controlled by the hydrogen in solution and is not affected by the amount of hydrogen in bulk hydrides.
Sponsorship	Pressure Vessels and Piping Division
Starting Page	1287
Ending Page	1295
Page Count	9
File Format	PDF
ISBN	9780791844564
DOI	10.1115/PVP2011-57624
Volume Number	Volume 6: Materials and Fabrication, Parts A and B
Conference Proceedings	ASME 2011 Pressure Vessels and Piping Conference
Language	English
Publisher Date	2011-07-17
Publisher Place	Baltimore, Maryland, USA
Access Restriction	Subscribed
Subject Keyword	Fracture (process) Zirconium Precipitation Hydrogen Yield strength Fracture (materials) Shapes Finite element analysis Pressure Diffusion (physics) Stress Thermomechanical treatment
Content Type	Text
Resource Type	Article

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Effects of Temperature and Thermal Cycling on the Threshold Stress Intensity Factor for Delayed Hydride Cracking in Zr-2.5Nb Pressure Tubes

Validation of Weight Function Based Process-Zone Model for Evaluation of Delayed Hydride Cracking Initiation in Zr-2.5Nb Pressure Tubes

Effect of Flaw Orientation on Delayed Hydride Crack Initiation in Zr-2.5Nb Pressure Tubes

Effect of Hydrogen Isotope and Concentration on Delayed Hydride Crack Growth Rates in Zr-2.5Nb Pressure Tubes

Effects of Hydrided Region Overload on Delayed Hydride Cracking Initiation From Flaws in Zr-2.5 Nb Pressure Tubes

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Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5 Nb Pressure Tube Material

Use of the Extended Finite Element Method in the Assessment of Delayed Hydride Cracking

The Effect of Load Reduction on Crack Initiation Behavior of Hydrides From Flaws in Zr-2.5Nb Pressure Tube Material

Effect of Hydrogen Concentration on the Threshold Stress Intensity Factor for Delayed Hydride Cracking in Zr-2.5Nb Pressure Tubes

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