Effect of radial hydride fraction on fracture toughness of CWSR Zr-2.5%Nb pressure tube material between ambient and 300 °C temperatures

• Published in: Journal of nuclear materials 2018-09, Vol.508, p.546-555
• Main Authors: Sharma, Rishi K., Bind, A.K., Avinash, G., Singh, R.N., Tewari, Asim, Kashyap, B.P.
• Format: Article
• Language: English
• Subjects: Alloys; Annealing furnaces; Compact tension; Cooling; Ductile-brittle transition; Fracture mechanics; Fracture toughness; Fracture toughness and hydride embrittlement; Hydride continuity coefficient (HCC); Hydrides; Hydrogen; Hydrogen charging; Hydrogen embrittlement; Machining; Niobium base alloys; Pressure; Pressure tube; Radial hydrides; Spools; Stress; Stress reorientation of hydrides; Stresses; Transition temperature; Transition temperatures; Zirconium; Zr-2.5%Nb alloy
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2018.06.003

Pressure tube spools fabricated from Zr-2.5%Nb alloy, were subjected to gaseous hydrogen charging and stress reorientation treatment to form radial hydrides in a specially designed fixture. Curved Compact Tension specimens of 17 mm width containing 100 wppm of hydrogen were made using electro discharge machining. Some of the samples were annealed at 300, 325 and 350 °C followed by furnace cooling. Metallographic examination of the samples revealed 100% radial hydride in the samples to which stress reorientation treatment was imparted. The radial hydride fraction decreased with increase in the temperature of annealing that followed stress reorientation. Fracture toughness of samples with variable radial hydride fraction was evaluated as per ASTM E1820-13 procedure between 25 and 300 °C. All the samples exhibited a sharp ductile to brittle transition behavior and the transition temperature was observed to increase with increase in the radial hydride fraction. Fracture toughness of Zr-2.5%Nb alloy pressure tube material containing samples 100 wppm of hydrogen with variable radial hydride fraction was evaluated as per ASTM E1820-13 procedure between 25 and 300 °C. All the samples exhibited a sharp ductile brittle transition behavior and the transition temperature was observed to increase with increase in radial hydride fraction. The critical crack length (CCL) behavior with respect to the temperature for material containing hydrides of different HCC is presented in Fig. 1. The CCL is calculated for 220 MWe pressure tube dimensions and maximum operating pressure. The CCL is calculated by iterative/recursion method with the formulation as described in CSA-N285.8–10. Fracture toughness properties were observed to improve after the annealing heat treatment due to the decrease in HCC as shown in Fig. 2. The ambient temperature fracture toughness property was observed to be 105.7 MPa√m with 100 ppm circumferential hydrides and decreased rapidly to 49% with presence of 16.7% radial hydrides as presented in Fig. 2. Further nearly linear reduction in the fracture toughness was observed with increase in radial hydride fraction. Ductile to brittle transition temperature in presence of radial hydrides was observed to increase with increase in HCC as shown in Fig. 2. [Display omitted]