The effects of texture and microstructure on stress-orientation of hydrides in zirconium-2.5niobium tubes

Zr-2.5Nb pressure tubes act as the primary containment of D2O coolant in CANDU reactors. As such, corrosion can cause significant amounts of hydrogen to enter the pressure tubes. If the hydrogen levels in the pressure tubes exceed the terminal solid solubility (TSS) limit, hydrides can form. Sharp flaws, sufficient amounts of stress, and excessive amounts of hydrogen can lead to a type of pressure tube failure called Delayed Hydride Cracking (DHC). Since hydrides align with the stress field during DHC, understanding and controlling the effect of stress on hydride orientation is of great importance for safe and economical operation of CANDU reactors. The purpose of the present study is to identify microstructures and crystallographic textures of Zr-2.5Nb that may better control the hydride orientation and hence prevent DHC.; Previous authors have shown that processing variables such as extrusion ratio, extrusion temperature or beta-quenching have an effect on the texture and microstructure of Zr-2.5Nb pressure tubes. By changing these process variables, micro-pressure tubes with varying textures and microstructures were obtained for this study. Hydrides were precipitated under a range of hoop stresses from 100 to 300 MPa using a new specimen design. Seven specimens, each subjected to nine different stresses were investigated. Some samples were reheated to reprecipitate the hydrides without stress. The following conclusions were drawn: (1) for the materials studied, grain structure is the dominant factor governing the effect of stress hydride orientation; (2) materials with equiaxed grain structures are more susceptible to development of radial hydride orientation distributions than materials with flattened elongated grain structures; (3) in materials with flattened elongated grain structures, coarse grained material is more susceptible to development of radial orientation distributions than fine grained material; and (4) materials with a fine elongated grain structure display a more gradual change in orientation with stress than specimens with a fine equiaxed grain structure. This is accompanied by a growth and decline in population of hydrides of intermediate orientation with increasing stress.