Anisotropy in impact behaviour of Zr-2.5Nb pressure tube alloy

Pressure tubes serve as pressure boundary for hot coolant in Pressurized Heavy Water Reactor (PHWR). Zr-2.5Nb alloy is used as pressure tubes in PHWR in cold worked and stress-relieved (CWSR) condition. The microstructure of the CWSR Zr-2.5Nb pressure tube material comprises of heavily elongated Zr phase with thin grain-boundary network of Zr phase. Due to both microstructural and crystallographic anisotropy, the mechanical properties in general and fracture behaviour in particular are anisotropic for this material. An estimation of the directional dependence of the impact energy is required for assessment of the safety of the pressure tubes under accidental conditions. In this work impact behaviour of the material was characterized using instrumented impact test. The impact tests were carried out on as-received as well as hydrided conditions. Impact samples of dimension 10x3.5x55 mm3 were machined from the pressure tube spools with notches oriented in the axial or circumferential direction. Impact tests were carried out over a range of temperature varying from room temperature to 300 deg C. Fracture surfaces were examined under scanning electron microscope. The test data were analyzed to delineate the effect of crack growth direction, test temperature and hydrogen content on crack initiation and crack propagation energy values. The absorbed energy values were found to be strongly dependent on the direction of crack propagation and test temperature. Significant differences in fracture appearance were also noticed between the specimens having notch in the axial direction and notch in the transverse direction. An attempt has been made in this work to rationalize the observation in terms of microstructure, influence of sample holding device, hydrogen content and test temperature. (author)