High temperature tensile properties of Zr-2.5 wt % Nb alloy pressure tubes used in IPHWR220

The pressure tubes that are being used in Indian Pressurized Heavy Water Reactor (IPHWR) of 220 MWe are made of Zr–2.5 wt % Nb alloy. The mechanical behavior of these components depends on the processing route adapted. These pressure tubes are made by the process of hot extrusion followed by two stages of cold pilgering with an intermediate stage of annealing and finally autoclaving. In this work, tensile tests of un-irradiated Zr–2.5Nb alloy tube were done in the temperature region of 25 – 750°C. The tests were performed under a constant engineering strain-rate of ∼1 × 10−4 s−1. Tensile properties such as 0.2% offset yield stress, ultimate tensile stress, and total elongation were determined at the test temperatures. At lower temperature, strain hardening was dominant at the onset of plastic deformation whereas at high temperature flow softening was observed. The material exhibited large strain at elevated temperature indicative of super plasticity. Fracture studies revealed brittle fracture at room temperature whereas at elevated temperatures the fracture mechanism was by ductile failure.