Influence of Soaking Temperature and Time on Microstructure and Mechanical Properties of Water Quenched Zr–2.5Nb Alloy

Abstract

In heavy water cooled nuclear reactors, Zr–2.5Nb alloy is used for pressure tubes under cold worked and stress relieved (CWSR) condition in Canadian Deuterium Uranium (CANDU) reactors and under quenched and aged condition in RBMK and Fugen reactors. Because of its ease of use and the lower cost of fabrication, most of the work reported in literature is focused on the development and characterization of cold worked and stress relieved pressure tube material. However, recent work showed that the tubes manufactured using quenched and aged route showed a lower and predictable rate of in-reactor deformation during reactor operation as compared to those fabricated using the CWSR route. One of the important stages in the fabrication of heat treated pressure tube material is solution heat treatment (SHT), which governs the microstructure and hence, the mechanical properties of pressure tubes. Usually, SHT is carried out in two phase (α + β) region at a temperature closer to the β transus. The present work characterizes the influence of different SHT parameters such as soaking temperature (850, 870, and 890°C) and duration (15 and 30 min) on microstructure, texture, and Nb partitioning between phases and mechanical properties such as tensile properties, hardness, and fracture toughness. Fracture toughness parameters were determined as per ASTM standard E1820-11. Optical microscopy was used for microstructure, X-ray diffraction for texture, and EPMA for Nb partitioning investigation. The fracture surfaces were examined using SEM.