Hydrogen concentration limit and critical temperatures for delayed hydride cracking in zirconium alloys

Abstract An experimental study was carried out to determine the hydrogen concentration limit as a function of temperature at which delayed hydride cracking (DHC) commences in Zr-2.5 Nb pressure tube material. For a given hydrogen content of the specimen, two critical temperatures were observed in this work — a DHC initiation temperature, T c at which DHC would initiate when approaching the test temperature from above the solvus (or terminal solid solubility) for hydride dissolution (TSSD) and a DHC arrest temperature, T h , obtained by heating the same specimen from T c after DHC had started. Both of T c and T h are close to, but below, temperatures defined by TSSD for the specific hydrogen content of the specimen. A theoretical analysis was carried out to quantitatively derive the hydrogen concentration limit and these critical temperatures. The theoretical por T c depends sensitivity on the particular solvus or terminal solid solubility curve for hydride precipitation (TSSP) used, since there is a wide range of values for TSSP depending on the thermal-mechanical history of the material. It is also suggested that T h is governed by the TSSP for hydride growth, in contrast to T c , which is governed by the TSSP for hydride nucleation. A model for a previously observed critical temperature ( T A ) is also proposed. T A is a DHC arrest temperature, obtained by approaching the test temperature from a lower temperature. The model suggests that T A is controlled by the energy difference between TSSD, TSSP and the hydrostatic stress at the crack tip.