Effects of oxidation on high-temperature mechanical properties of alloy 600: application to cracking in primary water of nuclear pressurized water reactors

Since the early 1970's certain component parts of the primary loops of nuclear pressurised water reactors, such as steam generator tubing and pressure vessel head penetrations, have been affected by intergranular stress corrosion cracking. This form of cracking, which occurs after several thousand hours, initiates on the component side exposed to primary water. The purpose of this study is to gain a better understanding of the cracking mechanism. The focus is on the interactions between the local mechanical loading conditions and the effects of an oxidising environment. Fatigue and creep fatigue crack propagation tests were carried out using CT specimens. The local mechanical loading conditions were determined by finite element calculations. The oxidation behaviour was studied by transmission electron microscopy. Creep tests were conducted on thin sheet samples to investigate the effects of environment on the creep properties. Intergranular crack propagation, as determined at 550°C in air and in vacuum, has been related to a creep damage mechanism. Dislocation creep deformation mechanisms, which appear to induce intergranular damage in this material at high temperature, are enhanced when the growing oxide scale is nickel- or iron-rich, compared to the results obtained in high vacuum with the protective chromium oxide scale. Thus, intergranular fracture, which occurs in all environmental conditions under creep fatigue cycles, occurs even under fatigue cycles when creep is enhanced. Fatigue crack propagation tests at 400°C gave essentially the same results as those obtained at 550°C. Crack propagation tests in deaerated water, either in low pressure vapour at 400°C or in primary water at 320°C, lead to the same cracking results. The fracture surfaces were identical to those observed after conventional stress corrosion cracking tests. For these environmental conditions, the enhanced creep model was also proposed to account for the experimental results. (author)