A Study On Electrochemical And Mechanical Properties In Stress Corrosion Cracking Prediction Model For Austenitic Stainless Steels Used In Nuclear Power Plants

Stress corrosion cracking (SCC) of structural materials such as austenitic stainless steels in high temperature water environments has been one of the major threats to the reliability and safety of nuclear power plants. In the work described in this thesis, a number of austenitic stainless steels used in nuclear power plants were studied. Based on the achievements in the world about SCC mechanism and quantitative prediction models, the electrochemical property in high temperature water environments, the internal friction and the mechanical properties were studied.On the electrochemical properties of austenitic stainless steels in high temperature water environments, the effects of temperature, chloride in water on the polarization curves and the re-passivation curves were studied. Main results showed that temperature decrease and chloride increase caused the decrease of the re-passivation ability, which would lead to longer exposure of bare metal surface at crack tip environment and then higher susceptibility to SCC.Based on the effect of crack tip strain rate in the SCC prediction model, the internal friction behaviors of the stainless steel from ambient temperature to 600℃and the effects of cold work and hydrogen were studied by means of torsion pendulum method. Results showed that cold work increased internal friction of 304L steel. Hydrogen in the steel also increased internal friction under 250℃. However the relations of internal friction to crack tip strain rate and then to SCC crack growth rate needs investigate further in the future.In the practical aspect of mechanical properties of austenitic stainless steels, statistical analyses of a large number of experimental data were performed to study the conversion and the corresponding regression equations of the relationships between Leeb-hardness, Vickers-hardness, yield strength and tensile strength of such materials. The conversion and the corresponding regression equations of the relationships were established. Based on the test results, it would be possible to estimate the strength value by measuring the Leeb-hardness and using the regression equation, and thus to predict the SCC performance of austenitic stainless steels in high temperatures water environments.As a part work of Key Basic Research (973) Project " Environmental behaviors and failure mechanisms of the key materials for nuclear power plants " funded by Ministry of Science and Technology, the thesis is a fundamental research in studying the stress corrosion cracking of austenitic stainless steels for nuclear power plants and has a certain value for studying the quantitative prediction models, and may provide a reference to build the conversion relationship between the strength and hardness of austenitic stainless steels.