Failure Mechanism Of CF8A Stainless Steel And Its Welded Joint In A Simulated PWR Environment

CAP1400 pressurized water reactor(PWR)nuclear power units were introduced by China based on digesting,absorbing and comprehensively mastering the third generation of advanced nuclear power AP1000.Through reinnovation,a large passive advanced PWR nuclear power units with independent intellectual property rights and greater power have been developed in China.The CF8A austenitic stainless steels with large dimension in length produced by centrifugal casting were used as the construction material for the primary pipes of CAP1400 nuclear power units.In order to optimize the performance and to allow operation under severe service conditions,the stress corrosion cracking(SCC)and corrosion fatigue(CF)behaviors of CF8A austenitic stainless steels and Inconel 52M/CF8A austenitic stainless steels dissimilar metal welded joints in simulated PWR environment were systematically studied,and the failure mechanism of CF8A austenitic stainless steels and its welded joints in primary pipes were fully revealed.The innovative research work of this paper includes:(1)The SCC behavior of CF8A austenitic stainless steel in simulated PWR environment was studied by means of slow strain rate tensile test(SSRT).It was found that the SCC crack size increases with the decrease of strain rate,and the number of SCC cracks increase with the decrease of strain rate.With the growth of SCC cracks,the crack spacing ratio becomes smaller,and the interaction between cracks becomes more and more intense.Individual cracks grow preferentially by breaking through the shielding effect between cracks,and have a significant inhibition effect on the surrounding cracks,and eventually become the critical crack leading to specimen fracture.(2)The oxidation behavior of CF8A austenitic stainless steel in simulated PWR environment under constant load and overload test conditions was studied.Compared with the constant load condition,the protective ability of the oxide film formed under overload condition to the matrix was seriously degraded,and the inner oxide changed from Cr₂O₃ to non stoichiometric spinel oxide(Ni,Fe)(Fe,Cr)₂O₄,which is caused by the increase of the chemical potential of the matrix and the change of the orientation between the oxide film and the matrix.(3)The microstructure evolution and oxidation behavior of CF8A austenitic stainless steel during CF cracks initiation and propagation in simulated PWR environment were studied.Under the action of alternating load,the near surface performance of the specimen deteriorates due to the formation of Cr poor region and slip band array,which causes the CF cracks to sprout from the slip band.With the growth of CF cracks,the strong anodic dissolution and the redistribution of stress and strain at the crack tip weaken the tendency of CF cracks growth,which makes the CF cracks present typical environment assisted transgranular fracture.(4)The SCC behavior and stress assisted oxidation behavior of Inconel 52M/CF8A austenitic stainless steel welded joint in simulated PWR environment were studied.Due to the difference of precious metal content and plastic deformation between Inconel 52M and CF8A stainless steel,corrosion couple was produced near the fusion zone during SSRT test in simulated PWR environment,which accelerated the anodic dissolution of the heat affected zone and resulted in the fracture of Inconel52M/CF8A stainless steel welded joint from the heat affected zone.