Accelerated Thermal Ageing Behavior Of Stainless Steel For Nuclear Power Plant Under Different Temperature

With the merits of both austenitic and ferrite steels, several excellent characteristics, such as high resistance to stress corrosion and excellent mechanical properties, casting duplex stainless steels (CDSS) have been widely used in the pressurized water reactor (PWR) for primary pressure boundary components. However, CDSS will become thermal aging embrittlement during prolonged exposure to service temperatures from 288℃ to 327℃ in PWR primary coolant piping, and with the deepening of thermal aging embrittlement, the critical crack size of pressure parts will decrease, which will threaten the structural integrity of the pressure boundary in the primary loop and the safe operation of PWR plant. Therefore, it is very important for engineering practice to study the microstructure and mechanical properties of CDSS after thermal ageing.Based on a review of application of Z3CN20-09M CDSS, the microstructure, micro-hardness, small punch (SP) properties and fracture analysis of Z3CN20-09M CDSS thermally aged for different time at 400℃ and 350℃ have been systematically investigated by OM, TEM, Vickers micro-hardness test and small punch test (SPT). The fracture morphologies of SP specimens have been observed by use of SEM and the corresponding fracture mechanism has been analyzed. The main conclusions obtained are as follows.The microstructures of Z3CN20-09M CDSS are composed of austenite and low ferrite with lacy and island type uncontinuously distributed on the austenite matrix and there is no obvious effect of short term thermal ageing on the content and distributions of the ferrite. After short-term thermal ageing at 400℃ and 350℃, the density of dislocation in austenite decreases while the density in ferrite slightly changes, but a little second phase particles separate out around the dislocation lines. And the number of these particles tends to decrease and the size of them increases slightly. The separated particles at 400℃ is more than those at 350℃.At the equivalent thermal ageing time at 350℃ and 400℃, the Vickers micro-hardness of ferrite phase and the value of Pu/t20 increase first and then decrease with the prolonging of thermal ageing time increases, which show peaks at 419h for 350℃ and 100h for 400℃, after the peaks, they tend to increase slowly. However, the micro-hardness of austenite phase and the value of Py/t02 have no obvious changes and the the value of Ef/t decline slightly with the time.Z3CN20-09M steel SPT’s fracture begins in the outer surface folding, and the folding appear in the phase interface of ferrite and austenite mostly. The macro morphology fracture shows ductile dimples of different size can be observed in crack propagation area, which are lengthen or squashed in the deformation process.After accelerated thermal ageing at 350℃ and 400℃ for various time, the fracture surface does not change obviously, which shows no characteristics of quasi-cleavage. The fracture mechanism is still microvoid coalescence and dimpled ductile fracture, which includes the nucleation, the expansion and the consolidation of cavitations.