Study On Thermal Aging And Nondestructive Evaluation Of Z3CN20.09M Stainless Steel For Primary Coolant Pipes Of Nuclear Power Plant

At present,the primary circuit piping of nuclear power plants in service mainly uses cast austenitic-ferritic duplex stainless steel,and has already achieved localization.The ferrite phase in cast duplex stainless steel(CDSS)will suffer thermal aging embrittlement after long-term service at intermediate temperatures(280-500 ?),which increase the brittleness of the primary circuit pipes and exists security risk.Effective monitoring and reliable evaluation of thermal aging embrittlement of CDSS used in primary circuit piping of nuclear power plants is important to the safe operation of nuclear power plants.At the same time,the mechanical properties data about the thermal aging embrittlement of the domestic CDSS are relatively scarce.Therefore,it is of great practical significance to carry out experimental research on the thermal aging embrittlement of domestic CDSS and collect basic data to ensure the safe operation and life management of nuclear power plants in China.In this paper,the accelerated thermal aging experiments of domestic CDSS Z3CN20.09M cut from primary circuit piping of nuclear power plants were carried out at 350,380,400 and 454??The longest thermal aging time was 10000 hours.The changes of material structure,mechanical properties and eddy current signals during thermal aging were studied,and the relationship between them was discussed.At the same time,the existing method for evaluating thermal aging embrittlement was studied,and the eddy current testing was used to evaluate thermal aging embrittlement of CDSS.These research results will contribute to the evaluation of the thermal aging embrittlement of DDSS.The results of mechanical property show that with the increase of thermal aging time,the Charpy impact energy of the material decreases significantly.Finally,it tends to a saturation value of 128J,which is much higher than the lower limit of impact energy value of 80J stipulated by relevant standards.The microhardness of ferrite phase increases significantly and tends to be stable gradually.The microhardness of the austenite phase remains unchanged.The higher the heat aging temperature,the faster the impact energy decreases,and the more obvious the microhardness of ferrite phase increases.The microstructure of domestic CDSS Z3CN20.09M is characterized by discontinuous ferrite distributed on the austenite matrix.The main morphology of ferrite is banded and island,and the ferrite content determined by metallographic method is 12.8%.Thermal aging has little effect on the morphology and quantity of ferrite.TEM observations indicate that the ferrite decomposes into an iron-rich and a chromium-enriched domains,and G-phases precipitate in ferrite.The G phase precipitation has a certain incubation period.Meanwhile,the number of dislocations in ferrite increases obviously,while austenite does not change substantially.Scanning electron microscopy was used to observe the effect of thermal aging at 400? on the impact fracture surface.It is found that with the extension of thermal aging time,the fracture is transformed from the ductile fracture with many fine dimples to the brittle fracture characterized by cleavage plane.According to the influence of thermal aging on the mechanical properties of materials,the Chopra model,which is commonly used to evaluate the mechanical properties of primary circuit piping in nuclear power plants,is analyzed.The calculation of thermal aging activation energy which is the key parameter in the model was optimized.Based on the change of microhardness of ferrite phase during thermal aging,a new method for calculating activation energy of thermal aging by exponential function fitting is proposed.In this paper,the thermal aging activation energy for domestic CDSS Z3CN20.09M was calculated to be 73.9 kJ/mol by exponential function fitting method based on the change of microhardness of ferrite phase during thermal aging.A formula for predicting the change of Charpy impact energy of domestic CDSS during thermal aging is obtained based on the calculated activation energy and Chopra model,which can better predict the change of room temperature Charpy impact energy of domestic CDSS during thermal aging.At the same time,the functional relationship between the microhardness of ferrite phase and the Charpy impact energy is established,which can be used to evaluate the thermal aging embrittlement of duplex stainless steel nondestructively.Eight detection frequencies between 20000Hz and 90909Hz were selected for eddy current testing of domestic CDSS after thermal aging at 380 and 400 ?.The results of eddy current test show that the amplitude of eddy current signal of domestic CDSS Z3CN20.09M decreases with the increase of aging time?The variation of eddy current signal amplitude with thermal aging time is similar to that of room temperature Charpy impact energy with thermal aging time.There is a good correlation between the mechanical properties and the amplitude of eddy current signal of domestic CDSS Z3CN20.09M after thermal aging.In the eight selected frequencies,the best detection frequency is 38461Hz.Comprehensive analysis of the relationships between the amplitude of eddy current signal and the Charpy impact energy,the amplitude of eddy current signal and Vickers microhardness at two temperatures.Using the percentage reduction of eddy current signal amplitude as a variable,the functional relationships between the percentage reduction of eddy current amplitude and Charpy impact energy,the percentage reduction of eddy current amplitude and ferrite microhardness were obtained.There is a good functional correlation between the percentage reduction of amplitude and Charpy impact energy when the eddy current detection frequency is 38461Hz.