Research On Fracture Toughness And Fatigue Crack Propagation Rate Of Z3CN20.09M Steel For Nuclear Power Plant

Cast austenitic stainless steel Z3CN20.09M is widely used for critical components of primary water coolant system in nuclear power plants. But the thermal aging embrittlement will occur in the Z3CN20.09 stainless steel under the reactor coolant operating temperature(288～327℃), and it will threat to the wholeness of the pressure boundary in the primary loop and the safe operation of nuclear power plants. So it is essential to study the variation of the fracture properties of Z3CN20.09M stainless steel after long term service for safe operation of nuclear power plants.Fracture tougness, fatigue crack propagation rate, substructure and macroscopic fracture morphology of Z3CN20.09M steel both made in china and France treated with thermal aging at 400℃for 0.3000.10000 hours were investigated by the fracture toughness test, the crack propagation rate test. TEM and SEM. The paper analyzes the size effects on the fracture toughness of the domestic steel and predict of the fracture toughness after long term thermal aging. The results are as follows:The fracture toughness of domestic and French Z3CN20.09M stainless steel both decrease with the thermal aging time increase. The fracture toughness of domestic steel were lower than that of French steel under the same state. The fracture toughness values of domestic steel decreased rapidly by 23% in aging time 3000 hours, at the edage of thermal aging late, the fracture toughness values decreased slow, approximately 19.6%; The fracture toughness of Frence steel decreased by 24.2% and 14.7% in aging 3000 hours and the edage of thermal late respectively. Ferrite embrittlement which indicates Fe-richαand Cr-richα' formed by spinodal decomposition, it is the main reason for the rise of Z3CN20.09M steel embrittlement.Microscopic fracture morphology indicates that the fracture mechanism of domestic and French steel is aggregation microporous fracture at the original state. With the thermal aging time increasing the fracture mode gradually transforme into mixed fracture including cleavage fracture and aggregation microporous fracture. Cleavage and tear of the ferrite and austenite characteristics can be seen from observation of microscopic fracture morphology of long term thermal aging Z3CN20.09M stainless steel.The results of size effect on the fracture toughness of Z3CN20.09 steel showed that the fracture toughness increases with the size of the specimens, and then decreases gradually to the plane strain fracture toughness. The ductile fracture toughness increases with the size in the range of 12-20mm. and then drops down from 20 to50mm. The tearing modulus increases with the decrement of specimens thickness. In the zone near the mid-plane thickness, the intergranular ductile fracture takes place, which leads to that large dimples prevail in the fracture surface, and near the outer surface of the specimens, the transgranular ductile fracture which results in many little dimples. Theoretical analysis by the energy dissipation in the crack initiation and propagation, the constraint level and the fracture affected volume was performed to investigate the physical mechanism of different size effect. With Finite Element Analysis simulation the distribution and variety of the stress triaxiality which as an important factor of fracture strain and fracture toughness be got.The fracture toughness of Z3CN20.09M steel estimated by embrittlement dynamic equation, fracture toughness J values with the thermal aging time t showing the exponential relationship. The error between measured values and theoretical values is less than 5%. Extrapolation prediction of the fracture toughness of aged 1.5 X 104 is performed by the power functions established in this study, its value is 223.76 KJ/m2±11.89kJ/m2. and the confidence probability is 95%.For original state of Z3CN20.09M stainless steeel. the fatigue crack propagation rate curve lower than that of thermal aged steel, and the date dispersion larger. The fatigue crack propagation rate curves have big slope at aging 3000 and 10000 hours, so its values increased very quickly. When△D is more than a critical value, crack propagation will occur, and fatigue crack propagation rate is faster than that of original state. With aging time while the microstructure of Z3CN20.09M steel has changed, strength and embrittlement of the steel increased but toughness decreased greatly, which result in fatigue crack propagation resistance decreases, and the crack propagation rate significantly increased.