Research On The Mechanism Of Casting Stainless Steels For Nuclear Power Main Pipeline Thermally Aged At Different Temperature

Cast duplex stainless steel?CDSS?is composed of austenite and ferrite duplex structure.Because of its high strength,good cast ability and strong resistance to stress corrosion cracking,it is widely used in the primary circuit main pipeline of pressurized water reactor?PWR?nuclear power plant and other important components.However,the aging and embrittlement of duplex stainless steel will occur in the temperature range 288³27?for a long time,which will lead to the decline of material properties,thus posing a great threat to the service safety and life extension of nuclear power plants.Therefore,it is of great significance to study the mechanism of thermal aging and the changes of structure and properties of duplex stainless steel after long-term thermal aging at service temperature.In this paper,the serrated yield activation energy of Z3CN20-09M steel for PWR primary circuit main pipeline was measured by tensile tests at medium and high temperatures.The effects of equivalent thermal aging at 400?and 350?and long-term thermal aging at 400?on the microstructure and mechanical properties of the steel were analyzed and compared by means of transmission electron microscopy,metallographic microscopy and scanning electron microscopy,as well as Vickers hardness,small punch test?SPT?and instrumented impact test,and then the mechanism of thermal aging embrittlement was explored.The main conclusions were as follows.The activation energy Q of Z3CN20-09M stainless steel in the range of 250⁴00?was about 112 kJ/mole based on the relationship between strain rate,critical strain and temperature,which was very close to that by chemical composition.By substituting the calculated activation energy into the Arrhenius kinetic formula,the ratio of the equivalent thermal aging time between400?and 350?was about 1 to 5.The microstructures of Z3CN20-9M CDSSs were composed of austenite and delta-ferrite with dot,strip and island type uncontinuously distributed on the austenite matrix and there was no obvious effect of long term thermal ageing at 400?and 350?on the distributions and content of the ferrite.With the increase of aging time,various dislocation configurations changed in austenite at 400?,the dislocation density and the degree of dislocation entanglement decreased.The second phase particles Cr₂N were precipitated in the ferrite and austenite phases of stainless steel after thermal ageing for 100 h at 400?;spinodal decomposition with dispersed and disteibuted Cr-rich??phase and Fe-rich?phase was found in ferrite phase after thermal ageing3000 h.After aging for 5000 h,the?/??phase boundaries of spinodal decomposition were slightly clear,and the spinodal decomposition structure aggregated and coarsened.The change trend of substructure structure at 400?was similar to that at 350?thermally aged for the equivalent time.The microhardness of ferrite phase rapidly increased at 400?thermally aged for 100 h and3000 h and at 350?for the equivalent thermal ageing time,and then slowly increased.The microhardness of austenite phase was not affected by thermal aging time.The specific yield strength?P_y/t₀²?and specific tensile strength?P_u/t₀²?of stainless steel increased slowly,while the specific fracture energy(E_sp)decreased slowly with the prolongation of thermal aging time at400?and 350?.The load-displacement curves in instrumented impact test of stainless steel at400?and 350?gradually narrowed and increased with the extension of equivalent thermal aging time.The impact energy values of W_iu,W_a and W_t decreased sharply at first and then slowly with the extension of equivalent thermal aging time.With the prolongation of thermal aging time,the fracture mechanism of SPT at 400?and350?were ductile fracture,but the number of dimples decreased gradually.whereas the instrumented impact fracture mechanism transitted from a tough fracture with a large number of dimples to a mixed fracture with dimples and cleavage step.Thermally aged for the equivalent thermal aging time at 350?and 400?,there was the silimar change trend on the microstructure and substructure,microhardness of ferrite phase and austenite phase,small punch properties and instrumented impact properties and fracture mechanism of SPT and instrumented impact test.After a long thermal ageing of 15000 h at 400?,G phase was precipitated in ferrite phase in Z3CN20-09M stainless steel,At the same time,the spinodal decomposition structures were further aggregated and coarsened,and the spinodal decomposition was close to equilibrium.The amount of G phase in ferrite increased after thermal ageing of 30000 h,and the boundary of spinodal decomposition?/??phase was clear,and the equilibrium was basically achieved.Under the synergistic action of G phase and spinodal decomposition,the mechanical properties of Z3CN20-09M stainless steel remained basically unchanged after thermal aging at 400?for 30000 h.At this time,the SPT fracture mechanism was mixed brittle fracture,and the oscillographic impact fracture mechanism was brittle fracture.