Microstructure And Mechanical Properties Of Z3CN20-09M Steels After Long-term Thermal Ageing In Different Temperature

Casting Duplex Stainless Steels(CDSSs)are widely used in the pressurized water reactor(PWR)for primary pressure boundary components such as primary coolant pipeline due to its high strength,excellent casting and weldability and high resistance to stress corrosion cracking.However,CDSSs will become thermal ageing embrittlement during prolonged exposure to service temperatures from 288? to 327? in PWR primary coolant piping,which will lead to performance reduction of CDSSs and threaten the structural integrity of the pressure boundary in the primary loop and the safe operation of PWR plant.So it is essential to conduct an intensive investigation on the microstructure and mechanical properties of CDSSs over a long period service and to clarify the thermal aging mechanism to provide a technological support for the ageing management and life extension in nuclear power plant.Based on the service conditions of Z3CN20-09 M CDSSs,thermal accelerated aging test for domestic centrifugal casting Z3CN20-09 M steel were respectively carried out in air and simulated working conditions at 350 ? and 400 ? in this paper.The microstructure,micro-hardness,small punch properties at 25 ? and 290 ? of Z3CN20-09 M CDSSs thermally aged for different times had been systematically investigated by OM,TEM,Vickers micro-hardness test and small punch test(SPT).The fracture morphology of the SPT specimen was observed by SEM,and the corresponding fracture mechanism has been analyzed.The results obtained are as follows.The density of dislocations in the austenite after the long-term thermal aging of Z3CN20-09 M steel is greatly reduced,and the dislocation network,sub-grain boundary and straight sessile dislocations were formed by the intersection and tangles among dislocations and the density of extending dislocations be reduced.The main reason was the interaction which dislocations,extending dislocations and among extending dislocations formed.Some small granule was appeared in the grain after aging for 1000 h at 350? and for 300 h at 400?.When aged for 3000 h at 400?,spinodal decomposition appeared in the ferrite phase,generated ? phase with Fe-rich and ?? phase with Cr-rich.With the deepening of thermal ageing,the degree of spinodal decomposition was gradually increased in the CDSSs.With the aging time extended to 10000 hours,the spinodal decomposition has not reached equilibrium.Microscopic Vickers hardness test show that:the micro-hardness value of austenite phase has no significant change with the extension of thermal aging under different environment and temperature.Thermal aging at 350?,the hardness of the ferrite phase increased slightly.The change of micro-hardness value is related to the thermal aging temperature and time.The effect of different environmental on micro-hardness value is not obvious.The SPT at 25? show that:with the increase of thermal aging time under the two condition at 350?,The ultimate strength Pu/t02 of are both slightly increases,and the fracture energy Esp slightly decrease,while the change of yield strength Py/t02 is not significant.With the deepening of thermal ageing in different circumstance at 400?,The ultimate strength Pu/t02 and yield strength Py/t02 of steels are slowly increasesed,the fracture energy Esp are persistently reduce.The change of SPT mechanical property is related to the thermal aging temperature and time.The effect of different environmental on SPT mechanical properties is not obvious.Analysis of the SEM fracture surface shows that there is no significant difference among the SPT macroscopic fracture.In the early stage of thermal aging,the microcircuit fracture of the material is mainly composed of microporous aggregation which is characterized by excellent toughness.Quasi-cleavage fracture charactered was appeared with the the thermal aging time extended.It shows that with the extension of the thermal aging time,the toughness of the material decreased,and gradually into the brittleness.The SPT at 290? show that: the ultimate strength Pu/t02 and specific yield strength Py/t02 of Z3CN20-09 M steel are increased slightly and the Esp gradually reduced with the deepening of thermal aging at different condition.Thermal aging time and temperature are the main reasons that affect the mechanical properties of small punch test.The effect of thermal aging environment is not significant.