Research On Thermal Aging Behavior Under Simulated Condition Of 17-4PH Stainless Steel For Nuclear Power Valve Stem

17-4PH stainless steel has been widely used in manufacture of secondary main steam isolation valve stem?MISV?components of PWR nuclear power plant due to its excellent characteristics,such as mechanical properties and good corrosion resistance in seawater and acid environment.However,17-4PH stainless steel would become thermal aging embrittlement during prolonged exposure to service temperature 283?in secondary loop environment,which will threaten the structural integrity of the secondary loop and the safe operation of the whole PWR nuclear power plants.Therefore,it is necessary to conduct an intensive investigation on the microstructures and properties of 17-4PH stainless steel over a long period service and to clarify its thermal aging mechanism to provide technological support for the ageing management and extension in nuclear power plants.In this paper,accelerated thermal aging tests were carried out on 17-4PH stainless steel in temperature 350?and pressure 16.5MPa to simulate the secondary loop steam environment.The microstructures,Brinell hardness,tensile properties,impact performance and corrosion resistance of 17-4PH stainless steel thermally aged for different times have been systematically investigated by metallographic microscope,scanning electron microscopy,transmission electron microscopy,X-ray diffractometer,brinell hardness tester,universal testing machine,Charpy impact machine and electrochemical workstation.The fracture morphologies of tensile specimen and impact specimen were observed by use of SEM,and the influence of thermal aging on fracture mechanism was analyzed.The results obtain are as follows.The matrix of 17-4PH stainless steel was composed of lath martensite and austenite which existed in grain boundaries?volume fraction accounting for about16.7%?.The martensite contained a fine dispersion?-Cu phase and a high density dislocations introduced during the process of martensitic transformati2on.In addition,spherical Nb C particles with a size of 0.65?m?0.70?m were also found in the tissue.During the process of thermal aging for 5000 hours at 350?and 16.5MPa,martensite and austenite did not change significantly.The microstructural changes mainly included precipitation of?-Cu particles in martensite lathes during the initial stage of thermal aging?less than 1000 hours?.When the aging time reached 1000 hours,the precipitation density of?-Cu particles is the largest and the average size is the smallest.At the same time,the spinodal decomposition take place in lathes that made the microstructure of mottled contrast appeared,the dark and light stripe were corresponding to the Cr-rich?'phase and the Fe-rich?phase,respectively.Then,as the thermal aging time continued to increase,?-Cu gradually ripened and spinodal decomposition became more complete.Nb C particles gradually coarsened at a lower rate throughout the thermal aging process.Thermal aged at temperature 350?and pressure 16.5 MPa which simulated secondary loop vapor environment for 5000 hours,the 17-4PH stainless steel became stronger but more brittle.Brinell hardness,yield strength and tensile strength of 17-4PH stainless steel increased by 25.96%,21.42%and 19.36%,respectively.Elongation and reduction of area decreased by 30.43%and 27.42%,respectively.More importantly,impact energy decreased by 81.05%.The fracture morphology of tensile and impact fracture showed that with the increase of thermal aging time,the area of shear region in macroscopic fracture decreased,while the area of crack propagation increased.The fracture mechanism of Ch impact was changed from the microporous coalescence to the fluvial cleavage fracture,which showed thermal aging embrittlement obviously.These mechanical properties were mainly related to the precipitation and growth of?-Cu during the thermal aging process,and the Spinodal decomposition of martensite matrix to form Cr-rich?'phase and the growth of Nb C.Thermal aged at temperature 350?and pressure 16.5 MPa which simulated secondary loop vapor environment for 5000 hours,the 17-4PHstainless steel also became weaker in different corrosive liquids such as 0.6MNa Cl,0.15M,0.3M,and0.6M H₂SO₄ and 0.1M,0.3M and 0.6M HCl.This was mainly related to the precipitation and growth of?-Cu particles during the thermal aging process,the growth of Nb C particles and the occurrence of Spinodal decomposition of martensite to form Cr rich?'phase.The precipitation and growth of these second phases would lead to the dissolution of martensite matrix in contact with them,and lead to the corrosion resistance of stainless steel decline.In addition,with the increase of H⁺concentration,the corrosion rate of passivated film on the surface of stainless steel increases,but the passivated film still has a higher protection.When the electrolyte solution contained Cl^-,the corrosion rate of stainless steel increased significantly,and the possibility of large pitting pits on the surface of stainless steel greatly increased.This is because of the small size of Cl^-,strong penetration,and easy to pass through the passivation film,which will not only accelerate the dissolution of stainless steel,but also lead to the formation of occluded Cell Corrosion,accelerating the expansion of the corrosion pit,leading to the corrosion resistance of stainless steel greatly reduced.