| The third generation nuclear power plant cooling pump adopts shielded nuclear main pump,and the water lubricated thrust bearing carries all the thrust of the nuclear main pump.Safety,stability and reliability are the key to the service of the thrust bearing.The new balanced ring supporting structure can balance the height difference between the thrust pads of the nuclear main pump,ensure the uniform distribution of the load on the thrust bearing,prevent the eccentric load of the shafting,and integrate the design,which reduces the complexity of the supporting system.The advantages of the new balanced ring supporting structure are shown.The service environment of the supporting structure is complex,and it is not only corroded by boric acid-sodium borate solution coolant at 55?85?in the nuclear main pump for a long time,but also subjected to tension-compression load cycle.Therefore,corrosion fatigue becomes the main threat to the failure of balance ring supporting structure.17-4PH stainless steel has been widely used in structural parts of nuclear power equipment because of its high strength,excellent corrosion resistance and easy processing.In this paper,17-4PH stainless steel is taken as the research object,and the new balanced ring support structure of thrust bearing of nuclear main pump is taken as the application background.the microstructure and mechanical properties of 17-4PH stainless steel are observed and tested.Especially,the corrosion properties and corrosion fatigue properties of17-4PH stainless steel were studied in order to investigate the possibility of using stainless steel as an alternative material for balanced ring support structure.The microstructure and mechanical properties of 17-4PH stainless steel used in the experiment,the corrosion fatigue properties in boric acid-sodium borate solution at room temperature?about 20??,and the corrosion fatigue properties at room temperature,The corrosion properties of boric acid-sodium borate solution at 55?and 85?were detected and analyzed.The results are as follows:?1?the microstructure of experimental 17-4PH stainless steel is lath martensite,the grain is fine and uniform,the size is 2030?m,and a large number of particles?-Cu strengthening phase is distributed in the matrix.The Vickers hardness of 17-4PH steel is between 3.5 and 4GPa,the yield strength is 900 MPa,the total tensile strength is 19.3%,and the cross section shrinkage is 63.2%,the tensile fracture has typical ductile fracture characteristics.?2?at room temperature,the fatigue limit of 17-4PH stainless steel in air?air-1ir-1 is 565MPa,the fatigue limit in boric acid-sodium borate solution?corr-1orr-1 is 555 MPa.There are three typical regions of corrosion fatigue fracture:fatigue source zone,crack stable propagation zone and transient fracture zone.The source of corrosion fatigue originates from the defects such as surface knife marks or wear marks,surface corrosion pits and internal inclusions.The morphology of corrosion fatigue crack propagation zone and transient fracture zone is similar to that in air.The corrosion fatigue fracture mechanism of 17-4PH stainless steel is in good agreement with the two theories of activation during deformation and accelerated fatigue crack generation by pitting corrosion.?3?in boric acid-sodium borate solution,the annual corrosion rate of 17-4PH stainless steel is 1.20?m/a at room temperature?about 20??,3.32?m/a at 55?and 6.84?m/a at 85?.With the increase of corrosion temperature,the corrosion rate increases.17-4PH stainless steel occurs corrosion at 55?and 85?,the micro-morphology is corrosion pit,and the size of surface corrosion pit at 85?is about 2 times that of 55?.The results of electrochemical corrosion show that the corrosion resistance of 17-4PH stainless steel in boric acid-sodium borate solution is uniform at room temperature,and its corrosion resistance is similar to that of Stellite alloy,but it is point corrosion in 3.5 wt.%NaCl aqueous solution.The corrosion resistance of 17-4PH stainless steel in NaCl solution is poor. |
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