| Nuclear power is one of the first choices of clean energy and one of the main adjustment directions of national power system in the future.Nuclear emergency diesel engine generator set ensures that the nuclear power plant can start quickly under unexpected power outage,reduces the waste heat of the reactor core,and executes the safe shutdown of the reactor,which can avoid the potential safety hazards caused by nuclear leakage.Because of the complex structure of emergency diesel engine generator set,camshaft is the core component of diesel engine.During the operation,the camshaft bears large torque and periodic impact load,and links up with many parts in the diesel engine.Therefore,its strength,surface wear resistance,accuracy and roughness have higher requirements.It takes a long time to manufacture camshafts,requires high-precision moulds,and passes through many processes such as lathe,high-precision machining and so on.It is difficult to manufacture camshaft of emergency diesel generator by traditional manufacturing method.The laser direct deposition forming technology can realize the near-final forming of complex parts,which has the characteristics of high manufacturing speed,low cost and high material utilization.Therefore,this paper carried out the research of laser direct deposition forming method to prepare camshaft material for nuclear power plant emergency diesel engine.In order to obtain the influence of laser process,hot isostatic pressing and post-heat treatment parameters on the microstructure and properties of 12CrNi2 alloy steel.Obtain basic data support for laser direct deposition nuclear power emergency diesel camshaft.The 12CrNi2 alloy steel block material samples were successfully prepared by laser direct deposition,and then hot isostatic pressing and conventional post-heat treatment were carried out to control the microstructures and obtain better comprehensive mechanical properties.The microstructures were characterized by OM,SEM,EPMA,TEM and XRI),and the properties were analyzed by microhardness tester,universal material tester and impact tester.The effects of laser direct deposition process parameters and hot isostatic pressing and heat treatment on the microstructures and properties were obtained.The experimental results show the laser directly deposited 12CrNi2 sample has good formability and no other defects such as cracks.Under the condition of other laser process parameters,when the laser power is increased from 700W to 900W,the number of micropores is slightly reduced,the density of the deposited samples is increased,and the average density is stable between 98.2%and 98.3%.Relative to the forged 12CrNi2 alloy steel).The samples deposited by laser are martensite in the area of 1-2 mm from the upper surface,and the structure in the middle is ferrite and granular bainite.The yield strength of as-deposited samples prepared by optimizing process conditions(laser power P=900W)is 665.2 MPa,the tensile strength was 703.7 MPa,and the elongation was 15.6%.The average microhardness of the upper surface martensite is 329 HV,and the average hardness of the central stable ferrite and granular bainite is 223 HV.Two batches of hot isostatic pressing(HIP)treatment were carried out on the deposited samples formed by laser direct deposition.The HIP treatment processes were 880? holding for 4 hours and 820? holding for 2 hours,respectively.The pressure was 120 MPa.Argon was used as the medium for pressure transfer,and the cooling rate was 15?/min.After hot isostatic pressing,the microstructures are ferrite and pearlite structure.When the hot isostatic pressing process is 880? for 4 h,the pearlite content is 30.8%,the size of pearlite structure is about 13 ?m,and the size of ferrite is about 16 ?m.When the hot isostatic pressing process is 820? for 2 h,the pearlite content is reduced to 19.2%,and the size of pearlite structure and ferrite is reduced by 12 ?m and 14 ?m,respectively.The structure of hot isostatic pressing can be controlled by reducing the temperature and holding time of hot isostatic pressing.The mechanical properties of the samples treated by hot isostatic pressing at 820? for 2 hours are better than those treated at 880? for 4 hours.The average microhardness is 155 HV,the yield strength is 381.2 MPa,the tensile strength is 488.8 MPa and the elongation is 26.7%.According to the requirement of camshaft,12CrNi2 alloy steel under hot isostatic pressing was heat treated to adjust its comprehensive mechanical properties.After hot isostatic pressing(820?/2h),after heatment(850?/30min+300?/2h),The average microhardness obtained by heat treatment is 329 HV,yield strength 960.8 MPa,tensile strength 1092.7 MPa,elongation 16.3%,impact toughness 140.26 J/cm2.The hardness values of different heat-treated samples were obtained preliminarily,and the mechanical properties could be judged preliminarily. |
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