Study On Microstructure Evolution Mechanism And Properties Of Laser Selective Melting 24CrNiMoY Alloy Steels

As a key component of high-speed train,the structure of brake disc is complex,its core needs high strength and toughness,and its surface needs good wear resistance.Domestic brake disc manufacturing technology reserves are weak,and the brake disc has depended on imports.Laser selective melting high-speed rail brake discs is one of the new research technologies in the frontier fields at home and abroad,the CrNiMo alloy steel is commonly used for forming brake disc.Because of its complex composition and various phase transformation,it is easy to cause deformation and cracking,and it is difficult to control shape and performances of samples prepared by laser selective melting technology.Therefore,24CrNiMoY alloy steel without cracks was fabricated by selective laser melting,and the research of its structure evolution mechanism and properties has scientific research significance and practical application value.The 24CrNiMoY alloy steel with different layers and process parameters was fabricated by using FL-Dlight02-3000W semiconductor laser.The microstructure the samples were characterized by OM,SEM,TEM,and the evolution of microstructures were studied.The nonequilibrium phase diagrams of 24CrNiMoY alloy steel at different cooling rates were simulated by Jmatpro software,the thermodynamic and kinetics of phase chage are analyzed according to experiemental results and nonequilibrium phase diagrams.The universal material testing machine and friction wear testing machine were used to test the performances of sample.The experimental results show that there are no cracks in 24CrNiMoY alloy steel prepared by laser selective melting at optimized process parameters.The microstructure of the samples upper position is lath bainite,granular bainite and ferrite,and the lower position is granular bainite and ferrite.The substructure of lath bainite is comprised of lath ferrite and short rod-like ?-carbides at an angle of 60° to 70° with ferrite lath.The substructure of granular bainite is equiaxed ferrite.The relationship between the phase content and laser power,laser scanning rate is a cubic analytic function,and the influence of laser scanning rate on the structure is greater than that of laser power.The results of thermodynamics and kinetics analysis of martensite and bainite transformation show that when the composition of alloy steel is constant,martensite and bainite transformation are only affected by cooling rate,cooling temperature and supercooling degree.According to the peak temperature and ambient temperature,the complex thermal history of laser selective melting alloy steel is simplified to six kinds of thermal cycles for the first time.Based on the six kinds of thermal cycles,a model of microstructure evolution of laser selective melting 24CrNiMoY alloy steel is established.The combination of different thermal cycle types and frequency will lead to the formation of different structures.With the increase of laser power and scanning rate,the average hardness,density,friction coefficient,yield strength and tensile strength of the sample first increase and then decrease.The performances of sample at the laser power of 1800 w and the scanning rate of 12 mm/s is the best,the average hardness is 353 HV0.2,98.9%of relative density,0.38 of friction coefficien,890 MPa of tensile strength,810 MPa of yield strength,and the elongation is 8%.