| With the continuous development of high speed and heavy load of China’s high-speed railways,higher performance requirements have been put on the brake discs of high-speed railways.As a key component of high-speed train safety braking,the brake disc also has extremely high requirements for its performance.The core of the brake disc needs high strength and toughness,while the surface part requires high strength and wear resistance.The traditional subtractive manufacturing method has a long processing time and is difficult to manufacture.Selective laser melting technology is a kind of additive manufacturing.In recent years,it has been widely used in the preparation of titanium alloys,nickel-based superalloys and stainless steels.The selective laser melting technology has the ability to process complex structural parts,and is suitable for preparing high-speed train brake discs with internal support structures.Therefore,this paper uses the selective laser melting(SLM)technology to prepare 24CrNiMo alloy steel brake discs.The selective laser melting technology is prone to generate pores at the overlap between layers and channels during sample preparation.Therefore,further hot isostatic pressing is performed on the formed 24CrNiMo alloy steel to improve the material density.Using EOS M290 metal 3D printer,optimized laser process parameters:laser power 320 W,laser scanning speed 950 mm/s,scanning pitch 110 μm,laser spot diameter 75 μm,stripe partition scanning width 10 mm,layer In the scanning direction of 67°,24CrNiMo alloy steel samples without macro cracks and defects were successfully prepared by selective laser melting under the condition of argon protection(oxygen content of 0.05%)in the forming chamber;hot isostatically formed 24CrNiMo samples were formed.Phase analysis and micro structure characterization using OM,SEM+EDS,TEM,EPMA and XRD.Use micro hardness tester and tensile testing machine to analyze its mechanical properties.Experimental results show that the microstructure of the formed 24CrNiMo alloy steel sample is composed of As fabricated(AS)and heat affected zone(HAZ).The AS zone consists of lath martensite,lath bainite structure,and a small amount of retained austenite;the HAZ zone consists of tempered martensite,lath bainite structure,and granular bainite structure.The density of the sample SLM 24CrNiMo steel is 7.7980 g/cm3.Referring to the density of CRH2 forged steel brake discs,the sample density ratio is 98.96%.The microhardness of the SLM 24CrNiMo sample is 458.2 HV0.2.The yield strength,tensile strength,and elongation of the samples were 1280 MPa,1370 MPa,and 11.2%,respectively.All meet the requirements for the use of brake disc performance of high-speed trains.The samples of 24CrNiMo steel prepared by selective laser melting were subjected to two batches of hot isostatic pressing subsequent heat treatment.Its hot isostatic pressing process parameters are:working pressure 120 MPa,700℃ for 4 h and 800℃ for 2 h,and high-purity argon as the pressure medium.The structure after hot isostatic pressing at 700℃ for 4 h was lath ferrite and granular carbide.The density ratio is 99.06%,the microhardness value is 309.6 HV0.2,the yield strength,tensile strength and elongation of the samples were 791 MPa,939 MPa and 15.9%.The microstructures of the samples after hot isostatic pressing at 800℃ for 2 h were polygonal ferrite,massive bainite,and granular pearlite with intermittent carbides.The sample density ratio was 99.50%,which was 0.5%higher than SLM 24CrNiMo sample.The microhardness of the sample is 385.5 HV0.2.The yield strength,tensile strength,and elongation of the samples were 744 MPa,1100 MPa,and 15.3%,respectively.Compared with the formed 24CrNiMo sample,the density is increased,and the strength is slightly reduced,but the yield ratio is reduced by 27.6%,and the toughness is increased by 36.6%.Obtaining good comprehensive mechanical properties can be used as the core material of brake discs for high-speed trains. |
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