Research On Microstructure And Mechanical Properties Of 12CrNi2 Steel Fabricated By Laser Additive Manufacturing

The nuclear power emergency diesel engine camshaft is a component for controlling the opening and closing of the diesel engine valve.It is generally prepared by forging and machining of 12CrNi2 steel because of its good strength and toughness.Laser additive manufacturing technology has the advantages of precision molding and high-performance molding.The use of this technology to manufacture camshaft has enormous advantages compare to conventional processes.In this paper,the forming process of laser additive manufacturing 12CrNi2 steel was studied.The microstructure and tensile properties of formed parts were analyzed and tested.In addition,the heat treatment process and surface WC_p/Fe composite strengthening of the formed parts were studied.In this experiment,the effects of laser power,scanning speed and powder feeding on the geometric characteristics of single-layer deposition layer were studied.It was found that 40%of the lap joint can obtain smooth surface of single layer multi-channel deposition layer.When the laser+5mm defocusing and the powder 0 defocus,the multi-layer multi-channel deposition process is unstable,and there is a periodic fluctuation of the plasma,which determines a well-formed block sample cannot be obtained.When the laser is-5 mm defocused and the powder 0 defocus,the deposition process is stable,and a well-formed block sample is obtained.When the flow rate of the powder feeding gas is15 L/min,a large amount of unfused defects exist in the block sample.When it is reduced to 5 L/min,the block sample and thin-walled sample which are well-formed and have no unfused defects are obtained.The microstructure evolution process of the block sample was analyzed by SEM,TEM and XRD,and compared with the thin-walled microstructure.Single-layer single-channel and single-layer multi-channel deposition layer are composed of low carbon lath martensite due to their fast cooling rate.The block sample has a certain heat accumulation effect,which causes the cooling rate slow down.From bottom to top,the microstructure of block sample is tempered martensite+lath bainite+small amount of ferrite,granular bainite+ferrite+small amount of tempering martensite,martensite+lath bainite.The remelting heat of the block sample causes the grains to be significantly smaller than the grains of single-layer single-channel and single-layer multi-channel deposition layer.Due to the severe heat accumulation effect of the thin-walled sample,the cooling rate is further slowed down than that of block sample.The microstructure from the bottom to the top is lath bainite+tempered martensite+small amount of ferrite,ferrite+carbide clusters+small amount of bainite,ferrite+carbide clusters,martensite+lath bainite.The tensile properties of thin-walled sample in different directions and the block sample were tested.The tensile strength of the block sample is up to 915 MPa,elongation is as high as 17.7%due to the fine grains.The tensile properties of the thin-walled sample are anisotropic,the strength of X-Z and X-Y directions are equivalent,but the elongation of X-Z direction is significantly lower than X-Y direction.Due to the huge difference in microstructure,the strength of thin-walled sample?685MPa?is significantly lower than that of block sample?915 MPa?,but the elongation?28.5%?is higher than that of block sample?17.7%?.The anisotropy of tensile properties of the sample is eliminated after quenching,the strength is decreased after tempering,but the elongation is improved.The tensile strength of thin-walled sample is significantly improved after quenching and tempering at 500?for 1h.A well-formed WC_p/Fe composite deposition layer is obtained by optimizing the deposition process parameters of 12CrNi2 steel.The composite deposition layers with WC addition amount of 20%,30%,40%and 50%?wt.%?have a large number of longitudinal through cracks.However,there are not longitudinal through cracks in the composite deposition layer with WC addition amount of 10%.During the deposition process of composites,WC reacted with Fe to partially decompose.The results of XRD and EDS showed that there were Fe₃W₃C and Fe₆W₆C in the composite deposition layer.Due to the formation of brittle hard phase and the solid solution of C and W in Fe,the hardness of composite deposition layer with WC addition amount of 10%increases to587.4 HV,which is 89.4%higher than that of 12CrNi2 steel,wear amount is only 9.8%of 12CrNi2 steel at the same condition.