Study On Modification And Bending Fatigue Properties Of 20CrNiMo Gear Steel Composite Surface

Gear is a key transmission component in high-speed rail,automobiles,aircraft,and other high-end equipment.In the process of service,the root of the gear is prone to impact stress and alternating bending stress,which often causes premature fracture due to insufficient toughness or bending fatigue property.Low carbon alloy gear steel has high strength toughness and impact resistance,but its hardness is low,so it needs to be strengthened by carburizing phase change in gear application.With the development of modern industry,the gear with single carburized phase change strengthening treatment cannot meet the service life requirements of high-end equipment.The fatigue resistance manufacturing of gear with composite surface modification is an effective means to improve its fatigue performance.For this reason,this thesis takes 20CrNiMo gear steel as the research object.The influence of composite surface modification on surface properties and bending fatigue properties of 20CrNiMo gear steel is studied by the carburizing phase transformation strengthening treatment of 20CrNiMo gear steel,and then the composite high-energy laser shock strain strengthening treatment.The mechanism of improving the bending fatigue property of 20CrNiMo gear steel by composite surface modification was also revealed.The research results can provide a theoretical reference for the composite surface modification technology of 20CrNiMo gear steel.This thesis mainly carries out research from the following aspects:（1）Theoretical analysis of 20CrNiMo gear Steel composite surface modification.Firstly,the mathematical models of the carbon concentration and hardness gradient of the phase change strengthened 20CrNiMo gear steel and the process parameters of carburizing were analyzed,which provided a theoretical basis for the selection of the process parameters.Secondly,through the theoretical analysis of the laser shock pressure peak and the plastic deformation of the 20CrNiMo gear steel sample,the mathematical model of the peak pressure and strain of the sample was established,and the suitable range of laser shock process parameters was obtained.Finally,the residual compressive stress field and grain refinement mechanism induced by surface modification of carburizing and laser shock were analyzed.（2）The process parameters of surface carburizing phase change strengthening were optimized.Based on the theoretical analysis of phase transformation strengthening of 20CrNiMo gear steel,in order to obtain an effective hardening layer depth of about 1mm on the specimen surface,under the conditions of strong penetration temperature of 930℃/carbon potential of 1.1wt.%/strong penetration time of 45min/diffusion temperature of 910℃/carbon potential of 0.8wt.%,Three groups of carburizing tests with diffusion time of 50min,100min and 180min were carried out respectively.The results show that the effective hardening depth of the three groups of samples is0.65mm,0.80mm,and 1.10mm,respectively.When the diffusion time is 180min,the surface microhardness of the sample is 740HV_0.1,and the surface residual compressive stress is-295MPa.（3）Effect of composite surface modification on surface properties of 20CrNiMo gear steel.The effects of carburizing,laser shock,and carburizing combined laser shock on the surface properties of 20CrNiMo gear steel were compared.It was found that the surface residual compressive stress and hardness of 20CrNiMo gear steel were significantly improved by composite surface modification compared with single carburizing and single laser shock peening.The surface residual compressive stress was-960MPa and the surface hardness was 845HV_0.1.The surface properties of the composite surface were improved by carburizing phase transformation strengthening,and then by high-energy laser shock strengthening.High compressive residual stress,high hardness,and grain refinement were obtained in the surface layer.（4）The process parameters of laser shock in composite modification were optimized.The influence of different shock energy and shock times on the surface properties of carburized20CrNiMo gear steel was investigated.The results show that the surface hardness of the composite modified sample increases from 740HV_0.1 to 958HV_0.1（29.5%）,and the depth of the affected layer is about 400μm when the laser impact energy is 4J and the impact frequency is 2 times.The surface residual stress is increased from-295MPa to-1300MPa（341%）,and the depth of the affected layer is about 800μm.The depth of the grain refinement layer is about 28.8μm.（5）The bending fatigue property of the 20CrNiMo gear steel composite surface modified was studied.By comparing the bending fatigue properties of the carburized specimens and the composite surface modified specimens,the results show that the bending fatigue life of the carburized specimens is increased by 5.25 times when the laser shock energy is 4J and the number of shocks is 2.The composite surface modification can transfer the fatigue source from the surface to the subsurface,and improve the bending fatigue property of 20CrNiMo gear steel effectively.