| The reliability of heavy-duty gear has a crucial influence on the maneuverability of heavy-duty equipment.In the service process,the gear needs to bear a large impact load,and the tooth surface is prone to pitting,wear and other failures.With the development of equipment technology,higher requirements are put forward for the high-end manufacturing of gears.The core problem is to improve the fatigue performance of gears,and for the tooth surface,it is to improve its contact fatigue life.Therefore,it is an urgent problem to develop a kind of strengthening technology to meet the requirements of high-end gear manufacturing.Pulsed magnetic processing is a strengthening technology developed in recert years,which is one of the magnetization treatments.The magnetic field obtained by pulsed magnetic processing is of higher intensity,which can change the mechanical properties of materials to a greater extent,and meets many requirements for high-end gear manufacturing.In this thesis,20Cr2Ni4A gear steel is taken as the object,the internal microstructure and mechanical properties of the material are changed by adjusting the pulsed magnetic field intensity:Combined with finite element simulation,the influence of pulsed magnetic treatment on the contact fatigue property of gear steel is discussed.This thesis mainly carries out the following works:1.In order to meet the requirements of pulsed magnetic processing,a pulsed magnetic processing testing machine is designed and built.A capacitor pulse power supply is used to improve the intensity of the discharge pulse by parallel connection of several power components.The height of the magnet is 50 mm,the center aperture is 16 mm,the winding wire of conductor is 2.5 mm×5.4 mm glass-coated flat copper wire,and each turn is reinforced with carbon fiber material.The magnetic field intensity can reach 30 T,which lays the fouodation for pulsed magnetic treatment experiment.2.The multi-physical field environment of pulsed magnetic processing is simulated by COMSOL software.Finite element models of magnets and samples are established.The results show that the induced current is formed in 20Cr2Ni4A material during pulsed magnetic treatment,which changes the distribution of magnetic field in the sample and results in skin effect.The temperature rise caused by eddy current can be reduced by adjusting the pulsed magnetic treatment time reasonably.Magnetostriction occurs in the sample in the pulsed magnetic field,but the magnetostriction is very small and can be ignored.The magnetostriction of the sample reaches the maximum at 9T,and 9T can be used as the critical magnetic field parameter in the pulsed magnetic treatment experiment.3.The changes of grain,precipitates,dislocations and magnetic domain under different magnetic treatment parameters are observed by electron backscatter diffractometer,transmission electron microscope and magnetic microscope.The results show that after pulsed magnetic treatment,the lath martensite grains in the material are obviously refined.There are precipitated compounds such as Fe,Ni and Cr in the sample.When the magnetic field intensity is 9 T,the precipitated phases are more evenly distributed.With the increase of magnetic field intensity,the dislocation movement intensifies.After 4 T magnetic field intensity treatment,the dislocation in the grain boundary moves inwards.After 9 T magnetic field intensity treatment,a large number of dislocations appear in the grain of the sample,and the distribution tends to be uniform.The domain changes with the direction of the magnetic field,and the labyrinth domain gradually changes into strip domain.Compared with the sample without pulsed magnetic treatment,the phase difference of the domain structure is improved.This indicates that the microstructure of the gear steel is improved effectively by pulsed magnetic treatment.4.The rolling contact fatigue experiments of 20Cr2Ni4A gear steel are conducted,and the fatigue resistance under the same stress cycle number and contact fatigue life at the same load are studied respectively.The results show that the pitting pits on the sample surface become less and more uniform with the increase of magnetic field strength.The contact fatigue life increases with the increase of magnetic induction intensity.When the magnetic field intensity is 9 T,the average life of the sample increases about 42.11%compared with that of the untreated sample.Hardness changes little with magnetic field intensity,and surface hardness increases slightly after magnetic treatment compared with that without magnetic treatment.Residual compressive stress increases obviously with the magnetic field intensity of pulsed magnetic treatment,so the residual stress is the main factor affecting the fatigue performance of the samples of pulsed magnetic treatment.The simulation results show that the surface compressive stress caused by magnetostriction increases,and the surface stress weakens the actual stress of the material,thus the rolling contact fatigue life is increased.In the pulsed magnetic treatment process,the magnetic stress of the sample can reach 0.1 MPa.The existence of Lorenz force breaks the dislocation movement barrier and promotes the dislocation proliferation,which reduces the local high stress caused by grain boundary heterogeneity,and makes the macroscopic residual stress distribution more uniform,thus improves the contact fatigue performance of the gear steel. |
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