| Gear,as an important transmission part in industrial production,is widely used in automobile,ship,aerospace and other fields.In practice,gear is combined with normal normal stress and circumferential shear stress.Therefore,the actual service life of gear is lower than the original design life,which will bring great hidden danger to production and life.Many scholars have studied the failure mechanism of rolling contact fatigue,but fatigue damage is caused by complex stress,and many phenomena need to be further explained and verified.At present,the research on the rolling contact fatigue of gears is mainly focused on the fatigue damage of hard tooth surface,but the research on the fatigue damage of soft tooth surface is rarely reported.Therefore,the paper uses experiments to simulate the working environment of gears,and studies the rolling contact fatigue damage morphology and crack growth of different hardness gear steel materials,which provides theoretical basis for tracking the source of fatigue and prolonging the service life of gears.In this paper,two kinds of gear steel 40 Cr and 18crnimo7-6 are taken as the experimental research objects,and the rolling contact fatigue experiment is carried out by CQHH-RCF-A linear rolling contact fatigue testing machine.The contact stress distribution on the roller contact surface is simulated by ABAQUS finite element software,which is compared with the subsequent experimental results.The normalized roller is used to study the effect of contact width on rolling contact fatigue failure,the quenched and tempered roller is used to study the rolling contact fatigue failure mode at lower stress level,and the carburized and quenched 18 crnimo gear steel is used to study the rolling contact fatigue failure mode at higher stress level.After the test,the wear morphology,cross-section crack,microstructure,phase composition and composition of the sample surface were observed and analyzed by optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD)and energy dispersive spectroscopy(EDS).The conclusions are as follows:(1)The simulation results show that the normal contact stress distribution of roller contact surface is higher than the theoretical value on both sides of the step,while the contact stress is consistent with the theoretical value in the middle of the step.The reason for this phenomenon may be that the roller step edge is not chamfered,resulting in stress concentration,and the maximum shear stress is 0.16 mm deep from the contact surface The distribution of contact stress is similar to that of normal contact stress.(2)Through statistical analysis of the fatigue failure parts,it is found that the initial position of the failure point is almost at the edge of the step,and the roller step deforms at the fatigue failure position,which corresponds to the results of finite element simulation.The results show that the fatigue failure mode of 40 Cr normalized roller is massive spalling accompanied by small pitting,which is contrary to the traditional view that the fatigue failure mode of low hardness roller is pitting.The failure mode of 40 Cr quenched and tempered roller and 18crnimo7-6 carburized and quenched roller is single large spalling pit.(3)The longitudinal section observation and analysis of fatigue specimens shows that the growth of fatigue cracks is discontinuous.When the contact stress is low,the crack growth direction is relatively single,but when the contact stress is high,the fatigue crack expands into a divergent network,which eventually leads to pitting or peeling off of the contact surface.(4)In the rolling contact fatigue process,the martensite content of the surface layer is significantly increased compared with the original sample content.It shows that the residual austenite changes into martensite in the rolling contact fatigue process.With the decrease of the content of the soft austenite phase on the surface,the harder martensite structure content increases,so the surface hardness is improved. |
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