| The high-end manufacturing technology of heavy-duty gears has always been one of the key research contents of industrial,military and research institutions at home and abroad.The core problem is how to improve the fatigue performance of gears,and the roots determine the bearing capacity and bending fatigue life of the gears.It is crucial to strengthen the roots.However,the current strengthening methods have certain limitations,so it is urgent to explore a technical method suitable for strengthening heavy-duty gears.In view of this,firstly,Quasi-static compression test and Split Hopkinson pressure bar test were carried out respectively for 20Cr2Ni4 A gear steels before and after refiniment.Based on the effects of strain,strain rate and temperature,the Johnson-Cook(JC)dynamic constitutive model was established and modified to describe the stress-strain relationship of materials during ultrasonic surface rolling process and to provide material parameters.And the effect of different inclusions on the dynamic mechanical properties of materials in two states was analyzed.The results show that the modified JC constitutive model can better describe the dynamic mechanical behavior of 20Cr2Ni4 A carburized gear steel under ultrasonic surface rolling process.And the inclusions have a significant effect on the mechanical properties of the material.The size of the inclusions in the refined material is smaller,the distribution is more uniformer and the number is smaller,which results in higher yield strength and better strain rate strengthening effect than material before refiniment.Secondly,the function model of composite dynamic impact force generated by the superposition of static pressure and high-frequency impact force was studied.The amplitude parameter was converted into the expression of force and added to the load parameter.The finite element model of ABAQUS surface single point impact was established.And the change of stress distribution on the surface of the material during a single impact was obtained.The results show that when the static pressure is 1300 N after the single impact,the residual stress on the surface of the material can reach more than 600 MPa.Thirdly,the influence of static pressure on the surface roughness,residual stress and microhardness of the material was studied.Combined with the influence of surface integrity on fatigue performance,the optimal process parameters were determined.The results show that when the other parameters are the same,with the increase of static pressure,the surface microhardness and residual compressive stress both increase first and then decrease.However,the change in surface roughness is opposite.When the static pressure is 1374 N,the surface roughness is 0.114?m,and the microhardness value and the residual compressive stress value are 828 HV and-612 MPa,respectively.Finally,the three-point bending fatigue test was carried out on the samples before and after ultrasonic surface rolling process,and their life curves(P-S-N curves)were obtained.Combined with the fatigue fracture morphology,the difference between the two microstructures was analyzed,and the strengthening mechanism was revealed.The results show that the average three-point bending fatigue life at the same stress level is more than doubled after the sample is subjected to ultrasonic surface rolling process.It can be found from the corresponding S-N curve at 90% survival rate that the point bending fatigue strength increased from 1053 MPa to 1190 MPa before processing,increased by 137 MPa.The crack source originated on the surface of the sample before processing.After strengthening treatment,a dense plastic deformation layer was formed on the surface of the sample,causing the cracking position to move down.At the crack propagation stage,the parallel spacing of the fatigue striations before processing is 30.29 ?m,and then reduced to 18.76 ?m after processing;the gradient of grain size,residual compressive stress and hardness valued along the depth are the causes of this phenomenon. |
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