| In the manufacturing process of bearings,grinding is an important process after heat treatment and turning,which has an important influence on the surface quality and surface accuracy of bearings.In the common bearing processing process,bearings need to go through two grinding processes,rough grinding and fine grinding,in order to reach the final surface quality requirements.In recent years,ultrasonic surface rolling technology has been widely used to improve the surface quality of parts due to many advantages in the process of machining.In this thesis,the surface ultrasonic rolling technology was introduced into the grinding process of GCr15 bearing steel workpiece.By comparing the influence of surface ultrasonic rolling technology on the surface integrity and surface wear resistance of coarse grinding and fine grinding workpiece,The feasibility of introducing the surface ultrasonic rolling technology into the existing bearing machining technology and the rational introduction sequence of the surface ultrasonic rolling technology into the bearing grinding process were analyzed.In addition,aiming at the above research objectives,the influence of surface ultrasonic rolling process parameters on the surface properties of GCr15 bearing steel was analyzed.A mathematical model of surface mechanical properties of GCr15 bearing steel was established by response surface method and multi-objective optimization was carried out to obtain a set of optimal process parameters for surface ultrasonic rolling of GCr15 bearing steel.The main research contents and conclusions of this thesis include:(1)By comparing the surface processing results of four combinations of rough grinding,rough grinding+fine grinding,rough grinding+surface ultrasonic rolling,rough grinding+fine grinding+surface ultrasonic rolling,it is found that compared with the workpiece treated by rough grinding process,the surface integrity and surface wear resistance of the workpiece treated by rough grinding+fine grinding process have been slightly improved.The surface integrity and wear resistance of the workpiece after surface ultrasonic rolling are greatly improved.The ultrasonic surface rolling treatment in this thesis will not reduce the surface shape and position accuracy of GCr15workpiece,but will optimize the shape and position accuracy of the workpiece within a certain range and improve the machining and assembly quality of the workpiece.Therefore,the surface ultrasonic rolling process has a stronger ability to improve the surface integrity and wear resistance of the workpiece than ordinary precision grinding processes.During the bearing manufacturing process,ultrasonic rolling processes unrelated to precision grinding can be used to replace the precision grinding process,and the surface quality of the workpiece being rolled is better.(2)The surface integrity index of GCr15 bearing steel workpiece treated with rough grinding+fine grinding+surface ultrasonic rolling is slightly better than that of GCr15 bearing steel workpiece treated with rough grinding+surface ultrasonic rolling.The improvement range of most surface integrity indexes of GCr15 bearing steel workpiece treated by two processes is no more than 4%.The average friction coefficient and wear volume of GCr15 bearing steel workpiece treated by the two processes are only 0.8%and 1.3%respectively.Combined with the surface morphology and surface microstructure changes involved in this study,it can be seen that although the process scheme of rough grinding+fine grinding+surface ultrasonic rolling has a slightly better improvement effect on surface integrity and tribological properties than that of rough grinding+surface ultrasonic rolling,the actual difference between the two is small.Compared with the process combination of rough grinding+fine grinding+surface ultrasonic rolling,the process combination of rough grinding+surface ultrasonic rolling can improve the processing efficiency of the whole surface strengthening process because it reduces the fine grinding process.Therefore,through the study of this thesis,it is considered that the combination of rough grinding and surface ultrasonic rolling is an optimal process scheme in the study of GCr15 bearing steel surface strengthening.(3)Based on ultrasonic surface rolling technology,taking the surface roughness and surface hardness of GCr15 bearing steel after ultrasonic surface rolling as response objectives,a mathematical model of surface ultrasonic rolling processing of GCr15bearing steel was established.The accuracy and reliability of the two models are verified by variance analysis.The influences of ultrasonic rolling process parameters on surface roughness and hardness of GCr15 bearing steel are obtained as follows:static pressure and rolling times have extremely significant effects on surface hardness and surface roughness;The effect of rotational speed on the two responses was not significant;The roughness model is affected by the interaction of static pressure and rolling number,while the hardness model is not.According to the influence of each process parameter on the surface properties,the degree of influence of each significant factor in the roughness model was ranked as P~2>P>T>T~2>n~2>P*T.In the hardness model,the degree of influence of significant factors on hardness was ranked as P~2>P>T~2>n~2>T>f.The multi-objective optimization of ultrasonic surface rolling mathematical model was carried out,and a set of composite optimization process parameters were obtained:speed=207 r/min,feed=0.34r/mm,static pressure=0.49MPa,rolling times=3.46.The experimental results show that the optimum surface quality can be obtained by the combination optimization of the process parameters.(4)The machining efficiency and surface machining quality of two kinds of rolling tool heads during surface ultrasonic rolling of GCr15 bearing steel were compared.The results show that the rolling effect of disc rolling tool head on GCr15 bearing steel is better than that of ball rolling tool head. |
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