A Study On The Effect Of Laminar Plasma Quenching On The Rolling Contact Fatigue Performance Of GCr15 Bearing Steel

As the key basic parts of support and rotation,bearings play an important role in the safety and stability of industrial equipment such as high-speed trains,aerospace and ships.With the development of society,machinery and equipment often operate under high-speed and heavy-load conditions,and the service conditions of bearings were becoming more and more complicated.Bearings were subjected to multi-axial stresses such as radial force,axial force and tangential force during service,which will cause rolling contact fatigue failure and cause huge economic losses.Therefore,it is necessary to study the surface strengthening of bearing materials,thereby prolonging the service life of the bearing and improving its service performance.In this thesis,GCr15 bearing steel was taken as the test object,and the surface quenching treatment of GCr15 bearing steel was carried out by laminar plasma,which has the advantages of high energy density,heat concentrated and good controllability and the MJP-30 type rolling contact fatigue testing machine was used to carry out the rolling contact fatigue test.Laser confocal microscope,super depth of field microscope and scanning electron microscope were used to analyze the microstructure and microscopic damage morphology of the samples.The microhardness test and wear amount analysis of the samples were carried out to study the rolling contact fatigue crack propagation mechanism of GCr15 bearing steel,which provides new ideas and solutions for improving the fatigue life of bearings.The effects of three factors,namely,scanning rate,quenching distance and input power,on the surface properties of GCr15 bearing steel were investigated by orthogonal test method.The surface of GCr15 bearing steel is composed of hardened layer,transition zone and matrix after plasma surface quenching treatment.The depth and width of the hardened layer where constantly changing with the changes of the three factors.The formation of a dense cryptocrystalline martensite structure in the hardened layer will increase the surface hardness of the material.Among the three factors,the scanning speed was the main factor affecting the surface properties of the material.Rolling contact fatigue tests were carried out on samples quenched at different scan rates to study their rolling contact fatigue properties.Laminar plasma quenching produces residual compressive stress on the surface of bearing steel,which can significantly improve the rolling contact fatigue life of bearing steel,and the thickness of the hardened layer was positively correlated with the fatigue life.The thickness of the hardened layer also affects the rolling contact fatigue mechanism of the sample.When the hardened layer was shallow,cracks develop into spalling pits under the action of contact stress after the cracks was initiated on the surface.When the spalling pits develop to a certain depth(the spalling pits are not After reaching the transition zone),the crack propagates to the matrix through the shallow quenching zone along the bottom of the spalling pit.When the hardened layer was deep,a small spalling pit was first formed on the surface of the sample,and this spalling pit develops into a large spalling pit under the action of the contact stress(the spalling pit has reached the substrate at this time,and the depth and width are large),and then the crack along the bottom of the spalling pit spreads to the substrate.The transition zone formed by high temperature tempering has higher microstructure and toughness,which hinders the growth of fatigue cracks in this zone and improves the fatigue performance of the samples.Based on the above test results,the evolution of rolling contact fatigue damage of quenched specimens was studied.After the cracks were initiated at the surface stress concentration,they first propagate into the material at 30° ～ 40°,and then the crack propagation direction gradually turns to be parallel to the surface of the sample.When the crack expands to a certain depth,a spalling pit will be formed,and the spalling pit will continue to become larger under the action of impact load and vertical load.Since the initiation and propagation of fatigue cracks are continuously carried out,the spalling pits will also be continuously generated and enlarged.