Surface Damage Behavior And Performance Regulation Of M50 Bearing Steel Under Dry Friction Condition

Aeroengine is the heart of aircraft.As the key safety part of engine,aeroengine bearing is also the most important core basic part of aeroengine.It is recognized as one of the main bottlenecks in the development of aviation equipment in China.The performance,life and reliability of bearings are directly related to the performance,life,reliability and safety of aeroengines and gas turbines.Therefore,breaking through the manufacturing technology of engine spindle bearing has become a major and urgent need for national defense security and national economic development.With the development of aeroengine towards higher thrust weight ratio,higher speed,long service life and high reliability,the requirements for engine bearings and the performance of bearing materials are becoming more and more stringent.Recently,the main failure forms of aeroengine bearings are contact fatigue failure,wear failure and corrosion failure based on surface damage.At the same time,M50 bearing steel（Cr4Mo4V）is still one of the main materials of aeroengine bearings in China.Therefore,improving the surface damage resistance of materials through different technologies is of great significance to improve the service life and reliability of bearings and break through the bottleneck technology of aeroengine bearings.Aiming at the prominent problems of aeroengine bearings and materials,this dissertation first systematically explores the mechanism of dry friction and wear failure of materials,and then uses the composite treatment technology of special preheating treatment process（controlled forging and cooling process）and ultrasonic rolling surface strengthening technology to explore the mechanism of improving the surface damage ability of materials such as fatigue,wear and corrosion resistance,and improve the surface damage resistance of materials.To improve its service life and reliability,the main conclusions are as follows:In the dry friction test,with the increase of ambient temperature,the friction coefficient of the sample gradually decreases from 0.78 to about 0.4.During the friction test,oxidation reaction occurred on the wear surface.The main oxide is Fe₂O₃ at room temperature to 200℃,but when the temperature is higher than 300℃,not only Fe₂O₃ but also Mo O₃ are formed.It has the effect of lubrication and wear reduction at high temperature,so the friction coefficient of the sample is reduced.The wear mechanism of the sample mainly changes from abrasive wear to adhesive wear（oxidation wear）,and the morphology of wear debris produced by friction gradually changes from granular to thin-walled.In the dry friction test,with the increase of speed,the average friction coefficient of M50decreases,but fluctuates greatly,and the wear rate increases nearly linearly.The oxidation products on the wear surface are mainly Mo O₃and Fe₂O₃.The wear mechanism is mainly oxidation wear;With the increase of load,the friction coefficient of M50 sample gradually decreases and the fluctuation of friction coefficient is small.The oxidation products of wear surface are mainly Fe₂O₃,Mo O₃,Fe₃O₄and Cr₂O₃.The main wear mechanisms are adhesive wear and oxidation wear.Speed has a great influence on M50 wear under harsh conditions.Among the rolling parameters strengthened by ultrasonic rolling,the greater the rolling pressure,the smaller the feed rate and the more rolling times,which has the greatest impact on the surface microstructure and properties of M50 steel;By selecting the optimal rolling parameters,the surface residual compressive stress and surface hardness of the sample are greatly improved,and the surface roughness is reduced.About 25μm strengthening layer is produced on the surface of the sample,and nanocrystals are found in the strengthening layer near the surface;Ultrasonic rolling strengthening treatment improves the wear resistance and corrosion resistance of M50 steel.After ultrasonic treatment,the fatigue life of L10 and L50were improved by 2.93 and 2.88 times,respectively.The fatigue life of surface preformed indentation defects and the L10 and L50 life of specimens strengthened by ultrasonic rolling are increased by 2.06 and 1.64 times respectively.After controlled forging and cooling treatment,the grain size of the material is updated evenly and fine,and the primary carbide is broken relatively fine;At the same time,the macro average hardness of M50 steel is increased by 0.5 HRC and the impact toughness is increased by 10%;The wear resistance of M50 steel changed little,and the friction coefficient and wear rate decreased slightly;The contact fatigue increased by 0.26 and 0.36 times respectively.After the combined treatment of ultrasonic rolling strengthening and special preheating treatment,the surface hardness of M50 steel can reach 67 HRC.At the same time,it improves the surface stress and stress depth,refines the grain and carbide dispersion strengthening;The dry friction coefficient and wear rate of M50 steel decreased by about one third after composite treatment;The rolling contact fatigue life L10 is increased by 3.6 times.