Research On Ultra-high Cycle Fatigue Properties Of 18CrNiMo7-6 Carburized Gear Steel

As the core transmission part,gears are widely used in industries such as automobile,machinery,aerospace,and high-speed railway.Gear steel is usually required to have a high fatigue performance to ensure that it does not fail during service.In view of the fact that gears are subjected to very high cycle（>10⁷）cyclic loads during the operation of high-speed railway,it is of great significance to carry out research on the very high cycle fatigue（VHCF）performance of gear materials.18CrNiMo7-6 carburized steel is a gear material commonly used for high-speed train,however its VHCF performance has not been reported.In this paper,the ultrasonic fatigue loading method is used to study the VHCF performance and failure mechanism of 18CrNiMo7-6carburized gear steel.In view of the difference in the mircostructure and property between the carburized layer and the core,the very high cycle fatigue properties of 18CrNiMo7-6 gear steel in terms of carburized layer and core at 10⁹ cycles were studied by using fully carburized samples and pseudo carburized samples,respectively.The mechanism of fatigue crack initiation and propagation was discussed,and the fatigue properties of the material at 10¹⁰ cycles was estimated.The effects of the material microstructure,inclusion level and other factors on the VHCF failure of 18CrNiMo7-6 carburized gear steel were analyzed.The main conclusions are as follows,The results of VHCF performance of the carburized samples show that as the stress amplitude decreases,the fatigue life of the carburized layer of 18CrNiMo7-6 gear steel increases,and VHCF failure will occur after 10⁷ cycles.The VHCF strength of the carburized layer of 18CrNiMo7-6gear steel at 10⁹ cycles is about 600 MPa,and the fatigue failure stress range at 10¹⁰ cycles is obtained by extrapolation method to be from 524 MPa to 570 MPa,which can meet the requirement of 10¹⁰ cycle life under the 515 MPa stress for high-speed rail gears.Fractography of the 18CrNiMo7-6 gear steel carburized sample after VHCF fracture show that fatigue cracks usually form at the internal non-metallic inclusions,and a fine particle layer is generated in the matrix around the inclusions.The martensite matrix is gradually debonded and connected into crack interface to form the granular bright area（GBF）area.After the size of the GBF area reaches a critical value,fatigue crack propagation follows the Paris formula,and then it forms a"fish eye"and ultimate fatigue failure.The critical value of the stress intensity factor in the GBF area of the test steel is 3～4 MPa·m^1/2.At a given stress amplitude,the VHCF life of the carburized sample increases with the decrease of the inclusion size,indicating that reducing the inclusion size can improve the VHCF proerties of the carburized layer.Fatigue results of pseudo carburized samples show that VHCF failure will not occur in the core of 18CrNiMo7-6 carburized gears.The fatigue strength at 10⁹ cycles is the same as that at10⁷cycles,being about 650 MPa.On the contrary,the pseudo carburized specimens in the carburized layer will experience VHCF failure,showing typical characteristics of VHCF failure.The fatigue crack growth rate of the core is slower than that of the carburized layer.During the crack growth process,the crack growth is hinded by the grain boundaries and martensite substructure,and there are more crack deflection and branching phenomena.A large amount of carbides in the carburized layer promotes the propagation of cracks,as a result the phenomenon of microstructure hindering crack propagation being reduced.The rate of fatigue crack growth of the carburized gear core is significantly lower than the carburized layer,so the carburizing method is beneficial to improve the fatigue life to a certain extent.