Research On The Effect Of Roughness On The Fatigue Life Of Carburized And Quenched 18CrNiMo7-6 Steel

As the core and key part of the transmission system,the design and manufacturing level of the gear is an important display of the comprehensive strength of the national equipment manufacturing industry.Due to the sensitivity of fatigue strength of high-strength components to stress concentration,fatigue becomes the main mode of gear failure,and the core of the third generation manufacturing technology-anti-fatigue manufacturing is surface hardening and surface modification,which is important for solving the problem of gear fatigue failure.The surface roughness formed during the machining process is like sharp notches and cracks,especially the stress concentration effect at the bottom of the valley promotes the initiation of slippage and fatigue cracks and reduces the fatigue resistance of parts.At the same time,high surface hardness causes a larger stress concentration factor,and the effect of roughness on fatigue performance is more obvious.Surface roughness is an important indicator that reflects the geometrical shape error of the micro-topography of parts.Compared with the two-dimensional surface roughness,which can only reflect the data characteristics of one line,the three-dimensional surface roughness can reflect the morphology features of a certain area,which is closer to the real surface morphology.Therefore,it is of great significance to investigate the effect of three-dimensional surface roughness on the fatigue properties of carburized and quenched materials.18CrNiMo7-6 steel is the leading application material for high-speed heavy-duty gears,shafts and other key components.After carburizing and quenching,it is provided with high tensile strength,surface hardness,and a tough core.The thesis takes carburizing and quenching 18CrNiMo7-6 steel as the research object,and carries out the research on the effect of roughness on the rotating bending fatigue life.The main research contents are as follows:?1?Optimizie the grinding process of rotating bending fatigue specimens.For efficient and high-quality grinding of standard rotating bending fatigue specimens,based on the dimension,shape and position tolerance and surface quality requirements of GB-T4337-2015,two grinding schemes are designed,one of which is selected as the best.After grinding,the coaxiality error of the sample is 0.0093 mm,the surface residual stress is-233.84 MPa,and the surface arithmetic mean deviation S_a is 0.208?m,which meets the requirements of the national standard;?2?Design of external axis axial grinding device.Based on the CNC grinding machine,the linear motor module is used for rapid reciprocating motion and software-controlled positioning features to achieve high-efficiency uniform axial grinding of the gauge segment of the fatigue sample;?3?Explore the influence of S_a on the rotating bending fatigue life of carburized and quenched 18CrNiMo7-6 steel under the fourth-level stress level,two sets of rotating bending fatigue samples with S_a of 0.025?m and 0.2?m were prepared by an external cylindrical axial grinding device.The results show that when the loading stress is 1184 MPa,the fatigue life of the S_a=0.025?m specimen is increased by 29.9 times than S_a=0.2?m;when the loading stress is 1216 MPa,it is 6.76 times higher;?4?Explore the influence of the processing texture direction on the rotating bending fatigue life of carburized and quenched 18CrNiMo7-6 steel under the fourth-level stress level,a group of fatigue specimens with S_a=0.2?m and circumferential texture prepared by shape grinding.The results indicate that:when the loading stress is1184 MPa,the fatigue life of the specimen with axial texture is improved by 3.57 times than that with the circumferential texture,and when the loading stress is 1144 MPa,it is 3.07 times higher;?5?Explore the effect of three-dimensional surface roughness on fatigue life when the loading stress is 1184 MPa,by selecting five three-dimensional surface evaluation parameters related to fatigue performance:surface root mean square deviation S_q,surface kurtosis S_ku,surface skewness S_sk,deep valley volume S_v,surface ten-point height S_z,prepare four sets of samples with S_a of 0.025?m,0.1?m,0.2?m,0.4?m for fatigue test.The results manifest that:S_q and S_z have the highest correlation with rotating bending fatigue life.