| With the rise of strategic marine industries such as coastal engineering,marine mining and underwater engineering,corrosion fatigue failure of key components of marine equipment has become a prominent problem in the field of marine engineering.The ultrasonic impact surface modification technology is an efficient and convenient surface strengthening technology,which is widely used to improve the fatigue resistance of key components of various high-end equipment.In this thesis,an experimental study on the surface modification of 20CrNiMo carburized gear steel,which is widely used in marine equipment transmission system,was carried out.The influence of surface modification of 20CrNiMo carburized gear steel on corrosion fatigue performance was investigated.The main research contents of this thesis include:(1)Study on surface modification mechanism of ultrasonic impact.Firstly,the components and vibration principle of ultrasonic shock system are introduced.Then the surface modification mechanism of ultrasonic impact is elaborated from three aspects:surface roughness reduction,microstructure refinement and residual compressive stress field generation.Finally,based on Hertz contact theory,the contact radius and maximum pressure of ultrasonic impact tool head and specimen are analyzed theoretically,and the distribution function of contact stress in specimen contact area is derived.(2)Theoretical analysis of ultrasonic impact process parameters.Based on the theoretical analysis of the plastic deformation of the specimen under the ultrasonic impact load,the mathematical model between the process parameters and the strain value of the specimen was established.According to the characteristics that the greater the plastic strain is in the ultimate strength of the ultrasonic impact specimen,the greater the residual compressive stress is.Combined with the stress-strain curve of the specimen material under high strain rate,the strain value range is 0.7ε_b to 1.1ε_b,and the ultrasonic impact static load range of the 20CrNiMo carburized gear steel under the condition of 12μm amplitude and 20k Hz frequency is calculated.Finite element simulation was used to analyze the distribution of plastic deformation and residual stress of the specimen.The results show that the maximum residual compressive stress of the specimen surface is 338MPa when the strain value isε_b.(3)Optimization of surface modification by ultrasonic impact.Ultrasonic impact experiments were carried out with the microhardness,residual compressive stress and shape of ultrasonic impact groove as evaluation indexes,and the influence of static load on ultrasonic impact effect was investigated.The experimental results show that ultrasonic impact can increase the surface hardness,generate residual compressive stress and improve the machining cut marks on the specimen surface.When the static load is 900N,the maximum residual compressive stress of surface layer is 319MPa,and the depth of affected layer is about 200μm.In addition,the effects of the feed speed on the surface microhardness and surface roughness of the specimen were investigated experimentally.The results show that the surface hardness of the specimen decreases gradually with the increase of the feed speed,and the surface roughness decreases first and then increases with the increase of the feed speed.When the feed speed is 0.2mm/r,the minimum surface roughness Ra is 0.067μm.(4)Corrosion fatigue experiment.First of all,the optimized process parameters were used to modify the surface surface of the specimen by ultrasonic impact.Then,the corrosion fatigue test was carried out after 72h of precorrosion together with the untreated specimen.The results showed that the corrosion fatigue life of the original specimen decreased from 193610r to 84711r after 72h of precorrosion.The corrosion fatigue life of the modified specimen under ultrasonic impact is1967328r,which is about 23.2 times that of the untreated specimen.Finally,scanning electron microscopy was used to observe the failure fracture.The results showed that there were a large number of micro corrosion pits on the surface of unmodified specimens after 72h of precorrosion,which would cause stress concentration and induce cracks from the surface.There is no micro corrosion pit on the surface of the specimen modified by ultrasonic impact,and the crack initiation position is pushed to the subsurface.The experimental results show that the corrosion fatigue property of 20CrNiMo carburized gear steel can be significantly improved by ultrasonic impact surface modification. |
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