| Precision forming is a method of gear manufacturing,and the forged streamline is distributed along the tooth profile after forming,which has the natural advantage of anti-fatigue punching.However,the influence of cold precision forming on the fatigue performance of carburized gear lacks experimental demonstration,and the influence mechanism has not been revealed.Carburized gear steel has high yield strength and high deformation resistance due to high alloy element content,which brings a new challenge for the cold forming of spur gear.The methods of theoretical analysis,numerical simulation,and empirical research were adopted in this paper.According to the material properties of high-strength carburized gear steel and the process difficul ties on cold precision forming of spur gear.The properties of high-strength carburized steel material,the cold precision forming technology of spur gear,the evolution of microstructure and properties during carburizing and quenching,and evaluation of g ear bending fatigue properties were taken as the main research contents.The cold precision forming technology of high strength carburized gear and the influence mechanism about bending fatigue properties were systematically studied.According to the material characteristics of 20 CrMnMoH carburized steel with high yield strength and tissue inheritance.The microstructure and properties of gear steel were deeply investigated combined with the law of austenite grain growth,the tissue inheritance,the grain refinement mechanism of the non-equilibrium structure,and the fusing mechanism of flake pearlite.By controlling the transformation temperature,transformation speed and transformation time of undercooled austenite.The microstructure was granular pearlite and the length reached to nano-scale,meanwhile,the tissue inheritance was eliminated and the yield strength was reduced.The problem of large deformation resistance in the process of cold precision forming was solved,and the tissue hereditary phenomenon of carburizing and quenching was eliminated.A pre-heat treatment process of the billet with secondary isothermal normalizing plus once spheroidizing annealing is proposed.The mechanical deformation behavior of the material in different microstructure states and the deformation characteristics of non-uniform plastic deformation were studied.Based on the principle of the dynamic die forming process and the design principle of three-layer prestress of die,the 3D simulation geometric models were established.The flow stress characteristics of the material were studied with t he Gleeble-3500 tester.According to the relationship between true stress and true strain,the Johnson-Cook constitutive model of the material was established.The load characteristics and material flow law during the cold forming were studied based on the finite element method.The principle of gear pressure relief and shunt forming was applied to the die structural design.Combined with the influence of forming gap on forming load,the best die gap for cold precision forming is 0.15 mm.The deviation between the numerical simulation forming load and the experimental testing load is 3.8%,which realizes the accurate prediction of the forming load.The corresponding relationship between the gradient distribution of forging streamline and the numerical analysi s model was established.The tooth profile precision of the formed gear is 7-8 grade,and the tooth direction precision is 7grade.A continuous distribution of forging streamlines is formed by the regular plastic flow of the material during the cold precision forming of gear.The distribution of forging streamlines in the subsurface of gear is the densest.The distribution of the streamlines in the tooth root and the tooth groove is denser than that of the tooth surface,and the distribution of the forging streamlines in the surface is gradient and non-uniform.The ferrite grains were elongated along the tooth profile,and the carbides were distributed in a granular form at the elongated grain boundaries.The distortion degree of grain in tooth root and tooth groove,the density of the streamline distribution,and the density of the carbide particle distribution are all greater than those of in the tooth surface.The martensite size in the tooth root of the formed gear is smaller than that of the cutting gear after carburizing and quenching,and the dispersion degree of carbide distribution is more uniform than that of the cutting gear.High grain boundary distribution density can be obtained after forging streamlines recrystallize during the carburization process,which is beneficial to the diffusion of carbon atoms during the carburizing process,and also is beneficial to obtain a higher carbon concentration distribution.The carbon content of the formed gear at the relative positions of the tooth surface,tooth root and tooth groove are higher than that of the cutting gear.The microhardness of the formed gear on the surface is higher than that of the cutting gear.Within the range of 0.50 mm from the surface after carburizing and quenching,the microhardness distribution on the surface of the formed gear appears to be a "platform" area.The microhardness of the cutting gear appears "head down" in the surface.Forging streamlines indirectly affect the carbon concentration distribution of the carburized layer through recrystallization.The cold precision forming process of spur gear can avoid the phenomenon of " head down ",which generally exists in the surface microhardness gradient of cutting gear after carburizing and quenching,which is beneficial to improve the bending fatigue performance of the gear.The bending fatigue life of cold forming gear is compared with that of cutting gear:when the bending stress is 700 MPa,the fatigue life of gear is increased by 7 times.When the bending stress is 740 MPa,the fatigue life of gear is increased by 8.8 times.When the bending stress is 780 MPa,the fatigue life of gear is increased by 9.4 times.When the bending stress is 860 MPa,the fatigue life of gear is increased by 12 times.When the bending stress is 941 MPa,the fatigue life of gear is increased by 10 times.This paper reveals the influence mechanism of forging streamline on the bending fatigue strength of gear in cold precision forming.A material-structure-property integrated evaluation method has been constructed for forming gear in the whole process of basic material,cold precision forming,carburizing and quenching,and fatigue performance evaluation.It builds a new method and provides a practical reference for the high-performance manufacturing of carburized steel gear in the aerospace field. |
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