| At present,domestic robot industry is developing rapidly.But there are still many problems in the process of its industrialization.Among them,the high performance harmonic reducer used by robots cannot be self-sufficient and has been limited by foreign for a long time.Based on the present situation,the relationship between microstructure and mechanical properties of flexible gear steel are studied.In this paper,40CrNiMo steel was studied by pre-quenching,austempering and tempering at different temperatures.The growth behavior of original austenite grain size with austenitizing temperature and the effects of austempering parameters and tempering temperature on microstructure transformation and mechanical properties were studied.The purpose is to further clarify the effects of the heat treatment processes such as austempering above and below M_s on the bainitic transformation and mechanical properties,and reveal the transformation and the strengthening and toughening mechanism of the sample austempered below M_s,which has important guiding significance for controlling the ratio of martensite/bainite multiphase and obtaining excellent mechanical properties.In the present paper,the microstructures and mechanical properties of the samples were studied by means of color metallographic technique,scanning electron microscopy(SEM),hardness test,tensile and impact test.The fracture morphology and secondary crack propagation path of the impact samples were characterized by SEM.(1)In order to obtain reasonable austenitizing temperature,the effects of different austenitizing temperatures on the microstructure and properties of 40CrNiMo steel are studied.The results indicate that austenite grain size increases slowly at first and then grows rapidly with the increases of austenitizing temperature.After 1000℃high temperature quenching treatment,the band segregation phenomenon of the sample is weakened,and the segregation zone is dominated by uncorroded martensite microstructure.At this time,the hardness is 56 HRC.Therefore,the optimal pre-quenching temperature of 40CrNiMo steel is1000℃.(2)Bainite and martensite microstructure are obtained in the sample after pre-heat treatment+austempering.The quenching temperature and isothermal time affect the ratio of bainite/martensite microstructure,then change the final mechanical properties.The considerable enhancement of bainitic transformation kinetics in the below-M_s sample is mainly ascribed to the formation of the pre-existing martensite,which obtaining fine bainite microstructure and optimizing the material properties.The results show that the pre-exsiting martensite with a volume fraction of 28%is obtained by isothermal quenching at 280 for 1h compared with the samples austempered above M_s,and strength and toughness are improved simultaneously.It is observed that the secondary cracks of the sample mainly expand along the interface between martensite and bainite/pre-exsiting martensite.(3)The effect of tempering temperature on the tempering stability and mechanical properties of martensite/bainite microstructure are revealed.Many fine and lamellar carbides are precipitated when the tempering temperature below 500℃.The strength and hardness of the sample decrease,and the ductility and toughness show a trend of decreasing first and then increasing.When the tempering temperature increases to 600℃,the microstructure recrystallizes and transforms into tempered sorbite microstructure.At this time,the strength and hardness is the lowest,and the impact absorption energy is as high as 147 J.(4)After austempering and tempering,40CrNiMo steel has excellent comprehensive mechanical properties.The optimal heat treatment process is 870 1h+280 1h,followed by 500tempering for 2h.And the yield strength is 1136 MPa and the impact energy is 76 J... |
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