| High speed precision angular contact ball bearing is the key structural component in high speed precision machining center.The mainstream products in the market are all made abroad.As one of the core manufacturing technologies,heat treatment has a significant gap with foreign countries that resulting in the overall lag of machine tool manufacturing in China.In order to offer some meaningful guidance to the manufacture of high speed precision angular contact ball bearing,this dissertation presents systemic theoretical studieson themicrostructurerequirement of bearing after heat treatment,research and development of ultrafine heat treatment process and distortion of bearing during heat treatment.First,the microstructure,hardness and residual stressof four kinds of high speedprecision angular contact ball bearing that adopted by the seven kinds of high-speed spindlewith highest market share have been detailedanalyzed.The chemical composition,matrix organization,carbide particle size,retained austenite,hardness;corrosion resistance and residual stress have been measured.The results show that:(1)The overall heat treatment that can ensure the whole section be hardened is the appropriate heat treatment for the GCr15 high carbon chromium bearing;(2)The final microstructure of the bearing should be cryptocrystalline martensite without troostite,the maximum and average diameterof the carbides should be less than 5?m and 0.45?m a respectively,and the content of residual austenite in the bearing ringshould be in the range of 3-5%;(3)The hardness distribution of the ring should be uniform andthe deviation should be controlled in HRC61±0.5,besides,the residual stress should be about-400 MPa.Second,the method to obtain uniform-fine-rounded carbides has been researched.A proper spheroidizing heat treatment is a guarantee to obtaining appropriate microstructure during subsequent heat treatment.Quenching after forging with high temperature tempering and low temperature duplex annealing process have been proposed in this paper.And then the morphology,size,distribution and quantity of the carbides have been quantitatively characterized.The results show that:(1)Precipitation,grew up and spheroidizing have been occurred during high temperature tempering,and tempering at 740? for 8h is the most appropriate spheroidizing process;(2)During low temperature duplex annealing,the carbides precipitated at the beginning 60 min,and then the precipitation and grew upcarried out simultaneously in the time 60-120 min,after holding more than 120 min the carbides begin to grow up rapidly,the best holding time is 60 min for obtaining ideal state carbides.Base on that,the influence of temperature and holding time on the size and distribution of carbides and its spheroidization mechanism under high temperature tempering and low temperature duplex annealing process have been revealed.The distortionof the bearing during heat treatment is a thorny problem to the subsequent grinding process.Accurate prediction of the deformationis beneficial to the setting of the grinding allowance that can dramatically improve the grinding efficiency.In this paper,a modelfor predicting dimensional change during heat treatment has been established based on the phase transformation and specific volume difference between different phases.The model can accurately forecasted the dimension of the bearing during overall heat treatment,including Spheroidizing treatment,quenching,subzero treatment and tempering.Besides,the content of phases canbe calculated from the dimension of the bearing.This model accuratelyforecasted the dimensionof 80 sets of bearing ringthat subjected to different heat treatment.The predicting resultsshow that the model has a good effect onpracticability and veracity.Finally,the effect of carbon content,austenitizing temperature,the decomposition of residual austenite during subzero treatment and tempering on dimension of the bearing have been discussed. |
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