The Microstructural Evolution And Dimensional Changes Of Bearing Materials Of Precision Machine Tools In Thermodynamic Field

Precision machine tool bearings are the key components supporting the spindle to rotate at high speed.However,due to the transition of metastable structures such as retained austenite and martensite,the dimension and accuracy of bearing rings will change during the long-term of storage and service,which will seriously affect the life of bearings.So it is necessary for bearings to study the microstructure evolution and the dimensional changes.The kinetics and mechanism of microstructure evolution of precision machine tool bearings matrix materials in the temperature field were studied.The activation energies of retained austenite decomposition and carbide transformation were obtained by means of DSC,which were 109.4 kJ/mol and 179.4 kJ/mol,respectively.The kinetic model of microstructure evolution at different isothermal conditions was established,and it showed that increasing aging temperature can accelerate the aging process.The results of aging experiments at different temperatures(140,160,and 180 °C)demonstrated the accuracy and reliability of the model,and suggested that raising aging temperature not only promoted the decomposition of retained austenite,but also increased the carbon concentration in the phases and improved its thermal stability.At the same time,carbides transformed in this way: transition carbides — metastable carbides — cementite,and the content,size and distribution of carbides also changed,which affected the performance of the material,so that the Rockwell hardness decreased gradually during the aging process.The aging kinetics and microstructure evolution of the precision machine tools bearing matrix materials in the thermodynamic field were studied.Thermal expansion curves showed that the tensile stress affected the temperature range of retained austenite decomposition and carbide transformation,accelerated whole aging process.The activation energy of two stages under tensile stress were obtained by further calculation,which were 121.5 kJ/mol and 94.7 kJ/mol,respectively.In addition,the aging kinetics under tensile stress was also established.Comparison with the activation energy in temperature field,it showed that tensile stress slightly improved the mechanical stability of retained austenite and greatly promote carbide transformation.The experimental results of continuous heating up to 300 ℃ under different tensile stress(0,20,40 MPa)and aging experiments in thermodynamic field also showed that tensile stress slightly increased the carbon content in residual austenite,and make carbide precipitation more homogeneous at the beginning of aging,their average diameter is smaller and the proportion of small particles is more.The association mechanism between microstructure evolution and dimensional changes was established,it based on the transformation kinetics and it was called the prediction model of dimensional change.The increase in size due to retained austenite decomposition and the reduction in size due to carbide transformation will jointly determine the dimensional change in the aging process.The model fully considers the kinetics of microstructure evolution,the change of carbon concentration,the source of carbide transition and so on,which is more accurate than the traditional method.The model also obtained the theoretical and experimental verification by the literature data and experimental data,and it can predict the size and trend of dimensional changes very well,so it also can provide theoretical guidance for the precision control of bearing rings.