Carbide Evolution And Bainite Transformation Behavior Of High-Carbon-Chromium Bearing Steel

The uniformity and ratios of microstructure of bearing steel has great influence on fatigue life of bearing, so the uniformity and thermal stability of microstructure should be guaranteed during the rolling and heat treatment process.Based on the regulating and controlling theory of thermodynamic and kinetic, the high carbon chromium bearing steel was taken as the research object in this thesis, and the key process parameters of TMCP and heat treatment was taken as the core. The evolution of carbide was analyzed qualitatively and quantitatively, the transformation behavior of martensite and bainite was discussed, and the precipitation and dissolution law of carbide and its influence on the subsequent transformation behavior of martensite and bainite was revealed. The main research contents and results are as follows.The draughting schedule, cooling rate, and alloying elements has great influence on the precipitation behavior of network cementite and then the properties of bearing steel. The precipitation temperature of network cementite is improved after applying certain compression deformation on austenite compared to the static transformation, and the CCT curve is moved to the left. The addition of Mo and enhancement of Si/Mn which moved the CCT curve to the right and improved the hardenability, has a certain inhibition effect on the precipitation of network cementite.The bearing steel needs to be spheroidized to improve the machinability and the microstructure is consisted of ferrite and spheroidized cementite. The JMAK equation is transformed mathematically, and the model of ferrite to austenite and carbide dissolution is achieved. The influence of heating rate on the kinetic parameter is taken into consideration. The model is checked with dilatometer experiments by comparison of the experimental and calculated length change.In order to investigate the carbide dissolution mechanism of high carbon chromium bearing steel during the intercritical austenitization, the database of TCFE7of Thermo-Calc and MOBFE of DICTRA software are used to calculate the elements diffusion kinetic and the evolution law of volume fraction of carbide based on the theory of interface local equilibrium. The result indicates that carbide dissolution is composed of two stages:initial quick growth, C elements is distributed quickly in austenite, but Cr elements is just get local equplibrium near the interface; subsequent slow growth. Cr elements is redistributed between austenite and carbide. At microscopic scale, the dissolution of carbide is based on the disruption of local equilibrium of austenite/carbide interface and diffusion of elements. Initial austenite growth controlled by rapid carbon diffusion is termed the NPLE mode, and subsequent slow growth controlled by diffusion of Cr elements is known as PLE mode.The microstructure of tempered martensite and bainite is analysed. The individual martensite plates interfere with one another, leading to an apparently tangled microstructure. The first plates to form are stopped only by the austenite grain boundaries, and create obstacles for subsequent plates with different orientations, leading to successively shorter lengths. During the tempering process, carbon diffuses and carbide nucleates at the martensite twin. The nose temperature of bainite transformation is300～350℃. The factors that influencing the kinetics of bainite transformation include the uniformity of alloying elements, C/Cr contents of austenite, holding time and holding temperature. Accompying the formation of bainite, the carbide precipitate. The plate of bainite ferrite appears parallel on one side of the initiating ferrite spine. Owing to the lower temperature, the diffusion of carbon decreases which fostering a finer dispersion of precipitations. The bainite structure has better strength and ductility compared to the tempered martensite, and almost the same abrasion performance.Mainly for the long duration of bainite transformation process, the kinetic is studied. The sub structure of grain and microstructure change of under cooled austenite formed by pre-strain and pre-quenching process accelerate the transformation rate at first, but don’t shorten the duration of full bainitic transformation process. A two-step process, through suitable adjustments of the temperature and holding time, could accelerate the full transformation of bainite without loosing mechanical properties and abrasion performance.