Research On The Transformation Mechanism Of Bainite In Steels

Bainite, the isothermal transformation product of austenite, was widely suspected from the day of discovery. Bainitic transformation mechanism was debated by displace and diffusion points, which has been more than half of the century. In the paper, isothermal experimentation and surface relief observation experimentation of 20CrMo steel, 35CrMo steel, 45CrNiMoV steel, S7 steel, 60Si2CrV steel, GCr18Mo steel, GCr15 steel and 9Cr3Ni steel of industrial steels were investigated by transmission electron microscopy(JEM-2100F), environmental scanning electron microscope(Quanta-400), scanning tunneling microscopy(Nanofrist-1000)and optical microscopy(LeiCa)and so on.The observation of the morphology showed that the upper bainite more than can be nucleated at the boundary and also can be stimulate-nucleated in the intragranular, the morphology gradually transmitted from the needle-like to feathered by increasing of carbon contents; needle bainite beam formed lower bainite; bainitic transformation shear mechanism and actual diffusion mechanism were inconsistent supported by the calculation of growth speed of bainite, the replacement of iron atoms and atomic diffusion coefficient, and the observation of surface relief; and non-concerted thermal activation transition of iron atoms of the interface and replacement atoms were proved in nucleation and growth process of bainite.At the same time, experimental analysis and theoretical research showed that bainitic transformation occurred at the beginning of nucleation of bainitic ferrite, also bainitic carbide of bainitic ferrite and original austenite interface nucleation formed as bainitic sub-unites by, which made diversely bainite morphology. The iron atoms and replacement atoms were conducted by atom non-concerted thermal activation transition way in the process of growing and nucleation of bainitic ferrite and bainitic carbide. The bainitic thermal activation transition mechanism was different displacement vector transition between iron atoms of the interface and replacement atoms, which made the bainitic ferrite/ austenite (BF/A) and bainitic carbide/ austenite (Bc/A) interface lapse to the austenite to formed as bainite, and also the atom transition distance within an atom.Finally, the bainite was formed by the system's function of self-organization, orderly combination and efficiency cooperation of composition phase of bainite, and also indicated that surface relief was must exist in surface transition, could not be studied as the characteristics of some phase transitions. In the paper, the experiments and theories were presented to support the mechanism of atom thermal activation transition of bainite transformation.