On Nucleation And Growth Kinetics Of Bainite Transformation Of Low And Medium Carbon Steels

Bainite can be formed at wide range of temperature in steels with various microstructures. A deep understanding of the bainite transformation mechanism is necessary for the development of novel steels. Based on the analysis of the existed definition about bainite, the paper focuses on the transformation mechanism of the products according with the general microstructure definition of bainite.Based on Boltzmann distribution and dislocation theory proposed by Cottrell, an exact formula was derived for calculating the carbon atoms number aggregating at edge dislocations, which increases with decreasing isothermal temperature and increasing carbon concentration.A model was developed to describe the relationship between carbon atoms drifting to edge dislocations and isothermal holding time. Furthermore, the radius of carbon depleted zone around edge dislocation was obtained. The theoretical results of aggregation process of carbon atoms to edge dislocations and bainite incubation time are consistent with the experimental data. By comparing the radius of carbon depleted zone with critical radius of displacive nucleus and considering the isothermal holding temperature, interfacial energy, dislocation morphology and stored energy etc., it is found that lower bainite displacive nucleation probably occurs at lower temperature rather than at higher temperature.The growth rates of bainitic ferrite in steels with varying alloy concentration were calculated by Zener-Hillert and Bosze-Trivedi equations after reevaluating the thermodynamic and kinetic parameters. A good agreement between experimental and theoretical results was found in Fe-C alloy. A slight overestimate of the theoretical results was observed in medium alloyed steels. However, the experimental data are two orders lower than theoretical ones in Fe-C-8.7Ni wt.% alloys. Taking the interface condition, supersaturation and solute drag effect into account, it is suggested that the bainite transformation mechanism may be related to steel composition.A kinetic model based on displacive mechanism was proposed after modifying the maximum nucleation driving force and autocatalysis nucleation factor. A satisfactory agreement with experimental data indicates that the model is capable of representing the relationship between the formation of bainite and isothermal holding time. It was found that the deviation of the theoretical maximum bainite fraction and the experimental ones is large at higher temperature. It is implied that the bainite transformation mechanism may be associated with transformation temperature.The effects of Nb content on the bainite transformation were studied. It is shown that Bs is increased about 20℃with cooling rates of 30℃/s and 20℃/s when the Nb content is raised from 0.01 to 0.2 wt.%. No significant effect of Nb content on Bf was observed. Upon continuous cooling the coarser bainitic ferrite plates are formed, followed by the formation of thinned bainitic ferrite plates. The process of the formation of NbC may concern with the diffusional process of Nb. The experimental results indicate the bainite formation is concerned with diffusional process.Coupling the obtained results, a possible mechanism of bainite transformation was suggested. If the carbon and alloy content are low, bainite may be transformed by diffusional processes at higher temperature. With decreasing the temperature, bainite transformation may proceed by displacive mechanism. In the case of higher carbon and alloy concentration, bainite transformation would not be expected by diffusional process due to the solute drag. However, bainite may be transformed displacively at lower temperature resulting from sufficient driving force.