Thermodynamic Mechanism Of Bainite Transformation And Carbide Precipitation Stability In Steel Under Steady Strong Magnetic Field

The transformations of iron and steel materials under strong magnetic field have become the focus of research in modern iron and steel materials.However,the change of microstructural composition and physical mechanism of phase change under strong magnetic field are the top priorities for exploring the effects of strong magnetic field on metal transformation.In this paper,combined with experimental observations and theoretical calculations,the structural transformation and thermodynamic mechanism of low-temperature bainite transformation were studied,and the carbon diffusion and carbon distribution during bainite transformation under magnetic field were explored.Among them,bainite ferrite models with different carbon contents were established based on experimental data.Combined with first-principles,Weiss molecular field theory and related thermodynamic data,the change of magnetic free energy was calculated,and the thermodynamic mechanism of carbon atom distribution under magnetic field was semi-quantitatively analyzed.At the same time,the thermodynamic stability of M₂₃C₆ carbide precipitated in advance under the magnetic field was also studied.The main conclusions are as follows:（1）The carbon content of bainite ferrite decreases under the action of a magnetic field.The EPMA line scan and spot scan experiments qualitatively and quantitatively measured the reduction of bainite ferrite carbon content under magnetic field,and the reduction of bainite ferrite nano-hardness value verifies this experimental conclusion from the side.（2）The lower the bainite ferrite carbon content,the more stable the structure.By calculating and analyzing the supercell models of bainite ferrite established according to experimental data,the lower the bainite ferrite carbon content,the smaller the formation energy and the smaller the change in the unit cell structure.（3）Under the action of magnetic field,the magnetic moment of bainite ferrite increases.This is because the magnetic field induces an increase in the magnetic exchange coupling,and the magnetic exchange coupling determines the magnitude of the magnetic moment.（4）The magnetic free energy change of bainite ferrite is-11.4 J/（mol*Fe）under the action of magnetic field,while the magnetic free energy of austenite is 0.This makes the driving force of the phase transformation between austenite and bainite ferrite increase,which promotes the bainite transformation.（5）Under the action of the magnetic field,the equilibrium carbon concentration at the interface between the bainite ferrite and austenite increases,causing the interface carbon diffusion flux to increase and more carbon atoms to migrate into the austenite.（6）Modeling and calculating the carbon diffusion in the austenite phase shows that the carbon diffusion flux in the austenite increases under the influence of the magnetic field.（7）Under the action of the magnetic field,the equilibrium carbon concentration at the interface between the cementite phase and the austenite phase increases,which promotes the precipitation of cementite.In order to maximize the promotion of the magnetic field,the cementite is precipitated in a spherical shape to increases the interface area.This allows part of the carbon atoms to be distributed into the cementite under the action of the magnetic field.（8）The Fe₃ and Fe₄ atoms in Fe₂₀Cr₃C₆ carbide are connected to form a stable lattice framework,Cr1,Cr2 and C atoms are located in the center of the frame;the magnetism of Fe₂₀Cr₃C₆ mainly comes from the 3d orbital contribution of Fe and Cr atoms,and their directions are opposite.There is sp or pd hybridization in Fe₂₃C₆ and Fe₂₀Cr₃C₆.（9）During the intermediate temperature phase transition,Fe₂₀Cr₃C₆ has a large magnetic moment under a strong magnetic field,and thus has a large amount of change in magnetic free energy,while the magnetic free energy of Fe₂C and Fe₃C is very small,almost 0.Therefore,Fe₂₀Cr₃C₆ has greater thermal stability under a strong magnetic field,which promotes the early precipitation of Fe₂₀Cr₃C₆ carbides.It is in good agreement with experimental phenomena.