Effects Of High Magnetic Field On Fe-C Phase Diagram And Microstructure Of High-Carbon Steel

In this work, the Fe-C binary phase diagram under magnetic field is theoretically simulated by using the LFG thermodynamics model and the Weiss model. The formation and evolution mechanisms of microstructure of high purity Fe-0.76%C and Fe-1.1%C alloys are systematically studied under high magnetic field.The simulated results show that magnetic field can shift the Ae1 line, GP line and Ae3 line to the higher temperature side, but the Aecm temperature is almost independent on the applied magnetic field. The eutectoid point shifts to the higher carbon concentration and higher temperature side under magnetic field, and the extent of the eutectoid point shift increases with the increase of magnetic field intensity. The phase zone areas ofα,α+γandγ+Fe3C are calculated. It is found that magnetic field can remarkably enlarge theαandα+γphase zone areas and can shrink theγ+Fe3C phase zone area.Introducing the Gibbs free energy difference between austenite and proeutectoid ferrite due to magnetic field, the theory which is based on classical solid state phase transformation can explain the experimental phenomenon that magnetic field can increase the number of proeutectoid ferrite grain, and the number of proeutectoid ferrite grain increases continuously with the increase of magnetic field intensity. And it can elucidate the reason why magnetic field can decrease the critical size of proeutectoid ferrite nucleus and the activation energy to form a proeutectoid ferrite nucleus and increase the nucleus rate of proeutectoid ferrite, which results in the increase of the number of proeutectoid ferrite grain.During the transformation from austenite to proeutectoid ferrite in Fe-0.76%C alloy, magnetic field elongates the proeutectoid ferrite grains along the magnetic field direction through the magnetic dipolar interaction between the proeutectoid ferrite grains and the iron atoms nearby in austenite, which makes the mean angle between the magnetic field direction and the major axis of proeutectoid ferrite of Fe-0.76%C alloy decrease with the increase of magnetic field intensity.It is also found that the magnetic field effect on the trend of proeutectoid ferrite elongation along the magnetic field direction in the Fe-0.76%C alloy is dependent on the sample position with respect to the field direction. When the normal direction of sample is parallel to the field direction, the effective magnetic field intensity is lower than that in the case when the normal direction of sample is perpendicular to the field direction. Therefore when the normal direction of a plate sample is perpendicular to the field, the mean angle between the magnetic field direction and the major axis of proeutectoid ferrite of Fe-0.76%C alloy is much lower, and the trend of the proeutectoid ferrite elongation along the magnetic field is more obvious.Because the eutectoid temperature increases with the increase of magnetic field intensity and the start temperature of the transformation from austenite to proeuectoid ferrite is also higher, the mean lamellar spacing of pearlite of Fe-0.76%C alloy increases with the increase of magnetic field intensity.Magnetic field can considerably increase the area fraction of proeutectoid ferrite of Fe-0.76%C alloy and the eutectoid carbon content, and the area fraction of proeutectoid ferrite and the eutectoid carbon content increases considerably with the increase of magnetic field intensity through shifting the eutectoid point to high carbon and high temperature side. Meanwhile, magnetic field can decrease the area fraction of pearlite and increase the mean lamellar spacing of pearlite, leading to the decrease of the macroscopic hardness of Fe-0.76%C alloy.Magnetic field considerably affects the abnormal microstructure in Fe-1.1%C alloy. Magnetic field can increase the area fraction of abnormal microstructure in Fe-1.1%C alloy by remarkably decreasing the Gibbs energy needed for the ferrite transformation. Simultaneously, due to the fact that the Ae1 line shifts to high temperature side with the increase of magnetic field intensity, the abnormal microstructure transformation has to stop and the pearlite transformation begins, therefore, the area fraction of abnormal microstructure firstly increase and then decrease with the increase of magnetic field intensity.When the normal direction of sample in Fe-1.1%C alloy is parallel to the field direction, the effective magnetic field intensity is lower than that in the case when the normal direction of a plate sample is perpendicular to the field direction, and the magnetic field intensity is enough(more than 4T) to end the abnormal microstructure transformation, and make the pearlite transformation begin. Therefore, the area fraction of abnormal microstructure in the case when the normal direction of sample is parallel to the field direction is larger than that in the case when the normal direction of sample is perpendicular to the field direction.