Formation And Evolution Mechanisms Of Microstructure Of Proeutectoid Ferrite In High Purity Fe-C Alloys Treated Under A High Magnetic Field

High magnetic field, as an extreme experimental condition, has been widely applied to heat treatment of materials. The effect of a high magnetic field on diffusional solid phase transformation in Fe-C alloys has been a burning and leading topic in this domain. It offers wide space for innovation.In this work, high purity Fe-C alloys were selected to exclude the effect of other alloying elements or impurities and heat treated without and with a 12T static magnetic field. The obtained microstructures of the proeutectoid ferrite were analyzed by optical microscopy and its crystallographic orientations were measured by SEM-EBSD. The microstructural formation and evolution mechanisms of the proeutectoid ferrite in the high purity Fe-C allys under a high magnetic field was systematically studied. These work have important significance to further enrich the theory of EPM (Electromagnetic Processing of Materials) and to offer new way to control microstructure and optimize property.The results of various heat treatments under a high magnetic field show that the high magnetic field obviously inhibits the formation of acicular Widmanstatten ferrite during the proeutectoid ferritic transformation and promotes the elongation and alignment of the ferrite grains in the field direction in the Fe-0.36%C and Fe-0.52%C alloys. The former effect is due to the fact that magnetic field enlarges the Gibbs free energy difference between austenite and proeutectoid ferrite and thus increases the transformation driving force; while the latter is owing to the joint action of the magnetic dipolar interaction between ferrite grains and the degaussing field (demagnetization energy). Although the ferrite grains elongate and align along the field direction, that shows obvious microstructure orientation, there is no preferential crystallographic orientation in the aligned grains detected by SEM-EBSD, which excludes the possible effects of magnetic anisotropy and magnetostriction on the formation and evolution of the proeutectoid ferrite microstructure.For magnetic field heat treatment, increasing the field intensity, lowering austenitization temperature, decreasing the holding time and reducing the cooling rate can also suppress even removing the formation of acicular Widmanstatten ferrite in high purity hypoeutectoid Fe-C alloys. Based on these results, a heat treatment method that can removing the thick Widmanstatten ferrite of hypoeutectoid steel under vacuum and high magnetic field was proposed by this work and has been patented.It was found that with the increase of carbon content in the alloys, the effect of the magnetic field on proeutectoid ferrite alignment in the field direction decreases. This is due to the fact that with the increase of carbon content, the amount of proeutectoid ferrite formed decreases; therefore, the magnetic dipolar interaction between proeutectoid ferrite grains becomes weak. Moreover, with high carbon content, the formation temperature of proeutectoid ferrite drops. And the diffusion of iron atoms and carbon atoms becomes difficult, which also hinders the formation of ferrite grains elongate in the field direction. Based on this observation, a new magnetic dipolar interaction model that explains the formation of proeutectoid ferrite with large inter-ferrite spacing in low carbon Fe-C alloys(at the beginning of ferrite transformation) or in high carbon (close to eutectoid composition) Fe-C alloys was proposed in this work.It was also found that the field effect on the formation of proeutectoid ferrite in the Fe-0.36%C and the Fe-0.52%C alloy is dependent on the sample position with respect to the field direction. When the sample normal direction of a plate sample is perpendicular to the field direction, the intensity of the demagnetization field inside the sample decreases. Consequently increase the availability intensity of magnetic field which act on the sample. The field effect in reducing the amount of Widmanstatten ferrite and in enhancing the alignment of proeutectoid ferrite grains in the field direction becomes more pronounced. And the overall amount of proeutectoid ferrite formed under a magnetic field is much more than the other cases. This further proves that demagnetization has effect on the formation and evolution of proeutectoid ferrite.The high magnetic field was also applied to the isothermal ferrite transformation in Fe-0.12%C above the paramagnetic Currie temperature. It was found that even in the paramagnetic state, the high magnetic field also tends to elongate and align the ferrite grains to the field direction. The magnetic dipolar interaction model was applied to analyze this phenomenon. This further reveal the mechanism of the ferrite grains elongate and align along the magnetic field direction.A diffusion couple made of purity Fe and high purity Fe-0.76%C was heat treated under a 12T magnetic field, which allows the carbon to diffuse inγ-Fe in three direction:in the field direction, against the field direction and perpendicular to the field direction. Results show that the field obviously hinders carbon diffusion when the field was applied in the perpendicular direction; While it slighly enhances the diffusion in the carbon direction which is against the field direction and parallel to the field direction. And then from two aspect to explain the behavior of diffusion of carbon inγ-Fe under high magnetic field, one is that the reduction of magnetic Gibbs free energy ofγ-Fe that is carbon content dependent, the other is that the magnetic dipolar interaction between iron atoms inγ-Fe result in the anisotropic diffusion of carbon inγ-Fe. This suggests that the anisotropic diffusion of carbon inγ-Fe could be one possible reason for ferrite grain elongation and alignment along the magnetic field direction.