Effects Of High Magnetic Field On Microstructure Of High Purity Fe-C Alloys

In this work, high purity Fe-C alloys were selected to exclude the effect of other alloying elements or impurities. A series of contrast experiments with different treatment conditions were done without and with a12T static magnetic field. The microstructural formation and evolution mechanisms of high-purity Fe-C alloy under a high magnetic field was studied by means of optical microscopy and SEM-EBSD analysis.No matter for the samples with their normal direction parallel to the magnetic field or the samples with their normal direction perpendicular to the magnetic field, The angle between the direction of the magnetic field and the observed surface-lamellar cut line is much larger.With the increasing of the magnetic field strength, the growing trend of angle between the direction of the magnetic field and the pearlite lamellae is more obvious. Under the same heat treatment conditions, compared with the samples with their normal direction parallel to the magnetic field, when the samples’normal direction perpendicular to the magnetic field, the angle is much larger.With the increasing of austenitizing temperature, the angle between the direction of the magnetic field and the observed surface-lamellar cut line is decreased. This is because the higher of the austenitizing temperature, the bigger of the austenite grains, and the composition is more uniform, which lead to the reduction of the pearlite nucleation rate and nuclei growth velocity during the decompose of austenite, the increasing of the pearlite lamellae spacing and the atoms diffusion distance.these lead to the decreasing trend of the angle between observed surface-lamellar cut line and the magnetic field.With the increasing of cooling rate, the angle between the direction of the magnetic field and the observed surface-lamellar cut line is decreased. This is because the samples stay a very short time at the high temperature with a rapid cooling velocity, which cause the carbon diffusion in austenite is insufficient. Austenite transformation occurs at a lower temperature, resulting in an exaggerated super-cooling degree, that is, the driving force of phase transformation is bigger, and then causes the increasing of nucleation rate of pearlite grains and a shorter process of nucleation and growth. Under this condition, the effect of magnetic field cannot be fully played. Thus the angle between observed surface-lamellar cut line and the magnetic field is decreased.Compared with the non-field annealed sample, the application of the magnetic field increases the number of texture components of proeutectoid ferrite grains and the texture tend to diversify. But, with the increasing of the magnetic field strength, the magnetic field significantly weakened the intensity of main texture components. It indicates that the application of the magnetic field can make the preferred orientation in annealed samples weakened. This may be attributed that ferrite grains can be easily magnetized to saturation state under a low magnetic field, that is,the magnetization intensity in various crystallographic directions was magnetized to saturation magnetization, and thus lose the magnetic anisotropy, and resulting in the no preferred orientation in proeutectoid ferrite grains in field annealed samples.The results of carbon diffusion in the pure iron show that high magnetic field can significantly affect the carbon diffusion in pure iron along the direction parallel and perpendicular to the magnetic field direction. Compared with the non-field carburized sample, with the increasing of the magnetic field strength, the diffusion distance of carbon in field carburized samples increased continually, whether the direction of the carbon diffusion parallel or perpendicular to the magnetic field direction. When the magnetic field strength is higher than1T, with the increasing of the magnetic field strength, the diffusion distance of carbon shows no obvious change, but the diffusion distance of carbon in field carburized samples is higher than that of the non-field carburized sample, this indicates that high magnetic field can promote the carburization in pure iron markedly.