Calculation Of Fe-C Phase Diagram And Evolution Mechanisms Of Microstructure In High Purity Low Carbon Steel Under A High Magnetic Field

High magnetic field as a clean, non-contacting physics field has been widely used in the investigation of many science domains such as physics, chemistry, biology, metallurgy and material science. Consequently, lots of new experimental phenomenon have been brought up with new research areas and directions, one of which is high magnetic material science. The use of high magnetic field as an extreme experimental condition in material preparation and processing has generated various experimental phenomenon. As the high magnetic field treatment equipments develop, more attention is paid to investigating the impact of a high magnetic field on high temperature diffusional solid phase transformation, especially, to the experimental study and theoretical explaination of the high magnetic field effect on the microstructure of high temperature diffusional phase transformation in Fe-C and Fe-C-X alloys.Based on the above, in this work, the Fe-C alloy solid-phase diagram under high magnetic field is theoretically simulated by using the thermodynamics calculation. High purity Fe-0.12%C and Fe-0.0272%C alloy are selected and treated without and with a 12T static magnetic field. The obtained microstructures are analyzed by optical microscopy and the evolution and formation mechanisms of the high purity Fe-C alloy under a high magnetic field are systematically studied. Results are as follows:The phase equilibrium in Fe-C phase diagram without and with magnetic field are calculated by introducing the demagnetization field to KRC thermodynamics model and the Weiss model to set up a calculated Fe-C phase diagram. It turns out that the calculated Fe-C phase diagram is more close to the experimental result, moreover, any Fe-C phase diagram under magnetic field could be calculated by this model even when the sample arrangement is different.During magnetic field heat treatment, magnetic field elongates the pearlite colonies in Fe-0.12%C alloy in the magnetic field direction through the magnetic dipolar interaction between proeutectoid ferrite grains. When magnetic field intensity increases, the influence of magnetic field on ferrite grains and the magnetic dipolar interaction between ferrite grains become stronger, so the elongation of pearlite colonies along the magnetic field direction increases.It is found that, when cooling rate is high, the distance between proeutectoid ferrite nucleations decrease due to the high nucleation rate, which reduce the elongation and alignment of ferrite grains in the field direction. Finally, the elongation of pearlite colonies in the field direction becomes weaker. Based on this, when Fe-0.12%C alloy is treated under 12T magnetic field, the elongation of pearlite colonies in the field direction decrease as the cooling rate increases.For magnetic field heat treatment with high austenitization temperature and long holding time, the austenite grains in Fe-0.12%C alloy are relatively coarse, so the distance between proeutectoid ferrite nucleations at austenite grain boundary triple junctions is larger, which promotes the elongation and alignment of the ferrite grains in the field direction. As a result, the elongation of pearlite colonies in the field direction becomes more pronounced.It is known that, the strength of the demagnetization field generated by magnetic field is dependent on the sample position with respect to the field direction. When the sample normal direction of a plate sample is parallel to the field direction, the overall amount of pearlite colonies of Fe-0.12%C alloy is less than the case when the field is perpendicular to the sample normal direction., and the elongation of pearlite colonies in the field direction is weaker.It is also found that the overall amount of pearlite colonies decreases gradually as the magnetic field increases. When the magnetic field reached 8T and 12T, the pearlite colonies can be hardly seen in Fe-0.0272%C alloy which sample normal direction is parallel to the field direction. This experimentally proves that the C saturation solubility in ferrite of Fe-C phase diagram shifts to the high carbon content side.Under magnetic field, paramagnetic cementite may generate magnetic cavities in the ferromagnetic matrix ferrite, and those magnetic cavities induce a demagnetization field effect. Besides it is known that the eutectoid temperature rises as the magnetic field increases. Based on the above, in Fe-0.0272%C alloy, the cementite elongation is more obvious under higher magnetic field. In the case of fast cooling, the undercooling degree is high and carbon atom diffusion is hindered. Therefore, in Fe-0.0272%C alloy, the tendency of the cementite alignment in the field direction becomes weaker.During magnetic field heat treatment, different sample arrangement results in different demagnetization field intensity. When the sample normal direction of a plate sample is parallel to the field direction, the amount of pearlite colonies of Fe-0.0272%C alloy is less than the case when the field direction is perpendicular to the sample normal direction. Consequently, the cementite elongation in the field direction becomes lower.