Study On Diffusion-Controlled Phase Transformation In Medium Carbon Si-Mn Casting Steel Under Magnetic Field

The main purpose of this paper has concerned with the influence of magnetic field on pearlite transformation in medium carbon Si-Mn casting steel. Experimental study has been divided into three main steps:(1) Carry out the related studies on the influences of low magnetic field (<1T) on continuous cooling phase transformation with different cooling rates, isothermal pearlite phase transformation at temperature below the eutectoid temperature (A1), and magnetic circling isothermal treatment at temperature above A1, in 30Si2Mn2 and 50Si2Mn3 steels. (2) Explore the possibility of applied high magnetic field (up to 12T) on inducing pearlite transformation at temperatures above the A1 point; (3) Build up the theoretical model of magnetic-inducing pearlite phase transformation (MIPT) above eutectoid temperature and put forward the related thermodynamic description of phase transformation. In this paper, the innovation lies in the fact that external magnetic field has been utilized to induce and has successfully induced pearlite transformation at temperatures above the eutectoid point; On the basis of experimental results, some theoretical issues related to the preceding phase of pearlite transformation have been discussed; Theoretically, a model of magnetic-induced pearlite transformation has been put forward.Studies carried out under weak magnetic field have indicated that:a) For continuous cooling phase transformation, there exists a problem of how long the sample expose to a magnetic field for. The contribution of magnetic field supplying to the driving force of phase transformation depends on multiple factors, such as the time length of applying a magnetic field, the intensity of the field, the chemical composition, and the heat-treating conditions. Applying a magnetic field can promote the completion of the high temperature-type phase transformation under slow cooling condition, the low temperature-type one under rapid cooling condition. For the steel with lower carbon content, the experimental parameters of heat treatment under the field can significantly affect the morphologies of bainite, which is the product of bainite phase transformation. b) For the isothermal pearlite transformation at temperature below A1, the applied magnetic field can shorten the incubation period. In the early stage of transformation, the formation of pearlite colonies shows the feature of " an evoked growing mode". A frame of pearlite colonies can quickly form at the very beginning, and a relative high transformed fraction can be reached within short time. Applied magnetic filed can enhance the " evoked growing-mode" happening in the early stage of transformation. c) Through the exploration of magnetic circling treatment at temperature above A1, the nucleation positions, preceding phase, nucleus morphology and growth mode of induced pearlites have been studied. Two phases of earlier induced pearlites do not have to parallel to each other, the morphology of earlier pearlite is confined by the configurations of pro-eutectoid ferrite that has nucleated at the grain corners and grain boundaries of austenite, but the growth of pearlite is still consistent with the cooperation-growth mode of two phases. During the nucleation and growth of pearlite, the interlamellar spacing does not hold constant but decreases with time, which indicates that the isothermal transformation shows non-steady-state mode. The eutectoid cementite can precipitate within ferrite plate. The nucleation and growth rate of cementite phase is one of main constraining factors in the formation of pearlite.Studies carried out under high magnetic field have indicated that:a) magnetic-induced pearlite mainly formed at austenite grain boundaries, the preceding phase of pearlite is ferromagnetic phase—ferrite; the stronger the applied field, the more the induced pearlite; the closer to Al point the isothermal temperature, the more apparent the influence of magnetic field on the transformation; the longer the isothermal holding time, the larger the volume fraction of induced pearlite. b) Pearlite mainly nucleate and grow on the basis of the formed ferrite grains, the morphology depends on the configurations of both ferrite and cementite:First kind is the one with bar-like cementite phase distributed within ferrite matrix; The second is the lamellar pearlite; The third is the scale-like pearlite in which cementite phase takes the form of scale. c) Through the diffraction patterns'analysis, so far, it has not been found that there is fixed crystallographic orientations between the precipitates (cementite phase) and the ferrite matrix. The phenomenon that pearlite colonies arranged along the direction of applied magnetic filed has not been observed. But, we observed the phenomena that several long bar-like ferrite phases parallel to each other within austenite, and some adjacent pearlitic colonies show nearly the same direction of lamellae. The fact that magnetic field can induce the pearlitic transformation at temperatures above A1 indicates:applying a magnetic filed, esp. high magnetic field, at higher temperature range, can degrade the hardenability of materials due to the formation of magnetic-induced ferrite or pearlite. In the theoretical research, from the view of magnetism and thermodynamics, a theoretic model of magnetic-induced pearlite transformation has been put forward, that is the magnetizing and nucleating model of fluctuating clusters or structural micro ranges, its theoretic basis is that:there are magnetic interactions and chemical interactions among adjacent atoms, the additional magnetostatic energy contribution can set significant influences on the nucleation and growth of new phases. At temperatures above A1, the preceding phase of magnetic-induced transformation is ferrite with body-centric cubic structure, ferrite phase formed on the basis of " ferromagnetic cluster, FMC" existing in the interior of austenite. Both the interactions between adjacent atoms and the spontaneous magnetostrictive effect below Curie temperature are the main causes of existing "FMCs" within paramagnetic phase. External magnetic field can cause the magnetic deflection of iron atoms and the magnetostrictive effect, hence, can promote the growth of existing "FMCs" and induce more new "FMCs" within austenite, the latter factor may increase the nucleation rate of ferromagnetic phase. The magnetic driving force of phase transformation is the magnetostatic energy difference between the transforming phases caused by the introduction of magnetic field. The chemical driving force item supplied by local "chemical order phases" cannot be neglected when considering the total Gibbs free energy.