| This study came to design the material heat treatment experimental device and data acquisition system. Key structure of the experimental device such as heating furnace, differential temperature system, temperature control system and data acquisition system were analyzed and described. Then, without magnetic field, the effectiveness was validated through the experiment, which included temperature control, signal acquisition, data recording and output. The results follow as, the heater unit could heat up to normal working condition and keep warm under the control of intelligent temperature controller. However, it couldn't realize the accurate control during the temperature declining process. So right amount of argon was used to achieve a constant input of rapid cooling. The single acquisition unit could precisely collect the signal of furnace temperatures. Signal expanding module would be used to enlarge the signal in the future study. The data recording unit could accurately record the temperature data from the signal acquisition unit. Besides, experimental result was outputted with excel charts and image.On that basis, the different isothermal heat treatment time for50Si2Mn3cast steels near the temperature around A1point was carried out in the strong magnetic field of12T. On the one hand, it validated the applicability of the device in a strong magnetic field. On the other hand, it has studied on the influence of magnetic field on both the volume fraction and the morphology of transformed pearlite by the comparison of the different technique. It has studied on the morphology of transformed granular pearlite and granular mechanism. The results of the study show that,the incubation time of pearlite phase transformation for50Si2Mn3cast steels decreased at a high temperature around A1point with12T field. In other words, magnetic field could promote pearlite phase transformation, which made the volume fraction of transformed pearlite increase evidently. Magnetic field of12T could raise the critical point of pearlite phase transformation, which made50Si2Mn3cast steels transition above A1point. The starting point of pearlite phase transformation was100min by isothermal heat treatment at the temperature of690℃. The starting point of pearlite phase transformation was30minutes and the ending point was5hours by isothermal heat treatment at the temperature of700℃. The microstructure of50Si2Mn3casting steels treated isothermally at temperatures around A1point with12T field were studied. The results showed that there are more granular pearlite colonies in the microstructures of samples through magnetic field treatment. The granular pearlite consists of two parts: one part is from the discontinuous granular cementite which precipitated directly from ferrite matrix; another part is from the "fusing break" of the lamellar cementite during isothermal treatment. Magetic field promotes the nucleation and growth of ferromagnetic phase---errite, so that more carbon atoms are "trapped" within the growing ferrite and form "granular cementite" later in the form of precipitation in ferrite matrix. The lower isothermal temperature is, the more obvious the effect is. In addition, the magnetostriction between two constituent phases of pearlite is quite different under the application of magnetic field, which cause the strain energies of both phases of pearlite to change in such way which benefit the cementite precipitation from austenite or ferrite in the shape of sphere, and it also makes a contribution to the volume fraction-of granular pearlite. |
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