| Steel has been in the centre of materials science and technology in a modern industry. Materials scientists at home and abroad take effort to find new steel materials by new methods. In recent years, electromagnetic processing of materials attracts more attentions due to it can control the microstructure and improve the properties of materials. With the development of low temperature superconducting techniques, high magnetic field are more widely used.(1) The effect of the strong magnetic field on tempering precipitation in an Fe-0.28% C-3.0% Mo alloy was studied in this paper. The specimens were heat treated under a magnetic field of 12T with different temperature for varying time. Then the morphology, precipitation sequence, type, size distribution and hardness of carbides were observed and analyzed by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and hardness tester. Based on the experimental results, the main conclusions are as follows:(2) The carbide precipitation and the morphology of martensite in Fe-C-Mo alloy could be analysed and observed by using extraction replica and thin film sample preparation method. Different types of carbide precipitation could be seen from Fe-C-Mo alloy at different tempering temperature(low, medium, high tempering temperature). The carbides would precipitated in the grain and at the interface respectively.(3) The magneticfield had no obvious effect on the morphology of martensite when the alloy was tempered at 200℃; but the number of carbide precipitation will increase, which may be related to the magnetic field accelerating the diffusion velocity of carbon. After long time tempering at 530°C ,the morphology of carbide precipitation changed, rod-like without the magnetic field changing to spherical in the magnetic field. The main reason is the difference of magnetization between carbides and ferrite: the difference of free energy reduced in a strong magnetic field. Furthermore, the strain energy produced by the magnetostriction was the reason. In the high-temperature tempering, the magnetic field to some extent inhibited recrystallization, mainly with the magnetic ordering or magnetic domain walls hindered grain boundary migration during recrystallization.(4) Tempered at 200℃, M2C carbide precipitated firstly. With the increase of tempering time, the Fe5C2 carbide precipitated. But Fe5C2 carbide was not observed without the magnetic field. Thus strong magnetic field promoted the precipitation of Fe5C2 type carbide. When tempered at 530℃, (Fe, Mo) 2C, (Fe, Mo) 3C precipitated in a short time, but application of the magnetic field, making steady state M6C precipitated in advance, which was related with the effect of strong magnetic field on free energy between different carbides. Tempered at 700℃, carbide precipitation sequence and type did not change because of magnetic field, which may be due to the strong magnetic field having no effect on free energy at 700℃.(4) Strong magnetic field had no obvious effect on hardness when specimens were tempering at each temperature, because that the increase of hardness due to precipitation of fine carbide in the high magnetic field compensate for decrease of hardness due to missing of carbon atoms in the matrix or coarsening of the matrix at high temperature.
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