Effect Of High Magnetic Field On Phase Transformation And Carbide Precipitation In Fe-0.28%C-3.0%Mo Alloy

To develop steels with good combination of high strength and good toughness by means of efficient and ecologicial way is becoming a main task for materials scientists and engineers so as to satify the stricter and stricter demand on modern industry application. The addition of microalloying elements to improve alloy design and invention of processing and heat treatment are the most important approaches. Heat treatment of steels with high magnetic field was emeraged in last decades. With the development of generation technology of high magnetic field, the application of high magnetic field has become in-depth and extensive. As a new kind of heat treatment technique, the application of high magnetic field on steel materials processing will bring great impact on research and development of new materials. Insight into the effects of magnetic field on phase transformation will be having useful contributes to the development of materials science field.In this thesis, an alloy of Fe-0.28%C-3.0%Mo was isothermally held with 12T magnetic field at different temperatures for various times. The ferrite transformation, carbide precipitation and pearlite transformation in this alloy were systematically investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and hardness indentation. The analysis and discussion on experimental results were made from the view points of phase transformation thermodynamics and kinetics. The main conclusions are as follows:(1) In the Fe-0.28%C-3.0%Mo alloy, degenerate ferrite was observed around and below the bay temperature in TTT-curve of ferrite initial transformation. The coupled solute drag-like effect and sympathetic nucleation was responsible for the formation of degenerate. It was also found that the degenerate ferrite transformation kinetics was accelerated in a magnetic field and that the bay temperature of TTT-curves increased in a high magnetic field. The bay temperature was about 610°C without magnetic field, whereas it became approxiamtely 640°C with a high magnetic field. This is because the free energy of solid phase and degenerate ferrite nucleation barrier decreased, and coupled solute drag effect was thus reduced.(2) The combination of X-ray diffraction, high resolution images and electron dispersed spectrum was utilized to identify carbides. It was found that there were three kinds of carbides: (Fe,Mo)2C, (Fe,Mo)3C and (Fe,Mo)6C. In addition, one kind of carbide could be transformed into another kind in a high magnetic field. The kinds of carbides in a magnetic field were less than than without magnetic field. High magnetic field promoted the precipitation of carbides. The sequence of carbide precipitation was changed in a high magnetic fiel. This is because that the free energy reuced for different kinds of carbides varied owing to their different magnetization.(3) The pearlite both had degenerate morphology with a high magnetic field on and off. The most distinguished/notable effects of magnetic field on pearlite transformation were that the interlamellar spacing of pearlite was reduced in a high magnetic field. Theortical calculation indicated that the transformation temperature was raised in a high magnetic field. The phenomena and experimental results were analyzed and discussed from the view points of thermodynamics.