Influence of High Magnetic Field & Surface Nanocrystalline Treatment on Carburizing of Pure Iron
|Keywords||Carburizing Diffusion Pure iron Fe3C High magnetic field Surface nanocrystalline Mechanical Milling|
Carburizing is one of the most widely used surface engineering techniques to enhance the surface hardness, anticorrosion property and wear resistance of metallic materials. However, industrial carburizing is usually performed in between 850℃and 950℃for more than 4 hours, which can cause equipment damage and energy consumption. So the carburizing conditions have to be changed. The purpose of this study is to reduce carburizing temperature, to control the thickness of carburizing and carbon content by high magnetic field and mechanical milling.In this work, two experiments were performed. Carburizing behavior of the sample under the magnetic field was studied, which intensities were different. In another experiment, we prepared a nanostructured surface layer on a pure iron by means of a mechanical milling (MM). Carburizing behavior of the sample after MM was investigated in comparison with the original materials. Samples were characterized by means of XRD, SEM, micrographic observation, hardness test and analytical chemistry test. The main results include:1. Carburizing of pure iron under the high magnetic field(1) When the magnetic field intensity and direction changes, the diffusion layer was affect to some extend. For an example, when the magnetic field direction and the carburizing direction are parallel, carburizing is restrained on the surface layer, and diffusion layer becomes thinner. When the magnetic field intensity increases, carburizing is restrained significantly. When the magnetic field direction is parandicalar to the carburizing direction, carburizing is accelerated on the surface layer, and diffusion layer becomes thicker. When the magnetic field intensity increases, carburizing is accelerated significantly.(2) The main reason of change in carburizing layer is that crystal structure may vary by the high magnetic field. Therefore, the solid solubility of carbon in theγ-Fe phrase is varied. The growth of cementite (Fe3C) along the magnetic field direction is energetically favored in minimizing the total interaction energy.2. Carburizing of mechanical milling (MM) pure iron(1) The carburizing kinetics of the MM iron sample was found to be greatly enhanced. Carburizing can be performed at lower temperatures. When MM iron sample was carburized at 750℃for 120 minutes, a certain diffusion layer was obtained. However, no diffusion layer was obtained for the original sample carburized in the same condition. When MM iron sample was carburized at 770℃or 780℃for 120 minutes, the diffusion layer of MM sample was thicker than that of the original sample.(2) Reduced carburizing temperature for the MM iron can be attributed to the nano-scale grains size with a large amount of defects such as grain boundary and dislocation. These defects can introduce a high canbon content on the surface. In this way, carburizing can be performed at lower temperature.