Research On Microstructures Of The Hypereutectic High Chromium Cast Iron Treated By Electric Current Pulse During The Heat Treatment

The hypereutectic high chromium cast iron(HCCI) specimens with the size of φ18×150mm were treated by electric currents pulse(ECP) during the heat treatment in the tube electrical resistance furnace. In this paper, the effect of temperature、working stage and treating time and the ECP parameters(electric voltage and frequency) on the carbides’ morphology、size and distribution in the heat treated microstructure of hypereutectic HCCI was studied. At the same, incorporation with the present theory, the mechanism that the ECP affects the carbides of hypereutectic HCCI was analyzed preliminarily.The reach result showed that, except the M7C3carbide, there is no new type carbide in the microstructure which was treated by the ECP during the heat treatment. But the ECP can accelerate the progress of the primary carbides and eutectic carbides refining and granulating, which makes the morphology of the primary carbides and eutectic carbides changed a lot obviously. The treating technology and parameters of ECP also have a great influence on the morphology of the carbides.Under the experiment condition, the morphology of the carbides were not improved by the ECP treatment at the room temperature. At the temperature of1100℃, the rod-shape primary carbides became thin and shot, the number of granulated carbides increased, the carbides in microstructure granulated obviously after being treated by ECP. As the temperature increased to1200℃,the degree of carbides’ granulating was improved, while the carbides grew bigger, the effectives of ECP were limited. The effectives of the ECP were different at different working stage. The ECP improved the morphology of the carbides a little during the cooling stage, and with the increase of the treating time, more and more black hole appeared, which means the morphology became worse. When the ECP was added during the holding stage, the size of the primary carbides reduced with the increase of the ECP’s working time, and the eutectic carbides turned into particles gradually. Within the range of the experiment, the optimum treating process of the electric current pulse were as follow:the treating temperature were1100℃, the ECP was added during the holding stage, and the treating time was5minutes.In order to study the effectives of the ECP parameters on the equivalent diameter and shape factor of the primary carbides, the experiments about ECP parameter were taken out under the optimum treating process. The results show that, with the increase of the ECP’s voltage, the equivalent diameter of the primary carbides decreased first and then increased; and shape factor of the primary carbides increased first and then decreased. The mechanism of the frequency is familiar with the voltage, the variation tendency of the equivalent diameter and shape factor of the primary carbides was also the same. Under the condition of U=1400V,f=40Hz, ti=100μs, the effectives of the ECP were the best, the equivalent diameter of the primary carbides was25.26μm, and the shape factor was0.51.After being analyzed preliminarily, based on the heat effect、electromagnetism force and the electromigration, the mechanism that the ECP accelerate the refining and granulating of the carbides in the hypereutectic high chromium cast iron during the heat treatment. The ECP can encourage the carbides broken and resolved, so the diffusion speed of the atom was improved, which made the carbides became smaller, and the granulating process accelerated.