Effect Of Electric Current Pulse On Carbide In Hypereutectic High Chromium Cast Iron

It has significant engineering practicability that refining the carbides as hard phase in hypereutectic high chromium cast iron. The high chromium cast iron samples (Φ18 X 150 mm) during the course of solidification were treated with electric current pulse (ECP) from the temperature of 1350℃in solid-liquid stage and 1360℃above the liquidus line in tube electrical resistance furnace. The effects of ECP treatment of starting temperature, treating time and ECP parameters, such as electric voltage, frequency and pulse width, on the shape, size, distribution and type of primary carbides was investigated. The mechanism of primary carbides refined by ECP process was also analysed preliminarily.The results showed that, besides the M7C3 carbide, some new types of carbide such as MC, M7C0.45, M23C6 were found in the high chromium cast iron sample treated with ECP. The primary carbides and eutectic carbides were refined significantly by ECP. The fraction of rod-shaped primary carbides were reduced, and the fraction of granular primary carbides were increased. The eutectic carbides were also refined. The effectiveness of refinement of the carbides was determined by the starting temperature and treating time. At the same starting temperature, the effectiveness was increased as the treating time prolonged. But the coarsening of carbides were appeared if the treating time was too long, which lead to non-homogeneous primary in size. There were an optimum treating time to obtained the finest and homogeneous carbides. The effectiveness of refinement of carbides was increased as the starting temperature increased. But the higher of the starting temperature was, the longer of the optimum treating time was. The optimum treating technology of ECP in this case were as follow: starting temperature is 1360℃and treating time is 3.5 min.Based on the optimum treating technology mentioned above, the orthogonal experiment was took to study the effect of ECP parameters including electric voltage, frequency and pulse width on the equivalent diameter, shape factor and volume fraction of primary carbides. The relation between ECP parameters and equivalent diameter of primary carbides was analysed by multivariate nonlinear regression analysis, and the mathematic regression model between these was established. The results indicated that, the equivalent diameter of primary carbides was decreased initially and then coarsening as the increasing of electric voltage, frequency and pulse width. The parameter which had the most obvious influence was electric voltage, the next was pulse width, and then was frequency. The finest granular primary carbides with 19.03μm in equivalent diameter under the following ECP parameters:the electric voltage 962.74 V, frequency 38.14 Hz, and pulse width 111.38μs.