| Refining solidification structure of metals is effective method to improving mechanical properties of metal materials. In the past few years, investigators utilized electric current pulse (ECP) for refining grain size during solidification, which become grandully current issue. This work systematically investigated process conditions of refining solidification structure by ECP, and theoretically analyzed refinement mechanism. At last, industrial experiments of refinement technique by ECP were carried out.Pure Al was firstly selected as the experimental material, and ECP employing modes were optimized. Experimental results showed that under the same conditions, refining effect by the parallel electrodes on the melt top surface was better than other modes used in the experiment. The refined equiaxed grain size and area rate of equaixed grain zone was about 0.3mm and 70%. The inserting depth of electrodes and the change of superheat degree had not influence on grain refinement effect. ECP can not influence the macrostructure of pure aluminium when treating at temperature above liquidus, treating of ECP at initial solidification or nucleating stage is most effective, its refinement efficiency was euqivalent to that of treating at the whole solidification processing, but treating of ECP at crystal growth stage also made structure change from columnar to equaixed (Columnar to Equiaxed Transition--CET). Based on all these results, the refinement method of ECP's agitation on the melt top surface was presented.The distribution of current and magnetic fields caused by ECP in melt was analysed by using electrodynamics theory. The results showed that action zone of ECP focused on the top surface because of skin effect, and presented exponential attenuation from surface toward melt inside. And, power loss had relation not only with peak value current and vibrating frequency, but also with pulse discharging frequency, effective width of electrode, distance between electrodes. Increasing of ECP's power loss in the melt could increase refining efficiency. It is considered that power loss prolong solidifying time near the top surface of melt, Joule heating casued by ECP in melt continually bring out floating crystal nucleus, which drift downward along the front of solid liquid interface by the natural convection and electromagnetic force. If the front of solid liquid interface exists undercooling, the floating nucleus will grandully grow up and sedimentate at the bottom of ingot, hence the fine equaixed grain is obtained.On the other hand, the influence of ECP on solidification structure of high carbon and chromium bearing steel was investigated. Results showed that under the laboratory conditions, the better refining effect could be achieved by ECP with low voltage and intermediate discharging frequency. With an increase in discharging frequency, the solidified structure and the central shrinkage porosity were gradually improved. With the hot top, lower power loss could achieve the better refining effects. Based on the investigation mentioned above, a large-power ECP generator for industry application were designed and made, subsequently plant trials about applying electric current pulse (ECP) to 3.7 ton weight high carbon and chromium bearing steel ingot were carried out in Baoshan Iron & Steel Co., Ltd. The inspection results by ultrasonic detection and macroscopic examination and so on suggested that compared with samples without ECP treatment, the inner quality of the forged billets treated by ECP was obviously improved, and the center porosity and shrinkage cavity were also reduced. For finished round bars, carbon distribution was more homogeneous. Moreover, maximum width and average width of carbide banding, especially average spacing between carbide bandings were decreased significantly. Therefore, ECP can refine the solidified structure of ingot and enhance its inner quality. |
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