The low spheroidizing grade ofnodular cast iron is mainly caused by high spheroidizing temperature (due to high pouring temperature). How to improve the spheroidizing rate under the condition of ensuring the pouring temperature, we first set a layer of spheroidizing agent at the bottom of one side of the dam of the dam type spheroidizing package, set a layer of covering agent on the surface of the spheroidizing agent and tamp it, place the cover plate on the covering agent, and then place the spheroidizing agent, the first inoculant and covering agent successively on the cover plate. The cover plate is stainless carbon steel plate or formed nodular cast iron plate.
Raise the temperature of the molten iron of the electric furnace to 1560 ~ 1600 ℃, remove the slag on the surface, sprinkle 0.05% ~ 0.15% silicon carbide pretreatment agent by weight of the molten iron, and keep it warm for 9 ~ 11min. Pour the molten iron after removing the scum in the molten iron ladle into the side without spheroidizing agent in the dam spheroidizing ladle. The inverted spheroidizing and inoculation process of lost foam casting nodular cast iron is to first set a layer of spheroidizing agent with the weight of 0.7% ~ 0.9% of the molten iron at the bottom of one side of the dam, cover the surface of the spheroidizing agent with a layer of covering agent with the weight of 0.05% ~ 0.15% of the molten iron and tamp it, place the cover plate on the covering agent, and then place the spheroidizing agent with the weight of 0.55% ~ 0.65% of the molten iron on the cover plate in turn The weight of the first inoculant is 0.05% ~ 0.15% of the weight of the molten iron and the weight of the covering agent is 0.05% ~ 0.15% of the weight of the molten iron. The top surface of the cover plate is flat, the shape of the bottom surface is the same as that of the package bottom of the spheroidized package, and the gap between the edge of the top surface of the cover plate and the inner wall of the package is ≤ 5mm. Therefore, when adding molten iron for spheroidizing reaction, due to the existence of intermediate cover plate, the whole spheroidizing reaction is carried out in two times. The first layer of spheroidizing reaction is that the spheroidizing agent on the upper layer of the cover plate reacts first to make the molten iron absorb part of rare earth magnesium. With the progress of spheroidization reaction, the cover plate melts gradually. When the cover plate melts, the spheroidizing agent arranged under the cover plate begins to react, that is, the second spheroidizing reaction. Through two spheroidizing reactions successively, the spheroidizing reaction time is prolonged, the absorption rate of rare earth magnesium is improved, the later burning loss is reduced, the spheroidizing grade is improved, and the final quality of lost foam casting nodular graphite castings is guaranteed. After the spheroidizing process is improved, the metallographic structure of lost foam casting nodular iron castings is shown in Fig. 3 and Fig. 4.
It can be clearly seen from Figure 3 and Figure 4 that through the process improvement, the number of graphite and pearlite content have been significantly improved, and the process improvement has achieved obvious results. We have explored the spheroidization process improvement in the melting of nodular cast iron in lost foam casting and achieved good results, but the pearlite content still needs to be improved, and the process needs to be further improved in the future work.