In addition to melting, the inoculation treatment of nodular cast iron is also a key link in the quality of molten iron. After the melt spheroidization, multiple inoculation treatments are adopted. Figure 1 shows the metallographic structure of different inoculation combinations of nodular cast iron, which are ① SiC pretreatment+floating silicon inoculation; ② SiC pretreatment+floating silicon inoculation+wire feeding inoculation; ③ SiC pretreatment+floating silicon inoculation+wire feeding inoculation+ladle inoculation; ④ SiC pretreatment+floating silicon inoculation+wire feeding inoculation+ladle inoculation+stream inoculation.

As shown in Figure 1 (a), if only SiC pretreatment and floating silicon inoculation are carried out, because SiC pretreatment is added to the furnace during the melting process, and floating silicon inoculation floats on the surface of the melt, the scope of action is limited, and the number of spheroidal graphite is very small, only 100-200/mm2. In the presence of floating silicon, the number of spheroidal graphite increases sharply and the diameter of graphite ball decreases rapidly after increasing wire feeding inoculation, as shown in Figure 1 (b). In order to prevent the inoculation of nodular cast iron from fading too fast during transportation, the inverted ladle inoculation is added. At this time, it should be added after the ladle bottom is filled with molten iron to avoid the inoculant sticking to the ladle bottom. In order to fully fuse the inoculant with the molten iron and improve the absorption rate, the addition speed should not be too fast. The amount of inoculant added in the ladle should be 0.2%, and the silicon content should be increased to promote the activity of carbon, so as to increase the nucleation and further increase the number of graphite spheres, as shown in Figure 1 (c). Stream inoculation belongs to late inoculation, and the amount of inoculant required is smaller than that of inverted pregnancy, generally 0.1%, and the inoculation effect of nodular cast iron is better. Because the added period is relatively late, there is basically no problem of decline in late inoculation, as shown in Figure 1 (d).

Figure 2 shows the shrinkage and looseness of steering knuckle with different inoculation combinations. If only SiC pretreatment and floating silicon inoculation are carried out, the number of graphite spheres is small and the shrinkage area is obviously large, as shown in Figure 2 (a). With the increase of wire feeding inoculation, the number of graphite spheres increases and the shrinkage and porosity are improved, as shown in Figure 2 (b). The shrinkage porosity is eliminated after the addition of ladle inoculation and flow inoculation, as shown in Fig. 2 (c~d).