The metallurgical process of the induction furnace is essentially different from that of the cupola. Induction furnaces are used to melt gray cast iron. Compared with the cupola, the superheat of molten iron is higher, and the melting and holding time is longer, which is easy to cause decarburization. Castings with the same chemical composition and the same casting mold have higher strength and hardness than cupolas, a greater tendency to cold white, shorter graphite lengths, easy-to-produce D and E graphites, poor fluidity of molten iron, and increased shrinkage cavities. Large, easy to cause various casting defects (porosity, cold shut, shrinkage cavity, shrinkage cavity, etc.). Since our factory adopted the induction furnace to produce castings in 2013, the melting process similar to the cupola furnace is still used in the initial stage of production. For the castings produced in the early stage, the casting rejection rate is as high as 10%.  In 2014, our factory pretreated gray cast iron with the pretreatment agent SiC. After adding silicon carbide, the shape of graphite becomes thicker and shorter, and the spacing becomes larger, which increases the number of eutectic groups, improves the strength and hardness of cast iron, reduces the supercooling degree of molten iron after inoculation, and improves the processing performance of castings.

1. Production conditions

3t intermediate frequency induction furnace is used for smelting, and the furnace charge ratio is stable: 20% pig iron, 40% scrap steel, and 40% recycled material adding the appropriate amount of recarburizer, ferrosilicon, ferromanganese, SiC, and FeS to adjust the chemical composition of molten iron. The time to add SiC is in The molten iron in the furnace is added when it is half melted, and the tensile strength and hardness are measured on a Φ30mm single-cast tensile test bar, which is formed by furan resin sand.

2. Test results and analysis

1. Effect on graphite morphology

As shown in Figures 1 and 2, sampling is sampling.

The chemical composition in Figure 1 is: wC=3.18%, wSi=1.85%, wMn=0.83%, wP=0.043%, wS=0.078%, and the molten iron has not been pretreated by SiC. Graphite morphology: the distribution of flake graphite belongs to type A, there is a small amount of type C graphite, and the graphite length is 4~5 grades.

The chemical composition in Figure 2 is wC=3.20%, wSi=1.83%, wMn=0.86%, wP=0.043%, wS=0.076%, adding 0.8% SiC to pretreat the molten iron. Graphite morphology: the distribution of flake graphite belongs to type A, there is a small amount of type C graphite, and the length of graphite is 5~6.

figure 1

figure 2

2. The effect of hardness on the tensile strength of cast iron

Electric furnace smelting was used for production tests. The composition of each furnace is the same, try to ensure the same chemical composition. The temperature of molten iron is 1515-1525°C, the pouring temperature is 1380-1430°C, and the bottom is covered with 0.3% silicon barium inoculant for inoculation treatment.  , the detected chemical composition and the corresponding strength and hardness are shown in Table 1. The test results show that under the same smelting conditions and similar chemical composition, the mechanical properties and hardness of molten iron pretreated by SiC are improved to varying degrees, compared with untreated molten iron.

3. Influence on the cutting performance of cast iron

As shown in Figure 3, this part is the left cover, which is a part of the company's product T165. Material, weight 45kg. The main wall thickness of this part is 8~10mm. In the early production process, we mainly improved its processing performance through the method of high silicon-carbon ratio. Because the induction furnace smelts cast iron, the supercooling degree is large, and the whitening tendency is strong. Under the same carbon equivalent, induction furnace-smelted cast iron has a higher degree of undercooling during eutectic transformation than cupola-smelted cast iron. Most of the castings produced in the early stage showed whitening at the edge of the casting during the machining process, which was difficult to process. Only by annealing the casting can the machining requirements be met. Our approach of increasing the carbon equivalent and using a high silicon-to-carbon ratio has not yielded good results. Subsequently, we tested the SiC pretreatment process on molten iron and used the method of high silicon-carbon ratio to verify the process. First, we confirm that the chemical composition of the workpiece is consistent with the original, wC=3.20%~3.30%, wSi=2.20%~2.30%, wMn=0.80%, wP≤0.50%, wS=0.050%~0.10%, pretreatment agent SiC The addition amount of the compound is still maintained at 0.8%. We have followed up on the processing of the two batches of 40 pieces produced later, and there is no phenomenon of white edge melting. The processing performance is good, and the phenomenon of hard-processed materials is completely alleviated.

image 3

3. Conclusion

(1) Pretreatment of molten iron with SiC in the process of smelting gray cast iron in an induction furnace can improve the shape and size of graphite.

(2) Pretreatment of molten iron with SiC in the process of smelting gray cast iron in an induction furnace can improve tensile strength and hardness. Under the same conditions, the tensile strength can be increased by 20~, and the hardness can be increased by 10~.

(3) Pretreatment of molten iron with SiC in the process of smelting gray cast iron in an induction furnace can significantly improve the cutting performance and reduce the tendency of castings to be white.