First trial production of cylinder head casting

1. Primary raw materials

Q12 pig iron, industrial scrap, vermicularizing agent (wre = 9% – 11%, WMG = 4% – 6%), calcium silicate inoculant.

The particle size of vermicularizing agent is 5-25 mm, and that of calcium silicate and inoculant is 3-8 mm.

The casting process of the cylinder head is the first mock exam of two parts, and some risers are added on the basis of the gray iron molding process (casting process is shown in Fig. 1).

2. Batching and smelting process

(1) At the beginning of trial production, in order to ensure the purity of the burden, the recycled burden and carburizing agent are not used temporarily. The burden is 5200kg pig iron, 800kg scrap steel, 65kg ferrosilicon and 33kg ferromanganese.

(2) During the test, 500 kg molten iron was taken from the vermicularizing ladle. Before the ladle was used, it needed to be dried and dried thoroughly. The vermicularizing agent was 2.8 kg (0.56%) and the calcium silicate inoculant was 6 kg (1.2%). Before the ladle was used, the vermicularizing agent was dried and preheated. The vermicularizing agent was evenly spread in the dam. About 4 kg calcium silicate inoculant was evenly covered on the surface of the vermicularizing agent and tamped, The surface coating slag agent (without inoculation particles), the remaining inoculant is poured into the 4/5 side of shovel into the side of vermicular reaction, and then the remaining 1/5 iron is poured. The side of vermicularizing agent can’t be impacted during ladle pouring. Three boxes of zc170 cylinder heads are poured in one ladle, with a total of 6 pieces. The furnace temperature is 1520 ℃, and the vermicularizing treatment temperature is 1470 ~ 1480 ℃. The test block is cast by single casting. One test block is cast by each ladle of molten iron, and the test block is poured after the pouring of cylinder head. The pouring time is controlled within 8 minutes after the treatment. A total of 72 blanks are poured by one ladle of molten iron.

3. Result analysis

(1) After inoculation, the fracture surface of triangular test piece in front of furnace is silvery white with slight depression on both sides, which is obviously different from triangular test piece in front of furnace of gray iron, as shown in the figure 2.

(2) The chemical composition test shows that C, Si, Mn, P and s meet the requirements of the target composition, re is 0.004% ~ 0.024% higher than the target value, and Mg is 0.012% ~ 0.022% higher than the target value. It can be seen that more vermicularizing agents are added. Sulfur has great affinity with all the vermicular elements. When the Vermicular Agent is added to the molten iron, it is first consumed in desulfurization and deoxidation. When WS ≤ 0.03% in the molten iron, the remaining vermicular elements can make the graphite vermicular. If the sulfur content of the original molten iron is low, there are more remaining vermicular elements besides desulfurization and deoxidation, which will make the graphite transiently vermicular and increase the probability of forming spherical graphite.

(3) Metallographic analysis through metallographic observation, the conclusion is that there are more graphite balls, and the creep rate is only 30% – 50%, as shown in Figure 3.

(4) The tensile strength is 432 ~ 451 MPa, the elongation is 5.37% ~ 6.41%, and the hardness is 172 H B W. The tensile strength is much higher than expected. Metallographic analysis shows that the creep rate is low, there are many graphite balls, and the tensile strength is high. This will affect other properties, such as the decrease of thermal conductivity, which is unfavorable to the high temperature operation of cylinder head.

(5) Appearance quality: no major defects were found in the appearance of the cylinder head cast for the first time. After processing, the pressure test was carried out, and all of them were scrapped due to air leakage. After dissection, it was found that the defect was obvious porosity. The defects are located in the concentrated hot spot of the upper part of the casting, such as tappet hole, guide hole and threaded hole. The defect morphology is shown in Fig. 4 and Fig. 5.

4. Summary of the first trial production

The shrinkage of vermicular graphite cast iron is between that of nodular cast iron and gray cast iron. The volume shrinkage of vermicular graphite cast iron is related to the vermicular rate. The higher the vermicular rate is, the smaller the volume shrinkage is, and finally close to that of gray cast iron; On the contrary, the lower the creep rate is, the larger the shrinkage is, and finally it is close to that of ductile iron. From the analysis of chemical composition, C, Si, Mn, P and s are in line with the requirements. There is no carburizing agent and recycled material used in proportioning. The sulfur content of the original molten iron is low (WS < 0.02%), and the vermicularizing agent added accounts for 0.56% of the molten iron, which is obviously too much. The spectrum analysis shows that the residual rare earth elements and magnesium elements are consistent with the analysis results.