1 Preliminary test
As the pores are non fissured, N2 pores are excluded. In order to reduce the source of H2, the moisture content of molding sand was first adjusted from 3.5% ~ 4.0% to 3.0% ~ 3.5%, and the loss on ignition of molding sand remained unchanged (3.0% ~ 3.5%). Internal verification results after moisture adjustment.
The results showed that the rate of the waste products of the subcutaneous pores was greatly improved after reducing the moisture content of the molding sand.
Even if the water content is reduced, the subcutaneous pores will still occur repeatedly, and the effect is not stable. Therefore, the molding sand water content is not the root cause of the increase of the rejection rate of the subcutaneous pores of 693 brake caliper.
2 Analysis
During the mixing process of green sand, a certain proportion of coal powder, bentonite and water should be added. When pouring molten iron, some of the gas produced by these substances will always enter into the molten iron. After the gas inside the molten iron accumulates to form bubbles, it rises to the liquid level under the action of the buoyancy of the molten iron. If there is no obstacle to the liquid level, the gas can be discharged smoothly along the gap in the sand mold, and the casting will not generate air holes [2-4]. If there is an oxide film on the liquid surface to prevent the gas from overflowing, there will be air holes in the casting after solidification. Therefore, delaying the formation of oxide film on the surface of molten iron is the most effective way to improve porosity.
In the investigation of production process, compared with the jaw structure of 693 brake caliper and 812 brake caliper, the jaw height and thickness of 693 brake caliper are higher and thinner, and the magma simulation results also show that the temperature at the jaw is lower (lower than 1320 ℃), therefore, when the casting of this shape is vertically poured, the jaw position at the top of the casting structure has a larger rise height of molten iron, In the process of rising, the temperature of liquid surface is more serious, and the oxide film is easy to form on the liquid surface. In addition, the amount of floating gas is more, so the gas can not all overflow, so it is easy to form subcutaneous pores.
After the occurrence of subcutaneous pores, the first consideration may be to reduce the water content and ignition loss (effective coal content). The effect of water is generally considered to be relatively large, but the effect of coal powder on the probability of affecting the occurrence of pores can not be ignored.
Good pulverized coal is bituminous coal, which contains a large amount of C, volatiles and a small amount of S. during the high-temperature pouring process, the pulverized coal will react as follows:
C + O2 = CO2 (when O2 is sufficient) (3)
C + CO2 = 2CO, 2C + O2 = 2C0 (under the condition of insufficient O2) (4)
C = 2H2O = CO2 + 2H2 (5)
The temperature of reaction (5) is only 400 ℃, the higher the temperature is, the faster the reaction proceeds. When the temperature reaches about 650 ℃, the coal powder begins to decompose and produce a large number of hydrocarbons; when the temperature reaches about 880 ℃, C + H2O = CO + H2 will occur, which can produce reductive h. CO and H are conducive to reducing Fe2O3 flowing into the cavity with the molten iron and further improving the quality of the molten iron. In addition, h can eliminate the original O2 in the air of the mold cavity and prevent the oxidation of the molten iron.
3 Scheme determination and verification
According to the above analysis, two schemes are adopted to test and verify. Scheme 1 is to adjust the water content of molding sand from 3.5% ~ 4.0% to 3.0% ~ 3.5%, and the burning loss of molding sand from 3.0% ~ 3.5% to 4.0% ~ 4.5%; scheme 2 is to adjust the water content of molding sand from 3.0% ~ 3.5% to 3.5% ~ 4.0%, and the burning loss of molding sand from 3.0% ~ 3.5% to 4.0% ~ 4.5%.
Compared with table 4 and table 5, it can be seen that even when the water content of 693 brake caliper is relatively high, the subcutaneous pores basically do not recur. Therefore, scheme 2 is selected for batch production to verify whether the repeated occurrence of subcutaneous pores is improved. Continuous tracking of 6 batches of castings, batch verification results are shown in Table 6.
According to the data in Table 6, after the brake caliper is processed, the porosity rejection rate is below 1.5%, which meets the customer’s requirements. The results show that the porosity can be improved by properly increasing the burning reduction of molding sand, and the repeated occurrence of porosity can also be controlled and improved.
The production efficiency of disa vertical automatic molding line is high, but the unavoidable disadvantage of vertical pouring is that the rising height of molten iron is large, the temperature drop of molten iron is more during the rising process, and the gas concentration at the top is more, and the key to prevent the subcutaneous porosity is to extend the overflow time of the gas inside the molten iron. Although the conventional way of reducing gas output (such as avoiding excessive gas intake in smelting process, reducing moisture content in molding sand, reducing coal content, reducing gas output of bentonite) is effective, the fluctuation of each link in the production process is difficult to avoid, and the repeated occurrence of subcutaneous pores causes batch scrapping accidents often occur. Therefore, while strictly controlling the smelting process and molding sand parameters, according to the different structure of the casting, the key measures to prevent the subcutaneous porosity are to establish reducing atmosphere in the mold cavity, delay the formation of the oxide film on the molten iron surface, and improve the internal gas discharge conditions of the molten iron.