Solution to the Scab Defect of grey cast iron engine block by green sand casting

Through the understanding of this defect, we start with the properties of molding sand and focus on the properties of molding sand [2]. According to the analysis of molding sand data in recent period, it is found that the moisture content is 3.7%, the mud content is over 11.5%, while the CB value (compaction ratio) is only 31% ~ 32%, and the moisture ratio is only 8.5 ~ 9.0, which is in a very low range. Through the analysis, it can be found that the main reason for the data change is the increase of mud content, which leads to the increase of molding sand moisture, while the CB value is in a relatively low range, which leads to the poor toughness of molding sand, resulting in the formation of sand inclusion and scab defects during pouring.

1. Adjustment of mud content

Through field investigation, it is found that in recent two months, the dust removal amount of molding sand is only 300-500kg per day. Under normal circumstances, the dust removal amount should be about 2T, and the dust removal amount is reduced, resulting in the continuous high mud content of molding sand, thus deteriorating the quality of molding sand. By changing the dust removal cloth bag and adding new sand continuously, the mud content of molding sand can be reduced rapidly. Through the circulation and adjustment of two shifts, the data of molding sand can be basically restored to normal, the mud content can be reduced to about 10.5%, the water content can be reduced to 3.2% – 3.5%, and the water ratio can be guaranteed to be 10-11.

After adjustment, 224 products were tested and poured, 32 of them had scab defects, and the area of scab defects became smaller. After processing and verification, 5 products were leaked, and the leakage rate was 2.23%.

The test results show that by adjusting the molding sand to ensure that the molding sand data is in a reasonable state, it can significantly improve the generation of casting scab defects, and greatly reduce the product leakage rate.

2. Adjustment of pouring temperature

After adjusting the properties of molding sand, although the processing leakage is basically contained, a large number of workpieces still have scab defects after pouring, and when the molding sand fluctuates, there will still be sand inclusion and scab defects, resulting in the risk of processing waste products.

According to the mechanism of scabbing, the temperature of molten iron is the decisive factor of water transfer rate. By reducing the pouring temperature and the strength change of the surface sand and the inner sand caused by the excessive migration of the sand moisture, the defects of sand inclusion and scab can be reduced or eliminated.

By adjusting the pouring temperature and carrying out small batch pouring test, the pouring test was carried out according to the decreasing pouring temperature of 1430 ℃, 1425 ℃, 1420 ℃ and 1415 ℃, each group of 36 pouring products.

Through the comparison, it can be found that, when the pouring temperature is reduced, the area of scabbed defect becomes smaller until there is no defect. Through the trial cutting, it is found that with the pouring temperature reduced to a certain range, the porosity defects have a sharp rise trend. Although reducing the pouring temperature can improve or even eliminate the scab defects, the porosity and sand hole defects are obviously increased, which can not improve the yield of processed products.

3. Coating

A layer of fire-resistant coating is evenly sprayed on the surface of molding sand. After being sintered by molten iron, the fire-resistant coating forms a solid film similar to ceramic shape on the surface of molding sand and firmly adheres to the surface of molding sand. The ceramic film can play the role of heat insulation and prevent the scar defects caused by the rapid migration of water.

By referring to the relevant data, we selected the aluminosilicate coating with high fire resistance and relatively low price, and the Baume ratio was 25-35 ° B é. In order to prevent the adverse effect of solvent volatilization on the properties of molding sand, we selected the aluminosilicate alcohol based coating, and used more than 95% industrial alcohol for the ratio to evenly spray on the surface of molding sand with defects 。

It is found through the test pouring that, after spraying the coating, even if the molding sand performance deteriorates, there will be no sand inclusion and scab defects in this part. Under the condition of the fluctuation of molding sand performance, the product quality can be guaranteed and the applicable process range of the product can be improved.

According to the analysis of the above data, it can be found that when the properties of molding sand return to the normal state, the defective rate of scabbing can be significantly reduced in the comparison test group 1 and test group 2.

Through test group 3, test group 4 and test group 5, it can be found that no matter whether the molding sand performance is in normal state or not, spraying fire-resistant coating at the place where the Scab Defect occurs can completely prevent the generation of the scab defect, so as to improve the product quality.

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