In order to verify the correctness of defect cause analysis, the project team carried out process verification test. The test was divided into group A and group B. the two groups had the same pattern and technology, and both adopted the pattern scheme shown in Fig. 1. The difference is that group A uses the cast iron coating produced by another company, while group B still uses the original coating.
The process parameters are the same, the same hot metal is used, the initial pouring temperature is 1490 ℃, the pouring time of each group is 20 ~ 23S, the negative pressure is – 0.05Mpa, and the holding time is 10min.
The test results show that there are no cold shut, wrinkle and other defects in the two groups of 30 blanks. After processing, it was found that 3 of 18 castings (group A) with new coating had slag inclusion defects, and 6 of 12 castings (group B) with original coating had slag inclusion defects. Among them, one defect in group A has regular shape, which is due to the defect that the coating of the gate part is not cleaned up during the molding process (as shown in Fig. 2), and the other two pieces belong to slight slag inclusion. Based on the above test results, it can be concluded that the main cause of slag inclusion is that the aggregate composition in the coating is mixed with the molten iron, and the slag inclusion defect is formed after the casting solidifies. However, there are still two problems about what makes the coating enter the molten iron. First, the fire resistance of the original coating is not enough to withstand the high temperature erosion of 1490-1510 ℃; Second, the pouring temperature in the original process is higher than that of the conventional cast iron coating.
Therefore, the project team redesigned the test scheme and optimized the original process, that is, changing the pouring temperature on the basis of the original process to control the pouring temperature at 1420 ℃ ± At 10 ℃, they were also divided into group A and group B. the new coating was used in group A, and the original coating was still used in group B. Other process parameters remain unchanged.
A total of 36 billets were cast, 18 in each group. After processing, no defects were found in group A, and the qualified rate was 100%; One slag inclusion defect occurred in group B. Therefore, it can be determined that the defect of the brake shell which has been bothering the foundry company for a long time is caused by the low fire resistance of the EPC coating, which can not withstand the high temperature scouring of 1490 ~ 1510 ℃.