Analysis of the cracking failure of the casting support plate

According to the test results of the support plate, the microstructure and mechanical properties of the normal position are qualified, but there are abnormal structures in the crack area. Combined with energy spectrum analysis and EPC process of support plate, these abnormal structures may be the surface wrinkle formed in the pouring process. The main reasons are as follows:

① The EPC material used for the support plate is EPS (expandable polystyrene resin) with high carbon content up to 92%. When molten iron with temperature up to 1400 ℃ flows into the mold, it will have complex chemical reaction with EPS, resulting in sharp decomposition of EPS. The main decomposition products are CH4 CH4, 4, Co, H2, Co, H2, 2, CO2, 2, toluene, ethylene, polystyrene, etc. due to the fast decomposition speed of EPS, coupled with the existence of dimer, trimer and polymer generated by multiple decomposition, a liquid similar to asphalt is formed. The residue of this liquid decomposition will be distributed on the inner wall of the mold cavity under the promotion of molten iron, One part is absorbed by the coating on the inner wall of the mold cavity, and the other part is formed by the reduction of CO into the scurf like and inclusion like bright crystalline carbon. The contact angle between the free carbon and molten iron is greater than 90 °, the wettability is weak, and the density is small, it will float above the molten iron, and form carbon deposition at the corner of the casting, near the surface layer of the last flowing area of molten iron, that is, wrinkled skin, commonly known as carbon deposition. There are more carbon and oxygen corresponding to the cracks analyzed by SEM. Because the density of bright carbon is small, it is similar to the porosity in the casting, it is easy to destroy the continuity of the workpiece surface, and it is easy to produce cracks and expand along the position of carbon deposition when subjected to external force, which is the main reason for the cracks.

② When the model material EPS decomposes at high temperature, it will produce free carbon. For materials with relatively low carbon content (such as alloy cast iron), the carbon saturation is small, and the free carbon can still be partially dissolved in molten iron, so as to reduce carbon deposition; however, the nodular cast iron has a high carbon content, The high concentration of carbon in high temperature molten iron makes the carbon decomposed by the die hard to be dissolved, and it is easy to form carbon deposition at the end or corner of the molten iron flow. The increase of local carbon concentration will lead to the decrease of the proportion of spheroidizing agent, which will lead to the lack of local spheroidization and the precipitation of A-type graphite.

③ From another point of view, due to the decomposition of the mold material and the reduction reaction produced by CO, the energy provided by the molten iron is needed to complete, which will lead to the decrease of the temperature at the front end of the molten iron, which is equivalent to the increase of the cooling speed in the area where the carbon deposition is located. The faster cooling speed may also lead to the carbon not being spheroidized before a or C There are more A-type graphite and A-type graphite around the carbon deposit. Because the crack propagates along the carbon deposit, there are more A-type graphite around the crack, and it is far away from the crack area, so the structure is normal and the spheroidization rate is good.

Based on the above analysis, it can be seen that the main cause of bearing plate failure is wrinkle (carbon deposition) on the surface, and the process improvement mainly starts from how to prevent carbon deposition. First of all, the carbon content of EPMMA (polymethacrylate) is only 60%, which can replace EPS to make pattern and reduce gas emission; second, raise pouring temperature and carry out top pouring, so that it can fully meet the heat loss in the process of mold material volatilization, so that the wrinkled skin can be distributed in the sprue and riser as far as possible under the buoyancy of molten iron; finally, the coating with better air permeability is used, Ensure the smooth passage of escape gas and smooth exhaust. Through the improvement of the above casting process, the surface of the new bearing plate has no wrinkle, and after 18 months of service, it is still in good condition.

(1) The microstructure and mechanical properties of the bearing plate with cracks were observed, and the spheroidization rate was grade 3; the metallographic structure was ferrite + pearlite, pearlite accounted for 25%; the hardness was 204hb, the tensile strength and yield strength were 559 MPa and 343 MPa, respectively, and the elongation was 8.0%, which met the technical requirements.

(2) The crack extends from the surface to the center with a depth of about 2.8 mm. There are more A-type graphite on both sides of the crack and more carbon and oxygen elements in the crack. During the pouring process, the ESP die material used for the support plate produces carbon on the surface, which is insoluble in molten iron, thus cutting the matrix structure of the metal, generating stress concentration and forming cracks when subjected to external forces, which is the main reason for the cracks.

(3) In view of the failure reasons, the process improvement was carried out by using EPMMA instead of EPS, increasing the pouring temperature, adopting top pouring and adopting the coating with better permeability, etc. measures were taken to reduce the gas generation. The results show that there is no wrinkle defect in the improved casting and the problem is solved thoroughly.