The causes of cracks and sand inclusion defects in motor shell steel castings are studied from the perspective of process optimization. The research on process optimization mainly includes four aspects.
① Optimization of molding sand process in sand casting. The addition amount of alkaline phenolic resin is 1.5% ~ 1.8% of the original sand, the addition amount of organic resin curing agent is 20% ~ 25% of the resin, the demoulding strength is 0.25 MPa ~ 0.30 MPa, and the strength of 24h molding sand is 0.3 MPa ~ 0.7 MPa. The experiments are carried out by using new sand molding scheme and recycled sand + 25% new sand molding scheme respectively. The test results show that the application of alkaline phenolic resin sand process in motor shell steel castings is feasible and can greatly reduce the number of crack defects of motor shell. However, considering the cost, the scheme of recycled sand + 25% new sand should be adopted, which is more economical. The process of alkaline phenolic resin sand is the same as that of furan resin sand. There are many sand inclusion defects in sand castings, which can only be improved from the process design.
② Structure and process optimization of sand castings. The poor structural manufacturability of sand castings is also one of the reasons for the crack defects of castings. The connecting fillet at the stiffener on the inner surface of sand castings is too small. After analyzing the product assembly and product performance, and communicating with the design engineer, the casting fillet at this position is increased. After the fillet is increased, the casting process is good, which can reduce the probability of crack defects.
③ Optimization of sand casting process. 1) The original top pouring process scheme is changed to the bottom pouring process scheme. The pouring system is made of ceramic pipe, ceramic gate cup, ceramic sheet and molding sand. 2) The riser is designed from ordinary heating riser to high heating insulation riser, the diameter is changed from ￠ 100mm to ￠ 110mm, and the riser height remains unchanged. 3) Optimize the sand mold design of No. 1 core, No. 2 core, No. 7 core and lower box. 4) The pouring process is optimized from the aspects of pouring temperature, pouring time, pouring height, pouring speed, point pouring after pouring and so on. The optimization of sand mold casting process is conducive to reduce the occurrence of cracks and sand inclusion defects.
④ Optimization and control of chemical composition of molten steel. By studying the effects of C, Si, Mn, P, s and Cu on the cracking strength of the alloy, it is determined that the chemical composition of liquid steel is 0.20 ≤ carbon content ≤ 0.25%, 0.25 ≤ silicon content ≤ 0.45%, 0.60 ≤ silicon content ≤ 0.68%, phosphorus content ≤ 0.020%, sulfur content ≤ 0.015%, and copper content ≤ 0.30%. The smelting process is controlled to ensure that the chemical composition of molten steel meets the technical requirements. The optimized chemical composition plays a positive role in reducing cracks and sand inclusion defects.