The sand inclusion casting defects of steel castings are relatively serious, because the proportion of product scrapping caused by sand inclusion casting defects has reached 4%, and the welding repair rate of sand inclusion casting defects is basically 100%. What is the reason why the sand inclusion casting defects of steel castings are so serious? Combined with the causes of sand inclusion casting defects and the actual production of ZHY Casting, the main causes of sand inclusion casting defects of steel castings are analyzed as follows:
① Unreasonable design of gating system
The gating system of steel castings is designed as top injection type. The height from the inner sprue to the bottom of the sand mold is more than 600mm. When the high-temperature molten steel begins to enter the mold cavity, it will wash directly at the bottom of the sand mold. Because the scouring force of molten steel on the bottom of the sand mold is greater than the strength of the sand mold, sand washing occurs, and the washed sand blocks or sand particles cannot float up into the riser, resulting in sand inclusion casting defects. All sprues are made of furan resin sand, and then the surface of molding sand is painted and dried. During pouring, the high-temperature liquid steel has been scouring and baking the sprue, and the molding sand cracked by the high-temperature liquid steel is directly washed into the mold cavity to form sand inclusion casting defects. After observing the sprue after sand falling, it is found that there is an obvious “bulge” on the sprue, which is the result of the falling off of molding sand at this place.
② Sand mold damage and local molding sand strength
In the process of molding and core making, the appearance and sand core are partially damaged during demoulding. At this time, the measure of manually repairing the sand mold is adopted. Because the mold and core made in the early stage have been hardened, although the sand repaired later can be bonded with the hardened core, the strength of the joint position is low, less than 80% of the strength of other parts of the sand mold, which is very easy to cause sand flushing. Round steel cold iron is used locally in the mold and core. During molding, the molding sand around the round steel cold iron is difficult to compact, which is easy to cause sand falling and sand washing. This is also one of the main reasons for sand inclusion casting defects of steel castings.