Local pitting defects of nodular cast iron parts generally appear at the concave corners of the upper surface of the casting and consist of many pits. The shape of the pit is a circular or irregular polygon, and its diameter is mostly between 1 and 3 mm. Gray oxides remain in many places on the pit wall, and graphite is not in the form of spheres but in the form of fine flakes or dots near the pits.
(1) Molding sand
There are three types of molding sand used in the custom cast iron bench leg factory's production, namely wet molding sand for small molding machine molding, wet molding sand for semi-automatic molding line (FBM) and self-hardening resin sand for resin sand production line. After analysis, it is found that local pitting defects mostly occur on castings produced by FBM production lines, and castings produced by small machine modeling rarely show such defects, even if they are relatively light, and such defects are found on resin sand castings. Never appeared. Obviously, this phenomenon indicates that the local pitting defects are related to the molding sand used in the casting.
(2) Melting temperature
There are many types of nodular cast iron products produced by the company, the sizes vary widely, such as ductile iron bench leg, ductile iron lamp postetc., and the order quantity of each type of casting varies. Sometimes, a furnace of molten iron needs to cast several kinds of castings with different sizes and thicknesses at the same time, which makes it difficult to determine the melting temperature. In order to ensure the casting of thin parts, sometimes the smelting temperature has to be raised to 1550 ~ 1570 ℃. After the smelting temperature is increased, although the casting of the thin part is guaranteed, it also brings some disadvantages. One of them is that as the smelting temperature increases, the slag becomes thinner, the fluidity increases, and it is not easy to separate from the liquid metal, which causes a larger number of silicate melts to enter the cavity with the molten iron. In addition, as the smelting temperature increases, the molten iron becomes more oxidized during the melting, spheroidizing treatment and subsequent casting, which increases the amount of oxides in the molten iron.
According to the above analysis, it can be speculated that the formation process of local pitting defects in the casting is roughly as follows: the liquid slag remaining in the molten iron and various metal oxides continue to float after the molten iron enters the mold, and finally the concave angle on the upper surface Obstructed everywhere and stay here. Here, liquid slag solidifies to form slag holes. Various basic metal oxides react with acidic substances in the molding sand under the action of high temperature, and the reaction products form defects such as slag holes and pores. In addition, the concave corner of the foundry outdoor bench chair leg is the sharp corner of the sand mold, which is surrounded by the molten iron, and is most seriously heated. Under the action of high temperature, the low melting point material on the surface of the sand mold melts, which increases the distance between the sand particles, and the molten iron enters, thereby forming hot sticky sand defects. Together, these defects form a local tingling surface here.
According to the cause of local tingling surface, 2 preventive measures have been taken in production:
① Reinforce the molding sand to improve its fire resistance and make its pH less than 7;
② Reasonably arrange production and control the melting temperature of ductile iron used in FBM molding line below 1550 ℃.
After the above two measures were taken, local pitting defects in the castings were effectively controlled, indicating that our analysis of the causes of such defects was correct, and the preventive measures taken were also effective.