yvonne@nydcasting.com 
1 Morphological characteristics of defects
Local pitting defects in ductile iron castings generally appear at the concave corners of the upper surface of the casting , and are composed of many pits. The shape of the pits is circular or irregularly polygonal, and its diameter is mostly between 1 and 3 mm. As can be seen from the figure, gray oxides are retained in many places on the pit wall, and near the pits, the graphite is not spherical but exists in the form of fine flakes or dots.
2 Causes and mechanisms of defect formation
2.1 Analysis of causes
Molding sand
There are three types of molding sand used in the company’s production, namely, wet molding sand for small molding machines, wet molding sand for semi-automatic molding lines (FBM) and self-hardening resin sand for resin sand production lines. After analysis, it was found that local pitting defects mostly appeared on castings produced by FBM production lines, while castings produced by small machine molding rarely had such defects, and even if they did, they were relatively light, and such defects never appeared on castings produced by resin sand. Obviously, this phenomenon shows that local pitting defects are related to the molding sand used in castings. Under the production conditions of our company, it only appears in castings molded by wet sand, and the most serious one is the FBM molding line.
The molding sand used by the FBM molding line and the molding sand used by the small molding machine are both made of quartz sand, bentonite and coal powder mixed with water, but the conditions of use of the two are very different. First, the molding sand of the FBM molding line is a single sand, that is, there is no distinction between face sand and back sand, and they are regenerated together after use. The molding sand used by the small molding machine is divided into face sand and back sand, and only a small part of the used molding sand is regenerated to make it face sand. The FBM molding line has a large amount of regeneration processing, and the amount of new sand added is relatively small. A large amount of old sand is repeatedly used and regenerated, and is constantly subjected to the burning and mixing of high-temperature molten iron. The specific gravity of dead ash continues to increase, thereby reducing the refractory of the molding sand. In actual testing, its sintering degree is about 1050℃; second, in the castings produced by the FBM molding line, the amount of core used is relatively large. The core is made of mold sand, and the proportion of mold sand with residual resin and curing agent in the molding sand continues to increase, causing the molding sand to be acidic. After testing, the pH value of the molding sand is between 7 and 8, which is slightly acidic.
The low refractory property of the FBM molding line molding sand and the large amount of acid contained in it are probably the important reasons for the local pockmarking of the casting.
Melting temperature
The company produces many varieties of ductile iron castings, with great differences in size, and the order quantity of each casting varies each time. Sometimes, a furnace of molten iron needs to be used to cast several castings of different sizes and thicknesses at the same time, which makes it difficult to determine the smelting temperature. In order to ensure the casting of thin parts, the smelting temperature must be raised to 1550-1570℃. After the smelting temperature is raised, although the casting and forming of thin parts is guaranteed, it also brings some disadvantages. One of them is that with the increase of smelting temperature, the slag becomes thinner, the fluidity increases, and it is not easy to separate from the liquid metal, causing a large amount of silicate melt to enter the mold cavity with the molten iron. In addition, with the increase of smelting temperature, the oxidation of molten iron intensifies during melting, spheroidization treatment and subsequent pouring, thereby increasing the amount of oxides in the molten iron.
Analysis shows that the high smelting temperature and the increase in the amount of slag residue and metal oxides in the molten iron are also an important condition for the local pitting in castings.
2.2 Formation Mechanism
Based on the above analysis, it can be inferred that the formation process of local pitting defects in castings is roughly as follows: the liquid slag and various metal oxides remaining in the molten iron enter the casting mold with the molten iron and continue to float up, and finally are blocked at the concave corners of the upper surface and stay there. Here, the liquid slag solidifies to form slag holes. Various alkaline metal oxides react with acidic substances in the molding sand under the action of high temperature, and their reaction products form defects such as slag holes and pores. In addition, the concave corners of the casting are the sharp corners of the sand mold, which are surrounded by molten iron and are most severely heated. Under the action of high temperature, the low-melting-point substances on the surface of the sand mold melt, increasing the distance between the sand particles, and the molten iron enters, thereby forming hot-bonded sand defects. These defects combined together form local pitting here.
3 Preventive measures
In view of the causes of local pitting, two preventive measures were taken in production:
Strengthen the renewal of molding sand to improve its refractoriness and make its pH value less than 7;
Rationally arrange production and control the melting temperature of ductile iron used in FBM molding line below 1550℃.
After taking the above two measures, the local pitting defects in castings were effectively controlled, indicating that our analysis of the causes of this defect was correct and the preventive measures taken were also effective.