Causes of shrinkage casting defects of nodular cast iron piston rings and preventive measures

Although the upper and lower wall thickness of the cylinder is relatively uniform when casting the nodular iron ring, the central part of the cylinder is still easy to produce shrinkage porosity; although there is also shrinkage porosity in the double ring casting, it is slightly, basically in the central part of the double ring, and can be cut off when machining slices.

1. Causes of shrinkage

Nodular cast iron has a large tendency to form shrinkage porosity, which is determined by the solidification characteristics of nodular cast iron. The volume of molten iron changes from pouring temperature to the end of solidification as follows: the liquid shrinkage before eutectic solidification, the liquid expansion during eutectic solidification, and the secondary shrinkage of residual liquid at the end of eutectic solidification. If liquid shrinkage and secondary shrinkage are not enough, shrinkage cavity and shrinkage porosity will be produced. Liquid shrinkage can be fed by pouring riser, and secondary shrinkage can only be fed by liquid graphitization expansion.

It can be seen from the solidification characteristics and cooling curve of nodular iron that the temperature range from the beginning of Eutectic Transformation to the end point of eutectic transformation is wide during the solidification process of nodular iron. The surface layer of the casting maintains the coexistence of solid and liquid for a long time, and the solidification proceeds simultaneously on the whole casting section, which is called “paste solidification”. However, the solidification layer on the surface of the casting is thin and easy to deform under pressure. When the graphite ball austenite eutectic group grows up to conflict with each other, the graphitization expansion will produce expansion force. Only a small part of the expansion force acts on the liquid between the eutectic groups, and most of the expansion force acts on the adjacent eutectic groups, which tends to squeeze the eutectic groups apart, and in the macroscopic, it will produce expansion pressure on the mold. If the mold rigidity is low, it will cause the casting to expand outwards, so as to enlarge the gap between adjacent eutectic spheroids. At this time, the channel between the adjacent eutectic group gap has been blocked. At this time, the graphitization expansion can not compensate the secondary shrinkage, so the casting will form shrinkage porosity after solidification.

2. Technological measures to prevent shrinkage

a. It can improve the compactness of the mold, prevent the mold from expanding, utilize the graphitization expansion of molten iron in the solidification process, and obtain the self feeding.

The double ring casting is a machine mold with box, and the hardness of sand mold is required to be 80-90; the cylinder casting is a manual de box mold, and the hardness of sand mold is required to be no less than 80. At the same time, the sand used for enclosure mold after de box must be compacted with special tools to make it close to the hardness of sand mold, so as to improve the resistance of sand mold to graphitization expansion.

b. After spheroidizing, the molten iron should be fully inoculated to ensure that the graphite can be fully analyzed, and the self feeding of the secondary shrinkage can be realized by graphitization expansion.

c. On the premise of hot metal spheroidization, reducing the mass fraction of residual magnesium and rare earth is conducive to reducing shrinkage porosity.

d. Proper increase of pouring temperature can bring in more heat, make graphite fully precipitated, which is beneficial to reduce shrinkage porosity.