SAND CASTING EQUIPMENT.

Shrinkage and porosity are more common in ductile iron than in ordinary gray iron. To prevent them, more attention and control must be given. The large and concentrated holes that can be clearly seen are called shrinkage holes, and the small and scattered holes that are not suitable to be seen are called shrinkage porosity. Most of the shrinkage holes are generated in the upper part of the hot joint of the casting. There are often scattered shrinkage porosities at the hot joints of the castings and below the shrinkage cavity. However, for some centers with uniform wall thickness, or at the center of thick walls, shrinkage may also occur.

Some shrinkage porosities are so small that they can only be found under a microscope. This shrinkage porosity is polygonal, sometimes continuous, sometimes discontinuous, and distributed at the boundary of the eutectic cluster. This kind of shrinkage is called microscopic shrinkage. After the austenite dendrites are solidified, the remaining molten iron finally solidifies between the dendrites, forming microscopic shrinkage due to the lack of feeding.

The shrinkage cavity and shrinkage volume of ductile iron are larger than ordinary gray cast iron, white cast iron and carbon steel. (There are tables and data descriptions in the article on the composition of cast iron), however, in production, the no-riser process can also be used to obtain sound ductile iron castings.

Xinyuanzhu Group combined with more than 10 years of casting production to summarize and summarize the reasons for the shrinkage and shrinkage of ductile iron. The main five points are as follows:

1. Spherical graphite precipitates in molten iron After spheroidizing treatment, spheroidal graphite will precipitate in molten iron immediately, and as the temperature gradually decreases, the graphite balls in molten iron gradually grow up. The process of graphite precipitation and growth is accompanied by the expansion of liquid metal.

2. Divorced eutectic transformation Nodular cast iron undergoes eutectic transformation in a divorced eutectic way. The coagulation method is porridge-like coagulation that is almost simultaneous inside and outside, so it is easy to form microscopic shrinkage.

3. The eutectic expansion is large. Due to porridge solidification, the casting will last a long time during the eutectic transformation. The eutectic time of ductile cast iron can be more than doubled than that of ordinary gray cast iron, which leads to eutectic transformation. The graphitization expansion is large.

4. During the eutectic solidification, the mold wall moves. Due to the solidification of the eutectic, the solidification layer on the surface of the casting is determined to be very thin, so that a solidified shell of sufficient strength cannot be established to inhibit the graphitization expansion during the eutectic solidification. The inner wall of the model moves outwards. In the case of insufficient mold rigidity, the size of the cavity is increased, resulting in a further increase in shrinkage and porosity.

5. The spheroidizing treatment increases the subcooling of the molten iron. After the spheroidizing treatment, the original hydrogen, oxygen, nitrogen and CO gas content of the molten iron is reduced, and the molten iron is purified, resulting in the reduction of external cores. Moreover, the higher the overheating temperature of the molten iron, the higher the degree of purification, and the resulting tendency of overcooling becomes more severe. In addition, both the spheroidizing element magnesium and rare earth can form carbides with carbon, thereby reducing the degree of graphitization and increasing the shrinkage tendency.

There are also 5 measures to prevent shrinkage and shrinkage. I hope it will be helpful to friends in the same industry.

1. The composition of molten iron The content of carbon, silicon, manganese, rare earth, and magnesium must be appropriate. High carbon content can reduce the tendency of shrinkage cavities and porosity, but too high carbon content can cause graphite to float. For thin-walled castings, when the carbon and silicon content is low, free carbides are easily produced. For thick-walled castings, lower carbon content can be used, and silicon content should be appropriately increased. Manganese is easy to form carbides and easily promotes the formation of shrinkage cavities and porosity. For this reason, efforts should be made to reduce the manganese content, especially for as-cast ferritic ductile iron. Under the premise of ensuring spheroidization, the residual amount of magnesium and rare earths should not be too high.

2. The state of the molten iron. The tendency of shrinkage cavities and shrinkage is small. The slope of the cooling curve of the molten iron should be small, and the degree of subcooling should be small. The expansion of the eutectic during solidification is small, and the secondary shrinkage is also small. The conditions to be met are:

(1) slow cooling rate 

(2) high carbon equivalent and high tendency to precipitate graphite

(3) There are many effective graphite cores in molten iron

(4) Good gestation effect.

3. Mold rigidity For green sand molds, the mold hardness must be above 90 (type hardness tester), and it is hoped that the tensile strength can reach 40MPa. In addition, the fastening of the sand box is also very important. For metal sand-coated (sand-coated thickness of about 8mm) and large-scale castings made of self-hardening sand, riser-free casting can be realized. The conditions to be met at this time are: G≥α1+α2+β where G-the expansion of molten iron caused by the graphitization of carbon; α1-the liquid contraction of molten iron; α2-solidification contraction β-mold expansion.

4. Casting temperature In order to prevent shrinkage cavities and shrinkage porosity, it is necessary to reduce the amount of liquid shrinkage, and low casting temperature is advantageous. However, for thin-walled (10mm) castings, carbides are prone to appear. At this time, it is difficult to use the feeder to feed. Therefore, the appropriate pouring temperature also depends on the structure and wall thickness of the casting.

5. The pouring system adopts a sequential solidification method, which is effective for the design and placement of castings, risers, riser necks, internal runners and runners, as well as external cooling iron settings and the use of metal molds when necessary. Effective measures to prevent shrinkage and shrinkage.

The ductile iron castings produced by xinyuanzhu group can be customized with drawings or samples according to the needs of customers, and can undertake mass casting production. Welcome to visit our foundry workshop.