Causes of Shrinkage Cavity and Shrinkage Porosity of Ductile Iron and Its Solutions
Sep. 02, 2020
Features: 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 easy to see 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 in the center of thick walls, shrinkage may also appear. Some shrinkage porosities are so small that they can only be found under a microscope. This shrinkage porosity is polygonal, sometimes continuous, sometimes intermittent, and distributed on the boundary of the eutectic cluster. This kind of shrinkage is called microscopic shrinkage. After the austenite dendrites are solidified, the remaining molten iron will finally solidify 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. (From the article on the composition of cast iron, there are tables and data descriptions), but in production, the no-riser process can also be used to obtain sound ductile iron castings.
Reasons for the increase in shrinkage and porosity of ductile iron: 1. Nodular graphite precipitates in molten iron. After spheroidizing treatment, nodular graphite will precipitate in molten iron immediately, and as the temperature gradually decreases, the The graphite balls grow up gradually. The process of graphite precipitation and growth is accompanied by the expansion of liquid metal. 2. Divorced eutectic transformation Ductile cast iron undergoes eutectic transformation in the form of divorced eutectic. 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 atheroma solidification, which determines that the solidified layer on the surface of the casting is so thin that a solidified shell of sufficient strength cannot be established to inhibit the graphitization expansion during the eutectic solidification, resulting in casting 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.
Measures to prevent shrinkage porosity and shrinkage cavities: 1. Liquid iron composition 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 stiffness For green sand molds, the mold hardness must be above 90 (B type hardness tester), and it is hoped that it can reach a tensile strength of 40MPa. In addition, the fastening of the sand box is also very important. For metal type sand-coated (sand-coated thickness of about 8mm) and self-hardening sand to make large castings, riser-free casting can be achieved. 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 shrinkage of the molten iron α2-solidification contraction β-the 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. We are cast iron weights manufacturers. If you are interested in our products, please feel free to contact us.