With the establishment of the company’s multi-ethnic market, the successful development of the multi-ethnic products, and the continuous expansion of the company’s business with the recovery of the world economy, not only the customers have higher requirements for product quality, but also the prices of raw materials have increased significantly with the recovery of the economy, leading to the difficulty of casting, an industry with small profits. If you want to make profits in today’s society, you must use scientific methods to reduce costs, To improve the yield and yield of castings, the important measures are to design reasonable risers and use cold iron. Our current casting yield is 60-70%, while that of the successful process (or more stable process) is more than 80%. The use of cold iron is much smaller than that of our current castings. Now the company’s ductile iron castings are more and more ordered with the development, and the riser and chill must be reduced to obtain greater economic benefits.
The design of riser and chill is the key to obtain high-quality ductile iron castings, and is also a major aspect to reduce the production cost of castings. A well designed riser and chill can not only reduce the riser and improve the yield of castings, but also ensure the quality of castings and meet customer requirements. We must know clearly the function of riser and cold iron. The function of riser: to provide metal liquid that needs to be fed due to volume change during casting forming, so as to prevent shrinkage type defects in the casting.
Role of cold iron
1) Accelerate the cooling rate of the hot spot part of the casting, so that the casting tends to solidify at the same time, which is conducive to preventing the deformation or cracks of the casting, and may reduce the segregation.
2) It is used together with the riser to accelerate the cooling of the local area of the casting, strengthen the directional (sequential) solidification conditions of the casting, and is conducive to the feeding of the riser and expanding the feeding range of the riser. In this way, it is not only conducive to preventing the casting from shrinkage porosity and shrinkage block defects, but also may reduce the number or volume of the riser and the feeding slope, and improve the process yield.
3) The cooling rate of some special parts of the casting shall be accelerated in order to improve the surface hardness and wear resistance of the casting and refine the matrix structure.
4) The cold iron shall be prevented at the position where it is difficult to set the riser or the riser is not easy to be fed to reduce or prevent shrinkage porosity and shrinkage cavity.
5) For nodular cast iron, chilling with cold iron can increase the temperature gradient on the surface or center of the casting, which is conducive to improving the utilization of graphitization expansion and improving the feeding effect of the riser.
Solidification mode of nodular cast iron
The solidification mode of nodular cast iron is paste solidification. When designing, we consider to design according to this solidification mode, and also need to do solidification simulation to verify whether the process we designed is correct. Of course, when considering the whole process, try to consider riser and chill, and combine them. According to STAF’s theory, we have the following consensus:
The riser must be able to transport enough molten iron to compensate for shrinkage.
1) Liquid shrinkage: above TL temperature: 1,5%/100 ° C.
2) Solid shrinkage: 0 – 2 – 5%, depending on the metallurgical quality of molten iron.
The riser must be functional: the dark riser must have Williams core and air hole, and the exposed riser must have a heating jacket.
The feeder has a certain feeding distance, which depends on the wall thickness and the metallurgical quality of molten iron. It can be:
3) Horizontal feeding distance: 10 to 2 times of section thickness
4) Vertical feeding distance: 15 to 4 times of section thickness.
It is important to realize that the ductile iron will produce a solid layer when it contacts the molding. This layer (2-3mm) is more important for thin wall parts than thick wall parts.
Cold iron can never make up for the shrinkage porosity of molten iron, it will only transfer this shrinkage porosity to other areas.
The maximum utility of cold iron is half of its thickness or half of its wall thickness, and the minimum value is selected between the two.
The chilled section will reduce the feeding distance of the riser. The “surrounding” part of the riser shall not be chilled. The closer to the riser, the smaller the thickness of the chill.
In the whole design, it should be soberly recognized and go out of the misunderstanding: cold iron can only reduce the casting modulus, and can not play the role of feeding. Now many people think that the cold iron can solve the shrinkage porosity vaguely, which means that the cold iron can remove the shrinkage porosity. In fact, the cold iron reduces the modulus at this place of the casting and shrinks the porosity to other parts. This is also an important aspect of STAF theory.