As we all know, the quality of nodular cast iron is related to the material and technology used. Besides, the design of riser and cold iron has a deep influence on its quality. Good design can not only reduce riser, improve the excellent rate ofcastings, but also directly reduce production costs and promote economic efficiency. Therefore, in order to achieve the above effects, we must understand the function of riser and chill and the problems to be noticed in the design process! The function of the riser is to provide the metal liquid that needs to be fed due to the volume change during the casting forming process, so as to prevent the defects of shrinkage type in the casting. The effect of cold iron 1) accelerate the cooling speed of the hot spot part of the casting, make the casting tend to solidify at the same time, which is beneficial to prevent the deformation or crack of the casting and may reduce the segregation. 2) When used together with riser, the cooling of local area of casting is accelerated, the directional (sequential) solidification condition of casting is strengthened, which is conducive to riser feeding and expansion of riser feeding range. In this way, it is not only beneficial to prevent casting from producing shrinkage porosity and block defects, but also possible to reduce the number or volume of riser and feeding slope, and improve the technological yield of nodular iron castings. 3) In order to improve the surface hardness and wear resistance of castings and refine the matrix structure, the cooling rate of some special parts of castings should be accelerated. 4) It is difficult to set the riser or the part where the riser is not easy to be fed shall be prevented from cold iron to reduce or prevent shrinkage porosity. 5) For nodular cast iron, chilling can increase the temperature gradient of the surface or center of the casting, which is beneficial to the utilization of graphitization expansion and the feeding effect of riser. The solidification mode of nodular cast iron is paste solidification. We should design according to this solidification mode in the design, and also need to do solidification simulation to verify whether our design process is correct. Of course, when considering the whole process, try to consider riser and chill, and combine them. Therefore, we must pay attention to the following problems in the design of cold iron and Riser:
The riser must be able to transport enough molten iron to compensate for shrinkage. Liquid shrinkage: above TL temperature: 1,5% / 100 ° C. Solid shrinkage: 0-2-5%, depending on the metallurgical quality of molten iron. The riser must be functional: the concealed riser must have Williams core and air hole, and the exposed riser must have heating sleeve. The riser has a certain feeding distance, which depends on the wall thickness and metallurgical quality of molten iron. It can be: horizontal feeding distance: 10 to 2 times of section thickness, vertical feeding distance: 15 to 4 times of section thickness. It is important to realize that a solid layer will be produced when the ductile iron contacts with the shape. This layer (2-3mm) is more important for thin-walled parts than thick walled ones. Cold iron can never make up for the shrinkage of molten iron, it will only transfer this shrinkage to other areas. The maximum effect of cold iron is half of its thickness or half of its wall thickness, and the minimum value is selected between the two. The quenched section will reduce the feeding distance of riser. The “surrounding” part of the riser shall not be cooled. The closer it is to the riser, the smaller the thickness of cold iron.
In the whole design, we should know clearly and get out of the misunderstanding: the cold iron only reduces the casting modulus, and can not play the purpose of feeding. Now many people mistily think that if cold iron can solve the shrinkage porosity, they think that cold iron can remove the shrinkage porosity. In fact, cold iron reduces the modulus of nodular iron castings, but shrinks to other parts.