Application of Proportional Solidification Technology of nodular cast iron in prevention and control of shrinkage cavity and porosity

By analyzing and mastering the proportional solidification technology of nodular cast iron into the cylinder, the cooling rate is changed by cold iron quenching, the temperature difference of cross-section is balanced from the leading position by the ladder gating system, and the liquid shrinkage is supplemented by riser feeding. On the other hand, from the previous solidification volume diagram of nodular cast iron, it is also a very effective measure to reduce the liquid shrinkage in the solidification process from the research of chemical composition optimization design and metallurgical quality improvement. The main process measures to prevent and control shrinkage cavity and porosity are as follows:

(1) In order to reduce the tendency of liquid iron shrinkage, the melting composition and process scheme of castings should be designed reasonably.

The chemical composition has a great influence on the casting properties of ductile iron. The selection of chemical composition should not only meet the fluidity and filling ability of molten iron at the thin-wall position of the inlet cylinder, but also reduce the tendency of shrinkage cavity and porosity by selecting the appropriate composition ratio. According to the existing research conclusion of the effect of carbon content on shrinkage cavity and porosity in our company, when the content of C is lower than 3.5wt.%, serious shrinkage defects appear at the top of the casting. By controlling the interference elements such as Ti, Al and P in the alloy charge, the tendency of shrinkage cavity and porosity will also be reduced. Therefore, through reasonable smelting chemical composition and smelting process design, the mechanical properties of castings can meet the standard, and the addition amount of relevant elements can be controlled to reduce the shrinkage cavity and porosity tendency, which is conducive to the realization of balanced solidification.

(2) The cooling scheme of cold iron is optimized to balance the wall thickness difference.

Cold iron is a block made of metal or non-metallic materials with different heat storage coefficient from sand mold or sand core. Its main function is to change the local cooling rate of the casting according to the requirements of the pre design. According to the actual demand, the material with different heat storage coefficient can be selected according to the casting material and chilling effect.

When the mold is filled with molten iron, the whole casting begins to solidify. At the location where the chill is set, the surface of chill zone and its surroundings begin to solidify in a short time. Due to the role of cold iron, the heat dissipation in this area is accelerated. Through the calculation and design of chilling capacity of cold iron, the solidification completion time of other parts can be synchronized, and the casting defects at hot spot position can be eliminated. Reasonable setting of chill can accelerate the solidification speed of the defect location, reduce the wall thickness difference of the casting, and effectively eliminate the shrinkage cavity and porosity defect in the thick wall position.

(3) The combination of sectional self feeding and riser feeding is convenient for liquid feeding.

The riser can be placed close to the hot spot on the basis of not interfering with the physical hot spot of the casting itself, so as to facilitate effective and sufficient feeding. For the selection of riser size, it is not that the larger the riser is, the better feeding is. The practical experience case study shows that new contact hot spot is often formed at the root of the riser due to large riser and riser neck, resulting in serious shrinkage cavity, shrinkage porosity and shrinkage sink.

Different from other castings, feeding amount, feeding position and feeding time of riser are very limited. The theory of Proportional Solidification of nodular cast iron shows that the thinner the wall thickness is, the greater the liquid shrinkage is, and the more technological measures should be taken for feeding. For the side wall of the inlet cylinder with thin wall thickness, it is impossible to feed by setting the riser, and the upper flange is used for sectional self feeding. Therefore, a series of necking risers are selected for feeding risers with thick and large static blade flange. The riser is placed on the thick flange surface, which is convenient for feeding, slag collection, exhaust and removal.

The design and application of riser and chill is an important process measure to prevent shrinkage cavity, porosity and crack in the intake cylinder casting. It is not only based on the balanced solidification characteristics of nodular cast iron castings, but also to meet the requirements of sequential solidification at the cross position of wall thickness of complex hot spot structure of inlet cylinder. Reasonable riser feeding scheme can not only achieve the purpose of using riser for liquid shrinkage feeding, but also make full use of its own graphitization expansion for self feeding; the optimized chill quenching scheme can realize the sequential feeding of castings from top to bottom, and realize the balanced solidification of castings as a whole. By adopting the proportional solidification technology of intake cylinder casting, the reasonable scheme design and optimization can ensure the dense structure of the casting.