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2021
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Features of ductile iron solidification, features of riser design
Generally speaking, the tendency of shrinkage cavities and porosity in ductile iron castings is much greater than that in gray iron castings, and preventing shrinkage defects is usually a very difficult problem in process design. In this regard, the experience summarized from actual production is very inconsistent, and each has its own views: some people believe that ductile iron castings can only be produced with small risers to produce qualified castings, sometimes without risers; some people believe that the principle of sequential solidification should be followed, and large risers should be placed at the solidification site to supplement the volume shrinkage of the casting during solidification.
Generally speaking, ductile iron castings have a much greater tendency to shrink than gray iron castings, and preventing shrinkage defects is usually a very difficult problem in process design. In this regard, the experience summarized from actual production is very inconsistent, and each has its own views: some people believe that ductile iron castings can only be produced with qualified castings with small risers, sometimes without risers; some people believe that the principle of sequential solidification should be followed, and large risers should be placed at the solidification site to supplement the volume shrinkage of the casting during solidification.
In order to improve the process yield as much as possible while ensuring the quality of the castings, it is not enough to rely solely on controlling the chemical composition of the cast iron. On the basis of understanding the solidification characteristics of ductile iron, control the entire process of cast iron smelting, spheroidizing treatment, inoculation treatment, and pouring operation, and control the rigidity of the mold.
Most of the ductile iron used in practice is close to the eutectic composition. Hypoeutectic composition is used for thick-walled castings, and hypereutectic composition is used for thin-walled castings, but it is not far from the eutectic composition. In eutectic and hypereutectic ductile iron, small graphite spheres first precipitate from the liquid phase during eutectic solidification. Even in hypoeutectic ductile iron, due to the increase in the degree of supercooling of the molten iron after spheroidization treatment and inoculation treatment, small graphite spheres will precipitate first at a temperature much higher than the equilibrium eutectic transformation temperature. The initial small graphite spheres are formed at 1300° or higher. In the subsequent solidification process, as the temperature decreases, some of the initial small graphite spheres grow, some redissolve into the molten iron, and new graphite spheres precipitate. The precipitation and growth of graphite takes place over a wide temperature range.
When the graphite spheres grow, the carbon content in the molten iron around them decreases, and an austenite shell surrounding the graphite spheres will form around the graphite spheres. The formation time of the austenite shell is related to the cooling rate of the casting in the mold: when the cooling rate is fast, carbon cannot diffuse evenly in the molten iron, and the austenite shell forms earlier; a low cooling rate is conducive to the uniform diffusion of carbon in the molten iron, and the austenite shell forms later.
Before the austenite shell is formed, the graphite spheres are in direct contact with the molten iron with high carbon content, and the carbon in the molten iron easily diffuses into the graphite spheres, causing the graphite spheres to grow. After the austenite shell is formed, it prevents the diffusion of carbon from the molten iron to the graphite spheres, and the growth rate of the graphite spheres drops sharply. Since the latent heat of crystallization released when graphite precipitates from molten iron is large, about 3600 J/g, and the latent heat of crystallization released when austenite precipitates from molten iron is small, about 200 J/g, the formation of the austenite shell around the graphite spheres and the growth of the graphite spheres are hindered, and the release of the latent heat of crystallization will be significantly slowed down. In this case, eutectic solidification relies on further lowering the temperature to generate new crystal nuclei. Therefore, the eutectic transformation of ductile iron should be completed in a considerable temperature range, and its solidification temperature range is twice or more that of gray iron, with typical pasty solidification characteristics.
Ductile iron
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