Quality Control Technology for Thick and Large Section As-cast Ferritic Ductile Iron Parts

- Nov 18, 2019-

1. Chemical composition control

1) Choice of C, Si, CE

Since the spheroidal graphite has little effect on the weakening of the matrix, the amount of graphite in the ductile iron has no significant effect on the mechanical properties. When the carbon content varies from 3.2% to 3.8%, there is no significant effect on the mechanical properties. . Therefore, when determining the carbon silicon content in the process, the main consideration is to ensure the casting performance, and the carbon equivalent is selected around the eutectic composition. The molten iron having the eutectic composition has the best flow performance, tends to form a concentrated shrinkage cavity, and has a high density of the microstructure of the casting. However, when the carbon equivalent is too high, it is easy to produce graphite floating, and at the same time, it has an effect on spheroidization to some extent, mainly because the required residual Mg amount is high. Increase the number of inclusions in cast iron and reduce the performance of cast iron.

In silicon ductile iron, the effect of increasing ferrite is larger than that of gray cast iron, so the silicon content directly affects the amount of ferrite in the ductile iron matrix. Silicon has a great influence on the performance of ductile iron. It is mainly manifested in the solid solution strengthening effect of silicon on the matrix. Silicon can refine graphite and improve the roundness of graphite balls. Therefore, the increase in silicon content in the ductile iron greatly improves the strength index and reduces the toughness. The spheroidized iron of spheroidal graphite cast iron has a large tendency to crystallize and cool and form a white mouth, and silicon can reduce this tendency. However, the amount of silicon is too high, and large-section ductile iron promotes the formation of broken graphite and reduces the mechanical properties of the casting. The data show that silicon in the ductile iron is added in a way that is bred to improve performance to some extent.

According to the above analysis, from the viewpoint of improving the casting performance, the carbon equivalent of the molten iron is preferably selected in the vicinity of the eutectic point, and at this time, the molten iron has the best fluidity, tends to concentrate the shrinkage hole, and is easy to be fed. However, if the carbon equivalent is too high, the graphite will float, and the thickness of the graphite floating layer will increase as the carbon equivalent increases. It should be noted that too high a carbon equivalent is the main cause of graphite flotation, but not the only reason. Casting size, wall thickness, and pouring temperature are also important factors.

The relationship between carbon equivalent, casting wall thickness and graphite floating is obvious that the thin carbon equivalent of the casting wall can be selected to be higher, and graphite floating will not occur. On the contrary, the carbon equivalent of the thick casting should be selected lower. In short, the upper limit of carbon equivalent is based on the principle that graphite does not appear to float. The lower limit is that no cementite is present, and the globalization is guaranteed. Under such a premise, the carbon equivalent should be increased as much as possible to obtain a dense casting.

2) Manganese (Mn)

Manganese plays a different role in ductile iron than gray cast iron. In gray cast iron, manganese can reduce the harmful effects of sulfur in addition to strengthening ferrite and stabilizing pearls. In nodular cast iron, spheroidized elements have a strong desulfurization capacity, and manganese no longer has this effect. Because manganese has a strong positive segregation tendency, it is often concentrated at the grain boundary of the eutectic group, which promotes the formation of intercrystalline carbides, which significantly reduces the toughness of ductile iron. For thick section ductile iron, the segregation tendency of manganese is more serious. At the same time, the manganese content is increased, and the pearlite content in the matrix is increased, so that the strength index is improved and the toughness is lowered. The control of manganese content in high toughness ductile iron should be more stringent.

Therefore, the lower the Mn, the better the raw materials are possible. The upper limit of control for manganese with large castings is Mn < 0.3%.

3) Phosphorus:

Phosphorus has a strong segregation tendency in ductile iron, and it is easy to form phosphorus eutectic at the grain boundary, which seriously reduces the toughness of ductile iron. Phosphorus also increases the tendency of the ductile iron to shrink. When spheroidal graphite cast iron is required to have high toughness, the phosphorus should be controlled below 0.06%.

4) Sulfur:

Sulfur in spheroidal graphite cast iron has a strong ability to combine with spheroidal elements to form sulfides and sulfur oxides, which not only consumes spheroidizing agents, but also causes spheroidization instability, and also increases the number of inclusions and accelerates the spheroidization decay rate. In the smelting, sulfur is involved in the recarburizing agent, and the process control minimizes the sulfur content in the raw materials, and the pre-furnace desulfurization measures are taken.

After treatment with Re-Mg alloy, the residual amount of sulfur is generally S<0.02%, which has no effect on spheroidization decay and sulfide slag. When S>0.02% in original molten iron, desulfurization treatment must be used.

 

5) Molybdenum:

Mo improves the high temperature strength and room temperature strength of the material. Due to the use, it is easy to form a certain amount of pearlite and carbide, and the toughness is lowered. For the ductile iron with Mo alloying, the material specification requires a Mo content of 0.3 to 0.7%.

 

6) Contents of magnesium and rare earth

Magnesium is the main spheroidizing element. The rare earth has desulfurization, neutralizes the anti-spheroidizing elements, has a protective effect on Mg, and improves the anti-recession ability of molten iron. However, since the rare earth element is a carbide forming element, the residual amount of the rare earth is controlled as much as possible while ensuring good spheroidization. Re=0.01~0.04%, and the spheroidization can be guaranteed when Mg=0.03~0.06%.

Based on the above analysis and calculation, the final chemical composition is as follows:

C: 3.3-3.8%; Si: 2.2-2.7%; Mn: <0.30%; S < 0.02%; Re = 0.01 to 0.04%; Mg = 0.03 to 0.06%, Mo: 0.3 to 0.7%


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