Properties of nodular cast iron are improved by adding rare earth

In terms of high strength and low alloy nodular cast iron, in addition to copper and molybdenum, nickel and niobium were also studied. Although the properties of medium manganese nodular cast iron are not stable enough, the systematic research and production application over the years have achieved remarkable economic benefits.

In terms of heat-resistant nodular cast iron, in addition to medium silicon nodular cast iron, the effect of the total amount of Si + Al on the growth resistance of rare earth magnesium nodular cast iron was systematically studied. The service life of rqtal5si5 heat-resistant cast iron developed in China is three times that of gray cast iron and two times that of ordinary heat-resistant cast iron, and is equivalent to that of cr25ni13si2 heat-resistant steel in Japan.

High nickel austenitic nodular cast iron has also made progress. It has been successfully applied in petroleum mining machinery, chemical equipment and industrial furnace devices.

In terms of acid resistant nodular cast iron, the structure of rare earth high silicon nodular cast iron produced in China is smaller, uniform and denser than that of ordinary high silicon cast iron. Therefore, the corrosion resistance is improved by 10% ~ 90%, and its mechanical strength is also significantly improved.

Rare earth can spheroidize graphite. Since H. morrogh first used cerium to obtain nodular cast iron, many people have studied the spheroidizing behavior of various rare earth elements and found that cerium is the most effective spheroidizing element, and other elements also have spheroidizing ability to varying degrees.

China has done a lot of research and development work on the spheroidization of rare earth. It is found that rare earth elements are difficult to obtain the complete and uniform spherical graphite as magnesium nodular cast iron for the common components of nodular cast iron (c3.6 ~ 3.8wt%, si2.0 ~ 2.5wt%); Moreover, when the amount of rare earth is too high, various abnormal graphite will appear, and the white mouth tendency will also increase. However, if it is a high carbon hypereutectic component (c > 4.0wt%) and the residual amount of rare earth is 0.12 ~ 0.15wt%, good spherical graphite can be obtained.

According to the poor iron quality, high sulfur content (cupola smelting) and low tapping temperature in China, the addition of rare earth is necessary. Magnesium is the dominant element in the spheroidizing agent. On the one hand, rare earth can promote the spheroidization of graphite; On the other hand, it is also necessary to overcome the influence of sulfur and impurity elements to ensure spheroidization.

Rare earth prevents interfering elements from damaging spheroidization. The research shows that when the total amount of interfering elements such as Pb, Bi, Sb, TE and Ti is 0.05wt%, adding 0.01wt% (residual) rare earth can completely neutralize the interference and inhibit the production of abnormal graphite. Most of China’s pig iron contains titanium, and some pig iron contains titanium as high as 0.2 ~ 0.3wt%, but rare earth magnesium spheroidizing agent can still ensure good graphite spheroidization because it can make the residual amount of rare earth in iron reach 0.02 ~ 0.03wt%. If 0.02 ~ 0.03wt% Bi is added to nodular cast iron, the nodular graphite is almost completely destroyed; If 0.01 ~ 0.05wt% Ce is subsequently added, the original spheroidization state will be restored, which is due to the formation of stable compounds between Bi and CE.

Nucleation of rare earth. Studies after the 1960s have shown that inoculants containing cerium can increase the number of balls in the molten iron throughout the retention period, so that the final structure contains more graphite balls and less white mouth tendency. The research also shows that the inoculant containing rare earth can improve the inoculation effect of nodular cast iron and significantly improve the anti recession ability. The reason why the addition of rare earth can increase the number of graphite spheres can be attributed to: rare earth can provide more nuclei, but its composition of nuclei is different from that of FeSi inoculation; Rare earth can make the original inactive crystal nucleus (existing in molten iron) grow up, resulting in an increase in the total number of crystal nuclei in molten iron.