construction technology

Adding rare earth to improve properties

In addition to copper and molybdenum, nickel and niobium are also studied in high strength low alloy ductile iron. Although the properties of medium manganese ductile iron are not stable enough, remarkable economic benefits have been achieved through systematic research and production application for many years.

In addition to Medium Silicon Ductile Iron, the effect of Si + Al content on the growth resistance of rare earth magnesium ductile iron was systematically studied. The service life of rqtal5si5 heat-resistant cast iron developed in China is three times as long as 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 made in Japan.

Progress has also been made in high nickel austenitic nodular cast iron, which has been successfully applied in petroleum mining machinery, chemical equipment and industrial furnace devices.

In the aspect of acid resistant ductile iron, the microstructure of Rare Earth High Silicon Ductile iron produced in China is finer, uniform and denser than that of ordinary high silicon cast iron. As a result, the corrosion resistance is increased by 10% ～ 90%, and the mechanical strength is also significantly improved.

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

A lot of research and development work has been carried out in China on the spheroidization of rare earth elements. It is found that it is difficult to obtain spheroidal graphite as complete and uniform as that of magnesium nodular cast iron for the common composition of nodular cast iron (c3.6-3.8wt%, si2.0-2.5wt%). Moreover, when the content of rare earth is too high, various abnormal graphite will appear, and the tendency of white cast iron will increase When the content of rare earth is 0.12-0.15wt%, the spherical graphite can be obtained.

According to the poor iron quality, high sulfur content (cupola melting) and low tapping temperature in China, it is necessary to add rare earth. On the other hand, the spheroidizing effect of rare earth elements on Spheroidization must be overcome.

Rare earth prevents the interference elements from destroying spheroidization. The results show that when the total amount of interference elements Pb, Bi, Sb, Te, Ti is 0.05wt%, and the addition of 0.01wt% RE can completely neutralize the interference and inhibit the generation of abnormal graphite. Most of the pig iron in China contains titanium, and some pig iron contains as much as 0.2-0.3wt% titanium. However, the rare earth magnesium nodulizer can make the residual rare earth in iron reach 0.02-0.03wt%, so it can guarantee the good spheroidization of graphite. If 0.02-0.03wt% Bi is added into the nodular cast iron, the spheroidal graphite is almost completely destroyed; if 0.01-0.05wt% Ce is added later, the spheroidized state will be restored again, which is due to the formation of stable compounds between Bi and CE.

The nucleation of rare earth. Studies after the 1960s have shown that the inoculant containing cerium can increase the number of balls in molten iron during the whole holding period, resulting in more graphite balls and less chill tendency in the final microstructure. The results also show that the inoculant containing rare earth can improve the inoculation effect of nodular cast iron and improve the ability of anti fading. The reason why the number of graphite spheres increases with the addition of rare earth can be attributed to the following: rare earth can provide more crystal nuclei, but the composition of nuclei provided by rare earth is different from that of FeSi inoculation; rare earth can make the original (existing in molten iron) inactive crystal nuclei grow up, resulting in an increase in the total number of crystal nuclei in molten iron.