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22
2021
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07
Construction technology of ductile iron
Foundry
In terms of high-strength low-alloy ductile iron, in addition to more research on copper and molybdenum, nickel and niobium have also been studied. Although the performance of medium-manganese ductile iron is unstable, it has achieved significant economic benefits in terms of systematic research, production, and application over the years. In addition to medium-silicon ductile iron, the effect of the total amount of Si+Al on the anti-growth ability of rare earth magnesium ductile iron was systematically studied.
In high-strength low-alloy ductile iron, in addition to more research on copper and molybdenum, nickel and niobium have also been studied. Although the performance of medium-manganese ductile iron is unstable, it has achieved significant economic benefits in systematic research, production, and application over the years.
In addition to medium-silicon ductile iron, the effect of the total amount of Si+Al on the growth resistance of rare earth magnesium ductile iron was systematically studied. The service life of RQTAL5Si5 heat-resistant cast iron in China is 3 times that of gray cast iron and 2 times that of ordinary heat-resistant cast iron, which is equivalent to the service life of Japanese Cr25Ni13Si2 heat-resistant cast iron.
High-nickel austenitic ductile iron has also made progress and has been successfully applied to petroleum extraction machinery, chemical equipment, and industrial furnace components. In terms of acid-resistant ductile iron, the rare earth high-silicon ductile iron produced in China has a smaller, more uniform, and denser structure than ordinary high-silicon cast iron, its corrosion resistance is increased by 10%~90%, and its mechanical strength is also significantly improved.
Rare earth elements can cause graphite spheroidization. Since H. Morrogh first used ductile 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 different degrees of spheroidizing ability.
We have developed a large number of rare earth spheroidization effects and found that it is difficult for rare earth elements to obtain complete and uniform spherical graphite like magnesium ductile iron for commonly used ductile iron compositions (C3.6~3.8wt%, Si2.0~2.5wt%); moreover, when the amount of rare earth is too high, allotropic graphite will appear, and the tendency will also increase. However, if it is a high-carbon hypereutectic composition (C>4.0wt%), a rare earth residual amount of 0.12~0.15wt% can obtain good spherical graphite.
The addition of rare earth is necessary because the iron quality is poor and the sulfur content is high (cupola melting) and the iron tapping temperature is low. Magnesium is the main element in the spheroidizing agent, and rare earth can promote graphite spheroidization on the one hand; on the other hand, it is necessary to overcome the influence of sulfur and impurity elements to ensure spheroidization.
Rare earths prevent interfering elements from destroying spheroidization. Studies have shown that when the total amount of interfering elements Pb, Bi, Sb, Te, and Ti is 0.05wt%, adding 0.01wt% (residual amount) of rare earth can completely neutralize the interference and inhibit the production of graphite. Most pig iron in China contains titanium, and some pig iron contains 0.2~0.3wt% titanium, but rare earth magnesium spheroidizing agent can make the rare earth residue in iron reach 0.02~0.03wt%, so it can still ensure good graphite spheroidization. If 0.02~0.03wt% Bi is added to ductile iron, the spherical graphite is almost completely destroyed; if 0.01~0.05wt% Ce is added, the original spheroidization state is restored because Bi and Ce form a stable compound.
Nucleation of rare earths. Studies since the 1960s have shown that bismuth-containing inoculants can increase the number of spheres in the iron liquid throughout the maintenance period, so that the final structure contains more graphite spheres and a smaller tendency. Studies have also shown that rare earth-containing inoculants can improve the inoculation effect of ductile iron and significantly improve anti-aging ability. The reason why adding rare earth can increase the number of graphite spheres can be attributed to: rare earth can provide more crystal nuclei, but compared with FeSi inoculation, the crystal nucleus composition it provides is different; rare earth can make the original (existing in the iron liquid) unactivated crystal nuclei grow, thereby increasing the total number of crystal nuclei in the iron liquid.
Ductile iron
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