Add rare earth to improve performance
In the high-strength low-alloy ductile iron, in addition to the research on copper and molybdenum, nickel and niobium have also been studied. Although the manganese fiber ductile iron is not stable enough in performance, it has achieved remarkable economic benefits for many years of system research and production applications.
In the heat-resistant ductile iron, in addition to the medium silicon ductile iron, the influence of the total amount of Si+Al on the growth resistance of the rare earth magnesium ductile cast iron was systematically studied. The RQTAL5Si5 heat-resistant cast iron developed in China has a service life of 3 times that of gray cast iron and 2 times that of ordinary heat-resistant cast iron, and is equivalent to the service life of Japanese Cr25Ni13Si2 heat-resistant steel.
Progress has also been made in high-nickel austenitic ductile iron, which has been successfully applied in petroleum mining machinery, chemical equipment, and industrial furnace devices.
In the acid-resistant ductile iron, the rare earth high-silica ductile iron produced in China is finer, more uniform and denser than the ordinary high-silicon cast iron. Therefore, the corrosion resistance is improved by 10% to 90%, and the mechanical strength is also significantly improved.
Rare earths can spheroidize graphite. Since H. Morrogh first used spheroidal graphite cast iron, many people have studied the spheroidizing behavior of various rare earth elements, and found that strontium is the most effective spheroidizing element, and other elements have varying degrees of spheroidizing ability.
China has carried out a lot of research on the spheroidization of rare earths. It is found that rare earth elements are difficult to obtain the same as magnesium spheroidal graphite cast iron for the common spheroidal graphite cast iron composition (C3.6-3.8wt%, Si2.0-2.5wt%). Complete and uniform spheroidal graphite; moreover, when the amount of rare earth is too high, various metamorphic graphites will appear, and the white mouth tends to increase, but if it is a high carbon hypereutectic composition (C>4.0 wt%), When the residual amount of the rare earth is from 0.12 to 0.15 wt%, good spheroidal graphite can be obtained.
According to China's poor iron content, high sulfur content (cupola melting) and low iron temperature, it is necessary to add rare earth. Magnesium is the dominant element in the spheroidizing agent, and rare earth can promote the spheroidization of graphite on the one hand; on the other hand, it is necessary to overcome the influence of sulfur and impurity elements to ensure spheroidization.
The rare earth prevents interference elements from damaging the spheroidization. Studies have shown that when the total amount of interfering elements Pb, Bi, Sb, Te, Ti, etc. is 0.05 wt%, the addition of 0.01 wt% (residual amount) of rare earth can completely neutralize the interference and suppress the generation of metamorphic graphite. Most of China's pig iron contains titanium, and some pig iron contains up to 0.2-0.3wt% of titanium, but the rare earth magnesium spheroidizer can ensure the graphite residue by 0.02-0.03wt%. Spheroidized. If 0.02 to 0.03 wt% of Bi is added to the ductile cast iron, the spheroidal graphite is almost completely destroyed; if 0.01 to 0.05 wt% of Ce is subsequently added, the original spheroidized state is restored, because Bi and Ce form a stable state. Compound.
The nucleation of rare earths. Studies since the 1960s have shown that bismuth-containing inoculants can increase the number of balls in the entire retention period, leaving the final tissue with more graphite balls and a smaller white-mouth tendency. Studies have also shown that rare earth-containing inoculants can improve the inoculation effect of ductile iron and significantly improve the ability to resist recession. The reason why the addition of rare earth can increase the number of graphite spheres can be attributed to: rare earth can provide more crystal nucleus, but it has different crystal nucleus composition compared with FeSi; rare earth can be original (exist in molten iron) The inactivated crystal nucleus grows up, resulting in an increase in the total number of crystal nuclei in the molten iron.