Nodular cast iron is a kind of high strength cast iron material developed in the 1950s. Its comprehensive performance is close to that of steel. Based on its excellent performance, it has been successfully used to cast some parts with complex forces and high requirements for strength, toughness and wear resistance. Nodular [SCALE STEEL]cast iron has been rapidly developed into a very widely used cast iron material second only to gray cast iron. The so-called "iron instead of steel", mainly refers to nodular cast iron.
Nodular cast iron is obtained by spheroidization and inoculation treatment of spherical graphite, effectively improve the mechanical properties of cast iron, especially improve the plasticity and toughness, so as to get higher strength than carbon steel.
Cast iron is an iron-carbon alloy with carbon content greater than 2.11%. It is obtained by industrial pig iron, scrap steel and other steel and alloy materials through high-temperature melting and casting. In addition to Fe, it also contains carbon in other cast iron precipitated in the form of graphite. If the graphite precipitated[SCALE STEEL] is lamellar cast iron called gray cast iron or gray cast iron, wormlike cast iron called vermicular cast iron, was flocculent cast iron called malleable cast iron or code iron, and was spherical cast iron called nodular cast iron.
In addition to iron, the chemical composition of nodular cast iron is usually 3.0 ~ 4.0% carbon content, 1.8 ~ 3.2% silicon content, the total content of manganese, phosphorus, sulfur is not more than 3.0% and an appropriate amount of rare earth, magnesium and other spheroidal elements.
Ductile iron castings have been used in almost all major industrial sectors, which require high strength, plasticity, toughness, wear resistance, severe thermal and mechanical impact resistance, high or low temperature resistance, corrosion resistance and dimensional stability. To meet these variations in service conditions, nodular[SCALE STEEL] cast iron is available in many grades, offering a wide range of mechanical and physical properties.
Most nodular cast iron castings, as specified in ISO1083, are produced primarily in a non-alloying state. Obviously, this range includes high strength grades with tensile strength greater than 800 Newtons per square millimeter and elongation of 2%. At the other end of the spectrum are highly plastic grades with elongation greater than 17% and corresponding low strength (as low as 370 Newton/mm2). Strength and elongation are not the basis for designers to choose materials. Other important properties that are decisive include yield strength, modulus of elasticity, wear resistance and fatigue strength, hardness and impact properties. In addition, corrosion resistance and oxidation resistance as well as electromagnetic properties may be critical to the designer. To meet these special uses, a group [SCALE STEEL]of austenitic nodules, usually called Ni Resis, is developed. These austenitic nodules are alloyed mainly with nickel, chromium and manganese and are included in international standards.
It is pearlescent ductile iron with medium and high strength, medium toughness and plasticity, high comprehensive performance, good wear resistance and vibration reduction, and good casting process performance. The properties can be changed by various heat treatments. Mainly used in various power machinery crankshaft, camshaft, connecting shaft, connecting rod, gear, clutch plate, hydraulic cylinder and other parts.
In the aspect of high strength and low alloy nodular cast iron, besides copper and molybdenum, nickel and niobium have also been[SCALE STEEL] studied. Although the performance of medium manganese nodular cast iron is not stable, the systematic research and production application over the years have obtained remarkable economic benefits.
In addition to middle silicon nodular cast iron, the effect of 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 3 times that of gray cast iron, 2 times that of ordinary heat-resistant cast iron, and comparable to that of Japanese Cr25Ni13Si2 heat-resistant steel.
High nickel austenitic nodular cast iron has also made progress. It has been successfully applied in petroleum mining machinery, chemical [SCALE STEEL]equipment and industrial furnace components.
In terms of acid-resistant nodular cast iron, the rare earth high-silicon nodular cast iron produced in China has a finer, uniform and dense structure than the ordinary high-silicon cast iron. Therefore, the corrosion resistance is increased by 10% ~ 90%, and the mechanical strength is also significantly improved.
Rare earths 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 varying degrees of spheroidizing ability.
China has made a lot of research and development on the spheroidization of rare earth elements. It is found that rare earth elements are difficult to obtain spheroidal graphite as complete and uniform as magnesium spheroidium iron for the common nodular iron components (C3.6 ~ 3.8wt%, Si2.0 ~ 2.5wt%). In addition, when the [SCALE STEEL]amount of rare earth is too high, there will be a variety of deformed graphite, white mouth tendency is also increased, but if the high carbon hypereutectic composition (C>4.0wt%), the residual amount of rare earth is 0.12 ~ 0.15wt%, good spherical graphite can be obtained.
The addition of rare earths is necessary due to the poor quality of iron in China, the high sulfur content (cupola smelting) and the low temperature of iron extraction. 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, [SCALE STEEL]it is necessary to overcome the influence of sulfur and impurity elements to ensure spheroidization.
Rare earths prevent interfering elements from disrupting spheroidization. The results show that when the total amount of interfering elements such as 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 abnormal graphite. The majority of pig iron in China contains titanium, and some pig iron contains titanium as high as 0.2 ~ 0.3wt%, but the rare earth magnesium spheroidization agent can still ensure good [SCALE STEEL]graphite spheroidization because the residual amount of rare earth in iron is 0.02 ~ 0.03wt%. If 0.02 ~ 0.03wt%Bi is added to nodular cast iron, the nodular graphite is almost destroyed completely. When 0.01 ~ 0.05wt%Ce was added, the spheroidization state was restored, because Bi and Ce formed stable compounds.
Nucleation of rare earth. Studies since the 1960s have shown that cerium-containing inoculants increase the number of pellets in liquid iron throughout the retention period, resulting in more graphite pellets in the resulting tissue and a less white-mouth tendency. The results also show that the inoculant containing rare earth can improve the inoculant effect of nodular cast iron and significantly increase the anti-decay ability. The reason of increasing the number of graphite spheres by adding rare earth can be concluded as follows: rare earth can provide more crystal nuclei, but the composition of crystal nuclei is [SCALE STEEL]different from that provided by FeSi inoculation; Rare earth can make the original (existing in liquid iron) inactive nuclei grow, resulting in an increase in the total number of crystal nuclei in liquid iron.
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