1. Carbon is the basic component in machine tool castings. It is not only the main basis for distinguishing steel or iron. Carbon content greater than 1.7% is iron, and less than 1.7% is called steel. Moreover, in the casting process, carbon affects the mechanical properties of castings. In casting, appropriate carbon promotes graphitization and reduces the tendency of white cast iron, that is, reduces cementite, pearlite, and ternary phosphorus eutectic, increases ferrite, thereby reducing hardness and improving processing performance; carbon promotes the improvement of magnesium absorption rate; improves spheroidization to achieve the expected effect; carbon can improve fluidity and increase volume expansion during solidification; carbon improves vibration absorption, friction reduction, and thermal conductivity. However, too high carbon content causes graphite floating and deteriorates mechanical properties, and too low carbon content is prone to shrinkage and shrinkage defects. Therefore, for castings with different quality requirements, reasonable selection of carbon content is generally a way to improve the quality of castings. For example, the carbon content of gray iron is mostly 2.6%-3.6%, and that of ductile iron is 3.5%-3.9%. Carbon has no obvious effect on the mechanical properties of medium manganese ductile iron. Generally, when the carbon content is higher than 3.9%, graphite floating is easy to occur, which affects the quality of cast iron. When the carbon content is lower than 3.0%, it is not conducive to graphitization. Therefore, it is generally appropriate to control the carbon content at 3.0%-3.8%.
Second, silicon is a beneficial element in large castings. Like carbon, it can promote graphitization. The effect of silicon added in the form of inoculant is more obvious. For as-cast ball-milled castings, increasing the silicon content has a dual effect. On the one hand, it reduces cementite, pearlite, and ternary phosphorus eutectic, increases ferrite, thereby reducing strength and hardness and improving the plasticity of castings; on the other hand, silicon solid solution strengthens ferrite, increases yield point and hardness; silicon improves casting fluidity and increases volume expansion during solidification; silicon can improve heat resistance and corrosion resistance. Increasing the amount of silicon, especially the amount of inoculated silicon, can significantly control the number of carbides. Therefore, silicon is a powerful element that inhibits the tendency of white cast iron in medium manganese ductile iron. Silicon within a certain range is conducive to improving strength and toughness, but reduces wear resistance. Therefore, an appropriate amount should be taken. Generally, the silicon content of gray castings is 1.2%-3.0%, and the silicon content of ductile castings is 2.0%-3.0%.
3. Manganese is one of the important elements of castings. An appropriate amount of manganese helps to generate texture structure, increase firmness, strength and wear resistance. Manganese, like sulfur, is a stable compound and an element that hinders graphitization. When coexisting with sulfur, manganese has a greater affinity with sulfur and will combine into compounds such as MnS. At the appropriate temperature, it not only does not hinder graphitization, but also neutralizes sulfur and plays a role in desulfurization. When manganese reaches a certain amount, it can make the casting have the advantages of high strength, high hardness, high density and wear resistance. At this time, the amount of silicon is also increased accordingly. Manganese is easy to segregate at the boundary of the eutectic group, and it is easy to generate carbides in the cast state. Increasing the amount of manganese will deteriorate the mechanical properties. Therefore, the manganese content should generally be low. However, manganese can stabilize austenite and promote the formation of austenite matrix, which can become weakly magnetic ductile iron with good wear resistance. Manganese is dissolved in austenite and forms a substitutional solid solution with iron. Moreover, since manganese has a stronger affinity for carbon than iron, it organizes carbon to diffuse and precipitate from the solid solution, which plays a role in stabilizing and expanding the austenite zone.
4. Phosphorus is a harmful element and is treated as an impurity. Phosphorus often affects the mechanical properties of castings, especially reduces toughness and density, and is the main cause of cracking of castings. Because phosphorus has a very low solubility in castings. If P<0.05%, it is dissolved in iron and has no obvious adverse effect on the mechanical properties of ductile castings. Phosphorus is an element that is easily segregated in cast iron. When the phosphorus content in the casting reaches 0.05%, phosphorus eutectic may form. For most castings, phosphorus eutectic will increase the brittleness of the casting and seriously deteriorate the mechanical properties. For example: in ductile iron, the phosphorus content increases from 0.04%-0.05% to 0.2%, the tensile strength decreases from 800Mpa-850Mpa to 650Mpa-700Mpa, and the elongation decreases from 3.5%-4% to 1.5%-2.0%. Therefore, the phosphorus content should be limited to less than 0.04%. However, phosphorus can increase hardness and improve wear resistance. In some wear-resistant cast irons, phosphorus is added to utilize the wear resistance of phosphorus eutectic.
Five. Sulfur is also an impurity and a harmful element. In casting, sulfur has a strong affinity with other elements such as Mn and Mg, produces stable carbides, hinders graphitization, consumes spheroidizing elements in molten iron, and forms residues such as MgS and MnS. Due to the consumption of sulfur, the effective residual spheroidizing element content is too low, which reduces spheroidization and promotes the formation of defects such as slag inclusions and subcutaneous pores. Sulfur reduces the spheroidization rate, accelerates the decline of spheroidization, and forms slag inclusions, which causes the mechanical properties to decrease or become unstable. The sulfur element should be removed and the content should be low. In ordinary gray iron, the sulfur content is generally 0.02%-0.15%, and in ductile iron, S≤0.02%, sometimes depending on the situation. It can be seen that cast iron is actually a very complex chemical process based on elements such as carbon, silicon, manganese, sulfur, and phosphorus. Among them, carbon and silicon are the basic components, and the manganese content is generally low and has little effect. Sulfur and phosphorus are often regarded as impurities, so they are often restricted. Each of these elements has a certain influence and effect on the quality, solidification crystallization, organization and performance of cast iron. This requires the caster to reasonably match the five elements during the casting process, which is a way to improve the quality of dense castings.
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