The influence of five elements on castings

The appearance of cast iron is convenient for human beings and promotes the progress of human society and civilization. With the development of science and technology and China’s national economy, all walks of life on the quality of cast iron parts put forward higher requirements, so the correct use of reasonable casting technology is to improve the quality of cast iron parts to ensure. The main factors affecting the quality of cast iron casting process are: cooling rate, chemical composition, temperature, gas, charge, etc., now take chemical composition as an example, the five elements of the casting of the impact.

Conventional elements affecting the quality of castings are mainly five, respectively, carbon, silicon, manganese, sulfur, phosphorus, the above elements we call the basic elements or commonly known as the five elements, they are directly affecting the casting physical properties of an important factor. Their main role is as follows:

1. Carbonis the most basic component of cast iron. Appropriate carbon in casting to promote graphitization, reduce the tendency of white mouth, that is, to reduce the carburite, pearlite, ternary phosphorus eutectic, increase ferrite, thus reducing the hardness to improve machinability: carbon can improve fluidity, increase the solidification of the volume of the expansion; carbon to improve vibration absorption, friction reduction of thermal conductivity. But the carbon content is too high to cause graphite floating, deterioration of mechanical properties, too low and easy to produce shrinkage holes loose shrinkage and other defects. Therefore, the castings of different quality requirements, reasonable allocation of carbon content is generally a way to improve the quality of castings. For example: most of the gray iron carbon content in 2.6%-3.6%, ductile iron in 3.5%-3.9%. Carbon on the mechanical properties of manganese nodular graphite cast iron is not obvious, generally higher than 3.9% of carbon is easy to graphite floating, affecting the quality of cast iron, carbon is less than 3.0%, is not conducive to graphitization of the general control of the carbon in 3.0%-3.8% is appropriate.

2. siliconis a useful element in castings, it is the same as carbon, can promote graphitization. For cast ball mill castings, increase the amount of silicon has a dual role, on the one hand, it makes the carburite, pearlite, ternary phosphorus eutectic reduction, ferrite increase, thus reducing the strength and hardness, and improve the casting plasticity; on the other hand, silicon solid solution ferrite strengthening, so that the yield point and hardness increase. Silicon to improve casting fluidity, increase solidification volume expansion; silicon can improve heat and corrosion resistance. Increase the amount, especially the amount of pregnant silicon, can significantly control the number of carbides, therefore, silicon is a powerful element to inhibit the tendency of white mouth in manganese ductile iron. Silicon in a certain range, is conducive to the improvement of strength and toughness, but make the anti-wear performance is reduced, so take the right amount. In general, gray casting silicon content in 1.2%-3.0%, ductile casting silicon in 2.0%-3.0%.

3. Manganeseis one of the important elements of castings, the right amount of manganese, help to generate texture structure, increase solidity and strength and wear resistance. Manganese and sulfur samples are stable compounds, is an obstacle to graphitization of the elements, when coexisting with sulfur, manganese and sulfur affinity is greater, will be combined into compounds such as MnS, in the appropriate temperature, not only does not impede the role of graphitization, but also neutralize the sulfur, plays a role in removing sulfur. When manganese reaches a certain amount, it can make castings with high strength, high hardness, high density, wear-resistant and other advantages, at this time, the amount of silicon is also increased accordingly. Manganese is easy to produce segregation at the boundary of eutectic group, easy to generate carbide in casting state, increase the amount of manganese, will deteriorate the mechanical properties. Therefore, the content of manganese should be low. But manganese can stabilize the austenite, prompting the formation of austenitic matrix, can become a weakly magnetic ductile iron, with good wear resistance. Manganese solid solution in austenite, and iron to form a replacement solid solution, and, because manganese has a stronger affinity for carbon than iron, it organizes the diffusion of carbon from the solid solution and precipitation, play a role in stabilizing and expanding the role of austenite zone.

4. Phosphorusis a harmful element and is treated as an impurity. Phosphorus tends to affect the mechanical properties of castings, especially reducing toughness and densification, and is the main cause of cracking in castings. Because phosphorus in the casting solubility is very low. Such as P < 0.05% when solidly dissolved in iron, ductile casting mechanical properties have no significant adverse effects. Phosphorus in cast iron is an easy element of segregation, when the casting of phosphorus content reaches 0.05%, there is already the possibility of the formation of phosphorus eutectic, the majority of castings, phosphorus eutectic will increase the brittleness of the casting, a serious deterioration of the mechanical properties. For example: in ductile iron, the amount of phosphorus from 0.04% -0.05% to 0.2%, tensile strength from 800Mpa -850Mpa, down to 650Mpa -700Mpa, elongation from 3.5% -4% down to 1.5% -2.0%. Therefore, phosphorus should limit the content of the best in 0.04% or less. But phosphorus can increase the hardness, improve wear resistance, in some wear-resistant cast iron, to add phosphorus, the purpose is to use phosphorus eutectic wear resistance.

5. Sulfuris also an impurity, is a harmful element. In casting, sparing elements and Mn, Mg and other elements with strong affinity, produce stable carbides, hinder graphitization, consumption of liquid iron in the spherical element, the formation of MgS, MnS and other residues, due to the consumption of sulfur so that the effective residual spherical element content is too low to reduce spherical, but also to promote the formation of entrapment, subcutaneous porosity and other defects. Due to sulfur to reduce the rate of spheroidization, accelerate the decline of spheroidization and the formation of slag, etc. so that the mechanical properties of the decline or instability. Sulfur element should be removed, should be low. In ordinary gray iron, sulfur content is generally 0.02%-0.15%, in ductile iron S ≤ 02%, sometimes depending on the situation.

It can be seen, cast iron is actually a very complex chemical process based on carbon, silicon, manganese, sulfur, phosphorus and other elements. Among them, carbon, silicon is the most basic component, manganese content is generally low when the impact is not significant, sulfur, phosphorus is often regarded as an impurity, and therefore often be limited. Each of these elements on the quality of cast iron, solidification and crystallization, organization and performance have a certain impact and role. This requires the foundry in the casting process should be the five elements of a reasonable proportion, it is a way to improve the quality of castings.