First, the role of silicon in cast iron
 The role of silicon in cast iron is multifaceted. Among them, the first thing we pay attention to is “promoting graphitization” and “solid solution strengthening”. In addition, silicon has many important functions. Here, simple The following two points are mentioned:
 (1) Silicon dissolved in liquid cast iron greatly enhances the anti-oxidation ability of molten iron, and silicon can also reduce the solubility of nitrogen in molten iron. It is because of this role of silicon that cast iron can be smelted under conditions of strong oxidizing and nitrogen-rich conditions. Among the various cast alloys, only cast iron can be smelted in an oxygen-rich, nitrogen-rich atmosphere using a smelting apparatus such as a cupola or an oxygen rotary kiln.
 (2) The silicon content in the cast iron is increased to 3.5% or more, and the oxidation resistance and heat growth resistance of the cast iron are greatly improved. In the early days, the standards for heat-resistant cast iron in various countries were all grades of silicon-based heat-resistant cast iron. In recent years, due to the consideration of energy saving, various internal combustion engines have increased the temperature of the exhaust gas. In various automotive industries, the application of heat-resistant silicon-molybdenum ductile iron castings has been emphasized.
 1. The role of silicon in promoting graphitization in cast iron
 Silicon in cast iron is the alloying element that promotes graphitization. The ability of silicon to promote graphitization is three times that of nickel and five times that of copper.
 Whether in liquid or solid cast iron, the combination of silicon and iron acts better than carbon.
 The presence of silicon in liquid cast iron reduces the solubility of carbon. The higher the content of silicon in the molten iron, the lower the carbon content, and more carbon is extruded.
 When the molten iron is a hypereutectic component, the silicon content is high. During the solidification process, more carbon is precipitated in the form of primary graphite until the remaining molten iron reaches the eutectic composition, and eutectic transformation occurs.
When the molten iron is a hypoeutectic composition, silicon is enriched in the primary austenite during solidification.
During eutectic transformation, silicon is enriched in the early crystalline eutectic austenite, inhibiting the formation of cementite by carbon and ironification, enhancing the diffusion rate of carbon in austenite, and promoting the precipitation of carbon in the form of eutectic graphite.
During the eutectoid transformation, the silicon dissolved in austenite still inhibits the formation of cementite between carbon and iron, enhances the diffusion rate of carbon in austenite, and promotes the precipitation of carbon in the form of eutectoid graphite.
In gray cast iron, ductile iron, vermicular cast iron and black core malleable cast iron, carbon and silicon are the main elements affecting the morphology and quantity of graphite. It is a white-heart malleable cast iron which is basically free of graphite. In the process of decarburization annealing, silicon promotes the diffusion of carbon in austenite, which also plays an important role in the decarburization of such malleable cast iron.
 In addition, both oxygen and nitrogen in cast iron have a stabilizing effect on carbides. The silicon contained in the cast iron can reduce the oxygen and nitrogen content, which indirectly enhances the effect of silicon on graphitization.
 2. Solid solution strengthening of silicon in ferrite
 In solid cast iron, silicon is almost completely dissolved in austenite and ferrite and does not enter carbides. The silicon atom and the iron atom can be combined into a silicon-containing ferrite having a strong covalent bond, which not only promotes the formation of ferrite but also has a strong effect of strengthening the ferrite.
 In order to understand the ability of silicon to strengthen ferrite and avoid the influence of graphite form and other alloying elements, in the 1950s, some people in foreign countries added different amounts of silicon to steels with a carbon content of 0.1% and no other alloying elements. Compare the effects of silicon on mechanical properties. The results are shown in Table 1. In Table 1, the properties of carbon steel with all pearlite and no other alloying elements are also listed for comparison.
 It can be seen from Table 1 that the effect of silicon-reinforced ferrite is obvious. When the silicon content is increased, the tensile strength and hardness are increased. However, the value of tensile strength and hardness of silicon solid solution-strengthened ferrite is still significantly lower than that of pearlite.
 In cast iron, it is beneficial to reduce or eliminate the use of alloying elements such as copper, nickel, tin, molybdenum, chromium, etc., by solid solution strengthening of silicon. However, for a long time. The foundry industry has not fully utilized this potential of silicon.
In the case of gray cast iron, since the action of the flake graphite cutting substrate is large, the strength of the cast iron is not high, and the elongation is generally not required. Although the strength of gray cast iron is improved, mainly by controlling the morphology and quantity of graphite, and reducing the size of the eutectic group, it is also impossible to enhance the matrix structure as much as possible. In addition to low-grade gray cast iron, which requires little demand, it is generally required that the matrix structure be entirely pearlite. In order to obtain a pearlite matrix, the silicon content in the cast iron is of course not too high. Therefore, colleagues in the foundry industry pay little attention to the solid solution strengthening of silicon.
In the case of ductile iron, all grades have stringent requirements for elongation. It can be seen from Table 1 that the amount of solid solution silicon in the pearlite increases, and the elongation decreases accordingly, especially when the silicon content exceeds 3%.
 In addition, similar data can be found in many test reports on the mechanical properties of ductile iron.
 After a long period of time, it has gradually formed the concept that the silicon content in cast iron is too high, which leads to a decrease in ductility and toughness. Therefore, the solid solution strengthening effect of silicon is often not taken seriously. In fact, some test data only consider changes in silicon content, ignoring the influence of other factors, and inadvertently exaggerating the "embrittlement" effect of silicon.
Second, the application of silicon solid solution strengthening
The solid solution strengthening effect of silicon in ductile iron has recently received widespread attention. When it comes to this, we cannot but mention the work done on grey cast iron in China more than 30 years ago.
 1. Solid solution strengthening of silicon in gray cast iron
 Gray cast iron above grade HT250, the matrix structure is pearlite. In order to ensure the strength is achieved, alloying elements such as copper, tin and antimony are usually added to the production.
In pearlite, ferrite accounts for about 90%. If the silicon content in cast iron is properly increased, solid solution strengthening is carried out in ferrite, and ferrite does not appear in cast iron structure. Of course, alloying elements can be saved. It also simplifies the operation.
 Around 1980, Zhong Xueyou and others from the Beijing Iron and Steel Institute (now Beijing University of Science and Technology) conducted research and experimental work in this area. Under the condition that the carbon equivalent of gray cast iron is about 4.05%, the silicon content is appropriately increased (S i / C ratio is about 0.78), and the tensile strength of cast iron can be maintained at 300 MPa or more without adding alloying elements.
 In the 1980s, this process was confirmed at several foundries and used in production.




Table 1
Mechanical properties of ferrite with different silicon contents
























Mechanical properties




Silicon content (%)


Full pearlitic carbon steel


Trace
0.82


2.28


3.40












Tensile strength / MPa
280~290
315


540


645
840


Elongation(%)
60
55.0


50.0


21.0
10.0


Hardness HBW
75
88


124


150
200


2. Solid solution strengthening of silicon in ductile iron
 The production of ductile iron castings, spheroidization rate, number of graphite spheres and average size of graphite spheres are basic quality requirements. Under the condition of normal spheroidization of graphite, the effect of cutting the matrix is greatly reduced compared with that in gray cast iron. By controlling the matrix structure, the mechanical properties of ductile iron can be adjusted to a large extent to meet the requirements of many different working conditions. In addition to austempered ductile iron and high-nickel austenitic ductile iron, conventional spheroidal graphite cast iron has more than ten grades, the tensile strength can be changed between 350 and 900 MPa, and the minimum elongation can be correspondingly 22 Change between % and 2%.
 QT450-10, QT500-7, QT550-5 and QT600-3 grades of ductile iron castings are controlled by the proportion of ferrite and pearlite in the matrix to ensure that the mechanical properties meet the requirements. Generally speaking, when producing such ductile iron castings, it is necessary to control the cast structure of the castings to meet the requirements by controlling the chemical composition of the cast iron and various technological conditions in the production process, so as to avoid time-consuming and energy-consuming heat treatment processes.
 In the case where the process control is insufficient to ensure the strength of the cast iron, it is also a common countermeasure to add a small amount of alloying elements such as copper and nickel. However, doing so not only increases production costs, but also consumes precious resources.
 With the gradual deepening of the understanding of ductile iron, more than a decade ago, Europe began to notice the role of silicon in strengthening ferrite in ductile iron. The Swedish research found that: QT500-7 brand ductile iron, which is widely used, The silicon content is increased to 3.5%, the matrix structure is all ferrite, and the elongation can be improved not only while maintaining the tensile strength at 500 MPa, but more importantly, the hardness of the casting is uniform and the cutting performance is remarkably improved.
On this basis, the international standard ISO1083 "Class of Ductile Iron" was revised in 2004, supplemented with a "high silicon ductile iron"
The grade is JS500-10.
European Standard E N 1563 "Spheroidal Iron Castings" was revised in 2011, supplemented with three "solid solution reinforced ferritic ductile iron" grades, see Table 2.
In 2012, Herbert Löblich of Germany published a research report on the mechanical properties of ferritic ductile iron for silicon solid solution strengthening.
In 2013, the Kyushu University and the Technical Development Department of the Sekisho Machine Co., Ltd. also conducted experimental research.
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