The role of silicon in casting

Entering the Iron Age is a milestone in the rapid development of human civilization. The production and application of iron castings promoted the early industrial revolution and promoted the progress of science and technology. Up to now, "casting" is still the foundation of the manufacturing industry, but the development of all walks of life has in turn stimulated the foundry industry and brought it into a new era of modernization.
In recent years, in order to meet the requirements of many aspects, various new processes and new materials have continuously emerged, and the application of light alloy castings and steel castings has also developed rapidly, but the demand for iron castings still ranks first. In 2012, the total output of various castings in the world was 100.83 million tons, of which, gray iron castings were 45.996 million tons, accounting for 45.6%, ductile iron castings were 25.167 million tons, accounting for 24.9%, and malleable iron castings were 1.275 million tons, accounting for 1.3. %. In other words, gray iron castings and ductile iron castings account for more than 70% of the total output of various castings in the world.
Although since entering the Iron Age more than 3,000 years ago, foundry colleagues cannot do without silicon. Humankind’s understanding of silicon has been continuously deepened with the accumulation of experience and the advancement of science and technology; however, today, we are in the cast iron with silicon. The cognition is not enough, and there is still plenty of room for further exploration.

1. The role of silicon in promoting graphitization in cast iron
In cast iron, silicon is a strong alloying element that promotes graphitization, and its ability to promote graphitization is 3 times that of nickel and 5 times that of copper. And whether in liquid or solid cast iron, the combination of silicon and iron is stronger than carbon.
Liquid cast iron contains silicon, which reduces the solubility of carbon. The higher the silicon content in the molten iron, the lower the carbon content correspondingly, and the more carbon will be squeezed out.
It is not difficult to understand from the iron-carbon phase diagram: when the molten iron is hypereutectic, 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. Eutectic transformation occurs; when the molten iron is hypoeutectic, silicon is enriched in primary austenite during solidification. During the eutectic transformation, silicon is enriched in the early crystalline eutectic austenite, inhibiting the formation of cementite from carbon and iron, enhancing the diffusion rate of carbon in the 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, spheroidal graphite cast iron, vermicular graphite cast iron and black core malleable cast iron, carbon and silicon are the main elements that affect the shape and quantity of graphite. It is a white core malleable cast iron that does not contain graphite. During the decarburization annealing process, silicon promotes the diffusion of carbon in austenite, which also plays an important role in the decarburization of this malleable cast iron.
In addition, the oxygen and nitrogen in cast iron have the effect of stabilizing carbides. The silicon contained in cast iron can reduce the oxygen and nitrogen content in it, thus indirectly enhancing the effect of silicon on graphitization.

2. The solid solution strengthening effect of silicon in ferrite
In solid cast iron, almost all silicon dissolves in austenite and ferrite, and does not enter carbides. Silicon atoms and iron atoms can be combined into silicon-containing ferrite with strong covalent bonds, which not only promotes the formation of ferrite, but also strengthens the ferrite.
Table 1 Mechanical properties of ferrite with different silicon content
In order to understand the ability of silicon to strengthen ferrite, in the 1950s, foreign researchers added different amounts of silicon to steel with a carbon content of 0.1% and no other alloying elements to compare the effect of silicon on mechanical properties. The results are shown in Table 1. Table 1 also lists the properties of carbon steel whose structure is all pearlite and does not contain other alloying elements for comparison.
It can be seen from Table 1 that the role of silicon strengthening ferrite is obvious. As the silicon content increases, the tensile strength and hardness will increase accordingly. However, the value of tensile strength and hardness of ferrite strengthened by silicon solid solution is still significantly lower than that of pearlite.
In the production of cast iron, the solid solution strengthening effect of silicon can be used to reduce or eliminate the strength-enhancing alloy elements such as copper, nickel, tin, molybdenum, and chromium, which is beneficial to reduce production costs and avoid the negative effects of alloy elements.
For a long time, our foundry colleagues have not fully realized and effectively utilized this potential of silicon.
For gray cast iron, due to the great effect of flake graphite in cutting the matrix, the strength of cast iron is not high, and the elongation is generally not required; moreover, except for the low-grade gray cast iron with low demand, the matrix structure is generally required. All are pearlite. In order to obtain the pearlite matrix, the silicon content in cast iron should not be too high. Therefore, the casting colleagues seldom notice the solid solution strengthening effect of silicon. Although increasing the strength of gray cast iron mainly depends on controlling the shape and quantity of graphite and reducing the size of the eutectic cluster, the effect of strengthening the matrix structure cannot be underestimated.
For ductile iron, all grades have strict requirements on elongation. It can be seen from Table 1 that the amount of solid-dissolved silicon in pearlite increases, and the elongation decreases accordingly, especially when the silicon content exceeds 3%. In addition, similar data can be seen in many test reports on the mechanical properties of ductile iron.
As a result, the concept of "too high silicon content in cast iron will lead to a decrease in ductility and toughness" has gradually formed. In fact, some test data only consider the change of silicon content, ignoring the influence of other factors, and inadvertently exaggerating the "embrittle" effect of silicon.

3. Other roles of silicon in cast iron
The role of silicon in cast iron is multifaceted. In addition to "promoting graphitization" and "solid solution strengthening", silicon also has many important functions. Here, I will briefly introduce two:
1) Silicon dissolved in liquid cast iron greatly enhances the oxidation resistance of molten iron, and silicon can also reduce the solubility of nitrogen in molten iron. Among various casting alloys, only cast iron can be smelted in an oxygen-rich and nitrogen-rich atmosphere with melting equipment such as cupolas and oxygen rotary furnaces. It is precisely because of this effect of silicon.
2) Increasing the silicon content in cast iron to more than 3.5%, the oxidation resistance and thermal growth resistance of cast iron are greatly improved. In the early days, there were silicon-based heat-resistant cast iron grades in the standards of heat-resistant cast iron in various countries. In recent years, due to the consideration of energy saving, various internal combustion engines have increased the temperature of exhaust gas. In the automobile industry of various countries, the application of heat-resistant silicon-molybdenum ductile iron castings has been attached great importance.

4. Application of solid solution strengthening effect of silicon
In recent years, the solid solution strengthening effect of silicon in nodular cast iron has gradually received widespread attention, but in fact, my country has done certain research and application in the solid solution strengthening of gray cast iron more than 30 years ago.
1) The solid solution strengthening effect of silicon in gray cast iron
The matrix structure of gray cast iron above HT250 is pearlite. In order to ensure that the strength is up to standard, alloy elements such as copper, tin, and antimony are usually added in the production. Pearlite is sandwiched between ferrite and cementite, of which ferrite accounts for about 90%; if the silicon content in cast iron is appropriately increased, it can play a solid solution strengthening effect in ferrite , Without the appearance of pure ferrite structure, of course, alloying elements can be saved, and the operation is also simplified.
Around 1980, Zhong Xueyou and others from Beijing Institute of Iron and Steel (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 (Si/C ratio is about 0.78), without alloying elements, the tensile strength of cast iron can be maintained above 300 MPa. In the 1980s, this process was confirmed in many foundries and applied in production.
2) The solid solution strengthening effect of silicon in ductile iron
When producing ductile iron parts, the spheroidization rate, the number of graphite balls and the average size of graphite balls are the basic quality requirements. Under normal conditions of graphite spheroidization, the effect of cutting the matrix is ​​greatly reduced than in gray cast iron. By controlling the matrix structure, the mechanical properties of ductile iron can be adjusted in a wide range to meet the requirements of a variety of different working conditions. Except for austempered ductile iron and high nickel austenitic ductile iron, there are currently more than 10 grades of conventional ductile iron. The tensile strength can be changed between 350 and 900 MPa, and the low elongation can be correspondingly 22. %～2%.
QT450-10, QT500-7, QT550-5 and QT600-3 grades of ductile iron castings are all achieved by controlling the proportion of ferrite and pearlite in the matrix structure to ensure that the mechanical properties meet the requirements. Generally speaking, when producing this kind of ductile iron castings, it is necessary to control the chemical composition of the cast iron and various technological conditions in the production process to make the as-cast structure of the casting meet the requirements to avoid time-consuming and energy-consuming heat treatment processes.
When the process control is not enough to ensure the strength of cast iron, adding a small amount of alloying elements such as copper and nickel is also a common countermeasure. However, doing so will not only increase the production cost, but also consume precious resources.
With the gradual deepening of the understanding of ductile cast iron, more than ten years ago, Europe began to notice the role of silicon in strengthening ferrite in ductile cast iron. Swedish research found that: The silicon content is increased to 3.5%, and the matrix structure is all ferrite. Not only can the elongation be increased while maintaining the tensile strength at 500MPa, but more importantly, the hardness of the casting is uniform and the cutting performance is significantly improved.
On this basis, when the international standard ISO 1083 "Classification of Ductile Iron" was revised in 2004, it added a grade of "High Silicon Ductile Iron" JS500-10.
In 2012, Herbert Löblich of Germany published a research report on the mechanical properties of ferritic ductile iron strengthened by silicon solid solution. In 2013, Kyushu University in Japan and the Technology Development Department of Hinode Waterway Machinery Co., Ltd. also conducted experimental research on this.

5. Conclusion
Silicon is a rich element in the earth's crust, and there is no danger of being scarce. In addition, in various cast irons, silicon is one of the main constituent elements. It has influence on the shape and quantity of graphite in the cast iron structure and the formation of the matrix structure. Very important role.
In recent years, in order to follow the concept of sustainable development, in addition to increasing requirements for the functions of iron castings, there are also increased requirements for light weight, low cost, energy saving and emission reduction, and resource conservation. Therefore, the foundry industry in various countries attaches great importance to the research and development of improving the quality of cast iron, especially the research and application of the role and mechanism of silicon in cast iron, which need to be further explored by casting colleagues.