In the industry, it is customary to determine whether gray ductile iron castings are qualified based on their tensile strength. However, the tensile strength of gray ductile iron castings is determined by the carbon content in addition to the basic chemical composition being qualified. As the carbon content increases, the tensile strength will decrease accordingly.
From the basic characteristics of gray ductile iron castings, it can be seen that if a small amount of carbon is added to their raw materials, the tensile strength can be improved. Therefore, in the carbon content of gray iron castings, it is advisable to achieve a carbon content of 2.6% to 3.6% and a silicon content of 1.2% to 33.0%. It is necessary to add some manganese appropriately while determining the wall thickness of gray iron castings.
Meanwhile, alloying methods can also be used to improve the tensile strength of gray ductile iron castings; Alternatively, adding an appropriate amount of trace alloys, such as chromium, molybdenum, tin, etc., is also one of the ways to improve the tensile strength of gray iron castings, but corresponding iron inoculation treatment should be done well.
It should be noted that carbon and silicon affect the strength of gray ductile iron castings in different ways. With a high carbon content, increasing the amount of graphite promotes the coarsening of graphite sheets, while the amount of austenite dendrites is relatively reduced, resulting in a decrease in tensile strength. So, while keeping the carbon equivalent constant, increasing the silicon content results in a relative decrease in carbon equivalent, a corresponding decrease in graphite content, and an increase in the amount of primary austenite. Silicon incorporated into ferrite has a strengthening effect, which is beneficial for improving strength.
However, silicon can increase the eutectoid transition temperature and coarsen the pearlite. Overall, increasing the silicon to carbon ratio when the carbon equivalent is low, or appropriately reducing the silicon to carbon ratio when the carbon equivalent is high, can promote the improvement of the tensile strength of gray ductile iron castings. However, a low silicon to carbon ratio can easily cause white spots in the castings, which requires inoculation treatment.
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