At present, ferrite and pearlite matrix castings account for more than 90% of the world’s annual output of 18 million tons of nodular cast iron. At present, most grades of ductile iron at home and abroad can be realized in as cast production. 80% of ductile iron castings produced by foreign automobile factories are produced as cast. Considering energy, environment and economy, it is imperative to produce as cast nodular iron castings.
In order to make the as cast nodular cast iron meet the material requirements of the service performance of the crankshaft, the process conditions and the chemical composition of the casting must be strictly controlled. Ensure that the castings have stable metallographic structure and chemical composition. This requires strict control of spheroidization, inoculation, solidification and other processes. At the same time, a series of practical technical measures are taken in various production links such as raw materials, casting process, smelting equipment, chemical composition inspection, metallographic structure analysis, spheroidization rate inspection and defect inspection, and a strict production management system is formulated.
In order to ensure the strength of as cast nodular cast iron, pearlescent volume is usually increased. The matrix structure of as cast pearlite is more difficult to form than the mixed matrix structure of as cast pearlite and ferrite. Pearlite matrix structure is realized by adjusting the chemical composition of raw molten iron, such as P content less than 0.06%; The total amount of interfering spheroidizing elements Pb, Sb, Bi and as is less than 0.01%: the content of Ti is less than 0.06%; Carbide elements V, Cr and B are strongly formed, and the total amount is less than 0.1%. Generally, the final silicon amount needs to be adjusted between 2.2 and 2.5. Cu, Sn and other alloy elements are added to improve the as cast microstructure and strength. Considering the cost reduction, the addition of alloying elements should be reduced or simplified as much as possible in the production of as cast pearlite matrix ductile iron.
As cast nodular cast iron crankshaft has a considerable cost advantage instead of forged steel crankshaft. The process does not need normalizing heat treatment, simplifies the production process, reduces energy consumption and production cost. In order to replace forged steel crankshaft, nodular cast iron crankshaft needs to ensure sufficient toughness and fatigue strength. During production, certain measures shall be taken to strengthen the fine grain; Refine the microstructure; Reduce the content of harmful inclusions; Spheroidization or passivation treatment to disperse weak phases (such as graphite) to reduce the stress concentration factor. In addition, the properties can be improved by generating or introducing discontinuous structures with good toughness (such as residual austenite in ADI ductile iron).
High strength as cast nodular cast iron crankshaft is mainly pearlite matrix. In order to ensure sufficient pearlescent volume, in addition to adjusting the chemical composition of raw hot metal, such as adjusting the amount of final Si and adding alloy elements such as Cu and Sn, appropriate inoculation treatment is also required to strengthen metallographic inspection; On the other hand, the cooling time can be controlled accurately, and the pearlite content can also be improved through normalizing process.