High-Temperature Mechanical Properties of Gray Cast Iron, Chilled Cast Iron, and Ductile Cast Iron
Gray cast iron, chilled cast iron and ductile cast iron have all become the most-preferred engineering materials used in a variety of applications in various industrial fields. This is due to their number of advantageous properties, including relatively high melting point, low cost, rust-resistance, wear-resistance, excellent thermal conductivity and good machinability. In this essay, particularly for its application in automobile industry, the high-temperature mechanical properties of the three different materials will be discussed and compared.
Gray cast iron, typically made from pig iron, is formed from the rapid chill of the molten iron poured into the mold. Microscopy observations indicate that the microstructure of gray cast iron is composed of lamellar Eutectic mixture of crystalline phase Graphite and ferrite. Gray cast iron consequently has both good machinability and castability with good wear-resistance and damping behaviour. In terms of its strength at elevated temperature, gray cast iron has shown good tensile and compressive strength, but it has a limited thermal conductivity, leading to possible cracking and spalling during thermal cycling.
Chilled cast iron is made using the same basic material as gray cast iron, however, it is produced by a process of cooling the molten metal slowly. Due to the slower chilling process, the microstructure of chilled cast iron is composed of higher and more uniform concentration of the Graphite lamellar structure, while small and uniformly distributed perlite and pearlite are also found. The higher Graphite concentration promotes better machinability and corrosion resistance, however, it has a lower tensile strength and hardness than gray cast iron. When subjected to high-temperature, chilled cast iron has relatively good thermal stability, but still inferior to gray cast iron.
Lastly, ductile cast iron is produced by a process of adding nodularisers (e.g. magnesium) to the molten iron, eventually forming a graphite-structured material. This material has a vastly improved tensile strength, elongation and improved machinability. In terms of the high-temperature properties, the graphite structure is stable at elevated temperatures, resulting in a material that is capable of withstanding thermal cycling while maintaining its ductility.
Compared to gray cast iron, the ductile cast iron is a more preferred material for applications in machines and engines due to its improved thermal stability, machinability and high-temperature strength. However, the relatively high cost of production and the potential for porosity makes ductile cast iron not suitable for some applications. Chilled cast iron has a compromise between gray and ductile cast iron as it has good machinability and better corrosion resistance than gray cast iron, with still limited thermal stability and strength.
Overall, gray cast iron, chilled cast iron and ductile cast iron, each have their advantages and disadvantages relating to their high-temperature mechanical properties. Gray cast iron is suitable for applications that do not require high-temperature specifications, such as the automotive industry components. Chilled cast iron, with its superior corrosion-resistance property, can be used when mechanical performance is more important than thermal cycling stability. Lastly, ductile cast iron is suitable for applications that need to withstand extreme thermal cycling while still retaining their operating mechanical properties.
