Which one of the following cast irons consists of carbon in rosette form?

• A. White cast iron
• B. Gray cast iron
• C. Malleable cast iron
• D. Nodular cast iron

Answer: (D)

The correct choice refers to nodular cast iron, also known as ductile iron. This type of cast iron is characterized by the presence of carbon in the form of spherical or nodular shapes, rather than as flakes or in other structural forms found in other types of cast iron. The nodular form of carbon significantly enhances the mechanical properties of the iron, leading to improved ductility and toughness compared to gray cast iron and white cast iron.

In contrast, gray cast iron has carbon predominantly in the form of flakes, which provides good machinability but lower tensile strength. White cast iron, on the other hand, features carbon present in a combined form, which can result in a hard and brittle structure. Malleable cast iron is a type of cast iron that is produced by heat treating white cast iron to convert the carbon into clusters of graphite, but it does not exhibit the same spherical nodular structure found in nodular cast iron. Thus, nodular cast iron stands out due to its unique microstructure, offering desirable mechanical properties for various applications.