ANS:C - cast iron

Cast iron is a group of iron-carbon alloys with a carbon content greater than 2%. It is characterized by its relatively low melting point, good fluidity when molten, excellent castability, and high compressive strength. Cast iron is produced by melting iron along with a specified amount of carbon and other alloying elements in a furnace, such as a cupola furnace or an electric induction furnace, and then pouring the molten metal into molds where it solidifies. There are several types of cast iron, each with unique properties and applications:

1. Gray Cast Iron: This is the most common type of cast iron. It contains graphite flakes dispersed throughout the matrix of ferrite and pearlite. Gray cast iron is known for its excellent machinability, damping capacity, and good wear resistance. It finds applications in engine blocks, pipe fittings, cookware, and machinery parts.
2. White Cast Iron: White cast iron is formed when carbon in the molten iron is in the form of cementite (iron carbide). It has a white, crystalline fracture surface due to the presence of cementite. White cast iron is hard, brittle, and difficult to machine. It is often used in applications requiring abrasion resistance, such as in grinding balls and liners for grinding mills.
3. Ductile Cast Iron (Nodular Cast Iron): Ductile cast iron contains nodules of graphite dispersed throughout its matrix, which gives it improved ductility and toughness compared to gray cast iron. This type of cast iron is achieved by adding magnesium or other nodularizing agents to the molten metal. Ductile cast iron is used in applications where strength, toughness, and resistance to shock and vibration are required, such as in automotive components, gears, and heavy machinery.
4. Malleable Cast Iron: Malleable cast iron is produced by heat treating white cast iron, causing it to undergo annealing and graphitization. This process converts the brittle white iron into a more ductile and malleable form. Malleable cast iron has good machinability and can be easily formed into complex shapes. It is used in applications such as pipe fittings, agricultural equipment, and decorative ironwork.

Cast iron is valued for its versatility, cost-effectiveness, and a wide range of desirable properties, making it a popular material in various industries. However, its brittleness and susceptibility to cracking under tensile stress limit its use in applications requiring high tensile strength.