Pig iron is converted into cast iron through a process called "remelting" or "re-melting." Pig iron is the product of the first stage of iron smelting and contains a high carbon content, along with impurities such as silicon, sulfur, manganese, and phosphorus. It is not directly suitable for many applications due to its brittleness and high carbon content.
To convert pig iron into cast iron, the pig iron is re-melted in a furnace and mixed with certain materials to adjust its carbon content and remove impurities. The process involves the following steps:
Charging the furnace: The pig iron, along with recycled scrap iron and steel, is charged into a furnace. Sometimes, additional materials like limestone, coke, or anthracite coal may also be added to the mixture.
Melting: The furnace is heated to high temperatures, usually between 2,600°F to 2,800°F (1,427°C to 1,538°C). The heat causes the pig iron and other materials to melt and form a molten pool.
Adjusting carbon content: The desired carbon content for cast iron is typically between 2% to 4%. To achieve this, specific amounts of steel scrap or other materials with lower carbon content are added to the molten pool of pig iron. This process is known as "adjusting the charge."
Removing impurities: During the remelting process, impurities like silicon, sulfur, manganese, and phosphorus are oxidized and rise to the surface as slag. The slag is then skimmed off the top of the molten pool, leaving a cleaner cast iron with reduced impurities.
Alloying: Depending on the desired properties of the cast iron, certain alloying elements like nickel, chromium, or molybdenum may be added at this stage to enhance specific characteristics of the final cast iron.
Casting: Once the desired carbon content and composition are achieved, the molten cast iron is poured into molds to create various cast iron products.
The casting process allows the molten cast iron to solidify in the shape of the mold. After cooling, the cast iron products can be removed from the molds and undergo further processing, such as machining or surface finishing, as needed.
It's important to note that while pig iron is the initial product of iron smelting and contains a high carbon content, cast iron is a refined version of pig iron with adjusted carbon content and reduced impurities, making it more suitable for various industrial and manufacturing applications.