Pig iron usually refers to an iron alloy with a carbon content of 2% to 4.3%, also known as cast iron. In addition to carbon, pig iron also contains silicon, manganese, and small amounts of sulfur and phosphorus. Pig iron cannot be forged, but it can be cast. According to the different forms of carbon, pig iron fall into steelmaking pig iron, cast pig iron and ductile iron.
1. Steelmaking pig iron
The carbon in steel-making pig iron exists in the form of carbides and has a white cross-section, usually called white iron. Steelmaking pig iron is hard and fragile, so it is usually used as a raw material for cast steel.
2. Cast pig iron
The form of carbon in cast pig iron is graphite flakes, which break into gray. Thus usually called gray iron. Because graphite is soft and has lubricity, it has good cutting, wear and casting properties. However, its anti-bit strength is insufficient. Therefore it cannot be forged. Gray iron is only used to make castings, such as casting various machine tool beds, iron pipes, etc.
3. Ductile iron
The carbon in spheroidal graphite cast iron exists in the form of spherical graphite. Meanwhile, its mechanical properties are far superior to gray cast iron, and are close to cast steel. It has excellent casting, machining and wear resistance, and has a certain degree of flexibility. Therefore, ductile cast iron has been widely used in the manufacture of crankshafts, gears, pistons and other high-grade castings as well as various mechanical parts.
In addition to the above, there are some iron alloys, which contain a lot of silicon, manganese, nickel or other elements, such as ferrosilicon, ferromanganese and so on. These ferroalloys are commonly used as raw materials for cast steel. Using these alloys, cast steel can obtain better physical properties.
Cast iron with a carbon content of less than 0.2% is called wrought iron or pure iron. Cast iron with a carbon content of 0.2-1.7% is called cast steel. Therefore, cast steel is a special kind of cast iron. The carbon content of more than 2% is called pig iron. Wrought iron is very soft, plastic and easily deformed. However, its strength and hardness are low, so it has not been widely used.