Pig iron
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- For the DC Comics superhero Pig Iron, see Peter Porkchops.
Pig iron is raw iron, the immediate product of smelting iron ore with coke and limestone in a blast furnace. Pig iron has a very high carbon content, typically 4-5%, which makes it very brittle and not very useful directly as a material.
Pig iron is typically poured directly out of the bottom of the blast furnace into pots to form ingots. The ingots are then used to produce wrought iron or steel, typically with a Bessemer converter or basic oxygen furnace, by burning off the excess carbon in a controlled fashion. Earlier processes for this included the finery forge and the puddling furnace.
The traditional shape of the molds used for these ingots was a branching structure, formed in sand, with many individual ingots at right angles to a central channel or runner, bearing some similarity in appearance to a litter of piglets suckling on a sow. When the metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the much thinner runner (the sow), hence the name pig iron. As pig iron is intended for remelting, the uneven size of the ingots and inclusion of a little sand was unimportant compared to the ease of casting and of handling. Modern steel mills, equipped as they are with large cranes, either cast large single ingots or transfer the molten iron to a ladle for immediate use as molten metal.
Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron, and taking various steps to remove undesirable contaminants and adjust the carbon content.
The Chinese were making pig iron by the later Zhou Dynasty (1122 BC - 256 BC). In Europe, the process did not become common until the 14th century.