Cast iron manufacture and properties

Cast iron is an iron-carbon alloy containing 2–4.5% carbon (plus Si, Mn, S, P), made by remelting pig iron. It is hard and brittle, and it is cast into shape because it cannot be forged or rolled.

Manufacture

Pig iron is first produced in a blast furnace from iron ore, coke (fuel and reducing agent) and limestone (flux). The pig iron is then remelted and refined into cast iron in a cupola furnace.

1. Blast furnace: A charge of ore, coke and limestone is fed from the top, and a hot air blast enters near the bottom. Coke burns to give CO, which reduces the ore ($Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$). Limestone combines with silica impurities to form slag. Molten iron collects at the bottom and is tapped as pig iron.
2. Cupola furnace: A vertical cylindrical furnace lined with fire bricks. Pig iron, scrap iron, coke and limestone are charged in layers. An air blast through tuyeres burns the coke and melts the iron. The molten metal is tapped from the bottom, poured into moulds, and cooled to get castings.

   Charge (pig iron+scrap+coke+flux)
            |
        +---v---+
        |       |  <- Cupola
  Air ->|=======|  <- tuyeres
        |       |
        +---+---+
            |  Molten iron -> Mould
            v  Slag tapped separately

Types

Grey cast iron (flake graphite), white cast iron (carbon as cementite), malleable cast iron (annealed white iron), and nodular (SG) cast iron.

Properties

• Carbon content 2–4.5%; melting point about 1200 °C (lower than steel).
• Very high compressive strength (about 600–1000 N/mm²), but low tensile strength (about 150–250 N/mm²).
• Hard and brittle; fails suddenly with little elongation.
• Good fluidity when molten, so complex shapes can be cast.
• Cannot be forged or rolled and is difficult to weld; grey iron can be machined.
• Good wear resistance and vibration damping.
• Resists corrosion better than mild steel; specific gravity about 7.2–7.5.

Uses: pipes, manhole covers, columns, brackets, drain fittings, machine beds and railings.