Iron and Steel
One element, and what a little carbon does to it.
Iron is the most used metal on Earth. Steel is iron alloyed with a small, carefully controlled amount of carbon. The difference between useful steel and brittle cast iron is a fraction of a percent of carbon, which is why steelmaking is fundamentally about controlling that one number.
Carbon content decides the material
| Material | Carbon | Character |
|---|---|---|
| Wrought iron | under 0.08% | Soft, tough, easily forged, corrosion-resistant |
| Mild steel | 0.05–0.25% | Structural workhorse; weldable |
| High-carbon steel | 0.6–1.5% | Hard, holds an edge, brittle. Tools and blades |
| Cast iron | 2–4% | Hard, excellent in compression, cracks under tension |
Smelting
Iron ore is mostly iron oxide. A blast furnace charged with ore, coke and limestone reduces it: the coke burns to carbon monoxide, which strips oxygen from the ore, while limestone combines with impurities to form slag that floats off. The product, pig iron, is high in carbon and brittle.
Making steel
Steelmaking is decarburising — blowing oxygen through molten pig iron to burn carbon out, then adding back a measured amount. The Bessemer process made this cheap in the 1850s and is a reasonable candidate for the single most consequential industrial development of the century; the basic oxygen furnace does the same job today.
Heat treatment and alloys
Quenching hot steel makes it hard and brittle; tempering — reheating gently — trades some hardness back for toughness. Adding chromium above about 11% forms a self-repairing oxide film, which is stainless steel.