Steel and Alloys Explained: How Carbon and Chromium Work
Steel is iron with a little carbon, and carbon atoms block the crystal defects that let iron bend, which is why steel is so much stronger than pure iron.
Steel is iron with a small amount of carbon, usually less than about 2 percent by weight. An alloy is any metal mixed with one or more other elements to change its properties. Carbon atoms make iron far stronger by getting in the way of the defects that let it bend, and other additions, such as chromium, make steel resist rust.
Why does pure iron bend so easily?
A metal is built from atoms arranged in a regular crystal lattice, with the atoms sitting in neat layers. Engineers describe how a metal responds to force in terms of stress and strain. When a metal is pushed beyond its limit, it deforms permanently because line defects called dislocations move through the lattice.
A dislocation moves a little like a ripple travelling under a rug. The rug shifts a fraction at a time, without the whole sheet being dragged at once. Each small step moves one part of a layer past its neighbor. In pure iron, the layers slide easily, so the metal yields at a low force and stays bent.
How does carbon make steel stronger?
Carbon atoms are much smaller than iron atoms, so they fit into the gaps between iron atoms. Each one distorts the lattice around it, creating an obstacle that a moving dislocation must push against or go around. More obstacles mean more force is needed before the metal yields.
Heat treatment changes how the carbon is arranged. Iron has one crystal structure at room temperature and another above about 900 °C, and the hot structure can dissolve more carbon. If hot steel is cooled quickly in water or oil, the carbon atoms are trapped in place, forming a very hard but brittle structure called martensite. Tempering reheats the hardened steel to a moderate temperature. This lets the structure relax, reducing brittleness at a small cost in hardness. Higher-carbon steel can be hardened more, while low-carbon steel bends further before it breaks.
What other elements are added to metals?
Alloying is an old craft. Bronze, made from copper and tin, gave its name to the Bronze Age, and brass, made from copper and zinc, was known in antiquity. Modern alloys include aluminum alloys for aircraft and titanium alloys for jet engines. The main alloys in everyday use include:
| Alloy | Main ingredients | Typical uses |
|---|---|---|
| Carbon steel | Iron with up to about 2 percent carbon | Beams, car bodies, railway rails |
| Stainless steel | Iron with at least about 10.5 percent chromium, often with nickel | Cutlery, surgical instruments, kitchen equipment, chemical plants |
| Bronze | Copper with tin | Bells, statues, bearings |
| Brass | Copper with zinc | Musical instruments, plumbing fittings, ammunition cases |
How is steel made from iron ore?
Iron ore is mostly iron oxide. In a blast furnace, the ore is heated with coke, a carbon-rich fuel made from coal. Carbon monoxide produced from the coke strips oxygen from the ore, a reduction reaction, leaving molten iron that holds about 4 percent carbon. This product, called pig iron, is brittle, so most of it is refined further.
In a basic oxygen furnace, oxygen is blown through the molten iron to burn off most of the excess carbon and other impurities. The desired alloying elements are then added. Electric arc furnaces make steel mainly from recycled scrap, melting it with powerful electric arcs.
Why doesn't stainless steel rust?
Chromium is a metal that reacts readily with oxygen, as its place in the periodic table would suggest. Chromium atoms bond with oxygen at the surface and form a very thin, tough layer of chromium oxide. This layer is passive: it blocks oxygen from reaching the iron beneath, and if it is scratched, it re-forms as long as oxygen is present. Stainless steel is not rust-proof. In salty or chloride-rich environments, such as seawater or road salt, the protective layer can break down at small points, causing pitting corrosion. Different grades resist this to different degrees, so the right choice depends on where the steel will be used.
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chemistry engineering manufacturing materials materials science
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