Answer: a specific percentage of carbon. Steel is ubiquitous in our lives: it’s a critical component in suspension bridges, train tracks, building reinforcements, automobile bodies, cables, machine components, hammer heads and kitchen utensils. It’s also deeply anchored in our culture. Its reputation for solidity can be found in our everyday language: you can have nerves of steel, give a steely stare, have a mind like a steel trap, or steel yourself to face something difficult. The hardness and strength of this metal alloy, of which iron is the main component, depend on very little, however – a very specific percentage of carbon in the mix, between 0.008% and 2%. Most steels are between 0.1% and 1.5% carbon. At less than 0.008 % carbon, the material is more ductile, and is referred to simply as “iron.” In quantities above 2% carbon, more carbides and graphite are produced, which make the metal more brittle. These are known as “cast iron,” and their low melting points (lower than pure iron which is 1536°C), make them easier to cast into molds. Between these two percentages, the hardness and strength of the alloy can vary across a wide range, from a soft or semi-soft steel to a mid-, high-, or extra-high tensile strength steel. “Steel is on of the world’s most complex alloys and has many varieties; a list could fill an entire volume,” says professor Andreas Mortensen, head of EPFL’s Mechanical Metallurgy Laboratory. In addition to carbon, other components can be included to confer certain properties. Iron rusts easily; thus it’s often necessary to make the alloy resistant to corrosion. Adding more than 12% chromium yields a protective layer that makes the alloy “stainless.” Adding at least 7% nickel also improves rust resistance. Innumerable possibilities With the addition of a bit of manganese, sulfur, which would otherwise degrade the steel, is neutralized. With chromium and molybdenum, the metal can be “tempered,” or made very hard when heated and then quickly cooled in water. In this process it turns into “martensite,” a very hard iron-carbon alloy. Finally, multiphasic steel can be treated or exposed to specific chemical environments in order to be locally enriched with particular alloy elements (carbon, nitrogen, boron), yielding steel that is very hard on the surface, but with a very strong core. Because of Iron’s high density, aluminum or other lighter alloys are considered better in certain applications, particularly in automobiles and transportation in general. It’s a reliable, inexpensive material because the raw material from which it is made is abundant – iron makes up 5% of the Earth’s crust – and because it has excellent mechanical properties. “This material is going to be with us for a long time; the innumerable possible alloy combinations continue to provide very interesting opportunities for new research,” Mortensen concludes.