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Steel is an alloy of __________.


Really tall houses are called skyscrapers. Skyscrapers, a bit like we humans, have a kind of skeleton which supports all the floors. However, the skyscraper's skeleton is not made of bone, but of another material: steel. What is steel? And why is it such a useful material?

Steel is a mixture of different substances: an alloy. An alloy always consists of at least one metal. Steel always contains the metal iron, and always has a little carbon, a non-metal. The amount of carbon in steel, the carbon content, varies between 0,15 and 2,0 percent. The carbon content determines the steel’s properties.

High carbon content gives a harder steel. This can be used to manufacture many types of cutting tools. Low carbon content gives a steel that is softer and easier to bend and shape without splitting. Low carbon steel is easy to stretch out, press, and weld: it is more malleable. This steel can be used to make steel wire, sheet metal, and girders to use in a skeleton for bridges and houses.

The main ingredient of steel, the metal iron, turns brown and decays when it comes into contact with oxygen and water: it rusts. To prevent steel from rusting, and becoming unusable, the metal chromium is added. Chromium creates a protective surface on the steel. The result is: stainless steel. This can be used to make, for example: stoves, saucepans, and cutlery.

The pharmaceutical and food industries use stainless steel because they need materials that do not contaminate medicine or food. To produce a steel that can withstand high temperatures, the metal tungsten is added. This kind of steel is used for cutting and drilling tools, which often get very hot. Iron, the main ingredient of steel, is magnetic. But if a steel with even better magnetic properties is needed, the metal cobalt is mixed in.

Steel which contains cobalt is therefore suitable for making magnets. Some steel must be soft and malleable, but also tough and hard wearing. For this, the steel is heated to a temperature between 750 and 950 degrees Celsius. The carbon then blends more evenly with the iron in the steel. Then the steel is cooled rapidly in water or oil.

This produces a harder steel. The process is called hardening the steel. But rapid cooling makes some areas contract faster than others, creating tension within the steel: thermal stress. The steel becomes brittle. So it is heated again, but to a slightly lower temperature: between 150 and 650 degrees Celsius.

The steel is tempered. This way, the iron and carbon in the steel can move around a little, allowing the stress in the steel to be relieved. The steel is now hard and durable, while also being malleable. Hard wearing steel, which is also elastic, can withstand being bent by forces such as strong winds. So it'™s a perfect material for building really tall houses.

Such as skyscrapers.