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What is steel?
Types of steel
Steels are a large family of metals. All of them are alloys in which iron is mixed with carbon and other elements. Steels are described as mild, medium- or high-carbon steels according to the percentage of carbon they contain, although this is never greater than about 1.5%.

Type of steelPercentage of carbon
Mild steelUp to 0.25%
Medium carbon steel0.25% to 0.45%
High carbon steel0.45% to 1.50%
Table 1 Carbon steels

Picture 1. Roll your cursor up the scale to see the percentage carbon content of some items.
Adding metals such as nickel, chromium, and tungsten to iron produces a wide range of alloy steels, including stainless steel and high speed steels.
Properties of steels
The properties of a steel are closely linked to its composition. For example, there is a big difference in hardness between the steel in a drinks can and the steel that is used to make a pair of scissors. The metal in the scissors contains nearly twenty times as much carbon and is many times harder. Notice how the percentage of carbon in the steel items in Picture 1 varies. Changing the carbon content changes the properties of the steel and the way that it is used.
Heat treatment
The heat treatment given to a steel can affect its properties too. Cooling a red-hot tool steel rapidly in cold water makes it harder and more brittle. We could have made the same piece of metal softer by keeping it at red heat for longer and then cooling it slowly. Heat treatment is another method that the steelmaker uses to make the properties of the steel match the job it has to do.
Question 8

Look again at Picture 1. Besides hardness, in what other ways does changing the carbon content affect the properties of the steel?