Iron and carbon are the most abundant materials present in steel. Pure iron is not particularly strong or hard on its own, so it is the addition of carbon that helps give steel its great strength.

The crude iron used to produce steel has a relatively high amount of carbon. Its carbon composition can be as high as 2.1%, which is the greatest amount of carbon a material can contain and still be considered steel.

However, iron can be processed further to reduce carbon. This manipulation of carbon alters several material properties, including:

Strength: The load a material can bear, measured by yield point and tensile strength. Yield point is the point at which a material deforms, but does not break, and tensile strength is the amount of stress needed to actually break a material.

Ductility: The amount a material can be stretched without becoming brittle. Ductility is measured by elongation, which is the percent the length of a material increases before it breaks.

Hardness: The wear resistance of material and machinability of material.  This is usually measured on the Rockwell hardness scale or Brinell Hardness scale.

The carbon present in steel is typically reduced so that it fits into three main categories of carbon steel: low (or mild), medium and high carbon steel. Each of these categories contain different levels of carbon, show in the chart below.