Iron and carbon are the most abundant materials in steel. Pure iron isn't particularly strong or hard on its own, so adding carbon helps give steel tremendous strength.

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

However, iron can be further processed to reduce carbon. This manipulation of carbon changes a variety of material properties, including:

Strength: The load a material can withstand, 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 required to actually break the material.

Ductility: The amount a material can stretch without becoming brittle. Ductility is measured by elongation, which is the percentage increase in length of a material before it breaks.

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

The carbon content in steel is usually reduced, so it can be divided into three main categories of carbon steel: low carbon steel (or low carbon steel), medium carbon steel and high carbon steel. Each category contains different levels of carbon, as shown in the image below.