• Low carbon Steel: Composed of mainly iron and from .1 to .3% carbon, this steel cannot be heat-treated but is easily welded.
  
• Medium carbon Steel: Composed of mainly iron and from .3 to .6% carbon, this steel can be heat-treated and gain toughness and hardness.
  
• High carbon Steel: Composed of mainly iron and from .6 to 1.7% carbon. The steel that contains from .6 to 1% carbon is the unalloyed steel that is often heat-treated. Steel that has a carbon content greater than 1% is seldom used.

Alloy steels that can be heat treated are:

• Tool Steels, which contain up to 1.3% of carbon
  
• Alloy Steels, which contain a few percent of alloying elements that enhance their heat-treat ability.
  

The table (right) shows annealed, maximum, and minimum values of tensile strength (in pounds per square inch) that a selected few materials can be heat treated to

Low Carbon (1020)

Medium Carbon (1040)

High Carbon (1095)

Alloy Steel (4130)