Heating and hardening methods
Steel has a critical temperature at which it becomes ductile and begins to change its chemical structure. When steel is heated above this temperature, it enters what is known as the austenite stage.

From this point on, engineers can create the correct shape and quality of steel that they need. There are many ways to heat treat steel, including but not limited to:

Annealing - heating and then slowly cooling the steel to refine it and make it softer
Carburizing - using heat and carbon-rich materials to add carbon to the surface
Surface Hardening - Carburizing and rapidly cooling the steel so that the center remains soft and the rest hardens
Cyanide Hardening - similar to surface hardening, but uses molten cyanide salts instead of carbon to form a hard shell
Decarburization - removing carbon from a steel alloy by heat or oxidation
Nitriding - adding nitrogen to the surface of steel using heat and a nitrogen-rich liquid or gas
Stretching or Tempering - reheating steel that has been cooled to a specific temperature to remove hardness