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