Heat treatment could be any one of a variety of controlled heating operations used to change the physical properties of a metal; there are five different types of heating processes. These would be case hardening, normalizing, hardening and tempering. Even though they all differentiate in results, they all share three simple steps: heating, soaking and cooling.
The first step in a heat-treatment process would be heating. The steal is heated to its final temperature. This helps in keeping a uniform temperature in the part being heated. This reduces cracking and distortion.
When a piece of metal is heated to the temperature in which a change in structure takes place, it has to stay at that particular temperature until the entire part has been evenly heated. The more mass it has, the longer it will take for t to be completely soaked, this is known as soaking.
After the metal part has been soaked, the last step us to cool. This process may force the structure to stay the same, revert to its original form or even change from one chemical composition to another. These changes are all predictable and for this reason metals can now be made to suit certain structure to increase the level of their, hardness, strength and ductility.
This is the process of hardening the surface of metal objects while the deeper metal underneath stays soft, this will form a thin layer of harder metal called the case. For other metals like iron and steel that have low carbon content the process would involve infusing additional carbon into the surface layer. Generally case hardening would be done after it has been formed into its final shape, carbon and alloy steels are suitable for case hardening.
Annealing the exact opposite of hardening, it relieves internal stresses, softens them and makes it more ductile. It is the process where a metal is heated to very high temperature; it would be held there for a period of time usually between hours and days and allowed to cool. For this process to go perfectly, the process should be slow down in steels and other ferrous metals. Condition in the furnace is controlled firmly to assure the expected changes are going to take place.