Steel and its alloys do not have enough hardness or strength to withstand the rigors of things such as modern machinery and automobiles after being forged or rolled. The component, after being machined from stock, is usually heated to a high temperature so that the structure and grains are realigned to make the component stronger. Additionally, the component has a lot of internal stress that is removed after heat treatment. In short, proper heat treatment makes the part stronger and less susceptible to failure. It is a highly skilled operation and an improper process could endanger lives.
There are various types of heat treatments that are used such as: hardening, annealing, tempering, spherodizing, normalizing, case hardening, nitriding, and subzero treatment, etc.
To perform metal heat treating, the metal must first be heated to between 200 and 500 degrees Fahrenheit. The metal is then held in the heat and cooled in something such as water, liquid nitrogen, or oil. The strength, toughness and the durability of the treated steel will depend on how long it is heated and how long it is cooled. Occasionally, different sections of the metal piece will be heated more than others, and sometimes certain areas of the metal will also be cooled at different rates. Metal heating is a complex procedure that takes a lot of skill because if the steel is cooled too quickly, it will become brittle. However, before steel can be heat treated, it has to go through a higher temperature process known as forging. In this process the steel metal should be heated over 900 degrees Fahrenheit.
The machinery comes with an iron carbide diagram which reveals the amount of dissolved carbon in steel and is used as a reference in high temperature heat treatment of steel. There are many types of metals that can be heat treated such as steel tools, all types of steel, aluminum, castings, and ferrous alloys, etc. Glass can also be heat treated. In some applications, heat treatment is used to make a part easier to machine. Heat treatment can also do things such as deform parts; so the proper allowances for finishing, grinding, etc. need to be considered when rough machining. Hardness is measured as per the Rockwell scale, shore, etc.