Heat treatment of steel is essentially processing to increase the hardness, durability, and usefulness of the metal. To achieve this result, the steel used must have a certain proportion of alloying elements so that hardening and tempering occur as the various elements are heated and cooled under controlled conditions. The chemical elements involved are the atoms of the metals which make up the steel itself and the atoms of carbon, which are suspended in the steel, but are not mixed with the metal atoms. The bonding of the metal atoms with the carbon atoms forms metal carbides and this is what changes the character of the steel and improves the steel for certain uses.
Most steel is purchased in its softest state, or what is known as the annealed state. Since this is the softest state, it is also the best state for machining steel. Once the machining has been done, however, it is often desirable to make the steel harder and more durable. To accomplish this, heat is used and this is where heat treatment begins.
Each steel has a critical temperature range, which is the temperature at which the steel goes into solid solution. Chemically, this is where the atoms of the metals and the atoms of carbon in the steel mix freely and ultimately bond together. Lower critical temperature is the heat at which this process begins. Upper critical temperature is the heat which must be reached to ensure that the process has been completed. The steel is actually heated to a temperature slightly above upper critical temperature. Depending on the analysis of the steel, this temperature will usually range from 1500° to 2300° Fahrenheit. Steel that is heated to the critical temperature is called austenite or is in the austenitic state.
Just as heating to the proper temperature is crucial to the success of the heat treatment process, cooling at the proper rate is also essential. If the atoms which are bonded together during the process of heating are not cooled properly, they will not stay bonded, and the desirable properties of the steel will be lost. Steel which has been heated to upper critical temperature (austenite) is cooled under controlled conditions using various cooling media such as oil, water, or air. Once steel has been cooled under proper conditions, it is called martensite or is in the martensitic state.
Steel that has been treated and cooled as described above (martensite) is very hard and brittle until it undergoes another process called tempering. The tempering process is simply reheating the steel to a temperature which is always lower than the lower critical temperature. This toughens and usually softens the steel. Some steels require one temper; other require two or sometimes three tempers to fully temper the brittle martensite.
Steels that have been subjected to the above procedures have been fully heat treated. This provides steel tools that are optimally useful in manufacturing.