Steel are mixtures (alloys) of iron and carbon and are generally divided into five groups: carbon steels, alloy steel, stainless steels, tool steels and special-purpose steels. Of great significance to the heat treater is the fact that iron is an allotropic elements, which means that it can exist in more than one atomic arrangement (dependent upon temperature).
The process by which iron changes from one atomic arrangement (crystalline form ) to another is called a transformation. Heating or cooling the material induces the transformation. When steel is heated to a high enough temperature, the resulting iron crystal structure is called austenite. When steel is rapidly cooled from above the austenitizing temperature, the austenite transforms to an extremely hard, almost brittle crystal structure called martensite. With some richly alloyed steel (such as tool steels), the transformation is not complete even at room temperature. Some austenite will be retained in the martensite structure unless the steel is refrigerated. This transformation from austenite to martensite is not time dependent, but related to the degree of cooling. Further cooling to cryogenic temperatures continues the transformation in all metals.
