Generally speaking, the process for converting a steel alloy into a premium knife steel is heat treating. Heat treatment is the most important stage in the evolution of an alloy into a performance knife steel. It is also one of the many factors that determines the grain size of the steel (a fine grain structure is more desirable for knife blades because it improves edge retention and enhances blade finish). The first step in the heat treatment process is to reach a critical (recrystallization) temperature which allows the carbon atoms to enter into the crystalline molecules of the iron which have expanded due to the heating. This temperature is held for a specific amount of time (depending on the steel being hardened) at which austenite is formed, i.e., the steel become austenetized. Next, the steel is quenched to achieve its maximum level of hardness. Quenching the steel causes the molecules to contract, trapping the carbon atoms inside. Austenite has the property of dissolving all the free carbon present in the steel. Quenching is then used to “freeze” the austenite changing it to martensite.
Although quenching steel causes the austenite to transform into martensite, all of the austenite usually does not transform. Further transformation of the austenite into martensite can be induced by cooling the metal to extremely low temperatures. This sub-zero treating (cryogenic treatment) generally consists of cooling the steel to around -115 ˚F, but that does not always eliminate all of the austenite. Cryogenic treating usually consists of cooling too much lower temperatures, often in the range of -315 ˚F to transform most of the austenite into martensite. Cryogenic treatment is typically done immediately after quenching, before any tempering, and will increase the hardness, wear resistance, and reduce the internal stresses in the metal.
At this point, the steel is too hard and brittle for practical use and thus tempering is of key importance in bringing the steel to its ideal hardness level (different knife steels perform best at different levels of hardness). These processes create internal strains in the steel; these are relieved by tempering (further heating the steel at lower temperatures). Tempering the steel decreases the hardness, strength and brittleness, however, it does increases the wear resistance and toughness. When tempering, it is important to understand the interaction between hardness and toughness. Therefore, proper tempering will result in a good balance between hardness and toughness of the finished knife blade. (See: Our Steel Glossary)