The hardness of steel denotes its mechanical resistance. Steel can be hardened with several different methods. These are also employed to make the steel for knife blades.
The mechanical resistance of steel is its so-called deformation resistance – how steel holds up to outside forces that might deform it (i.e. alter its external shape). The mechanical resistance of steel is essential for many objects of daily use made from the material, including knife blades. It can be increased by changing the microstructure of steel, which can be achieved by heating the steel and then cooling it rapidly. Steel is a compound (alloy) of iron, carbon and other elements, mostly metals. After the steel is melted and cooled, the iron is shot through with carbon, but the carbon atoms are arranged in a way that leaves the steel relatively soft. In order to harden it, the carbon content needs to be increased, which can be achieved with three different methods: martensitic hardening, precipitation hardening and work hardening. Of these hardening processes, martensitic hardening is the most important.
The steel is heated until the ferrite changes to austenite, which dissolves much more carbon. The metal is then quenched to inhibit carbon diffusion and bind the carbon permanently in the metal, which also changes its microstructure. The new structure is called martensite. This process needs to be repeated several times since the (soft) austenite is only partly changed into (hard) martensite with each repetition. Basically, the transformation happens faster if the temperature difference between heating and cooling is higher, which can be controlled with several different cooling media (e.g. air, water, oil).
The heating and quenching process causes tension within the piece, which can lead to cracks and warping. The tension is relieved by reheating the metal at a lower temperature after quenching (annealing). This gives the steel its final hardness.