Välikarkaistus

Välikarkaistus, also known as tempering, is a heat treatment process applied to hardened steel to reduce its brittleness and increase its toughness. Hardening steel typically involves heating it to a high temperature and then rapidly cooling it, a process known as quenching. This quenching transforms the steel's microstructure into martensite, which is very hard but also brittle.

Tempering is performed after hardening and involves reheating the steel to a temperature below its critical transformation point. The specific temperature and duration of tempering depend on the desired properties of the steel. Higher tempering temperatures result in lower hardness but greater toughness and ductility. Conversely, lower tempering temperatures maintain more of the hardness achieved during quenching but offer less improvement in toughness.

The process involves heating the steel to the chosen tempering temperature and holding it there for a specific period to allow the microstructure to change. During tempering, carbon atoms diffuse within the martensite structure, forming fine carbides. This diffusion process reduces internal stresses within the steel, which are a major cause of brittleness. After holding at the tempering temperature, the steel is cooled, usually in air, to room temperature.

Tempering is a crucial step in achieving the optimal balance of strength and toughness required for many steel applications, ranging from tools and machinery parts to structural components. Without tempering, hardened steel would be too prone to fracture under impact or stress. The careful control of tempering parameters allows metallurgists to tailor the mechanical properties of steel to specific end-use requirements.