järelkõvenemise

Järelkõvenemise, also known as after-hardening or secondary hardening, is a metallurgical phenomenon observed in certain steels. It refers to the increase in hardness and strength that occurs when hardened steel is tempered at elevated temperatures, typically between 450°C and 600°C. This process is distinct from the initial hardening achieved through quenching.

The effect of järelkõvenemise is primarily due to the precipitation of fine carbide particles within the steel's microstructure. During the initial hardening, carbon atoms are trapped in a supersaturated solid solution known as martensite. When the steel is subsequently tempered at higher temperatures, these carbon atoms diffuse and combine with alloying elements like chromium, molybdenum, and vanadium to form new, hard carbide phases. These precipitated carbides impede dislocation movement, thus increasing the steel's hardness and tensile strength.

Järelkõvenemise is particularly significant in high-speed steels and other tool steels that contain substantial amounts of carbide-forming elements. The controlled application of this tempering process allows for the optimization of tool performance, providing superior wear resistance and hot hardness. Without this secondary hardening, these steels would not achieve their desired properties for demanding applications. The exact temperature and time for tempering to achieve optimal järelkõvenemise depend on the specific alloy composition of the steel.