Jäännöskovettuminen

Jäännöskovettuminen, also known as residual hardening or secondary hardening, is a metallurgical phenomenon that occurs in certain alloy steels, particularly those with high carbon and alloying element content like high-speed steels. This hardening process takes place during tempering at elevated temperatures, typically above the first tempering transformation.

The mechanism behind jäännöskovettuminen involves the precipitation of very fine alloy carbides within the martensitic structure. During the initial quenching and tempering stages, a relatively soft martensite is formed, and some of the alloying elements are dissolved in this matrix. When the steel is then tempered at higher temperatures, these dissolved alloying elements diffuse and precipitate out as extremely fine, hard carbide particles. This precipitation significantly increases the hardness and strength of the steel, often to a level exceeding that achieved by the initial quenching alone.

The effectiveness of jäännöskovettuminen is dependent on the specific alloy composition. Elements like molybdenum, tungsten, vanadium, and cobalt are known to promote this secondary hardening effect by forming stable, finely dispersed carbides. The tempering temperature and time also play crucial roles; exceeding optimal conditions can lead to the coarsening of carbides, reducing hardness and potentially causing embrittlement. Jäännöskovettuminen is highly desirable for tools and components that require exceptional hardness and wear resistance at elevated operating temperatures, such as cutting tools.