välikarkaistuksen

Välikarkaistus, also known as tempering, is a heat treatment process applied to hardened steel. Following the hardening process, which typically involves heating steel to a high temperature and then rapidly cooling it, the steel becomes very hard but also brittle. Tempering is performed to reduce this brittleness and increase the toughness of the steel, making it more suitable for practical applications.

The process involves reheating the hardened steel to a temperature below its critical transformation point, which is significantly lower than the hardening temperature. The exact temperature and holding time are crucial and depend on the specific type of steel and the desired properties. Lower tempering temperatures result in higher hardness but less toughness, while higher tempering temperatures increase toughness and ductility at the expense of some hardness.

During tempering, the internal stresses induced by the rapid cooling of hardening are relieved. Microstructural changes also occur, such as the precipitation of fine carbide particles within the steel matrix. This controlled reduction in hardness and brittleness allows the steel to absorb more energy before fracturing, improving its resistance to impact and shock. Different tempering temperatures produce different microstructures and thus different combinations of hardness and toughness. For example, tempering at lower temperatures might be used for cutting tools where high hardness is paramount, while higher temperatures are suitable for components subjected to significant bending or impact loads.