Karkaisevuus

Karkaisevuus is a term used in metallurgy to describe the hardenability of a material, especially steel. It denotes the propensity of a steel alloy to develop martensitic hardness deeper than the surface when subjected to heat treatment. The word stems from the Finnish verb karkaistaa, meaning to harden, with the abstract-noun suffix -vuus. In practice, karkaisevuus characterizes how far into a cross-section a given cooling rate will produce a hardened structure, rather than the surface hardness alone.

Measurement and interpretation: Hardenability is commonly assessed with the Jominy end-quench test, which yields a hardness-versus-distance curve. From this curve, one can compare steels and predict the required quenching conditions for a desired cross-sectional hardness. Hardenability is influenced by carbon content, alloying elements such as chromium, molybdenum, nickel, vanadium, and silicon, as well as microstructure, grain size, and heat-treatment history. Quenching media and component geometry also affect measured hardenability.

Applications and limitations: Hardenability informs steel selection and heat-treatment design for components where deep hardness is required, such as gears, shafts, and tools. It is not a direct measure of surface hardness; rather, it describes the distribution of hardness through the section. Predictive tools include time-temperature-transformation (TTT) and continuous cooling transformation (CCT) diagrams and other models that relate composition and cooling rate to phase transformations.