karkaista

Karkaista is a Finnish verb that refers to hardening a metal, typically steel, through heat treatment. The aim is to increase hardness and wear resistance, often at the cost of ductility. The process is a central method in metalworking and tool-making.

A standard karkaistus involves heating the metal to a high austenitizing temperature, holding it at that temperature to transform the microstructure, and then cooling rapidly (quenching) in water, oil, or another quench medium to produce a hard martensitic phase. After quenching, tempering—a controlled reheating to a lower temperature—may be performed to relieve internal stresses and adjust the balance between hardness and toughness.

Karkaistus can be performed in different forms. Deep hardening (syväkarkaistus) aims to harden the entire cross-section of the part, while surface hardening (pintakarkaisu) creates a hard outer layer while leaving the core softer and more ductile. The choice of method, temperatures, soaking times, and quench media depends on the steel grade and the desired properties.

The practical effects of karkaista include increased surface hardness, improved wear resistance, and generally better fatigue performance, but with reduced elongation and notch toughness if not tempered. The term is commonly used in engineering, manufacturing, and material science contexts, and it distinguishes this method from other heat-treatment processes such as annealing, normalizing, and various tempering regimes.