Kastopäällystys

Kastopäällystys, also known as quench hardening or case hardening, is a heat treatment process used to harden the surface of a metal part while leaving the core relatively soft and tough. This is achieved by introducing carbon or other alloying elements into the surface layer of a steel workpiece, followed by rapid cooling (quenching). The resulting surface layer has a high carbon content, which, upon quenching, forms a very hard martensitic structure.

There are several methods for achieving kastopäällystys. Carburizing is a common technique where steel parts are heated in a carbon-rich atmosphere, causing carbon to diffuse into the surface. Nitriding involves introducing nitrogen into the surface of the steel, often at lower temperatures than carburizing, leading to the formation of hard nitride precipitates. Carbonitriding combines both carbon and nitrogen diffusion. Induction hardening and flame hardening are surface hardening methods that heat the surface layer rapidly using induction coils or a flame, respectively, followed by quenching.

The primary benefit of kastopäällystys is the creation of a wear-resistant surface on a component that retains its ductility and toughness in the core. This combination of properties is crucial for parts subjected to friction, abrasion, and impact loads, such as gears, shafts, bearings, and cutting tools. The depth of the hardened layer and the achievable hardness depend on the specific process parameters, the base material, and the desired application.