case hardening - Carbonitriding
Case hardening (charcoal and cooling) and carbonitration (nitrogen addition, used to increase the wear resistance and surface hardness) is a process used in the production of steel articles when a core combined with a hard and durable surface is needed.
The carbonitriding process combined with hardening, and the principle, has been known from the early history of iron for the production of steel from soft iron.
If you have steel with a fairly low carbon content, which becomes carbonitrically on the surface and hardened, after hardening the steel gets a surface layer with high durability and strength and the core has a high toughness with minimal brittleness.
Case hardened steel usually has a carbon content of less than 0.2%.