Cyaniding

a method of case hardening involving the diffusion of carbon and nitrogen into the surface layer of steel in cyanide-salt baths at temperatures of 820°–860°C (medium-temperature cyaniding) or 930°–950°C (high-temperature cyaniding). Its principal purpose is to increase the hardness, wear resistance, and fatigue limit of steel products.

During cyaniding, the cyanide salts are oxidized with the liberation of atomic carbon and nitrogen, which diffuse into the steel. In medium-temperature cyaniding, the cyanide layer formed, containing 0.6–0.7 percent C and 0.8–1.2 percent N, has a thickness of 0.15 to 0.6 mm, while in high-temperature cyaniding (a method often used instead of carburizing), the cyanide layer, containing 0.8–1.2 percent C and 0.2–0.3 percent N, has a thickness of 0.5 to 2 mm. After cyaniding, a product undergoes hardening and low-temperature tempering.

The disadvantages of cyaniding are high cost and the toxicity of the cyanide salts, the latter necessitating the adoption of special measures to protect workers and the environment. The difference between cyaniding and nitrogen case hardening (or carboni-triding) is that in the latter the diffusion of nitrogen and carbon is achieved from a gaseous medium.