1. Mechanism of Cyaniding: Cyaniding is a liquid case-hardening process. The steel components are immersed in a molten salt bath, typically containing sodium cyanide ($NaCN$), sodium carbonate, and sodium chloride. When heated above the lower critical temperature:
• Carbon Addition: Carbon from the cyanide decomposes and diffuses into the surface layer of the steel.
• Nitrogen Addition: Simultaneously, nitrogen is absorbed into the surface.
This dual addition creates a thin, extremely hard case containing iron carbides and nitrides.
2. Comparison with Other Processes:
• Carburising: This process involves the addition of
carbon only to the surface of low-carbon steels to improve surface hardness after subsequent quenching.
• Nitriding: This involves the diffusion of
nitrogen only into the surface of alloy steels, usually in a gaseous ammonia atmosphere. It does not introduce carbon.
• Spheroidizing: This is an annealing process used to produce a structure of globular carbides in a ferrite matrix. It is a bulk heat treatment used to improve machinability, not a surface hardening process.
3. Practical Application: Cyaniding is highly effective for small parts like screws, nuts, and small gears where a very hard, thin case is required quickly. However, due to the extreme toxicity of cyanide salts, it requires rigorous safety protocols.