Question

The process, which involves addition of carbon and nitrogen to carbon steels and alloy steels to increase hardness at the surface, is known as:

• A. Carburising
• B. Cyaniding
• C. Nitriding
• D. Spheroidizing

Hint:

Remember: Cyaniding starts with 'C' and 'N' sounds (C-ya-N-ide). It adds Carbon and Nitrogen. This distinguishes it from Carburising (Carbon only) and Nitriding (Nitrogen only).

Answer 1

The Correct Option is B

Surface hardening processes are metallurgical techniques used to create a wear-resistant outer shell on a component while maintaining a tough, ductile core. The specific process that introduces both carbon and nitrogen is known as cyaniding. 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.