Cyniding

Definition

Cyniding is a case-hardening process used to harden the surface of low-carbon steel components by diffusing both carbon and nitrogen into the surface layer. This process, also known as liquid carburizing, is conducted in a molten cyanide salt bath, resulting in a hard, wear-resistant surface with a tough, ductile core.

Main Content

1. Purpose of Cyniding

• It is primarily used to increase the surface hardness of steel parts that require a durable exterior to withstand friction.
• It enhances the fatigue strength of components by introducing compressive stresses on the surface.

2. The Role of Cyanide Salts

• The molten bath typically consists of sodium cyanide (NaCN), sodium carbonate, and other salts.
• The chemical reaction releases carbon and nitrogen atoms which penetrate the austenite phase of the steel at high temperatures.

3. Surface Characteristics

• The resulting surface is a "case" that is typically 0.05 mm to 0.5 mm thick.
• Because it introduces nitrogen, the surface attains higher hardness compared to standard carburizing.

Surface Layer (Hardened Case)
|--------------------------|
|  Carbon + Nitrogen Rich  | <--- High Hardness
|--------------------------|
|     Core (Tough Steel)   | <--- High Ductility
|--------------------------|

Visual representation of the hardened case on a steel cross-section.

Working / Process

1. Preparation and Heating

• The steel component is cleaned thoroughly to remove any oil, grease, or scale that might inhibit diffusion.
• The part is preheated to avoid thermal shock and then immersed in a molten cyanide salt bath maintained at 850°C to 950°C.

2. Diffusion (Soaking)

• Once immersed, the cyanide salt decomposes, releasing nascent carbon and nitrogen atoms onto the steel surface.
• The duration of immersion depends on the desired thickness of the hardened case; longer soaking times result in deeper case depths.

3. Quenching

• After the diffusion process, the component is removed from the bath and immediately quenched in water, brine, or oil.
• Quenching locks the carbon and nitrogen into the steel lattice, transforming the surface into hard martensite.

Advantages / Applications

• Rapid Hardening: Compared to pack carburizing, cyniding is much faster, making it ideal for high-production environments.
• Superior Finish: It produces a clean, bright surface with minimal oxidation or scaling, reducing the need for post-process grinding.
• Applications: Widely used for small machine parts like gears, bolts, small shafts, and bushings that need high wear resistance and a tough core.

Summary

Cyniding is a chemical-thermal surface treatment that uses molten cyanide salts to enrich the exterior of low-carbon steel with carbon and nitrogen. By heating the steel in a bath followed by a rapid quench, engineers create a hard surface shell that resists wear while maintaining a strong, flexible interior. Key terms to remember include Case Hardening, Diffusion, Molten Salt Bath, and Quenching.