Cyaniding is the heat treatment process that results in a very hard surface, while the core remains soft and ductile. Let's understand the process of cyaniding and how it achieves this outcome.

Cyaniding Process:
Cyaniding is a case-hardening process in which the surface of a metal part is hardened by introducing carbon and nitrogen into its surface layer. The process involves the following steps:

1. Preparing the part: The metal part that needs to be hardened is thoroughly cleaned to remove any contaminants or oxides from the surface.

2. Packing: The cleaned part is then packed in a container with a material containing carbon and nitrogen, typically a mixture of sodium cyanide and anhydrous ammonia. The part is packed in such a way that it is surrounded by the cyanide mixture.

3. Heating: The packed container is heated to a specific temperature, typically around 870-980°C (1600-1800°F), in a controlled atmosphere furnace. The heating temperature and time depend on the material being treated and the desired hardness.

4. Diffusion: As the part is heated, the cyanide mixture decomposes, releasing carbon and nitrogen gases. These gases diffuse into the surface layer of the metal part.

5. Quenching: After a sufficient diffusion time, the part is rapidly quenched in oil or water to cool it down quickly. This quenching process locks the carbon and nitrogen atoms into the surface layer, creating a hardened case.

Outcome:
The cyaniding process creates a hardened case on the surface of the metal part while keeping the core soft and ductile. The high carbon and nitrogen concentration in the surface layer forms hard nitrides and carbides, which significantly increase the surface hardness. At the same time, the core of the part remains relatively unaffected by the process, maintaining its original properties.

The hardened surface achieved through cyaniding provides excellent wear resistance, reducing the chances of abrasion, galling, or fretting. It is commonly used to improve the surface hardness of low-carbon steels, enabling them to withstand harsh operating conditions and extend the lifespan of the part.

In summary, cyaniding is a heat treatment process that introduces carbon and nitrogen into the surface layer of a metal part, resulting in a very hard surface while keeping the core soft and ductile. This process is effective in enhancing the wear resistance of parts and is widely used in various industries.