Explanation:
In the nitriding process, only nitrogen is absorbed by the metal surface to harden it, not both carbon and nitrogen. The correct statement should be "In the nitriding process, nitrogen is absorbed by the metal surface to get it hardened."

Flame Hardening:
Flame hardening is a heat treatment process used to harden the surface of a material by heating it using an oxy-acetylene flame and then quenching it. The high temperature of the flame heats the surface of the material above its transformation temperature, which allows for the formation of a hardened layer upon quenching. This process is commonly used for hardening the surface of steel components such as gears, shafts, and tools.

Induction Hardening:
Induction hardening is a heat treatment process that uses high-frequency electromagnetic induction to heat the surface of a material above its transformation temperature. The high-frequency current generates a localized heat in the material, which results in rapid heating and subsequent quenching. The high frequency is kept high to ensure efficient heating and rapid quenching, which leads to a hardened surface layer. This process is commonly used for hardening gears, shafts, and other components that require wear resistance.

Nitriding Process:
Nitriding is a surface hardening process that involves the diffusion of nitrogen into the surface of a material to create a hardened layer. The process is typically carried out in a controlled atmosphere that contains nitrogen. The nitrogen diffuses into the surface of the material and forms nitrides, which are hard and wear-resistant compounds. This process is commonly used for steel components to improve their wear resistance, fatigue strength, and corrosion resistance.

Carburising, Cyaniding, and Nitriding:
Carburising, cyaniding, and nitriding are all case hardening processes used to increase the surface hardness of materials.

- Carburising involves introducing carbon into the surface of a material by heating it in a carbon-rich atmosphere. The carbon diffuses into the material, forming a hardened layer known as a "case." This process is commonly used for low carbon steels.

- Cyaniding involves introducing carbon and nitrogen into the surface of a material by heating it in a bath of molten cyanide salts. The carbon and nitrogen diffuse into the material, forming a hardened case. This process is commonly used for low carbon steels.

- Nitriding, as mentioned earlier, involves introducing nitrogen into the surface of a material to create a hardened layer. This process is commonly used for steel components.

In summary, the incorrect statement is option C, which states that both carbon and nitrogen are absorbed by the metal surface in the nitriding process. In reality, only nitrogen is absorbed to harden the surface of the material.