Carburizing is a process to increase carbon content of steel surfaces by heating in carburizing compounds and diffusing carbon on low carbon containing steels. Carbon is an indispensable element for hardening steel, and higher the carbon content the higher the achieved hardness will be.
When carburized steel is hardened, the carburized layer will be hardened and will acquire abrasion resistance, but non-carburized inner layer will not harden and will remain tough.
Carbonitriding is a process of simultaneous diffusion permeation of carbon and nitrogen, and generally uses atmosphere of carburizing gas mixed with a few percent ammonia gas.
The steels that carburizing is applied to are, among mechanical structural steels, steels with 0.1~0.2% carbon content as case hardened steel.
Carburizing methods can be classified based on carburizing compounds and carburizing equipment used, as shown in [Table 1]. Solid medium carburizing is poor in working environments and efficiency, and liquid carburizing has cyanide pollution problems. Therefore, gas carburizing currently has become of the mainstream.

[Table 1] Types of Carburizing

The solid medium carburizing is a method is where the subject steel is sealed in a heat proofed steel box with carburizing compound mainly comprised of charcoal and heated to a prescribed temperature. 20~30% barium carbonate or sodium carbonate will be added to the carburizing compound.
The principle of carburizing is as follows: The charcoal heated to a high temperature (900〜950℃) reacts with oxygen and generates carbon dioxide in the carburizing box. Furthermore, this carbon dioxide converts into carbon monoxide by reacting with the charcoal. The carbon monoxide reacts with iron and the carbon dissolves into the iron.
The steel that has been carburized is extracted from the heating box and chilled, hardened, and annealed to be strength increased.
Liquid carburizing is a method of carbon diffusion by immersing in salt baths mainly comprised of sodium cyanide or potassium cyanide. In this case, the process will be carbonitriding process since nitrogen will also be penetration diffused along with the carbon at the same time. The cyanide contained in the bath solution becomes cyanic acid, then thermolytically becomes into carbon monoxide. The carbon from this penetration diffuses into the steel and carburizing takes place. Similarly, nitrogen in the cyanide nitrides the steel.