The current mainstream of carburizing is Gas Carburizing. There are various methods of gas carburizing, as mentioned in the [Table 1] of the previous volume.

(1) Converted gas method

Converted carburizing gases are generated by heating the mixtures of propane C₃H₈ or butane C₄H₁₀ and air in nickel catalyst to 1000℃.
This gas reacts with the oxygen in the air and carbon monoxide (CO) and hydrogen (H2) are generated, reacting with the steel for the carburizing to proceed. Also, in order to increase the carbon content within the furnace, propane or butane gases are introduced as carbon content increaser.

(2) Cracked gas method

For the heat cracking gases, organic liquids such as methanol CH₃OH are used at carburizing temperatures by heat cracking. A characteristic of this heat cracking gas carburization is that it does not require gas generator furnace, and the organic liquids are directly drop applied into the carburizing furnace. Thus, this method is called Drip Feed Carburizing.
The methanol drip fed into the furnace is thermally cracked into carburizing carbon monoxide and reductive hydrogen, and the carbon monoxide reacts with the steel for the carburization to proceed. Also in this case, propane or butane gases are used as carbon content increaser gas.

(3) Carburizing method not requiring gas generator furnace

Other carburizing methods that do not require the gas generator furnaces are: Nitrogen base method, Direct carburizing method, Vacuum carburizing method, and Plasma carburizing method.

(4) Others

In general, carbon content of carburized layers is 0.8~1.0%, though lately, high carbon concentration carburizing of up to 2~3% is being implemented.
Difference of gas carburizing from solid carburizing and liquid carburizing is that the gas component control in the furnace is possible.
Based on experiments, it has been found that the carbon monoxide and methane gas in the furnace contribute to carburizing, but water, carbon dioxide, and oxygen will rather promote de-carburization. By accurately controlling these gas component concentrations, desired carbon concentrations can be obtained.
For measurement and control of the atmospheric gas, dew point meter to measure the water amount, infrared gas analyzer to measure carbon dioxide, and oxygen sensor to measure the oxygen concentration are used.