June 2013 Archives

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.

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.