Decarburization is a surface degradation phenomenon in the forging and heat treating of steels. Decarburization may be described as a metallurgical process in which the surface of steel is depleted of carbon, by heating above the lower critical temperature or by chemical action. Steel forgings are usually decarburized. This process can happen as a side effect during a process, or can be performed intentionally.
The amount of carbon contained in a metal influences its hardness. During decarburization, the carbon diffuses from the surface of the metal, thus weakening the metal. This diffusion increases at higher temperatures. The effect of decarburization not only brings down the strength, but also increases the shear strain below the metal surface. The fatigue resistance is decreased while the rate of crack growth and wear rate is increased.
In plain carbon steel, the carbon content will decrease until the ferrite layer is reached, after which decarburization is inhibited by the ferrite layer. Excessive decarburization will lead to defective products. Decarburization can be detrimental or advantageous depending on the application for which the decarburized steel is used for.
Decarburization occurs when metals are subjected to high temperatures, and the carbon present on the surface reacts with oxygen or hydrogen and diffuses. The reaction between carbon and the atmospheric gases usually begins around 700°C (1292°F).
During heat treatment of metals, decarburization happens as a side effect, which is undesirable. Hence it must be prevented by maintaining an inert atmosphere and switching over to resistive heating techniques.
Some of the beneficial applications of decarburization are listed below:
- Production of electrical steel
- Production of stainless steel
- Grain boundary segregation of plain carbon steel
- Microstructure analysis of nitride steels.