Carburizing is a metal treatment process that adds carbon to the surface of metal that has a low carbon content to increase the hardness of the metal. The metal is heated at an elevated temperature in an atmosphere rich with carbon. Heat will cause carbon atoms to diffuse into the metal surface. The process is done below the melting point of the metal being carburized. There are five carburizing methods — pack, gas, liquid bath, vacuum, and plasma.
Pack carburizing uses a furnace to heat the metal parts to be carburized that are packed inside a container with a sufficient amount of carbon powder. The heating process will last for 12 to 72 hours at a high temperature. This method is noted to be slow compared to the other methods and has heating inefficiencies because of the difficulty in maintaining an even temperature.
Gas carburizing follows the same procedure applied in pack carburizing. It, however, feeds carbon monoxide (CO) to the furnace to improve diffusion, which is not done in the pack method. The process has safety issues because CO is a poisoning gas that is odorless and colorless and could be inhaled by persons working inside the plant. The gas method is preferred in carburizing large volumes of metal.
In the liquid bath method, the metal parts are submerged in molten salt with abundant carbon. Cyanide (CN), which is a deadly poison, was previously used as the main component in the liquid bath. It was replaced by nontoxic bath components that can achieve the same carburization results as cyanide.
The vacuum method requires a heating environment that is free from oxygen and carburizes metal at low pressures. The method uses a furnace that has a complex structure and a single component to create a carbon-rich environment — methane (CH4). The oxygen-free environment allows the temperature to rise significantly, thus increasing the solubility of carbon and the rate of diffusion. No surface oxidation results from the process.
The plasma method utilizes a vacuum furnace to heat the metal. Once the metal attains a temperature suitable for processing, a rare gas containing hydrocarbon gases like CH4 is supplied to the furnace. High DC voltage is introduced between the metal and the furnace to generate a glow discharge that will cause the gas ion and the DC plasma to have an electrochemical reaction to accomplish carburization.
The choice of carburizing method must be guided by social and economic considerations. The method must be environmentally friendly and should not cause harm to the health of the workers and the community. It should also be cost-effective.