Carburizing adds carbon to low-carbon metal surfaces to increase hardness. Five methods include pack, gas, liquid bath, vacuum, and plasma. Each method has advantages and disadvantages, and the choice should consider safety, environmental impact, and convenience.
Carburizing is a metal treatment process that adds carbon to the metal surface that has a low carbon content to increase the hardness of the metal. The metal is heated to a high temperature in a carbon-rich atmosphere. The heat will cause the carbon atoms to diffuse into the surface of the metal. The process is performed below the melting point of the metal to be cemented. There are five methods of cementation: pack, gas, liquid bath, vacuum and plasma.
Pack cementing uses a furnace to heat the metal parts to be cemented which are packed inside a container with a sufficient amount of carbon powder. The heating process will last 12 to 72 hours at high temperature. This method is known to be slow compared to the other methods and has heating inefficiencies due to the difficulty of maintaining a uniform temperature.
Gas cementation follows the same procedure applied in pack cementation. However, it feeds carbon monoxide (CO) to the furnace to improve diffusion, which is not the case in the pack method. The process has safety concerns because CO is a colorless and odorless poisonous gas and could be inhaled by people working inside the plant. The gas method is preferred when carburizing large volumes of metal.
In the liquid bath method, metal parts are immersed in molten salt with plenty of carbon. Cyanide (CN), which is a deadly poison, was previously used as the main component in liquid bath. It has been replaced by non-toxic bath components which can achieve the same carburizing results as cyanide.
The vacuum method requires an oxygen-free heating environment and carburizes the 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 increase dramatically, thereby increasing the solubility of carbon and the rate of diffusion. No surface oxidation results from the process.
The plasma method uses a vacuum furnace to heat the metal. Once the metal reaches a temperature suitable for processing, a rare gas containing hydrocarbon gases such as CH4 is supplied to the furnace. A high DC voltage is introduced between the metal and the furnace to generate a glow discharge which will cause the gas ion and DC plasma to have an electrochemical reaction to realize carburization.
The choice of cementation method must be guided by social and economic considerations. The method must be environmentally friendly and must not cause harm to the health of workers and the community. It should also be convenient.
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