Metal gas carburising is a heat treatment that increases the surface hardness of low carbon steel by adding additional carbon atoms to the outer layer of the metal. The process involves heating the steel in a furnace with a carbon-rich gas, such as methane or carbon monoxide, which decomposes on the metal surface and diffuses the carbon into the steel lattice. The carbon-enriched layer, also known as the case, becomes hard and wear-resistant when quenched in water or oil, while the core of the steel remains soft and hard.
Metal gas carburising is widely used to improve the mechanical properties of various components such as gears, shafts, bolts and crankshafts. The process can produce case depths of up to 6.4 mm, depending on temperature, time and gas composition. The carbon potential of the gas, the ability of the gas to provide carbon to the metal surface, can be controlled by adjusting the ratio of hydrocarbon and carrier gas (such as endothermic gas). The higher the carbon potential, the faster the carbon diffusion and the greater the case depth.
Metal gas carburising has several advantages over other surface hardening methods such as solid or liquid carburising. It provides better control over the temperature and atmosphere inside the furnace, resulting in a more uniform and consistent surface finish. It also enables complex shaped parts to be machined evenly because the gas can penetrate holes and narrow cavities. It is also more environmentally friendly than liquid carburising, which contains toxic chemicals and produces hazardous waste.
However, metal gas carburising also has some limitations and challenges. One of these is the distortion and cracking of parts due to thermal stresses and phase transformations during heating and cooling. To minimise this problem, careful steel grade selection, preheating, post-carburising annealing and tempering are necessary. Another challenge is to prevent decarburisation or oxidation of the metal surface during heating, which can reduce case hardness and quality. This can be avoided by using protective coatings or inert gases in the furnace.
Metal gas carburising is a versatile and effective technique for increasing the surface hardness of low carbon steel parts. By understanding the principles and parameters of this process, one can optimise its performance and quality for a variety of applications.