thumb|A modern computerised gas carburising furnace
Carburizing, or carburising, is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. The intent is to make the metal harder and more wear resistant. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard due to the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.
This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and ductility; and it produces case hardness depths of up to . In some cases it serves as a remedy for undesired decarburization that happened earlier in a manufacturing process.
Method
Carburization of steel involves a heat treatment of the metallic surface using a source of carbon. Carburization can be used to increase the surface hardness of low carbon steel.
- Conventional Furnace (Atmosphere Furnace): conventional carburizing is an atmosphere controlled process with control of the carbon potential.
- Low Pressure Carburizing Furnace (LPC): low pressure carburizing is running without any oxygen and with injections of carburizing gas (ex. acetylene).
Hardening agents
There are different types of elements or materials that can be used to perform this process, but these mainly consist of high carbon content material. A few typical hardening agents include carbon monoxide gas (CO), sodium cyanide and barium carbonate, or hardwood charcoal. In gas carburizing, carbon is given off by propane or natural gas. In liquid carburizing, the carbon is derived from a molten salt composed mainly of sodium cyanide (NaCN) and barium chloride (BaCl<sub>2</sub>). In pack carburizing, carbon monoxide is given off by coke or hardwood charcoal.
Geometrical possibilities
There are all sorts of workpieces that can be carburized, which means almost limitless possibilities for the shape of materials that can be carburized. However careful consideration should be given to materials that contain nonuniform or non-symmetric sections. Different cross sections may have different cooling rates which can cause excessive stresses in the material and result in breakage.
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! Work material properties
! Effects of carburizing
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| Mechanical
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- Increased surface hardness
- Increased wear resistance
- Increased fatigue/tensile strengths
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| Physical
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- Grain growth may occur
- Change in volume may occur
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|Chemical
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- Increased surface carbon content
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Workpiece material
Typically the materials that are carbonized are low-carbon and alloy steels with initial carbon content ranging from 0.2 to 0.3%. The workpiece surface must be free from contaminants, such as oil, oxides, or alkaline solutions, which prevent or impede the diffusion of carbon into the workpiece surface.