Carburizing

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Carburizing is a heat treatment process in which iron or steel is heated in the presence of another material (but below the metal's melting point) which liberates carbon as it decomposes. The outer surface or case will have higher carbon content than the original material. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard, while the core remains soft and tough.^[1]

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 0.25 inches (6.4 mm).

Method

Carburization of steel involves a heat treatment of the metallic surface using a gaseous, liquid, solid or plasma source of carbon. Early carburization used a direct application of charcoal packed onto the metal (initially referred to as case hardening or Kolsterising), but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide or methane). The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.

Plasma carburization is increasingly used in major industrial regimes to improve the surface characteristics (such as wear and corrosion resistance, hardness and load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steels. The process is used as it is environmentally friendly (in comparison to gaseous or solid carburizing). It also provides an even treatment of components with complex geometry (the plasma can penetrate into holes and tight gaps), making it very flexible in terms of component treatment.

The process of carburization works via the implantation of carbon atoms in to the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic crystalline lattice, the implanted carbon atoms force their way into the crystal structure of the metal and either remain in solution (dissolved within the metal crystalline matrix — this normally occurs at lower temperatures) or react with the host metal to form ceramic carbides (normally at higher temperatures, due to the higher mobility of the host metal's atoms). Both of these mechanisms strengthen the surface of the metal, the former by causing lattice strains by virtue of the atoms being forced between those of the host metal and the latter via the formation of very hard particles that resist abrasion. However, each different hardening mechanism leads to different solutions to the initial problem: the former mechanism — known as solid solution strengthening — improves the host metal's resistance to corrosion whilst imparting its increase in hardness; the latter — known as precipitation strengthening — greatly improves the hardness but normally to the detriment of the host metals corrosion resistance. Engineers using plasma carburization must decide which of the two mechanisms matches their needs.

In oxy-acetylene welding, a carburizing flame is one with little oxygen, which produces a sooty, lower-temperature flame. It is often used to anneal metal, making it more malleable and flexible during the welding process.

A main goal when producing carbonized workpieces is to insure maximum contact between the workpiece surface and the carbon-rich elements. In gas and liquid carburizing, the workpieces are often supported in mesh baskets or suspended by wire. In pack carburizing, the workpiece and carbon are enclosed in a container to ensure that contact is maintained over as much surface area as possible. Pack carburizing containers are usually made of carbon steel coated with aluminum or heat-resisting nickle-chromium alloy and sealed at all openings with fire clay.

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 chloride, or hardwood charcoal. In gas carburizing, the CO is given off by propane or natural gas. In liquid carburizing, the CO is derived from a molten salt composed mainly of sodium cyanide(NaCN) and barium chloride(BaCl). In pack carburizing, carbon monoxide is given off by coke or hardwood charcoal.

Change in material properties

Geometrical possibilities

There are all sorts of workpieces that can be carbonized, which means almost limit less possibilities for the shape of materials that can be carbonized. 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.

Dimensional changes

It is virtually impossible to have a workpiece under go carbonization without having some dimensional changes. The amount of these changes varies based on the type of material that is used, the carbonized process that the material undergoes and the original size and shape of the work piece. However changes are small compared to heat-treating operations.

Workpiece material

Typically the materials that are carbonized are low-carbon and alloy steels with initial carbon content ranging from .2% to .3%. The workpiece surface must be free from contaminates, such as oil oxides, alkaline solutions, which prevent or impede the diffusion of carbon into the workpiece surface.

Cost elements

Cost elements include the following

• Setup time
• Tooling/fixturing costs
• Direct labor rate
• Overhead rate(maintenance, power, insurance, etc.)
• Material costs
• Amortization of equipment and tooling costs

Comparing different methods

In general, pack carburizing equipment can accommodate larger workpieces than liquid or gas carburizing equipment, but liquid or gas carburizing methods are faster and lead themselves to mechanized material handling. Also the advantages of carburizing over carbonitriding are greater case depth ( case depths of greater than .3 in are possible), less distortion, and better impact strength. The disadvantages include added expense, higher working temperatures, and increased time.

Choice of equipment

The needed equipment depends largely on size of the workpiece, equipment capacity, and required case depth. In General Gas Carburizing is used for parts that are large. Liquid carburizing is used for small and medium parts and pack carburizing can be used for large parts and individual processing of small parts in bulk.

See also

• Nitridization
• Cementation process
• Crucible steel
• Carbonitriding

References

Robert H. Todd, Dell K. Allen and Leo Alting Manufacturing Processes Reference Guide. Industrial Press Inc., 1994. pgs. 421-426

External links

• "MIL-S-6090A, Military Specification: Process for Steels Used In Aircraft Carburizing and Nitriding" (PDF). United States Department of Defense. 07 JUN 1971. http://www.everyspec.com/MIL-SPECS/MIL+SPECS+(MIL-S)/MIL-S-6090A_8810/.