Carburizing
Carburizing, or carburising, is a
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
Method
Carburization of steel involves a heat treatment of the metallic surface using a source of carbon.[3] Carburization can be used to increase the surface hardness of low carbon steel.[3]
Early carburization used a direct application of
Plasma carburization is increasingly used to improve the surface characteristics (such as wear, corrosion resistance, hardness, load-bearing capacity, in addition to quality-based variables) of various metals, notably stainless steels. The process 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 diffusion of carbon atoms into the surface layers of a metal. As metals are made up of atoms bound tightly into a metallic
Gas carburizing is normally carried out at a temperature within the range of 900 to 950 °C.
In
A main goal when producing carburized workpieces is to ensure 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 nickel-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 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 (BaCl2). 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.[4]
Dimensional changes
It is virtually impossible to have a workpiece undergo carburization without having some dimensional changes. The amount of these changes varies based on the type of material that is used, the carburizing process that the material undergoes and the original size and shape of the work piece. However changes are small compared to heat-treating operations.[4]
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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.[4]
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 lend themselves to mechanized material handling. Also the advantages of carburizing over carbonitriding are greater case depth (case depths of greater than 0.3 inch are possible), less distortion, and better impact strength. This makes it perfect for high strength and wear applications (e.g. scissors or swords). The disadvantages include added expense, higher working temperatures, and increased time.[4]
Choice of equipment
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. Vacuum carburizing (low pressure carburizing or LPC) can be applied across a large spectrum of parts when used in conjunction with either oil or high pressure gas quenching (HPGQ), depending on the alloying elements within the base material.[4]
See also
- Carbonitriding
- Case-hardening
- Cementation process
- Crucible steel
- Harvey armor (also known as Harveyized steel), an early application of carburizing
- Hayward A. Harvey, a pioneer in the development of carburizing
- Krupp armor
- Nitridization
References
- ^ "Carburizing of Steel". The Free Dictionary By Farlex. Archived from the original on 2011-08-31. Retrieved 2012-05-25.
- ^ Oberg, E., Jones, F., and Ryffel, H. (1989) Machinery's Handbook 23rd Edition. New York: Industrial Press Inc.
- ^ a b "Low-carbon steels". efunda. Retrieved 2012-05-25.
- ^ a b c d e f Robert H. Todd, Dell K. Allen and Leo Alting Manufacturing Processes Reference Guide. Industrial Press Inc., 1994. pp. 421–426
Further reading
- Geoffrey Parrish, Carburizing: Microstructures and Properties. ASM International. 1999. pg 11
External links
- "MIL-S-6090A, Military Specification: Process for Steels Used In Aircraft Carburizing and Nitriding". United States Department of Defense. 7 Jun 1971. Archived from the original (PDF) on 29 August 2019. Retrieved 19 June 2012.