Carbonitriding is a surface hardening process that improves wear resistance of metal parts by adding ammonia to the carbon source, resulting in shorter cycle times and lower temperatures. It is more economical than case hardening and has benefits such as increased resistance to hardening distortion. The process is used for low-cost, mass-produced parts such as gear teeth, bearings, and tools.
Carbonitriding is a metallurgical surface hardening process used to improve the wear resistance of metal parts. Essentially similar to traditional gas carburizing processes, carbonitriding involves the inclusion of a nitrogenous component in the form of ammonia. Because of the shorter cycle times and lower temperatures involved, the process is more economical than case hardening, thus making it a more desirable process for low-cost, mass-produced parts. The process also has additional benefits such as increased resistance to hardening distortion. It is used to cement parts such as gear teeth, bearings and tools of various surface hardness values and average casing thicknesses.
Surface or case hardening metal parts is one of the oldest and most effective methods of increasing their wear resistance. The process creates a thin layer, or casing, of harder, more wear-resistant material on the surface of the part, while the internal structure remains malleable and strong. Carburizing is the oldest form of carburizing and involves heating the piece in the presence of a carbon-rich material. Carbon permeates the surface of the workpiece during this process, thus creating a very hard surface layer when cooled or rapidly cooled. Carbonitriding is similar to carburizing with the exception of adding ammonia to the carbon source, lower process temperatures, and shorter cycle times.
During the process, ammonia is introduced together with the carbonaceous material as a nitrogen source. The inclusion of nitrogen allows for shorter heat exposure cycles and lower process temperatures. The average temperature of a conventional carburizing process ranges from 950°F to 3100°F (510°C–1700°C) with carbonitriding temperatures ranging from 1560°F to 2820°F (850°C–1550°C). These factors represent significant savings in manufacturing costs, making it an ideal case hardening process for low cost, high volume parts. The process also has the benefits of lower quench distortion rates, higher resistance to metal fatigue, and higher impact strength.
The surface hardness values possible with the carbonitriding process range from 55 to 62 HRC and average case thicknesses of 0.003 to 0.02 inches (0.07-0.5 mm) can be obtained. In cases where specialized compound surface characteristics are required, the part can be case-hardened initially to a depth of 0.1 inch (2.5 mm) and then exposed to carbonitriding afterwards. These composite hardening processes have the advantage of providing deeper and longer lasting carburized areas with the higher impact and fatigue resistance of the carbonitriding process. Parts that are commonly case hardened by carbonitriding include fasteners, lock pins, gear teeth, bearings, shafts, cams, and tools.
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