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What is Carbonitriding?

Paul Scott
Paul Scott

Carbonitriding is a metallurgical surface hardening process used to improve metal parts' resistance to wear. Similar in essence to the conventional gas carburization processes, carbonitriding features the inclusion of a nitrogen component in the form of ammonia. Due to the shorter cycle times and lower temperatures involved, the process is more economical than carburizing, thereby making it a more desirable process for mass produced, low cost parts. The process also features additional benefits such as increased resistance to quenching distortion. It is used to case harden parts such as gear teeth, bearings, and tools of various surface hardness values and average case 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 case, of harder, wear-resistant material on the surface of the part while the internal structure remains malleable and tough. Carburization is the oldest form of case hardening and involves heating the part in the presence of a carbon rich material. The carbon permeates the surface of the part during this process, thereby creating a very hard surface layer when quenched or cooled rapidly. Carbonitriding is similar to carburizing with the exception of the addition of ammonia to the carbon source, lower process temperatures, and shorter cycle times.

Man with a drill
Man with a drill

During the process, ammonia is introduced along with the carbon material as a source of nitrogen. The inclusion of nitrogen allows shorter heat exposure cycles and lower process temperatures to be used. The average temperature of a conventional carburizing process is 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 a significant saving in production costs, thus making it an ideal case hardening process for large volume, low cost parts. The process also features the benefits of lower quenching distortion rates, greater resistance to metal fatigue, and higher impact strength.

Surface hardness values possible with the carbonitriding process range from 55 to 62 HRC, and average case thicknesses 0.003 to 0.02 inches (0.07–0.5 mm) are attainable. In instances when specialized combination surface characteristics are required, the part may be carburized initially to depths of 0.1 inch (2.5mm)and then exposed to carbonitriding afterward. These composite hardening processes have the benefit of offering deeper, longer lasting case hardened areas with the increased impact strength and fatigue resistance of the carbonitriding process. Parts which are commonly case hardened by carbonitriding include fasteners, locking pins, gear teeth, bearings, shafts, cams, and tools.

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