Carbonitriding

Carbonitriding

The purpose of this Section is to establish methods for specifying carbonitrided gearing. the information in carburizing will generally apply to carbonitriding, with noted exceptions.
Typically carbonitriding is carried out at lower temperatures, 1550-1650_F (843-899_C), and for shorter times than gas carburizing. Shallower case depths are generally specified for carbonitriding than is usual for production carburizing. Its effect on steel is similar to liquid cyaniding and has replaced cyaniding because of cyanide disposal problems. Normally 2.5 to 5 percent anhydrous ammonia is added to the carburizing atmosphere when carbonitriding. Specified case depths are usually from 0.003 to 0.030 inch (0.076 to 0.76 mm) maximum.

Applications (Advantages and Limitations).
Use of carbonitriding is more restricted than carburizing. It is limited to shallower cases for finer pitch gearing since the process must be conducted at lower temperatures than carburizing. Deep case depths require prohibitive time cycles. One of the advantages of carbonitriding is better case hardenability in lower alloy or plain carbon steels. The carbonitrided case has better wear and temper resistance than a straight carburized case. Carbonitriding can be used to minimize distortion in finer pitch gearing because lower austenitizing and quenching temperatures can be used along with less severe quench techniques and still achieve hardness. These facts, along with lower alloy steels, result in the lower core hardness mentioned previously, thus reducing tooth growth and distortion. However, if higher core hardness and deeper case depths are required for bending resistance, carbonitriding may not be applicable.
Materials.
Typically carbon and low alloy steels such as 1018, 1022, 1117, 4022, 4118 and 8620 steels are used for carbonitriding.
Specification and Inspection.
Case depth, microstructure, hardness, etc. for carbonitrided parts can all be specified and evaluated as prescribed in the section for carburized gearing. Case depth is specified andmeasured as effective or total, depending upon application. Cases shallower than 0.010 inch (0.25 mm) are generally specified as total case depth. The advantages and limitations as described herein should be fully understood before specifying carbonitriding for industrial gearing.