There are two common types of nitrocarburizing: ferritic nitrocarburizing (FNC) and austenitic nitrocarburizing (ANC). In ferritic nitrocarburizing, the part is held below the steel’s critical temperature and exposed to an atmosphere of nitrogen and carbon. Temperatures are typically within the range of 1020­°F–1110°F in an atmosphere of 50% endothermic gas and 50% ammonia. Other atmosphere combinations are also possible.

Nitrocarburizing requires a controlled atmosphere furnace. Like most case hardening processes, nitrocarburizing increases fatigue and wear resistance. Additionally, nitrocarburizing can be used on inexpensive steels to create high-hardness and shallow cases.  Since nitrocarburizing occurs at a lower temperature than carbonitriding or carburizing, there is greater distortion control, making it a good option for design engineers looking for a combination of wear resistance and strength.

Austenitic nitrocarburizing occurs in the temperature range of 1250°F–1425°F. It creates a surface compound layer of epsilon carbonitride and, after quenching, a subsurface layer of bainite and/or martensite. Austenitic nitrocarburizing is best suited for intermediate-stress or point-contact-resistance applications.

Source: Dan Herring, The Heat Treat Doctor®, Vacuum Heat Treatment, Volume 1, pages 383–384.