Carbonitriding is a variant of case hardening and not a nitriding process (not to be confused with nitrocarburizing). Carbonitriding entails, not only carbon, but also nitrogen being added to the steel surface through the inclusion of ammonia in the furnace atmosphere during carburizing. It can be applied through both gas and low pressure carburizing.

The addition of nitrogen greatly improves surface hardenability, meaning that its presence enables the same level of hardness to be obtained with a lower alloyed steel. The increase in hardenability also counteracts the decrease in hardenability caused by the depletion in alloying elements in the surface caused by internal oxidation (gas carburizing) or manganese effusion (low pressure carburizing).

Ref. Steel and its Heat Treatment – A Handbook (8.4.9)