Carbonitriding

Carbonitriding process is most suitable for low carbon and low carbon alloy steels. In this process, both Carbon and Nitrogen are diffused into the surface. The parts are heated in an atmosphere of hydrocarbon (such as methane or propane) mixed with Ammonia (NH3). The process is a mix of Carburizing and Nitriding.

Carburizing involves high temperatures (around 900 ºC, 1652 ºF) and Nitriding involves much lower temperatures (around 600 ºC, 1112 ºF). Carbonitriding is done at temperatures of 760 - 870 ºC (1400 - 1598 ºF), which is higher than the transformation temperatures of steel that is the region of the face-centered Austenite.

It is then quenched in a natural gas (Oxygen free) atmosphere. This quench is less drastic than water or oil-thus less distortion. However this process is not suitable for high precision parts due to the distortions that are inherent. The hardness achieved is similar to carburizing (60 - 65 RC) but not as high as Nitriding (70 RC). The case depth is from 0.1 to 0.75 mm (0.004 to 0.030 in). The case is rich in Nitrides as well as Martensite. Tempering is necessary to reduce the brittleness.
source : http://www.efunda.com/processes/heat_treat/hardening/diffusion.cfm#PageTop