Nitriding is a process of hardening steels.  The material to be hardened is heated in an atmosphere of ammonia at a high temperature below the final tempering temperature of steel.    Nitrides are formed in the surface. 

The formation of nitrides hardens the surface.  Nitriding can be done only in steels that contain elements, which can form nitrides, such as molybdenum, aluminium and chromium.

Unlike other methods of hardening, nitriding does not require quenching.   After nitriding, the components are subject to stress relieving processes.  Sometimes, the surface formed is ground to remove the outermost layer which may be brittle.