I worked on an application where a shaft on which the spider gears of a differential ran steel on steel. Diametral clearance was critical, but without a white layer, seizure failure was assured. The white layer has an inherently low coefficient of friction, i.e. good lubricity, making it very beneficial when in a high speed but low load application. The following is a brief and simplified explanation of the difference between the white layer, sometimes referred to as the compound layer and the nitrided case. The white layer is formed by direct contact with the nitriding atmosphere and is where the ammonia dissociates into its component, the most important being nascent nitrogen, which reacts with the steel surface to form the white layer.
Diffusion then occurs from the white layer into the steel.
But more importantly, the nitrogen remains in solid solution in the steel. The nitrogen atom's much smaller size permits it to nest within the steel alloy atoms, strenghtening the surface thereby increasing its hardness dramatically. I just wanted to know what other engineers in the industry call out for the white layer on their nitrided gears. I think that the greates problem is the 64 Diametral Pitch gears that we have and the excessive case depth form poor process control. The name by which you'll be known in the forums.