.................... The cutting process does take away some of the hardened material but some guys do a cryo and a micro polish to regain some of the rockwell hardness on the dog face that is lost when cutting them ...



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Cryo treating affects the core material of the gear.  Cryo will not regain any of the case hardening that is lost when it is machined away during the back cutting process.

Carburizing (case hardening) is a process where the gear is in a carbon rich environment at a high temp (usually 1500 to 2000 deg F) for many hours.  The length of time in the oven determines the depth of the case hardening. 

Gears cannot be re-casehardened after they are back cut.  Carbon infuses in the the surface of the gear as well as building up, changing the dimension of the gear.  Compensation for the case hardening  has to be made in the design and machining of the gear so that the gear will have the correct dimensions after case hardening.