Case Hardening 8620

We have several pinion shafts made from 8620, approx 8" long with 1"-2" varying diameters. Parts are case hardened .050" depth, to Rc 62. We are looking to machine through the case on one of the diameters, to add a keyway, and then send out for re-carburizing and re-heat treatment. Is it possible to perform this re-heat treatment process without detrimental effects on the already case-hardened surfaces? Would we be required to mask off the rest of the part, and carburize only the keyway? Any feedback would be appreciated. thanks!