We have several parts ranging in size from 3" OD X .8" ID X .7" deep (largest) down to 1" OD X 3/8" ID x 11/32" deep (smallest). We are trying to get away from our current process of ion nitriding 4340 and are looking at a number of options. Achieving the case hardness is the most important aspect and core hardness should not be a concern.
I feel there has to be another reason besides case hardness that you are using 4340, that steel easily hardens through on your largest part, and Ion Nitriding is going to gibve you a extremely hard case, though thin.
I believe that you should be able to attain the surface hardness with carbonitriding as opposed to straight carburizing, but with the lack of alloying the effective depth will be shallow.
Based on past experience with similar specification UK steels you will need to carbo-nitride rather than carburise this steel to get Rc56 (we used to get Rc59-61).
I agree with NickE that you had better carefully evaluate the change in material. Case hardness and applicable surface treatments are one consideration, but when you change alloy composition, other parameters need to be considered for the intended service; core mechanical properties and notch toughness (if applicable).
Ion Nitriding being an expensive process and with few facilities available, many are considering to switch over to conventional case hardened parts. However, this change over needs to be done with caution as surface hardness is not the only criteria as advised by others in this post. Core properties are also to be considered. A candidate material quite often being considered now is 8620,this might make a safe transition to case hardening.
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