Case hardening is still used for very many applications. One example is the common CV Joint: The Outer and Inner Races are fabricated from SAE 8620 steel that is case hardened to a depth of around .010 to .015 in. This material and process results in hard and tough components that probably cannot be equaled with a through-hardening process.
Case hardening results in a composite structure. Theoretically, if a crack initiates at the surface, propagation will be halted by the tougher interior. Also, the act of hardening the surface (martensitic transformation) results in a surface with residual compressive stress (thereby increasing hoop strength, a very good thing in the case of the receiver of a rifle).
But which is better in the long run - surface or through hardening? I honestly don't know. That is why I am soliciting information.
