2-p, re case hardening adding strength to the hardened part:

Take a look at the modulus of various steel alloys both soft and hardened. I think you will find this number to be quite similar, esp. for plain carbon steels. This means that up to the yield point of the core the case will have the same strain and will not add anything beyond its "soft" strength. Above the yield point for the core, it will be permanently deformed while the case will attempt to return to original dimensions.

Rumnate on that and let me know.

DDA