I more or less agree with Phariss, but I'd like to point out that "properly casehardened" means some number of hours in a red-hot pack of wood charcoal/bone charcoal/charred leather. Kasenit, which we all have used, just puts on a very, very light file-hard case. Great for keeping your engraving from wearing out, will do no harm on hammers. But, IMHO, is too shallow for decent life with tumblers and sears.
My first experience with metallurgy was the local blacksmith handing me a can of Kasenit, pointing me to the forge fire & telling me how to harden the frizzen on my $5 Miller Bedford flint lock. It sort of sparked the few times I used it.
Kasenit is now banned, Our Gov't says the potassium ferrocyanide in it is bad for you (if you read the lable on your grocery store brand of salt you may find it there as an anti-caking agent).
There is a modern replacement for Kasenit, I personally know nothing about it or how it works.