I don’t see a picture, but from the wording, you are going to need some new
parts. Internal
parts. The case hardening could be on the stuff that holds the gears in there, or it could be on the
gear end it’s self. Not able to tell without pictures or more of a description. Either way, you’re going to need those new parts if that is happening.
Basically case hardening is a type of hardening on metal that is on the outer layer of it. If you overheat metal from say, not having any lubricant in the
gearbox, that can change the hardening of the metal. Make it weaker, brittle, it can do all sorts of things. So it sounds like it overheated and needs to be replaced.