Good point. I was assuming you could turn the speed down on a bench grinder (I don't have one myself to know yes/no). Heat could definitely destroy temper in some metals (I'm thinking spring steel). I don't know about something solid like an ax head. I remember grinding a "waist" in a flat spring from one of my firearms to lighten it's springiness (the loading gate spring from a lever action rifle) - I was grinding that with a Dremel tool and holding the piece in my hand, not in a vise or with pliers. As soon as I felt any warmth (only took a few seconds to develop), I would dunk the piece in a cup of water to cool it. I was worried about ruining the temper of the spring. Same thing may go for an ax head, but since it's not a spring, I don't really know.