[rick-in-seattle]

This is the type of machine I use.

http://www.b-cal.com/indentron.htm

What a rockwell test measures is the resistance to penetration of a diamond point under 150 Kg.

Mike,
You asked "How hot is too hot?".

My tests demonstrate that a Dark Straw color will still be adequate for carving.
It also shows that should you over-heat the steel and it turns blue, you must either grind out the blue area or re-heat-treat the steel.

I am thinking of doing a more in-depth test using high quality tool steel.
Perhaps showing the hardness as purchased, after annealing, after hardening, and after tempering at different colors. Who knows, I might even learn something.
It might be an interesting subject for a webpage.