What kind of heat are you talking?
Most Carbides will easily stand up to temperatures used to braze it to steel
shank holders.
However as with most things it won't do it any good if too much heat is
applied.
Gradual heating and cooling is ok. Quenching with water or compressed air may
lead to cracking of carbide.
If you are talking heating during grinder, you still want to avoid over
heating and cooling to quickly.

Kori wrote:

When I am making a tool bit out of tungsten carbide, do I have to keep it
cool? Does heat change it's hardness, or does this just apply to tempered
steel?

Kori