I concur with Rngr on NOT using a hot tool on your beard. Aside from the risk of scorching your follicles into brittle tinder, you are in danger of burning your face! Your scalp skin is tougher than your facial skin, and your face is more susceptible to getting blistered.

Look for McBeard's thread. At mid-length he was using a straightening implement and quit doing it because of the damage it caused.