Powder Coating Torsion Bars

Firm Feel powder coats all of the torsion bars they sell, as do some of the other torsion bar manufacturers. Unless you heat them up until they're darn near glowing red you won't hurt the spring rate.

Says right in the description they're powder coated, and I can confirm all the sets I have bought from them are. 60k+ miles on the set in my Challenger.
Firmfeel Mopar Suspension and Steering

***Edit***
Hotchkis powder coats their torsion bars as well
41 in. 1.03 in. Forged Torsion Bars for Mopar B and E Body Models