Not too many ovens go to 600 deg. Old timers method of annealing springs that are first hardened/quenched is to dunk in motor oil, and just burn off the oil. But a bullet casting thermometer can read to 600 deg, so you can use an electric hotplate, with a 50/50 solder bath to dunk the spring in to bring it to 600, let cool slowly(out of the solder!).

hippie redneck geezer