With drill rod springs, I always bend red hot rather than annealed and cold. I suspect more than a few would crack with cold bending. Also with drill rod springs, the colour to go for is when dark blue just changes to grey (when drawing the temper from brittle to spring) By using molten lead, you get a uniform heat with no hot spots or cold spots. I use a high temperature thermometer but before I had one, I floated a thin piece of mild steel on the surface of the lead and when the temperature was reached, pulled the plug and quenched the hardened future spring in the lead and kept it there for as long as I dared without the lead freezing.
Some material like car leaf springs appear to require much higher temperatures to be drawn to spring temper, at least in my experience
cheers mooncoon