Heating a spring to red hot, is not something you want to do. Heating a spring destroys the temper, softens and weakens it. (I cut and fit springs several times a day)
".....With knowledge of the composition and phase diagram, heat treatment can be used to adjust from harder and more brittle to softer and more ductile.
In the cases of copper, steel, silver, and brass, this process is performed by heating the material (generally until glowing) for a while and then slowly letting it cool to room temperature in still air. Copper, silver[1] and brass can be cooled slowly in air, or quickly by quenching in water, unlike ferrous metals, such as steel, which must be cooled slowly to anneal. In this fashion, the metal is softened and prepared for further work such as shaping, stamping, or forming......"
https://en.m.wikipedia.org/wiki/Annealing_(metallurgy)