I don't think you can compare car springs to gun springs. Just re-hardening it without knowing the proper temperature will result in spring either breaking or not working, and the only one that would know would be a good gunsmith, a good one.
There is a certain temperature for all hardened parts and a certain temperature for annealing them. This work must be done in an oven and not with a torch, as the temperature is achieved too fast.
If you have a replalcement, some are oversize and I suggest you measure the old one and grind the new one to shape, keeping a dish of water so as to not over-heat during grinding.
Keep the old one for a copy.
Most likely, the other spring was not of the right temperature to begin with.
There are some guns that I have that are over 120 years and the springs in the locks and top lever are still strong.
