Quote Originally Posted by ggggr View Post
Could it not just be thrown in the oven to retemper? I'm sure someone on here straightens old springs and puts them in the oven for half an our then turn the oven off?
No, you can't retemper a spring that is too soft just by heating it, but you can retemper a spring that is too brittle and make it softer if you heat it to a temperature higher that in was originally tempered at.

The trouble is you only find out that a spring is too brittle after it has snapped!

The heat treatment sequence for making a spring is to first heat it to a red heat and quickly quench. That makes it extremely hard and brittle. Then you remove the brittleness by tempering, which means heating it again, but to a much lower temperature and letting it cool naturally. The higher the tempering temperature the softer the result. The trick is to get the correct temperature for tempering, as a good spring has very specific requirements.