Hi Mike,
Re:
CHANGING THE TEMPER OF A COIL SPRING
You will not really be changing the temper. That would mean annealling & then re-heat treating it/them.
You can get less strength; actually, you would be eliminating all of the strength. I would suggest working ( heating ) at the end of a coil(s) depending upon the actual design of the spring(s).
Effectively, you are taking the heat treatment/spring rate out of the spring(s) by heating them/it.
I am thinking that maybe this is on that new replica car that you bought. If so, I would be very, very careful with playing around with the suspension. I would look for springs with a different spring rate that would fit. They can be very hard to find ( if never to be found ).
You might Google spring manufacturers and then contact a couple of them for their input.
Just my $0.20, it is worth what you paid for it. :yikes:
Jerry Baumchen
A long-time retired Mech Engr; but I did study spring rates about 34 lifetimes ago.