Here is a generic formula for heat treating simple carbon steel springs. First, you will need a little can a vermiculite or hard wood ash, a can of oil for the quench ( olive, peanut, transmission fluid).
The spring needs to be annealed first, heat it to a very dull red in a dark room, imediately put it in the vermiculite /ash. It will help alot of you can heat a larger bar of steel to go in with it. The keey is to cool to room temp SLOWLY. Do this three times. This refines the grain of the steel and greatly enhances the toughness. Polish, make sure there are no deep scratches anywhere
Next, using a magnet to check for proper temperature, heat the spring EVENLY until the magnet has no attraction to it. Quench it in the oil until room temp. Clean it off, polish it again and put it in the oven at 500 degrees for two hours, let it cool, temper it again, same temp for two hours. It should be a bright blue color.
I've made litteraly thousands of springs. The most important step is the anneal prior to hardening. You can't skip this step as it puts the steel in a known state ( dead soft with as fine a grain structure as you will achieve). The fine grain structure will more than double the toughness and flexability of the spring over "normal" grain.
If you have access to a digital furnace, even better. The temps are 1350 for the anneal, simply leave it in the furnace and turn off the furnace to cool. Heat to 1450, quench and temper at 500.
