The basic process for 1075 spring steel is to heat it cherry red and quench it in oil, which renders it "dead hard", then to reheat it to draw it back (tempering). 500°F is one temperature that is used for this. It leaves the steel springy, but a bit brittle. Another temperature drawing range is 750°F. This leaves the spring slightly maleable, so you can bend it a bit, though it is more prone to taking a set than the lower tempered spring.
Under no circumstance use a temperature between 500°F and 700°F to draw the spring. That is the temper embrittlement range for common carbon steels. If drawn back within that range, the steel not only loses tensile strength, as normally occurs as the drawing temperature is increased, but at the same time it actually becomes more brittle than it was at 500°F. By about 700°F the ductility has returned to what it was at 500°F, and tempering at higher temperatures from that point upward continues the decrease in tensile strength and increase in ductility. Note this only applies to conventional carbon steels. Other alloys can have different temper embrittlement ranges. 416 barrel steel, for example, has a temper embrittlement range that is something like 700°F to 1100°F, IIRC.
The old fashioned method was to quench the spring in motor oil, then hold the oily piece in tongs while you set it on fire. The oil fire would bring it up to the lower temper range. The problem is that while that works OK on a heavier flat spring like a mainspring for a flintlock, it can overheat a thin spring.
The rolled flat spring stock at Enco will already be heat treated for cut and paste spring making. You cut it and form it, like music wire. It is the higher temper (temper comes from the same root as temperature, so "high" temper means softer and more ductile and "low" temper means harder and more brittle).
Here is a link to some
spring tempering information.
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