Mr. dghq:
As you know, hardening a spring is a simple matter of heating it above the upper critical temperature and quenching it. Tempering, now there's the rub. There are two ways to approach it, one is art and the other science, both work fine.
Heating to the proper oxidation color or putting it in a tin of oil and burning the oil and a few other methods I have heard of are art. Or alchemy. They work very well for those who have mastered the techniques through experiment and experience. I am in awe of that sort of skill.
The other way is science. You simply heat the hardened part up to the proper draw temperature for the steel, soak it at that temperature for a period of time and let it cool. The draw temperature is dependent on the alloy chosen, but for a spring steel will be typically in the neighborhood of 700 degrees F. This information is available in tables that specify the draw temperature required for a desired hardness for a specified steel.
There are a number of ways to heat to a specific temperature. A heat treating furnace would be nice. <g> Howsomever, if you happen to cast bullets you can use your electric lead pot and thermometer. Just get the lead to the right temp and soak the spring in it. Dipping the spring in oil before immersing in the lead will help keep from tinning it. I use a cheap Lee lead pot with Nitre Bluing salts in it the same way. No worries about tinning and the spring comes out real pretty. <g>
A precision electric hot plate will work well, but they ain't giving them away either. Perhaps the cheapest route would be a temperature indicating product like Temp-Lac or a Temp-Stick. These are compounds that melt a a very specific temperature. Put a mark on the spring and heat on an electric stove top until it melts, remove the spring and let it cool. A temp stick will cost about $10 from MSC or J&L. I suspect Brownells will have them as well.
My goodness, this got wordy...
Glenn
