Correct or Incorrect? The answer is maybe. If you look at tempering color charts for steel Ken, you will see different temperature ranges and colors for slightly different tool steels. If you know what you have for spring stock, and have used it successfully many times before at a given temperature or color to make a good spring, then what you are assuming is correct. If you anneal some unknown spring steel, shape a complex spring, harden it and temper it exactly as you did with another spring steel, your spring may or may not be satisfactory. Some folks refer to tempering as annealing, but generally in the U.S., annealing is the process of heating steel dull red for an extended period, usually in an inert gas atmosphere to prevent scale formation, and then very slowly cooling to achieve a dead soft state. I've seen annealing processes as long as 28 hours. Tempering is a partial annealing process that brings steel down from it's maximum hardness.

https://www.google.com/search?q=steel+tempering+colors+temperatures&tbm=isc

It is the clarity of molten Potassium nitrate that makes it nice for tempering or drawing springs, or for blueing small parts because you can actually see the color and quench when you get there. Going all the way to the 633 degree F melting point, or beyond, may not give you a good spring. When you coat a part in powdered graphite and immerse it in molten lead, you are relying on temperature alone, and a thermometer of some sort is necessary since you may not have 100% pure lead, and your lead pot can get much too hot. Using stump killer that is not pure Potassium Nitrate may slightly change the melting point just as different alloys of lead have different melting points. You can heat either lead or potassium nitrate far beyond their melting points, as their boiling points are much higher. In short, using an accurate temperature controlled oven or knowing the exact temperature of your molten lead or potassium nitrate bath will only be useful for known alloys of steel or prior success with a given steel. Confused enough?

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