Here's an old blacksmith's and gun maker's method for tempering that I use for carbon steel knife springs and it hasn't failed me yet.
After hardening the spring, I put it in a pan and pour in just enough oil to cover it. (I use used crankcase oil). I heat the container of oil with a torch until the oil is hot enough to keep burning and leave it until the oil is burned off and it cools. I don't know the temperature of burning oil, but it's quick, simple and it works every time.