With this instructable, we will take a cheap toaster oven and turn it into an accurate, temperature controlled tempering oven that will be able to achieve a stable and accurate temperature controlled by a microprocessor.
We will combine some common and off-the-shelf components to easily and safely achieve this goal.
Why do we want to do this? After heat treating steel, it's in a very hard and brittle state. So we can use it with less risk of it snapping, we want to temper it. Tempering will slightly soften steel, but more importantly will restore flexibility to it. Most steel manufacturers specify particular temperatures to temper steel at so you can achieve a given hardness rating.
Having the temperature accurate to a fraction of a degree isn't critical for tempering, but I want more accuracy and repeatability than the simple bimetallic strip thermostat that most inexpensive ovens use. ±10°C could easily be a difference of 1-2 Rockwell C points in hardness.