Since I'm working on my tempering oven maybe I'll explain a little abut heat treating and tempering, what it does and why I do it. I use 1095 high carbon steel to make knives. There are many steel's a person can use but 1095 makes a good strong blade that holds an edge. I also look at keeping cost in line and availability which lately has become hard. 1095 like any other steel is malleable which isn't good for a knife. When steel is heat treated it become very hard but also very brittle, so tempering comes into play removing the brittleness but the steel still remains hard. Without getting technical heat treating involve heating the steel to the correct temperature and then cooling (quenching & tempering) at a controlled rate. 1095 is high carbon so it is magnetic, in heat treating as the steel get red hot it becomes non magnetic. So I watch for the color of the blade and it becoming non magnetic to know when its at the right temperature to have a proper heat treat. The blade is then put into oil that is preheated (quenched) and after the quench it goes into the tempering oven for 2.5 hours. In tempering you need to maintain the temperature at close tolerances. Some temper their knives in a regular household oven. I have 2 problems with that. 1 the wife wouldn't let me do it and 2nd household ovens doesn't hold temperature very accurately. What I did was modify an everyday toaster oven. I looked for and found a used toaster oven that had the right kind of heating element. I removed all the stock controls and built my own temperature control sytem capable of running a PID loop. Working in the dairy industry as long as I did I had plenty of knowledge making and programming this. The toaster oven when I first got it with stock controls had a 75 degree swing at 450 degrees in an hours time, this wouldn't do to properly temper a blade. After I was done modifying I am able to hold a 2 degree tolerance over a 3 hour period at 575 degrees. To help make the oven more efficient I insulated to outer shell. I also had an old pizza stone that hasn't been used in some time so I cut it up and placed pieces in the oven to help hold heat. The update to my oven is a new longer thermocouple to make it a little more accurate, I also put a second layer on insulation on as the first was getting beat up from all the use. I temper my knives in the oven @ 450 degrees for 2.5 hours, then I turn the oven off and let them cool in the oven which takes another 1.5-2 hours.
