All carbon steels will have their own particular temperature that will cause them to become non magnetic. The temperature range that this happens in is small, but important. The quenching temperature to harden is about 50 to 100 degrees higher. Very much higher than that and you will experience a rapid grain growth in the steel. This is what causes the steel to break even after tempering. This was a very expensive lesson for me to learn during the first year or so of forging. The time that you hold a steel at a given temperature will depend on the thickness. The one old blacksmithing book I used to have, said to quench on the rising temperature. I assume this would make sure that it didn't get too hot.
I only heat the part I want to harden and I can check it with a file and then after cleaning, put in ferric chloride for about 3 minutes. The hardened area will be darker. If you didn't over heat it, and the area of hardening isn't large enough for you, you can reheat and harden again.
The 52100 that I use will show a difference in color between the steel that is hot enough and the rest. On O1, I think that there is supposed to be a soak time if you are using an oven. An oven will eliminate the over heating possibilities when hardening steel.
The best way to find out the temperature needed to make a steel harden it to start out on the low side and reheat till it becomes hardened. This will work for just about all the carbon steels.
Hope this helps.
