Depth of penetration to case harden 1040 steel

I was attempting to find out the approximate depth of penetration of 1040 steel packed in graphite. The furnace temperature is 1800 degrees F and the time is 1 hour. I was wanting to have about 20 mil penetration and I am not sure if I am allowing enough time.

Thanks for your help.

Just packing in graphite without an activator mixed in, is not likely to give you much carburising. But anyhow, the way to assess the effectiveness of the process is to run it and observe the result, then adjust time/temperature/carburiser to move the result in the direction you want. As well as depth of penetration, you will also want to get the right distribution of carbon within the case, otherwise you'll risk cracking the case in the subsequent hardening heat treatment.