....For hardening to take place it is necessary the part be above its critical temp & then quenched....
....Often times the temp will be raised somewhat for the Carburising stage & then dropped for the hardening phase, but, not below that critical temp....
....Most of the warpage & size change occurs at the quench & the higher the temp above that which is an absolute requirement the more likely it is to occur....
Only conversation Miller. I think games can be played with critical temperature, not necessarily for good heat treating practices.
I've mentioned before and it's easy to verify. Critical temperature is not necessarily the by the book the best temp. for quenching, but it's a practical temp. that can be verified with a magnet. On the way up in temp., carbon steel will loose its magnetism at a very sharp point. But, if one lets the temp. slowly drop, it will be very apparent that the steel becomes magnetic again slowly at a much cooler temp. than it did when hit critical on the way up. Anyway, quenching non magnetic steel will harden it, even if it's not good heat treating principles.
I believe that's a possible intended, or unintended, benefit of delaying quenching the contents of a charcoal pack. The parts may be at a lower temperature than the oven was set at, and the part may have significant temperature differences from thinner to thicker areas, affecting color distribution. It may also be a strategy to attempt to minimize warping. Again, only thoughts, nothing more.
