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Much of this depends on which 52100 you buy. Much of it on the market today is heavily spheroidized. By heavily spheroidized I mean that the carbon in the steel is locked up in larger spheroidal carbides, which are very stable, and slow to dissolve. This will necessitate a longer soak at precise temperatures and in some instances that is not enough. Forging at proper temperatures is your normalizing and will undo much of this, it is also beneficial to follow up with proper normalizing. If you are stock removing this steel there is no forging to undo the spheroidization so normalizing becomes even more necessary if it is heavily spheroidized.
Liquid nitrogen for retained austenite on a low alloyed steel such as 52100 is merely putting a band-aid on the problem rather than avoiding it to begin with. If one sees a huge jump in HRC from freezing 52100 they overheated it in hardening. You may encounter this problem when heating in a forge if your eye isn’t really good at holding the temp you want.
If you have the ability to dial your gas forge down and have access to a pyrometer, this may still be not much problem. Do all of your heavy cutting and machining (especially drilling) then normalize the blade with a little final grinding left to do. Then you will be going from a very fine pearlitic/upper bainitic microstructure into the heat treatment, this will eliminate much of the need for demanding soak times. If you keep your steel just above decalescence (the shadows have just left the blade) and then quench, you should be able to get pretty acceptable results.
There are different levels of results, terrible, better, acceptable, real good and optimum. As we have already discussed, with all the wackadoodle heat treatments people are using on this steel and still liking the results, it is safe to say that there is a very wide latitude of results that will still make a serviceable blade for most people. We sweat over one or two percent because that is what drives us to get better, but for the most part is beyond what most will notice.
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