Seems like there is a lot of this question still up in the air.
My understanding is, the normal steels a person would forge (10xx, 15N20, W1/W2, O1, O2, L6, 5160, 6150, 52100) do not have enough retained austenite to need cold treating, let along cryo treating.
In Kevin's example of I-T curve with higher and lower soak temps (amount of carbon in solution) and the change in Ms/Mf (lower soak temps lowering carbon in solution and Ms/Mf), a person can find enough retained austenite at room temp that cold treating is an advantage. Commonly a gain of 1-2 points RHc. That gain bringing the RHc up to the max. level you would find in a reference (like Heat Treaters Guide) for one of the above listed steels. If the steel is HT-ed to spec in the beginning, the room temp RHc will be at max range and cold treating won't gain anything noticeable outside of a lab... that is... not findable in use.
Likely the most problematic of the common forging steels for retained austenite is 52100. Partly it's the chrome in it but mostly a higher retained austenite level comes from knife maker tendencies to HT it quite a bit differently than industry does, it seems to me.
The other common forging steels with chrome can have slightly higher retained austenite if HT-ed in ways that promote it. That they are usually HT-ed closer to industry spec limits the amount of retained austenite to a level where cold treating is not necessary.
Of the above listed steels, only O1 has an "optional" recommendation for cold treating (not cryo treating) in Heat Treaters Guide. Kevin Cashen does not cold treat his O1 or O1/L6 damascus and he would if he could find the difference in use.
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An aside...
There seems to be a tendency to run low-end austenitizing temps in HT-ing and it seems the primary driver of it is to avoid grain growth. There are two things going on here, it looks to me. There is an advantage for a knife to have small grain size... for a hardness, a knife with smaller grain size will be tougher. To have small grain size a person has to make them small, then keep them small. I think this last is where "the phobia" comes in... avoidance of above low-end austenitizing-range temperatures in the heating-to-quench phase.
Two things... grain growth does not really become problematic until reaching 1695F - 1795F, and getting steel to the desired temp quickly shortens the time grains can grow in. There is more-middle ground in austenitizing temps that won't grow grains and won't produce significant retained austenite.
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The other-than-forging-steels have higher retained austenite due to higher alloy content (commonly from higher chromium levels but not inclusively). All the "A" and "D" series steels have "optional" recommendations for cold treating (not cryo treating). The 440 series recommendation is to cold treat (not optional) "for minimum retained austenite and maximum dimensional stability (-100, +/-20) with continuous cooling from austenitizing temp".
Except for the 440 series, all other cold treating temps listed are at -120F.
I know other high alloy steels have cryo recommendations. I don't use them and I don't have industry reference to them.
Mike