Something I have been wondering about is the ETA carbides that form in cryo, but not sub-zero. IIRC, they are only there after cryo, and once tempered they dissolve. Is this true? At a structural level, what improvement would cryo result in over sib-zero, as long as Mf is reached by -110f? It seems obvious that Mf at -200f would benefit from cryo, but that's not what we are talking about here.
The stuff I read indicated the eta carbides did not form directly from the cryo treatment, but the ultra cold temperatures conditioned the martensite and led indirectly to the formation of eta carbides during tempering.