There appears to be a misunderstanding about sub-zero quench; perhaps it is the name's fault. You do a normal quench (typically oil for O1, then, after it reaches room temperature it is placed in a sub-zero environment. You don't go from cherry red to sub zero, this will crack many metals.
Going colder than liquid nitrogen is a waste of time (and money). You can actually get great results with resting the steel between layers of dry ice (frozen CO2 which is at negative 110F). The more ideal cooling medium would be one which gets you down to negative 135ish, or a bit more.
