cryo treatment does tug on me now and then. I want to ask what happens to a tube that was first cryoed, changing the grain structure of it's metal parts. Then when heated to operating temps, does that undo the 'goodness' of cryo treatment in the first place? Kind of like taking the temper out of a blade. All in all, I think that there is much that we don't know, but that doesn't mean it is suspect.
Using steel as an example... iron has a particular phase/crystal structure (atomic arrangement) at room temperature. If you raise it above a certain point, it will actually change structure, with some minor accompanying changes in properties. If you lower the temperature back slowly, it will revert. If you do it quickly (such as with quenching), you will "freeze" the structure in place before it can change back. The quench creates a lot of internal stresses (because this second phase does not normally exist at room temperature) and the formation of little carbides. Both of these lead to large increase in hardness, but also brittleness.
Tempering is reheating at a much lower point (well below the point where it changes phase), to remove some of the internal stresses and maybe change the shape of the carbides. Internal stresses are relieved by recrystallization (new grains) and/or grain growth, both of which literally eat up the highly stresses regions of the metal (predominantly in the grain boundaries and in lattice imperfections).
To understand part of how cryo works you need to understand how phase changes occur, which is easier with heating. When materials change phase, you are essentially moving them from one (relatively stable) energy state to another. However, to move from one state to another requires getting over a high energy wall/fence in simplistic terms. You have to push it over before it can fall back down and stabilize on the other side. So this is in heat treating where you heat something up to bring up its energy level and push it over a wall, then quench or do some other trickery to get it to stay on that side of the wall when you're done.
Internal stresses muck with things and add to the energy state of your material, meaning (but not always) that you need to put in less energy to reach over the wall. Also, moving over the wall in this case can permanently remove those residual stresses, so they don't come back later (sometimes). Thick of it like a shoddy step ladder that breaks as soon as you get over the fence
Cryo treatments... if I'm keeping to this analogy... a blizzard just rolled in and dumped 6 feet of snow, and the plows just built a mountain on your yard. What fence? The kids can sled down the hill like nothing's there. Ok I'm exaggerating. So uh where was I? You have a potential reduction in residual stress, recrystallization, and phase changes.
So coming back to the question of "undoing" the cryo treatment of a tube... well, uh, yes and no. Heating makes changes regardless. The cryo just changes (maybe) your starting point.
I think I saw something like that on TV recently. I remember that the reason the skin didnt immediately freeze was because of a layer of frozen H2O or air that protected the skin for short time that it was submerged.
The heat emanating from your hand will literally boil off the liquid nitrogen before it can reach your hand... For a very short time. It basically just creates a protective gas layer. Don't try this if you're a particularly hairy guy though, as the hairs will wick the liquid more quickly to your skin.
The inverse can also be done by dipping your fingers/hot dogs into molten metal. Moisture/airbubble will protect your fleshy dangly bits for a very brief moment, although the weight of the molten metal will make it push in much quicker than liquid. In any event, don't try that one at home and just watch the mythbusters episode.
edit: typos and clarification
