Several post above indicate a proper way to cryo and the way it's usually done. Can someone explain to me the difference?
I got this information from a professional heat treater, whom people who have had their knife blades heat treated, and cyroed, by him hold him in very high regard.
People trained in metalology would have a better understanding of this than I have, but when steel is hardened, it is said it goes to a austinizing temperature. Then, it is quenched in water, brine, air or oil, depending on the type of steel and it's alloying content.
This rapid cooling transforms the austinite to martinsite. I don't know what this is, but I understand that martinsite is good and austinite is bad.
When the steel is quenched, not all of the austinite is transformed into martinsite.
Now, here is the difference. As I understand it, when it is cooled, from the heat treating temperature to room temperature, it needs to continue the cooling cycle down to approximately -320 F without interrupting, for the austinite to martinsite transformation to be complete, and for as much as the austinite to transform as possible.
If this cooling is interrupted, as in quenched to room temperature, then tempered, there is still a lot of austinite left in the steel, and further cooling it does not transform any more austinite.
If cyro quenched at a later date, say next week or next year, and after the steel is tempered, the austinite to martinsite transformation does not start over from room temperature to -320 F.
A far as I know, when the cooling cycle is interrupted, and the steel is tempered, then cyroed, there is no change. What ever austinite was in the steel at room temperture is still present.
So, if a barrel is cyroed a year after it is heat treated I don't think there would be any change. I have seen advertisements offering cyro treating on rifle and shotgun barrels after they have been in service for years.
I would not pay the expense to cyro treat a barrel, or a knife blade, if the cycle has been interruped for any length of time. And by this, I mean, after it is cool enough to handle with your bare hands, it goes right into the liquid nitrogen.
Some people claim cyro treating will stress relieve the steel. It doesn't. When the steel is placed into the heat treat furnace, it is brought up to about 1000 to 1200 F, depending on the steel and it's alloy content, where it soaks at this temperature, before being brought up to the austinizing temperature. This is stress relieving. Freezing it is not stress relieving.
In short, the steel has to be taken down from the austinizing temperature to -320F in one complete cooling cycle for cyro treating to have any effect.
