hmm. This has been interesting in a bleeding out the ears sort of way...
I don't know a lot of the particulars involved in making a rifle barrel, but I make knives and do my own heat treating, and have read a fair bit on it.
Maybe it will help some people if I lay out the very simple process that goes into heat treating one of my knives.
I start off with annealed precision ground steel. It is garuanteed within a certain tolerance for uniform thickness, but due to handling and stuff its never quite straight. Basically it has been bent, and due to it bein soft, it has taken a set bend (some stress build up, more importantly this is the shape it wants to stay in, like a warped peice of wood)
I do all the cutting grinding and machining, and by the time the blade is ready for HT, it looks completely straight. The grain/crystal structure still remembers the previous bend though, I just cut and ground stuff off till it was flat. So before the actual hardening process, I normalize the steel. This is simply heating it to critical temp (it becomes non magnetic, for the O1 I use this is around 1440 deg) and then cooling it off very slowly. By doing this you excite the atoms (or even molecules if you feel better calling it that
) They start moving an slidin around past each other, if you heat it too much they can move so freely that it becomes liquid. So now you've got the little guys movin around some, and you cool the stuff down slowly, they gradually relax into a more fixed state. And they settle there with the peice in its present form. No more stress, the crystal structure is aligned with the present shape.
Now its time to actually harden the steel. Pretty simple, heat it to critical temp and quench in oil. This was explained pretty well earlier in the thread. Austenite into Martensite, carbide matrix etc. All that complicated stuff. Now the steel is much too hard to to be used for anything, its very brittle.
So its time to temper (heat at 400 deg F for 2 hours, let cool slowly, repeat). This releives stress and it brings the steel into a useable range of hardness. Basically you have the same thing going on as the the normalizing process from earlier only on a smaller scale though.
Now for the cryo. I don't do this to my knives, but had it done on a few that were professionally heat treated (I can't do stainless, takes too much heat and too complicated) The cryo treatment is supposed to complete the transition from austenite(not good) to martensite (good) Some very accomplished bladesmiths I have talked to, seem to be of the opinion that if the heat treat is done properly to begin with, there is no need for the cryo. That makes sense. Basically the cryo is insurance.
I hadn't ever heard much of cryo being used as stress relief, but that makes sense. The same basic concept I talked about for normalizing takes place. Your dealing with a sort of phase change. Only you don't actually go far enough to get the change. Think of water, when its warm its in a liquid (disordered) state. When its cold its a solid (more ordered) state. So your using the extremely low temp of the liquid nitrogen to bring the steel into the most ordered state possible.
Is it needed for a gun barrel? I'd say yes and no. Its like insurance. If everything was done properly, the barrel should be fine without it. If there was a problem somewhere, it could possibly solve them.
Anyone else vote this for the most boring thread in the short history of THR? All I really care about is does it work? Beyond that I would like to chalk everything up as "01magic"
