A few words re: cryo stress relief

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COPYRIGHT

I just had to mention copyright....thanks for the attribution.

The subject of cryo treating various metals at various stages of production and the benefits derived is -- has been --- and will probably be -- kicked around machine shops for years.
In all the processes of metalworking, heat treating is one of the last that retains a "little bit of art and a little bit of magic." It is a demanding science and process. Cryo is, relatively, the new kid on the block. It will be used, explored, developed and perfected. Is it the answer to all problems? No. A professional discussion of cryo can get as fractious as .45 vs. 9mm.
Cryo is a fascinating subject. It is a subject Modern Applications News will continue to cover. How do I know?
Because I am the new editor of the magazine.:rolleyes:
 
I don't know why it didn't occur to me that my copying and posting the article re cryo might have been a violation of copyright. Certainly didn't mean to step on anybody's toes.

I've been aware, from the beginning of this discussion, of my own ignorance regarding metallurgy. I found the article and thought some might find it interesting. My ignorance of metal structure is irrelevant to the question at hand: "Does cryo treatment affect the structure (whatever that structure is) of barrel steel in a positive manner?? Does cryo relieve stresses in metals??"

Apparently, there are many metallurgists who believe that cryo works, and, from the article, there seem to be many end-users of cryo'd products who have found that cryo works.

According to our resident expert, cryo didn't work, period. Not in tool steel....not in any steel.

Having said everything that I've said so far, I'd like to add that I wouldn't spend $.05 of my money having a barrel cryo'd. (Not that I don't think the cryo would work, but that any marginal accuracy increases would be irrelevant to me.)

Either cryo works, and the many companies now offering cryo treatment are offering a beneficial service, or all those companies are frauds and scams. Time will tell.
 
I have no doubt that cryo works. It's more a question of application. There may be reasonable uses on some sort of firearm, but I doubt that the process has any use on a $600 hunting rifle.
 
Cryo, as a process, is neither a fraud nor a scam. Also, it is not the universal answer. In some applications it works very well. In other applications it's benefits are small. Yet, in others the benefits are still being discussed.
A quick point, the stage at which the process is used is critical. As an example, I've read where a premium barrel manufacturer uses the process with fluted barrels. However, the cryo treatment is employed after the fluting of a barrel.
Also, the material being treated is most important as is the process itself, like other heat treatments.
Heat treating is one of the most complex technologies in all of metalworking.
P.S. Metalworking and firearms have a fascinating shared history and future.
Thanks for reading.:D
 
I'm not smart enough to know if cryo is art, science or pixie dust. I am smart enough to appreciate the results: I own a light barreled rifle that would spray bullets all over the place as the barrel heated up. It suddenly became a tack driver with no other change but a $40 cryo treatment.
 
Aw, just stay with the regular dosage, Badge. :)

I'm still rolling this atom/molecule thing around. Part of the problem for me is that Inorganic/Quan/Qual was back around 19mumble-mumble. Before JFK, if that's a hint. :) And then my engineering world turned to dirt, as in dams and reservoirs...

"Air molecules" are spoken of, but air is a mix--yea, verily, like unto an alloy. I realize ya got two atoms each of Oxygen and Nitrogen, making O/2 and N/2, yada, yada. But how about the inert, trace gases?

Are two Oxygen atoms chemically bonded as a molecule of O/2? But a gaggle of Fe atoms merely hang out together?

Art
 
"Are two Oxygen atoms chemically bonded as a molecule of O/2? But a gaggle of Fe atoms merely hang out together?"

Correct.
 
Hokay. A summary of all this seems to be that, first, we know that stress-relief of machined or welded steel fabrications is beneficial.

In the past, some sort of heating to temperatures well above ambient and then cooling at some controlled rate has been the normal procedure.

Cryo-cooling as stress-relief is still mostly unknown, still in the cut-and-try stage, at least in the gunsmithing world. There are still arguments about repetitions, about rates of cooling and of re-warming to ambient, and even whether a further heating/cooling might be desireable.

Close enough?

Reports from shooters vary as to the success for their particular rifles. Some found it beneficial; others have reported no notable change (harking back to some TFL posts).

Art
 
Here's my take:

If you take a piece of typical medium carbon steel and heat it good and hot, the internal crystal structure rearranges itself into a structure called austenite.

If you then cool this steel slowly it transforms again as it cools to a stucture composed of grains of more or less pure iron (ferrite) and grains composed of layers (lamellae) of ferrite and of iron carbide. The lamellar structure is known as pearlite. This mixed structure is comparatively soft.

If, on the other hand, you cool the hot steel very quickly, the austenite structure undergoes a rapid transformation to martensite, which is composed of very fine needles of carbide in a ferrite matrix. This is glass-hard and very brittle. You can then temper this structure by more or less warming it, to allow some of the carbide to begin to rearrange itself into a less hard but tougher arrangement (under a microscope the needles appear to get less sharp). By the way, the austenite-martensite tranformation is accompanied by a small change in volume, as the atoms and molecules (eg Fe3C) rearrange themselves.

Now, here's where cryogenic quenching comes in: in some alloys and under some circumstances no matter what you do there's some untransformed austenite left at room temperature. This can be a problem in some situations as retained islands of austenite in a martensite matrix cause internal stresses (due to the volume difference) and soft spots. This has been a particular issue in certain tool and die alloys, where the austenite-martensite transition is not complete at room temperature. Cryo has been a real success here, in driving the martensite transfomation to completion, and this is indeed the mode by which it works to relieve internal stresses and improve physical properties _in those specific alloys_.

Typical rifle barrels don't have this problem: the martensite transformation is complete at room temperature in the alloys typically used (eg 4140, 416). IMHO Cryo treatment is just superfluous for rifle barrels.
 
Hokay, Daniel, lemme run this by you (and anybody else). It's a lot of inductive reasoning, I guess; I'm not in love with the idea:

If the piece of steel that will become a rifle barrel is not homogeneous in its mixture, there will be stresses created by the machining process as one cuts across areas of non-homogeneity. After machining, stress relief via heating and quenching, etc., makes everything all wonderful--or is supposed to.

We know that a barrel acts as a spring; we know it is subject to rapid heating and cooling. Stress relief apparently/allegedly allows more uniform vibrations during different conditions of temperature. (That is, it should vibrate the same for the first shot of a string as for the fifth shot.)

Now, whether some barrel steels inherently need further cooling below ambient to complete a martensite transformation, I dunno. However, since there are some reports of improvement via cryo-cooling, it seems to me that its benefits cannot be rejected out of hand.

I guess if one knows his barrel is of 4140 or of 416, worrying about cryo-treatment is unnecessary nit-picking.

Art
 
My last comment here guys. "stress relief via heating and quenching" no, heating and cooling slowly. Let me describe things this way Suppose you make a barrel that is not straight. You bend it to straighten it , this puts residual stress in the barrel. As you shoot it this temporarely relaxes the stress and the barrel goes back to to the original nonstraight condition .as it cools it will return to the straightened condition. This doesn't do much for accuracy. If you heat the barrel to stress relieve it then as it heats or cools during firing there will be no movement. Remove stresses and you get a more accurate barrel .Simple as that !!!
 
Hokay, so pardon the nitpicks*. I'd like to make sure I got the jargon straight, in talking about stuff where I "sorta know" some of the generalities:

Quenching = rapid cooling, having to do with tempering?

Stress-relief temperature changes are slow, whether in the heating or the cooling?

Art

* For instance, I don't fuss at people who speak of a reservoir's "silting up". Reservoirs trap sediments, some of which might be silt. :)
 
Art,

Let me correct you slightly.

Reservoirs are where you get oil and gas from. Silt is useful as a seal to keep the oil and gas in place so we can come along and drill into the reservoir and produce that oil and gas.

By the way, Hi from East Texas to West Texas (where all the good reservoirs are!).
 
So what does all this techno info have to do with accuracy in rifle barrels, tests show not much.

In tests conducted by the NRA recently the only advantage seemed to be with standard weight rifle barrels of production line quality not match barrles or heavy match barrels and the improvement in accuracy in my opinion was so small that other variables probably had as much to do with the increase in accuracy as the cryo treatment.

In short you are wasting your money by having your match barrel cyro treated.

At our own shooting range heavy barrel match rifles that were not cryo treated often outhshot those that were with groups often going into 1/4 inch at 100 yards out of semi-auto AR15's that were not treated.
 
G'day Art

"If the piece of steel that will become a rifle barrel is not homogeneous in its mixture, there will be stresses created by the machining process as one cuts across areas of non-homogeneity. After machining, stress relief via heating and quenching, etc., makes everything all wonderful--or is supposed to"

Well, um, yes and no. There may be residual stresses due to machining, especially if done badly (e.g. blunt tooling -> high friction - > localised heating). Residual stresses may also be due to plastic deformation, such as occurs in button rifling or straightening or, to a more significant degree, hammer forging. Such residual stresses may or may not be significant, and may or may not be a problem, depending on the end-use of the item. In the case of rifle barrels however, _unevenly-distributed_ residual stresses may cause the barrel to "walk" as it heats up. In the general run of barrel steel therefore these stresses may be relieved by normalising, being controlled heating and slow cooling.

"We know that a barrel acts as a spring; we know it is subject to rapid heating and cooling. Stress relief apparently/allegedly allows more uniform vibrations during different conditions of temperature. (That is, it should vibrate the same for the first shot of a string as for the fifth shot.)"

Yep.

"Now, whether some barrel steels inherently need further cooling below ambient to complete a martensite transformation, I dunno. However, since there are some reports of improvement via cryo-cooling, it seems to me that its benefits cannot be rejected out of hand."

In certain very specific situations, there is a different mechanism which can lead to residual stresses. This is that due to the difference in volume of austenite retained in a martensitic matrix, and is usually a product of specific alloy compositions, often exacerbated by long soaking at austenitising temperature. Stresses due to _this mechanism_ may be relieved by ensuring all the austenite is transformed to martensite, and in some cases, especially with certain tool steels, cryogenic quenching is necessary to do this.

Retained austenite is not at all likely in the steels used in rifle barrels, which complete the transformation to martensite at well above room temperature.

What I'm getting at is that cryo is not a generalised stress-relieving process. It is specific to a particular cause of residual stresses in certain specific situations, where the results make it very worthwhile. These situations don't include the treatment of steels typically used in rifle barrels.

BTW most rifle barrels these days are either of medium-carbon CrMo steel, usually 4140, or a machinable martensitic stainless, usually 416R.
 
Art

"Quenching = rapid cooling, having to do with tempering?"

Yes, and sort of. In the context of heat treating typical martensitic steels quenching means cooling sufficiently rapidly to cause the transformation to martensite: cool to slowly and it won't happen. The rate of cooling required depends on the alloy and the cross-section of the workpiece. Some alloys for example, require very severe quenching, into brine for example, to produce martensite to acceptable thicknesses. These are said to be of low hardenability. Othe alloys will be sufficiently quenched in oil or even air (oil-hardening and air-hardening respectively).

Now once you have quenched to martensite you have an extremely hard but brittle product, which is not terribly useful. Tempering addresses this by controlled heating of the workpiece to cause some diffusion of the structure, thus progressively increasing toughness and lowering hardness with time and temperature.

"Stress-relief temperature changes are slow, whether in the heating or the cooling?"

For these types of steels, broadly speaking, yes.
 
G'day BHP9

"In tests conducted by the NRA recently the only advantage seemed to be with standard weight rifle barrels of production line quality not match barrles or heavy match barrels and the improvement in accuracy in my opinion was so small that other variables probably had as much to do with the increase in accuracy as the cryo treatment."

I'm not surprised. I've yet to see any persuasive evidence for cryo treatment of rifle barrels.
 
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Daniel,

SHHHHHH, sheez, don't you know the gun Nazis will hear you speaking about, well, GUNS. I didn't know you guys could own guns down there anymore.

Seriously, is there anybody down under who is doing this cryo stuff?
 
G'day Badger

Yeah, we can still own guns, at least for the time being :(

As for cryo treatment, there are outfits offering it for tools and dies, and certain other applications. One is run by a bloke I used to work with many years ago.

I'm not aware of any advertising it for improving rifle barrels, but there again we have fairly strict laws on misleading advertising;)
 
Peter, no argument about them there subterranean reservoirs of awl and natgas.

Howsomever, ya got the Lake Conroe reservoir and the Lake Houston reservoir...and Sam Rayburn and Toledo Bend and other examples of that good French word, so beloved of the USCE, the TWDB and various river authorities. Ya cain't ignore the Beaver Clan!!! It's one of the biggest dam clans there is!

(I once worked with a dam engineer who, when he was irate, would exclaim, "Grand Coulee!" 'cause that was the biggest dam(n) he knew of.

:D, Art
 
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