Ballistix's thread about his barrel problem got me concerned. Can any of you share what you would look for to determine if a bulge or dent had been repaired.

Was that "repaired correctly" you wanted?

If a dent it properly removed it is very hard to see, next to impossible at times. As long as the barrel wall thickness has not been compromised, a dent is rarely an issue. Bulges are a very different matter, they are hard to remove and they have changed the characteristics of the metal once it has stretched beyond its elastic limit. If you can not see these issues with your naked eye, a bore micrometer helps a bunch. So does a bore scope, but they are usually quite expensive.

A bulge almost always affects the area between the ribs as well. I doubt there are many repairs of bulges where the gunsmith strips the ribs off and does any cold forming of the area between the ribs. So, a bulge will likely show as a 'wave' in the bore in the area of the ribs. It will be a subtle change in the light reflection in the bore. It might be difficult to see, but should be visible. I've seen it even on subtle bulges.

I would not be overly concerned with small bulges, especially out in the thin section of the barrel past the forend. But a large bulge, especially as it gets in around the forend is a big concern to me and I would pass on a gun that had one or be thinking sleeving it when assessing whether or not to buy it.

Any cold working of the steel affects the properties. Dents and bulges alike. For the most part, they probably do more to strengthen the area than reduce strength. A review of a typical stress/strain tensile test chart of a steel in the range of hardness of barrels would show you this. Just after the point at which the force required to yield (permanently deform) the coupon, the force required to yield it further is less. Then, the force required to yield it further goes up well beyond the original yield point. This is a result of "work hardening" the steel. As the grain structure is changed from the "work hardening", it becomes stronger until it finally fails. This is also why hammer forged barrels are very strong. Of course, "work hardening" can be over done and lead to earlier failure as well. Some of the barrel failures of the stainless Sako bolt guns may have been an example of this. For us with ole shotguns, I wouldn't worry about work hardening as a problem. Rather wall thickness and crack detection would be comforting to me. Your comfort requirements may vary.

I found a typical low strength steel stress-strain chart.

Note the dip in stress then the gradual increase in stress.

BTW,
If the steel is work hardened too much, it can also be annealed to return it to a softer, more ductile condition.

So Chuck, what you are saying is that a bulge repair if done very precisely should be at least as strong as the original metal prior to the bulging incident? I'm not a physicist or an engineer and this is a little over my head. Also, where would a bulge lie on the stress deformation axis? Right at yield or could it be way far to the right?

Buzz,
I'm not sure I'd use the words "...done very precisely..." when describing beating on a piece of steel with a hammer. But, yeah sorta. The main thing would be ensuring there were no cracks prior to attempting repair.

Any thinning of the steel from stretching does indeed reduce the capability, same as filing would. Which brings up filing as a way that someone may try to hide that last little bit of the bulge that won't seem to go away. If you see signs of a repaired bulge in the bore, I'd measure the wall at that point to determine how much thinning was done.

Interesting Chuck. I learn someting new on this great forum almost every day. But where does the bulge lie on the stress deformation axis, right at yield or could it be way to the right and how would you know?

There is the situation of not raising the dent but honing out the protrusion in the bore. That would be a "worst case" attempt at "dent removal" imo. I have not attempted to turn mandrels to raise dents by the tippytap method but did return one muzzle of a "Sterlingworth Co." 12 gauge to round by inserting a lathe dead center in bore (chucking taper down and tapping away. Chuck et al, while we're at it, is there a particular steel which lends itself to turning these plugs? I'd really like to try this as I worry a bit about orphaned frames when I send barrels out to Mike Orlen. Right now I've got a Sauer 66 (Euro model) o/u with two dents in the lower barrel near muzzle (one of them an appreciable crease dent. Yes, there are a couple of others on other guns; I hope no more!

jack

Buzz,
Typical stress strain charts show elongation, often in percentage. If you had such data for your steel or felt you had such data applicable for your steel, you could measure the bulge and calculated the elongation to a comparable value and make your assessment.

The issue is really about getting applicable stress/strain data. Different alloys and hardness of steels will have different amounts of elongation.

That's what some of us were trying to do when we had a member going by "Zircon", or similar handle a few yrs ago, who was intending to do these tests on a variety of barrel samples he collected from members. I volunteered to make the test fixture and he was going to use it in a tensile test machine where he worked. It's been a few or more years since we last heard from him. The value of the data he was going to generate would be very valuable to making assessments of strength. As you also just discovered, it would be helpful for this question as well.

Jack,
My preference is to use a leaded steel like 12L14 or similar steels designed for screw machines. These steels are very easy to machine and leave a very smooth finish. Common cold rolled steel would be a second choice, but can leave a rough finish if certain techniques are not used. Very high surface speeds and some cutting lubricant will help with machining common coldrolled steel. As you read in Shotgun Technicana, brass is also an alternative, but I prefer the plug to be a little harder than brass.

Thanks Chuck for your as usual wealth of knowledge on these scientific issues and for your patience in explaining the complicated technical topics to those of us more ignorant in understandable laymen terms.

The metal (at the repair site) would be at least as strong (as the original metal), BUT the thickness of the metal (almost certainly) would not be as thick as the original. Thus, even though the metal was stronger, the place of the repair would be weaker because there was less metal there.



Following the stress-strain diagram starting from zero, the part of the diagram going from zero to the yield point is the area where the metal behaves "elastically", i.e., the metal deforms under the stress to a degree described by the curve but, when the stress is removed, the metal returns to its original shape/size. The stress-strain relationship in the elastic range is usually linear. Ideally, the gunbarrel will never exceed the yield point (leave the elastic range) ever.

Once the stress applied to the metal reaches the yield point, the metal will move from elastic deformation into plastic deformation. Any stress in excess of the yield point will result in a permanent "plastic" deformation of the metal. A bulge, a bend, a dent - these are all plastic deformation. Plastic deformation results in permanent deformation and thus damage to the metal. While, as described above, the stress and deformation can be relieved by annealing, the shape has been permanently changed. Banging out the dent or bulge can reshape the metal to the original, desired shape, but it does not restore the strength.

There is a point on the stress-strain diagram immediately following the elastic limit where the amount of stress needed to create a certain amount of strain (movement) decreases. Thus, the little dip. Following that point, the amount of stress needed to create a certain amount of strain (movement) increases. That increase is work hardening. The stress-strain relationship in the plastic range is usually non-linear.

Plastic deformation and work hardening will continue until the metal reaches the plastic limit, where it fractures. Which is what we're worried about.

Bravo Dave and Chuck. You guys have explained this complicated process where even a bonehead like Buzz can have a general understanding. Thanks.

Dave,
Thanks much for explaining further.

While I think I understand why you stated " Banging out the dent or bulge can reshape the metal to the original, desired shape, but it does not restore the strength.", I would temper that statement with a more so we don't alarm people unnecessarily about dents and their removal/raising.

As can be seen in the diagram, any deformation beyond the dip (yield point) results in higher stress required to further deform it. In the example, this higher stress is around 20-22% or so. But all deformation (aka elongation) beyond the yield point results in "necking" (thinning) of the test coupon, yet it still took 20-22% more force before necking reduced the cross section enough to have a negative effect on the strength of the coupon.

This shows that even though the metal continued to get thinner as it stretched, it still got stronger up to the crest of the curve where the reduction in cross section (thinning) was so much that it affected load carrying capability of the metal. If we knew what amount of stretching has occurred in the metal, we can use a chart like this to evaluate whether we are near a fracture. We can also measure wall thickness and compare to see if the barrel is weaker due to thinning or if the net effect is insignificant.

Also, that portion of the diagram from the yield point to the failure is the basis of why hammer forging barrels makes them stronger. As you can see it also requires close control of the process.

Unfortunately, I think there are a few problems with trying to ascertain how close a barrel is to fracture is by measuring deformation.

One dilemma is we don’t know what the barrels stress/strain curve would look like. A generic curve may not be so useful because some strain hardening and residual stresses will have occurred during original manufacture. This could be remedied by testing (lots) of barrels.

It is even more convoluted after the repair. The fix will leave some residual stresses in the fixed bulge or dent. Working the metal probably also imparts some unknown directional properties.

The biggest puzzel is using the stress/strain relationship to predict fracture. This is an interesting idea but, I can’t think of any applications which use Poisson’s ratio outside of the elastic range. Once the yield point is reached, things get unpredictable. I'm not sure predicting fracture is a correct application of the theory.

It is relatively easy to predict at what stress something will yield. Predicting what happens outside the elastic range is like trying to predict how flames dance in a fire.

Ryan,
I appreciate you point. However, as we all know, everything from the seat brackets in your car to the barrels on a Remington 700 are cold formed. I have a friend making a huge living cold forming parts for automotive.

I just don't want to leave the readers here with the notion that a dent raised will result in a weaker condition and less safe or capable. There's just no evidence nor practical data to suggest that. An arguement could be made for an increased strength.

Why would the thickness at the site of a removed bulge be thinner than the original thickness? If the metal is ever so slowly put back into place by peening over a snug fitting plug mandrel wouldn't the metal flow back to where it was originally? If not, where would it go?

I'm just a layman trying to better understand this. I have removed most of a bulge by light peening, and dents as well. I probably won't mess with bulges on a double anymore, dents are no big deal, IMO.

Stan

First, you're all welcome. I knew that engineering degree would come in handy one day.

Now, on to the other points. I think most of what's been said on the thread is correct, so please note I'm not going after anyone. I am disagreeing in some aspects and giving my reasons.

Chuck, you said:

The point is, and I think I should have been clearer in the beginning, that once metal is stressed past the yield point into plastic deformation, there are stresses introduced into the metal which are permanently there (and can only be removed by annealing). Reshaping the metal (banging out the dent) is merely adding more plastic deformation to that which got us into the problem in the first place. You are correct that raising dents in barrels is not something to lead them to be alarmed unneccessarily. But they should be concerned nonetheless.

This is because the plastic deformation manifesting as damage to a shotgun barrel happens under uncontrolled conditions. The conditions of starting and ending temperatures, the amount of force and rate, vectors and sharpness of the object(s) imparting the force, and every other thing acting upon the barrels are neither controlled nor recorded. So, any claim to a scientifically precise approach to determining the risk from a dent and from dent removal is pretense.

You continue:


All true. But, it elides the problem. Because the plastic deformation manifesting as damage happened in uncontrolled (and un-measured) conditions, we have no way at all of knowing where on the stress-strain diagram the barrel currently lies. We'd have a better chance of being accurate if we were throwing a dart at the diagram. That understates the problem because it's entirely possible that different parts of the dent are at different points on the stress-strain diagram. Again, that's because the conditions under which the damage took place were uncontrolled and un-measured.


You continue:


Again, true, but also unhelpful. That increase in strength that took place as plastic deformation continued is work-hardening, which is accompanied by an increase in brittleness. So, while the metal may be stronger, it will also be more brittle and break unpredictably. Thus, banging out the dent will both strengthen the metal and make it more brittle. And, as noted above, given the uncontrolled conditions we cannot know the amount of stretching which occurred in the metal and more importantly, how. Measuring the remaining barrel thickness can give a rough index of whether there is sufficient metal and, derivatively, non-brittle strength, remaining. It needs be remembered that, contrary to some uses, the loads applied to shotgun barrels (by firing) are sudden, sharp impulses, i.e., the very kind of stress application that intuition tells us is most likely to shatter something brittle.

You continue:


Also quite true. It's the controlled process which distinguishes the plastic deformation that goes on in hammer-forging from denting shotgun barrels and banging out the dent. Moreover, and please correct me if I'm wrong, but I recall that in hammer-forging gunbarrels there are any number of annealing and tempering stages to relieve the internal stresses caused by the hammer-forging. I think if a gunsmith who was going to bang out that dented barrel was to take a torch to it too, he'd likely get a response like that received by the re-colored Parker and the gunsmith/gunseller the other week. The torch-wielding gunsmith might be doing exactly the right thing - trying to relieve stress in the metal - but I wonder who here would go along with him.

I agree that under closely controlled conditions, predetermined and designed by skilled engineers and scientists, plastic deformation is a useful tool in making metal stronger. I also agree that it's entirely possible to dent and later remove dents from shotgun barrels and have them remain safe to use. But the latter do not take place under the controlled conditions of the former and, consequently, caution must be exercised. How much risk of failure remains after a repair is almost impossible to determine and people are notoriously bad judges of risk/probability anyway (One is far more likely to be hit by lightning than win the big prize in the lotto, yet we come in out of thunderstorms and line up at the lotto terminals.). And, even in the most stringent settings, failure may come never, decades from now or, as in the case of Ballistix' pretty 20 ga, 2 weeks after passing proof.

So, using a gun with repaired barrels is an undefined and undefinable risk, and only the user can decide whether it's too much of a risk to accept.

Dave,
In the dozens of dents I've taken out, only one even gave me any concern that it was anywhere near the local elongation to be concerned. That one had what looked like it had been hit with a corner of something. All the rest were typical little dents of shallow depth. I am personally confident that none of these others would be anywhere near the upper limits of yield of the fairly ductile steel used in the respective barrels. Most dents are just little things. I'll give you that some severe dents are out there that may fall into what you describe, but I just don't believe anything this side of something hit with a chisile falls into what you describe. If it did, every press brake and forming die would be making scrap.

I do disagree that annealing would be the right thing. Most of the barrels I've been around were harder than annealed steels. Some up as high as in the thirties RC, some much lower. Annealing would likely weaken the barrel more than raising a dent, IMO.

But if you believe common dent removal is risky, you should indeed stay away from guns that have had them. I just don't know how you would ever know if they were done with the full treatment of restriking and light honing.

I think, Chuck, we're in agreement: there is some (unquantified and unquantifable) risk in using a shotgun with barrels that have been dented and repaired, and only the user can determine for himself whether he is willing to accept that risk or not.

I'll stick with buying lotto tickets and coming in out of thunderstorms to avoid being hit by lightning. YMMV and your choices may, too.