How is a gun barrel heat treated?

[indoorsoccerfrea]

I've read a lot of guides on how to make your own barrels, but nothing about the proper heat treat/tempering for them. Anyone know something more than nothing about this?

 

[sloughfoot]

Maybe because barrels are not heat treated or tempered? Receivers are, barrels aren't.

 

[lawrra]

I work for a company that heat treats steel bar, and some of our customers are firearm manufacturers. We get the steel in lengths of 20+ feet and ship them out roughly 8-12" shorter. It's easier to ensure a uniform hardness when the steel is processed in bar stock. The stock is then cut to length and machined into a barrel.

 

[lawrra]

Maybe because barrels are not heat treated or tempered? Receivers are, barrels aren't.

Where did you get your information?

 

[tv1217]

YHM, Yankee Hill Machine Co., Inc. – Makers of High Quality Firearms, Accessories & Sound Suppressors | AR-15 Barrels

I guess they're lying then

 

[lawrra]

YHM, Yankee Hill Machine Co., Inc. – Makers of High Quality Firearms, Accessories & Sound Suppressors | AR-15 Barrels

I guess they're lying then

Of course they are! Damn yankees...

 

[kludge]

As the button is pulled down the bore and impresses the grooves and lands, metal is displaced. This causes stress in the metal, which can lead to inaccuracy, such as bullets walking off the target as the barrel heats.
Heat treating relieves that stress. Lilja worked as an industrial engineer for the John Deere Company in Iowa before he started in the barrel business in 1985. Part of his time at Deere was spent in the heat treating department. Lilja used to heat treat his barrels himself in a small oven in his shop. His increased production over the years, though, has made it easier to send out the barrels to Spokane, Washington, for heat treating.

Lilja Precision Rifle Barrels - Articles: The Making of a Rifle Barrel

Heat treatment of metal is done by raising the temperature to a point where there is a change in the crystal structure of the metal (an expert can look under a microscope and see what has been done). But the actual crystal structure is on a molecular level. Depending on what you are trying to do, the metal is either quenched to prevent the crystal structure from changing, or allowed to cool slowly.

Crystal structure is different depending on whether it was cast, forged, or tempered in the first place. Button rifling is basically a forging process and can put stress in the barrel, and the heat treatment can remove that stress be bringing it back to it's pre-forged condition.

Queching steel while it is hot hardens the steel, but it can also make it brittle, so you have to know the right temperature for the application. Often a tempered steel piece is added to a forged piece for toughness (wood cutting chisels for example).


ETA: here's a link that shows "body centered" and "face centered" cubic crystal structures on the moleculer level.
Heat Treated 4130

 

[sloughfoot]

You know, I just read the OP. I think I misunderstood the original question. Barrels are heat treated to relieve stress from the rifling and threading and other manufacturing processes. Not hardened like receivers on bolt guns.

I am always open to learn new things, but when I can cut military and civilian barrels with a hacksaw and then recrown the muzzle with simple handtools, that indicates that they have not been hardened by heat treating.

I have never cut down a military AR barrel though. I have a couple I can play with.

Sorry to use up bandwidth with my mis-understanding of the question..

 

[lawrra]

You know, I just read the OP. I think I misunderstood the original question. Barrels are heat treated to relieve stress from the rifling and threading and other manufacturing processes. Not hardened like receivers on bolt guns.

I am always open to learn new things, but when I can cut military and civilian barrels with a hacksaw and then recrown the muzzle with simple handtools, that indicates that they have not been hardened by heat treating.

I have never cut down a military AR barrel though. I have a couple I can play with.

Sorry to use up bandwidth with my mis-understanding of the question..

Hardened steel can be cut with a hacksaw. Just because steel is hardened doesn't mean it's indestructible. Carbide and tool steel are harder than barrels, and explains why you can cut them.

 

[indoorsoccerfrea]

Well, essentially my issue is that I will be casting a receiver from aluminum around a mosin nagant barrel. Obviously the heat treat of the barrel, whatever it is, will be ruined by having molten metal cast around it...Once the aluminum cools, I will then unscrew the barrel (the whole point of casting around it in the first place is just to get the threads, since there is no such thing as a MN receiver thread tap available to the public) and re-heat treat the barrel...at least that is the plan...

 

[kludge]

Since barrels are not tempered or hardened steel I doubt you will hurt it. I would preheat the barrel though, and after just let it cool. Even if you do hurt it, it's just a MN.

since there is no such thing as a MN receiver thread tap available to the public

Have someone custom make a tap for you.

That said, I wouldn't want to pull the trigger on a receiver I cast myself, much less from aluminum.

 

[Phil502]

Lilja Precision Rifle Barrels - Articles: The Making of a Rifle Barrel

Heat treatment of metal is done by raising the temperature to a point where there is a change in the crystal structure of the metal (an expert can look under a microscope and see what has been done). But the actual crystal structure is on a molecular level. Depending on what you are trying to do, the metal is either quenched to prevent the crystal structure from changing, or allowed to cool slowly.

Crystal structure is different depending on whether it was cast, forged, or tempered in the first place. Button rifling is basically a forging process and can put stress in the barrel, and the heat treatment can remove that stress be bringing it back to it's pre-forged condition.

Queching steel while it is hot hardens the steel, but it can also make it brittle, so you have to know the right temperature for the application. Often a tempered steel piece is added to a forged piece for toughness (wood cutting chisels for example).


ETA: here's a link that shows "body centered" and "face centered" cubic crystal structures on the moleculer level.
Heat Treated 4130

I operated a heat treat furnace for about 5 years, that was 20 years ago.
In general we heated forgings in a 1600 degree furnace that moved on a belt toward the quench for 50 minutes.

After the quench the forgings continued on a belt and were reheated in the draw furnace for about 50 minutes at a temp anywhere from 1100-1400 degrees. The whole process took about 2.5 hours.

Using a Brinell Hardness tester, forgings were very hard after the quench and much softer after the draw. The draw readings were the most important.

 

[yotewacker]

I have my precision rifles cryonized "spelled wrong" put in a chamber to -300f it make the gun shoot straighter and improves barrel life.

 

[CountryBoy19]

I can cut military and civilian barrels with a hacksaw and then recrown the muzzle with simple handtools, that indicates that they have not been hardened by heat treating.

As has already been stated, your assumption that "hardened" = "can't be worked by hand" is incorrect.

There are lots of hardened metals that are worked by hand everyday.

Chainsaw chain/teeth are hardened steel alloys that are hard-chrome plated (extremely hard metal, equivalent to the hard-chrome lining in a chrome-lined barrel), yet people sharpen them with a hand file all the time.

 

[indoorsoccerfrea]

Have someone custom make a tap for you.

Methinks this would be very pricey...

That said, I wouldn't want to pull the trigger on a receiver I cast myself, much less from aluminum.

I understand your fear, and I share it. I would be sure to test fire numerous times and check for stresses/cracks/etc. before I ever tried to fire it myself.

 

[sloughfoot]

I'm not sure what is not understood here. Barrels are heat treated to relieve any stresses from the rifling and threading operations. They are not tempered (hardened) by this process to a Rockwell standard hardness. It is only to create uniformity.

Barrels SHOULD NOT be hardened, either case hardening or deep hardening. It would create a grenade situation in the event of an obstructed bore. A heat treated barrel typically splits along the axis precisely because it is not hardened.

Barrels can be cut with a common hacksaw because they are not hardened.

Again though, I have to admit I have never put a hacksaw to a military M16 barrel, but the question from the OP is about the MN barrel.

When I sharpen my chain saw blade, I am using a file or a grinding wheel, both of which are considerably harder than a common hack saw blade.

 

[sloughfoot]

Methinks this would be very pricey...


I understand your fear, and I share it. I would be sure to test fire numerous times and check for stresses/cracks/etc. before I ever tried to fire it myself.

Sir, I don't know of any receiver to be used with 45,000 to 60,000 psi pressures that is made out of any type of aluminum.

For one hundred plus years, they have all been made with case hardened steel for strength and durability.

Why are you thinking about going where no engineer has dared go before?

It would be far better to trim off the existing barrel threads and rethread the new shank to fit into a steel receiver of your choice. But again, what is so magical about the MN barrel that you would spend the time and money doing this?

How do you intend to create the bolt lugs inside your cast aluminum receiver?

 

[J10]

Steel barrels (not stainless steel) are heat treated and stress revealed after machining and rifling. A mill spec M16/M4 5.56 barrel is hardened to around 30 Rockwell C scale (i think the exact specs are 28 to 33 Rockwell). So technically they are "hardened" but not to the extent that most people think of hardened steel. 30 Rockwell is saw-able and file-able but much stronger than regular steel (which can vary from 3 to 12 Rockwell depending on type). The mil spec barrels are made from chromoly steel usually the 41xx series of alloy steels.

For a comparison, most steel disc targets are hardened to around 52 Rockwell, which is still sorta file-able but is very hard, If it was taken to 60 Rockwell you would pretty much roach the file.

Assuming the barrel you are using is made of the 41xx series of alloys then heating it to anything above 500 degrees could have adverse effects.

Here is a PDF that compares Brinell to Rockwell. www.universitymachineservices.com/rockwell.pdf

(By the way, this is just what i have picked up over the years as a machinist, i am not a metallurgical engineer by any means.)

Just my 2 cents, hope i helped!

 

[sloughfoot]

Thanks J10. You have a way with words that I lack. And certainly more specific expertise. Good explanation.

 

[lawrra]

Barrels are hardened to give the steel better mechanical properties. Heat treated barrels can withstand higher pressures without failing because the yield strength has been increased (yield strength is lowest stress that permanently deforms the steel e.g. ballooning, etc). There's an order of Colt steel sitting at work. I'll check what grade it is, but the 7xxx grade comes to mind. They want their steel at 30-34 Rc, in whole bars, to be shipped off for machining.