Forged???

[jad0110]

Next time an anti says we are just a bunch of gun loving, stupid, ignorant hicks, I'm whipping this thread out!

A great read everyone, I've really enjoyed it.

 

[Slamfire]

Assuming two bolt-action receivers of identical dimensions and
weight -- one constructed in forged steel and other in cast steel
(both of the best steel, process, and QC) -- would anybody here
assert that the cast version is of equal or superior tensile strength?

The metallurgical discussion is way beyond my education, but the question of which one is stronger, seems to me to be a cart before the horse question.

How often does someone design something based only on the material considerations? I am certain you could design an action, setting as your primary design criteria, “optimization of forging properties”, but why do that? I would think other goals are more important: cost, weight, function, safety, vendor supply, tooling, production processes, just do the system engineering. What do you think are the five most important characteristics of a rifle action,? And what are you willing to pay to get that?

If your number one goal is lightweight, then the solution can be radically different then if your number goal is safety. And if safety is not an issue, only cost, you can build a cheap gun out of PVC tubing, a rubber band loaded nail, and duct tape. It will be a one shot device though!

The material choice and fabrication technique support the overall design, they are not the design, or the program will surely fail. If you have ever sat down and wrote a system requirements document, and then done the trades, you know you have to trade things off to get an acceptable design. Yes, material choices influence the design, fabrication influences the design, but so does human factors, reliablity, maintainabilty, interchangablity, size, weight, etc, etc, etc. But there is one thing that totally dominates all commerical designs, and that is cost.

On the commercial market, you have to be price competitive or the market walks to the cheaper product. And of course you cannot ignore features. The automotive industry found that people will buy one $40,000 vehicle over another, just because of cup holders.

I want to thank everyone for keeping the discussion civil and very interesting.

 

[daniel (australia)]

Are you contending that gun parts/receivers are generally made of
softer pearlitic steel (subsequently heat treated) vs. martensitic steel?

You ask me that as if you think they are different things…

Further, are you contending that heat treating transforms pearlite into martensite?

Well, yeah, of course. What did you think?

You see, if you take a piece of your medium carbon steel and cool it slowly from a high temperature (ie from the austenite phase) you’ll end up with a microstructure composed of pearlite and ferrite, depending on carbon content, as I said in my last post. It doesn’t matter whether the steel was forged or cast. In this condition it is said to be annealed (if furnace cooled) or normalised (if air cooled) and it is comparatively fairly soft but ductile, stress free, and reasonably easy to machine.

Now whenever you want to - once you’re done machining it perhaps - you heat treat it by raising its temperature high enough to transform it all back to austenite, and then you quench, which is to say cool it so fast that ferrite and pearlite don’t have time to form again but instead you end up with a transformation to martensite, which is an extremely hard but brittle metastable phase at room temperature.

Your steel is not much use in that condition though, as it is too brittle, so you temper, by raising the temperature again to an intermediate level (below the austenite transformation temperature) to allow a certain amount of decay of the martensite structure and thereby regain some degree of toughness, though at the expense of some loss of hardness – the degree of tempering depends on the balance of these properties you want in the finished article.

BTW the process is reversible too: you can take your quenched and tempered article and heat it then slowly cool to end up back at pearlite + ferrite again, and vice versa.

I will respond to the rest of your reply when I can, but meanwhile reiterate
to you my previously unanswered question:
Quote:
Assuming two bolt-action receivers of identical dimensions andweight -- one constructed in forged steel and other in cast steel(both of the best steel, process, and QC) -- would anybody hereassert that the cast version is of equal or superior tensile strength?
To me, this is the real question.

Neither I nor McClung disallow that a sufficiently heavier cast
receiver can have a similar working strength as a lighter forged one -- just
that such is 1) heavier, and 2) less robust over time. Your disagreement
with 2) is certainly noted, but are you also disagreeing with 1)?

I thought I had answered it, but perhaps not directly enough.

I had already given the example of the aircraft parts, which have in recent times reached the stage of being no longer subject to the “casting factor” of the past.

In terms of rifle receivers though, relying on my old first edition of De Haas' Bolt Action Rifles I can give you this comparison: The weight of the Winchester Model 70 action is listed as 48 oz (45 oz in the Featherweight), the Mauser 98 as 45, Sako L61 44 oz, Brno ZKK600 43 oz and the Ruger 77 long action as 42 oz. (Perhaps someone with a more recent edition or other data could chime in with figures for some more recent designs)

All of these actions are about the same size, and for the same sizes of cartridge. I would suggest that all have proved quite sufficiently strong and serviceable, and they’re all much the same weight (though the Ruger, with the investment cast receiver, bolt, and other parts, is in fact the lightest by a small margin ) In practical terms then the answer to your question must be yes.

It is however a bit of an oversimplification, and I think Slamfire makes a very good series of points. Bottom line though is that investment casting as a technique is well established as a process capable of producing an action just as strong, safe and serviceable as forging or milling from tube or bar stock.

 

[JohnBT]

Knowing next to nothing on the subject, I learned a lot from Bill Caldwell's posts in this 2002 thread. You can google his name for info on his custom knives and guns. John

"Forged Frame, Cast Frame : View from the Hills"

www.pistolsmith.com/viewtopic.php?t=7912

Excerpt from one of his posts in his thread:

"The so called forged frames should be called drop forged frames. Proper forging technique calls for heating the steel to a certain temperature, then hammering in a proper sequence and with varying strength hammer blows, until the steel cools to a temperature where hammering should be stopped. Then the steel is reheated and hammered again. This is repeated until desired shape and dimensions are reached. A skilled blacksmith can refine the grain structure and grain direction. No frames are forged this way. Frame forging is done by heating a chunk of steel, placing it in a die, a matching die is slammed into this, and this is the 1911 forged frame. A quick way to get the general shape of the frame. This doesn't do anything to help the steel, and unless stress relieved and annealed, the frame is loaded with stress. It will certainly move during machining and heat treating. If the steel was not at the proper temperature when the hammer falls, lots can happen. All bad. Too hot: giant and very weak and brittle grain structure. To cold: inclusions, cold shuts. Not a good foundation to build on. "

 

[Boston T. Party]

Yes, heating pearlite into austenite first, of course.
I thought you meant skipping the austenite phase,
and going from pearlite directly to martensite.

____________
Regarding the weight issue, I found this:

Cast-Steel

We won’t waste much time discussing cast-steel rods because they’re poorly suited to any type of serious performance use. Though the casting process is very inexpensive and results in “near net” shapes that require minimal machining, the lack of a cohesive grain pattern and compromised molecular binding yields brittle parts. Trust us, brittle connecting rods are the last thing you want in a performance engine.

In the ’60s and ’70s, American Motors, Cadillac, Buick, and Pontiac all used cast rods in a wide variety of engine designs. In an effort to improve molecular binding and strength, the molten metal was injected into the mold cavity under high pressure. The resulting castings may have been good enough for use in everything from GTOs to Jeeps, but they have no place in anything other than the most fanatical numbers-matching restoration effort. Worst of all, these cast parts had to be made heavier than comparable forged rods to maintain strength. When you consider that a cast “Arma-Steel” Pontiac 455 rod weighs 31.7 ounces and a stock Chevy 454 forged rod weighs 27.4 ounces, you’ll agree they’re the automotive equivalent of recycled cardboard.

http://www.hotrod.com/techarticles/choosing_the_right_connecting_rods/index.html

More later,
Regards,
Boston

 

[hubel458]

Boston- You're not comparing the same things. Those cast
rods used in those different small and big block engines
were not investment cast and properly heat treated like gun
actions and other stuff is built nowadays. And we've busted all
kinds from mickey mouse engines up to Packard PT boat engines.

Above quote-the Winchester Model 70 action is listed as 48 oz (45 oz in the Featherweight), the Mauser 98 as 45, Sako L61 44 oz, Brno ZKK600 43 oz and the Ruger 77 long action as 42 oz.

And the Ruger is strongest......Ed

 

[Boston T. Party]

Regarding the "Ruger 77 long action as 42 oz", that is likely the MkI
given the 1st edition of Haas's book. What do the MkII actions weigh?
I'm away from my library, so I cannot now check this.


OK, Daniel, these near-net shape investment cast parts are:

1) slowly cooled down to annealed (or normalized) pearlite/ferrite, then
2) machined (if necessary) to net shape, then
3) reheated back up to austenite transformation temp, then
4) quenched into martensite, then
5) tempered

I follow that perfectly. I think we all do by now.

However, assuming, arguendo, that when the molds were poured
the carbons were (somehow) very nicely distributed in austenite
solution, that carbon content later segregates in stage #1
(as laminar cementite Fe3C strips in pearlite/ferrite).

When that pearlite/ferrite is reheated into austenite, any previous
quality carbon distribution (i.e., in the first austenite phase
during the investment casting) is lost as this second austenite
phase is derived from pearlite/ferrite (with its carbide segregated
in cementite strips).

And how are those carbides randomly distributed in the (second)
austenite then?

The second austenite solution has none of the benefits that the
first austenite solution enjoyed, such as (you mentioned) of induction
furnace churning, gating, pouring, or ultrasonic vibration. Near-net
shape pearlite parts are simply reheated to austenite and then quenched
into martensite. (Rather, it's re-melted but not re-stirred.)

If you counter that such processes (induction furnace churning,
gating, pouring, or ultrasonic vibration, etc.) are not necessary for
a well-mixed second austenite solution, then why were such
even done at all the first time during the original casting?

I.e., if simply reheating pearlite into austenite #2 effectively "stirs" the
solution, then why bother (very elaborately) stirring it in austenite #1?

McClung's point has been that attempts to drive pearlite segregated
carbides into austenite solution are notably unsuccessful. Hence,
investment casting relying on austenite-->pearlite-->austenite-->martensite
process will have, according to his thesis, segregated carbides in the
grain boundaries. In short, mere heat treat of pearlite does not create
a well-mixed austenite (#2) and thus you can't avoid carbide strings in
your martensite.

If you have specific countering evidence on this very issue, please post
it because we've arrived (finally) at the very crux of the dispute.

___________
hubel458, are you saying that those cast con-rods were sand
vs. investment cast?

. . . I know that the Win wouldn't of held.

And the Ruger is strongest......Ed

You've been saying that anecdotally, but without any offered
empirical evidence. Just because you claim to "know" that the
Win M70 wouldn't have held up to that load your M77 withstood . . .
does not make it so.

Regards,
Boston

 

[hubel458]

Ruger ran shear tests of bolt lugs/bolt seats and
they were about 25% higher than what the strength of
the lugs and seats would have calculated out with tensile and
yield strengths of a forged action the same size.

As for the rods, broken ones looked just like a
maleable iron. The auto mfs I don't feel
could set up to do the bother, all the steps, and
slowdown of proper heat treatment Ed.

 

[Boston T. Party]

. . . would have calculated out . . .

Would have calculated out?
That's empirical evidence?
Sounds pretty thin to me.

btw, the strongest bolt-action that author Haas could find
was an Arisaka. It was almost impossible to blow up.
A forged steel action . . .

Boston

 

[hubel458]

I'm reporting Rugers tests, and compared with the
strength of forged based on the tensile and yield strength
of same size forged bolts and actions others built it
was stronger. Simple math, get the cross section of parts,
apply the math using the strength of metals and it gives
a result. Yes Arisaka is almost impossible to blow up,
which is great, just like a Ruger is almost imposible to blow
up. I've seen charts where properly made investment parts heat treated
right had a tensile strengths over 300,000 lbs. So all the
talk about what happens at boundary layers don't mean nothing.
Investment with treating is more uniform through out,Making for
higher strength. Ed

 

[daniel (australia)]

Boston

the lack of a cohesive grain pattern and compromised molecular binding yields brittle parts

How do they dream this stuff up?

Leaving the truly laughable to one side though, I thought I’d dealt with the conrods, several times. You aren’t comparing apples with apples, as the method of production is different: these rods they're talking about are or were usually cast in sand moulds, though the article you posted also describes something that sounds like pressure diecasting (?), not investment casting. Incidentally I note that the article describes experiments carried out in the 1960s and 70s - which was, um, rather a long time ago. Where the cutting edge is for high-performance conrods nowadays is squeeze cast metal matrix composites.

The other point you continue to miss (and it isn’t made very well in the article either) is that it is fatigue life not strength which is the determining performance factor in conrods. If you read the article more closely you’ll see that: as they point out your basic as-forged rod is not that great for serious high-performance either, but can be dramatically improved by surface machining, radiusing, peening and polishing, all techniques which actually make it no stronger at all, but dramatically increase fatigue life. Fatigue really isn’t anywhere near being a factor for a rifle receiver though.

Regarding the "Ruger 77 long action as 42 oz", that is likely the MkI
given the 1st edition of Haas's book. What do the MkII actions weigh?
I'm away from my library, so I cannot now check this.

I’d be prepared to bet the difference isn’t enough to help you Anyway, the comparison remains valid, as the Mk I had investment cast receiver, bolt and sundry other parts. The differences between it and the Mk II aren't really material in this debate.

OK, Daniel, these near-net shape investment cast parts are:

1) slowly cooled down to annealed (or normalized) pearlite/ferrite, then
2) machined (if necessary) to net shape, then
3) reheated back up to austenite transformation temp, then
4) quenched into martensite, then
5) tempered

I follow that perfectly. I think we all do by now.

However, assuming, arguendo, that when the molds were poured
the carbons were (somehow) very nicely distributed in austenite
solution, that carbon content later segregates in stage #1
(as laminar cementite Fe3C strips in pearlite/ferrite).

When that pearlite/ferrite is reheated into austenite, any previous
quality carbon distribution (i.e., in the first austenite phase
during the investment casting) is lost as this second austenite
phase is derived from pearlite/ferrite (with its carbide segregated
in cementite strips).

And how are those carbides randomly distributed in the (second)
austenite then?

First of all the carbide doesn’t “segregate”. It is contained within the grains of pearlite, very finely distributed. When you heat it up into the austenite range the crystal structure reorganises – in the solid state – and the carbide molecules are re-dissolved in that structure in solid solution. The molecules don’t have to diffuse far to do that, and diffuse they do, resulting in a nice uniform composition. In fact austenitising is a method also used for addressing any microsegregation of alloys in alloy steels.

When you then quench this, the speed of cooling doesn’t allow the iron and carbon to diffuse back into the ferrite + pearlite structure – that is rather the point in fact. Instead the carbon is retained in the crystal lattice as an interstitial solid solution, giving rise to a wholly different set of properties such as high hardness etc.

BTW (and not for the first time) the same mechanism is at work regardless of whether the steel was cast or forged.

The second austenite solution has none of the benefits that the
first austenite solution enjoyed, such as (you mentioned) of induction
furnace churning, gating, pouring, or ultrasonic vibration. Near-net
shape pearlite parts are simply reheated to austenite and then quenched
into martensite. (Rather, it's re-melted but not re-stirred.)

If you counter that such processes (induction furnace churning,
gating, pouring, or ultrasonic vibration, etc.) are not necessary for
a well-mixed second austenite solution, then why were such
even done at all the first time during the original casting?

I.e., if simply reheating pearlite into austenite #2 effectively "stirs" the
solution, then why bother (very elaborately) stirring it in austenite #1?

No, the “second” austenite solution is formed entirely in the solid state, by heating the steel from lower temperature, unlike the “first”, formed from nucleation and grain growth by cooling from liquid. When you are producing the casting from the liquid steel you are trying to ensure the liquid is well mixed, so as to avoid segregation or variation the composition in the casting. This is more an issue with really big castings btw, including the cast billets and slabs from which you get bar stock for forgings.

Now if you’ve done it properly though your comparatively small receiver casting will solidify as a nice uniform austenite and, with decreasing temperature, this will transform to ferrite and pearlite. In the case of this transformation the diffusion distances the carbon travels are pretty small though, and when you heat it up again the carbon just has to diffuse back these same small distances to dissolve into a new and at least equally uniform (if not even more uniform) austenite.

McClung's point has been that attempts to drive pearlite segregated
carbides into austenite solution are notably unsuccessful.

which in the context of the alloys we're talking about is utter nonsense.

Hence,
investment casting relying on austenite-->pearlite-->austenite-->martensite
process will have, according to his thesis, segregated carbides in the
grain boundaries. In short, mere heat treat of pearlite does not create
a well-mixed austenite (#2) and thus you can't avoid carbide strings in
your martensite.

Again, utter nonsense. For starters the carbide doesn’t form on grain boundaries in these hypoeutectoid steels but as lamellae in the pearlite structure in grains of pearlite. I’ve posted micrographs to show you this. The pearlite completely transforms to austenite when you heat it above the appropriate temperature, and when you quench the carbon molecules are trapped as an interstitial solid solution in the martensite.

Not only that but forgings of these alloys undergo precisely the same cycle: they are forged in the austenitic condition at elevated temperature, normalised to form ferrite + pearlite so as to allow machining, and then back up to austentising temperature, quench to martensite and temper.

If this idea you attribute to Mr McClung were correct you’d have the same issue with forgings then: you'd have a nice even structure upon forging, but all the carbide would "segregate out" on cooling to ferrite + pearlite, and you'd never redissolve it on subsequent heat and quench heat treatment. Ironically this would put us back on a level playing field again for castings v. forgings - if this theory wasn't so wrong. Surely you can see that?

If you have specific countering evidence on this very issue, please post
it because we've arrived (finally) at the very crux of the dispute.

I’ve put up the "evidence". Where’s the evidence for your (or your mate’s) position?

btw, the strongest bolt-action that author Haas could find
was an Arisaka. It was almost impossible to blow up.
A forged steel action . . .

The weakest too: the “unsafe” Herter’s Plinker .22 Hornet

 

[Reyn]

Is someone honestly seeking the truth here or just refusing to accept it?

 

[Matt-J2]

Every time I see the thread title on the page, I start thinking documents.

After that, having read through this thread, I think of the symbol of the snake eating it's own tail.

 

[Slamfire]

btw, the strongest bolt-action that author Haas could find
was an Arisaka. It was almost impossible to blow up.
A forged steel action . .

.
From Hatchers’s Notebook : Japanese 6.5 mm Arisaka, “appear to be made of ordinary carbon steel similar to SAE 1085”

The ground zero for some of de Haas comments on the Arisaka came from a P.O. Ackley’s “Vol II Handbook for shooters and reloaders”. Mr. Ackley conducted blowup tests, of forged military actions, and made comparisons. The Arisaka was the strongest.

Read Stuart Otteson’s Book, “The Bolt Action”. After you read Otteson’s book you will understand that the Arisaka breech was extremely strong, because it was designed to be extremely strong. The Japanese created a design which gave superior support to the cartridge case. Creating a better design allowed them to use cheap plain carbon steel as a material, with plenty of safety margin.

The US used nickel steel in both the Springfield and the Enfield, in an attempt to improve on action strength. But chromium and nickel are rare strategic materials. The Japanese were able to design out those additives.

To get back on my soap box, the Japanese action was strong because they designed it to be strong. There are tradeoff’s, but the Japanese did not design this action as a demonstration of “optimial forging properties”, rather, their design goal was to create a strong, safe and simple service rifle.

Putting the materials or the fabrication process before the design is putting the cart before the horse. And an purposeless argument. It is design that creates a strong action.

Surprising to me, was the Japanese heat treat. From Ackely’s book: “it is obvious that this receiver was not only carefully, but even elaborately heat treated”…”Its heat treatment appears to be superior the average Mauser, Springfield and Enfield”….”Differential hardening, such as used on the bolt and firing pin…..”

Does anyone know of any modern actions that use differential hardening?! Maybe the Japanese used their famous sword smiths to heat treat their receivers, eh Gad!

 

[JohnBT]

The stuff I watch on my new HD tv. If cast is good enough for the CAT 797B it's good enough for me.

I didn't know they have cast frames. They cast these half dozen huge pieces and weld them together. The truck will haul a load of approximately 380 tons.

From the CAT site:

Weights - Approximate

Gross Machine Operating Weight
1375000 lb

Chassis Weight
473600 lb

Body Weight Range
38 000 - 63 000 kg (84,000 - 140,000 lbs)

 

[Gewehr98]

The bottom line?

How many investment-cast receivers have failed, compared to their forged counterparts?

Akin to counting the number of angels that can dance on the head of a pin, methinks.

 

[Boston T. Party]

Thanks for your reply, Daniel.

Not only that but forgings of these alloys undergo precisely the same cycle: they are forged in the austenitic condition at elevated temperature, normalised to form ferrite + pearlite so as to allow machining, and then back up to austentising temperature, quench to martensite and temper.

All forgings?

Are not some-many forged parts quenched directly into
martensite (thus purposely avoiding the pearlite phase)
and then slightly annealed for easier machining?

In the case of this transformation the diffusion distances the carbon travels are pretty small though, and when you heat it up again the carbon just has to diffuse back these same small distances to dissolve into a new and at least equally uniform (if not even more uniform) austenite.

Can you post any micrographs to support that?

Regards,
Boston

 

[Boston T. Party]

Update from Boston

Having spoken to Ron Smith at length about all this at 2008 SHOT,
I am obliged to relay the gist of the conversation here.

Ron termed the overreliance upon forged steel as "Celtic/Druid bullsh*t"
which floored me coming from such a master of forged steel gun parts
and receivers. He said that cast steel receivers (as he pointed to one of his
own M14s at the booth) could be manufactured to be just as strong as forged,
". . . but the problem is keeping your foundry honest" regarding quality of
steel, molds, etc. He explained that cast steel parts need such stringent
quality control (e.g., individual X-raying) that it makes more sense for some
firms to make forged steel parts instead -- not for any advantage in strength
but in reduction of QC costs.

I then asked him if he had the same confidence in his cast M14 as he has
in his forged M14, and replied that he did, especially after testing his cast
M14 to the point of catastrophic failure (and it still held).

Ron and Kevin McClung know each other (I introduced them) and are friends.
Ron expressed sincere respect for Kevin's knifemaking skill and knowledge,
but asked rhetorically "How many rifle receivers has he made?"

When I earlier posted that:

Regarding my cast-steel receiver SA M1, Ron spoke of
the cast steel being the issue vs. the heat-treat.

. . . it turns out that I'd misunderstood Ron's remark years ago in his shop when
he examined my rifle, as he corrected me at SHOT that he was speaking
of Springfield Armory's poor heat-treat vs. the cast receiver itself. This was
my error, which I will retract in both Boston's Gun Bible and in this thread.
I apologize for misreporting that point, and Daniel was entirely correct when he wrote:

Stretching indicates the steel was soft, and that almost certainly has nothing to do with whether it was cast or forged. More than likely a heat treatment issue, or perhaps the wrong steel.

_________________
When McClung emailed me in 2006:

I guess that what I am trying to say is that if you respect me enough to ask me for an engineering opinion on something, at least respect me enough to believe what I tell you.

. . . it was sufficient for me to publish his thesis in Safari Dreams, but it can no longer
stand alone as a pedestal for that thesis. While I personally still prefer a forged steel
rifle receiver over cast, it's appearing that McClung's thesis so far has no outside
engineering and metallurgical support. In the name of thoroughness I will still try find
corroborating evidence for it, but by now it really is time that McClung himself step up
and either successfully defend his own thesis or admit its flaws. Suffice to say that
the first reprinting of Safari Dreams will be modified accordingly.

Meanwhile, I thank Daniel and others here for their effort and patience during their many
informative posts. It's been a most interesting thread.

Boston

 

[JohnKSa]

Let me be the first to commend you for your very impressive intellectual honesty in re-opening this thread to post this update and also to thank you for the information.

 

[Boston T. Party]

Thanks, John, it's very kind of you to say so.

Boston

 

[Slamfire]

Let me be the first to commend you for your very impressive intellectual honesty in re-opening this thread to post this update and also to thank you for the information.

I'll second this.

 

[TimH]

Let me be the first to commend you for your very impressive intellectual honesty in re-opening this thread to post this update and also to thank you for the information.

I'll second this.

Well I guess I'l third it since I'm the origanal autor of the thread

 

[Boston T. Party]

M1A barrel failure and KB

Somewhat related to this thread, and interestingly
it regards the failure of a roll-forged barrel because of poor heat-treating.

I've quoted it below, but refer to the tech article for accompanying photos.

The backstory of this KB at:
http://www.thegunzone.com/m1akb.html

Boston

http://www.thegunzone.com/m1akb/762r.html

This polished section shows two features: a longitudinal crack and sulfide inclusions in the steel. The steel used in the barrel appears to be a re-sulfurized steel. The inclusions seen in the figure have been elongated as a result of the forging/rolling of the barrel. They enhance the machineability of the steel. These inclusions are well spaced and reasonably fine sized and do not form any noticeable network to act as a source of the failure. Shown in the top of the figure is an incipient crack. A number of these cracks were examined. The cracks appear to initiate at the surface, dive straight in and then turn to run parallel to the bore axis. The cracks seen in the transverse section, Figure 5, are these cracks as they appear from the end on. These specimens were etched to reveal the microstructure of the steel. This is shown in Figures 7 and 8.

Figure 7: Microstructure of barrel steel showing ferrite/pearlite structure and crack propagating through the ferrite grains Again, a crack can be seen running parallel to the bore axis. The dark areas are pearlite and the white areas are ferrite. These are the two major constituents of the steel. It should be notice that the ferrite forms a coarse network around the pearlite. Also notice that the cracks run through the ferrite. They appear to initiate in one end of the ferrite grain where it meets the surface, propagate down through the grain, follow it along the axis and then go back to the surface where the ferrite grain again surfaces.

Figure 8: Typical microstructure of failed M14 barrel showing elongated grains. A little background on steel; steel is pretty amazing stuff because it so versatile in the properties that can be produced. One of the reasons is the allotropic phase trans*for*ma*tion that takes place when iron is heated above a certain temperature. It changes its crystal structure, the arrangement of atoms. Above this temperature, the austenitizing temperature, it can dissolve carbon. However, as the steel is allowed to cool and the phase transformation occurs the solubility of carbon in iron decreases. Think of making rock candy; as the water is heated, more and more sugar can be dissolved. As the water cools, sugar begins to precipitate out and form solid sugar crystals. In much the same way, this is how steel works. In steel, the way the carbon precipitates (comes out) is largely dependent on temperature and time. Cool it fast and the carbon can't get out fast enough and is trapped in the iron. Cool it very slowly and it can come out uniformly. All the different phases seen in steels are basically governed by this reaction. It largely evidences itself in the hardness and mechanical properties of the resultant steel. The two phases seen in this steel are ferrite and pearlite. The ferrite is nearly pure iron. The pearlite is a combination of two phases that contains most of the carbon, trapped inside. Ideally, one would want a nice uniform grain size and uniform distribution of the two constituents. This results in uniform properties. The steel used in these barrels is rolled from a starting billet to produce a rod which is than bored out to make the barrel. The rolling or forming process causes these grains to become elongated as the bar is deformed from a large diameter to a smaller one. This is normal. The problem with this barrel2 is that the ferrite grains are quite large and interconnected through the pearlite grains. Because ferrite has so little carbon in it, it has roughly the strength of iron. It is the weaker constituent of the steel. Because the grains are large, interconnected and many sit on the surface of the bore, they are prime sites for cracks to form.

"This steel was most likely held at too high a temperature for too long."
That is what happened here. The other factors to consider are that the forces or stresses in the barrel which are tangential, i.e., want to cause radial cracking, are the highest at the bore sur*face and the bore surface is also the location of the stress concentrations at the land and groove interfaces. The ferrite grains are governed by the grain sizes in the steel while it is held at the high temperatures above the austenitizing temperature. This steel was most likely held at too high a temperature for too long. This allowed the austenite grains to grow too large and resulted in the large ferrite grains. Hardness measurements were also made of the sections removed from the barrel; they average out at about Rc 30. This would be typical for this type steel as processed for a rifle barrel. However, hardness measurements don't always pick up the anisotropy seen in this material. In summary, the failure of the M-14 barrel was the result of a poorly formed microstructure in the steel.

There was no evidence of excessive inclusions that would have contributed to the failure. The cause of the observed non-uniformity in the microstructure was most likely due to poor quality control during the austenitizing/rolling operations. The appearance of the microstructure is indicative of a 4xxx high strength low alloy steel. This is certainly an appropriate steel alloy. There should not have been any problems if conventional practices had been followed. Most likely, the effects of the large grain size after processing could have been detected by conducting mechanical property tests on a sample of the barrels prior to machining. The highly deformed microstructure resulting from the anisotropy would result in different mechanical properties depending on whether the test specimen was cut such that it aligned with the barrel axis or was cut transverse to the barrel axis. Higher toughness and ductility would be measured parallel to the axis and lower toughness and ductility for the transverse specimens in this case.

 

[gvnwst]

Dang, long going thread

 

[TimH]

Dang, long going thread

[proud papa} Thats my baby [proud papa]