Firearms Metallurgy case hardened vs through

[Faraday]

Hello,

I've been following these threads for a while and have learned a tremendous amount from the members here.

Something that I find fascinating is the metallurgy of firearms. Especially, how it has evolved over time and why it is what it is today (strength, cost, manufacturing, etc.)

I believe that many modern rifle actions are made from 4140 and similar, through hardened to Rc 30's or so, as compared to the old case-hardened mild steel actions of old.

If one were to make say a mauser 98 action today, would it be better/more desirable to make it from a modern case-hardening steel such as 4320 or 8620, or rather a through hardenening steel like 4140 or similar?

What would be the benefits/disadvantages to either approach? I'm guessing a case-hardened action would be more expensive with the additional steps? But could it potentially be better with a very tough/ductile core and harder outer surface for hard wearing and smooth cycling?

Or is through hardened modern alloy steel really the better approach?


There is no specific reason behind asking this, just hope to start an interesting discussion and keep learning about this subject. Thanks!

Best,
Faraday

[44 AMP]

But could it potentially be better with a very tough/ductile core and harder outer surface for hard wearing and smooth cycling?

What's good for a sword blade isn't necessarily good for a firearm. The type of stress the metal has to endure is quite different.

Or is through hardened modern alloy steel really the better approach?

Hardening the steel is a process, using a specific method of heating and cooling. It is normally done AFTER the major machining processes are completed. The base alloy (4140 or whatever) is machined THEN hardened. This allows the cutters to last longer. The cutting tools are very hard, and are intended to cut steel softer than they are. Cutting, milling drilling hardened steel wears out the tools at a very high rate, so whenever possible the steel is machined, THEN hardened. The hardening process is closely watched and if a part warps its nothing but scrap, but losing the occasional part to warpage is less costly then continuous replacement of expensive cutting heads.

Case hardening is a process that hardens the surface of the metal. Those pretty colors come from burning certain other materials with the steel (bone is one). It can provide a very hard surface layer, but once that is worn through or broken through the steel beneath is relatively soft.

There are many different processes that can be used under the heading of "hardening" or "heat treating" the steel. And they yield different results.

Some make it hard on the surface only, (face hardened) some to a given depth, some all the way through. The devil is in the details, and different degrees of hardness are made each being most suited for the intended purpose, AND ease of manufacture.

4140 steel (four point carbon steel), for example, can be "mild" steel (un hardened) or it can be hardened to nearly any degree and depth, entirely dependent on the makers, desires and the specific processes used.

For some applications, mild steel is entirely suitable. For others, only steel hardened to a specific amount and degree is used.

Some of the early black powder guns used steel that was essentially iron, with just enough alloy to technically call it steel. For lead bullets and black powder pressures it worked tolerably well.

As bullets got harder and faster, pressures went up, and the steel had to be stronger. At one time, if it was hard enough on the outside (or harder than needed) the inside didn't matter. Today we know more, and that's why you don't see much surface only hardening outside of certain specific applications.

Hope this helps...

[mete]

Since I'm a metallurgist I can confuse you better !
Guns in general were made of case hardened iron or low carbon steel .The case is added at and near the surface to give the part a wear resistant surface and tough core.. Today we use 4140 which is a great steel that gives excellent performance for many uses. It is through hardened .
As for which is better some of that depends on history of the company [what they used to make ] .Ruger's investment casting vs Remington's machining , both are capable of making good products but just different ways of getting there .
What we don't want is a brittle steel ,that's dangerous . So 35 HRc is all you want for a receiver if through hardened .
Many changes along the way to get a better balance of cost and performance .
Mauser M98 was made of a low carbon steel like our 1018, case hardened certainly during WWII .Many a sporterized rifle was made and served for years with a case hardened receiver.
Other factors like pollution enter the picture also .Lots of choices out there.

[dakota.potts]

To contradict what 44 AMP said, some makers of guns do 90% of the machining, case harden it, then machine the critical dimensions after heat treatment. This is to avoid warpage of the critical dimensions during heat treating. I know for sure that Gunwerks does this with their custom receivers.

If I remember correctly, some submachine gun bolts like the Uzi and Sterling, have bolts that exceed 50 HRC. These are a real pain for home builders who have to anneal and/or use carbide bits to machine them.

Another cool tidbit is the black nitride process. This is becoming increasingly more common on guns due to its excellent material properties. One of these is a nice case hardening effect. I have seen one supplier quote gun grade steels as reaching 50-70 HRC for a depth of around .0015" while leaving the rest of the temper relatively unaffected. Let me tell you from experience, though, nitrided barrels are not easy to cut even with carbide.

Other processes might have similar affects. Glock slides are notoriously difficult to cut and I speculate this has something to do with their finish as well

[Faraday]

Thanks for the replies!

What's good for a sword blade isn't necessarily good for a firearm. The type of stress the metal has to endure is quite different.

Certainly agree. Although, and not to tangent, I remember Hatcher stating that the famously-strong Arisaka rifle was made from SAE 1085 or similar, which I thought was a peculiar choice for a receiver steel (makes a decent knife though). Hatcher's notebook didn't mention anything about the heat treatments of the receiver, which I imagine would be quite different than the typical heat and quench given to a knife or sword.

1085 seems like a lot of carbon for a rifle receiver, anyone have any idea why it may have been chosen and why the action made from it is so strong?

Mauser M98 was made of a low carbon steel like our 1018, case hardened certainly during WWII

I understand that the Mauser for example is a quite strong action by design (geometry, etc.), even with the materials originally used which were nothing special.

Guess I am curious that because the mauser 98 was plenty strong already even using case hardened 1018, would a mauser 98 made of properly case hardened alloy steel (e.g. 4320, 8620 or so) or properly treated 4140 be nigh indestructible?
-probably unnecessary for any real world purpose but still fascinating.

Thanks again for the input everyone
Best,
Faraday

[mete]

Let's include use in the discussion. Some SMGs were designed mostly as throw aways ! The original MAC 10 was IIRC. When they were made for the civilian market they had to change the design
Wartime manufacturing often meant cost cutting to make things cheaper and faster to make .

[44 AMP]

To contradict what 44 AMP said, some makers of guns do 90% of the machining, case harden it, then machine the critical dimensions after heat treatment. This is to avoid warpage of the critical dimensions during heat treating.

I don't really see that as a contradiction. perhaps I wasn't clear enough, but I said hardening was done after the major machining was completed, not that hardening was done after ALL machining was completed.

What I meant to convey was the idea that one does not start with a blank billet, then harden it, then cut away everything that isn't a an action or frame.

I remember Hatcher stating that the famously-strong Arisaka rifle was made from SAE 1085 or similar,

Something to consider about Hatcher's blow up tests, they were blow up tests. And while they tell us something, what people think they tell us isn't quite what they really tell us.

Say rifle A lets go at 32% overpressure and rifle B doesn't blow up until 46% overpressure (numbers pulled out of thin air for illustration only).

Both A and B are identical at service AND standard proof pressures.
Rifle B is widely touted as "stronger", because it too more to blow it up. Does that make any difference at all to the normal working and use of the rifle at standard pressure levels? Not that I can see.

There is a difference between "stronger" being something useful to the ordinary operation, and stronger being "it took more to destroy it".

1085 seems like a lot of carbon for a rifle receiver, anyone have any idea why it may have been chosen and why the action made from it is so strong?

Why they used it might be as simple a reason as it was what they had or could get or make in sufficient quantity, and it got the job done.

its been ages since I read Hatcher's book, but I don't recall any detailed stage by stage explanation of what the rifle went through, before it blew up. More like "rifle survived X pressure, failed at Z pressure, not "noted .xyz setback of locking lugs at X pressure, .abc setback at Y pressure, failed at Z"...( it could be in there, and I just don't remember, and I don't have a copy to check, if you do, let me know if I'm right, or its just my poor memory, please...)

would a mauser 98 made of properly case hardened alloy steel (e.g. 4320, 8620 or so) or properly treated 4140 be nigh indestructible?

No. Nothing is indestructible.

But that doesn't matter much as its always the brass that lets go first.

I have personal knowledge of a reloading accident where a Remington 600 rifle was subjected to 90-110,000 psi chamber pressure. The case let go, and escaping gas broke the extractor, extractor pin, and bolt stop pin, and also struck the shooter on the cheek! (ALWAYS WEAR SAFETY GLASSES!!!!!)
The rifle was otherwise undamaged, and after repair, returned to use.
At the time, Remington said they proofed their rifles to 90,000.

have no idea what it would take to blow up that rifle, and no intention of finding out. Point is, while its nice to know, I suppose, how much past normal limits you have to go to blow a rifle up, isn't important to the user. Could be useful to a designer, but not an ordinary shooter.

[Scorch]

Just a few observations:
"Hardening" steel is an inexact term. Heat treating is what it is called. And most steels have more tensile strength in the unhardened (annealed) state, but they are also malleable and ductile (easily deformed) in that state, so you heat treat them to a point where they still have high tensile strength as well as being relatively hard so as to not be easily damaged.

The Arisaka was made of whatever steel the Japanese could import, they have little commercially mined iron ore in Japan. Most of the steel Japan imported before WW2 was scrap iron from the USA. In fact, placing an embargo on steel against Japan is thought to be one of the reasons Japan declared war when they did (if they had waited any longer they would not have had enough steel on hand to wage a war).

Mausers were made of mild steel on purpose. Hard surface to resist wear, and soft core for maximum tensile strength. Modern commercial Mauser 98 actions are made out of 4140 or similar steel, but are really not much stronger than the originals.

Roy Weatherby started his company with Mauser actions made in Germany, Belgium, France, Sweden, and Finland. He estimated a Mauser action could handle about 70,000 psi safely. He wanted more like 200,000 psi, so he had Sauer develop the Mark V action. But the action is not the weak link in firearms.

Cast iron was used to make many rifles in the late 1800s, including early Winchester, Stevens, Ballard, etc, so-called "powerful" rifles (black powder firearms only generate about 20,000-25,000 psi). Then-current technology for case hardening made the rifles wear-resistant. It also made them pretty. We still color case harden guns for pretty.

Many military arms are said to be "better" than others based on how much we respected the users, appearances, or features that had no impact on their serviceability (Mauser good, Mosin bad, etc, etc). Take the Carcano: it was declared to be cheap and dangerous due to its appearance and the fact that the Italians had surrendered millions of them, and yet PO Ackley had trouble blowing one up. OTOH, the Springfield 1903 was "superior" even though the first 10 years of production produced rifles that were famously fragile.

[5whiskey]

But that doesn't matter much as its always the brass that lets go first.

This is an important point if we're discussing the design of a firearm to withstand higher pressures. Case support of the brass is a fundamental design feature, and even with very good support brass will eventually rupture. And when it does parts of the bolt (at a minimum) will rupture with it, normally.


With that being said, it is in fact an interesting discussion. I have no doubt that most modern bolt action rifles are quite capable of handling 60k psi, repeatedly. I would wager that there are designs, to include the heat treatment/hardening method, that can handle 100k psi, once, will wear faster with a steady diet of 60k psi rounds than the firearm that can only handle 80k psi once. It's an interesting topic though. I hope more guys that are well versed will chime in. I have only dabbled in hardening/annealing so can't contribute much.

[Faraday]

But that doesn't matter much as its always the brass that lets go first.

I guess that really is the answer. There probably isn't much point making something ridiculously stronger than it needs to be for what is already the limiting factor: it would only add cost for no practical benefit. Still fascinating though!

And most steels have more tensile strength in the unhardened (annealed) state, but they are also malleable and ductile (easily deformed) in that state

Not to pick an argument or anything, but I don't believe that is entirely accurate? My understanding is that, within reason, both tensile strength and yield strength of most steels (such as 4140) increase with hardening/heat treatment.
For example, a brief google search gives about 95k psi tensile strength and 60k psi for 4140 annealed.
googling 4140 pre-hard (Rc28-32) gives figures of about 150k psi tensile and about 130kpsi yield.

However, this increase in strength usually accompanies a significant decrease in toughness/ductility as you say, which are absolutely important in a firearms application which is subjected to sudden shocks. In fact, isn't that one reason that the 41xx (and 43xx) steels are so good for firearms? They can be heat treated to a reasonable hardness for a good wearing action that won't deform under pressure but still maintain excellent toughness?

If someone knows, please chime in! I'm learning quite a lot and hope others are as well.

Best,
Faraday

[zukiphile]

But could it potentially be better with a very tough/ductile core and harder outer surface for hard wearing and smooth cycling?

Doesn't that describe melonited 4140 or 4145?

[Dfariswheel]

A couple of points....
There's two types of case hardening.... case hardening and color case hardening.
Case hardening is a process that gives a very thin outer layer of steel a glass hard surface to prevent wear, but leaves the internal steel softer to prevent breaking under shock.
This type is invisible, it can't be seen.

Color case hardening was developed to not only harden the surface, but also to give it an attractive appearance. This is the only use for color case hardening today.

Mauser rifles had a rather soft receiver that was case hardened only in critical areas like the locking lugs.
Early gunsmiths who gave Mauser receivers a hardness test found the receivers to be soft and thought the receivers were substandard.
Since the locking lugs are difficult to get into with a hardness testing machine they didn't try to test those areas.
This gave the old Mauser's a glass hard wear surface on critical areas but a receiver soft enough to withstand firing stress without shattering.

Today metallurgy has progressed to the point where most gun parts are through-hardened and the same hardness all the way through.
A modern exception is MIM (metal Injection Molding) that is usually case hardened after molding.
An example was the sintered molded steel parts used in the Colt Mark III series revolvers and later models like the Mark V and King Cobra.
These are soft inside but have that glass hard "crust" on the surface.