Differences among "case hardenings"?

[Naphtali]

I have viewed photographs of at least three different case hardening processes: a charcoal gray-ish color case; a mottled brown-ish color that I think was identified as "pack hardening"; and something identified as "chemical case hardening."

1. Please describe differences among them, and other variations if there are any, excluding the obvious difference in appearance.

2. What are the purposes for which the processes are intended? If each process has a different purpose, why?

3. These processes appear to be used on frames, locks, receivers, small parts, etc. - but I have not seen a barrel having the process. Why?

4. Since most firearms do not have parts processed, there may be liabilities to processing beyond any additional cost. What are liabilities?

5. Case hardening does not, I believe, build onto a metal surface. Rather it penetrates the surface to alter metal's surface chemistry. How deeply do different processes penetrate, if each penetrates differently? What problems occur with too little penetration? What problems occur with too much penetration?

[JCKelly]

You are asking for a whole lesson in metallurgy.
In order to harden a piece of steel that steel must contain some reasonable amount of carbon. A nail, for example, has about 0.1%carbon in it. Heat it bright red, quench in water and it comes out just as soft as it was when you bought it. Probably softer.
If a piece of steel has, say 0.4% carbon then it can be heated to the appropriate red heat temperature and quenched to be rather hard. To be useful, that metal must be reheated a bit, which these days is called "tempering", anywhere from 700 to 1100F to make it tougher. That might be how a high strength bolt is made, for example.
If you want a file, or maybe a truck spring, one starts out with a high carbon steel, such as AISI 1095. This commonly available steel has almost 1% carbon by weight in it. Heat it 1440F, quench straight down into salt water & it will be hard as a file. Literally, as that is how files are made.
Too hard for a spring, so take that thing out of the quench and reheat (temper) it to maybe 800F or so. Then it will still be strong, but not so hard as to break in use as a spring.
One may take a piece of low carbon steel and put it into a box surrounded by charcoal, sometimes with other stuff added to speed up the process. Heat it red hot, anywhere from 1500F up to maybe 1750F and carbon will slowly diffuse into the surface. When you quench the thing, the surface becomes file hard but the bulk of the piece, the "core" remains soft and tough. This is done for things like pinion gears and impact wrench parts.
No, one does NOT do this to a gun barrel. Would make it too brittle. As well as distort it from the quenching process.
"Color casehardening" is done to make the thing pretty, with only  0.001 or 0.002" hard case on it. Done to make guns look pretty. Not an industrial thing in recent century or so.
Chemical? I dunno, though it might mean someone dipped the thing in a bath of molten sodium cyanide. When oil quenched gets pretty colors and a very wear resistant case. Gov't doesn't like people doing this now. In the bad old days we just did not mix the cyanide with our lunch, thankee.

[Acer Saccharum]

traditional pack hardening uses charcoal to surround the parts in a closed container. this entire container is brought up to heat and held for a certain amount of time. This creates a carbon-rich atmosphere inside the container. At the right heat, somewhere around 1500F, the steel or iron will absorb carbon into its surface. The longer the metal is held at this heat, the deeper the carbon penetrates.


The carbon gets absorbed at a certain rate. It's quite slow, maybe .01" per hour? On thick parts, no matter how much time, the carbon just won't reach the center of the parts; but on thin parts, the carbon can completely penetrate, and you have a high carbon part.

A high pack temp will cause poor colors, tending toward grays. A lower temp, while the case is not as deep, makes for better colors. One can vary the charcoal in the mix, wood and bone charcoal. I have heard of peach pits, which have a natural occurrence of cyanide.

This method is expensive by modern standards.

The cyanide method was found to be more cost-effective, but is too toxic for home practice.

[FL-Flintlock]

The C actually comes from the CO produced by the pack contents which is why the packing of the pack can make a big difference.  Killed steels are preferred for casing and the surface C content is typically limited to 0.9% as excess C can result in retained austenite and brittle martensite.  There is a process called superquenching which does allow some alloys having <=0.3% C to be hardened without additional C being required.

[keweenaw]

Barrels are not hardened by casing as we want them to be somewhat ductile and stretch under pressure.  A hard, high carbon piece of metal is brittle and fractures not good in a barrel!  Just try bending a case hardened trigger without going to red heat if you want to see what happens.  Snap! Equal results in terms of infusing carbon into the steel can be had by any of the methods you mentioned.  Depth of case is simply a function of time, CO concentration and temperature.  How much depth one wants depends on the application and the parent metal.  Impact resistance requires more depth than is required for a wear surface.  Much commercial carburizing is done in a CO gas atmosphere in a closed furnace, much simpler than using anything else as a secondary source of CO but no colors.  Probably more commercial stuff is carbonitrided than simply carburized.  They use a mixture of CO and Ammonia gases in the furnace.

Tom

[44-henry]

I agree that it is not a good idea to caseharden barrels, but that being said I have seen a few originals pistols where this had been done. Most recently was a set of French target pistols with beautiful case colors still evident on the barrels. In a very low pressure application and very shallow case depths I would guess that it could be done, particularly if the quench was done under critical temperature which will still yield dramatic colors, but the warpage issue would be a definite concern. All in all I can't really see any desirable reason to attempt it.

[James Rogers]

Although there is some 17th century mentions of hardening barrels, do you think it possible the French brace you saw just had temper blued barrels? I have seen some with a beautiful array of color and that procedure was common.

[FL-Flintlock]

I agree that it is not a good idea to caseharden barrels, but that being said I have seen a few originals pistols where this had been done. Most recently was a set of French target pistols with beautiful case colors still evident on the barrels. In a very low pressure application and very shallow case depths I would guess that it could be done, particularly if the quench was done under critical temperature which will still yield dramatic colors, but the warpage issue would be a definite concern. All in all I can't really see any desirable reason to attempt it.

I side with James R, there are methods of inducing colors resembling case-color without inducing any appreciable hardening of the surface, only way to tell for sure what you're looking at is to have it tested.

[44-henry]

I have done more color case hardening using the bone and charcoal method than most people have with my university research and it is my opinion that these barrels were color cased. Like I said earlier, it does not mean that they are hardened as they could have been quenched below critical temperature, but I highly doubt that they were artificially colored using any process that I have ever seen. Also, this is not the first time I have seen an antique pistol with this treatment. Just because something does not fit with modern beliefs does not mean that it has never happened.

I probably can't provide the link, but here goes:

http://www.rockislandauction.com/viewitem/aid/58/lid/1182

If that doesn't work simply google French Underhammer Pistols Color Case and look at the images.

See for yourself.

[kutter]

I've seen antique firearms (small short bbl'd pistols usually) w/ what appears to be color case hardened bbl surfaces. No telling if they are really hard though or not.

The beautiful pistol in the link looks like it has an octagon bbl sleeve. If you use your mouse to run around the picture of the muzzle of the bbl on the site you can see a thin edged but uniform sleeve (IMO).
No telling how thick or thin it is deeper in.
Perhaps that was color hardened and the bbl placed inside afterwards.
Can't think of a reason to go to the trouble of making a sleeved assembly otherwise.

Most of the case coloring we did in the 90's was just that,,coloring. There was no surface hardness to it at all.
It looked nice and the collectors and dealers paid big prices for it. But when it came to an action being used continuously in the field, problems with galling and early wear were quickly evident.
It was no trick to case color w/o hardening the part. It was done on purpose to avoid warping, cracking and reassembly problems.

[44-henry]

I think that is just a slight chamfer that is reflecting light rather than a sleeve. I cannot even begin to imagine how much work it would be to fit an internal octagon sleeve that precisely, particularly after it had been case colored/hardened.

Since warpage would be a concern I suppose it is possible that the barrels on the above pistols might have been finish bored and rifled post case coloring. Regardless of what they did it had to have been a lot of work, but they are some of the finest underhammer pistols that I have seen.

[JCKelly]

Agree w 44-Henry, these barrels are color-case hardened.
Usually that means heating in the pack at fairly low temperature, somewhere around 1100 - 1400F, 44-H should know more exactly. The case is very thin, just a couple of thousandths. Not enough to harm the barrel's ductility, and probably done only on the outside.
These barrels would have been wrought iron, which is a reasonably ductile material. A skilled 19th century guy could have straightened what little warpage occurred. If they quenched these barrels straight into the water, rather than belly-flopping them, there would have been little distortion.
As far as ductility is concerned, the cold-drawn 12L14 used today for most muzzle loaders will elongate about 6 - 11% across the grain before breaking. Some of you know I am not at all fond of this grade for pressure containing applications.

[sse]

 "The case is very thin, just a couple of thousandths."

That must be how many mfg's often did shotgun receivers in the first part of the 20th century.  Very pretty at first, but it's nearly impossible to find one in which the case colors are reasonably intact.

[JCKelly]

Well, the case colors and the case depth are two different things.
How fast the colors wear off doesn't have a lot to do with how deep that high carbon, hardened steel case is.

The only published work on color casehardening, of which I know, is Oscar Gaddy's two part series Winter 1996-Spring 1997 in The Double Gun Journal. Gaddy says the colors come from some interaction between the hot steel and calcium phosphate from the bone charcoal during the quench. These colors are NOT the same as "temper colors", which one gets by heating steel in the 350 - 600F range. Temper colors are very thin and readily wear off. Proper case colors are more durable, though they are not the HARD part of the case. That thin layer of hardened steel will remain long after the colors are gone.

In college I recall asking my professor, ex-Springfield Armory (the real one) guy about case colors. He'd not a clue. Modern industry is interested in case depth, hardness &c but only old gun nuts want Colors. Case hardening is a very common industrial practice. It is no longer done in a pack. It is done in a controlled atmosphere of roughly 20% carbon monoxide, 38% hydrogen, 38% nitrogen, very low water and maybe 2-4% of methane to provide the carbon.

In those French pistols the smith cold have bored the barrels, color case hardened, then straightened them, and rifled them last if he so chose.

Boy, I sure wouldn't suggest anyone doing this today with a contemporary muzzle-loading barrel. One perhaps could learn how to do it, but I think one would go through several barrels before getting it right.

[Dphariss]

There were, and probably still are import guns with color but virtually no hard case. Though now that most makers understand that we actually USE the things this may have improved a little.
Where I used to work breech blocks, for example, done when the quench water was too hot would be too soft and would tend to crater.
Casehardening needs to be done by someone who knows how and CARES about his work. The case is usually only about .005 deep and this is really all that is needed for most firearms parts.
If someone wants pack hardening its best send it to someone who does it all the time. I use Wyoming Armory in Cody WY and have never regretted having them do anything from lock plates to complete single shot actions.

Dan

[Acer Saccharum]

I didn't know that I couldn't pack harden at home, so I did it. I didn't know what I was doing, but lady luck was smiling on me.