dont know about this method of heat treating, except that i have heard it is quite dangerous...any info re the process would be in appreciated...

and does anybody currently do this kind of work?

I’m not aware of any that do. More importantly why? Flame hardening yields a much harder, deeper and durable case with none of the inherent hazards of using sodium cyanide.

Dave

Bobby Tyler of Tyler Gun Works does it.

dtm: flame hardening? tell us more about that...

was wondering what current manufacturers of shotguns, such as csmc,beretta and the turks, are doing to case harden shotgun parts...are any of them using a cyanide process...

Flame hardening is just that, a high temp flame is used to heat the steel then quenched in water or oil. Which liquid used for quenching used is determined by the carbon content of the steel and the desired hardness. The flame is either an oxyacetylene or oxyhydrogen one resulting in a deep hard martesite layer over a soft inner core. The flame can be applied via a single head torch or a custom design predicated on the shape of the object to be hardened.
Cyanide hardening produces a shallow hardness layer of .5 to 5 mils in depth with a hardness of 50 to 65 HRC. Pack hardening is the type used which we are generally familiar yields a case depth of 5 to 60 mils with a hardness of 55 to 65 HRC. Flame hardening yields a case depth of .5 to 8 mils with a hardness range of 45 to 70 HRC.
Large parts such a some gears and machine toolways are flame hardened.

dtm: be aware that flame hardening, aka torching of shotgun receiver parts and forends, is frowned upon by some members of this forum...with that said, do you have an opinion you would express here?

Yes I aware that flame hardening is frowned upon by some here on this forum as is cyaniding. If used correctly, however its results are superior to cyanide hardening.
If one were restoring a firearm, then there is but one choice and that would be to anneal the parts to remove hardness and temper and use the methods employed at the time they were manufactured, ie bone and charcoal pack hardening. Bear in mind the coloration of the steel is secondary to the purpose of hardening it. A welcome side effect if you will.
If one is attempting not to restore but to place back in service a tired old worn piece, flame hardening would be preferable to cyaniding. It is safer and yield better results.

This thread is stupider than Wonko’s hockey thread, but not quite as stupid as the Old Bed thread.


_________________________
Like watching someone play Pong.

Why is this a stupid discussion? I realize I am new here but I am not new to metal working. The information I provided came right out of an ASM white paper on hardening processes.

The ways of many lathe beds were listed as being Flame Hardened. These were normally made of cast iron & used the carbon in the cast iron for hardening. This was often done by a flame traveling down the ways, followed by a stream of quenching water.

I do not recall having ever seen a reference to a gun frame being so hardened. What is "Frowned" upon here is the process of using the tip of an acetylene torch to Spot Temper Color a frame. Instead of imparting hardness this actually draws the existing hardness & in an irregular fashion to boot & is very rightly frowned upon.

Cyanide hardening is done by submerging the part in molten Cyanide which is above the critical temperature of the steel being treated. Carbon is absorbed into the surface of the steel just as in pack hardening, though generally not as deep. The part is then quenched which imparts the hard case. Cyanide hardening generally has more of a "Striped" appearance than "Color Case Hardening" done by the pack method & "In My Opinion" is not near as attractive.

Cyanide hardening is quicker & less expensive than pack hardening & was used on a lot of lower priced guns, such as single barrels & hardware store doubles etc. It was not used by most of the makers of higher quality guns until cost cutting methods came into effect in their later years when they were for the most part just trying to stay in business.

There is a method of coloring gun parts, including frames, which for want of a better term I will call "Faux Case Coloring". This process was described by Ithaca back when the were importing the Perazzi. The Perazzi was made of a heat treated alloy steel & not suitable for Case Hardening. As described by Ithaca the part was heated to around 200°F or just a bit below the boiling point of water. A Cold Blue solution was then squiggled on with a Q-tip to the desired pattern. With a bit of practise one can do a rather good looking job similar to Cyanide colors. I would not of course recommend it on anything other than an inexpensive gun. I once did a Stevens 94 for a friend this way which had belonged to his father but had been neglected prior to his obtaining it. He just wanted to pretty it back up & hang it on the wall. He was extremely pleased with the way it turned out.

Due to the low temp this method does nothing to detract from the original hardness of the part. Hot bluing is done at near 300°F & even in rust bluing, either slow or express, the part is subjected to water at a full boil.

Sorry Dtm but ed has an agenda, and we've endured it since about 2006. He of late has been having trouble recalling our discussions so this might help

http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=128436&page=1
Heating any shotgun receiver without controlling the temperature and heat duration is very dangerous and can damage guns and potentially harm shooters. in additional, if the receiver is heated to very high temperature, such as that required for the bone charcoal process to be effective, then the structure of the metal has been changed and should be tempered back to provide some elasticity. otherwise, the receiver is as brittle as glass and may crack or shatter upon firing of the gun. in my opinion, a low, controlled heat process, combined with specific chemicals is the safest way to recolor a shotgun receiver. so long as the heat is kept low and controlled the metallurgy is not changed and no harm is done to the gun nor is there any potential for danger to the shooter, due to incorrect heat treating, which is the inherent flaw in the use of the high heat bone charcoal process when applied to shotgun receivers.
this is exactly the potential disaster that can result in the use of the high heat, bone charcoal process to recolor a shotgun receiver. the gun becomes a potential bomb, without proper tempering of the metal after the coloring process is completed. the high heat bone charcoal process may work fine for some winchester rifle receivers, but it should never, never be used on a shotgun receiver.
i utilize the services of Ed Lander, for most of my repair and enhancement needs. Old Ed has over 60 years of experience in the gunsmithing trade and does fine work at fair prices. He has recolored literally hundreds if not thousands of shotguns receivers using his low controlled heat, chemical process. my criticism of those who case color shotgun receivers via the high heat bone charcoal method is well known, particularly by those who practice that black art as a business.
i do not know much more about old ed's case coloring process. he has developed his techniques over many years of trial and error. he even has different chemical formulas that simulate different factory colors for different guns, depending on when they were made. i believe he may use a potters kiln to precisely control heat. what i do know, is that his top priorities in all of his work are shooter safety and customer satisfaction.


He will now play the victim, simply seeking the truth, while cowardly hiding behind internet anonymity.
I can be contacted through the website listed below

dtm: in order to clarify, the above post appears to be an amalgamated, cut and paste, version of narratives that i have posted on this forum many times, over many years... the post above, accurately reflects some of my opinions and others not so...there is no agenda beyond expressing my opinions based on my own observations and that of others with far more knowledge than i...fact is, due to costs and potential risks involved; since 2010, i have not commissioned the recoloring of shotgun receivers, via any methodology...

and the fact that the above off topic post was even presented here, in its cut and paste form, makes one ponder who has the agenda...

Ed Good & dtm.....one and the same????? Hmmmmm.

le: now dats cool...maybe you could start an ot thread, suggesting which members here have more than one id...boy could i come up with a few...

LeFusil, I can assure you that I am no one but myself. I do not know Ed Good nor anyone else on this forum.

As for recoloring parts, whichever method is used, the coloration is but a single part of the process. Anyone that knows anything about heating and hardening steels, knows that they must be tempered post hardening or else you do indeed end up with a very hard yet very brittle part.

Dave Myrick

I think 2-piper summed it up well. I suspect, Dtm, you are referring to actual heat treating and not coloring for appearance. I think torching to simulate case colors is frowned upon because it’s not a flattering gun finish appearance, but it may also spot anneal in unknown variations.

An important consideration could be that gun parts may not necessarily need a very high surface hardness. Case coloring for appearance may be the practical goal without getting into the worry of distortion for various reasons. If a part was truly flame hardened by the book, that property may be lost if some degree of heat is used to create colors that’re desired. Only thoughts is all.

Yes I was speaking to the process. As I said any coloration would be secondary to the actual hardening of the steel itself and properly flame hardened and quenched parts would not have the colors of pack hardening.

Dave Myrick

dave: you seem to be a ray of calm and sunshine, re the stormy, cloudy subject of metal treatment...ed

+1 for 2-p. Hardening steel ranges from simple to verrrry complex and difficult; depending on the alloy and what result is needed. historically, gun parts have been made from low carbon steels that will not heat harden. They were plenty strong mechanically. However, they were subject to rapid wear. Fortunately, case hardening was reasonably well understood.

DDA

While on this subject of "Color Case Hardening" I have a question. In all my years as a machinist whenever a part was Case Hardened (Carburized & heat treated, though often in one process) it was always "Drawn" after quenching, usually at around 300°-350°F. I once saw a young man who was going to trade school & had built a set of what we referred to as 1-2-3 blocks. He had case hardened them & ground to finish within +/- .0002" tolerance & was very proud of them. He however did not draw them & the next day they were simply SCRAP. The entire surface of the case was checked & cracked. The Drawing process was for the elimination of this surface cracking of the brittle surface.

To date I have never seen this drawing mentioned in relation to the Color Case Hardening of firearms parts. Was this simply not done & if not what kept them from developing these surface checks, which would be totally unacceptable.

In order for a piece of iron or steel to harden it must have sufficient carbon content, generally at least 30-35 points (.3-.3½%). In case hardening exposing the surface of the part to a carbon rich substance while under heat causes the surface to absorb the carbon. Deeper depths of case require longer periods of heat. For hardening to take place requires the part be quenched from a above its "Critical Temperature". At this temp the molecules are rearranged & the part loses its ability to magnetic properties.

Any time a piece of steel is heated to this point & quenched there is a possibility of some warpage &/or size change. For this reason personally I have never decided to have one of my guns re-cased hardened.

Colors can be obtained by re-absorbing carbon into the surface & quenching from below this critical temp. To do so however would Draw the temper of the original process to a very low surface hardness, not an actual anneal, but close to it. Personally I would much prefer to retain my original case hardness than to restore the colors at the cost of the surface hardness & would never have a gun of mine so colored.

It is of course true that even the mild steel the frames are made of are sufficiently strong to contain the forces of firing, in fact they were normally proof fired "In the White" but the hard case was put there for a purpose & I prefer to retain it.

It is noted when these guns were built they were both Soft & Hard Fit. The soft fitting was the mating of the parts prior to heat treatment. After the hardening there would be some warpage. The "Hard Fitting" consisted of literally Hammering them back to a fit.

"IF" any desire to have a frame re-color cased be certain you trust it to someone who knows fully what they are doing & understands the possibilities. Also from my standpoint I would say make certain they are actually going to re-harden it & not just re-color it. If anyone tells you there is no chance of warpage in their process "AVOID THEM LIKE THE PLAGUE" they either don't know whereof they speak or they are outright LIARS.

Turnbull draws his parts as do some others. I just had a gun in re-hardened by another shop with the top tang snapped off, too brittle. I repaired the gun but either too hard or too soft does happen. I agree with everything you posted Miller.

yes, yes, yes...sigh...vindication at last...

SKB;
Thanks for the info that Turnbull as well as some other present day case hardeners do draw their work. This had always been a concern of mine.

As stated my experience with case hardening has been machine shop related & was not done for color, only the effect of the soft core with surface hardness.

While as stated I have not had any of my guns recased per my present knowledge if I were to do so I most likely would pick Turnbull. I do have confidence that he fully understands the process & has the capability of doing a proper job.

kudos, re turnbull's expertise...

Just curious Steve, did the tang seem to have a soft core or did it seem to have enough carbon in the core to through harden?

I think by the book heat treating will generally say that a good bit more than .3% carbon ends up in the case. If it’s hardened, it still may end up in the relatively brittle range as tool steels go, but I believe that case is only supposed to be a few thousandths thick, and the core is not supposed to be able to harden. I’m pretty sure drawing is generally part of the process, but it may have to be limited to not change the colors that formed.

If hardening is truly part of the gun finish process, I’d think a shop could promise a rockwell hardness. I don’t know that they are able to. It may just be that high carbon steel with a skin of oxide on it is more wear resistant than than low carbon steel under it. I’m not saying hardening isn’t happening, only that I think there’re other priorities when it comes to gun finishes. Thanks for commenting about your experiences.

Sometimes the thinnest portion of a part will through harden and this seemed to be the case in this situation. The client tried tapping the rear tang hole after the gun was re-hardened, popped off the section behind the hole. Had to weld that piece back on, torch draw and re-tap, make a new screw etc.

Hardening properly is the first consideration for me, looks second. I want a hard surface and a ductile core. Some shops do not see it this way.

I had to do the hard fitting on a Turnbull hardened boxlock for a client last year. The tang warped down substantially and the exterior was very hard. Because of the distance I needed to move the tang I torch drew it even though I know Turnbull draws his parts. I managed to move it to where it needed to be but it took some serious hard fitting. Not for the faint of heart but required at times.

Hard fitting is part of the process even if clients seldom hear about it. I have re-bent shotgun tangs top and bottom, forend irons, buttplates, lock plates and related, mainly thin parts.

One of the lockplates, top tang and forend iron warped on this project.


This has the case colors removed for French Gray;the action pinched in.
The worst are single shot actions. By worst, I really mean scariest as this is the most difficult and terrifying process I've undertake in four decades of gun work.

When a Sharps or Hagn action pinches in from the sides of the breech block mortice to the point the block will no longer go in it requires drastic methods to move the metal a couple of thousandths of an inch. As there is always some spring-back, it has to be moved a bit more than you want it to end up. I've done this with a large nut and bolt ground to fit the area and with two wedges, from top and bottom.The wedges worked best. They were made to fit with a very shallow taper.

Both parts a final lapped to fit with very fine compound. Mind you, this work is for Final Assembly of a case colored, engraved action costing a couple thousand dollars on a custom rifle valued at in the low five figures.
I've got another Hagn rifle soon to be ready for all that in the near future.

Years ago there was a firm called Heinzelmann and Sons in NJ that did cyanide hardening. They warped a Sharps '77 action for me...
To the best of my knowledge they went out of business a decade or more ago.

Fred Heinzelmann and Sons reportedly did some of the original case-hardening of A H Fox guns, but went out of business some time ago. I tried unsuccessfully to reach them for about two months, five years ago. I finally called the post office in Carlstadt, NJ and inquired. They said it was an undeliverable address.

SRH

Machinery's Handbook;
Casehardening;
In order to harden low-carbon steel it is necessary to increase the carbon content of the surface of the steel so that a thin outer "Case" can be hardened by heating the steel to the hardening temperature and then quenching it. The process, therefore, involves two separate operations. The first is the carburizing operation for impregnating the outer surface with sufficient carbon, and the second operation is that of heat-treating the carburized parts so as to obtain a hard outer case and, at the same time, give the "Core" the required physical properties. The term "Casehardening" is ordinarily used to indicate the complete process of carburizing and hardening.

To absorb the carbon the iron or iron alloy material is heated above its transformation temperature in the presence of a carbonaceous material which may be solid, liquid or gas. The longer the part is kept at heat the deeper will be the case. Cyanide is normally used for only shallow cases while the pack process is normally used when the deeper cases are desired. Depths of up to around .030" are quite practical with the pack (Solid) hardening process.

Note also the carburizing & hardening process can be done as separate operations or combined into one where the part is immediately quenched from the carburizing box.

Also note traditional Rockwell testers are normally useless on case hardened parts. The "Penetrator" will break through the case & give a false reading. The usual method of testing the hardness of the case is to use a "Shore Scleroscope". This instrument drops a diamond tipped hammer (Not pointed) of about 40 grains from a height of about 10" onto the metal. The rebound or bounce is then recorded, the harder the metal the higher the bounce. On hardened steel it may bounce to a height of about 6¼".

Case hardened steel, even when drawn at the low temp which is usual will be too hard for a file to cut in, it simply skates over the surface. This degree of hardness is impossible to obtain with a steel containing 30 points of carbon. A file normally has a carbon content in excess of 1% or 100 points. A case hardened part should have a carbon content up in that range.

Even though freely admitting the soft steel frame has the strength to withstand the firing of properly loaded shells, even the proof loads, I personally am Totally & Absolutely Un-Convinced that the case hardening process adds No Strength to the frame. I believe that it makes the frame both stronger as well as more wear resistant.

Kreighof and the Turks each paid Oscar to film him and record his directions as he case hardened an action. He of course used bone meal and charcoal, +, + + .

bill

2-p, re case hardening adding strength to the hardened part:

Take a look at the modulus of various steel alloys both soft and hardened. I think you will find this number to be quite similar, esp. for plain carbon steels. This means that up to the yield point of the core the case will have the same strain and will not add anything beyond its "soft" strength. Above the yield point for the core, it will be permanently deformed while the case will attempt to return to original dimensions.

Rumnate on that and let me know.

DDA

R-Man;
When I was actively employed I could always look up info such as this in books we kept in the shop. Now that I have been retired for some years I simply do not have them at home. I only have an old copy of Machinery's Handbook which has quite limited data.

It gives case hardened 1020 steel drawn to 400°F as having tensile strength of 80K psi & a yield of 50K psi (No case depth given). "Soft" open hearth steel in annealed condition shows 50K & 28K respectively. I do not have figures for the 1020 other than in the case hardened condition, but believe it to fall between these two figures.

1035 steel when through hardened, not cased, oil quenched & then drawn varies in it tensile & yield when drawn from 800°F to 1300°F. At 800° tensile is 96K with yield @ 65K. when drawn to 1300° these figures drop to 83K & 51K. With 1045 under same heat treat conditions tensile varies from 115 K to 95K & yield from 80K to 62K.

I really do not see how the addition of this hard case around the soft core can fail to add some amount to the strength, even though not necessarily a large part.

2-p, I agree with the yield and ultimate tensile you quoted. The catch is that the stress/strain (load/stretch) is similar for high and low tensile steel. However, the low tensile will yield at a given load whereas the high tensile steel will continue to stretch under increasing load without yielding. The hard steel has a higher yield point only by virtue of a greater stretch. So, as I see it, it is adding no extra strength beyond what it would add if it was soft.

DDA

If the harder case gives part of the frame, the skin, a higher yield point, even if it is by virtue of a greater stretch before the yield, could that not be interpreted in layman's terms as "stronger", Don? Aside from the abrasion and wear resistance the hard skin affords, an increased resistance to yielding can easily be seen as strength, to non-engineers.

I think this is another case where an engineer and a layman has a simple disagreement over the definition of a term, in this case............... "strength". I have to sit with Miller on this one, on the layman's pew, though I bow to both of your superior understanding of steels.

Ruminating, over a cup of joe.

SRH

I would suspect the actual case hardening of a low carbon steel would make it 'stronger'. Not because the original material was heat treated, but because the case is an entirely new steel that has the ability to harden. I think the trade off with 'hardening' a gun steel is that in a softer, relatively, state, it yields slower. I think hardened steels tend to yield quickly once that point is reached.

Some, not all, of the pictures of failures look like steels that were inadvertently hardened, with little apparent yielding before letting go. But, the grain of the steel might look very course, like pebbly sand instead of smooth and consistent. It may have unintentionally been exposed to heat that didn't just contribute to generic hardening or annealing. Just thoughts.

The non-deep thinkin' I only play a metallurgist on DoubleGun version

The makers of guns (and aircraft jet engines and fan blades) seek the optimal balance of tensile strength (measured by resistance to deformation by stretching), hardness/abrasion/wear resistance (measured by resistance to deformation by denting), and ductility/brittleness (measured by the amount of plastic deformation before fracture and expressed as the % elongation in tensile testing) Malleability is deformation under compressive stress, related to but not the same as hardness.
Manganese sulfate increases malleability. Done wrong, you get manganese sulfate 'stringers'. Too much phosphorus increases brittleness, which was why Swedish and Mesabi Range iron ores were of such value.
In carbon steels, more carbon = harder and stronger. Most turn-of-the-century actions were AISI 1020 which had the optimal balance of the 3 factors, and was easily case hardened. Obviously lock parts needed greater resistance to abrasion, and firing pins both strength and hardness. Barrels greater ductility.
Damascus barrels were made with Wrought Iron and 1002-1005 steel.

All 3 factors can be modified by heat treating, and today by cryogenic treatment, and of course alloys. Done wrong...things go wrong. Some Avis Gun Barrel Company/ American Gun Barrel Manufacturing Company Springfield 03' receivers failed related to improper heat treating.

Rockwell B Hardness.....Tensile strength
Grey Cast Iron - 63.....25,000 psi
Wrought Iron - 65........50,000 psi
AISI 1002 - 66.............40,000 psi
AISI 1020 - 68.............60,000 psi
AISI 1030 - 80.............70,000 psi
AISI 1040 - 93.............90,000 psi

A (uniformly) machined barrel segment loaded in the tensile machine and fitted with an extensometer.

It seems to me that a point of some significance is being overlooked. Whether or not some strength is added to the affected steel by the case hardening process is really not the point. Even if it is (or not) the film-like depth of the hardening contributes little and the OVERALL measurable fail point of the action as a whole is unlikely to be significantly, as in measurably, strengthened. And as best as I understand it the case coloring had no intended result of contributing to the strength of the action. Comparing case hardening (superficial) to heat treating (to the core) does not seem to me to be a tenable position.

JMO of course

On advice of a gunsmith, I didn't colour case-harden a Sterly during restoration because of chance of warping. Was I on the right track?

Drew;
I served my apprenticeship in an Air Force installation (AEDC, Tullahoma TN) which had their own metallurgical lab. During my time there I made quite a few of those little tensile pieces to be pulled apart.

Craig;
Note in the figures I gave above the % of the yield to the ultimate does tend to decrease as the steel gets harder. If one went to still higher carbon %age the tendency would be more pronounced. In the case of the case hardened 1020 the yield vs tensile would be much closer together in the case than the core.

Note that Drew's chart shows 1020 as having a tensile strength of 60K psi @ Rockwell B68. On the case hardened 1020 a tensile of 80K psi is shown. No hardness given, only that it was drawn to 400°F. This would give a quite hard case. Rockwell B68 is about the equivalent of Brinell 121 when tested with the standard 10mm diameter ball @ 3,000 KG load. This is quite soft & is basically off the Rockwell C scale which is normally used for heat treated steel. From these two figures we see the case hardening gives an increase in tensile strength of around 33%. This I believe is due primarily to the case as there would be little change in the strength of the core at the 20 point level of carbon due to heat treating it.

It appears, depending on the process, that color case hardening can increase steel strength
http://www.sst.net/advantages-case-hardening/

I am however looking for numbers...which are better than 1000 "expert" opinions

I should have also mentioned "cold rolling" which was used on decarbonized steel barrels as a means to increase tensile strength. The carbon steel numbers I gave are for cold rolled, not heat treated steel.

AISI 8620 is a chromium, molybdenum, nickel low alloy steel often used for frames today. It is easily carburized and machined when annealed
Carbon - .18 - .23%
Manganese - 0.7 - 0.9%
Chromium - 0.4 - 0.6%
Molybdenum - 0.15 - 0.25%
Nickel - 0.4 - 0.7%
Phosphorus - 0.035% max.
Sulfur - 0.04% max

Of interest, though slightly OT

H.P Leighly, professor emeritus of Metallurgical Engineering at University of Missouri – Rolla published a study of the steel used in the hull of the RMS Titanic in the January 1998 issue of Journal of Metals, the publication of the American Institute of Mining, Metallurgical and Petroleum Engineers
http://www.tms.org/pubs/journals/jom/9801/felkins-9801.html

Thomas Andrews, chief naval architect, and managing director of the design department at Harland and Wolff specified 1” hull steelplate with a yield strength of 40,000 psi and 30% elongation.
The “acid-lined open hearth” (not Bessemer) steel used in the construction of the hull, from the steelworks of David Colville and Co., was similar to AISI 1018 but with a slightly higher phosphorus, much higher sulfur, and lower manganese concentration. The ultimate tensile strength was 65,000 psi, yield strength 41,000 psi with 29% elongation. The low Mn:S ratio made the metal more brittle (lower impact strength) in the cold temperature.

Photomicrographs showed “dirty steel” with both silicate and sulfide (iron sulfide and manganese sulfide) inclusions; slag.

The Titanic was completed in 1912. It has been asserted by other researchers that the steel plate used was the standard for ocean liners of that period, and no steel available in 1911 could have withstood the impact with the iceberg.

So if the test sample is small and the cased component comprises the majority of the (also) heat treated material how is that supposed to apply to a shotgun action?
And I have to admit that fond as I am of CC'd guns this whole discussion ranks right up there with flame colored screws on the importance scale. Yet another topic of seemingly no importance whatsoever to the guns. Whatever it was the manufacturers did to the guns and why is not something that shooters and collectors are gonna be changing very likely.
Simple minded as I am I am faced with the burning question of "who cares" and "what difference does it make?" I suspect that would just be me.
Oh, yeah - casing can warp an action. A common concern.

The little test specimens I made were not for case hardened steels but alloy steels.
I have no idea how the tests were done for the case hardened 1020 steel I noted. Obviously it was not done in this same manner or the results would have been vastly different. If the test part was so small that the "Case" met in the middle then it would have been totally useless for case hardened parts as one would now have a High Carbon steel which was essentially through hardened.

As to who cares "Inquiring Minds Want to Know". "IF" one simply does not care I don't understand why they are still reading on page 5. I totally Skip many threads after about the first page because I have no interest in that particular thread. I am still posting here on page 5 because obviously I do Care.

Maybe, one could also add in previous case recoloring attempts after it left the maker, as possibly relevant. It would seem that casing the tang on SKB’s example to a depth of a few thousandths couldn’t impart enough depth of hardness to cause it to crack off. I’d think predictability matters, and Steve probably cared?

One thing which has always been a concern of mine. When the part was carburized in manufacture the carbon was added to some depth to the surface. This was then hardened to form the "case". Even though the colors fade the carbon is still there & unless annealed it still has its hard case. When the part is re-carburized what effect does this have on the original. what if a different medium is used, is it absolutely going to be compatible with what was used the first time etc. Does this re-carburization add to the depth of the case, perhaps making it more brittle. All things to be considered I believe.

Well I've foolishly (per Wonko) read through pages of internet "expert gun guy" verbiage about case coloring firearms. 95% is a repetition of the same stuff somebody smart must have once said, with no textbook or scholarly journal references.
The non-deep thinkin' version is that traditional charcoal color case hardening IS a form of heat treatment, and, DONE RIGHT, can increase the strength of the steel.
Still looking for numbers that are applicable to firearms but Miller's post re: case hardened 1020 is noted and appreciated

And another but relevant topic. There have been periodic posts over on Trapshooters.com by guys who wanted to fancy up their case colored Krieghoffs with custom engraving...requiring annealing...then requiring re-heat treatment...leading to fractured receivers...leading to very unhappy customers

Heat treatment tutorial
http://navybmr.com/study%20material/14250a/14250A_ch2.pdf

I do not know enough about the Krieghoffs to know what their base metal was for the frames. I know that at least some Perazzis were made of an alloy steel & heat treated & then given "Fake" colors. If an alloy steel such as this was given a traditional Case Hardening it would leave them excessively hard & brittle, could be the problem with the Krieghoffs.

king: in my opinion, your gunsmith gave you good advice...

What I find most interesting is how deep of a case depth you can achieve at the lower temp ranges. Something some so called "experts" say can not be done. It's nice to have numbers to back up what I have always suspected and observed in the trials I have run over the years. I have also found warpage and quench cracking to be much less of a problem when staying below 1500 deg.

I wouldn’t think there would be much disputing that higher temps and longer times could allow available carbon to migrate deeper into lower carbon steel. For gun finish, wouldn’t the question be, is it necessary? I don’t think the colors have been shown to be caused by the carbon, so would packing material intended create colors be affected by excessive heat and time.

It also could be that industrial case hardening of low carbon alloy steels benefit from much more precise controls and higher available carbon than inside of a char coal crucible. I’d think short of taking multiple samples out of a gun part for analysis before color casing, experience in the art of it may come into play.

Temper colors are produced at temperatures well below that necessary for hardening. This would of course normally give a uniform color to the entire part, not the mottled effect of Color Case Hardening. For hardening to take place it is necessary the part be above its critical temp & then quenched. This temp varies according to the carbon content of the base steel. The higher the carbon content the lower the critical temp. The listed 1400° is no doubt above the critical temp for the steel being used for those results. Often times the temp will be raised somewhat for the Carburising stage & then dropped for the hardening phase, but, not below that critical temp. After carburizing the carbon rich case does not need quite as high a temp to harden as it needed to absorb the carbon. Most of the warpage & size change occurs at the quench & the higher the temp above that which is an absolute requirement the more likely it is to occur.

As I stated I have not been involved in any Color Hardening, but it is my understanding to get good colors some form of Animal Charcoal is essential. Just a pure Heat Temper will not give these mottled effects.

DDA

craigd, note that color is a by-product of hardening and not even a reliable indication of hardening.

The fact that guns were often sent out unengraved "in the white" for the season is testimony to the strength of an unhardened gun being adequate. I am reasonably sure that protection of the engraving was a strong reason for case hardening.

DDA

Only conversation Miller. I think games can be played with critical temperature, not necessarily for good heat treating practices.

I've mentioned before and it's easy to verify. Critical temperature is not necessarily the by the book the best temp. for quenching, but it's a practical temp. that can be verified with a magnet. On the way up in temp., carbon steel will loose its magnetism at a very sharp point. But, if one lets the temp. slowly drop, it will be very apparent that the steel becomes magnetic again slowly at a much cooler temp. than it did when hit critical on the way up. Anyway, quenching non magnetic steel will harden it, even if it's not good heat treating principles.

I believe that's a possible intended, or unintended, benefit of delaying quenching the contents of a charcoal pack. The parts may be at a lower temperature than the oven was set at, and the part may have significant temperature differences from thinner to thicker areas, affecting color distribution. It may also be a strategy to attempt to minimize warping. Again, only thoughts, nothing more.

I'd agree and suspect that it is the intent of a process and not necessarily for the metallurgy.