doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com cyanide case hardening of shotgun parts

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-.3�%). 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