doublegunshop.com - home Forums General Discussion DIY Gunsmithing @ doublegunshop.com color case hardening

hereford: any comments re tempering techniques and temps?

My understanding is that this process, color case hardening, is done because the steel of the part does not have enough carbon in it to harden by conventional heat treating. I believe given the correct conditions created by this process, carbon defuses into the part. It seems to have been shown to require a low carbon steel for carbon to diffuse into the part and it seems to have been shown to only penetrate a few thousands?

Are you thinking that a crack formed in that few thousands on the surface, or did the entire part become somehow brittle and crack? If the entire part cracked, was it a good candidate to attempt this process, or was the process done incorrectly, or some of both? Possibly, not entirely related to just tempering?

craig: our understanding of the definition of case hardening is essentially the same, with color being a by product of the process...

it is my understanding, that if the metal is not tempered back and left brittle like glass, small parts, like tangs and levers may crack and break...and if surface hardened only, larger parts like frames and trigger plates, may develop spider web type cracks, under the stress of firing, if they are not tempered...

it seems plausible that if a large part, like a frame or receiver breaks, it is because the part was hardened all of the way thru and not tempered enough?

Ok� as far as heat treatment, steels fall into several categories: low carbon, med carbon and high carbon. Low carbon steels are generally steels with less than .25% carbon, these steels cannot �self-harden�. Steels such as 1095, 4140 with more than .25% carbon will self-harden, meaning when heated to a certain temperature and quenched in the appropriate medium, have enough resident carbon to harden all the way thru.

Low carbon steels do not have this capacity, thus carbon is added� thru the use of Bone/Wood, above a certain temperature the charcoal off gasses carbon in the form of carbon dioxide and carbon monoxide which is adsorbed by the steel at these elevated temps.

Case hardened parts, have a hard outer layer .001 -.003 and a soft inner core, which make it ideal for gun parts. Hard outer shell to provide wear resistance, soft inner core to absorb shock.

Steel can only absorb carbon, it cannot desorb carbon. Annealing does not remove carbon, it only allows it to spread within the steel, diluting it somewhat. IF a part is case hardened enough times, it can change the steel from a low carbon steel to a medium carbon steel; so from a case hardening steel to a through hardening steel. Thin parts are especially susceptible to this. If you have a thin piece of steel �say .030 thick, it will quickly absorb enough carbon to become thru hardening�.. Remember, unless blocked, carbon is absorbed from all sides top, bottom, front back.

I have had parts (done by someone else) that were so hard that the owner snapped a butt plate attempting to install it. He sent me all the CCH parts, and I could immediately tell what had happened. Quenched at too high a temperature and quenched in brine water; which greatly increases cooling�makes water wetter. I annealed the parts, welded the butt plate and re-cased� properly.

is it correct to say that most sxs shotgun frames made prior to ww2 were made of low carbon steel? hence the need for case hardening of certain metal parts, in order to increase their durability...

and how is annealing different than tempering?

also, what kind of shotgun was it that had a steel butt plate? an old parker hammer gun maybe? or maybe something german?

Annealing is softening the metal to work it. Typically, heat it to cherry red and let it cool slowly.

Tempering is toughening the metal after hardening. Once hardened (typically done by heating to non-magnetic / cherry red, then cooling quickly by quench), the steel is 'glass hard' and glass brittle. It will break easily.

To Temper, the hardened steel is heated to a moderate heat in order to essentially undo some of the hardening. Typically this is done either with a torch (by color) or in an oven set to the right temp for the steel, which varies

Annealing is the process of softening metal by heating at or above it�s critical range, then allowed to slowly cool.

Tempering is the process of heating a substance to a temperature below its critical range, holding and then cooling. Tempering is useful in reducing the brittleness of quenched steel.

Lynton McKenzie used to say something to the effect that, getting the correct hardness is no trick and neither is getting nice colors. Getting both without damage or warpage is difficult.

I've always said that heating any steel part red hot and dumping in a bucket of water is simply foolish~~

When I taught at the Summer gunsmithing classes at Trinidad State years ago I witnessed a case coloring session when a glowing red-hot Ruger #1 action was dumped in the quench tank. I cringed~~Six months later a visitor to my shop proudly showed me that same action. I cringed again~~ IMO 4140 steel (40 points of carbon) should not be subjected to that treatment. I know it is being done at lower temperature, but I would never do it.
The Hagn action shown above was specifically made from 8620 (20 points of carbon) so it could be properly heat treated by case hardening. This hardening is to provide a slick skin, increases wear and corrosion resistance. The beautiful colors are a bi-product.

Bertuzzi sidelock from my shop. Lovely case colors (Turnbull) hiding a couple thousand dollars of Mike Dubber engraving.

I have been placing the freshly colored parts back in the heat treat oven at 350F and allowed to set for 30min and then turned off. At this point I have not had issues. Seems to darken colors. At this point the majority of my experimentation has been achieving colors more in line to the pre 1913 LC Smith's as the later marlin era guns as well as earlier Syracuse appeared different as expected. Currently the majority of my project guns that warrant a restoration fall in the hunter arms era so that is the look I am shooting for. As Mike has mentioned it takes looking at 97%+ condition guns colors to compare as what many are shooting for are not nowhere near close to factory even though they are nice.