doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com cyanide case coloring also provide hardening?

From Wikipedia.

Cyaniding

Cyaniding is a case-hardening process that is fast and efficient; it is mainly used on low-carbon steels. The part is heated to 871-954 �C (1600-1750 �F) in a bath of sodium cyanide and then is quenched and rinsed, in water or oil, to remove any residual cyanide.

2NaCN + O2 → 2NaCNO
2NaCNO + O2 → Na2CO3 +CO + N2
2CO → CO2 + C

This process produces a thin, hard shell (between 0.25 - 0.75 mm, 0.01 and 0.03 inches) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours so the parts have less opportunity to become distorted. It is typically used on small parts such as bolts, nuts, screws and small gears. The major drawback of cyaniding is that cyanide salts are poisonous.

Sure there are other ways to do it but two interesting points are speed, 20-30 minutes and less opportunity to become distorted.

I don't believe that warping or distortion occurs during heating, but during the quench. It is hard to quench pieces of varying cross section uniformly. When I have made things such as punches that had to be heat treated, I always tried to put them into the quench endwise, or they will often come out bent.

I fully agree with Tom on the distortion bit. I also seriously question the "Fact" that Cyanide will result in a higher hardness that pack hardening. "Carburizing" is simply the part which adds carbon to the skin (Case) of the steel. The carburizing only minimally hardens it if at all, the quench hardens it. A part can definitely be carburized deeper than .003" by the pack carburizing method.
They can be "Colored" by heating below the critical temp, but they are then not hardened as Tom said. When the critical temp is passed the molecules are re-arranged, the sudden quenching "Locks" them into this new arrangement. If allowed to cool slowly they revert to their original condition, this is what annealing does.

A couple references I looked at claim that cyanide case hardening produces a higher surface hardness than charcoal pack hardening because the cyanide process nitrides the surface as well as carburizes it. I don't know if this is true or not, and one source said that it was tough to prove by Brinell hardness testing because the hardened skin was so thin that it would deflect under the point of the Brinell tester due to the softer material underneath. There is also a lot of conflicting and very obviously incorrect information on the cyanide process, but what KY Jon posted seems accurate and explains why it was less costly than bone pack coloring. I'd like to confirm the part about nitriding of the surface but it can be seen that nitrogen is certainly available.

Apparently, both time of exposure of the red hot steel to a carbon source and pressure affect the depth of carbon penetration. It is claimed that there will be a slight pressurization with a tightly packed crucible with a very small vent hole which will cause deeper carbon migration, as will heating in a pressurized carbon rich atmosphere such as a carburizing flame or a furnace rich in carbon monoxide. That made me wonder if that has as much affect upon the resulting colors as the composition of the charcoal pack, i.e. bone charcoal, willow charcoal, charred leather, charred fruit pits, etc.? I'm interested in what might have been different in producing the deep predominantly blue colors seen on original Lefevers with lots of remaining case.

I found that later too Keith, and apparently it's not just a carburizing process. A thing that may matter though is that the hardenabilty is increased, but not necessarily the hardness.

Another interesting thing about cyanide case colors is that the colors seem to be more durable than bone charcoal colors. I don't think it's my imagination. I've seen a lot of cheaper guns like Hunter Arms Fultons, H&R's etc. that retain a higher percentage of case colors as bone charcoal pack colored guns with similar amounts of overall use and wear to the gun. My very first shotgun, a Stevens 220 hammerless single shot 20 ga. that I bought with my paper route money is an example. I still have it, and it still has very strong colors in spite of a lot of use by me and prior owners.

Is that a possible clue that some component of sodium cyanide... nitrogen perhaps... could be introduced to the crucible to increase the durability of bone pack colors?

Keep sorted in your mind that hardening and coloring are differing processes. You can harden without coloring, color without hardening, and harden with coloring. Also, note that differing steel alloys react somewhat differently to the various hardening/coloring processes. Further note that low carbon steel is sufficiently strong (assuming adequate section thicknesses) without case hardening (the only hardening available for low carbon). The case is for surface wear reduction. Higher carbon steel alloys (which will through harden) are more likely (within my knowledge) to be colored separate from hardening. Witness the W-W M-21; the original plan was a case colored finish on a through hardened receiver. Said plan didn't work out satisfactorily, so they through hardened and blued.

Cyanide case hardened steel may or may not have increased surface hardness; I can't say. However, I am sure it is not to be worried about other than as a scientific fact. Aesthetics are, of course, your individual call.

DDA