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

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 6�".

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?