Zebulon said
The person to ask is Turnbull.

I’m not an engineer but, at least in a lever action rifle like the 86,  I would think it is the chamber area of the barrel, the breechbolt, and the rising blocks that must be heat treated to withstand the entire chamber pressure curve. 

Even in a modern  bolt action high velocity rifle, I don’t believe the entire receiver is heat treated, rather only certain spots like the ring.

That begs the question to what extent the receiver of a modern 1886 is heat treated at the factory.

I have wondered whether the traditional case hardening of 19th Century Colt SAA frames and Winchester 1873 receivers was to allow them to withstand the strains of firing or merely to toughen them from damage and wear in hard use.

My limited understanding of charcoal.blueing is that the temperatures are insufficient to harden and toughen the steel to any material degree,or to anneal the steel. 

  

The simplified answer is that it’s perfectly safe to color case harden the medium carbon steels used in most modern reproduction lever actions, primarily 4140.  Without going into a metallurgy dissertation, the short answer is that those of us who color case harden, do so well under the critical temperature (approximately 1650 degrees F) of 4140 and like steels.  Critical temperature is the point at which steels go through phase change and recrystallization (softening).

The temperatures that we use to get the best colors are only slightly higher than what is commonly used to temper 4140 steels after the hardening process.  (Tempering is the process of drawing back or making the brittle, newly hardened steel more pliable).  While color case hardening does have an effect on the strength and elasticity of modern medium carbon steels used in firearms manufacturing, it’s very minimal.  The outer skin of the part is now hardened, and the inner portion has only been nominally affected.  It’s my opinion that the case-colored part is perfectly safe.  If there has been a failure of a modern reproduction lever action firearm after case coloring, I’m not aware of it.  We do it regularly using our Winchester-type recipe with great results.  The modern steels seem to really take on nice colors. 

It’s also worth noting that Winchester lever actions (after the Henry, 1866 and early cast steel 1873’s) were made exclusively of mild (soft) steel up until Winchester advertised the model 71 with a proof steel receiver.  Receivers and internals take only a minor portion of the thrust from firing, so hardening the receiver steels wasn’t necessary.  Modern medium carbon steels that have been color case hardened are far superior in strength than original, mild steel receivers.

Brass cartridges seal up to the barrel and it’s the barrels that take the vast majority of the pressure.  Decades ago, PO Ackley was testing the effects of excess headspace with an 1894 in 30-30.  He fired rounds with progressively larger headspace until there was enough room for the primer to completely back out of the case, while the cartridge held firm in the chamber.  He then removed the locking block completely and test fired the rifle. The case held in the chamber and the action stayed closed.  However, he strongly warned against firing any firearm with oil in the chamber.  That condition can allow the case to slip, and the thrust taken by the bolt, locking blocks and receiver – a very hazardous condition for the firearm and the shooter.

The reality is that receivers are capable of taking far more abuse than what they are subjected to from firing.  When there is a catastrophic failure due to massive over-pressure, it’s almost universally the barrel that fails.  I’ve seen blown up barrels from over-pressure.  I’m sure there may be examples out there, but I’ve never seen or even heard of a catastrophic lever action receiver failure, whether it’s made of mild steel, through-hardened steel, or either of those that have been color case hardened.  Mark

Anthony said
Mark,

Once again, you’re explanation of this topic, is second to none, as with many other, hands on, works of yours.

It goes hand in hand with you’re Cinnabar case color hardening videos.

Thanks for sharing,

 

Anthony

  

You’re very welcome, Anthony. 

I’ve had a rather steep learning curve on color case hardening since I started this gunsmithing journey.  Especially so, since purchasing Wyoming Armory where CCH is our core business.  Mark