CBTO and neck tension

[RegionRat]

At what hardness level is cartridge brass considered to be ruined?

A new factory virgin case is not one harness level. It has different target values from the neck to the case head, so there isn't a simple answer to your question.

A case must take pressure without flowing near the primer pocket and case head, as well as stand up to the extractor. We get work hardening while taking a slug and doing the deep draws it takes to form a case blank, and then more as the primer pockets and final details are formed. Some of the work hardening of forming the case head is intentionally not annealed and left to form for the value it brings to do it's job. So when you consider that a softened head can weaken the function, it should make sense that there is more than one hardness value for a case, and thus not one answer to your question.

As a hobbyist, you often hear that 400F Tempilaq is placed on the case body to show how far down the shoulder and body the heat flowed while attempting to treat the neck. The idea is to preserve that higher level in the body and the case head. We don't want softening there.

Somewhere, there was a published a write up on a hardness survey that shows a hardness value along the whole section, I will try and find it and paste it in here.

https://apps.dtic.mil/sti/pdfs/ADA497469.pdf

ETA: this one was more of a deep dive into popped primers, but it shows the spec values along the way, as well as the difficulty of the study. There are lots of ways to screw up starting with mounting the sample before you section or poke it. It shows the accept and reject levels for a typical M855 (5.56) style case specification.

 

[mikecr]

One problem that has become ubiquitous is people not being willing to read anything that is longer than a few lines or taking the time to fully understand things that are one notch above dead simple. With that in mind, I suggest that everyone who wants to have a better understanding of case annealing read this more than once, and take notes. https://www.6mmbr.com/annealing.html

I probably took note of this much brought into the article (about 40yrs ago):
The critical time and temperature at which the grain structure reforms into something suitable for case necks is 662 degrees (F) for some 15 minutes. A higher temperature, say from 750 to 800 degrees, will do the same job in a few seconds. If brass is allowed to reach temperatures higher than this (regardless of the time), it will be made irretrievably and irrevocably too soft.

We probably all know that annealing is tied to timing, but then traditionally, brass has been heated with a source temperature capable of over-annealing(annealing beyond our use). Don't want that, so don't overdue it. I haven't seen a lot, or any source until recent, that suggested over annealing was a potential at temperatures below those understood as full annealing temperatures. The typical graph goes like this:

But maybe I've misunderstood this. Never have dipped for more than ~30sec myself, and that's only for a deep body dip in pure lead.
I've also read implications of supermen and dangers of dipping decades ago. This from folks who didn't do it, didn't know how to do it, tried & failed at it, etc.
I admit it wasn't easy to figure out how to be successful with it. But safety-wise a pound of pure lead at ~800degF is no more dangerous than a flame at ~3600degF next to it's fuel.
But hey, even with salt instead of lead, don't drink or eat it kids.

 

[BoydAllen]

It seems to me that most of the academic articles that I have read on this subject have not been done with actual cases, and because of this omission they cannot evaluate whether a given time at a given temperature will result in softening of case heads.

The flame annealing that I was involved with was with two torches and if I remember correctly, for unturned, and slightly turned magnum case necks the time in flame was abut 8 seconds. A second less did not result in the uniform shoulder bump that we were looking for, so we added a second and the problem was solved, with necks hard enough to work with heavy bullets, magnum recoil, and magazine feeding. It seemed that we accidentally found a way to test for degree of annealing.

 

[mikecr]

The case head needs to stay below ~450degF
None of our methods would violate that -without great efforts.