When metal is heated, the electrons in the metallic atoms become excited and release energy as they jump around in their orbits. At some level of excitement the energy released crosses into the visible spectrum, causing the metal to glow. This temperature is different for different metals but falls into a narrow range for common metals, including cartridge brass. I think that threshold is about 900*F, maybe a little less for 70:30 cartridge brass. Controlling the ambient light allows you to detect the first signs of the glow as soon as possible and stop heating at that point.
I've also read that the flash anneal point for 70:30 brass is something like 1200*F which I assume is the "dead soft" point. I've never been there or done that and it is obviously something that should be avoided.
Annealing is a kinetic (time dependent) process and will continue to progress as long as the brass remains hot enough. Quenching in water consistently stops the anneal a fraction of a second after the case enters the water. It does serve a purpose.
In summation, you can heat brass with a torch while slowly rotating to about 900*F by watching for the first signs of a glow in a dimly lit room. Quenching allows the anneal to be stopped from progressing further while the brass remains hot and also stops the heat from migrating beyond the neck area. In the end, the brass will be successfully annealed. In my world that means increased case life without neck cracking or any loss of accuracy.
If you're really interested in the mechanics, try these references. I warn you, it's not exactly, "War and Peace" but if you are into metallurgy or a working (or formerly working?) machinist, these are pretty good references.
pp478 /
Elements of Metallurgy and Engineering Alloys
Cold-worked a brasses are also subject to stress-corrosion cracking, originally called season cracking. The term season cracking was used to describe the spontaneous cracking of stored cartridge cases in India during the monsoon season. It was particularly prevalent when the damp atmosphere contained ammonia emanating from nearby cavalry stables. Alloys with less than 15 wt% Zn have good corrosion and stress-corrosion resistance. Alloys with more than 15 wt% Zn need a stress-relieving heat treatment at approximately 260C (500F) to avoid stress-corrosion cracking.
pp286/
Heat Treater's Guide: Nonferrous Alloys
Stress Relieving
This process relieves internal stress in materials or parts without appreciably affecting their properties. Treatments are applied to wrought or cast copper and copper alloys. During the processing or fabrication ofcopper or copper alloys by cold working, strength and hardness increase as a result of plastic strain. Because plastic strain is accompanied by elastic strain, residual stresses remain in the product and can result in stress-corrosion cracking of material in storage or service. unpredictable distortion during cutting or machining, and hot cracking during processing, brazing, or welding. In brasses that contain more than 15% Zn, stress-corrosion cracking, or "season cracking," can occur if sufficient amounts of residual tensile stress and trace amounts of atmospheric ammonia are present. Other copper alloys, such as cold-worked aluminum bronzes and silicon bronzes, may stress-corrosion crack in more severe environments.
Stress-relief heat treatments are carried out at temperatures below those normally used for annealing. Typical temperatures for selected coppers and copper alloys are given in Table 2 (wrought products) and Table 3 (cast products).
Table2 Typical stress-relieving temperatures for wrought coppers and copper alloys
[...]
C26000 Cartridge brass [Tubed/Tubes] 320C(610F)- [Tubed/Parts] 260C(500F)
________________
I copied those a while back for a local bud who hunts track properties when he was thinking about getting into reloading for his hunting rifles. I emailed him a bunch of info including prices which are no longer valid, sadly. Or, maybe not so sad since some prices on gear have come down since 2021. I need to find out if he ever got a press. Time gets away from me a lot lately.
My pix of the tables are garbage. Not worth posting. I can get you the ISBN's if you need them when I get home but you should be able to find these books on Amazon just with the titles. I don't have them at work or the time to look them up, sorry.
Basically, cartridge cases are considered tubed parts from a stress/hardening point of view and you're looking at two sets of temperatures: stress relief and annealing. From a case-life point of view, I've found stress relief is the bigger factor. Sometimes the two terms are merged to simplify the conversation and that's wrong but it happens and there's no sense arguing semantics. Just know the range of temperatures and exposure is much more broad for practical application than certain advertising and internet lore imply.
