Makes sense. Another Q. If I understand correctly this process is to soften the brass which has become hard from working it and firings. Wouldn't the rapid cooling affect of the water make it harder? I always thought a slow cooling process would make things soft and a rapid cool hardens them?
Jeff, your question involves a bit of metallurgy (science of metals). I didn't get an A in metallurgy, but I did pretty well in the lab. I'll attempt to explain, hopefully I can remember what Dr. Forgeng & Dr. London taught me...
You're thinking is correct, for carbon steels, because of phase changes and precipitation of a secondary phase.
However, cartridge brass is different.
Cartridge brass, (alloy C260) is 70%Cu (copper), 30%Zn (zinc). It is a single phase alpha FCC structure, from room temp to about 1700F. Therefore, temperature cycling at/below this range does not produce a phase change or precipitation hardening.
Cold working (forming, sizing, etc) breaks up the grain structure in the brass, which blocks dislocation motion (structure defects) thereby increasing the yield strength and hardness. Although firing is still considered cold working, it has less effect than our normal reloading procedures of forming, sizing and seating.
The process of annealing cartridge brass recrystallizes the grains into new strain-free grains about the same size as the original grains prior to cold working. The grain size after recrystallization is more dependent on the degree of cold working, than on the annealing temperature.
The rate of re-crystallization is governed by temperature and the degree of prior cold working. However, long periods of time at lower temperature can recrystallize also.
The purpose of the quenching is to stop the recrystallation process prior to grain growth (bad).
That's the "Book Learning", which nobody really cares about anyway.
We really just want to know how to anneal the brass and not ruin it in the process...